Metal Finishing Guide Book


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Table 1. Five-Stage TMC Sytem Process Stage 1 2 3 4 5 Comments Option 1 Clean Rinse TMC Rinse Seal Legacy Iron Conversion Option 2 Clean Rinse Rinse TMC Rinse/Seal New TMC Line Design Seal Rinse/Seal Clean-Coater TMC Option 3 Clean/TMC Clean/TMC Rinse colorimeter to measure the transition metal concentration. • TMC pretreatments work by passivating the substrate with respect to corrosion, and enhancing mechanical/physical paint bonding—the same as traditional pretreatments. CONVERTING FROM PHOSPHATE TO TMC There are generally two types of conversions: the washer is an existing traditional metal phosphate application, or the washer is newly constructed for application of TMC pretreatments. The two applications require different approaches. A legacy metal phosphate washer will typically feature a cleaning stage, one rinse, a pretreatment application stage, one or more rinses and perhaps a seal rinse. The best practice for TMC is stainless steel for the TMC stage, good cleaning, very good rinsing, and perhaps provision for a seal rinse. At first glance it would be considered fairly straightforward to simply replace the traditional metal phosphate stage with a TMC stage. Unfortunately, this overlooks the need for very thorough rinsing ahead of the TMC stage. More frequently, a conversion of the legacy metal phosphate stage to a rinse, followed by conversion of a legacy rinse to a TMC stage is more successful. New washer construction takes into account the requirement of sufficient rinsing ahead of pretreatment by inserting an extra rinse after cleaning. Three-stage washer configurations present a particular challenge because the process must both clean the metal and provide a conversion coating. Surfactants cannot be incorporated into zinc phosphate baths so the cleaning and pretreatment must be in separate stages, separated by a minimum of 1 rinse. Zinc phosphate requires a minimum of 5 stages and more typically 7 stages. Iron phosphate is commonly applied in a three-stage process, where the cleaning and conversion coating is accomplished in the first stage. The cleaning is achieved the addition of both the surfactant and/ or solvent into the formula. When parts are heavily soiled with mill oils and other metalworking fluids, it is not uncommon to use a tank side additive to improve cleaning and extend the useful life of the bath. As previously stated, TMC do not contain phosphate. While this is an environmental benefit, it diminishes the cleaning capability of the chemistry because phosphates are a detergent and help cleaning. TMC must rely solely on surfactants and solvents in the formula to degrease the metal. cleaner-coater TMC products are available and are best used when the soil load is light and consistent. The most common way of using TMC in three-stage pretreatment processes is to apply them in the final stage. The clean, rinse, coat configuration is ideal 121

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