Desaiination, 44 (1983) 265-276

265Elsevier Science Publishers B.V ., Amsterdam--Printed in The Netherlands

APPLICATION POTENTIAL OF A MODIFIED ALUMINIUM ALLOY IN ARABIAN GULFDESALINATION PLAITS

ZAKI ARMED *and SHADMAN RASHIDI9

* Associate Professor, Mech .Eng .Dept ., University of Petroleum & MineralsDhahran, Saudi Arabia .

o Research Assistant, Mech .Eng .Dept., University of Petroleum & MineralsDhahran, Saudi Arabia .

ABSTRACT

This paper describes the results of a development program designed to producebetter aluminium alloys for applications in desalination plants and brackishwater environments . On the basis of the results obtained from polarization,potential-time, long term exposure and metallographic studies, the modifiedalloy 2778 CR appears to show, promising application potentials .

Es werden die Ergebnisse eines Entwicklungs programns fur verbesserte Alu-miniumlegierungen mitgeteilt, die mit Vorteil bei Meerwasserentsalzungs anlagensowie verallgemeinernd im Bereich von Kustengew$ssern verwendet werden . Unter-suchungen von zeitabhangigen Polarisationspotentialen and metallographische .Studien an uerkstoffen aus Langzeitversuchen zeigten, dab die modifizierteLegiering 2778 CR besonders geeighet ist .

Cette publication dkrit les resultats d'un programme de developpement des-tine a ameliorer les alliages d'aluminium applicables aux installations de d€-salement et de traitement des eaux saumatres . Le resultat des recherches enpolarisation . en temps de potentiel, en exposition a longue duree, et enmetallographie ont nontre que 1'alliage "2778 CR" possedait a la fois desproprietees de resistance a Is corrosion et mecanique . L'alliage permet doncd'entrevoir des applications pour le d€salement et le traitement des eauxsaumatres .

INTRODUCTION

Aluminium alloys have been used successfully by the chemical, petroleum and

power industries and the literature contains numerous references to their appli-

cation potential in desalination plants (ref .l-5) . A number of studies on the

localised corrosion have been made and alloys of the 5000 series are reported to

have shown good resistance to corrosion (ref .5-10) . The use of these alloys in

desalination has been restricted by high temperature and controlled water che-

mistry . Attempts have been made to reduce operating temperatures and control

the environment in order to improve the performance reliability of such alloys

(ref .ll-13) .

This paper describes the result of a development program undertaken to pro-

duce better aluminium alloys for application in brackish and sea water environ-

0011-9164/83/$03.00 © 1983 Elsevier Science Publishers B .V .

266

ments (ref .14) . The investigations presented show the corrosion behavior of the

modified aluminium alloys in brackish water of the Eastern Province of Saudi

Arabia . Results of similar studies in Arabian Gulf sea water are also presented .

Studies related to mechanical properties and corrosion resistance of these al-

loys in North Sea water have been separately reported (ref .15) . The corrosion

data obtained from these investigation has been used to evaluate the application

potentials of these alloys for desalting units employing either brackish or sea

water. The bulk of the investigations have been carried out in local brackish

water and the results obtained from investigations in Arabian Gulf and North Sea

water have largely been used for comparison of corrosion behavior .

MATERIAL

Four modified aluminium alloys havingg the composition shown in Table 1 were

were cast and homogenised for 12 hours at 350°C . The alloys were first hot rol-

led to a thickness of 5 mm and later cold rolled to achieve a final thickness of

1 .75 mm . This form has been designated as cold rolled and abbreviated as C .R .

A selected quality of the sheets was heat treated for 2 hours at 360°C and cool-

SPECIMEN PREPARATION

For long term testing samples measuring 2 .5 cm x 2 .5 cm were used . Circular

discs of one square centimeter area were used for polarization measurements .

Specimens were mounted in a holder designed for flat specimens as specified un-

der ASTM designation 461-78 . All samples were treated separately with boiling .

petroleum benzene and ethanol for 10 minutes and finally degreased with 5% ace-

tic acid for five minutes . They were washed with demineralized water and allow-

ed to dry for fortyeight hours in a dessicator . Sample for metallographic stu-

dies were treated with a H3P0u-Cr 2 O3 mixture to remove corrosion products prior

to examination . Specimens for long term testing were exposed up to 8000 hours

in the test media . The composition of the two type of water is shown in Tables

2 and 3 .

ed in furnace .

