Environmental Technology Letters, Vol. 9, pp. 625-636 © Publications Division Selper lid., 1988

CORROSIVITY OF THE N.W. SPANISH SEA WATER

L. Espada,* X.R. Novoa,* and P. Merino ..

* Dept. Enxeneiria Quimica .. Dept. Enxeneiria Mec8nica e MaterhUs

E.T.S. de Enxeneiros Industri8is, Aptdo. 62, 36200 VIGO, SPAIN

(Received 3 November 1987; in final form 8 March 1988)

ABSTRACT

The corrosivity of the Galician coast water is studied by the coulombstatic method, which is particularly appropriate to this sort of marine system. Samples taken at different depths in areas of Galician coast were analysed using low carbon steel as the metal of study. The obtained results show that these waters present a moderate level of corrosion (between 0.1 and 0.2 mm.year- 1 penetration) compared to other data quoted in the literature. It is also observed that the corrosion rate decreases quickly with the depth as far as 1,200 m, and diminishes less quickly at further depths.

INTRODUCTION

Through history, a great number of researchers have devoted themselves to the study of sea water corrosion, due to the great amount of metal structures that man has exposed to the marine medium, either on the coast (marine atmosphere), floating (partial immersion) or in total immersion.

The development of marine agriculture during the last few years has increased the number of these structures.

Traditionally, Galicia is the most important fishing area in Spain with respect to the number of boats and port instalments. This fact alone would justify an exhaustive study of the corrosion of this marine environment. Nevertheless, another important reason to carry out this work is the ideal conditions that the area presents for the development of marine farms. .

This preliminary study attempts to obtain general information about the corrosive characteristics of the Galician marine environ­ment.

STUDY'S TECHNIQUE

There are two ways of appproachi~g the study of sea ~ater corrosion. Firstly, there are the natural tests, from which very precise information is obtained since the conditions of test and work (the actual marine environment) are the same, and secondly, there are electrochemical tests. The first ones have the disadvantage of needing long periods of time to obtain the results, and this is

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frequently unacceptable, tests.

we shall concentrate on the electrochemical

Corrosion can be defined as "the destruction or deterioration of a material because of its reaction against its environment" (1). Among the many definitions existing in the literature, this is the most suitable for the situation under study. Metals introduced into sea water suffer a reaction of an electrochemical nature, which can be separated into two semi-reactions: an anodic one, or dissolution of the metal, and a cathodic one, where some chemical matter prevai­ling in the surroundings absorbs the released electrons in the anodic zones of the metal's surface.

In the case of steel submerged in the sea, the following two processes occur on the metallic surface:

Anodic process:

Fe -----------) Fee· + 2e-

and ~athodic process:

De + 2H~D + 4 e- -----------) 4 OH-

The anodic process represents the effective loss of steel as it becomes hydroxide, according to the global process:

2 Fe + De + 2 HeO ------------) 2 Fe(OH)e

The Fe(OH)e produced in the first instance will develop until reaching Fe(OH)~, which is "rust", in the following way:

2 Fe(OH)e + HeO + 1/2 De -----------) 2 Fe (OH)~

Corrosion rate or loss of material per unit of time can be evaluated gravimetrically, as it is done in natural tests, or electrically using the electrochemical nature of the process, as there is a direct relation between the electricity exchanged in the anodic and cathodic processes, according to the following equation:

M Corrosion rate = ------- Icorr

n F

were M is the atomic weight of the dissolving element, n is the number of electrons exchanged in the corrosion process, F is Fara­day's number and Icorr is the corrosion current denSity.

In the case of iron, it would be:

1 ~A cm-e = 1.16 x 10-e mm year- 1

because the specific weight of iron is 7.86 gem-a.

The classical electrochemical techniques of direct current are based on the determinations of the amount of electricity interchanged during the corrosion process ,when the anodic and cathodic semi­reactions are separated into two different electrodes. This separa-

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tion means that the sample under study must be exposed to an external potential (anodic and cathodic) to be able to measure a net current between the electrodes, and, therefore, the system undergoes some disturbance.

