The protective effect of metal coatings
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, the protective effect of coating on steel base should be divided into two cases.
The first case. Some metal coatings, such as galvanized coatings (whether hot or electric), are anodized to the steel base!The protective effect of coating on steel base is divided into two stages.In the first stage, when the coating remains intact, the coating ACTS as an "isolation and protection", that is, the steel base is separated from the outside corrosive medium 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 the surface is corroded, resulting in the insoluble transformation of zinc hydroxide (Zn (0H) 2), zinc oxide (Zn0), zinc carbonate (ZnCo), etc.Compound.
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 act as an isolating protection to the steel base. When the local corrosion of the galvanized layer penetrates, there is no isolation 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, micro battery formation, galvanized layer become: the anode, the steel base as the cathode, anode oxidation reaction, to produce zinc corrosion;
The corrosion of iron was inhibited by the reduction of cathode. This is the "sacrificial anode protection" of zinc. Zinc as an anode sacrifices itself to protect the iron as a cathode from corrosion by its own corrosion loss.The experimental results show that the electrochemical protective effect of the galvanized layer on the non-coated steel surface is attenuated with distance. The area adjacent to the galvanized layer is a complete protection zone, where steel is not corroded. In addition to the complete protection zone, it is a part of the protection zone, where steel corrosion does not occur seriously. Outside some protected areas, there is no protected area. The steel in this area is seriously corroded to the same extent as the bare steel. The results of the atmospheric exposure test using "zinc/steel couple sample" abroad show that the width of the complete protection zone is ~1mm, and the width of some protection zones is ~3.5 m. That is to say, the galvanized layer has a total protective distance of ~1m and a partial protective distance of ~4.5mm.So, whether the galvanized layer played a sacrificial anode protection, not depends on whether the area of the base steel exposed to cannot be dominated by the electrolyte (water film, for example), but depends on whether the area of the base steel exposed beyond the size of "reserve" completely, or whether beyond "thoroughly reserve + part of the reserve" size. Beyond that, even if there is a water film completely covered, the steel base also occurs, corrosion.The second case. Some metal coating, such as tin plating layer (both hot tin and tin plating), a cathode for steel base, sex, with isolation protective coating on the steel base only, not with a sacrificial anode protection. If the tin coating is partially damaged.When the electrolyte is covered, the tin becomes the cathode and the iron becomes the anode, which in turn will aggravate the corrosion perforation of the steel substrate. So, this kind of coating is only before its corrosion penetration, rely on isolation effect to protect the steel base, regardless of the anode sex coating for the steel base, or Yin polarity coating for the steel base, its working life depends mainly on coating, thickness (coating weight per unit area) and the environment atmosphere. The thicker the coating, the heavier the plating, the longer its service life.