Corrosion and incrustation in steam boilers (Paper ...

CORROSION AND INCRUSTATION IN

STEAM BOILERS.

BY MR. K. L. MURRAY.

MR. PRESIDENT AND GENTLEMEN,

When commencing my little paper, I could not help feeling that there must be many members of the Association whose practical experience with steam boilers would fit them to intro-duce the subject of "CORROSION AND INCRUSTATION" much better than I can.

My attention was drawn to the matter some time ago when considering the best method of protecting the earth-plates used in practical telegraphy from corrosive action, and believing that it was only necessary to open the subject in order that it might be thoroughly and exhaustively treated by our members. Believ-ing also, that electrical action will be found to be the cure, as well as the principal cause of corrosion, I undertook the duty of compiling from the records to which I had access a few facts and theories which I hope will prove acceptable.

What, then, are corrosion and incrustation, or scale, and what harm do they do ?

Corrosion in boilers is, briefly, a combination of the iron of the boiler with oxygen, which, forming oxide of iron, or rust, necessarily wears away the metal ; and incrustation, or scale, is a deposit of a usually calcareous incrustation in the inside of the boiler. It generally consists, principally, of salts of lime, and is a very bad conductor of heat. When, therefore, a deposit of any thickness occurs, the heat from the furnace cannot get to the water as it should, so that not only does a great waste of fuel result, but, as the iron of the boiler is separated from the water by a substance which is practically a non-conductor of heat, it (the iron) will be likely to burn and twist. This is a very serious

74 Corrosion and Incrustation in Steam b'oile,s.

evil, and the necessity of fighting it has set many minds to work to devise schemes for getting rid of scale, the result of which has been the trying of scores of expedients with varying, but, until lately, not very great success, so that the old and rough plan of chipping off the scale with the hammer and chisel has generally been resorted to. Among the remedies which have been adopted with more or less success for boiler incrustation, Molesworth gives the following twelve :-

1. Potatoes: 1-50th of weight of water prevents the adherence of scale.

2. 12 parts salt, 2 caustic soda, é extract of oak bark, z of potash.

3. Pieces of oak wood, suspended in boiler and renewed monthly, prevent deposit.

4. Two ounces of muriate of ammonia in boiler twice a week prevents incrustation and decomposes scale.

5. Coating of 3 parts black lead, 18 tallow, appled hot to the inside of a boiler every few weeks, prevents scale.

6. 131bs. of molasses fed occasionally into an 8-horse boiler prevented incrustation for six months.

7. Mahogany or oak saw-dust in limited quantities. The. tannic acid attacks the iron, and should therefore be used with caution.

8. Slippery elm bark has been used with some success. 9. Carbonate of soda. 10. Chloride of tin. 11. Spent tanners' bark. 12. Frequent blowing off. Besides these, which are all simple enough, many other reme-

dies have been used. Mr. L. E. Fletcher, Chief Engineer to the Manchester Steam Users Association, in his report, dated November, 1883, dealt at considerable length with the subject of incrustation in steam boilers. He referred to a number of anti-incrustation compositions, the component parts of many of which were veiled in mystery ; many proved injurious to the boilers on actual trial, some lined the plates with a glutinous coating, which, while it kept off scale, also kept off the water, so that the furnace crowns became over-heated, strained, and bulged out of shape. As these compositions were costly, blowing out was too often given up when they were used, which was very greatly objected to. Good soda ash was recommended by the Steam Users Association, .who, at the same time, pointed out that it was not to be introduced at the man-hole or safety-valve in large, intermittent charges when the steam was down, but pumped in with the feed water at the rate of about 31b. a day for large boilers, so that the boiler would . regularly be fed with weak

Corrosion and Incrustation in Steam Boilers. 75

soda-water ; blowing out was not, on any account, to be neglected. Soda ash was recommended as being better than soda crystals, because the latter contains so much water that the ash is more economical in the long run. But the Association remarked that soda was not always successful, and that it was best to analyse the water used, so as to see what would likely be the most effectual composition to use as a remedy.

Above all things, it was desirable to purify the water before pumping it into the boiler, instead of (as all boiler compositions claim to do) treating the water afterwards.

Mr. John A. Rowe, Surveyor to the Board of Trade, in a paper upon Boiler Preservation, written last year, went largely into the question of scale formation. After referring to the evil effect of allowing a non-conducting substance to deposit on the heating surfaces—an evil felt in land as well as marine boilers—he pro- pounds the following questions : What is it that deposits on boilers ? " " And why is it deposited ? "

Sea water holds in solution the following substances :—Chlo-ride of sodium (or common salt), chloride of magnesium (a magnesium salt), bromide of magnesium (also a salt of .magne-sium), sulphate of lime (or gypsum), sulphate of magnesium (or epsom salts), carbonate of lime (or chalk), and organic matter. Those which give most trouble by forming hard, non-conducting incrustations are salt, gypsum, and chalk ; and the order of their deposition is said to .be-1st, chalk ; 2nd, .gypsum ; 3rd, salt. The two former appear to decrease in solubility from 100°'F. upward until, at a temperature of about 300° F., they are com-pletely insoluble. Sea salt is never deposited in an appreciable extent in modern marine boilers, which are fed by water from the surface condensers ; chalk and gypsum are deposited, because the carbonic acid which holds the chalk in solution is expelled from the water by an increase of temperature ; the higher the temperature the •more rapidly the deposit takes place. Hence some boilers (principally land ones) show a heavy deposit close to where the feed water is admitted. Gypsum becomes less soluble with an increasing temperature. As the temperature rises the attraction of the particles of gypsum gradually becomes greater than the attraction between them and the water, and deposition commences. When the minute particles of solid gypsum separate from the water, they adhere to the iron, and, by gradual secretion, build up a non-conducting scale. This deposit takes place not only upon flat, horizontal surfaces, but upon even vertical surfaces, such as tube plates, and if not removed in time they soon assume a thickness sufficient to pre-vent the heat passing from the fire to the water, and the conse-quent waste and damage is experienced,

76 Corrosion and Incrustation in Steam Boilers.

Some of the compounds which have been invented to prevent incrustation may be useful, and their influence do good in softening the scale and facilitating its removal. The best of them, however, contain an acid, and the great objection to their use is that the acid tends to injure the boiler by assisting chemi-cal and galvanic action, which are the most formidable enemies to contend with.

But what are these actions? Chemical action is that by which two or more substances unite to form a fresh substance, or by which a substance leaves that to which it had previously been united to take up with another for which it has a greater affinity. Thus, as a familiar example, water is formed by the union of two gases, which are neither of them like the water that results from the union. Rust is another example; expose iron to the action of damp air, and oxygen leaves the element it had pre-viously been united with to combine with some of the iron, for which it has a greater affinity, rust or oxide of iron resulting.

Now the gas, oxygen, is an all important element in the consideration of our subject, and we must remember that there are some substances for which it has a much greater affinity than it has for others. We know that it has a very great affinity for metals, and a much greater for some metals than for others ; thus it combines very readily with iron, and more readily still with zinc. It can easily be proved that it has a strong affinity for super-heated iron, so that, as corrosion is a combination of the iron with oxygen, we need not be surprised to find in practice much corrosion resulting in boilers wherever there is flame on one side of a boiler plate, and steam on the other.

Now, what is galvanic action? If we take two pieces of metal, one of a kind easily oxidizable, or that has a great affinity for oxygen, and the other of a kind less oxidizable and plunge them into an acidulated liquid, we shall find that chemical action will take place, and they will corrode in proportion to their respective affinity for oxygen. If, now, we connect them together with a piece of metal outside the liquid, or permit them to touch under the liquid, we shall find that the action will be almost wholly confined to the more oxidizable of the two metals, the other being practically unaffected. Thus, if we use plates of iron and copper for the two metals, we shall find that, if contact between them . be maintained, the iron, which is the more oxidizable metal, will be the one to suffer by the oxidation, the copper not being affected at all.

If, now, we remove the copper plate, and substitute one of zinc for it, we shall find that the iron will scarcely be attacked at all, but that the action will be transferred to the zinc, which

On onion and IneruNtotion la Steasi Boilers. 77

is more oxidizable. Another important point to note in connec-tion with this investigation is that the plate attacked will not suffer all over its surface equally, but will show a number of places where the action has been much more intense than else-where ; and this peculiarity we shall find without the two metals being in contact. The reason is that in the metal plates there are impurities, between which and the plates themselves a sepa-rate action has been going on ; and the action which produces these results is known as galvanic action.

Now, if we apply these facts to the action which is constantly going on in boilers, I think we cannot fail to come to the conclu-sion that the wearing away of the metal plate in a galvanic couple is very like the wearing ,away of boiler plates by corro-sion ; and the wearing of the galvanic battery plate in irregular holes is very much indeed like the pitting of boiler plates. In-deed, a close study of the subject will, I think, infallibly lead to the conclusion that the action which has in the past more than any other caused the life of a steam boiler to be so short is gal-vanic action.