TABLE 1

Composition of

These alloys are designated as annealed .

the modified alloys (weight percent) .

AlloyDesignation

Si Fe Cu Mn Mg Cr Zn Ti Al

2775 .68 .18 .01 < .01 < .01 - < .01 < .O1 Balance2776 .77 .19 .01 < .01 2 .56 - < .01 < .01 Balance2777 .67 .17 .01 .01 .03 .26 < .01 < .O1 Balance2778 .71 .19- .01 .02 2 .60 .29 < .01 < .O1 Balance

TABLE 2

Eastern Province brackish water analysis .

Milligrams per liter

Ions

Na

Ca

Mg

S04

Cl

C03

HCO3

Total

647

288

99

707

1,221

6

79

3,047

TABLE 3

Arabial Gulf sea water analysis .

Milligrams per liter

Ions

Na

Ca

Mg

504

Cl

C0 3

HCO3

Total

19,136

704

2,400

4,894

34,080

18 189

61,471

267

METHODS

Weight loss technique

These studies were conducted in accordance with the procedures specified by

ASTM under designation G-31-72 .

Polarization measurement technique

Potentiodynamic and polarization resistance techniques were used to evaluate

the corrosion mechanism of the alloys . The measurements were conducted in

accordance with ASTM practices specified under designation ANI/ASTM-G61-78 . and

G59/78 . The investigation were supplemented with Tafel extrapolation technique

to check the reproducibility between various techniques . The specimens were

allowed to remain in test solutions for 2 hours to allow equilibrium to be reach-

ed prior to commencement of polarization . Electrochemical parameters were deter-

mined from the measurements and corrosion rates calculated .

Potential-time studies

All potential measurements were made with respect to a saturated Calemel elec-

trode and a high impedence voltmeter was used for determination of open circuit

potentials . Potentials were constantly monitored by a potentiometer recorder .

Surface Morphology

A scanning electron microscope was employed to examine surface morphology and

localised corrosion of the alloys . Pit depths were measured by a metallurgical

microscope in accordance with the recommendation of ASTM specified under desig-

nation G46-76 .

268

POTENTIAL-TIME STUDIES

The trend of variation of potential of the alloys with time is of a consider-

able importance in establishing long term behavior of alloys in environments .

Typical potential-time plots for alloys 2778 and 2776 in smoothly stirred

brackish water is shown in Fig . 1 . The tendency of alloy 2778 CR to show a

pronounced shift in the noble direction is obvious . This tendency appears to

diminish with time, however, the alloy remains at potentials relatively noble to

the starting open circuit potential . This tendency is also shown by alloy 2776

to a large extent . Alloy 2777 shows a decrease in the magnitude of surge of po-

tential in the noble directon and a similar behavior Is shown by alloy 2775 .

o Alloy 2. 778 (CR)•

Alloy 2778 (Annealed)x Alloy 2776 (C^,)A Alloy 2776 (Annealed)

Fig .l . Variation of potential with time in smoothly stirred Eastern Provincebrackish water (temperature 25°C) .

Accurate monitoring of potential for the first 60 hours appears to be important

as the mechanism of pitting is to a large extent governed by the potential surges

in the initial period of exposure . The exceptional behavior of alloy 2778 CR -

appears to be due to the tendency of chromium to stimulate film formation .

Chromium addition in aluminium appears to be effective only in presence of Mg as

alloy 2777 containing Cr without any significant amount of Mg fails to exhibit

the behavior shown by alloy 2778 CR . The initial surge in the noble direction

suggests the onset of pitting followed by repassivation as shown by a gradual

increase of potential with time . The tendency to repassivate is increased by Cr

addition as reflected by the behavior of alloy 2778 CR . Magnesium and chromium

contents both appear to influence the re-passivation characteristics of the

alloys, and the combined effect further strengthens this mechanism . Detailed

studies have been separately reported (ref .15) . The increased resistance of

alloy 2778 CR to pitting is also reflected by the continuous decrease .i n the

loss of weight of this alloy with time . Repeated potential fluctuations in ei-

ther directions shown by alloys 2775 and 2777 suggest the existance of adsorp-

tion and dissolution processes leading either to film breakdown or film forma-

tion. The alloys showed an interesting behavior under conditions of dynamic

flow in sea water . With the exception of alloys 2778 and 2776 in cold rolled

state, the potential of all other alloys show a trend to shift towards relative-

ly more negative values . The potential of alloys 2775 and 2777 shows a repeated

peak and valley pattern corresponding to rise and fall in the values of potential

