Effect of Surface Treatment of Steel on Zinc Phosphate Coatings

摘要

The effect of surface treatment stages such as mechanical cleaning, degreasing, pickling and activation on the phosphatability of steel at the early period of reaction were thoroughly investigated. Different zinc phosphate baths contain Ni, Ca, Mn, polyphosphate, organic acids and surface active agents were also used. It was shown that the surface conditioning step either with colloidal titanium phosphate solution or dilute organic acids prior to phosphating has a great effect on the coating characteristics over the other surface treatment stages including the chemical composition of the zinc phosphate baths. The incorporation of -ionic species in the phosphate solution has a little effect on the phosphatability of steel and can be arranged in the following decreasing order Mn-Ca-Ni according to the grain size. Mild alkaline degreasers having pH value less than 10.5 without acid pickling are the most favourable pretreatment stages prior to activation. Mechanical treatment is the best method for cleaning rusted parts. An improvement in the aging stability of the activation and consequently in the performance of the coating were found by addition of anionic surfactant of the type tall oil fatty acid ethoxylates. Make up water must contain less than 60 ppm Ca + Mg with electric conductivity not higher than 200 礢cm{sup}(-1) at a pH range from 8-9.5.