A recent technique, described in the electrochemical literature, has proved to be very useful both in acid media and neutral ones (2,3) and, on the whole, is applicable when the electrochemical system corresponds to the scheme 'of figure 1.

Since this method does not require a net circulation of electri­cal current between working and counter electrodes during the overvoltage measure, its application implies minimal changes in the double eletrochemical layer and, consequently, a higher degree of reproductibility.

The working of the system is as follows: when a small amount of charge pulse is supplied to the electrical double layer of a metal test piece at a corrosion potential Ec~rr, the potential jumps to another potential E~.

The time of charge is supposed to be short enough for the circuit in figure 1 to be opened by C; that is to say, the reaction of corrosion does not occur. In these conditions, all the charge supplied to the double layer is spent in charging it and an initial overvoltage is obtained in the eletrode, no, that is worth:

(ll

When the impulse stops, the reaction of corrosion starts, the circuit in Fig.1 is closed by C and the electrode is discharged in the solution, without net flow of current between the working electrode (W.E.) and the counter electrode (C.E.).

Cdc

~y Rp C

Fig.l: Equivalent 'circuit for the test cell where R, Rp, Cdc correspond to solution resistance and double layer capacitance, respectively. C is an imaginary point in order to explain the method (see text).

Assuming that at small polarizations <Jnol < 5 mV) the differen­tial capacity per unit of area, Cec, remalns constant, the quantity of charge consumed, at the end of the time t, ~ q~, in the process of corrosion will be:

[2J

where nt is the overvoltage at the end of time t

The increment of charge supplied to the electrode is consumed progressively by the corrosion reaction and the potential drifts back to its initial corrosion potential, Ecorr •

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If it is assumed that the concentration of polarization is negligibly small in the corrosion process, the relation between overvoltage n~ and the Faraday ~urrent density I~ is given by the next equation (4):

[3]

where Ic~rr is the corrosion current density, Q_ and Q- are the transfer c~efficients of the anodic and cathodic reactions, and n_, and n- are the numbers of electrons concerning the anodic and cathodic reactions, respectively. R, T, and F are gas constant, absolute temperature and Faraday's constant, respectively.

An adequate combination of equations [2] and [3] and assuming that:

dt [4J'

leads to an expression for the discharge of the circuit in figure 1, as a function of the parameters of the electrochemical system:

Polarization resistance, Rp, is considered to be one of the most' important parameters concerning corrosion rate, which is inversely proportional to the corrosion current. In this way, the linear polarization technique, widely used for the kinetic study of corro­sion reaction, is based on the measure of R~.

Stern and co-workers (4,5,6) founded equation (6], the basis of this technique:

(6J

where I is the Faraday current density, Icorr is the corrosion current denSity, a~ and ac are the Tafel slopes (5) for anodic and cathodic reactions, respectively, and n is the overvoltage.

With respect to equation [3]:

and

Usually, equation (6] appears as:

B Ic~r.r = ------- [7]

Rp

Where 8 is:

The values of a~ and ac can not be obtained by the linear polarization technique, and therefore the value of B must be estima­ted from the determinations of.a. and ac made with the polarization curve.

The coulombstatic method allows the direct evaluation of a~ and ac (2,7). Nevertheless, in this work, the value of 8 = 26 mV will be used to obtain the corrosion rate, because it is a typical value in sea water media a~d adequate for purposes intended.

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EXPERIMENTAL

The experimental process consists of supplying to a test piece (W.E. in fig.3) known quantities of charge and registering with a plotter or a digital oscilloscope, the falling of the electrode potential with respect to time (figure 2). The coulombstate used was designed by ourselves based on the circuit proposed by K. ichi Kanno et al. ( 3) •

Fig. 2: Typical Polarization Potential (n~) - time (t) curve for a C-steel in sea water. Charge supplied is 1.75 ~C (testing point 2, depth 60 m).

