Metal Finishing Guide Book

2012-2013

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Table II. Typical Operating Conditions for Electrocleaning Steel and Copper Brass Zinc Alkalinity (as NaOH), g/L 50���100 15���20 15���20 30���60 Temperature, ��C 60���90 50���70 50���70 50���80 1���5 1���3 1���3 1���5 Time, min Nickel and Chromium Alloys Solution Conductivity. This in turn is a function of cleaner concentration and temperature at a given voltage. The higher the concentration and temperature (up to a practical level), the higher the conductivity and the amount of gassing. Voltage Applied. Current increases with voltage, although the latter is kept to maximum values of 10 to 12 V. Higher values are known to cause ���burning��� or roughness on parts. Surface Area Being Cleaned. This parameter controls current density and, for a given rectifier setting, will directly affect the cleaning efficiency. Adequate recommended current density ranges for different base metals are summarized in Table I. Values below these produce light to marginal electrocleaning. Higher values generally lead to etching and roughness of the surface. Anode to cathode area ratios of 1:1 are adequate for most applications. The ratio is not critical as long as the specified current density values are maintained. PROCESS CONSIDERATIONS There are general considerations in the selection and proper use of electrocleaners. Electrocleaner Formulation Several proprietary formulations are available covering a wide range of applications. These formulations should provide the following properties: ��� A suitable degree of alkalinity for the metal processed, e.g., high alkalinity for steel, lower for zinc and brass. ��� A proper ratio of hydroxide to silicate to prevent insoluble silicate films from adhering to the work and affecting plate adhesion. Silicates are often used to prevent burning of steel at high current density. Nonsilicated cleaners, using different types of inhibitors, are also available. Water softeners and conditioners should be considered in hard water areas. ��� An adequate amount of wetting agent and emulsifiers. Although high levels serve as cleaning agent for excess oil and grease, they inhibit the gassing action at the electrode surface and reduce desmutting characteristics. Efficient desmutting electrocleaners will have just enough surfactants to reduce solution surface tension and promote a thin foam blanket to hold down gas misting during electrolysis. Bulk oil removal should be a reserved function of the preceding soak cleaner. Typical operating conditions of electrocleaners are given in Table II. Suppliers of proprietary electrocleaners usually tailor the parameters to specific applications, which may vary from the values shown in Table II. Alkalinity figures expressed as NaOH may represent 20���80% of the total product formulation. 63

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