Steel will rust only if water and oxygen are both present and the rusting process is greatly accelerated by pollutants in the atmosphere, such as sulfur dioxide from the burning of oil, coal, or gas and chlorides from de-icing salts or marine atmospheres. Corrosion and rust prevention is essential for metals that used in environments where water, high humidity, mist and salt are factors. At Surface Technology we have a wide selection of anti-corrosion coatings that overcome these challenges.
“Paint” is the most commonly used material to protect steel and it can cover a wide range of materials with different properties. The application of paint is very easy and have no limitation on the size of steelwork that can be treated.
For steel equipments the paint film thickness is important for lasting protection so best results are obtained when coatings are applied in heated enclosed workshops; units for bridge and building structures can be coated in this manner before erection and additional coats may be applied on site, if required.
It must be recognized that paint manufacturers compete with each other to offer their products under their own brand. It is difficult for the user to be certain what type comparisons are valid. Many standards and guidance documents have gone a long way towards explaining and clarifying the purpose of the constituents of paints and in specifying minimum composition guidelines. More information on these constituents relevant to protective coatings for structural steel is given in the following sections.
3-layer anti-corrosion coating system:
Protective paint systems consist of primer, undercoat and finish coat.
The primer is applied onto the cleaned steel surface. Its purpose is to wet the surface and to provide good adhesion for subsequently applied coats. Primers that used to coat the steel surfaces, are also usually required to provide corrosion inhibition. Primers pigmented with metallic elements anodic to steel. When a break in the coating exposes the steel substrate, the anodic metal corrodes sacrificially in preference to the steel. This effectively stifles steel corrosion and under-rusting of the primer until the anodic metal is exhausted. Zinc-rich primers are the most commonly used.
The undercoats are applied to ‘build’ the total film thickness of the system. Generally, we have to know that the thicker coating the longer the life!
This may involve the application of several coats. Undercoats are specially designed to enhance the main protection and, when highly pigmented, decrease permeability to oxygen, water and wet atmosphere. The incorporation of laminar pigments, such as micaceous iron oxide, reduces or delays moisture penetration in humid atmospheres and improves tensile strength. Undercoats must remain compatible with finishing coats when there are unavoidable delays in applying them. The most common coatings that are used for this layer are High-Build Epoxy coating and MIO epoxy coating.
The topcoats provide the required appearance and surface resistance of the coating system. Polyurethane paint is the best choice to apply as a topcoat. Depending on the conditions of exposure, it can provide the first line of defence against weather and sunlight, open exposure, highly polluted atmospheres in chemical plant, impact and abrasion at floor or road level, and bacteria and fungi (in food factories and farms).