SECTION 6 

 ALUMINUM ALLOYS 



The resistance of aluminum and its alloys to cor- 

 rosion is due to a relatively chemically inert film of 

 aluminum oxide which forms on its surface. As long 

 as this oxide film remains intact the good corrosion 

 resistance is preserved. In oxidizing environments 

 where a sufficient amount of oxidizing agent or 

 oxygen is present to repair any breaks in this protec- 

 tive film, the corrosion resistance of the aluminum 

 alloys is maintained. The usual corrosion protection 

 (passive) film that forms on aluminum in waters at 

 temperatures below 70°C is bayerite 

 (j3-Al 2 3 -3H 2 0). 



In general, oxidizing conditions favor the pre- 

 servation of this passive film, while reducing condi- 

 tions destroy it. Chloride ions are particularly 

 agressive in destroying this passive film. 



When aluminum is immersed in water, the oxide 

 film thickens much more rapidly than it does in air. 

 The rate of growth decreases with time and reaches a 

 limiting thickness which depends on the temperature, 

 the oxygen content of the water, the ions present, 

 and the pH. In seawater this naturally formed protec- 

 tive film breaks down more readily, and its repair and 

 growth are retarded by the chloride ion. 



The corrosion of aluminum alloys in seawater is 

 usually of the pitting and crevice types. Pits begin by 

 breakdown of the protective film at weak spots or at 

 nonhomogeneities. The breakdown is followed by the 

 formation of an electrolytic cell, the anode of which 

 is a minute area of active metal and the cathode of 

 which is a considerable area of passive metal. The 

 large potential difference of this "passive-active" cell 

 accounts for the considerable flow of current with its 

 attendant rapid corrosion at the small anode (pitting). 



Pitting is most likely to occur in the presence of 

 chloride ions (for example, in seawater), combined 

 with such cathodic depolarizers as oxygen or 

 oxidizing salts. An oxidizing environment is usually 

 necessary for preservation of a passive protective film 

 with accompanying high corrosion resistance, but, 

 unfortunately, it is also a condition for the 

 occurrence of pitting. The oxidizer can often act as a 

 depolarizer for "passive-active" cells established by 



the breakdown of passivity at a specific point or area. 

 The chloride ion in particular can accomplish this 

 breakdown. 



As discussed above, aluminum alloys generally 

 corrode in seawater by pitting and crevice corrosion; 

 therefore, as much as 90 to 95% of the exposed 

 surface can be uncorroded. With such low percentages 

 of the total exposed area affected, corrosion rates 

 calculated from weight losses as mils penetration per 

 year (mpy) can give a very misleading picture. The 

 mpy implies an uniform decrease in thickness, which 

 for aluminum alloys is not the case. 



Another manifestation of localized attack in 

 aluminum alloys is oxygen concentration cell 

 corrosion in crevices (usually known as crevice cor- 

 rosion). This type of corrosion occurs underneath 

 deposits of any kind on the metal surface, underneath 

 barnacles, and at the faying surfaces of joints. The 

 area of the aluminum alloys which is shielded from 

 the surrounding solution becomes deficient in 

 oxygen, thus creating a difference in oxygen concen- 

 tration between the shielded and unshielded areas. An 

 electrolytic cell is created with a difference in 

 electrical potential being generated between the high 

 and low oxygen concentration areas; the low concen- 

 tration area becomes the anode of the cell. Corrosion 

 occurs at the small anodic area and, because the 

 cathodic area is much larger, the rate of attack is 

 considerably greater than if no such cell were present. 



There are two other types of localized corrosion 

 often found in aluminum alloys: intergranular and 

 exfoliation. Intergranular (intercrystalline) attack is 

 selective corrosion of grain boundaries or closely 

 adjacent regions without appreciable attack of the 

 grains or crystals themselves. Exfoliation is a lamellar 

 form of corrosion, resulting from a rapid lateral 

 attack along grain boundaries or striations within the 

 grains parallel to the metal surface. This directional 

 attack results in a leafing action, aggravated by the 

 voluminous corrosion products that causes the 

 uncorroded strata to be split apart. 



Low weight losses and low corrosion rates 

 accompany these manifestations of localized 



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