Steel is one of the strongest of construction materials and is able to withstand many extreme environmental conditions. However, this depends on an efficient and effective protection against corrosion.
Iron is the principal component of all steel grades. Without adequate corrosion protection, it will react on exposure to oxygen in the atmosphere, creating pockets of iron oxides. This process begins as small localized dots of oxidation but may quickly spread to consume the entire piece of steel. Rust can produce six times the volume of iron oxide compared with the original volume of steel material.
Corrosion may be controlled by coating the steel with materials that form a solid chemical bond on the metal surface and eliminate any elements or reactants that can lead to corrosion. These are:
• Pigments that add colour, reflect light and contribute to steel strength
• Resins or binders, also called non-volatile materials, that can hold coatings onto to the metal surface
• Solvents, or volatile materials, that reduce the coating’s viscosity and allow it to flow on the surface
• Additives to correct any defects, inhibit foam formation and enhance colour.
These components combine to form different types of coatings that steel suppliers can choose depending on the steel’s ultimate application:
• Zinc is probably the most successful and commonly used steel coating. This can be made from powdered zinc metal mixed with inorganic chemicals such as a silicate solution. Powdered zinc can also be mixed with organic chemicals such as urethanes, epoxies,vinyls and chlorinated rubbers
• Epoxies are commonly used for structural steel coatings and storage tank linings. They generally are combined with polyamine or polyamide hardeners
• Acrylics that provide colour retention are often used for interior environments that are ore benign and essentially non-corrosive
• Polyurethanes for hard outer coatings
Galvanising is the lowest cost method of coating steel, usually with zinc. The coating binds metallurgically to the steel surface and provides a tough protection to the steel whether in transit or in place. The galvanizing process may be mechanical, electrical or hot dip galvanizing.
The hot dip process involves cleaning the steel first in acid and then immersing it in molten zinc at 455 degrees C. The process provides a complete protection to any steel piece including recessed parts and crevices.
Duplex coating for steel
Sometimes the galvanizing and painting process can be combined into a duplex, or dual system. First the steel is galvanized and then it is painted with a coating. The crucial factors are that the galvanized surface should be clean, the paint should not react chemically with the zinc but the two coatings should adhere together.
One of the advantages of duplex coatings is that the combined protection is better than the sum of its parts. The galvanizing protects the metal even if the paint begins to flake under extreme heat or cold, but at the same time it also extends the operational life of the outer paint layer.