Resistance of AluminumMagnesium Alloys to Attack by Sodium Carbonate Solutions R. B. MEARS AND L. J. BENSON Aluminum Company of America, New Kensington, Penna.
concentrated sodium carbonate solutions than were alloys containing no magnesium. A possible electrochemical explanation for this maximum was investigated but was found to be inconsistent with solution potential measurements. It seems probable that differences in the continuity of the film of corrosion products which forms on the specimens is responsible for the changes in rate of attack with changes in magnesium content. Supersonic agitation of the sodium carbonate solutions definitely stimulated the corrosive action of these solutions, probably by facilitating removal of hydrogen bubbles from the surfaces of the specimens.
The rates of attack of a series of high-purity aluminum-magnesium alloys and also of several commercial aluminum alloys by sodium carbonate solutions at 31° C. were studied. Sodium carbonate solutions of 0.001 or 0.01 per cent concentration caused relatively little attack on any of the alloys, but solutions of 1 or 10 per cent concentration were definitely corrosive. Alloys containing substantially more than 3 per cent of magnesium proved to be much more resistant to the action of the more concentrated sodium carbonate solutions than did the other alloys. However, alloys containing from 1.2 to about 3 per cent magnesium were less resistant to the action of the more
LUMINUM-base alloys containing u p to 10 per cent magnesium have come into extensive use both in the United States and Europe during the past decade. These alloys combine relatively high physical properties with outstanding stability t o most neutral salt solutions (5, 8, IO). It has also been stated (4, 7 , I 2 ) that these alloys are particularly resistant to alkaline solutions. This is a point of considerable interest, since alkaline cleaning solutions are often used in contact with aluminum equipment. Attack by such alkaline solutions can be largely or entirely prevented by the addition of inhibitors (1, $3, 6, 9). However, in some cases i t is undesirable or impossible to make such additions. Thus alloys possessing a higher resistance to alkaline solutions are sometimes advantageous. Since magnesium itself is quite resistant to alkaline solutions, i t might be expected that additions of magnesium to aluminum would form alloys with a higher resistance to such solutions than does pure aluminum. However, it was surprising t o find from tests in sodium carbonate solutions that small additions of magnesium (2 to 3 per cent) resulted in a decrease in resistance t o corrosion and that more than about 4 or 5 per cent magnesium was required to produce alloys possessing a substantially greater resistance to attack than did pure aluminum.
A
Method of Test The specimens used in the main series of tests were cut from sheet prepared some years previously for equilibrium diagram
determinations (3). High-purity aluminum and high-purity magnesium were used in making up the various alloys. (The aluminum was 99.984 per cent pure and contained 0.004 per cent silicon, 0.008 per cent iron, and 0.004 per cent copper; sublimed magnesium, 99.98 per cent pure and containing 0.01 per cent group 2 metals and 0.01 per cent aluminum was used.) Alloys containing 0.00, 0.46, 1.01, 1.54, 2.05, 3.08, 3.99, 5.17, 6.10, 7.25, 8.77, and 10.13 per cent magnesium were made. After casting, the ingots of the various alloys were given a homogenizing anneal. They were then hot-rolled to 0.64-cm. thickness, annealed, and cold-rolled t o 0.064cm. thickness. This material was aged a t room temperature for 6 years prior to use. Specimens 1.9 X 6.4 cm. in size were then c u t from the sheet. They were degreased twice in carbon tetrachloride just prior to exposure. All tests were conducted in duplicate. For the solutions Raker's c . P. grade sodium carbonate and distilled water were employed. Care wad exercised so that the solutions were not freely exposed to laboratory air for any extended period prior to use. The specimens were partially immersed in 100-cc. volumes of the carbonate solution in Pyrex glass beakers, which were placed in desiccators in an air thermostat at 31 * 1" C. A small amount of distilled water was placed in the bottom of the desiccators to prevent evaporation of the solutions during the exposure period. In addition, a steady stream (approximately 25 cc. per minute) of purified air was passed through the desiccators in order t o maintain a uniform atmosphere during the entire test period. This air was purified by being passed successively through 10 per cent (by weight) sulfuric acid solution, 10 per cent (by weight) sodium hydroxide solution, and finally through a glass wool filter. The specimens were weighed prior to exposure and were removed from test at previously selected intervals, washed in distilled water, dried in an oven a t 105" C., and reweighed. At the end of the test (after one week of exposure), some of the
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35
30
d 925
E g20
cI
E 15 3 E
q
10
5
0
220 CASTING
aoi
aio
10
CONCENTRATION
10 0
OF
SODIUM CARBONATE IN PERCENT
FIGURE 1. RATESOF ATTACKON ALUMINUM-MAGNESIUM ALLOYSPECIMENS AND ON COMMERCIAL AND EXPERIMENTAL ALUMINUM ALLOYSPECIMENS EXPOSED ONE WEEK IN SODIUM CARBONATE SOLUTIONS AT 31" C.
