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Troubleshooting Decorative Electroplating Installations, Part 5

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Troubleshooting Decorative Electroplating Installations, Part 5 Troubleshooting Decorative Electroplating Installations, Part 5: Plating Problems Caused Article By Heat & Bath Temperature Fluctuations by N.V. Mandich, CEF, AESF Fellow Technical Technical In previous parts of this series, emphasis was given The fast-machining steels must then be carburized to troubleshooting of the sequences for pre-plating or case-hardened to obtain a surface with the hardness and electroplating over metals, Parts 1 and 2;1 required to support the top chromium electroplate. the causes, symptoms and troubleshooting for Case hardening is the generic term covering several pores, pits, stains, blistering and “spotting-out” processes applicable to steel or ferrous alloys. It changes phenomena, Part 3;2 and troubleshooting plating on the surface composition of the top layer, or case, by plastic systems, Part 4.3 Here in Part 5, causes and adsorption of carbon, nitrogen or a mixture of the two. some typical examples of problems that occur in By diffusion, a concentration gradient is created. The electroplating as a result of a) thermal, mechanical heat-treatments and the composition of the steel are surface treatments, b) the metallurgy of the part to additional variables that should be addressed and taken be plated or c) effects of plating bath temperature into account in the electroplating procedure. on plating variables and quality of the deposits When discussing the effect of heat-treatment on are discussed. subsequent electroplating processes it is necessary to zero in on the type of heat-treatment involved. We Nearly every plater has at one time or another had the can defi ne the heat-treatment process as changing the experience of trying to plate parts that simply would characteristics of the parts by heating above a certain not plate. The usual assumption is that the parts are temperature and quenching it in a suitable media, not clean. If parts pass a water-break test, conventional generally water or oil. A secondary process is then wisdom then suggests that the likely cause is that one used to achieve the desired hardness range. This type or more solutions are not as they should be. After all, of heat-treatment, commonly called neutral hardening if the parts cannot be properly plated it is the plater’s or through hardening, depending on the alloying fault, right? Wrong! characteristics in the steel or its properties, gives platers The plater is often the scapegoat even if the problem few problems since the surface has not been chemically is caused by the condition of the basis metal before the changed to any signifi cant degree. part enters the fi rst plating solution. Some problems Most heat-treatment problems occur when the cannot be completely prevented by the best solutions surface of a material is changed to achieve certain, and the fi nest techniques currently known to the prescribed characteristics. There are a few ways of electroplating industry. Often the most difficult doing this: problems for the plater to solve come from heat-treating, polishing and buffi ng or barrel fi nishing operations. 1. In nitriding, the part is subjected to nascent nitrogen Nevertheless, it is important for the plater to be able to and the nitrogen is absorbed into the surface of identify these problems correctly, if for no other the solid ferrous alloy to obtain defi ned hardness reason than to protect his or her own interests. In characteristics. Quenching is not needed to produce some cases the defects can be solved by suitable a hard case. pretreatment, if the plater identifi es what is causing 2. In carbonizing, the part is exposed at a high the particular problem. temperature to a source of carbon that is absorbed into the skin of the part surface. Influence of Heat Treatment & Mechanical 3. Carbonitriding is a case-hardening process in which Preparations a suitable ferrous material is heated above the lower Heat Treatment transformation temperature in a gaseous atmosphere Steel is frequently plated with functional chromium of such composition as to cause simultaneous or sometimes with hard nickel for improved wear absorption of carbon and nitrogen by the surface and resistance. It is vital that the substrate be hard enough by diffusion create a concentration gradient. Cooling to withstand the pressure that will be applied to the top at a rate that produces the desired properties in the chromium electrodeposit. If this substrate is too soft, it workpiece completes the heat-treating process. This may “cave-in,” causing failure of both the electroplated process seems to give platers the greatest amount deposit and the part. One way to get a surface hard of diffi culty. The hardness is relatively shallow. The enough for such applications is to use deep-hardening penetration generally is less than 500 µm (0.020 in.) steels that are heat-treated to produce the desired and in commercial practice, it generally is 75–250 mechanical properties. This would be no problem if µm (0.003-0.010 in.) deep. It produces a very hard, only a few parts are required. Where the application abrasion-resistant surface, while preserving the requires large numbers of parts, fast-machining soft ductility of the core. steels are generally preferred. 