Pitting aluminum

Inorganic acids, bases, and salts with pH values <4 and >9 are not compatible with aluminum. Pitting corrosion of aluminum can occur when it comes in contact with salt water and copper halide salts. 

Aluminum Pitting Galvanic Bacterial Concentration cell... 

Schmidtt CR (1986) Anomalous microbiological tuberculation and aluminum pitting corrosion — case histories. In Dexter SC (ed) Biologically induced corrosion proceedings of the international conference on biologically induced corrosion. National Association of Corrosion Engineers, Houston... 

F.D. Wall and M. A. Martinez, A statistics-based approach to studying aluminum pit initiation— Intrinsic and defect-driven pit initiation phenomena, J. Electrochem. Soc. 150(4), B146-B157 (2003). 

For aluminum, pitting corrosion is most commonly produced by halide ions, of which chloride (Cl ) is the most frequently encountered in service. The effect of chloride ion concentration on the pitting potential of aluminum 1199 (99.99-i-% Al) is shown in Fig. 6. Pitting of aluminum in halide solutions open to the air occurs because, in the presence of oxygen, the metal is readily polarized to its pitting potential. In the absence of dissolved oxygen or other cathodic reactant, aluminum will not corrode by pitting because it is not polar-... 

Crevice Corrosion. Crevice corrosion is intense locali2ed corrosion that occurs within a crevice or any area that is shielded from the bulk environment. Solutions within a crevice are similar to solutions within a pit in that they are highly concentrated and acidic. Because the mechanisms of corrosion in the two processes are virtually identical, conditions that promote pitting also promote crevice corrosion. Alloys that depend on oxide films for protection (eg, stainless steel and aluminum) are highly susceptible to crevice attack because the films are destroyed by high chloride ion concentrations and low pH. This is also tme of protective films induced by anodic inhibitors. 

Softer metals such as aluminum and its alloys can be blast cleaned using abrasives that are not as hard as those used on steel. Garnet, walnut shells, corncobs, peach pits, glass or plastic beads, and soHd carbon dioxide have been used successfully. 

Two types of localized corrosion are pitting and crevice corrosion. Pitting corrosion occurs on exposed metal surfaces, whereas crevice corrosion occurs within occluded areas on the surfaces of metals such as the areas under rivets or gaskets, or beneath silt or dirt deposits. Crevice corrosion is usually associated with stagnant conditions within the crevices. A common example of pitting corrosion is evident on household storm window frames made from aluminum alloys. 

Pitting is also promoted by low pH. Thus, acidic deposits contribute to attack on stainless steels. Amphoteric alloys such as aluminum are harmed by both acidic and alkaline deposits (Fig. 4.4). Other passive metals (those that form protective corrosion product layers spontaneously) are similarly affected. 

Figure 6.2 Severely pitted aluminum heat exchanger tube. Pits were caused hy sulfate-reducing bacteria beneath a slime layer. The edge of the slime layer is just visible as a ragged border between the light-colored aluminum and the darker, uncoated metal below.

Rolling oil tanks were corroded on surfaces contacting the emulsion. Small pitlike depressions were present beneath aluminum soap deposits. Each pit was surrounded by a lightly etched region exactly mirroring deposit patterns (Fig. 6.26). 

Figure 2-11 shows weight loss rate-potential curves for aluminum in neutral saline solution under cathodic protection [36,39]. Aluminum and its alloys are passive in neutral waters but can suffer pitting corrosion in the presence of chloride ions which can be prevented by cathodic protection [10, 40-42]. In alkaline media which arise by cathodic polarization according to Eq. (2-19), the passivating oxide films are soluble ... 

The system aluminum/water belongs to group II where represents the pitting potential and lies between -0.8 and -1.0 V according to the material and the medium [22,23,36,39,42]. Since alkali ions are necessary as opposite ions to the OH ions in alkalization, the resistance increases with a decrease in alkali ion concentration (see Fig. 2-11). In principle, however, active aluminum cannot be protected cathodically [see the explanation of Eq. (2-56)]. 

Other passivating materials suffer pitting corrosion by chloride ions [62] in a way similar to stainless steels (e.g., Ti [63] and Cu [64]). The pitting potential for aluminum and its alloys lies between = -0.6 and -0.3 V, depending on the material and concentration of chloride ions [10,40-42]. 

A higher content of AI2O3 and SiOj is critical for the composition of the protective films in the tubing, assuming the water contains silicates or silicic acid. The protective films have a maximum thickness of 1.5 mm and cannot grow further. The corrosion process can be stopped even in copper pipe networks with type I pitting [21] by providing a reaction tank with impressed current aluminum anodes. 

Aluminum was tried in the reboiler of an aqueous amine plant but it was found to pit very quickly. 

The aluminum smelter solid wastes, in the form of spent pot lining, are disposed of in engineered landfills that feature clay or synthetic lining of disposal pits, provision of soil layers for covering and sealing, and control and treatment of any leachate. Treatment processes are available to reduce hazards associated with spent pot lining prior to disposal of the lining in a landfill. Other solid wastes... 

This frontier s practical opportunities were first developed with submarines, which until the nuclear ones were limited to depths of only a few hundred feet. Many thousands of feet can now be navigated. The crushing pressures below the surface, which increase at a rate of about V2 psi per foot of depth, make corrosion a major threat to the operation and durability of many materials. For example, the life of uncoated magnesium bolts in contact with steel nuts is less than seventy-two hours, aluminum buoys will corrode and pit after only eleven months at just four hundred feet, and low-carbon steel corroded at a rate one-third greater than in surface waters. 

Copper reduces the corrosion resistance of aluminum more than any other alloying element. It leads to a higher rate of general corrosion, a greater incidence of pitting, and, when added in small amounts (for example, 0.15% ), a lower rate of pitting penetration. 

Iron reduces corrosion resistance. It is probably the most common cause of pitting in aluminum alloys. 

Corrosion has been encountered infrequently to date and has been a surface type, as opposed to pitting corrosion that can result in perforations. Entrapped air in the beverage or in the cans headspace increases the corrosive action of the product according to Koehler et at (21). As with beer and other canned foods, aluminum ends provide electrochemical protection when combined with tinplate or tin-free-steel can bodies. The level of iron pickup is reduced while the amount of aluminum dissolved in soft drinks increases without detrimental effect. Aluminum containers with vinyl epoxy and vinyl organosol coatings are compatible with carbonated soft drinks. 

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