specimens were coated with adherent Urns of corrosion products; they were cleaned in 40 per cent nitric acid by weight prior t o the last reweighing. Some tests were also made with standard commercial aluminum alloys (Table I). The specimens of these alloys were 2.5 X 10 cm. in size and generally 0.16 cm. thick. However one of the alloys employed (No. 220) was in the cast form and specimens of this material were 0.63 cm. thick.
solutions as the magnesium content increases. The alloy containing 10 per cent magnesium only loses l/68 as much as does pure aluminum in the 10 per cent sodium carbonate solution. This is a substantial decrease in the rate of attack. I n Figure 2 the losses in weight of the various commercial alloys are also given. Again, alloys which contain over 3 per cent magnesium are considerably more resistant to the action of sodium carbonate solutions than is pure aluminum. The relation between duration of exposure and rate of atTABLE I. NOMINAL PERCENTAGE COMPOSITION OF COMMERCIAL tack in the 10 per cent sodium carbonate solution is shown AND EXPERIMENTAL ALLOYS in Figure 3. After the initial 5 or 6 hours, the rate of attack Alloy Si Mn Mga Cr becomes practically constant up to the longest periods 2S-l/2Hb ... .... 0.0 .... 3S-l/2H .... 1.2 .... 0.0 examined. 53s-w 1.3 0.7 1.21 0.25 I n general, these results confirm those from earlier tests in 52S-l/2H 2.31 .... ... 0.25 Exptl. 2 .... ... 3.48 0.25 indicating that the addition of substantial amounts of mag0.25 ... 4.09 .... Fb%b;H .... ... .... 5.22 nesium markedly decreases the susceptibility of aluminum No. 220 casting .... ... .... 10.06 to attack by alkaline solutions. However, none of the Determined by analysis. previous workers mentioned the occurrence of the sharp 6 Commercially pure aluminum, 99 %, AI minimum. peak in the curve. For this reason additional tests were run to study this more closely. A new group of alloys was made u p using lots of aluminum and magnesium which differed Results of Tests slightly in composition from the earlier materials. (The The test results are summarized in Figures 1, 2, and 3. composition of the aluminum was as follows: 0.003 per cent Figure 1 shows that the rate of attack varies greatly with all silicon, 0.004 iron, 0.011 copper, and 0.002 magnesium; the of the alloys as the concentration of the sodium carbonate magnesium used contained approximately 0.010 per cent solution is altered. Attack does not become appreciable each of copper, iron, and nickel with only spectrographic in any case until a concentration of 0.10 per cent is attained. traces of manganese, sodium, silicon, chromium, and calFor the group of special alloys the type of attack in the more cium.) The ingots of these alloys were given a h o m o g e n i ~ concentrated solutions ranged from a moderate to a very uniing anneal, hoerolled to 0.63 cm., then annealed and coldrolled to 0.063 cm., usually with an intermediate anneal. A form etching on all specimens except those containing no portion of this cold-rolled material was annealed a t 343' C. magnesium. On these latter specimens (in the 10 per cent sodium carbonate solution) , severe special attack had d e for 2 hours. Three specimens of each alloy in each temper veloped adjacent to the edges and a t isolated spots elsewhere were exposed to the 10 per cent sodium carbonate solution under the conditions described above. It was found in these on the surface. At some of these latter areas actual perforacases also that maxima were obtained (Table 11). However, tion of the specimens had occurred. I n the more dilute solutions (0.001, 0.01, and 0.1 per cent) attack was very unithe position of the new maxima differed from that previously form in all cases and often resulted in the formation of a dull obtained; it occurred a t 1.9 per cent magnesium for the coldbrown or black film. rolled material and a t 2.55 per cent for the annealed maFigure 2 shows the relation between loss in weight and magterial instead of a t 3.08 per cent. The explanations of these maxima were not clear. Hownesium content of the specimens. The alloy containing 3.08 ever, they might have been associated with the solubility per cent magnesium loses considerably more weight upon limit of magnesium in aluminum. At 200" C. about 3 per exposure to either the 1.0 or 10.0 per cent sodium carbonate solutions than do any of the other alloys of the aluminumcent of magnesium is soluble in aluminum a t equilibrium conditions (S), but a t room temperature the solubility is magnesium series. Alloys containing more than 3 per cent magnesium are less and less appreciably attacked in these probably considerably lower a t equilibrium conditions. 0
