Automotive requirements for phosphate and oil coatings, as applied to a variety of parts such as nuts, bolts, washers, fasteners, hood hinges, and brackets, are primarily based on the corrosion protection provided by the coating system.
The automotive specifications covering the degree of corrosion resistance required, for a given application, normally stipulate the neutral salt spray test as a means of evaluating the quality of the coating system and may vary 24-168 hr of exposure time.
To comply with these corrosion resistance requirements, part suppliers use a number of proprietary phosphatizing processes in combination with an even larger number of specially compounded rust preventative petroleum products, either as water emulsions or solvent cut-backs. Some of these combinations of phosphate and oil systems have been more successful in consistently meeting the corrosion resistance requirements, as measured by the neutral salt spray test.
This paper presents the results of two independent evaluations of the corrosion protection afforded by a variety of phosphate and oil coating systems. In addition, the corrosion protection of these systems as measured by a new test method is briefly discussed.