The corrosion of aluminium alloys in hot brine

Dmahurfion - Ekvier

Publishing

Company.

Amsterdam

THE CORROSION OF ALUMINIUM J. N. WANKLYN-,

- Printed

in The Netherlands

ALLOYS IN HOT BRINE*

N. J. M. WILKINS. D. R. V. SILVESTER, C. E. AUSTING

AND

P. F.

LAWRENCE So&l Sfats DiwXon, A.E.R.E.. (RerxivcctJanuary

Hrtrwell, D&of,

12, 1971; in revised

form

Rerkr. (U.K.)

April

14, 1971)

SURYMARY

Aluminium and several of its alloys have been tested in flowing, acid-treated brine at 120” and 140°C. All materiafs showed similar rates of uniform corrosion, and even after relatively short times these had fallen to around O.O?S mm/year. However

localised

corrosion,

in the form

of both

pitting

and crevice

corrosion

occurred in many tests. Here there were significant differences between the alloys, those containing manganese and manganese + magnestum showing superior resistance. The pH of the brine, and also its retention time in the test appara:us, had an important influence on localised corrosion. An explanation of these effects, based on electrochemical measurements, is offered. tNTRODUCTtON

In the construction of distillation desalination plants there is a strong incentive to find materials for the heat-exchange surfaces that are cheaper than the conventional copper-base alloys. Ahtminium alloys offer an attractive possibility, but there has been considerable uncertainty about their corrosion behaviour in hot, concentrated brine. The studies described in this paper were undertaken to reproduce. as far as is possible in the laboratory, the corrosion conditions which afuminium would encounter in an acid-dosed flash plant, and to determine what alloy selection, design precautions, and operating conditions would be appropriate. There is already considerable experience of the behaviour of aluminium aiioys in sea water at ambient temperatures, for example, in ship superstructures. There is also much information on the corrosion of aluminium alloys in hightemperature pure water, obtained in connection with water-cooled nuclear reactors (1-3). However, the desalination application, involving temperatures intermediate between these two extremes (h 120°C) as well as high concentrations of chlorides, which are well-known stimulators of corrosion, was a relatively unexplored field. Nevertheless, when the present work started there were favourable indications * Presentedat the Third Int~atio~ Symposium on Fresh Water from the Sea, held in Dubrovnik, Yugoslavia, September f3-16.1970. l * Presentaddress fntemational Nickel Ltd.. Wiggin Street, Bikini

t6, England.

246

J. N. WANKLYN41 a!.

from trials of a smaI1 aluminium plant working at a relatively low temperature (4) as well as a fairly extensive laboratory study (5) of the behaviour of aluminium alloys in sodium chloride solutions at 150°C. The latter suggested that., with appropriate pII control, aazeptable rates of uniform corrosion could be achieved, but that loealised attack coultj be serious. The present work confirms that the greatest risk is from localised corrosion but suggests that its incidence is likely to lx less serious in brine than in sodium chloride solations. More recently. very favourable results have been reported from a pilot plant using aluminium alloy tubing (6), but valuable as this experience is, it provides little systematic information on the influence of alloying elements and brine chemistry on the corrosion prowsses. It was to provide this information that the present work was undertaken. EXPERIMMTAL

Scaling down in size makes it impossible to reproduce in the laboratory all the features of an actual plant, and the first apparatus was designed to incorporate the factors thought most important for the corrosion of aiuminium. These were acid dosing and subsequent removal of air and carbon dioxide, concentration of t.Se brine, a Bow velocity comparable to that in condenser tubes, and finally the ability to “replenish” the brine at a rate (in relation to the area of aluminium exposed) at least as high as that in a flash plant. The last factor is prominent in studies of aluminium alioys in pure water at around soO”C, where high ‘*replenishment rates”, car, signiiiwntIy increase the rate of uniform uwrosion (I)_ Fig. 1 is a schematic diagram of the loop, which for spe& of construction, was built of 2.5 cm id_ Pyrex glass components.

