Metallic coatings such as Zn, Al, Cu, Ni, Cr, Ag, Au, etc. are used on carbon steel and other metallic and non-metallic substrates for corrosion control, decorative or aesthetic purposes. In the context of carbon steel, Zn and Al coatings are anodic in nature while Ni, Cr, Ag, and some other coatings are cathodic in nature. Metallic coatings on steel can be applied by using an electrical process (e.g. electroplating of Ni or Cr) or a non-electrical process (e.g. application of Zn on steel by hot dip galvanizing or the application of Ni coating on steel through chemical deposition, also known as Electroless Ni plating).
Electroless Ni (EN) plating produces a very thin, uniform, adherent hard coating on the surface of carbon steel components even with intricate geometries, and on the internals of valves and piping. As such, the use of EN coatings as a corrosion control option in mildly corrosive environments is quite popular and prevalent. However, in very corrosive environments, the use of EN coatings or other cathodic coatings on steel is very risky since any breach of the coating will lead to accelerated localized (galvanic) attack of the substrate. A case study involving the failure of EN coated carbon steel fittings installed on the composite piping system of an oil and gas production facility is presented which will show how severe and rapid this localized attack can be, resulting in the through wall corrosion failure of multiple fittings.
Metallic coatings are frequently used on carbon steel and other metallic and non-metallic substrates for corrosion control, decorative or aesthetic purposes. The coating metal commonly used includes Zn, Al, Cu, Ni, Cr, Ag, Au, etc., singly or sometime in combination. When one of these metal coatings is applied on a metallic substrate, it is very important to consider the relative positions of the coating material and the substrate in the galvanic or EMF series. When carbon steel is the substrate, Zn and Al coatings are anodic in nature since their corrosion potentials are more active (negative) than carbon steel while Cu, Ni, Cr, Au, and some other coatings are cathodic in nature since their potentials are more noble (positive) than carbon steel.
Anodic and cathodic coatings behave differently in corrosive environments. Thus, when there is a breach in an anodic coating on a carbon steel substrate, the coating protects the carbon steel from corrosion damage. However, in the case of a cathodic coating, the carbon steel substrate suffers accelerated attack due to galvanic action. Figure 1 (a and b) illustrates what happens with anodic and cathodic type of coatings in corrosive environments.