Metal products will inevitably corrode to varying degrees during use, and the corrosion of steel parts is called rust. All coatings can have a certain protective effect on the base metal, so that the base is not corroded. However, different coatings have different protective effects on the base metal . According to the nature of the protective effect, or according to the electrochemical relationship , the coating can be divided into cathodic coating and anodic coating.
We also know that the degree of activity of metals is different from a chemical point of view, and their activity is judged by whether they can replace hydrogen ions or other metal ions from acids. This order of activity is the electrochemical order of metals. It is also an important basis . Accordingly, we can respectively define the cathodic coating and the anodic coating as follows.
(1) Cathodic coating
The activity of the metal substrate is greater than that of the metal coating. Under the conditions of use , only when the coating completely covers the substrate can . If the coating is incomplete or has pores or damage, when corrosion occurs, the substrate will be corroded and damaged, but the coating will not corrode, and thus the substrate will corrode faster. This kind of coating is cathodic coating, such as copper plating on steel, baht plating, etc.
(2) Anodic coating
The activity of the coating metal is greater than that of the base metal. Under the conditions of use , when the coating meets a corrosive medium, the coating will corrode first to protect the substrate from corrosion, which has the characteristics of sacrificial coating to protect the substrate. This coating is an anodic coating, typically galvanized on steel.
Since the cathodic coating has a protective effect , anodic coating should be used as much as possible from the perspective of protection. If a cathodic coating is to be used, the thickness and porosity of the electroplating layer must meet the requirements of the user.