Tbe test temperature was 120°C. Initially the concentration was x 2.0 brine, but dueto the long retention time, the precipitation of calcium sulphate (anhyd&e) was excessive; and most of the work was done at a concentration of x IS. Normally the acid dose was 100 ppm, added before evaporation in the separate vacuum evaporator which ako served to de-aerate the brine. The ci~lation rate Desa&zXi& 9 (1971)24~2!!8

CORROSlON OF ALUMlNlUM

ALLOYS IN HOT BRINE

247

___

Fig. 2. Schematicdiagram of “once-through”apparatus. was 25

L/min and specimens were normally in the form of rods 1.8 cm in diameter, held axially within the tubes by Teffon supports giving a flow velocity of 1.5 M/set (Fig. 3). Fresh brine was fed to the loop at rates up to 1 L/hour, and with 8 spccimens present this gave a replenishment rate of 1.3 ml/cm*/hour, (cf. plant values which are of the order 2 mt/cm*/hour). Experiments at very short retention times, to make sure that there was no increase in uniform corrosion, were carried out at 120°C in a special “once-through” apparatus (Fig. 2), and similar experiments were carried out at 95°C in simple glass units heated in an oil bath. Both these

eFig. 3. Specimen-ganam. ~esufnntion. 9 (1991) 245-258

1. N. WA~~~CLYN

248

et al.

tests also provided high replenishment rates, around 40 ml/cm2/hour, but in view of the limited supply of brine, Row velocities were very low. At a later stage tests in flowing brine at 140°C were required and, to avoid the precipitation of calcium sulphate, a second loop was built (Fig. 1) in which, in addition to a slow replacement (ca. 1 L/hour) to take account of possible corrosion effects. there were also arrangements to retu& brine at a high rate, via a cooler, to a large holding reservoir from which it was again fed into the loop. lt was hoped that calcium sulphate nuclei would either redissolve or settle out in this large cool vessel, so avoiding deposition: retention time above 100% was 1 hour.

and the pumping

rate was such that the

Electrochemical experiments were carried out in both loops, and a convenient method of introducing specimens and counter-electrodes is illustrated in Fig. 3. When reference elarodes were required, plain silver wires were found to be stable and reproducible to about 5-10 mV, presumgbly by establishing a Agf AgCl potential. These were introduced into the system by glass seals and were placed uy to 50 cm from the corroding electrode, away from the polarisicg curretit flow. Considering the high conductivity of brine at 12O”C, the maximum error due to 1R drops would be about 20 mV.

Brine

was prepared from natural sea water obtained from Ejortland and

stored in plastic-coated stainless steel tanks for periods up to two months. normally dosed with 100 ppm of sulphuric acid, and then evaporated

it was

under vacuum at a temperature of about 40°C. The final pH was typically 7.841, in agreement with published information (7). For special experiments lower or higher acid doses were used; and with overdosing the pH was readjusted with sodium hydroxide solution, added t0 the brine reservoir or fed dire&y into the loop. In norm& tests, the pH of the lotip efiluent fell to 6.247, mainly as a n&t of the corrosion reaction. Brine concentration was monitored by chloride analyses, x 1.5 brine being for convenience taken as 30 g/L chlori&. Molerio& Table 1 lists the alloy *-Med. Normally rod specimens were used, illustrated in Fig. 3; and the ends of these proved useful for study&g crevice corrosion. Some materials were available only as tubing: these specimens were supported in a similar way using internal Tefion plugs, and due to uncertainties about conditions inside them, such specimens could be used only for observations on localised c&rosion. Special specimens screwed axially to other alloys or to stainless steel were used to study galvanic effects, and a~prop~ately~ll~ tube specimens gave conditions of highly turbulent flow. Specimetis were normally prepared by etching in 5% sodium hydrokk solution, followed by 5% nitric acid; but in special cases this treatment

was followed

by anodising,

steam exposure

or other pre-treatments.

CORROSION OF ALUMINIUM TABLE

I

WOMMAL

CVMPOSTION

ALLOYS

OF ALLOYS

_-__-_,-_______-__

249

IN HOT BRINE

TESTED

(wr”/oo)

_____-.-.-__c___

h'o.