10 -> .se c; :: 6 c

" o 0. 4

o

j~

1\ 1 ~

1 1 1 I

" , "' ~' .....

........ ........ ....... -0..5 1.0 t (s)

The electrochemical cell used (figure 3) was also designed by ourselves in order to get a homogeneous distribution of electric field lines on the electrode of study and therefore a homogeneity of the differential capacity per area unit on it (to be able to write the equation [2]).

Fig. 3: Electrochemical test cell. C.E. and W.E. are the counter electrode and the working electrode, respec­tively.

WQ~r l~v~1

p. V. C. tu be's

Pt wir.

T~t pi~c~ (W. E.'

As reference electrode one of Ag/AgCI saturated was used.

C 0.096

The percentage composition of the carbon steel tested was:

Si 0.183

Mn 0.350

P 0.022

5 0.033

Cr 0.123

Ni 0.161

Mo 0.060

Cu 0.460

5n 0.041

Test pieces were submerged in aliquot parts of sea water collected in different places of the Galician coast and at different depths.

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Fig.4 shows the geograp~ical distribution of the sampling points and table 1 gives their exact location as well·as the depths at which the samples were taken.

Fig. 4: Distribution of test points (see table 1)

Table 1: Location of sampling points.

'9" •

'4

'3 7.

42

41

GO

10 «) 8 7

6

This corresponds to the so called "Atlantic Ditch" at 4,200 m. The sample has not been taken at such depth, but at 2000 m, and the salinity and temperature of the sample have been adjusted to those that exist at 4,200 m.

TEST POINT No. DEPTHS (m) LATITUDE (N) LONGITUDE (W)

1 0,20,60,100 42°10' 9°5'

2 0,20,60,100 430 9°22'

3 0,20,60,100 43 0 28' 9°11'

4 0,20,60,100, 44°15' 7°55' 400,1200,2000

5 0,20,60,100, 43°6' 9 0 51 ' 400,1200,2000

6 0,60,100,400, 42°59' 10 0 27' 600

7 0,60,100,400, 430 10°27' 1200,2000

8 0,60,100,400 43°53' 11°13' 1200,2000

9 .... 4200 46° 16°

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Table 2 gives the physical parameters of the test points.

Metallic material was prepared by polishing with Si/C papers, numbers 2 and 3. Then they were cleaned with acetone and degreased with trichloroethylene, and left in a dryer for at least twelve hours before starting the test, according with ASTM standards.

The sea water samples were treated in a thermostat in an appropiate climatic chamber at the same temperature as the collection site, in order to balance the dissolved oxygen before starting the tests.

RESULTS AND DISCUSSION

As previously noted, natural tests are important when finding an accurate evaluation of the marine environment aggressiveness. In this connection, 'Reinhart's investigation (8) over eight years of 475 alloys between 0 and 1,800 m depth in the Pacific Ocean (California) shows that in general, the corrosion rate is reduced as depth increa­ses, becoming up to three times as low as the surface rate.

The values found for marine corrosion rate vary widely from 0.6 mm per year, obtained in natural tests carried out in Kure Beach (North Carolina) (9), down to 0.06 mm per year found in Tortuga Island, after 31 years exposure (10).

It is known that oxygen plays a fundamental part in marine corrosion but it must be borne in mind that, in natural conditions, the accumulated corrosion products will act as an additional barrier to the gas diffusion on the metal. This will make corrosion rate decrease in time as can be observed in Reinhart's work.

Among the laboratory tests made to determine the corrosion rate, using electrochemical techniques, can be mentioned that of Climent (11) because of its similarity with the steel employed in our experiments. Climent uses synthetic sea water at a temperature of 35 °C and determines corrosion rate by the potentiometric method. The comparison of the values so obtained with those of natural t~sts, in test-tubes of the same type, shows that the corrosion rate is almost three times greater by the potentiometric method, than the parallel natural test (0.75 mm per year). This is explained by the author as a result of the simplifications introduced into Stern's formula (5).

