As a rule, coatings have two primary functions: decoration and protection which are of considerable economic importance. Functional coatings may be applied to change the surface properties of the substrate, such as adhesion, wetability, corrosion resistance, or wear resistance. In the steel industry, paint coating or powdering coating mainly protect weld steel pipe from corrosion, as well as maintain a pretty appearance of pipe.
Paints and lacquers are two major kinds of substance used for coatings in use. Technically, paint is the most commonly used material to protect steel in a mill. Paint systems for steel structures have developed over the years to comply with industrial environmental legislation and in response to demands from bridge and building owners for improved durability performance. Each coating ‘layer’ in any protective system has a specific function, and the different types are applied in a particular sequence of primer followed by intermediate / build coats in the shop, and finally the finish or top coat either in the shop or on site. Powder coating also is widely used for cold rolled steel tube with dry powder paint to a metal part for surface protection. In normal wet paint application the coating is suspended in a liquid carrier that evaporates into the atmosphere leaving the coating protecting the surface. A powder coated part is cleaned and the powder coating is electrostatically charged and sprayed onto the object to be coated. The object is then placed in an oven where the powder coating particles melt to form a continuous film.
Without a protective coating, steel or iron is easy to produce rust -- a process known as corrosion. To prevent this, steel pipe manufacturers galvanize steel pipes by coating them with a thick layer of zinc. They either dip the pipes in a vat of the molten metal or use electroplating techniques. Before shipping the pipes, manufacturers often coat the galvanized metal with oil to retard reaction of zinc with the atmosphere. When this oil coating wears off, the reaction of zinc with oxygen produces a fine whitish film that changes the color of the metal from gray to an even less appealing whitish-gray. When hot dipping galvanized steel pipe needs importing, this type of pipe usually has a passivator film that protects the metal from corrosion in the salt-water environment as the metal travels across seas or oceans on cargo ships.
Today, much progress has been made in the practice of using coating technology to offer corrosion protection to offshore structures, inner-hull tanks in fuel tankers, ship hull, underwater pipes, etc. New methods have been developed to repair and protect concrete and steel structures in coastal and offshore waters, such as the all-polymer encapsulation technique to repair and protect structures in the splash zone. Long-term structural or mechanical requirements for a particular application can be assured through corrosion protection, through either coatings or a combination of cathodic protection and coatings.
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Post time: May-03-2018