Mg

Mtl

c-r

Si

Al

99SA BA 2711

2.7

0.8

0.1

-

99.5 min.

BA60 BA 21 BA 25

2.25 0.7 5.0

1.2 0.4 0.5

0.3

-

-1.0

Balance &lance Balance Balance

1.0

I.2

-

-

Balance

l_l-_--__CI-_____~_-_.__-_--

BA 18

-

-

--.

Balance

After test. specimens were examined visually, after which scale deposits were removed by wet scrubbing with a nylon brush. If there was no significant localised attack, specimens were then stripped of adherent corrosion films with chromicphosphoric acid, and weighed. For. the identification of adherent films, sheet specimens were used, suitable for disso!ution of the metal in bromine:mcthanol. Film and scale samples were examined by X-ray powder diffraction and, when sample quantities permitted, by chemical analysis. The properties of various surfaces in relation to the cathodic part of the corrosion reaction were investigated, after test, by polarising specimens cathodically at room temperature in sodium

chloride solution or x 1.5 brine. A cruder, quicker method of assessing these properties immersed

was the measurement in chloride solution.

of the resistance Room temperature

(at 1592 ZHZ) of a known experiments on localised

area eor-

rosion were also carried out using the “model pit” shoivn in Fig. 3. Under anodic polarisation the end of the wire corroded rapidly enough to recede within the tube. so setting up the condi:ions of restricted diffusion that characterise a pit. RESULTS

All weight-loss values, at both 120” and 14O”C, lay within the three “bands” in Fig. 4. Though there was some variation between one test and another, the rates in all cases fell with time and approached 0.025-0.050 mm/year at the longest times studied. In some tests at 120°C an attempt was made, by adding sodium hydroxide to the loop, to maintain a pH higher than the value of 6.2-6.7 to whic‘h, under the influence of the corrosion reaction, it normally fell, Only with large additions of alkali could the pH be kept above 6.8, and high corrosion rates were recorded. Though in the glass loop some alkali certainly reacted with the glass. the same eifect was found in the stainiess steel loop; and reaction with the aluminium, causing enhanced corrosion must also be assumed. In contrast, some tests included galvanic couples which corroded very DesuIimfimv9 (1971) 245-248

250

J. N. WANKLYN

Fig.

et a/.

Summary of weighr-Ios results.

heavily and added a large amount of aluminium corrosion product to the system. In such cases low rates of uniform corrosion were found with normal specimens in the same test. A number of the tests at 95% were devoted to this effect, and large reductions of corrosion were produced both by recirculating the same brine so that corrosion products could accumulate and by adding dissolved aluminium (as chloride) to the brine. The results (Table II) were also noteworthy in that, although aluminium additions above about I ppm appeared to be precipitated, the corrosion values fell sharply onty when the addition reached about S-10 ppm. TABLE It

CORROSION OF ALUMINIUM

ALLOYS IN HOT BRINE

251

At the opposite extreme, there was no significant increase of uniform corrosion at 120°C in loop tests with replenishment rates of 25 ml/cm’/hour or in “once-through” tests with rates up lo 30 ml/cm*/hour, the latter corresponding to retention times down to 15 minutes. No results were obtained for typical ‘-high-temperature” alloys containing about I 9; of iron and nickel, because these corroded excessively fast in preliminary tests at 100°C; but among the other alloys no consistent influence of composition on uniform corrosion could be detected.

in view of the rather

favourable

rates of uniform

corrosion.

rosion was soon recognised as the major problem in the exploitation

localised

cor-

of aluminium

alloys. Such corrosion took the form both of crevice attack and of pitting on the free surface, but both forms were rarely found on thedame specimen. On standard rod specimens (Fig. 3A) the crevice formed z?t the T&on sclpport provided a site for attack (Fig. 5). If su-face pitting occurred, normally only one or two pits developed on a specimen (Fig. 6). in crevices the attack was usually severe, up to

