Metal Finishing Guide Book

2012-2013

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phosphates in water is a strategy aimed at reducing eutrophication (Footnote 2). Adopters of TMC pretreatments often claim a green pretreatment strategy; the disposal procedures are generally inexpensive and uncomplicated. TMC pretreatments are very reactive so heat is not needed to drive the reaction of the zirconium with the metal at the surface of the part. Thus, TMC pretreatments can run at ambient temperature, whereas the traditional metal phosphates require significant heat to drive the deposition reaction. This saves significant energy cost. Most TMC pretreatments operate between 90 and 105��F. The heat carried in by the parts coming from the heated cleaner stage and the energy generated by the pump in a spray system are typically enough to maintain this temperature range. Early adopters of TMC pretreatment technology enjoyed a minimum of 15%, to as much as 40% lower costs when converting from traditional metal phosphate pretreatments. These kinds of savings persist with the modern renditions of TMC pretreatments. Another key benefit of TMC pretreatments is much better corrosion performance in service, as well as in accelerated testing, when compared to the legacy metal phosphates (10% to 30% longer salt spray hours and more intervals of cyclic corrosion testing have been observed with the first versions of this new class of chemistry). Several suppliers of pretreatment chemistry have developed TMC pretreatments that are approaching the perforFigure 3 Top: Heavy zirconium oxide mance of zinc phosphate. Because of high coating on hot rolled steel. Bottom: Heavy zirconium oxide coating on cold rolled operational and disposal costs associated with steel. running a successful zinc phosphate process, OEM���s are investigating substituting TMC for zinc phosphate pretreatment, and several organizations have successfully made the transition. There are several reasons why TMC pretreatments provide excellent corrosion protection. As previously noted, TMC contain elements that are near chromium on the period table; the oxides of these elements are relatively chemically inert so they do not dissolve as easily as phosphate metal coatings. Zirconium oxides are so stable that hydrofluoric acid, which is extremely corrosive and aggressive, is needed to dissolve them. Secondly, TMC are made of much smaller particles than amorphous iron phosphate coatings or zinc phosphate crystals. Because the parFootnote 2: Eutrophication: or more precisely hypertrophication, is the ecosystem response to the addition of artificial or natural substances, such as nitrates and phosphates, through fertilizers or wastewater, to an aquatic system. 115

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