Aluminum alloys pitting corrosion

H. Kaesche, Pitting Corrosion of Aluminum and Intergranular Corrosion of Aluminum Alloys, Localized Corrosion NACE 3, R.W. Staehle, B.F. Brown, J. Kruger, and A. Agrawal, Ed., National Association Corrosion Engineers, 1974, p 516-525... 

H. Kaesche, Pitting Corrosion of Aluminum and Intergranular Corrosion of Aluminum Alloys, Localized Corrosion NACE 3, B.F. 

Corrosion Properties. In addition to corrosion problems such as pitting and exfoliation ced by conventional (monolithic) aluminum alloys, galvanic corrosion caused by the potential difference between the graphite fibers and the aluminum matrix is a reason for concern in these composites. As shown in Table 1, graphite appears at the cathodic (noble) end of the galvanic series while aluminum is at the anodic (active) end of the series. Graphite/aluminum composites have been shown to corrode 80 times faster than monolithic aluminum alloys in an aerated 3.15 wt% sodium chloride (NaCl) at room temperature (Ref 8). 

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. 

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 ... 

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]. 

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

Pitting potential increased with increase in chromium contents >20 wt%, and molybdenum of 2-6 wt%. Recent results, applying microelectrochemical techniques, confirmed that even in the superaustenitic stainless steels molybdenum strongly improves the repassivation behavior but has no influence on pit initiation.27 The corrosion resistance of aluminum alloys is totally dependent on metallurgical factors.52, (Frankel)5... 

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... 

In aluminum alloys exposed to aqueous chloride solutions, corrosion fatigue cracks frequently originate at sites of pitting or intergranular corrosion. Initial crack propagation... 

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. 

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