Fig. 5. Crevice corrosion on end of rod specimen.

x 2.

several mm deep, and the corrosion product contained basic aIuminium suIphate, sometimes mixed with Bshmite. Surface pits, on the other hand, tended to spread rather than to penetrate deeply, and pitted areas were often found to be active only at the edges. In such cases the central area was free of corrosion product and no longer corroding rapidly. Occasiondly however deep Jocalised pits were formed, and these had the characteristic cap of corrosion products, mainly Biihmite. A relatively complicated series of experiments was carried out on localised ~esai~~arioa, 9 (1971) 245-258

J. N.

252

Fig. 6. Pittingon surfaceof rod s~rnen.

WAhXLYN

et 01.

x 2.

corrosion; and these are best summarised by notes on the influence of each factor studied. Go&a& coupling. Accelerated corrosion was found in 120°C tests of most alloys when connected to areas of staintess steel comparable to the specimens’ own area, but small areas of stainless steel were tolerated in tests of 99.5% aluminium and BA 271. No significant effects were produced in couples between 99.50’ aluminium and BA 271. though these incfuded crevices between the two mate&&. CoupIing BA 271 to smalI electrodes of super-purity aluminium appeared to give piotection against pitting, but there were signs that the corrosion of such sacrificial elements could initiate attack in the crevice between the super-purity aluminium and the specimen to be protected. Specimens of BA 50 dad with an AllI yO Zn alloy showed excessive corrosion of the tfadding atfoy; but this particular test was inconclusive as lo protection by the cladding, because neither clad not unctad specimens showed any pitting. Allo), mmposifion and sfrucmre. As the tests proceeded it became ctear that the alloys BA 271 and BA 60 were more resistant to localised corrosion than ahtminium of 99.5:: purity. Certain other alIoys, notably 3004, seemed at least as susceptible as 99.5 ‘;‘o atuminium, perhaps more so. The superiority of BA 271 seemed to consist both of a lower likelihood of pitting or crevice corrosion and of a reduced depth of attack if iocalised corrosion did occur. To restrict the number of specimens, only BA 271, BA 40 and 99.5 a~uminium were usualiy tested in later experiments. In contrast to the infIu&cc of composition, no marked effect of meta&qica.l condition was apparent, though this was not studied extensively. With BA 27s

CORROSION OF ALUMINIUM

ALLOYS

IN HOT P
253

and 99.S aiuminium, simulated welds seemed no more likely to pit than normal rod specimens; and seam-welded/drawn tubing of BA 21 showed no differences at the weld. Ittrergrmttdarcorrosion. This form of attack, characteristic of alloys containing more than about 37;; magnesium at temperatures above -70°C. was neither anticipated nor found in most of the work. However, a few specimens and loop components of the ST/, magnesium alloy did show the beginnings of intergranular corrosion after esposures of about 260 hours. Investigations elsewhere (13) show that severe intergranular corrosion would develop in this alloy after longer times. Pre-filming rtnd artodisittg. Early in the work it seemed possible that Iocal corrosion, especially in crevices. might be discouraged by the presence of a sipificant oxide film from the very start of exposure, and pre-filming treatments in steam and water (8) were tried. Little benefit in pre-filmed crevices was obtained. It was also thought th.11 diflerences in the degree of filming between a crevice area ac.i the adjacent open surface mighr set up potential differences able to stimulate crevice attack. and experiments with and without anodic films in these areas were tried. As expected anodising within the crevice area was protective, but so also was anodising of the adjacent surface. This result was explained by electrical measurements which showed that anodic films had high resistances and strongly inhibited the cathodic reaction. Turbulence. No accelerated local corrosion was found inside tube specimens of BA 21 incorporating a sudden enlargement of flow area at which the velocity was -7 M&c, nor were turbulence effects ever noted at such features as edges, drilled holes, etc. Osrgen contettt and residtmi afk-afirtirywere eliminated at an early stage by tests with added oxygen (in the”once-through” apparatus) and by tests with varying levels of acid dose. Dissolved copper is a well-known stimulator of pitting in the tow-temperature corrosion of aluminium, and similar effects have been reported at 60°C (4). though it has been suggested that higher concentrations of copper can be tolerated at higher temperatures (9). Experiments were carried out in the “once-through” apparatus, using a copper addition of I ppm, and this produced incipient crevice corrosion of BA 271. However. copper additions of O-1 to 1.0 ppm in tests of BA 271 at 95°C gave only one instance of Iocalised attack, but rather surprisingly caused a slight increase in the rate of uniform corrosion (Table II). pH_ This proved a very difficuft variable to study, because not only d!d the corrosion reaction alter the pH of the brine, but other reactions caused pH changes in loop experiments in which no specimens were present. The behaviour of brine was different from that of sodium chloride solutions; but it was established that both in brine and in sodium chloride the maintenance of low pH values, especially at the start of a test, made local&d corrosion more likely. The critical pH in ~esffiina~i~n, 9 (1971) 245-258

