Theoretical and Practical Considerations of Sheet Steel Surface Cleanliness

Paint failure on cold-rolled sheet steel occurs by cathodic delamination of the paint film. Steel exposed at discontinuities in the paint coating becomes the anode of an electrochemical corrosion cell, and adjacent areas under the paint become the cathode of that cell. Highly alkaline conditions are created under the paint severely weakening the coating system bond at the pretreatment/steel interface. Nonuniformly distributed amorphous surface carbon contamination is primarily responsible for the variable paint performance observed. Adjusting mill practices in pickling, tandem rolling, and annealing resulted in improved steel surface cleanliness.