Coated steel components

Metal spraying with molybdenum powder, wire or strip is used for coating steel components. The layers reduce friction and improve wear performance. 

Alloy 400 (NiCu30Fe, 2.4360) was used in three stages of the evaporator/crystallisa-tion unit for waste water from the absorption fluid of a flue gas desulphurisation plant at 364 K (91 °C) and a pH value of 8-10. A corrosion rate of 0.04 mm/a (1.57 mpy) was measured after an operating time of 4 years. The alloy was used because the previously fitted rubber-coated steel components required frequent repairs [74]. 

The modern motor car is made from steel, zinc or zinc alloy-coated steel and some plastic parts, all of which require painting. The main component is the body shell, made from the above metals, and this is coated in a continuous production process. A full finishing system with all four coatings is usually applied for maximum protection and a high quality appearance. 

The common surface treatments for ammo are plating, dipping coating. Small components of steel are usually plated with Zn or Cd. Various chromate dippings (such as "Cronak, "Irridite "Yellow ano-zinc ) may also be... 

Since commercial dispersions and bonded coatings became widely available, the use of powder-burnishing has probably declined, but applications to ball-bearings have been reported " , and a 1981 report described a cold-extrusion machine for steel components in which a burnished film of molybdenum disulphide was applied to the steel billets automatically prior to extrusion . This resulted in a reduction in extrusion loads and improved quality. 

The application of selected protective coatings enables iron and steel components to be used in many otherwise prohibitively aggressive enviromnents. In this way, the obvious advantages of cheap unalloyed ferrous metals (e.g., good mechanical properties, ease of fabrication, etc.) may be economically combined with the increased resistance to corrosion, heat, and/or wear endowed by such surface treatments. 

Casting aluminum around steel components has been used where properties such as high strength, corrosion resistance, good heat transfer, or good electrical conductivity are required of a fabricated article. The surface is first prepared by the formation of an iron-aluminum intermetallic onto which the aluminum can be cast. Good adhesion is obtained in this manner, but coating thickness is limited by the difference in expansion coefficients. 

Above 900°C, the life of aluminized steel is limited, although extended over that of the bare metal (e.g., at 900°C-980°C, the life is increased 20 times, at 980°C-1000 C, five times). Below 750°C, the life of aluminized steel components can be considered to be indefinite. Results obtained between 750°C and 900°C are conflicting, although the presence of silicon has been shown to be detrimental (Figure 10.15) at all temperatures [135]. Aluminum-coated steels have also been shown to be particularly resistant to sulfurous atmospheres [131]. In this respect, such coatings are particularly useful for the protection of heat-resisting grades of steel. 

This process is applied mainly to steel components which are sprayed or dipped in a phosphoric acid solution containing iron, zinc or manganese phosphate. The most common processes use zinc or manganese phosphate and are employed in the treatment of car bodies and domestic appliances before painting. Where long-term corrosion protection is required, the phosphate coating must be sealed with oil lacquer or paint... 

The problems associated with the oxidation of mild steel components of Magnox reactor interned, structures cure now widely known-Exposure of mild steel to carbon dioxide reactor coolant at elevated temperatures, around 360 C, produces a magnetite coating on cd.1 free surfaces, at the interfaces between bolted ass blies and even growing in the unfused regions between plates joined by unsealed welds. The mechanical effects caused by this oxidation include loss of clecurance between moving components cuid the failure of bolts euid welds. 

Additional corrosion may occur where the supply of oxygen at interfaces between joined components is limited. This is not bimetallic corrosion, but its effect at bimetallic joints can be as detrimental as true bimetallic corrosion. However, no crevice corrosion occurred in couples of zinc-aluminum alloys and polyethylene in 10-year atmosphere tests (Noranda, personal communication) nor around nylon bolt heads. Jointing compounds are useful in preventing crevice corrosion as well as bimetallic corrosion. Also, stressed parts of the surface tend to be anodic to unstressed parts, but this effect is not usually of practical significance with zinc and zinc-coated steel. 

The physical contact between the porous silicon and the various stainless steel components, in particular the rotor fingers, does present a route to contamination. While the porous silicon is relatively soft, trace metallic elements such as Fe, Cr, and Ni can be transferred from the stainless steel the levels can be low (e.g., < lO s ppm) but may be sufficient to cause concern depending on the ultimate application. Titanium- and tungsten-coated rotors are available that can minimize contamination. 

---