at fixed intervals . Under the dynamic state of flow the behavior of alloys 2776

and 2778 is found to be exceptional and similar to that observed in smoothly

stirred state . The above trend appears to brought about by rig addition which

stimulates the formation of the oxide layer more vigorously in presence of chro-

mium and the combined effect of the two is reflected by the behavior of alloy

2778 CR in sea water flowing at a velocity of 0 .46 m/s (Fig .2)(ref .16) . The

effect of Cr and Mg addition on shifting the alloy potential to less negative

values in dynamic sea water is again reflected by shift of potential from a maxi-

mum initial potential of -900 mV to a near steady state value of -700 mV . A

similar trend is shown by alloy 2776 CR, however, the potentials in the steady

state region attains relatively more active value . This behavior also corres-

ponds with the decrease in the loss of weight with increasing time of exposure .

Metallographic examination has shown that alloys 2776 and 2778 have a lesser ten-

dency to pitting than alloys 2775 or 2777 . This observation agrees with the

potential-time behavior presented above .

CEO

U

E -800C

Ny

-900YOd

-1000

46

a0

120

160

2b0

240Time (in hrs)

Fir, . 2 . Variation of potential with time in dynamics Arabian Gulf sea water(terperature 25°C, velocity .46 m/s) .

00

o Alloy 2778 (CR)•

Alloy 2778 (Annealed)

269

270

Annealing on the whole results in displacement of the potentials to more ne-

gative values as shown by the potential-time behavior of the modified alloys in

dynamic sea water, whereas alloys 2776, 2778 and 2777 in the annealed state

show the opposite effect in smoothly stirred brackish water . Alloy 2778 (an-

nealed) shows a very pronounced shift of potential to less negative values . The

results obtained in Arabian Gulf sea water are similar to those obtained in

North Sea water . Full investigations on the variation of potential are separate-

ly reported (ref .17) . The shifting of the potential of annealed alloys to less

negative values in un-stirred brackish water can be attributed to the greater

degree of alloy homogenity resulting in a uniform film thickness . Velocity ad-

versely effects film stabilization and causes shift of potential in direction

opposite to that observed in un-stirred brackish water . The modified alloys in

Arabian Gulf sea water show trends similar to those in North Sea water (ref .18) .

Alloy heterogenity due to cold rolling does not allow the potentials to reach

values sufficient to initiate pitting . Alloys 2776 and 2778 in the cold rolled

formed showed excellent resistance to pitting in the experimental environments .

The results appears to be in excellent agreement with potential-time studies .

The outstanding corrosion behavior of the two alloys can be used with advantage

in sea water or brackish water environments .

WEIGHT-LOSS STUDIES

All experimental alloys showed' low corrosion rates in brackish and Arabian

Gulf sea water . The corrosion resistance of the alloys increased with increas-

ing period of exposure . A similar trend was also observed in dynamic North Sea

water . The behavior of alloys 2776 and 2778 in the cold rolled state was out-

standing and the results are described in Fig . 3 (ref .20) . As shown by brackish

water studies the initial loss in weight corresponded with onset of localised

corrosion in the form of pitting . This is also shown by the sudden surge of

potential of alloys 2776 and 2778 CR in the less noble direction . In many ins-

tances pitting is not observed and the attack appear to be confined to the grain

boundaries . Pits were found to insufficiently developed in both alloys and

alloy 2778 CR in particular showed higher resistance to pitting . Detailed in-

vestigation on pitting are separately described (ref .l8) . A typical pit morpho-

logy is shown in Fig . 4 . The localised corrosion appears to be due to the im-

balance between the process of competitive adsorption and dissolution of species

resulting in instability of corrosion behavior within the first 700 hours of ex-

posure . Between 700 to 2500 hours alloy 2778 CR shows a gradual decrease in the

rate of corrosion with increased exposure . Alloy 2778 CR shows an extremely low

rate of corrosion in the range of 0 .02 mpy to 0 .03 mpy beyond 2500 hours as shown

in Fig . 3 . The alloy did not show any significant increase in the rate after

being exposed to brackish water for two years . The corrosion rate of alloy

3 1000 2000

3000

4000

5000

6000Time (in hrs)

o Alloy 2778 (CR)•

Alloy 2778 (Annealed) .344x Alloy 2776 (Annealed)A Alloy 2776 (CR)

.258

.172

.086

A

Fig . 4 . A typical pit growth in alloy 2778 CR after 4200 hours exposurein brackish water (x 500) .