In the year 1874 a committee was appointed by the Admiralty to take evidence and report upon the question of corrosion in boilers. This committee collected an immense mass of evidence from the most experienced engineers and others connected with steamships, and made exhaustive experiments, both in the laboratory and with boilers at actual work, to test the adequacy of the means they considered best to be employed to remove the cause, and so prevent corrosion. This committee was dissolved before its labours were completed, and in 1878 another was ap-pointed to continue the investigation. The experiments made by these committees were to determine the effect of working boilers with the water at m» oh higher degrees of density than had usually been considered desirable ; also, to decide the rela-tive resistance of various kinds of iron and steel to different forms of corrosion, and in different parts of the boiler ; also, to see in what way, if any, a highly oxidizable metal, such as zinc, could be employed to prevent the corrosive action upon the iron plates ; and, also, to test which were the best kinds of oils to be used for lubrication.

The result of their labours was contained in reports which were published some three or four years ago, and a summary of their conclusions may be briefly stated as follows :-

That fatty acids, resulting from the use of vegetable and animal oils for lubrication, were a fertile source of injury, and they recommended that only mineral oil should be used.

They recommended, too, increased density in the water, and that boilers should be emptied as seldom as possible.

G

78 Corrosion and Incrustation in Steam Palen.

Their final conclusions were that the destruction of boilers was due chiefly to chemical and galvanic action, and they recommended the use of zinc for the purpose of setting up a galvanic action opposite to that which was so destructive. The result of the committee's reports was the adoption in the ships of H.M. navy of a system of fixing to stays or other convenient parts of the boilers slabs of zinc as pure as possible. The number of these slabs must be sufficient to provide a surface of zinc equal to 1 to 150 of the surface of the iron. It will be seen that a very large quantity of zinc would be used under this system, and the Admiralty committee's report mentioned that the expense of the zinc was a serious drawback to the employment of the system.

I remember thinking, when reading an epitome of the com-mittee's report, that the quantity of zinc used was in excess of what was required"and my experience since has confirmed me in that belief.

The Admiralty instructions issued to their engineer officers are as follows : Zinc slabs will be suspended in convenient parts of all boilers, and the engineer officer in charge of the machi-nery, on each inspection of the boilers, is to examine these slabs, and their condition is to be noted in the engine-room register. The object of these slabs is to form a galvanic battery with the shell of the boiler, and, by making the surface of the latter below water the negative pole, throw it into a condition in which it cannot be corroded: To effect this object special care must be taken—(a.) To insure perfect metallic contact between the zinc slabs and iron stays, or part of the plating of the boiler to which the zinc is attached ; the surfaces in contact of iron with iron, or iron with zinc, being filed bright, and means adopted to secure a good grip. (b.) To place the slabs in such positions that every portion of the iron surface may be protected. (c.) To replace, by new slabs, any found upon examination to be deteriorated. The zinc slabs appear to deteriorate either by gradually wasting away or by a gradual change of substance. In this latter case, which is the more common, the zinc slab gradu-ally becomes black in colour, and friable, and when it has been long exposed, mere pressure of the hand is sufficient to reduce it to powder."

Now, the fact that bhe zinc " changes in substance " and " becomes black and friable," as described in the Admiralty instructions, would appear to indicate that there is not " perfect metallic contact between the zinc and the iron ; " and that is the weak point in the Admiralty committee's plan, and the reason they require to use such a large surface of zinc. If we examine the details of the plan, we shall see why this " perfect

(lorrosion crud Ineryst ition in Steam Boilers, 79

metallic contact " does not exist for any length of time. The slabs of zinc are simply clamped to the iron stays, and an oxide of zinc soon forms between the zinc and iron. This oxide, being a very bad conductor of electricity, introduces great electrical resistance, and entirely takes away the usefulness of the zinc, which, to protect the iron, must oxidize In perfect metallic contact with it.

This weak spot in the Admiralty's plan was seen by Mr. Hannay, an electrician and chemist of Glasgow, who wasstudy-ing and experimenting with a view to devise a cure for incrustation in boilers ; and he hit on a simple and, he claims, most effective method to prevent not only incrustation, but corrosion. Mr. Hannay's plan is to get zinc of the best quality, and cast it in a solid ball on a copper rod, which becomes an axis to the ball. The casting of the zinc on the copper causes the union between the metals to be so perfect as to form brass at their juncture, so that corrosion between the metals is impossible. The zinc ball is then subjected to a good hammering, which converts its crystalline character into a laminated one, and so prevents it splitting as cast zinc is inclined to do. Mr. Hannay then solders copper wires to the ends of the copper rod. The ball is fixed below the water line, and the wires are soldered to some parts of the boiler, so that, when once they are firmly united to the plates, contact between the zinc ball and the iron frame of the boiler will be complete, and instead of the corrosion taking place upon the iron and consuming it, the action is on the zinc ball, which gradually wastes away.

An important point about the use of zinc in this way should be mentioned here.

In a galvanic couple the metal which wears away is called electro-positive to the other metal, and zinc is electro-positive to nearly all the other metals. Now, in a boiler there may be several metals in contact, and in the iron plates there are sure to be impurities which I have referred to as the cause of pitting, and, as zinc is electro-positive to them all, the action, which without it would be continually wearing away these other metals, will be confined to the zinc, which alone will suffer from the galvanic action.

Again, not only are different metals differently affected by galvanic action, but different plates of the same metal. Mr. Rowe, whom I quoted before; says of this :—" Lying side by side, rivetted together, are two iron plates under the water level. . To all appearances they are alike ; their tensile value and ductility may be alike, but it has been proved by experiments that one plate is electro-positive to the other ; it accordingly wastes, while its neighbour remains unaffected. Here seems to

80 Corrosion and Incrustation in Steam Boilers,

lie the chief cause of corrosion below the water line. We cannot obtain a couple of dozen plates equal throughout in electrical qualities. Probably no two plates possess equal capa-bilities for evolving electricity, and, therefore, when they are metallically united, and in contact with an exciting liquid, galvanic action proceeds. Even supposing one could obtain plates of perfectly uniform quality, we have still rivets, stays, tubes, and furnaces (which are generally made of a superior quality of iron) as causes of galvanic activity. So; then, to account for corrosion below water, we must point to plates electro-positive to other plates, or to plates electro-positive to rivets, or to rivets electro-positive to tubes, or electro-negative to them, and in contact with these must be water holding in solution some chemical substance capable of corroding metals. Given these galvanic action must proceed.

" What engineer has not, in his experience, heard of boiler tubes (smoke-box tubes) being destroyed in a few months by heavy pitting ? And the maker of the tubes, no doubt, suffered in reputation, and probably in pocket. Viewed in the light of electro-chemistry, it is plain that galvanic action destroyed the tubes which their very superior qualities caused them to be electro-positive to the plates around them." And let us remem-ber that zinc is electro-positive to all of them, and, if used, would monopolise the corrosion, and save the boiler plates.

Mr. W. Parker, the Chief Engineer Surveyor of Lloyds, has given the results of a number of interesting experiments, show-ing the relative susceptibility of different qualities of iron and steel to corrode under galvanic action. It was found that of eleven different specimens suspended inside a boiler, and insu-lated from each other, the steel and best qualities of iron corroded more rapidly than inferior brands of iron, the reason being that the superior plates were relatively freer from electro-negative impurities, such as carbon and phosphorous. Mr. Parker also tested properly prepared iron and steel plates, of similar size and shape, against each other in a boiler protected by zinc plates, and in a boiler unprotected by zinc plates. The test pieces lost twice as much of their weight where there was no zinc present as in the other case.

There is another galvanic action which, I think, has a great deal to do with corrosion in boilers. It is due to an electric current being set up between different parts of the boiler which are at different degrees of heat. It is known that if two pieces of metal are in contact, and one raised to a higher temperature than the other, that galvanic action will result, and the cooler piece of metal become positive and corrode. The zinc ball would be a cure also for this, because it would be positive to


both pieces of metal, and the action, therefore, would be confined to it.

One thing more. In thinking of the action of a galvanic battery, we must not forget that electrolysis of water takes place, and that, while the oxygen _ attacks the positive metal, the other constituent of water, hydrogen, settles on the negative plate. This fact Mr. Hannay relies on to prove his claim that the electrogen (as he calls the zinc ball he has arranged to use in boilers) prevents incrustation. We know that in electro-plating, if the current is too strong, the escape of hydrogen interferes with the regular deposit of the metal, and Mr. Hannay claims that the electric current which is present in a boiler where a zinc ball, properly connected, is used, is strong enough to disturb the deposition of scale, the decomposition of water being so great as to evolve sufficient hydrogen at the negative metal (the internal surface of the boiler) to preclude the possibi-lity of incrustation.

