Evaluation of Metallic coatings as related to their corrosion Resistance. A Review

An important difference between metallic and nonmetallic coatings is that the former can significantly affect the corrosion behaviour of the coated metal at sites where the coating is incomplete or damaged whereas generally organic coatings have no such effect. Metallic coatings afect the behaviour of the basis metal because an electrolytic cell is established in which either the basis or the coating metal will act as an anode and corrode at an enhanced rate. When the coating is the anode the basis metal will form the cathode and usually will corrode at a lower rate than in the complete absence of the coating. However, it is well known that simple considerations of anodic and cathodic behaviour do not explain all the phenomena involved in the corrosion and protection of metal coated metals. Other factors of importance include the formation of insoluble corrosion products and the influence of time and temperature on this process, the electrochemical polarisation behaviour of the metals, and the nature of the corrosive or inhibitive environment. The uses and properties of some types of metal coatings will be described in relation to these factors and the implication of these to test methods discussed. One particular system viz. galvanised steel in water containing a corrosion inhibitor will be described in more detail in terms of electrochemical behaviour.