254

J. N.

WANKLYN

41 a/.

brine (* 5.: r secmcd lower than that in sodium chloride solution (6.0-6.3). Such pH values required the addition of extra acid either to the feed or to the loop itself. In brine without such additions the pH moved to values in the range 6.2-6.7. In the “once-through” apparatus. presumably due to the shorter retention time, the pH vaiues were higher (6.8-7.2). This may have been partly responsible for the lack of local&d corrosion in these tests. Scale deposiriott and rerenrion rime. Tabie III summariscs the retention times and occurrence of localised corrosion in various tests, together with some information on the scales formed. In the early tests in the glass loop calcium sulphate was the predominant scale, and this seemed logical in view of the long TABLE Iti SUMMARY -

OF

-..._.

No.

-_-

OBSERVATIOSS -.._ -._

of

tpsIS

_

.._-.

OS

Luc~lised

tinws

99.5 AI

_.. . .-.._ _-

20 hrs.

6 1 3 8

10 hn. 13 hrs. 13 hn. 1 hr. 75 mins.

TIME,

--._.__

Retention _.._.-..- _---..

6

P.RESll0X

._ _,_-_-_

.__,.

LOCAUSED ___.-..

corrosion*

CORROSIDN,

.__-_.____

AX0

. .._.__.

Sc0I.e

SCALE

.-_.

FORMATIOS

- .-__. ___.-_

BA 271

_-.-- -._ .- .-._ ..- - --.---.-- ---_...- --.- .-- ..._,-

Yes Yes Yes YU

Yes Yes Yes

Anhydrite Anhydrite Anhydrite

NO

“W”

‘i

NO

NO

“W

I

-.-...- -..___

\ g Glass Stainless Steel

No 3 NO “Once-through” 14 20-75 mins. No No -_._-_.. * “Yes“ indicates focalised corrosion of at lcast one specimen per test. -I.-------_..-

retention

.--.

time.

-

____

Appurartrs

..-._.---_._-

In the “once-througW

__.__.-__.

..--.

-

apparatus

-...

-.---

-.,-.__--.

calcium

-

sulphate

---..---.

.--.

diminished

-.--_

in

importance and a new type of scale made its appearance. Localized corrosion was never found in this apparatus, in spite of the brine conditions being apparently similar to those in the glass loop, or sometimes more severe (high concentrations . of oxygen, copper). This suggested a fink between localised corrosion and the

deposition of anhydrite, though it was difficult to envisage a mechanism to explain this. The first tests in the stainless steel loop at 120°C supported this idea. but later tests at 140°C showed no crevice corrosion or pitting. in spite of the clear presence of anhydrite. This suggested that the retention time itself was the cruciaf factor, influencing both anhydrite deposition and Iocaiised corrosion, probably by different mechanisms. Short retention times appeared to discortrage local&d corrosion; and this focused attention on the unknown states formed under such conditions and first noticed in experiments in the ‘*once-through” apparatus. These materials, designated for brevity type l ‘W”, aIf contained significant quantities of magnesium, so much in fact that it could only have come from the brine. They covered a range of broadly similar compositions and X-ray Drurlinurion, 9 (19771) 245-258