I must say that, though I thoroughly believe in the efficacy of the electrogen to protect boilers from corrosion, and at very small cost, I was hardly prepared to expect also a protection from incrustation, but it is very well worth a trial ; and I may say that, since I wrote this paper, I have directed a firm of steamship owners in Melbourne how to try the efficacy of zinc balls properly fixed to the boilers, and I hope to hear shortly of the result.

There cannot be a doubt that to galvanic action the short life of boilers in the past is mainly due, and I firmly believe that, to again quote Mr. Rowe, " Boilers, if properly constructed, pro-perly worked, and properly protected, should, like their owners, reach the good old age of three score years and ten, instead of being a worry for five or ten years, and then pass into a dis-honoured grave."

Since writing the above, I have seen a very valuable paper upon corrosion and scale, which was read before the Society of Arts by Mr. G. Swinburn King. His description of the electrogen is so interesting that I am tempted to quote it :-

" I will endeavour," Mr King says, " as briefly as possible to narrate his (Mr. Hannay's) proceedings. The investigation was commenced for the Allan line of steamers from Glasgow, at the request of Messrs. Allan Brothers, who, like all owners of steam power, were deeply interested in the boiler question.

The common theory held at that time was that free oxygen and carbonic acid in the water were the active causes of corrosion, and Mr. Hannay's first experiments' were directed to absorb the oxygen by the ordinary methods known to chemists, with the result, however, that corrosion continued. When an alkali was

82 Corrosion and Incrustation in Steam :Boilers.

added to absorb the carbonic acid, priming was caused to such an extent as to be dangerous to the safety of the machinery. He may be said to have exhausted chemistry in his endeavor to find a means of stopping the decay ; but, although he succeeded in removing every trace of free oxygen and carbonic acid, the corrosion still continued, after six months of patient trial. He concluded, therefore, that, while free oxygen and carbonic acid might help to corrode the boilers, they were certainly not the chief causes.

" It next occurred to him that certain parts of the boiler, more highly heated, might, for reasons familiar to science, have their surfaces so altered as to cause them to become electro-negative to the colder parts. This view was particularly impressed upon him from the fact that corrosion so often took place along certain well-defined lines, as, for instance, along the sides of the fire-tube. Sometimes the corrosion was so deep that there was reason to apprehend collapse of the furnace. The cold blast going to feed the fire kept the part where the corrosion was quite cool, while the flames kept the top very hot.

" Starting, then from these facts, he deduced the theory that thermo-electric currents were set up between the colder and hotter parts of the boiler, and that the colder part, being positive, corroded by the natural law of galvanic action.

" To test this theory, experiments were made with a boiler specially constructed to allow it to be heated in sections, and to stand a pressure of 200 lbs. to the square inch. Two iron plates were fixed in the boiler—one near the top, and the other near the bottom—and both were connected with the galvanometer, so that a current of electricity passing from one plate to the other could be detected and measured. The boiler was heated alternately more strongly at the .top or the bottom, with the constant result that, whenever the temperature rose above the boiling point, as in a steam boiler, the cooler plate became positive and wasted away. Thus the theory was lifted into the region of ascertained fact.

" Attempts were made to keep boilers in actual use more uni-formly heated ; but if corrosion was stopped in some places it was sure to break out in others. It appeared, therefore, that the only way to prevent this corrosion was by making the iron all negative by a stronger current than that set up in the iron itself by differences of temperature. The current was found to be very small ; a weak battery was fitted up, and the positive wire passed into the water of the boiler, the negative being soldered to the outside of the boiler. After six months' trial with this arrange-ment it was found that corrosion had entirely ceased. Two important facts, therefore, were now made clear : natural electric


currents, so to speak, caused corrosion, and a stronger artificial current could be made to cure it.

" The experimental boiler was then again tried with the same arrangement as before, but it was first filled with dilute acid. The current was kept going for three months, when it was found that the interior of the boiler was still quite free from corrosion, the acid having been powerless to injure it.

" The actual natural current between the metals being so very small, it was thought that, instead of a battery, a simple galvanic couple, formed by a mass of zinc within the boiler, properly con-nected with the iron, might be sufficient to overcome it. As zinc had been previously used for this purpose, inquiry was next instituted to discover the cause of failure in zinc as ordinarily employed.

" It was found that, as zinc had been previously used, no proper arrangements were made for ensuring a true and lasting metallic contact. To show how this fact was demonstrated, and to what pains it was ascertained, a narration of one of the numerous experiments made will be interesting. A boiler was fitted with rolled zinc plates attached to studs, on Mr. Phillips' principle. Every precaution was used to give perfect metallic contact. The stud was filed bright and made slightly conical, and the hole in the zinc plate made to fit tightly, and the nut on the stud screwed home, so as to drive the plate into thorough contact with the clean iron. An insulated wire was fixed to the plate, and led through a stuffing box packed with india rubber. Another wire was soldered to the outside of the stud.

" When a small battery and galvanometer were put in circuit the current passed from the zinc to the stud, and so round again to the battery, proving that the contact was perfect. There were six plates put in on trial altogether, and the result was that, after three days' boiling, two of the plates had become metallically disconnected from the boiler—that is to say, no current could pass through the circuit. After five days another plate became useless, owing to the same cause. At the end of twelve days, only one plate was in electrical contact with the boiler. The boiler was opened after thirteen days' steaming, and it was found that none of the plates were really used up, but that a layer of oxide of zinc had formed between the plate and the stud, and the zinc was thus rendered useless. It 'must be remembered that these six plates were all specially fitted, and ought to have acted thoroughly well. The sixth plate was sent away with the ship again, but only remained active two days. It was eaten through when the boiler was again opened. By this, and other experiments, it was proved that no mere mechanical attachment of the zinc will suffice to ensure continued maintenance of the galvanic

84 (Jorrosion and Incrustation in Steam, 13oí/ens.

current; because, no matter how closely the zinc is fitted to the stud, or bolted to the iron, the water creeps in between, and soon destroys the metallic contact,

It was also found that the use of plates of zinc was faulty. If they are cast, they split up and fall to pieces in a few days, and, if rolled, they are only about a quarter of an inch in thickness, and soon dissolve away.

" To meet the various defects in the use of zinc plates, Mr. Hannay designed a ball of zinc, with a copper conductor cast through the centre of it, the copper being so combined and amalgamated with the zinc at the junction of the two metals as to form brass, and thus no corrosion could form between them to stop the galvanic current. The zinc is well hammered at a certain temperature, ensuring long existence in an efficient con-dition. This ball of zinc is called an electrogen. It is fitted in any part of the boiler by a simple device, and a wire from each end of the copper conductor is soldered firmly to the iron. From this moment the electrogen keeps of an uninterrupted galvanic current, and the whole of the interior of the boiler is absolutely protected from corrosion so long as any of the zinc remains.

" It was ascertained by further experiments that a very small surface of zinc was sufficient to afford protection for a radius of twenty-five feet from the point of contact, and the spherical form of the zinc was adopted because it would maintain perfect pro-tection with a minimum of waste, the large surface exposed by plates, in proportion to their bulk, being quite unnecessary. Herein, therefore, was the means of avoiding that waste which the Admiralty committee stated was much greater than that due to the protection of the boiler, and for which they sought a remedy.

" Two electrogens are found, in practice, sufficient to protect an ordinary, single-ended marine boiler, in which, by some engineers, forty or fifty plates would have been considered necessary. The electrogens will last for about six months, while the plates would probably corrode away in as many weeks.

" The advantages that Mr. Hannay claims for his system as compared with any employment of zinc plates, are that it is less expensive and more effectual, and that the protection it affords does not depend upon a chance contact that may be destroyed at any moment. But a further gain, perhaps even greater than these, is that it does not allow scale to form in a boiler at any time to a much greater thickness than that of an egg-shell or a coat of paint.

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The zinc ball, with its perfect contact, generates a current of greater intensity than zinc plates mechanically fitted, and the consequence is that a portion of the water is slowly decom-posed, and the hydrogen that is evolved at the negative metal all over the surface of the boiler iron, and underneath the scale, forces off the scale in thin flakes, by mechanical action, as soon as it becomes thick enough to be impervious to the hydrogen. In this way the scale is kept forming and re-forming, hanging in loose flakes, or falling off as it becomes detached from the iron.

" Thus all the evils attending incrustation, which have been before enumerated, are avoided. Fuel is saved, burning of the iron is prevented, and chipping becomes no longer necessary. The reason why scale becomes more hard and coherent under the zinc plate method, as used in the navy, is that, while the galvanic current set up acts in retarding corrosion, it has not sufficient intensity to decompose the water and deposit a layer of hydrogen on the iron; so the scale grows on a firm surface, and is not pushed off by gas evolved beneath it.

