Alloying elements, effect aluminum

Effect of alloying. The additions of alloying elements to aluminum change the electrochemical potential of the alloy, which affects corrosion resistance even when the elements are in solid solution. Zinc and magnesium tend to shift the potential markedly in the anodic direction, whereas silicon has a minor anodic effect. Copper additions cause marked cathodic shifts. This results in local anodic and cathodic sites in the metal that affect the type and rate of corrosion. 

Most commercial uses of aluminum require special properties that the pure metal cannot provide. The addition of alloying elements imparts strength, improves formability characteristics, and influences corrosion resistance properties. The general effect of several alloying elements on the corrosion behavior of aluminum has been reported by Godard et al. (2) as follows ... 

Alloying elements or impurities in the zinc coating have been found to have little effect other than that observed on addition of copper or aluminum. The action of copper, which increases the corrosion resistance of the coating, is the complete reverse of this elanent s effect on aluminum coatings. Aluminum decreases the corrosion resistance. 

Alloying elements that are particularly effective for improving oxidation resistance at high temperatures are aluminum, beryllium, and magnesium for example, at 256 °C, a 2% Be-Cu alloy oxidizes in Ih at 1/14 the rate of copper [42]. Maximum improvement by aluminum additions occurs at about 8% [43]. 

The most efficient alloying elements for improving oxidation resistance of iron in air are chromium and aluminum. Use of these elements with additional alloyed nickel and silicon is especially effective. An 8% Al-Fe alloy is reported to have the same oxidation resistance as a 20% Cr-80% Ni alloy [51]. Unfortunately, the poor mechanical properties of aluminum-iron alloys, the sensitivity of their protective oxide scales to damage, and the tendency to form aluminum nitride that causes embrittlement have combined to limit their application as oxidation-resistant materials. In combination with chromium, some of these drawbacks of aluminum-iron alloys are overcome. 

The usual alloying additions to aluminum in order to improve physical properties include Cu, Si, Mg, Zn, and Mn. Of these, manganese may actually improve the corrosion resistance of wrought and cast alloys. One reason is that the compound MnAle forms and takes iron into solid solution. The compound (MnFe)Alg settles to the bottom of the melt, in this way reducing the harmful influence on corrosion of small quantities of alloyed iron present as an impurity [27]. No such incorporation occurs in the case of cobalt, copper, and nickel, so that manganese additions would not be expected to counteract the harmful effects of these elements on corrosion behavior. 

A major issue, for the passivation and corrosion resistance of aluminum alloys, is the existence or not of second phase inter-metallic particles resulting from alloying with elements that have low solubility in aluminum (Rynders et al., 1994 Kowal et al., 1996). These particles are detrimental to the resistance of the passive film to breakdown (the first stage of a localized corrosion process). In contrast to stainless steels, this factor often overwhelms the beneficial alloying effects. However, it must be pointed out that alloying elements such as copper in solid solution are beneficial (Muller and Galvele, 1977). Other elements, such as chromium, molybdenum, titanium, tantalum, and niobium, seem to improve the corrosion resistance of aluminum, but their solubility is too low for them to be used in conventional alloy processes, and they require the use of rapid quenching processes or some sort of nonequilibrium surface deposition. 

Figure 3-2. Effects of alloying elements on the corrosion potentials of aluminum alloys.

The relationship between alloying elements and alloy types illustrated in the Schaeffler diagram (Figure 9.1) is an important concept in understanding stainless steels. It has been established that certain elements, specifically chromium, molybdenum and silicon, are ferrite formers. Aluminum and niobium are also ferrite formers, although their effect is dependent on the alloy system. There are also elements that tend to promote the formation of austenite. The most often used are nickel, manganese, carbon, and nitrogen. 

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