Principles of Other Anti-Corrosion Methods

The principles behind corrosion protection based on CP coatings are best understood if we take a quick look at the three other methods most widely used at present for protection of metals from corrosion  

The second method is the use of chemical conversion coatings to alter the metal surface for protection. This is illustrated by the most widely used- and the most successful to date- chemical conversion coatings, chromates, for A1 protection. Etching agents such as HF are first used to remove native A1 oxides. The chromates are then applied, with the overall reaction  

The alumina and the Cr(OH)3 formed together serve as a chemical barrier to corrosion. This barrier is however not entirely immune to pinholes and cracks. Additionally, the chromates are highly toxic, and there is a strong movement at present to eventually replace them due to environmental considerations. Commercially available chromate conversion coatings include Elf-Atochem s Alumigold and Accelagold and Parker Amchem s Alodine 1200S . 

The third widely used protection method is that of cathodic protection , where a small negative potential is continuously applied to the metal surface to render it passive. Its counterpart, anodic protection can also be used to keep a metal in a permanently oxidized state, rendering it passive to corrosion. Quite evidently, this method is more cumbersome and expensive than most methods, although it does find niche uses where it is more practical, e.g. metal pipelines which have periodic control stations on the pipeline. 

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