Hard chromium plating

Electroplating. Chromium is electroplated onto various substrates in order to realize a more decorative and corrosion- or wear-resistant surface (24—32). About 80% of the chromium employed in metal treatment is used for chromium plating over 50% is for decorative chromium plating (see Metal surface treatments). Hard chromium plating differs from decorative plating mostiy in terms of thickness. Hard chromium plate may be 10 to several 100 p.m thick, whereas the chromium layers in a decorative plate may be as thin as 0.25 p.m, which corresponds to about two grams Cr per square meter of surface. 

Eunctional or hard chromium plating (169,175) is a successfljl way of protecting a variety of industrial devices from wear and friction. The most important examples are cylinder liners and piston rings for internal combustion engines. Eunctional chromium deposits must be appHed to hard substrates, such as steel, and are appHed in a wide variety of thicknesses ranging from 2.5 to 500 ]Am. 

Chromium is conventionally deposited from chromic acid solutions containing at least one anionic catalyst, which is usually the sulfate ion. The weight ratio of chromic acid to catalyst is important and, for sulfate-cataly2ed solutions, is maintained about 100 1. Formulations and conditions for operating hard chromium plating solutions are shown in Table 5. 

Several high-efficiency hard chromium plating baths are now available commercially. A solution which does not contain fluoride, and does not therefore attack steel or aluminium, has been described by Schwartz . At 50 A/dm and 53°C the cathode efficiency is about 25%, enabling deposition to be carried out at the rate of I m/min, with a consequent substantial saving in power and time. The deposit is bright, and has a hardness of about 1 050 Hy. 

Hard chromium plating provides excellent resistance to atmospheric oxidation both at normal temperatures and at temperatures of up to 650°C. It is unattacked by many chemicals, owing to its passivity. When attack takes place, this usually commences at cracks in the chromium network hence the most corrosion-resistant deposits must have a very fine structure, such as is obtained from relatively high solution temperatures using low current densities. 

Namgoong E, Chun JS (1984) The effect of ultrasonic vibration on hard chromium plating in a modified self-regulating high speed bath. Thin Solid Films 120 153-159... 

A filter is normally installed in the suction line to the high-pressure pump to protect the pump and the hard chromium-plated scraped-surface heat exchanger cyhnder from any foreign matter in the margarine emulsion. 

Chromium is a refractory metal having a melting point of 3375°F (1857°C). Neither chromium metal nor chromium-based alloys are widely in the hydrocarbon or chemical industries. Chromium plating is useful for aesthetic purposes, and hard chromium plating finds some use in hardface applications. It is extensively used as an alloy addition to low-alloy steels (usually for the purpose of stabilizing carbides) and in cast irons (to produce wear-resistant products) and nickel alloys (for increased corrosion resistance). Chromium is the main alloying addition in the 400-series stainless steels and is used extensively in the 200- and 300-series stainless steels. 

The only galvanic process of importance to ceramic material extrusion is hard chromium plating, in particular in cases in which a high quality sur-... 

Thin coatings of chromium [139] are applied over another electrodeposited layer, usually nickel or copper, as a decorative, nonstaining finish these thin (250 pm) layers are porous. Hard chromium plating is used where advantage may derive from the high hardness and wear resistance, or the low coefficient of friction, of such deposits. Chromium electrodeposits are not suitable for use at high... 

An alternative method to irradiation of the whole plating tank is to apply ultrasound directly to the cathode. Using this technique for hard chromium plating a better quality of plate was obtained in terms of both brightness and increased microhardness [34]. 

Duan S, Li H, Zhang X, Sun G, Dash I (1995) Hard chromium plating from a trivalent bath. Plating and Surface Finishing, pp 84-86... 

Hard chromium plating is extensively used to protect steel parts and provide a silvery bright finish. 

Chromium on a hard chromium plated component is not harder than a decorative layer, but because of its layer thickness it can withstand harder wear. 

Make the wearing surface hard through the use of hardfacing, diffusion heat treatments, hard chromium plating, or more recently developed vapor deposition techniques or high-energy processes (e.g., ion implantation). 

Hard chromium plating is usually applied directly to steel parts in thickness up to about 300 xm to provide resistance to wear, abrasion, and corrosion. It is also used to build up worn or imdersized parts. In the thicker applications it may be impervious but is subject to microcracks. Chromiiun is a metal with low cathode efficiency, and substantial amounts of hydrogen are deposited on the part along with the metal being plated. Because of this, parts mxist he baked as soon as possible after plating to drive off the hydrogen and prevent embrittlement. 

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