Amphoteric 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. 

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). 

Certain alloys frequently used in cooling water environments, notably aluminum and zinc, can be attacked vigorously at high pH. These metals are also significantly corroded at low pH and thus are said to be amphoteric. A plot of the corrosion behavior of aluminum as a function of pH when exposed to various compounds is shown in Fig. 8.1. The influence of various ions is often more important than solution pH in determining corrosion on aluminum. 

Next we consider the Pourbaix diagram for iron, which is, of course, of paramount importance for the understanding of corrosion of ferrous alloys such as the many types of steel and stainless steel. This is a rather complex diagram, since two oxidation states of iron exist both in the liquid and the solid state and the metal is amphoteric to some extent. Figure 16M is a simplified version of the diagrams shown in the original work of Pourbaix. The two soluble species in acid solutions are Fe and Fe. The relevant equilibria are... 

Mild steel may be suitable as a material of construction for handling caustic soda at ambient temperature. At elevated temperatures, >60°C (140°F), corrosion may occur. Nickel and/or its alloys are most suitable for caustic handling at all temperatures and concentrations, including anhydrous molten caustic up to 480°C (896°F) (Leddy et al. 1978). Polypropylene, fluorocarbon plastics, and flberglass/vinyl ester resins are being used for many applications. Aluminum, tin, zinc, and other amphoteric metals should not be used in construction materials. 

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. 

The presence of a protective oxide layer on aluminum is the main reason why aluminum alloys are so broadly used with success in indoor and outdoor environments provided they fall within the passivation potential/pH boundaries shown in Fig. 4.14. The aluminum susceptibility to corrode in both acidic and basic environments is referred to as an amphoteric behavior. While the aluminum oxide will form naturally on aluminum, it is common practice to produce this oxide in a controlled process called anodization. As described in Chap. 5, the quahty and properties of the protective oxide can thus be greatly enhanced, providing various finishes for a multitude of applications. 

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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