Metal plating on the surface of the steel plate is equivalent to the steel plate wearing protective clothing. The basic composition of steel plate is iron, coating on steel base!There are two ways to protect it.
The first case. Some metal coatings, such as galvanized coatings (whether hot galvanizing or electrogalvanizing), are anodic to the steel substrate, and the protective effect of the coating on the steel substrate is divided into two stages.
In the first stage, when the coating remains intact, the coating ACTS as an "isolation and protection", i.e. the corrosion of the steel base and the outside world.The corrosion medium is separated to protect the steel base from corrosion. At this time, the surface of the galvanized layer is exposed to impure water and other corrosive media, and its surface is corroded, producing non-soluble compounds such as zinc hydroxide (Zn (OH) 2, zinc oxide (ZnO) and zinc carbonate (ZnCo3). These corrosion products are precipitated in the form of precipitation and merged into a dense film, which has a retarding and retarding effect on the further development of corrosion. Therefore, the corrosion loss of galvanized layer is relatively slow. As long as the galvanized layer is not corroded through, it will continue to play a role of separation protection on the steel base. When engraved zinc is local decay penetration inducement, here lost zone off protection.
Happened in the second stage, when the coating after local damage, whether caused by corrosion penetration damage or mechanical scratch injury, because of the zinc electrode potential is lower than iron, when the corrosive medium, such as not pure water cover, forming micro batteries, zinc coating to become· anode, steel base becomes cathode, anode produces oxidation reaction, zinc corrosion; Reduction of cathode occurs, iron corrosion is subjected to!Inhibition. This is the "sacrificial anode protection" of zinc. As an anode, zinc sacrifices itself to protect the cathode iron from corrosion by its own corrosion loss.