Metal Finishing Guide Book

2013

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the particles are so small, they are able to pack closer together. This results in less void space within the matrix of the TMC when compared to conventional phosphate metal coatings, so there is less room for air, moisture, and salts to travel to the substrate and cause corrosion. These coatings also inhibit galvanic corrosion because the transition metal has electrons that would be sacrificed prior to the electrons of the iron in the base metal. Paint/powder coating adhesion and corrosion resistance also benefit because of the efficiency of the reaction. As stated, the efficient reaction results in very little sludge formation, so there is much less suspended solids in the pretreatment solution. As the pretreatment bath ages and the level of insoluble suspended solids increases, they can become incorporated in the phosphate coatings and/or dry down on top of them despite rinsing. The result is a powdery appearance on the parts that provides an inferior surface for adequate paint or powder coating adhesion. If you have managed an iron or zinc phosphate pretreatment process, you have likely made the decision to dump the bath at the end of its useful life due to powdery part appearance in your past. The reader may be thinking, "If it saves costs, increases environmental compliance, and gives better performance, what's not to like?" The market agrees, and adoption of TMC pretreatments is therefore rapidly increasing in the marketplace. HOW ARE TRANSITION METAL COATINGS DIFFERENT (FROM TRADITIONAL PHOSPHATE)? New users observe several differences when converting from the legacy phosphate pretreatments. • TMC are best applied at cool temperatures, not warm-to-hot like phosphate. • TMC are MUCH more reactive than phosphates during application, yet they sludge much less. They benefit from a continuous filtering regimen to remove iron solids. • TMC can be (and are) used in mild steel washers, but are best applied from stainless equipment. • TMC are equally as well applied via spray, immersion, and pressure wand. • TMC require excellent rinsing and low-salt content applications along with a very clean surface. HOW ARE TRANSITION METAL COATINGS THE SAME (AS A PHOSPHATE)? New users of TMC pretreatments are delighted to find that there are many similarities with the traditional metal phosphates. • TMC pretreatments are usually applied from a washer and generally will change the color of the metal substrate (if it's steel). The color change can give a good visual indication of a properly running process. • The application mechanism of TMC pretreatments is somewhat similar to phosphate, with pickling of metal and depositing of coating. There is a bit of a difference in that the substrate metal is not generally believed to be a participant in the deposition reaction mechanism. • The application requires some measure of control and attention to the process. Typical measurements are for pH, acidity, and perhaps a 120

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