" When zinc is merely pressed against iron, the two metals really touch each other at minute points only, and thus great resistance is introduced. Resistance, in this case, means that the current is destroyed to a certain extent as electricity, and converted into heat, just as the resistance of the brake destroys the motion of a train, and converts it into heat. Then the water, creeping in between the two metals and forming a non-conducting oxide between the two surfaces, increases the resist-ance, and ultimately prevents the passage of the current altogether.

" The value of any discovery that will prevent the formation of hard scale in land boilers can scarcely be overrated. The boilers in which fresh water is used do not suffer so much from internal corrosion, but the calcareous scale which form in them has always been a great source of trouble ; compositions have failed, and zinc plates are ineffectual to remove it.

The electrogen, however, seems to have solved the problem, and to make it sufficiently active in fresh water a small quantity of salt is added, by means of which, as an exciting agent, an electric current is stimulated, which entirely disposes of incrustation.

" Land boilers, in many districts, would become quite un- workable through the accumulation of scale if it were not chipped off every five, six, or eight weeks, of course at considerable expense, and the boiler lying idle during the process. With electrogens it has been proved that boilers will work more than twice the usual time without any necessity for

u

86 Corrosion and Incrustation in Steam Poiters.

opening them, that then the loose flakes of scale may be cleared out with a broom and hose. Meantime, no thick scale being allowed to form, it becomes perfectly harmless ; the coal con-sumed does its full work, and steam is made more freely.

" Engineers who have witnessed the results in several recent trials have stated their opinion that the discovery will revo-lutionise the treatment of land boilers."

In conclusion, Mr. President, let me reiterate the statement with which I began, that my little paper pretends to be nothing but a compilation of facts and theories ; that in this form I hope it will be sufficient to introduce a discussion upon a most interesting and important subject, which those of our members who are so much better fitted than I am to deal with will take part in.

Corrosion and Incrustation in Steam, Boilers. 87

DISCUSSION.

Mr. WARNOCK said : I think there may be more in the theory than most marine engineers are inclined to believe. To say, however, that all corrosion in steam boilers is principally caused by electric action is going a little too far.

My own experience of marine steam boilers is, that if you can keep the seams and rivets tight, and the external surface dry and protected by paint, cement-wash, or oil, after ten or even twenty years' service the iron will be in good condition.

Respecting internal corrosion, most marine engineers who have from time to time surveyed steam boilers will have formed a theory of their own, but all are agreed that if a thin scale of salt be kept on the surface of the parts that are under water it will effectually protect the plates from corrosion ; but if the scale is broken off by expansion and contraction, or by the yielding of the plates by the varying pressure, they will be corroded at those parts.

The theory propounded by Mr. Hannay, and referred to by Mr. K. L. Murray, that the difference of temperature in the different parts of the boiler set up electric action and the colder parts are corroded, is worthy of some consideration, although my own observations do not quite agree with it. 1 For example, it is well known that all boilers suffer greatly round the stays that support the flat parts, particularly those that support the walls of the box boilers. Then the plates are corroded through round the stays that are connected to the furnace sides in about ten years. This fact, if taken alone, would support the temperature-electric theory, the boiler side being the coldest ; but when we know that in the side of the surface, if also flat, the same effect is produced there also, the theory becomes inverted, the furnace side in this case being the hottest part.

Another example which, to a certain extent, favours the temperature-electric theory came under my notice in a boiler with a flat top. The top was stayed by long stays reaching down to


the sides of the furnaces and riveted to them. The roof of this boiler corroded very rapidly, and in twelve months it quite alarmed me. I did all I could think of to protect it, but with little success, and in five years the roof had to be cut out and renewed. Up to this point it would favour the electric theory, but as the newroof, which has now been in constant use about four years and shows very little corrosion, it does not appear to me to favor it, seeing that all the connections are the same, except that the stays are coupledi by a screwed coupling instead of the solid bar. I would very much like to have Mr. Murray's opinion on this altered condition. My own opinion is that the hair felt bagging had something to do with the corrosion in the old roof, and that the improvement is due to the non-conducting composition now used instead.

There is another fact which does not favor the electric theory —that is, that marine boilers generally cost most for repairs below the fire-box.

It must be understood that my remarks refer to boilers fed with sea water, because when fresh water is used the result is very different. I think, under those conditions, nearly all the corrosion or thinning of the plates is due to a mechanical action caused by unequal expansions and yielding of the plates by the variation of pressure, causing leaks and throwing off the salt, and leaving clean iron for fresh oxidation. The boiler that will last longest is the one that is fed with fresh water, is so constructed that the varying pressure will not affect its shape, and has the most uniform temperature.

MR. WALTER MURRAY : I may state that I am going to speak on the subject from a practical Marine Engineer's point of view. At the same time, I am glad to say that in many particulars my practical observations fully bear out the value of Mr. K. L. Murray's remarks.

It has already been pointed out to us that almost every variety of incrustation is formed of material non-conductive of caloric, and, consequently, consumption of fuel is increased by the incrustation, and the danger resulting from the same is too well known, from the numerous instances which have occurred, from the plates becoming overheated and yielding to a pressure which could have been carried with safety if the plates had been free from deposit and had freely conducted the heat to the water.

We are aware that when water is heated until a pressure is attained of 1001bs. to the square inch, its temperature is about 327 degrees Fahrenheit, at which temperature the iron retains at least its normal strength; but the deposit of any non-conducting material soon causes the iron to become heated to a temperature


of 600 degrees, when it loses its strength, and collapse speedily follows.

I have lately had an opportunity of examining a boiler of 13ft. lin. diameter and 16ft. 6in. long, the shell of lin. good boiler plate, the fire-boxes and furnaces being of the best Lowmoor plates, ‚in. thick, the pressure of steam 651bs. to the square inch, and the temperature of same 312deg., and although the boiler is nearly nine years old the shell is scarcely worn or corroded ; but the furnace and fire-boxes exhibit certain signs of decay from corrosion and pittings, and chiefly in those parts which are exposed to the greatest heat, which points to the fact that mechanical formation in various details is principally the cause of it.

1st. The water runs with a greater force or velocity along the seams or joints of the plates, thereby causing a channel or gutter, which prevents the steam from freely escaping to the upper part of the boiler.

2nd. The double thickness of plates abruptly checks the regu-lar expansion and contraction of the adjoining single plates, thereby causing a buckling and unbuckling of the same, which distorts and weakens the grain of the iron.

3rd. The double plates at the riveted laps are not such good conductors as the single plates, and I have known, in boilers with a quick combustion, a more rapid wearing away of the parts named than in the body of the single plates.

4th. From bad caulking, in not making the seams tight by staving, and caulking up the edge of the overlapping plates, but carelessly allowing the caulking tool to make a groove in the hard surface of the under plate, which separates and disturbs the grain of the iron, and is the cause and commencement of a fracture.

At the present time the usual practice is to suspend boxes (freely perforated) on to the stays with the zinc inside, so that when the zinc decomposes its solution may be diffused through-out the whole of the water in the boiler—about lcwt. in the boxes, and the same quantity in the bottom—to be attacked by the copper depositing in solution. And, in corroboration of Mr. K. L. Murray's remarks, I may say that I have always found that the difference of temperature produces totally different results on the zinc. Here are sample pieces of zinc that have been suspended in the 'steam chest of a boiler six weeks at a temperature of 312deg., and this is a piece through the holes of which all the feed water from the surface condenser has passed to the boiler, at a temperature of 110 or 112deg., for a period of nearly nine years, which proves conclusively the influence of

li


heat on the same material used in the same machinery, (Sped. mens laid on the table.)

Mr. Elder, of Randolph, Elder, & Co., recommends that 11b. of zinc be used to one indicated h.p., so that in boilers of 2000 horse-power nearly one ton of zinc would be required.

The remedies proposed by Mr. Molesworth may be of some service under certain circumstances. Some may be useful to neutralise the effects of very bad water ; but, with the rapid ebullition in our high pressure tubular boilers of the present day, I think the remedy in most cases would be almost as bad as the disorder, for nearly all foreign substances which are put into a boiler are ballooned to the surface of the water, where most likely the steam will carry them over, or a portion of them into the slides and cylinders, and from there into the condenser. For instance, the boiler which I have been speaking of holds 6080 gallons of hater, and 1-50th part of potatoes, as recommended, would be 121 gallons, or about half a ton weight.

