Optimization Process Parameters of Manganese Phosphate coating developed on En-19 Steels by Statistical Design of Experiment

Abstract

Phosphating is a chemical conversion process in which constituents of a metal surface with a view to produce thin adherent protective coating of insoluble crystalline phosphate by reaction with phosphoric acid solution. This conversion coating should have strong interfacial adhesion and porous enough. The growth of phosphate coating is influenced by method of cleaning of metal surface to be treated before treatment, use of surface activating rinses and acid or alkaline content of phosphating solution. Manganese phosphate coating can produces a dark grey black phosphating layer in combination with suitable post treatments on En-19 steel samples. The quantitative assessment of coating was done by stripping method and corrosion resistance was evaluated qualitatively by salt spray test according with ASTM B 117 Standard. The process parameters employed to produce phosphating have been optimized using statistical design of experiments. The regression equations were derived for each set of experiment and depending on the numerical value of the coefficient of each parameter its influence was assessed. The parameters like Thermoil Granodine 112 content, steel wool content and process time showed the synergetic effect on the resulting phosphating quality. Keywords: acid ratio, manganese phosphate coating, stripping method, salt spray testing, statistical design of experiments