If appropriate temperature corrections are made (Climent works at 35°C and in our tests the margin is 2-12 oC) and accepting that the corrosion rate is doubled when the temperature is increased by 10 QC (12), the values obtained in the present work trough the coulombs­tatic method are very close to those of Climent's natural test.

The data given in Table 3 show that Galician littoral waters exhibit a moderate level of corrosivity, compared to other zones (8,9,10).

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Table 2: Physical parameters of the test points. These data as well as the samples have been provided by the Instituto de Investigaciones Pesqueras de Vigo (Vigo Fisheries Research Institute) •

SAMPLING DEPTH (Oe J T pH SALINITY POINT (m) (mmol·m-~) °C (g·1- 1 )

0 300 12.38 8.26 35.354 1 20 282 12.55 8.21 35.398

60 266 12.60 8.19 35.614 100 258 12.37 8.16 35.596

0 297 12.22 8.22 35.561 2 20 293 12.20 8.20 35.560

60 279 12.17 8.18 35.574 100 261 12.10 8.15 35.606

0 271 12.00 8.12 35.581 3· 20 271 12.00 8.15 35.583

60 272 12.00 8.15 35.582 100 272 11.95 8.15 35.585

0 263 11.87 8.16 35.535 20 263 11.88 8.16 35.529 60 264 11.88 8.16 35.530

4 100 265 11.83 8.16 35.530 400 235 11.22 8.11 35.536

1200 194 9.96 8.04 35.853 2000 251 4.55 7.98 35.074

0 262 12.58 8.16 35.641 60 261 12.56 8.16 35.641

5 100 262 12.51 8.16 35.639 400 227 13.30 8.07 35.555

1200 192 10.76 8.04 36.073 2000 255 6.97 7.98 35.252

0 261 12.55 8.17 35.655 60 262 12.57 8.17 35.656

6 100 261 .12.53 8.17 35.653 400 219 11.20 8.05 35.539 600 . 209 10.70 8.02 35.544

0 263 12.30 8.16 35.646 60 263 12.32 8.16 35.643

7 100 263 12.28 8.16 35.641 400 223 11.18 8.08 35.544

1200 201 9.18 8.02 35.776 2000 266 4.15 7.98 35.027

0 263 . 12.20 8.16 35.604 60 263 12.20 8.17 35.603

8 100 263 12.13 8.17 35.602 400 231 11.90 8.09 35.515

1200 197 9.75 8.04 35.854 2000 265 4.33 7.99 35.012

9 4200 246 2.06 .. -- 34.920

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Within this moderate corrosion level (penetrations between 0.1 and 0.2 mm per year after immediate immersion), an increase of corrosion with distance from the coast can be observed, finding the maximum at point 5, where due to the local equilibrium between rivers and sea, water reaches the highest temperature and the standard level of salinity.

From table 3, it can also be concluded, as it might be expected, that the variation in the corrosion rate with depth follows the line of dissolved oxygen concentration <shown in table 2). This factor is, together with temperature, the principal determinant of corrosion rate in these media; oxygen, because it is the "fuel" of the cathodic reaction and the temperature because it is closely linked to the kinetics of the process.

Table 3: Corrosion rate in the different testing points. Values given are 'in }-,Acm-e and between parenthesis, the penetration in mmyear- 1 • Remember for this case: 1 ~Acm-e = 1.16 x 10-e

mmyear- 1 •

DEPTH TEST POINTS

(m) 1 2 3 4 5

0 19.6(0.23) 19.5(0.23) 13.3(0.15) 13.2(0.15) 22.4(0.26)

20 12.9(0.15) 12.9(0.15) 17.7(0.21) 14.2(0.16) 14.2(0.16)

60 13.4(0.16) 17.2(0.20) 12.8(0.15) 14.4(0.17) 21.5(0.25)

100 15.0(0.17) 13.3(0.15) 15.2(0.18) 15.1(0.18) 18.3(0.21)

400 15.0(0.17) 14.9(0.17)

600

1200 9.2(0.11) 10.2(0.12)

2000 7.8(0.09) 9.5(0.11)

4200

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Table 3 (Cont.)