Relative to the exclusion of air in boilers, a method has been patented for feeding the boiler with the air extracted from the water—the effect of which I have not learned—but, from my own observation, I am inclined to think that the water should be well saturated with air, and that its absence is injurious, and most especially injurious in condensed water. The boiler which I have taken in illustrating my remarks is connected by steam pipes to a donkey boiler which supplies the winches on deck with steam, and it was found, when the donkey boiler was in use in port, that the drippings of condensed steam or water from a leaky cock to the top of the fire-box caused very deep pitting to take place in a very few months, and when a strong high pressure valve was fitted in place of the leaky cock that the pitting immediately ceased. Lately some zinc has been placed suspended in the donkey boiler, which seems to prevent the deposit of any scale, excepting a very thin film of alabaster appearance and hardness in the bottom of the Galloway tubes.

The s.s. Gabo, belonging to Messrs. Howard Smith & Co., is fitted with an electric apparatus. The only difference between it and Mr. Hannay's is in the balls being laid in cups, and wire soldered to the furnaces ; Mr. Hannay's are balls fixed to copper rods. The chief engineer positively asserts that at present the boilers show no signs of pitting, the iron being as clean as the first day of steaming.

But I fear that the want of homogeneity in the iron of which boilers are constructed, the difference of temperature to which the various parts are subjected, producing unequal expansion and contraction, the drilling of holes irregularly spaced, the want of technical knowledge in caulking, and the want of

Corrosion and Incrustation in Steam boilers. 91

thorough inspection by really qualified men, present insuperable difficulties in extending the age of boilers to the term which the hardiest amongst us can only expect to reach.

In the one case I have mentioned these boilers were fed with a filter. It was quite a new idea of the chief engineer, and he opened his boiler to allow me to go in and take the samples which you see here on the table. I went right down from the top to the bottom of the boiler, and here are the results of my examination. They came from the steamship Nelson, which has a very fine boiler. There was a peculiarity about this boiler not found in any other in the colony—that there was a filter for the feed-water. The water that passes from the condenser, instead of passing clean through into the well to be pumped by the feed-pumps, goes into and passes through a filter filled with ordinary gas coke that has been well washed. That has been nine years in use at a temperature of 210deg. The corrosion takes place a little above where the flame cuts across the fire level. The boiler is fired from both ends. It has six furnaces, but a single boiler. In the forward end of the boiler and the open end there is only one plate gone. It appears to me that the different quality of iron has positive]v arrested corrosion. You must also consider that where there are overlapping seams, the steam cannot rise, because it is foiled. Bad caulking tools will cause defects. If you cut the skin of the iron underneath you injure it. If the skin is left intact it will last longer. Only the left side of the right furnace went, while the other lasted nine years.

MR. R. MooRFIELD: With respect to this subject, I may state that I have placed the electrogen in one of our boilers since the matter was brought forward so ably by Mr. K. L. Murray at our previous meeting. I have endeavoured to place it in as complete a manner as possible so as to form a perfect connection between the boiler plates and zinc ball, by screwing a half-inch copper rod, on which was cast twenty-four poundssof zinc, with a little alloy of tin and lead, made in the form of an egg cut in two. I also placed lock nuts on both sides of the boiler plate, the plate being tapped so that free conduction might take place without leakage. I opened the boiler the other day in order to determine whether any particular change had taken place with the electrogen, or if any beneficial result cofild be observable or otherwise. As far as I can say at present—though the time is rather short as a test—I noticed that the rust shale over some pit holes had fallen off, thus indicating that an electric action had taken place. This would tend to show that, where the oxidization is thrown off, it might possibly prevent corrosion alto-gether, so long as the electrogen was used and kept in proper order,

Corrosion and Incrustation in Steam $oilers.

In reference to boilers decaying and becoming weak in various places, as mentioned by previous speakers, I think there are two aspects in which we may look at the subject—the internal and external decay. Internally, boilers in constant use become weaker where sedimentary deposits have been permitted to settle upon any part of them—such, for instance, as in the bottom and each end of Cornish boilers. When a boiler is silent in the night after the previous day's work a great part of the deposits become solidified, and stick to the boiler, not being able to mingle with the water during next day's working. This de-posit accumulates from day to day, and, being a very bad conductor of heat, often is the cause of the plates becoming overheated, destroys their tenacity and strength, and in course of time, in many cases, causes explosions. The accumulation of scales of different kinds is particularly noticeable in portable, locomotive, and marine boilers around the fire-boxes, crown of box, and boiler bottoms. Careful attention, occasional blowing off when convenient, and well washing out, with a good use of long and short wash-out rods, is necessary. Externally, boilers become very defective from causes of leakage through bad joints, defective flanges— any of them being too narrow on rim—leakage from water pouts, rain, and other causes of dampness, brickwork, &c., causing rapid oxidation. I think much can be said about the quality of the plates, there being a difference in the manufacture, which may account for the variation of decay in the same boiler to a large extent. The water used is a serious matter, some water, taken out of side cuttings and similar places for locomotives, being very often not fit for use, causing much damage through rapid incrustation in places which I have already mentioned. Yan Yean water wherever it can be obtained, to my mind, is the best. I have used it for many years, and find that it never wales, but forms a soft, miry deposit like the sample before you. It was taken out of one of our boilers, in which there was not a solitary ounce of scale. This boiler had been working for about two months without requiring washing out, with the exception of periodical blowings off. There would be about 26lbs. of this deposit in its thick, wet state.

Ma. K. L. MURRAY.-My experience has shown that, where steam is used, it will make its way between the points of contact. However closely the rod may be fixed mechanically, that cannot be entirely prevented. It must be soldered. I know that putting zinc into boilers, even with a very imperfect connection, has a very good effect. Therefore, in country places, such as referred to by Mr. Moorfield, it would be very advantageous, but it would riot have the effect described in my paper as the result of Mr, Ilannay's experiment,


Mx. D. MUNRO.—I think the subject has been pretty well threshed out so far as marine boilers are concerned, though I do not pretend to much experience of that branch. My own opinion is that a great deal depends upon the quality of the iron. I have seen one plate of a boiler go when others were nearly intact. I believe that was caused by a defect in the iron itself. This is, no doubt, a very scientific and interesting subject, and it should be thoroughly tested. I have no doubt that salt water is more injurious to boilers than fresh water. My experience of the deposit left by the Yan Yean water is that it is slimy, like soft soap. Used with care, however, boilers fed with this water will last fo r a great many years. I would like to see zinc tried in boilers used on the goldfields. It is surprising with what rapidity corrosion goes on where the water is brackish. I know a place in Northern Queensland where, in nine months, the tubes of a boiler were eaten away with corrosion. They are now using Cornish boilers, and will not have multitubular boilers. The mineral water in mines generally becomes stronger as the shaft is sunk deeper, and consequently becomes more injurious to the boilers. I think we should place ourselves in communication with Mr. Couldery, of Gympie, or Mr. Plant, of Charters Towers (who are both large mineowners, and have had considerable difficulty through bad water), and invite them to give the Association the benefit of their experience, and to state the means they adopt to prevent, as far as possible, the corrosion in boilers.

MR. A. C. WANNAN.—I recollect hearing a paper read some months ago at Ballarat in which it was stated that there was continual action in the boiler plates which had the effect .of weakening the iron. The principal reason for using zinc is to prevent a weak plate being eaten away by a stronger one. In many boilers one Pate will wear clean away while another will not be at all affected. The zinc is a positive, and it prevents the weaker plates being affected by the stronger ones. But I have never yet heard what effect the electricity passing through the plate has in weakening it. I think this Association and the general public who are interested in the subject are greatly indebted to Mr. K. L. Murray for having brought the question under discussion. It has not been sufficiently ventilated, because it is a difficult subject, and a little expensive in the experiments.

MR. JOHN NASrroR.—I think corrosion differs in rapidity in different localities owing to the varying qualities of water. Where the whole space is filled up round the boiler a great deal of leakage takes place. A great deal of damage occurs to Cornish boilers through building in the centre wall. A split will avoid that. I have not had an opportunity of reading the paper, but


I may say that I have seen zinc prove very effective where boilers are fed with mineral waters.

THE PRESIDENT, Major Shakespear read a precis of the report of the meeting of the Institution of Mechanical Engineers at Cardiff in August, 1884, on the question of corrosion in steam boilers, bringing out the principal points touched upon on that occasion in a paper by Mr. Hallett.

" Steel boilers are generally used, yet they corrode as easily as iron.

Defective design and management are principal causes of corrosion.

" In boilers first look to the circulation, for there can be no doubt that the coolest places in boilers are below the level of fire bars, and here the greatest deterioration is found, as the water contains here the greatest percentage of solid matter.

" In a trip from England to Rangoon (72 days), the boilers were blown down, and at the end of journey only a thin scale was on, but no pitting or corrosion.

" Carelessness in treatment is one of the most frequent causes of deterioration.

Introduction of oil, tallow, and grease cause corrosion in upper parts.

" Zinc has been tried, both cast and rolled, with favour ; but it requires large quantities, as it oxidises quickly.

