Chromium deposits

In 1827 Isaac Tyson discovered chromite ore deposits on the border between Maryland and Pennsylvania in the USA. This made the U SA the leading supplier for many years. Twenty years later high-grade chromite ore was found near Bursa in Turkey. After some years Turkey became the main source of supply. This continued until the mining of chromium ore began in India and South Africa around 1910. 

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Organochromium Catalysts. Several commercially important catalysts utilize organ ochromium compounds. Some of them are prepared by supporting bis(triphenylsilyl)chromate on siUca or siUca-alumina in a hydrocarbon slurry followed by a treatment with alkyl aluminum compounds (41). Other catalysts are based on bis(cyclopentadienyl)chromium deposited on siUca (42). The reactions between the hydroxyl groups in siUca and the chromium compounds leave various chromium species chemically linked to the siUca surface. The productivity of supported organochromium catalysts is also high, around 8—10 kg PE/g catalyst (800—1000 kg PE/g Cr). 

In 1979, a viable theory to explain the mechanism of chromium electroplating from chromic acid baths was developed (176). An initial layer of polychromates, mainly HCr3 0 Q, is formed contiguous to the outer boundary of the cathode s Helmholtz double layer. Electrons move across the Helmholtz layer by quantum mechanical tunneling to the end groups of the polychromate oriented in the direction of the double layer. Cr(VI) is reduced to Cr(III) in one-electron steps and a colloidal film of chromic dichromate is produced. Chromous dichromate is formed in the film by the same tunneling mechanism, and the Cr(II) forms a complex with sulfate. Bright chromium deposits are obtained from this complex. 

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. 

The CASS Test. In the copper-accelerated acetic acid salt spray (CASS) test (42), the positioning of the test surface is restricted to 15 2°, and the salt fog corrosivity is increased by increasing temperature and acidity, pH about 3.2, along with the addition of cupric chloride dihydrate. The CASS test is used extensively by the U.S. automobile industry for decorative nickel—chromium deposits, but is not common for other deposits or industries. Exposure cycle requirements are usually 22 hours, rarely more than 44 hours. Another corrosion test, now decreasing in use, for decorative nickel—chromium finishes is the Corrodkote test (43). This test utilizes a specific corrosive paste combined with a warm humidity cabinet test. Test cycles are usually 20 hours. 

Triple Nickel and More. As an extension to the dual nickel, a thin, higher sulfur-containing nickel strike is deposited between the sulfur-free and the bright nickel. The sulfur content of this minimumally 2.5 p.m-sttike is 0.15—0.20 mass %. Ttiple nickel and dual nickel are covered by ASTM specification B456 (89). A fourth nickel deposit has shown improved protection by the effects it has on subsequent chromium deposits. Highly stressed, these nickel strikes have been used to aid in producing microcracked chromium. 

Tin—Nickel. AHoy deposits having 65% fin have been commercially plated siace about 1951 (135). The 65% fin alloy exhibits good resistance to chemical attack, staining, and atmospheric corrosion, especially when plated copper or bron2e undercoats are used. This alloy has a low coefficient of friction. Deposits are solderable, hard (650—710 HV ), act as etch resists, and find use ia pfinted circuit boards, watch parts, and as a substitute for chromium ia some apphcafions. The rose-pink color of 65% fin is attractive. In marine exposure, tin—nickel is about equal to nickel—chromium deposits, but has been found to be superior ia some iadustfial exposure sites. Chromium topcoats iacrease the protection further. Tia-nickel deposits are bfitde and difficult to strip from steel. Temperature of deposits should be kept below 300°C. 

Ni-30 Cu Alloy 400 also undergo fogging, but alloys containing 15% Cr or more do not exhibit this phenomenon. Fogging is prevented by a very thin film of chromium deposited on the surface—a fact which forms the basis for the bright appearance of decorative chromium-nickel plate (see Sections 13.7 and 13.8). 

