Metal Finishing Guide Book

2013

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Figure 2a. Pretreatment comparison. TMC* Iron Phosphate Zinc Phosphate Amorphous Amorphous Crystalline Typical Coating Thickness ~50 nm ~250 nm 1000 nm Typical Coating Weight 50-150 mg2 5-15 mg/ft2 300-700 mg/m2 2565 mg/ft2 2-3 g/m2 180-300 mg/ft2 Coating Structure *TMC = Transitional Metal Coating Much like traditional phosphates, TMC pretreatments can exhibit an array of colors, from nearly colorless, to tan, gold, and iridescent blue. Investigation has revealed that the appearance of pretreated metal is related to the thickness of applied coating. When the substrate is mild steel, the coating color goes from the original appearance of the substrate to light gold or tan, to a deep gold, to light blue and gold, to blue, to deep iridescent blue as the coating becomes more complete and increases in coating weight or thickness. As with traditional phosphates, the coating will become higher with increases in one or more of the following variables: chemical concentration, contact time, pressure (spray) or agitation (immersion), or temperature. Another significant variable that impacts appearance of the coating is the type of steel a finished good is made from and the fabrication steps required to produce it. Heat treatment, welding, grinding, bending, blasting and other common manufacturing processes impact the amount of carbon (or scale) and iron at the surface of the part. The more carbon at the surface, the less reactive the surface is to the pretreatment solution. The more iron at the surface, the more reactive the surface is to the pretreatment solution. The photos below show a mild steel and hot rolled steel panel. Both panels were processed through a five-stage pretreatment process (Clean, Rinse, TMC, Rinse, Final Seal). The cold rolled steel is an even deep blue and the hot rolled steel is an even grey color because of the high carbon content at its surface. WHAT ARE THE BENEFITS OF REPLACING PHOSPHATE CONVERSION PRETREATMENTS? The primary benefit of replacing traditional metal phosphate pretreatments is significant and measurable cost savings in the operation/application of TMC pretreatments. A significant benefit of replacing traditional metal phosphate with TMC pretreatments is the elimination of phosphorus from the waste stream. Phosphorus is becoming increasingly regulated, especially in areas near large bodies of fresh water such as the Great Lakes region; watersheds such as the Chesapeake Bay; and other areas where municipalities are trying to reduce phosphates in the Footnote 1 - Note that a nanometer is on one billionth of a meter so the "smallness" of the concept is difficult to grasp. Put more succinctly, a nanometer is to a meter, what a marble is to the earth. 118

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