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pfonline.com/experts Voids at Welding Points Q. We stamp and weld two CRS brackets together then send them out to be e-coated. More often than not, brackets come back to us with voids in the electrocoat around the welds, approximately the same diameter as the electrode we used to weld the brackets together or a little smaller. The e-coater wants us to tumble the brackets prior to e-coating, but that leads to the parts being bent. Do you have any suggestions?—D.O. A. The intense heat generated during the welding operation causes the silicon (Si) in the steel substrate to diffuse to the surface, cool and harden. Because of surface tension differ- ences between the Si and the welding material, the hard deposits typically appear round, and dark brown or amber in color. One characteristic of the Si spots is that they have poor electrical conductivity. When electrocoating is involved, this condition produces voids or partial coats in the weld beads. The intense heat generated during the welding operation not only introduces the challenges of weld spots, but also challenges with the surface adjacent to the welding seam or bead. This area is typically called the heat-affected area, and it is characterized by its dark brown or black discoloration. Although this heat -affected area is conductive and will electrocoat, the appearance and adhesion of the electrocoat will be marginal. This marginal performance is due to the inability of the phosphate system to develop a good phos- phate crystal over such a heavily heat-oxidized surface. Fabrication shops typically pre-wash parts with alkaline cleaners prior to welding to prevent burning or baking residues onto the metal surface and leaving it contaminated with excess carbon and other inorganic residues. If there is no pre-washing prior to welding, then additional perfor- mance issues will appear. The best process to make a welded part ready for elec- trocoat would involve cleaning the Si spots, as well as the oxides and residues on the heat-affected areas. The solutions to addressing both conditions are those employing mechanical forces such as blasting, wire brushing, sanding or tumbling, or employing chemical forces such as etching. Some cleaning methods are preferred over others, and the right choice heavily depends on conditions such as the substrate, size of parts, type of manufacturing employed, type of electrocoat machine employed, etc. In your case, tumbling would be excluded because it bends the brackets. Bend Test Failure Q. We have a problem with our e-coat failing our mandrel bend test. We use a cathodic e-coat and a 13-stage pretreat- ment with conditioner and zinc phosphate, alkaline cleaner, heated stages 1 and 2, oil skimmer, etc. A cleaned down panel with e-coat only has passed the test. Does this mean the pretreatment is causing the failure? Could it be the tri-cat phosphate?—H.T. A. It definitely could be the zinc phosphate. The mandrel bend is a very inter- esting physical test for multilayer systems, because the bend radius of the external layer is greater than that of the internal layers. Because of the differ- ential pulling forces between the multiple layers, they generate great stress and thus have a tendency to produce disbondment between the layers. Typically, the more finishing layers, the more difficult it is to pass the mandrel test. In your example, the e-coat-only panel passed, and the phosphate plus e-coat failed (one layer versus two layers). For multilayer systems such as galvanized, zinc phosphate, electrocoat and topcoat (four layers), the mandrel bend test is very difficult to pass unless flexible coats are used. In my experience with the mandrel bend test on zinc phosphate and electrocoat, the most important contributing factors are: 1. Maintain low phosphate weight and small crystal size. The lower the weight and crystal size, the better, while still maintaining the minimum requirements for corrosion performance. Iron phosphates perform better on mandrel bending than zinc phosphates because of their lighter coating weights. 2. Maintain low cure conditions for the electrocoat, espe- cially temperature, although time also impacts flexibility. The higher the cure temperature, the less film flexibility. Un-flexible films typically perform poorly in mandrel testing as the films get harder and more brittle. 3. Maintain low electrocoat film thickness. The lower the film thickness, the better, while still maintaining minimum requirements. Thicker films increase the differential bend radius between layers, thus increasing the chances for mandrel bending failures. 4. Maintain or use electrocoats with low p/b ratios of more resin and less paste. The resin is more flexible than the pigments, therefore resin-rich systems perform better in mandrel bending. Keep in mind that galvanized steel, because of the addi- tional zinc layer between the metal and the electrocoat, tends to perform worse than steel when everything else remains the same. Again, the more layers and the greater their thickness, the worse the performance. Other factors such as type of resin (epoxy or acrylic) and type of the elec- trocoat may also contribute to poor mandrel bending. 40 JUNE 2014 — pfonline.com JOSE A. TIRADO / Consultant electrocoat@pfonline.com ELECTROCOATNG C L I N I C 0614_PF_EcoatCLINIC.indd 40 5/16/2014 1:43:19 PM