"° The electrogen of Hannay's invention is gaining favour, as it is simple. Boilers showing a tendency to corrosion look healthy after it has been applied to them.

" Mr. J. R. Fothergill agreed that defective design and bad circulation caused corrosion, though he says that the greatest cause was the atmospheric air introduced with the water by the air pump into the hot well, and thence into the boiler by the feed pumps.

" Experiment.—Suspend a strip of iron plate in the passage from condenser to air pump, no corrosion takes place ; but, sus-pend it in feed pipe, and it becomes destroyed in a very short time, and this he attributes to the presence of the air.

" If feed water were delivered to boiler free from grease, he considers corrosion would take place to only an extent of 90 per cent. of what otherwise occurs.

" He therefore considers that the water should be pumped or run out when cool, and not blown out by steam.

" Pitting would, he considers, be averted by scraping the plates, washing with a strong solution of soda, and then wash-ing with Portland cement.


" Mr. Sydney F. Walker considers corrosion from an electric point of view.

He ascribes pitting to different qualities of iron, and to impurities in it.

" That a boiler should be made of one substance to prevent electric action, or the boiler becoming a galvanic battery.

" That when zinc is put in, it protects the iron as long as any surface of the zinc can be attacked by the salt, and that pure zinc is invaluable.

" That zinc suspended in a boiler creates a galvanic battery. " He suggests amalgamated zinc in the absence of pure zinc. " He believes that a piece of amalgamated zinc connected by

wires to the boiler would have the same effect as the electrogen, or to suspend both a copper and a zinc plate with a wire of its own metal to the iron boiler.

" Mr. Charles M. Jacobs says that the parts of plates that were pitted were cleaned and washed with petroleum, and cemented with Portland cement, the boiler washed out, and the pitting did not again occur that there was only one structural alteration connected with the corrosion, and that was in the position of the feed pipe, which was removed, and the feed water delivered into the steam space, thus getting rid of some of the air, which he believed to cause corrosion. By preserving the cement, the boiler is as good as ever.

" Mr. T. C. Marshall considers that corrosion is caused by a galvanic action on the surface of the plate.

" He did not find a greater tendency to pitting in iron tubes than in steel.

" Mr. Hallett has used the electrogen, and found that it is extremely satisfactory.

" That through the decay of the zinc the boiler is saved. " Says that in H.M.S. the zinc saves the steel from corrosion,

and also neutralises by its deterioration the hurtful influences of the water.

" A slab of zinc,12in. x 6in. x lin., is required for every 20 I.H.P. Mr. Hallett has filled pitted holes with red lead with great

success. MR. E. J. HUYSMANS, Chief Engineer Naval Forces, read the

following notes commenting on Mr. K. L. Murray's theory :-

DURABILITY OF BOILERS. The question of the durability of boilers is one of the greatest

importance as regards the continued efficiency of steamships, for a steamer's efficiency may be measured mainly, if not wholly, by the efficiency of her machinery, so that if her boilers are injured the evil cannot be estimated by their value alone, but it


must be measured by the fact that the efficiency of the ship for the purpose she is designed is decreased; therefore, any measure to prolong the life of the boilers must be advantageous to the owners. Necessarily, the greater part of the work is professional, and depends upon the engineer (the human factor cannot be ignored), but many points relating to the preservation of them are in the hands of the owners, who, by giving the engineer the necessary time and means to clean, examine, and preserve them, thus prolong the life of the boilers, and save their own pockets.

BOILERS—CAUSES OF DETERIORATION, AND REMEDIES.

Some of the deterioration has been attributed to the corrosive influence of water distilled over and over again, to fatty acids, galvanic action arising from the various metal fittings ( such as condenser tubes, &c.), to inferior quality of iron, and want of homogeneity, which tends to cause local corrosion, or pitting. But experience has clearly demonstrated that the most serious cause of the corrosion which occurs in the interior of boilers, when under steam, is due to the combined action of air and water upon the unprotected surfaces of the iron, and, when not under steam, to the action of air and moisture.

Dry air, or water free from air, has no action upon iron, but moist air, or water containing air, are two powerful agents of corrosive action. The chlorides contained in sea water render it a more powerful agent than fresh water, but even sea water, de-prived of air by boiling, has scarcely any action upon iron.

EXPERIMENTS—SIX MONTHS DURATION.

One piece of iron immersed in water, air excluded, when taken out found quite bright, and no loss in weight. Another immersed in water, air admitted, in the same time lost 1-72nd of its weight, and found deeply pitted. Experiments have been tried by the Admiralty and by Jack of Liverpool with condenser tubes of iron and all feed pipes of iron, and the corrosion has gone on as before. The Cerberus' boilers had a very short life, owing to air and moisture, not from galvanic action as she had very little steam-ing ; and another great drawback with them was their inaccessi-bility, as, owing to the number of stays, it was impossible for a man to get inside to clean, examine or dry them--they were more like a bird's cage than a boiler. In the present ones, designed by Mr. Wilson, all parts are accessible, and - you can almost walk about in them. Accessibility is a grand thing to help the life of a boiler. But it is an established fact that in the Royal Navy boilers deteriorate much -faster than in the Mercantile


Marine, because H.M, Ships are so little under steam compared to the Merchant Service, and that the boilers are thereby (much more exposed to the action of moist air.

MEANS TENDING TO LESSEN OR PREVENT DETERIORATION. .

Use mineral oil for internal lubrication. Corrosion in the boilers is lessened by keeping the water alkaline by the admission of a little soda, which neutralises any acidity of the water, and by being admitted with the exhaust steam it tends to clean the condensor tubes. By increasing the density of water in boilers keeping them 3 to 4 times the density of sea water, there is no increase of scale or corrosive action, the evaporation being the same and below twice the density the scale is increased when the evaporation is constant. Increasing the density of water reduces the quantity of air admitted, but it is desirable to supple-ment this by extracting the air from the feed water. This has been effected in land boilers by the use of open top feed heaters whence the free air escapes, and the air held in solution is largely expelled by the heat. Feed tanks, for containing a reserve supply of feed water, and give more time for the air to free itself from the water as it comes from the hot well, have also been fitted to engines generally for some time. Filters have been fitted to some ships in the Royal Navy, consisting of small coke, sponge and fearnaught, for the feed water to pass through but the end did not justify the means, so was abandoned. But the most important protection seems to be the use of zinc in the water spaces, and coating the surfaces of the steam space with a thin wash of Portland Cement, about three coats applied with a paint brush, to form an artificial scale.

THE USE AND APPLICATION OF ZINC. The importance of zinc as a protective agent for the preven-

tion of corrosion in boilers has been demonstrated. Its applica-tion is a practical method of preventing, under water, corrosion of boilers under all conditions of working.

It has been proved that a comparatively small distribution of zinc surface, something like 1 to 150, if suitably distributed, especially as to intercept the feed water, will protect the iron to which it is attached from corrosion, but the zinc gradually decays so that you have to fit an excess of zinc surface at first as will ensure sufficient protection until an opportunity occurs to renew it.

When the boilers are new, the whole of the surface under water has to be protected, hence probably, the rapid decay. After a time, as the surfaces become covered with a slight scale (one of

98 Corrosion and Incrustation in Steam, Boilers.

the best medicines you can apply to prolong the life of a boiler), the area requiring protection becomes less, and of course the zinc lasts longer, and its area may to some extent be reduced.

ZINC SURFACE. The following, based upon a large number of instances, are

sufficiently near for all practical purposes :— (a) In new boilers, or boilers which have not been in use

for some time, 7 square inches per I.H.P. or 1 slab per 20 I.H.P. (size of Admiralty slab, 12in. by 6in. by lin.) or 1 slab for 50 square feet of tube surface.

(b) In boilers which have been in use for some time, but not lately had zinc fitted, 6 square inches per I.H.P., or 1 slab per 25 I.H.P, or 1 slab for 60 square feet of tube surface.

(e) The boilers which have been in constant use for some time and have had the zinc renewed as necessary, 5 square inches per I.H.P., or 1 slab per 30 I.H.P., or 1. slab for 70 square feet of tube surface.

Boilers in which fires are alight less than one-third of the time during which the water is in should be fitted the same as in new boilers.

The above refers to surface condensers; in jet condensers less is required—about one-third.

It must be borne in mind that the I.H.Power is not that at which the engines are being worked, but that of the boilers as measured by their grate or tube surface. Working the boilers at a reduced power does not reduce the amount of zinc to protect them. With regard to renewal, it appears if the zinc be fitted in the pro-portions mentioned it requires to be renewed frequently at first, but at periods of two to four months afterwards. With new boilers it will be found that the first set are decayed after a. few days' steaming ; the second set may last over 10 or 12 days ; and the third and succeeding sets should be examined after steaming 50 days, or after water has been in 100 days, if hot, and 150 days, if cold.

