Metal Finishing Guide Book


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OPERATIONS CARRIED OUT IN COMMON PLATING PROCESSES Substrate preparation using chemical etchants: In order to achieve a robust deposit, it is extremely important to prepare the substrate to be free of soils, scales, rust, and oily residues. The parts may be mechanically polished, buffed with varying degrees of abrasive compounds, belt polished, undergo simple tumbling, and/ or impact blasting in the presence of various types of media. It is imperative to remove any residual contaminants from these operations prior to starting the plating sequence. For removing shop soils on parts, the workpieces are soaked in organic solvents such as petroleum naptha, stirred to loosen residues, de-scaled, pickled, and de-smutted. For de-scaling, usually 20–30% aqueous solutions of HCl or H2SO4 in the presence of proprietary inhibitors are employed, and their strength can be monitored by titrations. The potency of these solutions should be frequently monitored depending on the surface area processed each day. Obstinate and aged scales are loosened by immersion in hot alkaline potassium permanganate solutions. The concentration and type of de-scaling chemistries should be selected for their compatibility with substrates. Alkaline cleaners: Alkaline cleaning solutions usually contain sodium hydroxide, sodium carbonate, trisodium phosphate, sodium metasilicates, and proprietary surfactants as wetting agents. Total alkalinity is monitored for such solutions. Ultrasonic cleaning may be achieved by introducing high-frequency sound waves (20–80 kHz) causing cavitation on parts, thus removing soils. Anodic and cathodic cleaning involves the liberation of hydrogen or oxygen, which promotes scrubbing action on the parts, causing the following reactions to occur: At anode: 4OH – ᒑ 2H2O + O2 + 4e– At cathode: 4H2O + 4e– ᒑ 4OH– + 2H2 Periodic reverse (PR) electro-cleaning is also employed in special cases. Table 2 presents a summary of the methods for analysis of soak cleaners and electro-cleaners for efficient cleaning and maintenance. For semi-transparent solutions, a suitable range refractometer may be employed for estimation of total detergent concentration. Nickel plating baths: Several nickel plating baths—such as matte, semi-bright, and bright—usually containing nickel sulfate, nickel chloride, nickel sulfamate, boric acid, and proprietary additive packages (comprising carriers, wetting agents or surfactants, brighteners, auxiliary brighteners, grain refiners) have been formulated. All sulfate, all chloride, high sulfate nickel, and black nickel baths are formulated for engineering applications. The primary parameters—such as nickel, boric acid, and pH—are routinely monitored (Table 3). The estimation of additives, carriers, and wetting agents are treated separately. Electroless nickel baths for low, medium, and high phosphorus content are monitored for pH, Ni, hypophosphite, and orthophosphite. There are a series of binary nickel alloy baths with other metals such as P, Co, Cr, Mo, Mn, Pd, S, W, Au, Cu, Ge, In, Ru, Ag, Tl, and ternaries like Co-Ni-P, Co-Ni-Fe, and Fe-Cr-Ni. Standard analytical techniques are used for bath control. 467

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