2778 CR obtained in brackish water (0 .38 mpy) after 500 hours of exposure in-

creases to 0 .432 mpy in un-stirred Arabian Gulf sea water and the same is found

to be true for other test durations . Alloy 2776 CR shows a sharp initial rise

in the rate of corrosion followed by a gradual decrease leading ultimately to

very low values in the steady state region . The corrosion rate shown by alloy

2776 after 800 hours is found to be higher than that shown by alloy 2778 CR .

In dynamic sea water tests (ref .18) alloy 2777 CR shows smaller variations of

corrosion rates with increasing velocities as compared to alloys 2776 CR and

2778 CA, however, its overall dissolution rate is twice as high . Alloy 2778 CR

0 .00

Fig . 3 . Variation of corrosion rate with time in Eastern Province brackish water(aerated) at 25°C .

271

272

shows a higher resistance to velocity than alloy 2776 CR under the same condi-

tions . The resistance of cold rolled alloys in general is found to be higher

than those of annealed alloys . Alloy 2778 CR exhibits highest resistance to

corrosion against experimental sea water velocities of 2 m/s . The beneficial

effect of chromium addition to 0 .71 Si +2 .60 Mg in optimal concentration is

cleanly reflected by the overall excellent corrosion resistance of alloy 2778 CR

in the environments investigated . The corrosion resistance of the alloy appears

to be heavily dependent on Cr/hlg concentration as shown by the experimental in-

vestigation . The higher corrosion resistance of the cold rolled alloys is appa-

rently due to greater heterogenity induced by cold work which prevents the hy-

drated oxide film to reach critical dimensions and stimulate pitting . The re-

verse appears to be true for annealed alloys . Alloy 2778 CR shows highest re-

sistance to corrosion followed by alloy 2776, 2775 and 2777, in decreasing order .

The outstanding corrosion resistance of alloy 2778 under experimental conditions

reflects its excellent potential for application in brackish water and desalting

units . A comparison of the corrosion resistance of this alloy with those of

5000 series largely used in marine applications has shown encouraging results

and further work on development and applications aspects is in progress .

POLARIZATION MEASUREMENTS

The results of polarization measurements of the alloys in brackish water and

Arabian Gulf sea water are described in Tables 4 and 5 respectively . Typical

polarization diagrams are- shown in Figs .S-7 . Polarization technique show good

reproducibility and the results are found to be in general agreement with the

weight loss studies . A comparison of potentiodynamic behavior of alloys Al-3Mg,

2776 CR, and 2778 CR shows an increase in the values of Ecorr and decrease in Bc

values . The corrosion reaction is cathodically controlled and an increase in the

rate of cathodic reaction is shown by the shift of cathodic curves to lower po-

tentials and higher current densities . The corrosion rate on the basis of catho-

dic reaction is found to be highest for Al 99 .5% followed by alloy Al-3Mg and

lowest for alloys 2776 CR and 2778 CR . These observations are in agreement with

the results obtained by Tafel extrapolation and polarization resistance tech-

niques in brackish as well as in sea water . In view of the small variation in

alloying constituents of the alloys it is difficult to detect an appreciable

difference in the corrosion rates with the same scan rate as examplified by the

polarization behavior of the alloys 2776 and 2778 . The experimental alloys show

enhanced cathodic reaction in sea water and the overall corrosion rate of the

alloys in Arabian Gulf sea water is found to be higher than those in brackish

water . The rate of anodic dissolution is hindered by the energy requirements to

surmount the barrieroxide film . The sharp rise in the anodic current density

is the result of oxide film breakdown and corresponds to onset of pitting . All

TABLE 4

Results of polarization and Tafel measurements in EasternProvince brackish water (temperature 25°C, pN 7 .6)