Slabs are occasionally found which last much longer than others in the same boiler. This may be due to the greater protec tion to the iron, by the deposit at that part of the boiler, or a greasy deposit on the slab, to impurities in the zinc, to a want of metallic contact, or to over-abundance of zinc in that particu-lar part.

1. The size of slab and mode of fitting have been found con-venient and satisfactory, as per sketch.

2. Rolled zinc is more suitable and more economical than cast zinc.

Corrosion and Incrustation in Steam Boilers, 99

The protection afforded by zinc is more prolonged in hot than in cold water. It protects an area of 4 feet in hot, and about 1 in cold around the slab. Be careful to get pure zinc. The principal impurities are lead and iron. There is a small per-centage present in all commercial zinc after the most careful re-casting, amounting to about 14 per cent. Care must be taken that, in re-casting, the surplus lead which falls to the bottom of the melting-pot is not poured out into the mould. These impurities set up a series of local galvanic currents, which assist in the de-struction of the zinc, and so reduce the time it would afford pro-tection. Slabs of ordinary cast zinc, as ordinarily sold, have a tendency to become distorted after a short-time exposure to the hot water in the boiler ; so much so as to cause fracture of the slab and its breaking up. In hot water the corrosive action rapidly penetrates the cast slab ; it becomes a mass of particles of metallic zinc, each surrounded by the products of corrosion. In this way the efficiency becomes rapidly less, and the slab, when apparently decayed, may contain as much as 70 per cent. of metallic zinc, which is useless for protective purposes, because it consists of isolated particles insulated by the accumulated pro-ducts of corrosion, which has permeated the whole mass.

In rolled, close-grained zinc the wear is entirely confined to the surface. It does not become distorted or broken up; on the contrary, it gradually wastes away until only a light shred, a sort of skeleton framework, remains to indicate what it has been. The result of the use of zinc is the formation of a harder lime scale on the boiler. It is worthy of observation that if the zinc fitted in any one boiler be insufficient in quantity, or not efficient otherwise, that side of the scale next to the iron surface will be more or less coated with black oxide of iron, whereas if the pro-tection be satisfactory, the traces of oxidation become exceedingly trivial, if they do not cease altogether. Zinc affords,no direct protection, if any, in the steam space.

PRESERVATION OF BOILERS WHEN NOT UNDER STEAM.

In the Victorian service the boilers of the torpedo boats have had at present only a twelvemonths' life. My practice is to fill the boilers with fresh water, and put in a certain quantity of soda (about 101bs. of soda per 100 gallons of water) ; but believ-ing that an engineer's pound consists in a great measure of an ounce of theory and fifteen of practice, I test it practically as follows :—draw off a little water from the boiler and immerse a nail, filed bright, for 24 hours; if there is no discoloration of the nail the quantity of soda is sufficient. I have tried lime as well as soda, but I prefer the latter. Also, I light a fire (to expel as


much air as possible through a cock and pipe fitted on the high-est part of the boiler). In the gunboats. the boilers are fitted with zinc plates in steam and water spaces, and the boilers filled with water. The interior of the Cerberus' boilers being so acces-sible, I keep them empty, and coat the inside with linseed oil, which, up to the present, has given satisfactory results.

(In reply.) I have had no experience of the electrogen, but have been told by engineers that, on examining boilers fitted with them, the wires have been found broken (whether on the day of sailing or of arrival they could not say).

EXPERIMENTS. To ascertain the comparative effect of copper, brass, tin, lead,

and white lead placed in contact with discs of iron in a boiler, also iron and zinc and iron alone, when immersed—

Loss. Loss. With Copper .. 1.78 per cent. With Brass 1.89 per cent. „ Tin 1.51 „ „ ;, Sheet Lead 1.69 „ „ „ White Lead 3.14 „ „ „ Iron and Zinc .52 „ „

And Iron alone .. 1.58 „ „ (In the proportion of 25 to 1.) Plates of metal of half the size of the iron plates, duration of

trial six months. The use of zinc is most beneficial with the slabs fitted in good

metallic contact, and, suitably distributed, the protection affor-ded is most marked. Whether the corrosion arises from the water itself, galvanic action, or from the air admitted with the feed, the Admirality recommend its general adoption.

MR. J. MEEKIsox.—I should like to ask Mr. Murray a ques-tion. Now, on the goldfields there are plenty of these boilers fed with mineral waters containing a great deal of arsenic and sulphur. At one place I know the boilers lasted only twelve. months. In another they put two plates over the boiler, and in six weeks it was eaten through. After the company laid on fresh water they had no trouble. In Egerton, Gordons, and China- man's Flat the water is very bad. Do you think this zinc would be of any use in these cases ?

MR. K. L. MURRAY : Most certainly. The process depends a great deal upon the solution. In the instance mentioned by the gentleman who spoke last, he refers to a case of chemical action upon the metals by the water, and when that chemical action is present there is always electrical action.

MR. J. WARNOCK : I supplement what I have already said with respect to corrosion in steam boilers. I may say I was much struck with the remarks of the President in his precis of the report of the meeting of a kindred association. They very much coincide with my own opinion. I may say I still feel doubtful as to this'electric theory, and what makes me sceptical


is this : On the bottom part of the fire-box of marine boilers corrosion takes place rapidly, but at no other part of the compo-sition chamber. All the corrosion in my experience takes place below the furnace crown. Every time the furnace door is opened there is a difference in the length of the furnace, which causes continual action in the plates, whereby there is a difference of temperature. At this particular place the plates are thin, espe-cially on the top of the fire-box and at the corners of the furnace where the tube plates join. Where there is any possibility of the plates giving away, I find those parts thin away very fast. In reference to this cement wash which Mr. Huysmans has referred to, on the last occasion I mentioned a boiler under my care which corroded very rapidly, and cement wash was one of remedies I applied. After the first six months I applied not less than two coats of cement wash, and after another six months' trial I could not see a bit of difference. I am, therefore, convinced that there is something in it. I got a plasterer to put a thin coat over the wall of the boiler, but it was no use whatever. This is one of the points I should like Mr. Murray to refer to. I want to know how he can account for the roof of a boiler going when there was hair-felt on it. The corrosion stopped entirely when the hair-felt was removed. It was remarked at the last meeting that thin plates were attacked much more than thick ones. The box bottom plates in marine boilers are the thickest. In dry bottom boilers many of the legs are 25 per cent. heavier than any other part. But the legs give way, and that convinces me that there is very little in the electric theory. It has been said that the tubes give way faster than any other parts of the boiler. That would favour the theory that thin plates give way, and that it is the heat. That makes me think that the temperature elec-tric theory is a little doubtful.

CAPTAIN F. C. ROWAN : I had not the advantage of hearing this paper read, or hearing the first discussion of it ; but, so far as I can gather, it seems that, with regard to electrical treat-ment, there is a great deal of theory so far, and that the amount of practical results to be recorded of this electrogen is very small. Now, the remark which was made just now is perfectly true :' that an ounce of practical experience is worth a ton of theory. I would suggest the possibility of carrying out a series of experi-ments under the auspices of the Association, so as to remove this matter beyond the region of doubt. I believe there are at present here in Melbourne a number of those zinc balls of Mr. Hannay's, and, if it were desired, I think I could easily make arrangements for procuring some.

MR. R. FULTON.—This paper read by our friend, Mr. Murray, is, I think, well worthy of our attention ; still, so far as I can

102 Corrosion and Incrustation, in Steam Boilers.

gather, Mr. Murray is the only gentleman capable of answering this paper, and I presume he can only do so after some little experience. I gather that the whole point is as to the efficacy of this zinc as applied by Mr. Hannay's electrogen. So far as I have been able to ascertain, no one has any reliable information respecting the action of that electric ball. After some little experience I think Mr. Murray will be able to give us the practi-cal results of what he has been able to accomplish. I know an intelligent engineer who will be willing to test Mr. Murray's theory, and any information he may convey will be quite accurate and reliable. I think it is quite useless to go into the question of how boilers are corroded without some . practical tests. Many boilers are corroded in very peculiar ways. From one end of a boiler in the Southern Cross steam was taken off, and no steam from the other end. It was repaired, and while one end was found to have been carried away, the other was perfectly good. We shall have to wait until we are better en-lightened as to this new theory. In his remarks on this paper, Professor Kernot referred to the locomotive boiler that burst some time ago. It was found that a certain plate was thin com-pared with another. These two plates were on the side of the fire-box, just on the crown where the fire-box turns round between steam and water. My impression is, those plates were in connection, and when steam was up they became exfoliated. I do not believe there was any electric action from one end of the boiler to the other. All the rest of the boiler was perfectly sound. I might refer to many other boilers with peculiar corrosion right throughout ; but I cannot attempt to explain the corrosion upon the theory set forth in this paper. If Mr. Murray can clearly solve these difficulties, he will not only deserve the thanks of this Association, but his services in the cause of science ought to be made known throughout the length and breadth of the world.

