The phosphate/primer interface has been identified as the site of attack in several corrosion studies. It is expected that all aspects of the pretreatment process including oiling, cleaning, and phosphating affect corrosion performance. The extent of the effects is likely related to substrate type. An alloy coated material such as galvanneal (with its range of surface morphologies) could be particularly susceptible to variations in the pretreatment process.
This paper reports a study examining the relationship of galvanneal surface morphology to phosphatability and resulting corrosion performance. Examination of phosphate crystal morphology, coverage, alkaline stability, and coating weight as a function of phosphate application mode and bath chemistry for conventional automotive phosphates was completed in the study. Electrochemical methods were used to evaluate phosphate porosity and to predict phosphate stability.
Painted galvanneal samples containing a wide range of phosphate qualities were exposed in the GM SCAB test. Galvanneal samples of varying Fe% and with different surface morphologies were also included in this cyclic test. Metallographic, SEM, and Electron Microprobe characterization of these materials after testing was also completed.
These studies indicated that galvanneal surface morphology (delta vs zeta surface phase) affects phosphate coating weight and crystal refinement. GM SCAB testing identified galvanneal coating weight and Fe% in the galvanneal coating as the most important variables controlling painted corrosion performance. Phosphate coating weight/quality also affects galvanneal corrosion behavior. The mechanism of galvanneal degradation in the GM SCAB test does not appear to be strongly dependent on galvanneal surface morphology. The chloride anion was identified as the primary actor inducing galvanneal degradation in the GM SCAB test. Details of the results of these studies are discussed in this manuscript.