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Why Use Coated Sheets?

Production Methods Applications of USS Coated Steel Sheets

Corrosion of low carbon steel sheet takes many forms.  It can be uniform, spreading over the entire exposed surface of a metal sheet; or it can be localized, occurring where special conditions create a corrosive environment.  It can occur microscopically small, or, as is more often the case for low carbon steel sheet, it occurs in a form that is macroscopic or clearly visible.

Uniform corrosion of steel sheet is usually readily visible, and as the name implies, the corrosion occurs on the entire surface. In addition to uniform corrosion, there are several other types of corrosion that are clearly visible.  Examples include  crevice corrosion that occurs within fissures and narrow openings where water may be trapped in such locations at gaskets, lap joints, bolts, rivets, weldments or scratches.  Poultice corrosion is common on vehicles, occurring when dirt and other debris accumulate inside wheelwells and other under-vehicle parts.  The accumulation of debris retains road salt and water and acts to keep the metal surface damp, promoting corrosive chemical action.  Erosive corrosion occurs when an abrasive stream of water strikes a metallic surface.  Still another form of corrosion, known as fretting, occurs when mating surfaces are subjected to an electrolytic solution (often a thin film of moisture) in combination with vibration.

Typically, for items manufactured using steel sheet, the causes of corrosion are clearly understood. For instance, the bottom skirts and panels of clothes washers may be splashed with water containing detergents and bleaches. The presence of detergents and bleaches in the water may accelerate the corrosion reaction substantially. Metal building roofing may be subjected to acid rain which contains sulfurous compounds that accelerate corrosion.  Of course, the very familiar corrosive reaction on vehicles in the presence of salt-containing road splash is well documented.

It is these types of environments that have led to the common use of coated steel sheet products. The understanding of corrosion and the documented benefits associated with the use of metallic coatings on steel have led us to where we are today. That is, the market demand for metallic-coated steel sheet products is increasing dramatically as the value of using coated steel sheet becomes more and more apparent.

Technology has advanced sufficiently today so that the influence of various contaminants in the atmosphere can be predicted. For instance, the corrosion-producing effects of the atmosphere vary from locality to locality according  to climatic conditions of temperature, humidity, amount of rainfall and the degree and nature of air pollution present.  Corrosion is severe-to-moderate along the coasts due to seawater spray carried inland by winds and storms. The application of a coated steel sheet product in any of these types of environments can thus be engineered to provide the best combination of coating type, coating thickness, etc. to achieve the desired life of the product.

U. S. Steel's Coated Steel Sheet, when properly selected and applied, can combat corrosion successfully in many applications.

Coatings protect steel sheet from corrosive attack by acting as a barrier to exclude air, water and other corrosion promoters from contact with the steel substrate.  All the metallic coatings on U. S. Steel Coated Steel Sheet products,  zinc (galvanized), galvannealed (zinc-iron alloy), zinc-aluminum (GALVALUME®), terne (lead-tin alloy coating) and paints provide excellent barrier protection.

In addition to barrier protection, zinc and zinc alloys have the ability to react at scratches and other damage through an electrochemical (galvanic) action between steel and zinc.  This galvanic action makes it possible for the zinc to protect breaks in the coating and prevent further damage.

Coating barrier protection

In this galvanic process, zinc actually sacrifices itself to protect and maintain the structural integrity of the steel.  This well-established behavior of a zinc coating on steel is known as sacrificial protection.  This sacrificial or galvanic protection is provided by zinc, zinc-iron and zinc-aluminum coatings.