by Dr E. Ramanathan
A metal surface should be free from crevice, pitting, galvanic and creeping corrosions, scales, tool marks, oil, grease and dirt in any form prior to chemical conversion coating. There are different types of corrosion viz., crevice, pitting, galvanic and cosmetic corrosions. Crevice corrosion is caused by stagnant electrolyte trapped in silent corners of joints or in surface deposits on the metal components. Pitting corrosion is caused by prolonged contact with the corrosive acids on a particular area on the metal components and results in a rough surface. When two dissimilar metals are in contact with each other galvanic corrosion occurs. Corrosion visible on scratches of paint layer coated on the sheet metal is called cosmetic corrosion [6]. Though it is an issue of appearance, it may lead to a creeping corrosion beneath the paint film and weakens the metal substrate.
Mechanical cleaning such as sanding with different grades of emery sheets and polishing increases the active sites on the bare metal. The electrochemical reaction is the initial corrosion step occurring on the bare metal. The acidic nature of phosphate coating increases more active sites that enhance the kinetics of nucleation of phosphate crystals i.e., quick phosphating and coating with smaller grains. If the grain size of such phosphate crystals is less, the porosity of the phosphate coat becomes smaller and imparts better corrosion resistance [7]. A rough steel surface has a greater surface area than a relatively smooth mild steel surface. The rough surface profile of metal substrate enables the formation of thick phosphate conversion coatings to protect against corrosion [8]. This difference in surface area can lead to differences in coating weight. When lubrication with corrosion protection is the desired property, thick phosphate coating with more porosity is recommended so that the phosphate layer with more porosity takes more oil and improves lubrication.