273

Results of polarization and Tafel measurements in ArabianGulf sea water (temperature 25°C, pll 7 .9)

Note: CTC

= Cathodic Tafel constant in v/decadeATC

= Anodic Tafel constant in v/decadeIcorr = Corrosion current in NA/cm2NPY

= Corrosion rate in mils per year .

experimental alloys show lower values of ga as compared with gc and the behavior

of the alloys can be judged by shapes of the polarization curves . . The polariza-

tion studies confirm the results obtained from long and short term exposure stu-

dies both in brackish and Arabian Gulf sea water . Alloys 2778 and 2776 show

excellent resistance to corrosion, however, the resistance of alloy 2778 CR is

more outstanding . The difference between the annealed and cold rolled alloys is

less obvious because of short exposure time in these techniques . The application

potentials of these alloys in brackish and sea water are reconfirmed by thepolarization studies presented .

Tafel Plot Potentiodynamicpolarization

Linear Po-larization

Averagecorrosionrate

CTC ATC Icorr IHPY CTC ATC Icorr NPY Icorr 14PY MY

2775 CR .215 .142 243 .107 .221 .151 185 .091 359 .158 .1182775 Annealed .106 .121 262 .115 .106 .161 262 .115 273 .120 .1162776 CR .190 .126 204 .090 .213 .150 207 .091 264 .116 .0992776 Annealed .131 .090 240 .105 .142 .104 349 .153 357 .157 .1382777 CR .094 .104 241 .101 .094 .114 246 .109 134 .059 .0892777 Annealed .121 .101 302 .132 .131 .109 215 .082 209 .080 .0982778 CR .119 .114 210 .081 .179 .167 216 .068 374 .064 .0712778 Annealed .138 .137 85 .037 .065 .138 45 .02 130 .057 .038A7-311g .096 .041 751 .329 .134 .025 905 .353 2371 1 .039 .573Al 99 .5 .522 .029 2736 1 .199 .629 .051 2832 1 .241 2312 1 .013 1 .151

TABLE 5

Tafel Plot Potentiodynamicpolarization

Linear Po-larization

Averagecorrosionrate

CTC ATC Icorr MPY CTC ATC Icorr IiPY Icorr IIPY MY

2775 CR .142 .192 625 .274 .185 .185 621 .272 1479 .648 .3982775 Annealed .153 .161 1202 .527 .144 .138 1974 .865 2101 .920 .7702776 CR .071 .076 1563 .685 .110 .163 944 .413 1663 .729 .6092776 Annealed .160 .243 1749 .766 .241 .245 1739 .762 1211 .531 .6862777 CR .164 .177 3821 1 .674 .072 .087 3538 1 .550 2052 .899 1 .3742777 Annealed .119 .185 1065 .467 .145 .262 2251 .986 2161 .947 .8012778 CR .063 .079 706 .315 .096 .075 418 .183 692 .303 .2672778 Annealed .069 .093 347 .151 .051 .069 821 .351 297 .131 .211Al-3i7g .031 .197 4535 1 .987 .080 .180 6910 3 .028 8272 3 .624 2 .879A1 99 .5 .062 .111 1642 .720 .065 .094 2277 .998 5353 2 .346 1 .354

274

volts

-0 .600

-1 .20002

-1,400

100

10°

102

104

106

108 1 o 9 nA/cn 2

Current density

Fin, 5 . Potentiodynamics polarization of alloy 2778 (CR) (temperature 25°C) .

Brackish water

sea water

10 1 10 2

103

10''

Current density

Fig . 6 . Tafel plot for alloy 2773 (CR) (temperature 25°C) .

1O 5nA/cn 2

Volts

3 .150

0 .25000m0

0.250

N

0 .750v0OC

1 .250

-0 .708

e -0 .724O

(1

-0 .772

-1 x 10 3

I .

275

0

1 x 10 3 nA/cm 2Current density

Fig . 7 . Linear polarization behavior of alloy 2773 (CR)(tenperature 25°C)

CONCLUSION

On the basis of the investigation the following conclusions are drawn ;

1 . Experimental alloys 2776 and 2778 in the cold rolled state show an excel-

lent resistance to corrosion in brackish and Arabian Gulf sea water both

in smoothly stirred and dynamic states . The alloys offer an outstanding

resistance to corrosion at velocities upto 2 m/s and the corrosion resis-

tance of alloy 2778 CR is found to be superior to that of alloy 2776 CR .