DEPTH TEST POINTS

(m) 6 7 8 9

0 21.5(0.25) 19.8(0.23) 18.5(0.22)

20

60 21.1(0.24) 16.2(0.19) 16.3(0.19}

100 17.8(0.21) 14.6(0.17) 15.5(0.18)

400 16.4(0.19) 17.2(0.20) 15.8(0.18)

600 13.8(0.16)

1200 11.2(0.13) 10.3(0.12)

2000 9.9(0.12) 8.6(0.10)

4200 8.1(0.09)

The apparent anomaly that at 1,200 and 2,000 metres the respec­tive corrosion rates are very similar even though there is a conside­rable fall of temperature between both levels, is due to the fact that below 1,000 m depth the oxygen concentration increases slowly, as can be observed from table 2.

The obtained data for point 9, the Atlantic Ditch, must be viewed with caution, since in the sea muds the cathodic reaction is not mainly going to be that of the oxygen but other cathodic reac­tions may have importance, such as:

2HeO + 2 e- ---------------) He + 2 OH-

(catalY2ed by sulphate reducting bacteria) (13)

This means' that the value of 0.09 mm point in table 3 represents the lower limit this, are obvious the conclusions about material containers stored in the area, if steel.

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per year given for this of corrosion rate. From

durability of radioactive they are made of carbon

The moderate corrosion level of this area, determined by the local environmental conditions (e.g. water temperature, salinity and oxygen concentration), allows us to conclude that a low-cost cathodic protection based system will protect adequately immersed steel structures in this environment.

The situation described above changes when approaching the coast line where the estuaries, called "ri"as" , present different levels of pollutants that influence the corrosivity of the environ­ment. Results including this aspect will be reported elsewhere (14).

The influence of steel composition has not been studied, because this work intends to evaluate the corrosivity of the marine environments in our area, and the effect of alloying elements is out of its scope. However, according to the literature (8,9), there is not much difference between the corrosion behaviour of low-carbon steels in marine environments. The situation changes, of course,when considering alloyed steels, due to the formation of protective films on the metal surface.

ACKNOWLEDGEMENTS

This research has been sponsored in part by the "Diputacion Provincial de Pontevedra". The authors thank the JlDiputacion Provin­cial" for their interest and encouragement.

REFERENCES

1.- M.G. Fontana and N.D. Greene, Corrosion Engineering, 2nd ed., McGraw-Hill, N.Y., 1983.

2.- K.S. Rajagopalan and others, J. Electrochem. Soc. India, L, 34-40, (1983).

3.- K. ichi Kanno, M. Suzuki and Y. Sato, J. Electrochem. Soc., ~, 1389-1393, (1976).

4.- M. Stern and E. Wiesert, Proc. Am. Soc. Test. Mater., 59, 1280-1286 , (1 959) •

5.- M. Stern and A. L. Geary, J. Electrochem. Soc., ~, 56-63, (1957).

6.- M. Stern, Corrosion, ~, 440t-444t, (1958).

7.- K. ichi Kanno, M. SU2uki and Y. Sato, Corros. Sci., 20, 1059-1066, (1980).

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8.- F.M. Reinhart, Corrosion of Metals and Alloys in th Depp Ocean, Civil Engineering Laboratory, Technical Report, R-834, Feb. 1976.

9.- F.L. La Que, Marine Corrosion: Causes and Prevention, J. Wiley, N. Y ., 1975.

10.- C.G. Gruelich, Civ. Eng., ~, 10-18, (1942).

11.- J. Climent, Rev. Metal. (Madrid),~, 148-154, (1976).

12.- U.R. Evans, An Introduction to Metallic Corrosion, E.Arnold, London, 1981.

13.- N.A. North, Stud. Conserve ,27, 75-83. (1982).

14.- L. Espada et al., to be submitted to Environ. Technol.Lett.

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