Oxidation alloying elements

Additional alloying elements can be added to the charge as ferro-alloys such as FeSi, FeMn, FeCr, or pure such as Cu, C, Ni. However most alloys are added to the molten metal to prevent metallic losses due to oxidation. Alloying elements are usually present in the foundry in small quantities and are always stored inside the building, preferably close to the melting installation. [32, CAEF, 1997]. 

The Wagner-Hauffe rules, illustrated here for a bivalent oxide, apply in fact to any oxide. Thus, in a general fashion, in those metals that form n-type oxides, alloying elements of lower valence will accelerate corrosion, and elements of higher valence will slow it. Inversely, metals that form p-type oxides oxidize more rapidly in presence of alloying elements of higher valence, and less rapidly when alloyed with elements of lower valence. The results presented in Table 9.26 serve as an illustration of the Wagner-Hauffe rules. 

For example,copper has relatively good corrosion resistance under non-oxidizing conditions. It can be alloyed with zinc to yield a stronger material (brass), but with lowered corrosion resistance. Flowever, by alloying copper with a passivating metal such as nickel, both mechanical and corrosion properties are improved. Another important alloy is steel, which is an alloy between iron (>50%) and other alloying elements such as carbon. 

The composition of turbine blades is a complex mixture of alloying elements in solid solution in nickel. A typical alloy would contain Al, Cr, Mo, Co or Ta in amounts up to 10 atom per cent of the particular elements which are chosen. Of drese all except cobalt form substantially more stable oxides than... 

Besides the use of anodic polarization with impressed current to achieve passivation, raising the cathodic partial current density by special alloying elements and the use of oxidizing inhibitors (and/or passivators) to assist the formation of passive films can be included in the anodic protection method [1-3]. 

Chromium is the most effective alloying element for promoting resistance to oxidation. Table 3.10 gives temperatures at which steels can be used in air without excessive oxidation. In atmospheres contaminated with sulfur, lower maximum temperatures are necessary. 

With copper alloys containing more noble metals the oxide will be substantially pure copper oxide since the oxides of the noble metals have higher dissociation pressures than the copper oxides. With alloys containing baser metals, however, the alloying element will appear as an oxide in the scale, often in greater concentration than in the alloy itself, and sometimes to the exclusion of copper oxides. The dissociation pressures of many oxides have been calculated by Lustman... 

Whether the rate of oxidation of an alloy of copper with a baser metal is less or more than that of copper will depend on the concentration of the alloying element and the relative diffusion velocities of metal atoms or ions in the oxide layers. There is extensive literature on the oxidation behaviour of copper alloys According to Wagner s theory the rate of oxida-... 

It has already been shown that bulk lattice diffusion is not generally considered to be the rate-controlling process for the oxidation of iron in most real situations. Hence the classical Wagner treatment, whereby the valency of the alloying element increases or decreases the number of lattice defects. 

With some alloying elements, e.g. Si and Al, the concentration of element required in the lattice to form their own oxide is so low that preferential formation of pure AljOj or SiOj, which are both highly protective, may occur. For the reaction... 

Effects of Specific Alloying Elements on the Oxidation of Iron... 

---