Alloy 2778 CR show a similar behavior in North Sea water . The beneficial

effect of chromium addition in combination withh magnesium is reflected by

the excellent corrosion resistance of alloy 2778 CR and its high resistance

to pitting . The corrosion resistance of cold rolled alloys is generally

superior to those of annealed alloys .

2 . Exposure studies of alloy 2778 CR up to 8000 hours in brackish and sea wa-

ter are found to be in agreement with the potential-time and polarization

studies . Alloy 2778 CR shows maximum resistance to pitting followed by

alloy 2776 CR, 2775 CR, and 2777 CR in decreasing order in the experimental

environments . Polarization measurements have been successfully used in de-

termination of corrosion resistance and localised corrosion, however, limi-

tations are encountered in comparison of alloys having very low corrosion

rate .

3 . As shown by the investigation modified alloy 2778 in particular shows bet-

ter corrosion resistance under the experimental conditions than Al-3Ng and

other alloys of 5000 series mainly recorinended for marine applications .

276

The alloy shows promising application potentials in local brackish water

and desalination environments .

ACKNOWLEDGEMENT

The help rendered by GKSS Forschungzentrum Geesthacht, West Germany in pre-

paration of the experimental alloys is highly appreciated . The authors are

grateful for the help and cooperation provided by the University of Petroleum &

Minerals . The help provided by Mr . S.I .Jameel in preparing the manuscript is

appreciated .

REFERENCES

1 E .T . Wandrer and 11 .4 . Wei, Metals Eng ., Quarterly 7, 3, 1967, 30 pp .2 R .J . Gainey, Corrosion, 17, 11, 1961, 526 pp .3 E .D . Vernik, Jr ., Proc . .'LACE, 25th Annual Conference, NACE, Houston, Texas,

1969, 339-341 .4 E .D. Vernik, Jr., Corrosion/71, Paper # 60, MACE, Houston, Texas .5 C .R . Schmitt, Rev . Coat. Corrs ., 4, 1, 1979, 95-112 .6 Pollack, Proc . Am. Pow. Conf ., Wisconsin Electric Power Co ., Report 7290740,

Vol .19, 1957 .7 G .I . Schrieber, D .D . Oakes, A .L . Whitted and H . Cole, Materials Protection

and Performance, 1213, 1973, 28 pp .8 R .A . Leganlt and W .J . Betting, Materials Protection and Performance, [larch

1971, 9 pp .9 Watanabe, et al ., Corrosion Engineering (Tokyo), 25, 1976, 323-331 .10 E . Wandrer, Met . Eng. Quart ., 7, August 1967, 30 pp .11 Alcoa, Water and Water Engineering, 72, August 1968, 330 pp .12 E. Wandrer, Metals Eng . Quart ., 7, 1967, 30 pp .13 Aluminium Courier, 88, 1969, 14-18 .14 Z. Ahmed, Development of Aluminium Alloys, Internal Report 1, UP[I, Dhahran,

Saudi Arabia, 1982 .15 Z . Ahmed, Designing of an optimum aluminium alloy for desalination application,

6th Int . Congress on Strength of Metals and Alloys, August 16-20, 1982, Mel-bourne, Australia .

16 Venkateswaran, GKSS Forschungzentrum, Internal Report # 82/E/19, Institute forWerkstofftechnolgie and Chenie, 2054, Geesthacht, West Germany .

17 Z Ahmed and H . Meissner, Long Term Testing of Aluminium Alloys in Arabian GulfWater (Third Middle East Corrosion Conference, to be held from May 16-18, 1983at Bahrain, accepted for presentation) .

18 Z . Ahmed, Second Report on Corrosion of Modified Aluminium Alloy under Desali-nation Condition, Department of Mechanical Engineering, UPM, Dhahran, SaudiArabia, 1982 .

i9 Z . Anmed and S . Rashidi, Evaluation and Pitting Control of Aluminium Alloysin Arabian Gulf brackish and sea water (Third . Middle East Corrosion Conferenceto be held in Bahrain from May 28, 1983 . Accepted for presentation) .