Ductile and easily buffed chromium deposits having satisfactory corrosion resistance have been produced thus 0.005 mm-thick chromium deposits applied to steel by chemical deposition or by eiectrodeposition gave simiiar results when subjected to a salt-spray test . 

A thickness of at least 0-8 xm is normally needed to ensure that the required crack pattern is formed all over a shaped part. Such microcracked chromium coatings have a slightly lower lustre than the thinner conventional chromium deposits and take longer to deposit. The improved resistance to... 

Engineering electrodeposits Engineering electrodeposits are used to give improved properties on new components, or to replace metal lost by wear, corrosion or mis-machining, or as an undercoat for thick chromium deposits. 

Using pulse plating techniques with a duty cycle of 50%, it is also possible to produce crack-free chromium deposits from a sulphate- or silicofluoride-catalysed solution with a hardness similar to deposits obtained by direct currenf . A high frequency (2 000-3 000 Hz) is required to give the hardest deposits at a current density of 40 A/dm and a temperature of 54°C. It is important to avoid conditions that will co-deposit hydrides. 

A crack count of 30-80 cracks/mm is desirable to maintain good corrosion resistance. Crack counts of less than 30 cracks/mm should be avoided, since they can penetrate into the nickel layer as a result of mechanical stress, whilst large cracks may also have a notch effect. Measurements made on chromium deposits from baths which produce microcracked coatings indicate that the stress decreases with time from the appearance of the first cracks . It is more difficult to produce the required microcracked pattern on matt or semi-bright nickel than on fully bright deposits. The crack network does not form very well in low-current-density areas, so that the auxiliary anodes may be necessary. 

Corrosion tests have shown that a system based on copper, double nickel and microcracked chromium gives good corrosion resistance, although automobile parts plated with microcracked chromium are not as easy to clean as those plated with crack-free chromium deposit. 

The use of a chromium deposit with a fine porosity pattern of 15 000 to 45 000 pores per square centimetre in the usual thickness results in a sharp... 

A further development is the use of a combined chromium-nickel-chromium or nickel-chromium-nickel-chromium deposit on steel- or zinc-base alloy articlesAn advantage of this system is that the first chromium layer need not be plated within the bright range of the chromium bath, so that plating can be carried out under conditions giving deposits of maximum corrosion resistance such conditions do not coincide with those under which fully bright chromium plate is obtained. 

Knapp reports that a chromium deposit of 0 000 25 mm from the usual type of chromium bath, followed by 0-013 mm nickel and a further 0-00025 mm of chromium gave protection equal to that of a nickel coating of double the thickness applied in the form of normal nickel and chromium plate. 

In the many reports on photoelectron spectroscopy, studies on the interface formation between PPVs and metals, focus mainly on the two most commonly used top electrode metals in polymer light emitting device structures, namely aluminum [55-62] and calcium [62-67]. Other metals studied include chromium [55, 68], gold [69], nickel [69], sodium [70, 71], and rubidium [72], For the cases of nickel, gold, and chromium deposited on top of the polymer surfaces, interactions with the polymers are reported [55, 68]. In the case of the interface between PPV on top of metallic chromium, however, no interaction with the polymer was detected [55]. The results concerning the interaction between chromium and PPV indicates two different effects, namely the polymer-on-metal versus the metal-on-polymer interface formation. Next, the PPV interface formation with aluminum and calcium will be discussed in more detail. 

Anantha, N. G., et ah, Chromium Deposition from Dicumene Chromium to Form Metal Semiconductor Devices, J. Electrochem. Soc., 118(1) 163-165 (1971)... 

Mandich, N.V., Chemistry and theory of chromium deposition—Part I Chemistry, Plating Surf. Finish., 84, 5, 1997. 

Thermodynamically, chromium deposition can occur by electrochemical reduction or chemical dissociation of high valent chromium species. Thus, the focus of the debate on the deposition of chromium during the 02 reduction on LSM based... 

Something of the complexity of factors affecting the efficiency of metal deposition—the competing reaction always being H co-deposition—can be seen in Fig. 7.171 where there is a maximum in chromium deposition efficiency at about 200 g... 

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