CORROSION

OF ALUMINIUM

ALLOYS

255

IN HOT BRINE

patterns, but in a number of cases the mineral Hydrotalcite, Mg,8A1,(OH),, (C0,),12H,O, (10, II) was identified. in contrast to the scales, the adherent corrosion film on the metal was biihmite. the normal fi!m material for aluminium exposed to water at these temperatures. Weight measurements before and after de-filming suggested that the biihmite films contained about half the aluminium lost by corrosion, implying the release of an equal quantity to the brine (and possible precipitation in scale compounds). Electrochemical measurements In the room temperature experiments on model pits all alloys. on standing unpolarised, adopted approximately the same rest potential. -0.75 to -0.80 volts 1’sSCE. Anodic polarisation by several tens of mV caused a large increase of current accompanied by pit growth, i.e. the classical breakdown of passivity in chloride solution. There were no significant differences between the alloys. nor did a period of standing unpolarised. after breakdown, bring about any differences in subsequent periods of polarisation. The liquid within the model pits became strongly

acid (pH

<

2.5) in agreement

with previous

findings

(12). A few cor-

rosion

tests in brine acidified to pH I with hydrochloric acid showed that BA 271 dissolved about half as rapidly as 99.5 aluminium. The polarisation experiments at 120X, summarised in Fig. 7, were similar to the low temperature results in that the specimens all adopted rather similar rest potentiais and, under anodic polarisation, broke down by localised corrosion (found by subsequent examination to be attack in the metal/Teflon crevice. Fig. 3). There was however an important time effect, absent at room temperature. Specimens of BA 271 after a period of exposure showed rather higher breakdown potentials than 99.5 aluminium: and they were further differentiated by a marked tendency to “heal” during unpolarized exposure, following a period of anodic treatment. After such “healing” the breakdown potential reverted to a high value. This effect was not found with 99.5 aluminium or BA 2 1, which, once broken down, always

failed again at the same potential

(approximately

- 0.65 volts w,r_t.Ag)_

In these experiments. the potential between the specimen and the polarising electrode was increased in steps of 50 mV every 4 minutes. and before breakdown the current tended to fall with time. During the anodic polarisation experiments at 120°C it was also possible to observe the cathodic behaviour of the aIuminium specimens used as counterelectrodes. In one experiment in which the polarising current was maintained for a number of hours, there was a large increase in the steepness of the cathodic curve, suggesting than an inhibiting deposit was being formed on the electrode. The room temperature measurements of cathodic polarisation showed no major differences between most of the surfaces, but a specimen from a test in brine containing added copper, which had visible copper particles on the surface, showed

256

J. N. WANKLYN

400 mV less polarisation an anodised

specimen

at 50 PA/cm2

had about

than most

other

1 volt more polarisation

specimens. at similar

f?l 01.

In contrast, currents.

The AC resistance measurements covered a wide range of values. from about 60 ohms (1 cmt area) for scale-free surfaces to around ZfXIOohms for scaled specimens, and several hundred thousand ohms for an anodised surface. There was a reasonably clear distinction between specimens with and without localised rosion, the resistance values consistently lying below and above 200 ohms,

corres-

pectively.

Fik 7. Polarisation

bchaviour

at 12WC.

DlSCUsTfON AND CONCLUSIONS

Uniform

corrosionraies

Though the tests reported here were relatively short, the rates of uniform penetration were already low enough to offer a satisfactory life for ahuninium tubing in a desalination plant. With the normal trend for corrosion rates to continue to fall for at least the first few months of expoburc. long-term rates well below 0.025 mmfycar can be anticipated, provided the pH is maintained around 6.5-7.0. Such values may reasonably be compared with the similar rates found for al~minium alloys in pure water at 120°C (2, 3), for present theories of passivity suggest that, unless the electrochemical conditions cause film breakdown, chloride

ions should

have little influence

The risk of penetration

on the rates of uniform

by local&d

corrosion

corrosion_

is much greater than that from