CAPTAIN J. KELLY : The gentlemen who have spoken to-night have referred to the effect of mechanical action in the boiler. Now, within the last few years it has come to be perfectly under-stood, and it has been demonstrated by Professor Thomson, that when iron or cast steel has been for a long time subjected to vibration it becomes fatigued and loses its strength. As no allowance is made for fatigue, the iron in boilers after they have been used for a certain length of time becomes worn out. A boiler is in a constant state of vibration while it is working and changing its temperature. I cannot find any instance where there was allowance made for deterioration in the strength of the boiler after being used for a certain length of time. I think that should be taken into consideration. With reference to the


heat theory, we should bear in mind that in water pipes, through which cold water runs in the city scale is deposited as well as in boilers. As to electricity, it is always passing, and there can be no mechanical or electric action unless an electric current is set on. Consequently, it is difficult to speak of it separately from mechanical action ; and it would be necessary to separate them and to lay down some definite line upon which inquiry should be conducted. But I think the mechanical theory should be carefully discussed.

MR. R. BUCHANAN : I perfectly agree that galvanic action has a great deal to do with corrosion. In boilers where there is movement in the plates there is bound to be a considerable amount of friction; there is bound to be a galvanic action in any boiler where there is movement in the plates, and I think the electrogen would counteract that to a great extent. But there is another aspect of the case which has not been touched upon, and that is the deposition of scale and the prevention of it. I find it is the effect of the steam rising that causes the scale to be deposited. Take a molecule of water composed of two of hydrogen and one of oxygen. One comes to the positive pole and two to the negative pole. But if the water is held in solu-tion the negative must be deposited, or else it must go into the rest of the water to increase the density. The water is being formed into gas, and, therefore, it deposits a solution. Mr. Hannay points out that the negative plate in electro-plating prevents the scale from going on ; but to my mind the two are not analagous, because in electro-plating there is not the same heat generated to fix it, while in the boiler there is intense heat passing through the furnace, so that whatever touches the plate is burnt on. I think, therefore, the analogy does not exactly hold true. I would like Mr. Murray to enlighten us upon that, and show us how he would fix the electrogen in the boiler. I do certainly think it would be a powerful preventive of internal corrosion.

MR. K. L. MURRAY : First allow me to say that I have been careful all through to disclaim any power to explain the causes of destruction of all boilers inwardly, and next, that I have never once said that the destruction of boilers is always due to electric action. Some of the gentlemen have referred to vibra-tion and mechanical causes of destruction. We know that if a person goes with a sledge-hammer and hits an iron plate, he will break it. No one will accuse me of saying in such a case that the destruction would be caused by electric action. I might just as well say that destruction by vibration or mechanical means was an electrical effect. I find a great deal of confusion of ideas with regard to electricity. Very few people have gone into the


study of electricity very deeply. I think, therefore, that it is a science which is not very clearly understood. A number of the members of this Association have given us a great deal of valuable information on the subject of the chemical causes of corrosion, and I do not join issue with them. Certainly, that chemical action goes on, but with it there is galvanic action. That chemical action takes place on the iron of the boiler. 1 call it electrical action, and I say that action, if an electrogen be used, will take place upon the zinc instead of the iron. An electric battery, as we know, consists of two pieces of metal placed in solution. The force of chemical action depends upon what that solution is, and the force of the galvanic action depends upon what the chemical action is. Therefore, the force of the galvanic action depends upon what the solution is. Now, when we have two plates in a battery, irrespective of the force with which the one or the other is attacked, if the two plates are connected only one will suffer to any extent. If we take copper and iron, we shall find that the iron will suffer. Take the copper away and put in zinc, and the iron will not suffer, but the chemical action will go on, and electrical currents will be pro-duced; but the iron will not be decomposed, it will be the zinc. So, with regard to the boiler, I maintain that there is always electrical action going on. There is chemical action and elec-trical action. In the paper I have read I mentioned two reasons for the electrical action—one the difference in the temperature, and the other the difference in the metals. It has been proved beyond any doubt to demonstration, and we can accept it as an absolute fact, that if there is a difference of temperature there will be galvanic action. It is also proved beyond doubt that if you have different kinds of metals there will be galvanic action. It has been proved that in boilers you will have corrosion taking place at one particular spot more than another, and it has been shown that at that particular spot there has either been a different kind of metal—that is to say, the iron plate differed from the iron plates surrounding it, or a difference of temperature at that particular spot as compared with the parts of the boiler that surrounded it. That being admitted, I maintain that putting in a piece of zinc, which is positive, to the other metals of the boiler, will cause the chemical action to be removed from the iron to the zinc. I mentioned at the last meeting that a zinc ball had been put into one of the boilers of a coasting steamer, and the day berore yesterday I looked into that boiler to see what had been the effect. The steamer is the Taramung. The result was very much like that observed by Mr. Hannay in the ships under his charge ; but instead of slabs there were round balls of zinc, The chief engineer will tell you

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if he is asked that there was very little difference between those two boilers—only one had no zinc and the other had. In the one without zinc there was very much more corrosion than in the one protected by the zinc. But I showed the engineer that he did not receive the full benefit from the zinc because he had not made that proper attachment between the zinc and the iron of the boiler which is absolutely necessary to ensure the protec-tion which the zinc should give. If you take a battery you want it to be expended in doing useful work. It can only do a certain amount of work, and if you allow it to have any other than the work you wish it to do, you wastefully dispose of a certain amount of its power, which is expended in overcoming the resistance. If you take the wires which connect two plates together and join them in a careless way, so that they are not in proper contact, the force of that battery will be expended in overcoming resistance at the points of contact, and the longer the battery remains in action the worse it will become, and in the end you will get no work out of it at all. This is the way with the zinc in marine boilers. Unless it is soldered to the iron you will not get the effect out of the battery that you desire to get. The power will be expended in overcoming resistance at the points of contact. And the longer it is in work the worse it will become, because corrosion will take place at the points where the zinc is connected to the iron, until at last it will cease to be of use. One gentleman says he bored a hole through the boiler and put in nuts and washers; but in spite of that the hot air, steam, and damp air would find its way between the points of contact. Oxide of iron was thus formed,. which would prevent the zinc from doing its work. Anyone who has had anything to do with zinc in boilers must acknowledge the usefulness of it. Almost everyone spoke of its good effects in the experiments that were made. But in using zinc you do not get half the good effects unless you solder the points of contact. The great advantage of Mr. Hannay's method is that he leaves no possibility of corrosion between the points of contact of the zinc and the iron. Whatever'work is done by his battery is, therefore, useful work—nothing is wasted. My friend, Mr. Huysmans, mentioned the use of cement wash, which is very beneficial, but you cannot use it under the water. It will wash off. It is only of use where you can put it on. Cement is of use because it prevents the action on the iron. You mentioned, Mr. President, in one of the extracts, the desirability of amalgamating zinc in the ordinary battery, where the common zinc you buy is used. You find the zinc plates become pitted very much in the same way that the iron plates are. That is because there are impurities in the zine, . There are little bits of other kinds of metal, and


between these and the zinc a small battery is set up, causing the zinc to be used up and these holes to be formed. These little impurities are covered over, and local action cannot take place. That is the reason why it is a good thing to coat boilers with some kind of substance that will prevent corrosive action. If you can coat them with a substance not soluble in water, you will perfectly save them ; but that is difficult. If zinc is used to form an electrical battery, you must fix it upon the scientific lines which electricians have discovered, and apply them in the way best calculated to secure beneficial results. Several members mentioned instances of corrosion which seemed to them not to be in accord with the galvanic theory. Mr. Warnock spoke of a case where a boiler plate wore away very rapidly, so that it had to be removed and another substituted, and that the latter, after four years' use, was very little corroded. That proves to my mind that the plate removed was electro-positive to the parts of the boiler to which it was attached, whereas the new plate is not. Mr. Huysmans said that the most serious cause of corrosion is the combined action of air and water upon the unprotected surfaces of the iron. Precisely. I call galvanic action what my friend speaks of as the " action of air and water." I can only supplement the very excellent advice Mr. Huysmans gives with regard to the use of zinc by urging him, as well as every other engineer, to solder their connecting wires instead of trusting to clamps. They will find that the protection afforded by the zinc will, by that means, last much longer, and that a less surface of zinc will be sufficient. (Applause.)

Library Digitised Collections

Author/s:Murray, Kynaston Noel Lapthrop

Title:Corrosion and incrustation in steam boilers (Paper & Discussion)

Date:1885

Persistent Link:http://hdl.handle.net/11343/24228

http://hdl.handle.net/11343/24228

Corrosion and incrustation in steam boilers (Paper ...

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