As automotive manufacturers continue to look for ways to reduce vehicle weight, aluminum is finding more utility as a body panel component. The substitution of cold-rolled steel and zinc-coated substrates with aluminum has led to new challenges in vehicle pretreatment. As a result, changes to traditional pretreatment chemistries and operating practices are necessary in order to produce an acceptable coating on aluminum body panels. These changes result in increased sludging and other undesirable characteristics. In addition to the chemistry changes, there are also process-related problems to consider. Many existing automotive pretreatment lines simply weren't designed to handle aluminum and its increased demands on filtration and circulation equipment. To retrofit such a system is capital intensive and in addition to requiring a significant amount of downtime, may not be totally effective. Thus, the complexities of pretreating aluminum body panels have actually had a negative effect on efforts to introduce more aluminum into new vehicle design programs.
Recent research into ways of reducing the negative effects has led to a new understanding of the nature of zinc phosphate bath - aluminum interactions. Many of the issues associated with the pretreatment of aluminum have been identified and can be mitigated with only minor changes to the zinc phosphate bath chemistry. The use of low levels of soluble Fe (II) ions, together with free fluoride, has been shown to dramatically improve the efficiency of a zinc phosphate system processing aluminum. Appearance of zinc phosphate coatings, coating weights and sludge are all benefited by this chemistry change.
