Pitting aluminum-based alloys

A similar reaction occurs during pitting corrosion of iron and its alloys. Partial hydrolysis, leading to the formation of Al(OH) and Al(OH) may also occur, but all such reactions lead to the formation of acid, making the solution inside the pit much more aggressive than outside. Measurement of the pH inside a pit is not an easy matter, but estimates based on various calculations and on measurements in model pits lead to values as low as 1-2 for chromium-containing ferrous alloys and about 3.5 for aluminum-based alloys, depending on experimental conditions. 

Pitting corrosion is usually associated with active-passive-type alloys and occurs under conditions specific to each alloy and environment. This mode of localized attack is of major commercial significance since it can severely limit performance in circumstances where, otherwise, the corrosion rates are extremely low. Susceptible alloys include the stainless steels and related alloys, a wide series of alloys extending from iron-base to nickel-base, aluminum, and aluminum-base alloys, titanium alloys, and others of commercial importance but more limited in use. In all of these alloys, the polarization curves in most media show a rather sharp transition from active dissolution to a state of passivity characterized by low current density and, hence, low corrosion rate. As emphasized in Chapter 5, environments that maintain the corrosion potential in the passive potential range generally exhibit extremely low... 

Measurement of the pH inside a pit is not an easy matter, but estimates based on various calculations and on measurements in model pits lead to pH values as low as 1-2 for chromium-containing ferrous alloys and about 3.5 for aluminum-based alloys, depending on experimental conditions. 

Steel phases have an influence on the rate of corrosion. Ferrite has a weak resistance to pitting. The presence of martensite can increase the hydrogen fragilization of steel. Intermetallic phases as Fe2Mo in high Ni content alloys can influence the corrosion resistance. The precipitate CuA12 in aluminum alloys the series 2000 is more noble than the matrix, with corrosion around the precipitate. The majority of case histories reported in the literature have involved austenitic stainless steels, aluminum alloys, and to a lesser degree, some ferritic stainless steels and nickel-based alloys.31... 

It has been recently shown that as-synthesized template-containing high-silica MFI coatings on aluminum alloys have superior corrosion resistance to chromate conversion coatings in strong acids, bases, as well as pitting aggressive and the in situ... 

When considering zinc-aluminum alloys, the surface oxide film normally present is likely to reduce any corrosion current. The risk of bimetallic corrosion is small in atmospheric exposure trials by Noranda have been in progress since 1984 on ZA alloys coupled to other common metals. No visual effects were noted at the 5-year examination (Barmhurst and Belisle, 1992). A zinc-25% aluminum-0.05% magnesium alloy coupled to other materials and exposed on the Noranda Research Center roof showed pitting attack on the zinc-based material (but only up to 0.38 mm deep in 10 years) when joined to copper, brass, or steel, but less when joined to stainless steel or lead and least when joined to aluminum. 

This system of atmospheric classification is now being revised to create a new approach based on dose-response functions for steel, copper, and zinc. Because the corrosion of aluminum occurs by a pitting or localized mechanism, the traditional approach of using mass loss to determine severity of attack is often misleading. Atmospheric corrosion problems with aluminum alloys are most frequently a result of metallurgical conditions rather than environmental conditions, and the behavior of aluminum may be excluded in the upcoming revision of the ISO 9223-6 documents. 

Figure 9.17 Effect of pH and oxygen content on seawater pitting of aluminum alloy tubes based on 30-day tests in environmental side unit tests. (From Vernik, E.D. and George, P.F. (1973). Mat. Pref. 12, (5). Reproduced by kind permission of NACE, Int, Texas, USA)...

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