Effect of Surfactants on the Porosity and Corrosion Resistance of Zinc-Phosphated Steel

hosphating is one of the most widely used metal pretreatment P processes for ferrous metals. It gives a reasonably hard, electrically nonconducting surface coating of insoluble phosphate that is contiguous and highly adherent to the underlying metal and is considerably more absorptive than the The phosphate coating serves to anchor the paint film and prevents the induction and spreading of corrosion.1-4 The mechanism of the phosphating process consists of the formation of numerous crystals of very different sizes that have spread from centers of nucleation to join and finally cover the surface. This process inherently results in fissures and channels through the basis metal at intercrystalline zones, indicating the porous nature of the coating. Usually, the porosity is of the order of OS-1.5% of the phosphated surface, and this can be reduced by approximately 50% by conventional chromic acid sealing. With the advent of electrophoretic painting and powder coatings, the need for a fine-grained, compact coating of reduced porosity and improved corrosion resistance was recognized. To keep pace with these needs, numerous modifications have been put forth in the processing sequence as well as in phosphating formulations. Of the various modifications, incorporation of special additives plays a vital role. Sankara Narayanan et al.5 have reviewed the role of special additives in phosphating. Some additives have been shown to result in a phosphate coating with lower porosity and higher corrosion resistance.6-8 Surface-active agents have a great influence on the growth of crystals and have the ability to alter crystallization kinetic^.^ This property can be effectively used to produce coatings with desirable properties. We therefore studied the ability of some surfaceactive agents to reduce the porosity and improve the corrosion resistance of phosphated steel. Generally, porosity is determined by the ferroxyl test and corrosion resistance by the salt spray test; however, the ferroxyl test is qualitative and not very effective in distinguishing the porosities of panels phosphated in different baths, and the salt spray test is time-consuming. Various publications have appeared in recent years with regard to the application of polarization studies for coating evaluation.l0-l2 Simplicity of operation and reliability make them useful for quick measurements of porosity and corrosion resistance. Moreover, they provide quantitative data from which mechanistic information can be derived. Of the various methods available, determination of porosity by cathodic reduction of oxygen in 0.01N sodium