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Cosmetic corrosion resistance of coated and cold rolled steel sheet products is being determined by use of test coupons (paint panels) mounted on full-size pickup trucks operating in the severe corrosive environments of Montreal, Quebec and St. John's, Newfoundland, Canada, Ten standard materials from the joint AISI/SAE corrosion test development program, including hot-dip galvanized, galvannealed, electroplated zinc and zinc-nickel alloy-coated, as well as phosphated and unphosphated cold rolled steel have been under evaluation for five years. The results of these evaluations are described in terms of resistance to paint undercutting (creepback) and red rust staining at scratches (scribes) through the paint film to the steel substrate. The effects of horizontal and vertical test coupon orientations are considered. Performance of the materials in this evaluation provides an excellent standard of real-world behavior against which the reliability of accelerated tests can be determined.

The American Iron and Steel Institute's (AISI) Task Force on Automotive Corrosion is making significant progress in its continuing efforts towards development of an accelerated laboratory test for ranking the cosmetic corrosion resistance of automotive steel sheet products. This paper provides an overview of this work and reviews major accomplishments to date. Accelerated tests conducted by the AISI and by the SAE's Automotive and Corrosion Prevention Committee (ACAP) Division 3 are compared to long-term on-vehicle exposure tests now in progress for four years in Montreal, Quebec and St. John's Newfoundland. A license-plate exposure test has also been initiated to broaden the basis for real-world performance. Statistical methods for comparing the results of the various tests are described. A designed experiment (Plackett-Burman L8) is underway to evaluate the effects of seven key cycle test parameters on the rankings of the test materials in a laboratory cyclic corrosion test.

A Task Force on Corrosion has been established by the Automotive Applications Committee of the American Iron and Steel Institute. The Task Force is composed of technical representatives from the North American sheet steel producers and experts from the major automakers and pretreatment suppliers, and it works closely with related committees of SAE and ASTM. The goal of the Task Force is to develop a laboratory accelerated test for cosmetic corrosion resistance that will provide a reliable ranking of automotive sheet steel products. Work to date has included reviewing available information on cyclic testing, designing new test cycles, identifying suitable test materials and procedures, and conducting tests at commercial laboratories. During 1988, four different cyclic tests were conducted on ten sheet steels coated with a full automotive paint system. The results of these tests are presented and discussed.

Effect of pretreatments on the performance of zinc and zinc alloy electroplated sheet in a cyclic corrosion environment has been evaluated. It is shown that Zn-Ni and Zn-Fe coatings are less sensitive to variations in the phosphating pretreatment than zinc and steel. These latter two materials benefit from modifications to the phosphate and show better scribe creep resistance when Ni++ or Ni++ and Mn++ containing zinc phosphates are used instead of the unmodified zinc phosphate. A nonchromate post-phosphate rinse improves the scribe creep resistance of steel, but it has little impact on the scribe creep resistance of zinc and zinc alloy coatings. Electrolytic Cr+CrOx pretreatment significantly enhances the scribe creep resistance of zinc. The likely reasons for the improvements are discussed in terms of the physical and chemical characteristics of the pretreatment layers and their stability in the underfilm corrosion environment.

Zinc-rich organic precoated steel sheet is an attractive product for corrosion-resistant automotive applications due to its formability, weldability, availability and cost. A disadvantage of this product is an inability of the coating to provide a significant level of cathodic protection to areas of the steel base exposed to the environment at sites of damage to the coating. To gain increased understanding of this behavior, we studied the galvanic properties of zinc-rich organic coatings with electrochemical, microscopic and electronic methods. We observed high electrical resistances within these coatings that preclude effective galvanic protection of steel by the zinc in the coating. Based on these studies, we discovered that corona-discharge treatment, an inexpensive electric discharge treatment, significantly increases the coating's conductivity and galvanic activity.

Electrodeposited Zn-Ni alloy coatings show a maximum resistance to salt spray corrosion at 12% to 15% Ni. To gain an increased understanding of this behavior, we studied the corrosion mechanisms of 13% Ni coatings in sodium chloride solutions with electrochemical, microscopic, x-ray, and Auger electron spectroscopic methods. We observed that the Zn-Ni alloy initially corrodes with the preferential dissolution of zinc. As the coating dezincifies, tensile stresses are created in the coating causing development of a fine network of cracks. As corrosion progresses, the coating transforms into a composite barrier layer consisting of a micro cracked nickel-rich metallic phase and zinc corrosion products. This behavior contrasts with that of pure zinc coatings which corrode by a simple mechanism of uniform dissolution.