The Corrosion Resistance of Aluminum and Its Alloys RONALD B. SPACHT The National Bronze and Aluminum Foundry, Cleveland, Ohio
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HERE is considerableconfusion about the resistance of aluminum and its alloys to corrosion. The teacher impresses upon his students the fact that aluminum, which is very high in the electrochemical series, maintains its luster and resists corrosion much better than many metals beneath it. He may leave the impression that aluminum never corrodes and may be safely used at all places. The housewife, too, is impressed with the noncorrosive properties of aluminnm, since no special care is needed to maintain its luster. She may wonder why boiling tomato juice or vinegar increases the luster temporarily, but unless she has tried to make soap in an aluminum pan, she accepts the fact that aluminum is corrosion resistant. Even the alkali cleaners she may use are usually sufficiently inhibited so that they make no serious attack on aluminum. On the other hand the industrialist may object to using aluminum because of fear of corrosion. If his fears are investigated, he probably has had corrosion difficultiesin his plant or in his product when aluminum was substituted for some other metal. The author has seen aluminum castings severely corroded and pitted after a year or two of exposure to nothing more corrosive than well water containing several hundred parts per million of chloride ion. When one considers the variety of conditions encountered by a modem industrial material, such as high temperatures, polluted atmospheres, polluted waters, salt water, and chemicals, it is not surprising that cases arise wherein a particular material such as aluminum fails in service. Aluminum is attacked directly by many chemicals. Oxygen, as we all know, attacks readily but soon stops because of the formation of a protective oxide film. Aluminum owes its supposed inertness to this Hm; however, if anythmg destroys or removes this Hm, then the attack goes rapidly until the aluminum is consumed. Alkalies, particularly sodium and potassium hydroxide, attack aluminum vigorously. Sodium hydroxide is one of the solvents used in the quantitative analysis of aluminum alloys. The attack is somewhat slower with trisodium phosphate and sodium carbonate. Disodium phosphate, sodium acetate, and sodium bicarbonate are practically without attack after long periods of exposure. Ammonia attacks aluminum very little, but suspensions of calcium hydroxide evolve hydrogen almost immediately. Since many agricultural spray solutions contain lime, aluminum should not be used for sprayers unless particular attention is paid to the selection of the alloy used in these sprayers. Aluminum also reacts with acids. The extent of re-
action depends upon the acid and upon the concentration and temperature of the acid. As expected, aluminum reacts only slowly with acetic acid and most organic acids, provided they do not contain chlorine. Aluminum tank cars are used to ship acetic acid, and many plants use aluminum piping and valves in its manufacture although some corrosion does take place. If you have ever tried making aluminum nitrate from aluminum and concentrated nitric acid, you know that it cannot be done. Aluminum is resistant to nitric acid attack, particularly if the acid is concentrated. Concentrated nitric acid may be shipped in aluminum containers. Nitric acid is often used to clean aluminum objects of corrosion products and to neutralize aluminum objects after an alkaline dip. Sulfuric acid reacts only slowly on aluminum a t low concentrations, rapidly a t 60 to 95 per cent concentrations, and very slowly above 95 per cent. Hydrochloric and other halogen acids attack aluminum vigorously a t all concentrations. In the quantitative analysis of aluminum alloys, mixed acids are often used. These mixtures usually contain sulfuric and nitric or sulfuric, nitric, and hydrochloric acids with some water added. The action of inorganic salt solutions depends on several factorsnamely, the ions composing the salt, the degree of hydrolysis, and to some extent the temperature and concentration. Aluminum tends to replace the heavy metals in the heavy metal salts. Thus, silver in silver nitrate is replaced rapidly and lead in lead arsenate is replaced if calcium hydroxide is present. Copper in copper sulfate is replaced slowly. Displacement reactions are slowed somewhat by the presence of oxide films, suggesting that the heavy metal salt must 6rst break down this film before the reaction can proceed. Freshly polished strips of aluminum displace silver almost immediately from silver nitrate solutions, while anodized aluminum strips show very little reaction for 24 hours and then react rapidly. In regard to the anion constituent, the chloride ion is the worst offender. Solutions of sodium and calcium chloride are corrosive to aluminum over long periods of time. In fact, chlorides are the worst natural corrosive agents of aluminum and its alloys. Most accelerated corrosion tests are carried out with sodium chloride solutions, sometimes with hydrogen peroxide added. The corrosive action of chlorides is probably due to the loosening of the oxide film which continually exposes fresh aluminum sutfaces. Chromate ions usually inhibit the corrosion of aluminum. Hydrolysis accounts for the action of sodium carbonate and sodium phosphate solutions upon aluminnm.
