What Does Passivity in Binary Alloys Mean?
This is a corrosion-inhibiting characteristic that occurs on the surface of binary alloys through the formation of films due to oxide formation. It is a quality achieved by heat treatment that makes alloys corrosion-resistant when used in saline and humid surrounding conditions. Passivity increase anodic polarization, which in turn results in the reduction of anodic reaction in the active sites—hence, corrosion is inhibited.
Corrosionpedia Explains Passivity in Binary Alloys
Alloying is one of the considerable ways of achieving passivation on metals other than using anodic polarization (electrochemical) and the action of inhibitors (chemical). In metallurgy, the composition of the alloyed elements in a binary alloy determines the passivation. It has been noted that minimizing the amount of alloying element to the host metal increases passivation, which in turn generates a corrosion-resistant surface.
Passivity in binary alloys is governed by four theories:
- The electron configuration theory – some of the transition metals become passive at certain compositions during alloy, which depends on the favoring of oxygen chemisorption by filling up the d-bands with electrons from the alloying element.
- Oxide film effects – the existence of an amorphous oxide film contributes to the corrosion resistance of binary alloys.
- The percolation theory – this defines passivity in terms of atomic arrangement in the elements used to enhance corrosion resistance.
- The graph theory model – it uses the concept of a connected network of -M-O-M- (M is the component of the binary alloy that produces passivity) bridges or chains present in the oxide film to define passivity in binary alloys.