Metal Finishing Guide Book


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can create corrosion cells. All that is needed is water or moisture and corrosion initiators such as chloride or sulfate commonly found in untreated water. Depending on the quality of your paint and your product���s service environment this may be an issue. WAR STORIES I���m not going to mention any names of companies or vendors. The TMCs Figure 1. Uniform coating on HRPO rims. have been around long enough that the stories have made it around our metal finishing world. Vendors are more in tune to this as we are all competing for business with new technology. There is an adage used by pretreatment vendors: every line is unique. As much as suppliers do research, it would be unreasonable to expect a research chemist to anticipate every unique situation. Before I start relating war stories, I want you to understand that I���m telling you about the bad things. My intent is not to make you paranoid but rather prepared for a TMC. Most TMC installations go without incident if there is sufficient planning and resources. The quality issues aren���t always unique to TMCs as they also happen with iron phosphating. Adhesion issues generally stem from improper cleaning and rinsing. Loose deposits will cause paint adhesion issues. Line stops can create flash rusting that is loosely bonded. Ergo, flash rusting can cause paint adhesion issues. A quick test for TMC adherence can be simply done with tape. If you can pull the coating off exposing silver metal underneath, then you will have a paint adhesion problem. This is pretty obvious but leads into a situation where a metal finisher had over a dozen consecutive line stops due to paint color changes. The question was posed: was there a particular area of the 5-stage washer where line stops create adhesion issues. Laboratory testing was conducted to determine if there was a particularly bad area for parts during a line stop. Lab variations simulated 3-minute line stops in the process stages with normal transfer times, 3-minute line stops in the drain Figure 2. TMC implementation changes how coagulants work in treating wastewater. 150

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