Chromium protective coating

Corrosion resistance The corrosion resistance of a copper deposit varies with the conditions under which it is deposited and may be influenced by co-deposited addition agents (see, for example, Raub ). Copper is, however, plated as a protective coating only in specialised applications, and the chief interest lies in its behaviour as an undercoating for nickel-chromium on steel and on zinc-base alloy. Its value for this purpose has long been a controversial issue. 

Interest in this metal comes from its remarkable inertness to atmospheric corrosion. Also, it is very hard and thus it forms an ideal protective coating. On the basis of its E° (1.18 volts higher than hydrogen) we expect chromium to be quite reactive in fact, it is vigorously reactive with... 

Main uses of the metal. Chromium can serve as a protective coating of alloys to prevent oxidation. It is an important component of several ferrous alloys, mainly stainless steels. 

New products are being developed to replace heavy metals in water-treatment and protective-coatings applications. The products have become available from suppliers of protective coatings under pressure from the U.S. EPA to remake paints and primers minus the solvents that contain volatile organic componnds. Mannfacturers have come np with a variety of formnlations that do not pose the environmental and health risks associated with chromium. These formulations are more expensive than chromium-based solutions, however, and are likely to remain so for the immediate fnture (Hairston, 1996b). 

The most important phosphate-containing pigments are zinc phosphate [5.67]-[5.69], [5.71] basic zinc phosphate [5.70], [5.71] chromium phosphate [5.72] aluminum triphosphate [5.73] barium phosphate [5.53] aluminum zinc phosphate [5.68] and zinc/iron phosphate [5.71], Phosphate ions form protective coatings of basic iron(III) phosphate on an iron surface. Their composition is described in [5.64]. 

Acidic soils are highly corrosive. Sulfur is a corrosive agent in automalive fuels and in the atmosphere (SO ) as well, and is frequently mentioned in connection with so-called acid rains. Sodium chloride in the air at locations near Ihe sea is strongly corrosive, especially at temperatures above 70 F (21.1 C). Copper, nickel, chromium, and zinc are among the more corrosion-resistant metals and are widely used as protective coatings for other metals. 

PROTECTIVE COATING. A film or thin layer of metal glass of paint applied to a substrate primarily to inhibit corrosion, and secondarily for decorative purposes, Metals such as nickel, chromium, copper, and tin are electrodeposited on the base metal paints may be sprayed or brushed on. Vitreous enamel coatings are also used these require baking. Zinc coating are applied by continuous bath process in which a strip of ferrous metal is passed through molten zinc. 

The standard oxidation potential for the reaction Cr(s)— Cr3+(aq) + 3e is 0.74 V. Despite the large, positive oxidation potential, chromium is used as a protective coating on steel automobile bumpers. Why doesn t the chromium corrode ... 

In addition to its use in making steels, chromium is widely used to electroplate metallic objects with an attractive, protective coating (Section 18.12). Chromium is hard and lustrous, takes a high polish, and resists corrosion because an invisible, microscopic film of chromium(III) oxide shields the surface from further oxidation. 

Electroplating is the process involving electrolysis to plate, or coat, one metal with another or a plastic with a metal. Often the purpose of electroplating is to give a protective coating to the metal beneath. For example, bath taps are chromium plated to prevent corrosion, and at the same time are given a shiny, more attractive finish (Figure 5.19). 

Chromium is a white, hard, lustrous, and brittle metal (mp 1903 10°C). It is extremely resistant to ordinary corrosive agents, which accounts for its extensive use as an electroplated protective coating. The metal dissolves fairly readily in nonoxidizing mineral acids, for example, hydrochloric and sulfuric acids, but not in cold aqua regia or nitric acid, either concentrated or dilute. The last two reagents passivate the metal in a manner that is not well understood. The electrode potentials of the metal are... 

The metals used to manufacture cans, ends and closures are either steel (tin plated or chromium passivated) or aluminium. In most cases they are coated on the food contact surface with a resinous or polymeric protective coating to avoid interaction between the foodstuff and the metal. However, there is a well defined sector of the tinplate food packaging market where no protective organic coating is needed or used. 

Interconnects are used to electrically connect adjacent cells and to fnnction as gas separators in cell stacks. High-temperatnre corrosion of interconnects is a significant issne in the development of SOFCs. Ferritic stainless steels have many of the desired properties for interconnects bnt experience stability issnes in both the anode and cathode environments. The dnal environments canse an anomalons oxidation for which a mechanistic understanding has yet to be determined. Protective coatings from non-chromium-containing condnctive oxides snch as (Mn,Co)304 spinels look promising bnt need further development. 

Uses.—Metallic chromium is used industrially in the manufacture of certain varieties of steel. It enters into the composition also of many non-ferrous alloys (see p. 17), the importance of which is increasing. It may be used as a protective coating for metals, either in the form of chromium plate (see p. 10), or by causing the metal to diffuse into other metals at temperatures below their melting-points ... 

Use Alloying and plating element on metal and plastic substrates for corrosion resistance, chromium-containing and stainless steels, protective coating for automotive and equipment accessories, nuclear and high-temperature research, constituent of inorganic pigments. 

Decorative protective coatings are primarily used for adding an attractive appearance to some protective qualities. Metals in this category include copper (Cu), nickle (Ni), chromium (Cr), zinc (Zn), and tin (Sn). 

Areas from which protective coatings have been removed physically or by corrosion (e g., chromium, copper, and nickel plates from steel)... 

Nickel metal is malleable, ductile, and a fairly good conductor of electricity and heat. Its most common use is in stainless steels, where it may be combined with various other metals (such as iron, chromium, chromium, copper, etc.) to form alloys that are highly resistant to corrosion. Nickel is also used to make coins (U.S. five-cent pieces contain 25 percent nickel), batteries, magnets, and jewelry to protectively coat other metals and to color glass and ceramics green, see also Coordination Compounds Inorganic Chemistry. 

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