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Microscopic Examination of Specimens Microscopic examination mas made of all the specimens which had been exposed to the 10 per cent sodium carbonate solution. I n the case of the first series of aluminum-magnesium alloys, some of the aluminum-magnesium constituent could be detected in the alloy containing 2.05 per cent magnesium but not in the alloys of lower magnesium content. Alloys of higher magnesium contents showed increasing amounts of precipitate. I n all cases except on the pure aluminum specimen the type of attack was nearly uniform etching. There was no evidence of special grain boundary attack in any of the specimens examined. The attack on the pure aluminum specimen was very irregular as mentioned above. TABLE 11. RESULTS OF ONE-WEEKEXPOSURE OF THE SECOND L O T O F SPECIAL ALUMINUM-MAGNESIUM ALLOY#TO 10.0 P E R CENT SODIUM C A R B O N a T E SOLUTION AT 31" c. hlg./Sq. Cm.--. --Aloy-Lossb, 0 temper H temper % A1 70 99.9s .... 17,8250 29 5716 1.90 20.6632 37 5619 98.OS 97.43 2.55 23.6218 14 7353 96 95 3.03 10.0886 13 6629 96 42 3.56 10,2097 12 3386 10 6188 95.95 4.03 8,4298 a Size of specimens, 0.0064 X 1.9 X 6.34 cm.; approximately E-om. leugth immersed. b 1 mg. per sq. cm. per week equals a b o u t 0.0076 inch average penetration per year.
w
I n the second lot of aluminum-magnesium alloy specimens, a slight amount of precipitate was detected in the cold-rolled specimen containing 1.90 per cent magnesium. Precipitation was more pronounced in the cold-rolled specimens of higher magnesium content. S o precipitate was noted in any of the annealed specimens from this second lot except in the one of highest (4.03 per cent) magnesium content. This showed a slight amount of precipitation a t the grain boundaries. The pure aluminum specimens from this lot of material, whether hard-rolled or annealed, also showed irregular attack, but attack of all of the other specimens was nearly uniform etching.
Possible Electrochemical Explanation of Maximum It is possible to explain the occurrence of this maximum by an electrochemical argument. (Such an explanation will be valid only if the corrosion of aluminum in sodium carbonate solution is largely or entirely electrochemical in nature; this point has not been established experimentally as yet.) I n the cold-rolled alloys of less than about 2 per cent magnesium content the metal is substantially a single-phase material. However, for the alloys above 2 per cent magnesium, a new phase (the magnesium-aluminum constituent) occurs. This new phase is cathodic to the solid solution in contact with it, but since the total area of the local cathodes is small, because they are few in number, the amount of current which flows between the large anodic area and these small cathodic areas is limited by cathodic polarization. As more and more precipitate is present in alloys of higher magnesium content, less cathodic and more anodic polarization will occur. The total current flow between the two phases will first increase; then when the anodic area is reduced still further by the increasing cathodic area, i t will fall off rapidly again since the anodic polarization will now control the reaction rate. If for electrode areas of equal size the anodic polarization greatly exceeds the cathode polarization, when sufficient precipitate is present, relatively little current will
EXPERIMENTAL
NO. 2
I
CONTENT
MAG~ESIUM
IN PERCENT
IO
FIGURE2 . EFFECTOF CHANGEIS MAGNESIUMCONTENTON RATESOF ATTACK O F A L r r n i I N u u - M . 4 G N E s I u ~ ~ ALLOYS-4ND O F C O M J I E R C I A L AND EXPERIMENTAL .4LLOYS EXPOSED ONE JvEEK I N 10 P E R CEN T SODlU?J CARBOSATE SOLUTION A T 31 C.
