Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 458 of 707

ing 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 mon- itored 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 pro- motes scrubbing action on the parts, causing the following reactions to occur: At anode: 4OH–t2HO + O + 4e– 2 2 At cathode: 4HO + 4e–t 4OH– 2 + 2H 2 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 solu- tions, a suitable range refractometer may be employed for estimation of total deter- gent 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 formu- lated. All sulfate, all chloride, high sulfate nickel, and black nickel baths are formulated for engineering applications. The primary parameters—such as nick- el, 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. Gold plating baths: Several types of gold plating baths have been formulated based on end uses, such as decorative, soft gold, nickel- and cobalt- hardened gold for industrial electronics, and miscellaneous applications involving repairs and 457

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