The final stage of production incorporates deep cleaning and coating of the metal. Carbon steel products are usually coated with powder polymers for protection from corrosion and better aesthetic appearance. Stainless steel products are usually cleaned and passivated (anti-corrosion coating). Sometimes it is necessary to apply additional polymer coatings to protect the products from highly acidic media.
The main additive in stainless steel is chromium (Cr). Reacting with oxygen, a thin (130 angstroms), but very strong layer of chromium oxide (CrO) is formed on the surface of the steel. This natural chromium oxide coating is the key component that makes this stainless steel resistive to corrosion. The greater the percentage of chromium alloy is, the more resistant the steel is to oxidants.
The most crucial aspect is the uniform concentration of chromium in the surface layer of the metal, which affects the formation of a uniform protective layer of chromium oxide.
During the manufacture of stainless steel products, their surface is constantly scratched by the machinery. The resulting scratches contain micro-particles of iron (Fe). The naturally formed protective coating (under exposure to oxygen) is formed non-uniformly due to the iron particles obstructing the way. After a while, signs of corrosion can be seen on the surface of the metal. Even in the inert gas media, if the metal is exposed to high temperatures, areas around the weld zone oxidize. This prevents the formation of a high-quality protective anticorrosive layer. To avoid this minor, but important flaw, we thoroughly clean and polish our products before applying coatings.
The products are chemically etched with an acid solution that removes organic matter, iron particles and various oxides. Besides that, as the iron particles react with the acid solution, salts are formed on the surface of the metal. This enriches the surface layer of stainless steel with alloying additives such as chromium, nickel and others (depends on the brand of stainless steel).
After neutralization of the etching solution, we passivate the stainless steel, which results in the formation of a strong, uniform chromium oxide film. The film allows stainless steel to resist corrosion easily even in aggressive chemical media
The photo above displays the matte-silver surface of a chemically processed product.
Powder coating of carbon steel
Powder coatings are solid dispersion solutions that contain special film-forming resins, hardeners, pigments, fillers and other additives.
After thermal processing, such coatings form a strong polymer film resistant to:
• water and salt solutions;
• high temperatures (up to 600 ºC / 1110 ºF);
Powder coatings include a large range of colors and various textures. Some of them conduct electricity.
The photo above depicts a finished carbon steel product with powder coating (RAL 7035). The surface is smooth and semi-matte. It is highly resistant to abrasion.
Polymer coating of stainless steel (fluorine polymer)
Teflon-based polymer coatings are used to protect the surface of stainless steel from aggressive oxidants.
The photo above shows two welded AISI-304 stainless steel plates, each 1.5 mm thick, covered with fluoride polymer produced by Solvey company – Halar ECTFE. The thickness of the coating is 300 microns. The coating is an excellent dielectric that resists concentrated acids and has a working temperature range from -80 ºС to +150 ºС.
Paint and varnish coating of carbon steel
Besides powder coatings, carbon steel can also be covered with conventional enamels that protect the surface from aggressive oxidants just as well as the powder coatings do. The painting process consists of two stages. First, the metal is primed, then it is painted into the desired color.
The photo above depicts an industrial cyclone in our hot air drying chamber. The final coating is applied and the product is drying.
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