'
flow between the two phases, and corrosion will fall to a low value. These relations are shown systematically in Figure 4. iJ7hen the values of current obtained from this figure are plotted against the cathodic area, the curve shown in Figure 5 is obtained. This curve shows a distinct maximum in current flow. If the short-circuit potentials of the anode and cathode areas obtained in Figure 4 are plotted against the cathode area as in Figure 6 , and if the proposed mechanism is effective, the potentials should shift as the cathode area is increased. Unless the potentials shift, in thig manner, the
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maximum in corrosion rate cannot be explained by this mechanism. Thus, it is possible to check this explanation by potential measurements. If this electrochemical explanation for the maximum is correct, the potentials of the aluminummagnesium alloys in sodium carbonate solution should decrease (become more cathodic) as the magnesium content is increased. The potentials of specimens of each of the aluminummagnesium alloys in 1 per cent sodium carbonate solution
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were measured and are given in Table 111. These results shorn that changes in the magnesium content from about 0.5 to 8.7 per cent magnesium have little effect on the potential of the alloys in 1 per cent sodium carbonate solution. The cold-rolled alloy containing about 10 per cent magnesium has a definitely lower potential than do the alloys of lower magnesium content, and a sample of aluminum-magnesium constituent (35.75 per cent magnesium) had the lowest potential of the group. TABLE111. RESCLTSOF POTENTIAL MEASUREMENTS IN 1 PER CENTSODIUMCARBONATE SOLUTION ON ALLOYS MADEFROM 99.98 PERCEST ALUMINUM PLUSMAGNESIUM ADDITIONS~ Mg Content
0.00 0.46 1.01
Potentialb, Volts -1.82 -1.84 -1.86
M g Content 3.08 3.99 5.17
Potentialb, Volts -1.89 -1.86 -1.86
Mg
Content 8.77 10.13 35.75 (AI-Mg constituent)
Potentialb,
Volts -1.86 -1.69 -1.11
6.10 -1.87 -1.86 1.54 7.25 -1.87 -1.85 2.05 a All alloys, except t h e aluminum-magnesium constituent, were heattreated a t 425' C., cold-rolled 90%, a n d aged a t room temperature f o r 6 years b A'gainst a 0.1 N calomel half cell.
t W
+ 0
a
u
n 0 I 4 0
I
ANODE 9 8 7 5 9 0 -CATHODE 125% (2) A N O D E 8 0 70 -CATHODE 20% (1)
(3) A N O D E 4 0 70 (4) ANODE 20 %
-CATHODE 60 7 0 -CATHODE 80 90
CURRENT-
FIGURE 4. EFFECTO F CH.4NGES
IN
AXODEAND CATHODE AREAS O N CORROSION CURRENTS (SCHEMATIC)
Since the falling off in potential in the range of the maximum in corrosion rate was not obtained, these results indicate that the electrochemical explanation suggested above is not probable or is incomplete.
Test for Inhibitive Effect of Magnesium Carbonate One possible reason for the beneficial effect of magnesium might be the inhibitive action of magnesium carbonate which builds up in the solution as attack proceeds. To examine this possibility a special series of tests was run. Specimens of 99.7 per cent aluminum were partially immersed in a 1 per cent sodium carbonate solution, and similar specimens were
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exposed in 1 per cent sodium carbonate solutions saturated with magnesium carbonate. (At the saturation point less than 0,009 per cent by weight of magnesium carbonate was present.) The rate of attack was actually greater in the solution containing magnesium carbonate. Evidently magnesium carbonate in the solution exerts no specific inhibitive action, although adherent films of magnesium carbonate formed on the metal surface could, of course, be protective.
