Chromium steel plated with

Fig. 12.1.8. Cyclone wear plate consisting of a mild steel plate with an abrasion resistant metal overlay such as chromium carbide. Such a plate is typical of that which may be installed in the cyclone s inlet target zone (just downstream, and in the line of sight, of the inlet duct)...

Stainless steel accessories in cars are usually plated with chromium to give them a shiny surface and to prevent rusting. When 5.00 g of chromium at 23.00°C absorbs 62.5 J of heat, the temperature increases to 50.8°C. What is the specific heat of chromium ... 

Another example of where ultrasound influences the discharge rate of chromium is in the chromium electroplating of steel plates. When cold-rolled steel plate is elecUo-lytically chromated from a silent aqueous solution containing chromates and dichromates, a chrome coating of 13 mg m is obtained after 1 s, whilst in the presence of ultrasound a coating of 45 mg m is obtained [25] in the same time. The product is also accompanied with an increase in the brightness of the metal. Similar results have been found by other workers [26]. 

Closely related to electrorefining is electroplating, the coating of one metal on the surface of another using electrolysis. For example, steel automobile bumpers are plated with chromium to protect them from corrosion, and silver-plating is commonly used to make items of fine table service. The object to be plated is carefully cleaned and then set up as the cathode of an electrolytic cell that contains a solution of ions of the metal to be deposited. 

Some firearms are plated with anodized aluminum, nickel, or chromium which gives durability and good looks, and some are made from stainless steel which is much less prone to rust than conventional steel. Electroless nickel coating is an alloy coating of 88% to 96% nickel and 4% to 12% phosphorus, which is produced by chemical (not electrical) reduction of nickel on to the metal surface. 

Ferrochrome, a high-chromium alloy with iron, is made by reducing chromite with carbon in the electric furnace. It is used for making alloy steels. The alloys of chromium are very important, especially the alloy steels. The chromium steels are very hard, tough, and. strong. They are used for armor plate, projectiles, safes, etc. Ordinary stainless steel contains 14 to 18% chromium, and usually 8% nickel. 

Protection against corrosion. Galvanized objects are steel coated with zinc. (Right) Steel is plated with chromium for appearance as well as protection against corrosion. 

Exposure to air and moisture causes the iron in steel to rust. Plating a steel surface with chromium protects it from corrosion. The appearance of some consumer products is enhanced by plating with bright, shiny chromium. 

When chromium is alloyed with iron, tough, hard steels or steels that are corrosion-resistant are formed. Chromium is also alloyed with other transition metals to produce structural alloys for use in jet engines that must withstand high temperatures. A self-protective metal, chromium is often plated onto other materials to protect them from corrosion. 

Mold design. PTFE resins are molded in molds similar to those utilized for thermosetting resins or metal powders. A complete mold consists of a cylindrical or rectangular die and upper and lower end plates and a mandrel for annular parts. These parts are normally made of tool steel to allow machining, and plated with chromium or nickel to protect them from corrosion. Occasionally, the end plates are made of brass or plastics such as nylon. A small diametrical clearance is designed in the end plates to allow easy assembly and air escape. 

Injection molds are machined from a variety of tool steels and then hardened or in some cases plated with chromium, nickel, or proprietary materials. Large molds use prehardened tool steels because they cannot be hardened after machining. 124 Stainless steel is employed for some smaller molds, particularly those used for optical and medical parts and for corrosion resistance. Since they provide better heat transfer and, thus, shorter molding cycles, materials such as beryllium copper are used as inserts in critical areas. Injection molds are usually cooled or heated with water, although oil or electric heater cartridges are employed for high-mold temperatures. Mold-temperature controllers pump water into the manifolds and then into cooling fine machine into the molds. 

Preliminary experiments on coupons which had been plated with a chromium layer and subsequently passivated by hot moist air treatment showed significantly smaller radionuclide deposition when inserted into a PWR steam generator for the duration of a fuel cycle (Asay et al, 1991 b). Other experiments with stainless steel coupons which were exposed over several fuel cycles in the hot as well as in the cold legs of a steam generator channel head showed that chromium-plated coupons exhibited very low levels of activity uptake, as Co and °Co were in all cases lower by more than an order of magnitude than on as-received or on electropolished/passivated coupons. The addition of a passivation step after the chromium-plating treatment had a detrimental effect by increasing the uptake of activity by up to a factor of two (Pick et al., 1992). 

Nickel. Nickel is a hard, malleable, and ductile metal that is highly resistant to corrosion. Like chromium, it is used to make stainless steel. Alloyed with copper, it is used for ship propellers and chemical industry plumhing. Other uses include rechargeable batteries, coins e, foundry products, plating, burglar-proof vaults, armor plates, and crucibles. 

Chromium plated parts on automobiles consist of steel substrates with an intermediate layer of nickel, or in some cases, layered deposits of copper and nickel. The thin chromium deposit provides bright appearance and stain free surface while the nickel layer provides the corrosion protection to the steel substrate. With this system it is essential that the nickel cover the steel substrate completely because the iron will be the anode and nickel the cathode. Any breaks or pores in the coating will result in the condition shown in Figure 16.3. This figure illustrates the reason for the corrosion of chrome trim on automobiles after World War II. 

Metal-plated articles are common in our society. Jewelry and tableware are often plated with silver. Gold is plated onto jewelry and electrical contacts. Copper is plated onto many objects for decorative purposes (Figure 21-5). Automobiles formerly had steel bumpers plated with thin films of chromium. A chrome bumper required approximately 3 seconds of electroplating to produce a smooth, shiny surface only 0.0002 mm thick. When the metal atoms are deposited too rapidly, they are not able to form extended lattices. Rapid plating of metal results in rough, grainy, black surfaces. Slower plating produces smooth surfaces. Tin cans are steel cans plated electrolytically with tin these are sometimes replaced by cans plated in a fraction of a second with an extremely thin chromium film. 

Chromium is a metal that is spontaneously passivated. That means that a thin, dense and transparent oxide layer with good adherence is formed on its surface in contact with air. This is the reason why chromium-plated components preserve their brightness even in markedly corrosive environments. Chromium steels share this property when the chromium content is more than 12% (see Figure 24.5). Because of that, stainless steels are steels with a content of more than 12% chromium. 

Crevice corrosion also develops under deposits or fouling, e.g. under the basalt plates of barnacles. The microbiological slime often observed on the surface of stainless steels can shift the free corrosion potential in the positive direction so that the pitting potential of the steels is exceeded [108]. Pipes made of a chromium steel with 26% Cr and 1% Mo failed within a brief period with biological deposits, but remained resistant in deaerated and sterile seawater in desalination plants [109]. 

---