Optimisation of conversion coatings in phosphoric acid media using statistical experimental designs

The process conditions used to produce conversion coatings on stainless steel as the basis for subsequent ceramic deposits have been optimised using statistical experimental designs. These conversion coatings should possess strong interfacial adhesion and must be highly porous; the specific area of the coatings has been measured by an electrochemical method. The effects of all influencing factors including temperature, time, and bath composition (acid, accelerators, and inhibitors), and also the interactions between these factors were evaluated using 2^5-1 fractional factorial design. To optimise the conversion coating, a methodological approach was supported by a Doehlert experimental design and the simplex method. The most important factors were found to be the concentrations of phosphoric acid, thiosulphate, and alcohol. The growth of the conversion coating was also influenced by the effects of interactions between other components of the bath. Under the conditions optimised in this manner, the real surface area obtained was about 100 m² m^-2. This conversion coating, modified by an alumina deposit, increases the thermal oxidation resistance of the stainless steel. A discussion of the use of experimental designs for the optimisation of conversion coating processes is also presented.