Experimental work studying the solubility of zinc phosphate, the inhibitive property of acid extracts, and the composition of the material formed on mild steel at pH4 showed that the inhibitive properties of zinc phosphate are limited. Where atmospheric pollution is high and the rain water has a pH of at least 4 a protective zinc phosphate film can form on steel. Without these conditions zinc phosphate is inert and not protective. Very little evidence was found to support the theory that zinc phosphate acts through soap formation and degradation. -- AATA