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:
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(i)
Cold-reduced steel (CRS)
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(ii)
Electroplated zinc-coated steel (EZ)
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(iii)
Electroplated zinc/13%nickel coated steel (Zn-Ni)
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(iv)
Galvannealed iron/zinc coated steel (IZ)
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(v)
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.