This content is not included in your SAE MOBILUS subscription, or you are not logged in.
Effect of Bimetallic Coupling on Automotive Pretreatment Quality
ISSN: 0148-7191, e-ISSN: 2688-3627
Published October 01, 1993 by SAE International in United States
Annotation ability available
The longer-term corrosion performance of an automotive body has become one of the most important items in the portfolio of packages being used by the automotive manufacturer to attract the customer. To support the lengthy corrosion warranties currently on offer, the automotive industry has increased the amount of zinc-coated steels used in the autobody construction. This means that steel is increasingly being joined to zinc-coated steel; in some cases, a zinc-coated steel is joined to another zinc-coated steel of a different variety.
This practice of bimetallic coupling has been reported in early investigations to affect the quality of the phosphate pretreatment employed on automotive painting lines. Poor pretreatment uptake at the joint areas, resulting in poor paint performance was reported. However, no clear indication was given as to whether the effect is the same over the range of bimetallic couples that may be created in the construction of the autobody. The work reported here was, therefore, undertaken to provide a clear understanding of the issue.
The pretreatment uptake across the bimetallic joints of ten couples has been studied. The couples were fabricated from a permutation of the following automotive materials:
Cold-reduced steel (CRS)
Electroplated zinc-coated steel (EZ)
Electroplated zinc/13%nickel coated steel (Zn-Ni)
Galvannealed iron/zinc coated steel (IZ)
Hot-dip zinc-coated steel (HDG)
Two different types of zinc phosphate-based pretreatments were used in the study. One of these pretreatments, a nickel containing trication zinc phosphate, is currently widely used by the automotive industry. The other pretreatment is free of nickel and chromate and was formulated as an environmentally acceptable alternative to current pretreatments. Both pretreatments were applied to a range of bimetallic couples constructed from the selected automotive materials. Samples from areas at the joints and areas away from the joints were examined by Scanning Electron Microscopy, Dispersive X-ray Spectrometry and paint adhesion tests.
The findings from the study reveal that the problem of poor pretreatment uptake is only confined to a limited number of bimetallic couples and is dependent on the type of pretreatment used. In general, the quality of the automotive pretreatment is dependent on the two metals being joined together and the pretreatment being applied.
|Technical Paper||Corrosion Resistance of Aluminum-Transition-Steel Joints for Automobiles|
|Technical Paper||Liquid Film Migration During Aluminum Brazing|
CitationQuarshie, R., "Effect of Bimetallic Coupling on Automotive Pretreatment Quality," SAE Technical Paper 932359, 1993, https://doi.org/10.4271/932359.
- Hoar T.P. ‘Report of the Committee on Corrosion and Protection’ HMSO 1971
- Department of Trade and Industry ‘Corrosion Control Checklist for Design Offices - Corrosion: Stop it now’ DTI 1981
- McArthur H. ‘Corrosion Prediction and Prevention in Motor Vehicles’ Ellis Horwood Ltd. England 1988 19
- SAE ‘CORROSION: Coatings and steels’ SP-649 Int. Congress and Exposition Detroit, Michigan Feb., 1986
- Shastry C.R. Townsend H.E. ‘Mechanisms of Cosmetic Corrosion in Painted Zinc and Zinc-alloy Sheet Steels’ CORROSION 45 2 1989 p103
- Quarshie R.L. ECSC Technical Reports, (Ref.7210.MB/803) 991 93
- Isaac J. John V. ‘Automotive Corrosion and Corrosion Control at Bimetaliic Couples’ SAE Technical Papers Int. Congress and Exposition Detroit, Michigan Feb.- Mar. 1990
- Harlfinger R. et al “Surface Quality Requirements for Sheet Steel Metal in Car Bodies” IDDR6 1986 231 38
- Claus G. et al “Phosphating Behaviour of Zn-coated Steel Welded to Uncoated Steel” Seminar on the Technical and Economic Aspects of the Manufacture and Application of Coated Steel Products 1990