Effects of Substrate and Phosphate Type on the Painted Corrosion Performance of Galvanneal Coated Sheet

Galvannealed sheet steel is gaining acceptance by automotive manufacturers due to its combination of properties and cost. Galvanneal can be produced by several processes including the most common method termed the “hot” line hot dip coating process, but also using “flux” line hot dip coating application. Similarly, several types of phosphate coatings: Zn, Zn-Ni, and Zn-Ni-Mn phosphates can be applied to these materials prior to painting to enhance paint adhesion and painted corrosion resistance. This manuscript reports the affects of commercial phosphate type and galvanneal type (based on production method and properties) on the corrosion performance of galvanneal. In addition, painted corrosion performance of galvanneal is compared to painted corrosion performance of electroplated zinc. The manuscript also describes the use of electrochemical methods as one alternative to conventional test methods resulting in substantial time savings. The studies reported in the manuscript show that phosphate type and galvanneal type effect the corrosion performance of galvanneal. They further show that cosmetic corrosion performance of galvanneal in the GM 9540P test is consistently better than that of electroplated zinc. Finally, the studies indicate that electrochemical methods can be used for the early estimation of scribe creep performance in the GM 9540P test.