Amphoteric alloys alkaline corrosion

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. 

The most harmful deposits are those that are water permeable. Truly water-impermeable material is protective, since without water contacting metal surfaces corrosion cannot occur. Innately acidic or alkaline deposits are troublesome on amphoteric alloys (those attacked at high and low pH—e.g., aluminum and zinc). 

Acidity and alkalinity affect the corrosion rate. Generally, alkaline conditions favor lower corrosion rates however, some metals, such as aluminum and zinc, are amphoteric and show increased corrosion at pH values above 9. For iron alloys, the corrosion rate is relatively steady between a pH of 4 and 10 at ambient temperature and where oxygen reduction is the primary cathodic corrosion reaction however, the corrosion rate increases rapidly below pH 4 [4]. Iron passivates and the corrosion rate decreases rapidly above a pH of 10, except at very high pH levels where the corrosion rate again can increase. 

Magnesium and its alloys are definitely anodic to the A1 alloys and, thus, contact with aluminum increases the corrosion rate of magnesium. For example, in sodium chloride solutions (3-6%), the potential of Mg alloys is -1.67 V/SHE while that of Al-12%Si and pure aluminum are -0.83 to -0.85, respectively. However, such contact is also likely to be harmful to aluminum, since magnesium may send sufficient current to the aluminum to cause cathodic corrosion in alkaline medium. Aluminum oxide is amphoteric and so it is soluble in acid as well as in alkaline solutions. The standard reduction potentials of these two half-reduction reactions are (-1.66 V/SHE) and (-2.35 V/SHE), respectively. Alkaline reaction of the possible existence of aluminum phase in sacrificial Mg anodes is ... 

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