Effect of substrate porosity on the corrosion behaviour of electro less nickel-phosphorus coated powder metallurgy iron compacts and strip

Abstract

Powder metallurgy (PM) sintered iron compacts having 2-9% porosity, and full density PM iron strip were electro less coated successfully with nickel-phosphorus alloy from baths containing sodium hypophosphite. Corrosion behaviour of such specimens in l·0M HCl was investigated. It was shown that anodic and cathodic polarisation plots of such PM iron specimens, irrespective of substrate porosity exhibit Tafel behaviour. Corrosion rates of PM iron compacts of 2-9% porosity and full density strip coated with electro less nickelphosphorus alloy for shorter periods, of the order of 0·5 h increase with substrate porosity. However, the corrosion rate of such PM iron compacts and strip coated for more than 0·5 h, first increases with substrate porosity and then remains practically constant.