“Effects of Steel Processing on the Surface Carbon of Cold-Rolled Steel” Technical paper presented at the American Society for Metals / American Deep Drawing Research Group Conference: Technological Impact of Surfaces: Relationship to forming, welding, and painting. April 14, 1981.
Phillip L. Coduti
Inland Steel Research Laboratories
I.Introduction
In 1956, Wirshing and McMaster of General Motors demonstrated the existence of wide differences in the corrosion resistance of apparently identical lots os cold-rolled steel (1). All lots were phosphate, painted and tested identically. At that time they were unable to identify the surface condition which would give one steel surface good corrosion protection and another steel surface poor corrosion protection after subsequent finishing, i.e., phosphating and painting operations. In 1959, Grossman offered substantial evidence that surface carbon was the principle contaminant which influenced the corrosion performance of painted steel (2). Grossman postulated that the source of this carbon contamination was from the breakdown of oils present on the steel surface during the annealing stage in the steelmaking process.
From economic and quality considerations, it is to the mutual advantage of both steel producer and steel user to share and understand the technical implications of the requirements for reproducible corrosion resistant, cold-rolled surfaces. Today both steel producers and users realize the detrimental effects surface carbonaceous residues have on the ultimate corrosion resistance of a finished steel surface. Over the last few years, the automotive industries have set tighter cold-rolled steel requirements. To meet these demands the steel producer was compelled to develop a deeper understanding of the steel processing-steel properties relationships. Previous to the need for lighter gauge sheet steel, greater fluctuations in steel corrosion resistance were tolerated. But to reduce weight and still maintain vehicle integrity it became imperative that cost effective, high strengths corrosion-resistant steel be used. These moves necessitated 1) a deeper understanding of the steel mill processing parameters which can affect the corrosion performance of finished steel, 2) the development of new methods or the improved use of existing methods for the detection, analysis, and quantification of surface contaminants, particularly surface carbon, 3) an understanding of the mechanism by which surface carbon affects the pretreatment and corrosion performance afforded by paint coatings, and 4) the use of dependable, high quality coatings to assure reliable corrosion protection.
