Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 76 of 707

At the cathode: 4H2O + 4e–® 4OH– + 2H2 During electrolysis, twice as much hydrogen is liberated at the cathode than is oxygen at the anode. TYPES OF ELECTROCLEANERS Electrocleaners are classified on the basis of two main criteria: (1) polarity of the work in the tank; and (2) the type of substrate being treated. There are three types of electrocleaning modes as defined by polarity of the work in process and the applicability of each to a given substrate. Anodic Electrocleaning The work is connected to the anode side of the rectifier and is positively charged. The process is also known as reverse electrocleaning, since the polarity is opposite that of plating. As described under electrolysis, oxygen is liberated at the surface of the work (the anode) when current is applied. As the gas rises to the top, it creates a mechanical scrubbing action that loosens and lifts the soils. Two other phenomena also take place. As oxygen bubbles are formed on the sur- face, they coalesce and grow before they rise in continuous layers. It is believed that the static charge holding fine particles to the surface is released through the layer of bubbles, facilitating their removal through the scrubbing action. Chemical effects, oxidation, and drop in pH also take place at the anode surface. If excessive, the effect of oxidation can be seen, for instance, on brass, zinc, and sil- ver as they discolor, stain, or etch. Special inhibited anodic electrocleaners are available for brass and zinc. When nickel is anodically electrocleaned it will quickly become passive and pre- vent further plating unless reactivated. A similar effect is experienced with stainless steels. Regular steels are not adversely affected by the process, whereas high-carbon steels are more sensitive and require moderation in electrocleaning. Alloys of lead, nickel–silver, and silver plate are attacked or tarnished by anodic electrocleaning. As oxygen is liberated at the anode, the net pH value tends to decrease at the inter- face. This effect can be noticed on steel if an electrocleaner's alkalinity is too low by design or for lack of bath maintenance. The steel is more rapidly oxidized, and pre- cipitated iron hydroxide forms on the surface. Parts exiting the tank will have a rusty or etched appearance, especially in high current density areas. The situation can be readily rectified by increasing the alkalinity of the bath or by reducing the current density below normal operating levels until the bath chemistry is adjusted. Cathodic Electrocleaning The work is connected to the cathode side of the rectifier and is negatively charged. This is also known as direct electrocleaning. In this case, hydrogen is lib- erated at the cathode. As seen from the net amount of electrolysis, twice as much hydrogen than oxygen is generated at the cathode. Consequently, more scrubbing action and cleaning ability are expected. The use of cathodic electrocleaning, however, has not found a widespread use in the industry as the main electro- cleaning mode for two reasons: (1) the concern with hydrogen embrittlement as a result of copious hydrogen release at the surface, and (2) the risk of plate out of charged impurities from the solutions on the cathodic surface. The latter may not be noticeable to the casual observer as the parts exit the tank, but it leads to poor 75

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