Electroplating corrosion resistance

Electroplated materials are generally employed for a specific property or function. There is, of course, some ovedap for example, decorative use certainly requires some degree of corrosion resistance. The various usages and the principal plating metals employed are as hsted. There are also smaller amounts of other metals and alloys used for specific apphcations. 

Plate thickness is an important factor in electroplating, in terms of both performance and economics. Corrosion resistance, porosity, wear, appearance, and several other properties are proportional to plate thickness. Minimum plate thicknesses are, or should be, specified as should the location, or check-point, where the thickness is to be measured. In some appHcations, such as threaded fasteners, maximum thicknesses should be specified. Root diameters of finer machine threads can be adversely affected by as Htde as 10 p.m of plating. 

Poor preparation of the substrate can result in loss of adhesion, pitting, roughness, lower corrosion resistance, smears, and stains. Because electroplating takes place at the exact molecular surface of a work, it is important that the substrate surface be absolutely clean and receptive to the plating. In the effort to get the substrate into this condition, several separate steps may be required, and it is in these cleaning steps that most of the problems associated with plating arise. 

The use of electrodeposited metals to protect corrodible basis metals from their service environments has been well established for many years and accounts for by far the larger part of the activities of the plating industry. There are many reasons for using an electroplated metal finish in preference to an organic finish or to making the articles concerned from inherently corrosion-resistant materials. 

Some of the reasons are aesthetic, and on many larger articles, such as motorcars, an attractive appearance is achieved by a careful visual balance between parts which are finished and protected by electroplating, and parts which are protected by organic finishes. In many other instances the manufacture of parts from corrosion-resistant materials is ruled out because of the relatively high cost of such materials, and in some cases the physical properties of the appropriate corrosion-resistant materials may render them either unsuitable for economic production or unable to perform the function for which the article is required. 

It will be seen that the design of articles to be electroplated can have a considerable effect on the corrosion resistance of the electrodeposited coating. The chief effects are the result of variations in deposit thickness, but also important are features which can influence the adhesion, porosity and physical properties of the deposit. Good design will also avoid features of the plated article capable of trapping liquids or solid contaminants which might cause more rapid corrosion. 

Electrochemical methods of protection rest on different precepts (1) electroplating of the corroding metal with a thin protective layer of a more corrosion-resistant metal, (2) electrochemical oxidation of the surface or application of other types of surface layer, (3) control of polarization characteristics of the corroding metal (the position and shape of its polarization curves), and (4) control of potential of the corroding metal. 

Electroplated CoPt [96, 97] and CoSnP [98] have also been suggested for thin-film media. Both alloys are reported to have good corrosion resistance. A plated 2.25" diameter disk uses Zn in the plating solution to control coercivity [24] details of the process are not given. 

The use of Zn-Cr(III) alloy plating has almost replaced the use of Cr(VI) in the electroplating industry due to its excellent corrosion resistance and its lower toxicity. Recently, a solvent extraction procedure for separating and selectively recovering the two metals, zinc and chromium, from electroplating wastewaters has been demonstrated [10]. 

Uses. In stainless and alloy steels, refractory products, tanning agents, pigments, electroplating, catalysts, and corrosion-resistant products... 

Uses/Sources. Corrosion-resistant alloys, electroplating, production of catalysts, nickel-cadmium batteries nickel subsulfide (NisS2) is encountered in the smelting and refining of certain nickel ores and may be formed in petroleum refining from the use of nickel catalysts. 

The process of electroplating metals is another important application of electrolysis. Metals that easily corrode (Fe and Cu) can be plated by metals that are resistant to corrosion. Chromo - plating steel parts used in vehicles is a good example of electroplating. Steel bumpers become more corrosion-resistant when they are electroplated with nickel first, then with chromium. 

Metal Coatings. Tellurium chlorides, as well as tellurium dioxide in hydrochloric acid solution, impart permanent and attractive black antique finish to silverware, aluminum, and brass. Anodized aluminum is colored dark gold by tellurium electro deposition. A solution containing sodium tellurate and copper ions forms a black or blue-black coating on ferrous and nonferrous metals and alloys. Addition of sodium tellurite improves the corrosion resistance of electroplated nickel. Tellurium diethyldithiocarbamate is an additive in bright copper electroplating (see Electroplating). 

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