Plating chromium

Several workers have investigated Cr(III) plating. Fig. 6.9 presents two typical conventional time-coating thickness curves [18,19]. 

At Coventry University [20] we have obtained similar results to Tu under conventional (silent) conditions. Our initial thoughts were that if the effect of the tail off was either a consequence of mass transfer to the electrode, or a consequence of some problem with the diffusion layer, then ultrasound might be expected to have an effect and thus improve the plating rate. Investigations in the presence of ultrasound and at various pH values did not significantly affect the plating characteristics i. e. the plateau effect still remained. However, the overall efficiency in the presence of ultrasound was affected (Fig. 6.10). 

The percentage efficiency observed in both conventional and ultrasonic plating is pH dependent and confirms the findings of Tu who observed maximum efficency in the absence of ultrasound at pH = 2.1 (Fig. 6.11). 

There was an improvement in efficiency, when compared to conventional plating, which increased as the pH increased. This latter point identifies a possible small technological benefit to the industry. For example, rather than curtail conventional plating at pH = 2.7, it is possible to continue plating (in the presence of ultrasound) at higher pH values. For example, at pH = 3.1 the efficiency is ap- 

It was suggested previously that one of the obvious variables to change in order to provide for increased plating rates was the current density. The more current applied (in a given time), the faster the coating should develop. Fig. 6.12 compares the effects of increasing the current density (CD) on the plating characteristics for Cr(VI) and Cr(III) [21]. Only for Cr(VI) is the effect as predicted. 

It is difficult to get a bright chromium layer. During the first decade of the 20 century, professor W. D. Bancroft at Cornell University in the USA conducted systematic and successful research into chromium plating. E. Liebrich s German patent of 1924 and C. G. Fink s US patent of 1925 are considered to be milestones. They clarified the essential importance of a high content of hexavalent chromium in the bath and a carefully adjusted percentage of sulfuric acid (1% of chromic acid). 

A typical conventional chromium-plating bath contains 250 g/1 CrOj and 2.5 g/1 H SO,. 

There are two principal types of electrolytic chromium plating. 

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Chromium Plating. Sodium selenate or selenic acid are added to chromium-plating baths to improve corrosion protection from pitting. 

RoUed-zinc products in the form of strip, sheet, wire, and rod have many and varied commercial appUcations. Strip is formed into dry-ceU battery cans, mason jar covers, organ pipes, grommets, eyelets, and many other objects, some of which are subsequentiy brass or chromium plated (jewelry, medaUions, bathroom accessories, etc) (132). The zinc—carbon dry-ceU appUcation accounts for about one half the roUed-zinc consumption in the United States (see Batteries). Sheet zinc is used in photoengraving and also in the constmction of roofing and other architectural uses. Special high grade zinc with a... 

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. 

Hard plating is noted for its excellent hardness, wear resistance, and low coefficient of friction. Decorative plating retains its brilliance because air exposure immediately forms a thin, invisible protective oxide film. The chromium is not appHed directiy to the surface of the base metal but rather over a nickel (see Nickel and nickel alloys) plate, which in turn is laid over a copper (qv) plate. Because the chromium plate is not free of cracks, pores, and similar imperfections, the intermediate nickel layer must provide the basic protection. Indeed, optimum performance is obtained when a controlled but high density (40—80 microcrack intersections per linear millimeter) of microcracks is achieved in the chromium lea ding to reduced local galvanic current density at the imperfections and increased cathode polarization. A duplex nickel layer containing small amounts of sulfur is generally used. In addition to... 

R. Winer, Electrolytic Chromium Plating, Finishing PubUcations Ltd., 1978. 

Around 1800, the attack of chromite [53293-42-8] ore by lime and alkaU carbonate oxidation was developed as an economic process for the production of chromate compounds, which were primarily used for the manufacture of pigments (qv). Other commercially developed uses were the development of mordant dyeing using chromates in 1820, chrome tanning in 1828 (2), and chromium plating in 1926 (3) (see Dyes and dye intermediates Electroplating Leather). In 1824, the first chromyl compounds were synthesized followed by the discovery of chromous compounds 20 years later. Organochromium compounds were produced in 1919, and chromium carbonyl was made in 1927 (1,2). 

Decorative chromium plating, 0.2—0.5 ]lni deposit thickness, is widely used for automobile body parts, appHances, plumbing fixtures, and many other products. It is customarily appHed over a nonferrous base in the plating of steel plates. To obtain the necessary corrosion resistance, the nature of the undercoat and the porosity and stresses of the chromium are all carefliUy controlled. Thus microcracked, microporous, crack-free, or conventional chromium may be plated over duplex and triplex nickel undercoats. 

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. 

Black and colored plates can also be obtained from chromic acid baths. The plates are mostly oxides (177). Black chromium plating bath compositions are proprietary, but most do not contain sulfate. The deposit has been considered for use in solar panels because of its high absorptivity and low emissivity (175). 

Decorative plating primarily over bright nickel is estimated to consume about 2270 t worldwide. Some 60% of this is used in North America, and about 900 t in Europe. A relatively new method in decorative chromium plating is based on trivalent instead of hexavalent chromium, and this is estimated to total about 10% of the decorative market and is expected to increase. Chromic acid, which cost about 2.76/kg in early 1993, is the hexavalent... 

Plate Thickness. Thickness of the plate should always be specified as should the locations on the work where the thickness is to be measured. Generally, thicker deposits perform better, but there are notable exceptions. Mating parts, eg, fasteners having fine machine threads, are not usable if over plated. Machine-threads are usually plated to 10 p.m or less, depending on tolerances. Additionally, gold-plate over nickel does not solder well if too thick thus, gold is usually 1—2 pm or less. Chromium, plated for decorative purposes from the conventional chromic acid bath, tends to macrocrack above about 0.7—1.0 pm. 

Chromium. AppHcations of chromium plating can be separated into two areas hard chromium, also called functional, industrial, or engineering chromium, and decorative chromium. The plating bath compositions may be the same for both. In most cases, the differentiating factor is plate thickness. Decorative chromium is usually less than about 1 p.m hard chromium can be from about 1 p.m to 500 p.m or more. 

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. 

Standard practices for chromium plating (93) and specifications for hard chromium (94) and decorative chromium (89) have been pubHshed. 

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