On the acid side the effect is not so noticeable, since most dilute solutions of acids attack aluminum only very slowly. Ferric chloride solutions, however, react rapidly on aluminum test samples with the liberation of hydrogen and finally precipitation of ferric hydroxide. Most organic liquids when pure do not attack aluminum; however, the presence of moisture and heavy metal impurities may lead to attack. Organic compounds containing chlorine may be corrosive. Alcohols attack when entirely free from water. Gases such as carbon dioxide and hydrogen sulfide have very little effect upon aluminum. Moist sulfur dioxide and moist chlorine will attack. A summary of the corrosion-resistant properties of aluminum can be found in a table prepared by Dix and Mears.' Selected data from a survey conducted by Chemical and Metallurgical Engineering2will be found in Table 1. This gives a fairly good summary of the material discussed so far. The second method of corrosive attack upon aluminum is called electrolytic or galvanic corrosion. There is no hard and fast dividing line between those two methods of attack and hence the division made here is purely arbitrary. This type of corrosion is usually much more prevalent in exposed aluminum than the purely chemical attacks described above. Bumsa and Brown4 are the authors of two outstanding papers on this type of corrosion. Burns says that electrolytic reactions are of the replacement type with the corroding metal replacing hydrogen from water. The metal ions go into solution a t the anode whiie hydrogen ions are discharged at the cathode. In such corrosion processes the anode becomes the corroded member and the cathode is the protected member unless side reactions take place. For electrolytic corrosion to proceed, an electrolyte must be present. Furthermore, which element becomes the anode or cathode cannot always be predicted from the electrochemical series, since corrosive environments are usually much diierent from those under which electrode potentials are measured. To illustrate from values given by Brown: aluminum has an electrode potential of -2.01 volts and zinc - 1.10 volts, thus making zinc cathodic toward aluminum. But if the potentials of zinc and aluminum are measured in 1 M sodium chloride solution, then the aluminum potential is -0.86 and the zinc - 1.15; in other words, aluminum is now cathodic toward zinc. In another example the electrode potential of iron is -0.78 and -0.37 for ferrous and ferric ions, respectively. In 1M nitric acid aluminum drops from -2.01 to -0.49 and iron becomes -0.58; hence again the potentials are reversed from those in the electrochemical series.
(1944). BROWN, R.H., ibid., pp,. 21-6.
Ammonia Aniline Sodium carbonate Sodium hvdroxide Sodium hipochlorite Acetic acid Carbonic acid Citric atid Hydroehiorie acid Hydroauorie acid Mired acids
Nitric atid
10
Pvre 10 1 5 (free Ch) 0.5 100 carbonated tap water 10 0.25 5 0.1 10 &SO, HNOI 90 4 80 20 80 5 60 40 40 60 20 80 20 55 5
Olcic acid Phosphoric atid Sulfvrie acid
-"
Sulfurour acid Tannie acid Ammonium suitate Calcium chloride Copper sulfate Ferric chloride Ferrous sulfate Sodium nitrate Sodium sulfate
Formeldehyde Glycerin Methanol
95 100 3 10 5 10 1 1 1 1
38 30
0.057 Pitted after 16 h n . boiling 0 . 3 3 (at 86'F.) 4.5 0.081 s t 7O'F 0.0013
~ ~ ~ 0.11 0.004 (at 135'F.) < 0.004 (at 135OF.)