4. Decarburizing is a loss of carbon on the surface layer of washing. They will plate readily after a simple, anodic the carbon containing steel. It occurs by the reaction of alkaline electrocleaning. carbon with one or more chemical substances from the What if conditions are not ideal? If there are leaks in the medium that contacts the surface. The process is another furnace or in the lines feeding the furnace, soot may be source of trouble for the suspecting or unsuspecting deposited. This can range from a faint trace to a heavy layer. plater. Heat-treating case-hardened parts that are to be Anodic electrocleaning will not remove the soot, causing hard chromium plated, for instance, may decarburize serious problems in subsequent electroplating steps. Fine the surface. When chromium is plated on this surface, a shot blasting, glass beading, vapor honing or grinding is three-layer structure results. The underlayer is hardened necessary to salvage such parts. Again, this is a costly steel, the middle is the decarburized surface and the additional step that should be avoided. overlayer is hard chromium. The decarburized surface All platers are discontent with the presence of soot, smut, is rather soft, and the hard chromium will not take the heavy oxides, greases or oil that have been baked on the part load nor will it have good adhesion. Poor adhesion is a during heat-treatment. When parts are adequately cleaned general problem with all steel that has been decarburized. before they go into the heat-treat furnace, the plater’s job is Problems also arise when the decarburized layer is rather defi nitely made easier. One way to accomplish this, wherever thick [500 – 750 µm (0.020-0.030 in.)] and 125 µm (0.005 possible, is to make the heat-treat foreman the supervisor in.) is removed in the grinding step to obtain a sound of the electroplating department as well. This may often clean surface. The ground surface appears smooth and be impractical and there will always be some baked-on plates properly, but the resultant product often does not soils to be removed. have good physical properties. Cleaning such parts in a solvent degreaser is not the best practice. It removes, by dehydration, the last trace of In hard chromium electroplating the surface metallurgy anything that could be solubilized, making cleaning more of the steel is rather important, if not the most important diffi cult. It is preferable to use a solvent emulsion cleaner parameter. Change in electrical resistance is an example of as the fi rst cleaning step. properties, which are infl uenced by metallurgical factors. Oxides on heat-treated parts are also a source of problems. Electrical resistance changes with the carbon content of the They are nearly always the result of improper furnace steel. Data from the literature4 shows that pure iron has a operation. Removal of an oxide film requires careful resistance of 10 µΩ•cm; steel, a mixture of pure iron and techniques to avoid damaging the parts. carbon, has a resistance of 16 µΩ•cm. The amount of current At times it is diffi cult to nickel plate a nitrided surface and required in electroplating varies with electrical resistance. cover it properly. Nitriding has an effect similar to that of a Hardening of steel causes a marked change in resistance. fi ne sand blasting. The effective area of this mildly rougher Heating above the critical temperature and quenching raises surface may be larger than the visual area by a factor of the resistance from 16 µΩ•cm to 46 µΩ•cm. Obviously two or three. In addition, the surface becomes more diffi cult the carbon content of steel is an important variable in to clean. In the pickling or acid treatment steps, with this electroplating. larger effective surface area, there is more acid attack and Since hardening is often necessary, and given that the a tendency to leave an insoluble smut on the surface. This resistance is noticeably changed as a result, subsequent in turn leads to poor metal coverage and poor adhesion in processing must take into account these changes. Current subsequent electroplating. density can be increased or other appropriate changes made Proper pretreatment of a nitrided surface uses mechanical that can improve throwing power to compensate for the cleaning methods. In chemical cleaning steps, some scrubbing increased resistance. Compensating for other changes caused or mechanical action is desirable, e.g., tumbling the parts by heat-treating, is unfortunately not quite so simple.
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