Film Theory of Protection At present the most likely explanation of the observed results appears to be one based on film formation. I n alkaline solutions of this type a visible film is formed. This film is probably largely hydrated alumina in the case of the specimens of pure aluminum. On the specimens of aluminum-magnesium alloy, the film which is formed undoubtedly contains some magnesium compounds, especially magnesium carbonate. As indicated above, magnesium carbonate is very insoluble in sodium carbonate solutions. For some physical reason alumina films containing small amounts of magnesium compounds are more porous than either pure alumina films or films containing a higher percentage of magnesium compounds. Thus, a maximum corrosion rate is obtained for aluminum alloys of intermediate magnesium contents; with alloys of higher magnesium content the finely dispersed particles of magnesium compound present in the film are sufficiently numerous to form a partially protective layer, once the surface atoms of aluminum are dissolved.
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about two thirds as great. Evidently supersonic vibration stimulates attack, possibly by reducing the hydrogen overvoltage.
Conclusions 1. The rate of attack of pure aluminum, pure aluminummagnesium alloys, and various commercial-wrought aluminum base alloys is slight by 0.001 and 0.01 per cent sodium carbonate solutions at 31 ’ C. but becomes appreciablc for some of these alloys at higher concentrations.
0
n 14
a
20
40
60
80
% CATHODE AREA
FIGURE 6. SCHEMATIC RELATION BETWEEN POTENTIAL AND SIZE OF CATHODE AREA(FROM FIGURE 4)
Z
CATHODE AREA
FIGURE5. SCHEMATIC RELATIOS BETWEEN LOCAL CURRENT FLOW AND SIZEOF CATHODE AREA (FROM FIGURE
4)
For this latter class of alloys the mechanism of the protection afforded is probably similar to that noted by Tammann and Brauns (11) in connection with “parting limits’’ of gold-silver alloys.
Effect of Supersonic Vibrations I n another special series of tests the effect of agitation of the solution with supersonic vibrations was studied. The frequency of these vibrations was 300 kilocycles. The oscillator was a modified Hartley circuit using two RCA 810 tubes in parallel. The output voltage, with a frequency of 300 kilocycles per second, was used to vibrate a circular quartz plate 2 inches (5 cm.) in diameter. During the experiment 100 watts were used in the plate circuit, but the amount actually utilized by the quartz plate was unknown. The quartz plate oscillated in an oil bath, and the beaker containing the sodium carbonate solution was partially immersed in the oil bath. I n a 10-minute exposure to 1 per cent sodium carbonate solution agitated supersonically a t 26 (2. (measured in the solution) the loss in weight for the specimens averaged 3.85 mg. per sq. cm., whereas the weight loss in a similar solution not agitated was 2.67 mg. per sq. cm. or only
2. Aluminum-magnesium alloys containing substantially more than 2 to 3 per cent magnesium are much more resistant to attack by 1 or 10 per cent sodium carbonate solutions than are aluminum-base alloys of lower magnesium content. 3. The addition of magnesium carbonate to sodium carbonate solutions does not inhibit their attack on pure aluminum. 4. Sodium carbonate solutions agitated supersonically are definitely more corrosive than are similar quiescent solutions. 5. The higher resistance of the aluminum alloys contsining more than 2 or 3 per cent magnesium to the action of sodium carbonate solutions can be attributed to the formation of a partially protective film of corrosion products probably made up, a t least in part, of magnesium carbonate on the surfaces of these alloys.
Literature Cited (1) Davis, Dairy Znd., 2, 438 (1937). (2) Edwards, Frary. and Jeffries, “Aluminum Industry”, pp. 58, 770,New York, McGraw Hill Book Co., 1930. (3) Fink and Willey, Trans. Am. I m t . Mining Met. Engrs., 124, 78 (1937). (4) Geller, 2.Metallkunde, 28, 192 (1936). (5) Holler, Aluminium, 20,539 (1938). (6) Lichtenberg, Ibid., 19, 504 (1937). (7) Muller and Liiw, Ibid., 20, 257 (1938). ( 8 ) Rohrig and Nicolini, Ibid., 17,519 (1935). (9) Seligman, Proc. World’s Dairy Congr., 2nd Congr., 1923, 1202. (10) Siebel, Aluminium, 17, 562 (1935). (11) Tammann and Brauns, 2.anorg. Chem., 200, 209 (1931). (12) Vosskuhler. Aluminium, 20,460 (1938).
