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ELECTROCOATNG A. Chrome-plated surfaces are smooth, hard and very chemically inert so that, although a clear electrocoat could be deposited over it with reasonably good initial adhesion, it would be very difficult to obtain adequate adhesion and durability over time. Some kind of adhesion promoter (must be conductive) or light acid etch (to provide a rougher surface) would be necessary as a pretreatment to enable the clear e-coat to deposit over the chrome-plated surface and provide improved adhesion and durability. There are companies that claim special pretreatments provide the long-term adhesion and durability necessary to e-coat over chrome-plated surfaces, but I can't confirm this. I have never known anyone to do it. Care must be taken to ensure that the special pretreatment does not introduce any color to the substrate and therefore cancel the benefit of the clear e-coat over the chrome plate. Typical automotive zinc phosphates do not improve adhesion over chrome-plated surfaces, as there is no iron in the substrate for the phosphate to properly develop. Bright, Custom E-coat Colors Q. We have a liquid paint system and would like to switch to a more environmentally sound coating system. Engineers in our company say that e-coat is not suitable for us because of the bright custom color we have. Can you customize electrocoat to match a specific color?—B.F. A. Epoxy electrocoat formulations do not have any constraints on the color of the liquid paint as received by the user, as long as it is a solid color, such as a RAL, and does not have any metallic or specific visual effects. You can choose any color to match your specific requirement or use clear coat, ranging in gloss from <10 to >65°. The issue with epoxy electrocoat formulations is that, although the color of the liquid paint as received is a perfect match, that color can't be obtained consistently in operation during the curing stage. The color of the cured parts will differ from piece to piece, depending on the specific degree of cure received by each part, depending of the mass of each piece. This issue with epoxy formulations is referred to in the industry as color stability during curing, and epoxies are known for having poor color stability during cure. For example, I have seen systems where the paint itself was a typical "construction yellow" in color, but some parts came out of the oven anywhere between light and dark brown. Epoxy systems must be specified and accepted as a color range or color ladder. This color ladder can be duplicated in the lab so that the color variation can be known and accepted. On the other hand, acrylic formulations or hybrid formulations (mixtures of acrylic, urethanes or epoxies) can enable the color of the cured parts to consistently match the color in the liquid form, independent of the degree of cure obtained. In this case, you can also choose any color to match your specific requirement as long as it CLINIC is solid and without effects. The gloss of the acrylic formulations can reach 90°-plus for highly visual applications. Minimizing Shear on Paint Q. If you split flow streams in an e-coat paint recirculation line, what is the best method for manipulating flow rates that would minimize excessive shear on the paint? Would it be better to vary the pipe diameter or throttle with a butterfly valve?—M.A. A. Fluid shearing refers to the development of external forces that strain or disrupt laminar flow. Thus, the pipe diameter increase will produce less shearing in the paint than installing a butterfly valve and throttling to decrease flow. Additionally, valves introduce significantly more pressure drop in the system than the roughness of the piping and require more energy to accomplish. Why E-Coat Over Silver? Q. I have heard that some fine silver jewelers use electrocoat. Why would anyone use electrocoat over silver?—M.P. A. Some jewelry manufacturers use clear electrocoats over sterling silver components such as wedding bands and rings to preserve them during storage. The clear electrocoat layer is very thin as its only objective is preservation against scratches and light dulling because of oxidation. Eliminating Voltage Rupture Q. Why are galvanized substrates more prone to voltage rupture than other metallic substrates? What process variables can you change to eliminate voltage rupture? — J.T. A. Galvanized substrates are characterized by a layer of zinc (Zn) deposited over a steel substrate. The Zn layer can be deposited by several mechanisms, including continuous or bath hot dip, electrogalvanizing, mechanical, etc. When the layer of galvanizing is not 100-percent tight or closed to the substrate (Zn layer voids or micro-cracks, small pinholes or large porosity), there is the possibility of creating a galvanic cell between the iron base and the zinc coating, because the two metals have different oxidationreduction potentials. These galvanic cells are typically the nucleation sites responsible for voltage rupture in electrocoatings. The interaction between the galvanic cells in the substrate and the applied voltage in the electrocoat bath can create points of excessive voltage. This excessive voltage can create burns or ruptures in the electrodeposited coating. To minimize or eliminate voltage rupture, first try reducing the voltage. In order to minimize application voltage and still meet minimum film thickness requirements, however, you also will have to modify other process variables to compensate, such as increasing percent solids, bath temperature, solvent levels or dwell time. You also can ask the galvanizer to improve the zinc coating to eliminate voids or porosity. In most cases, galvanized coatings are more resistant to rupture than galvanealed coatings because there is less iron in the coating. PRODUCTS FINISHING — pfonline.com 35