Protective effect of hot galvanizing on metal coating,
The surface of the steel plate is coated with a metal coating, equal to the steel plate wearing protective clothing, the basic component of steel plate is iron, coating on the protection of the steel base, to be divided into two cases
: in the first case, some metal plating layers, such as galvanizing (either hot-galvanizing or electrogalvanizing), are anodic to the steel base
Sections. The first stage, when the lock layer intact, coating play "isolation protection", which separates the corrosive medium of steel base with the outside world, protect the base steel from corrosion, at this point, the galvanized layer surface contact corrosion media such as pure water, its surface from corrosion, hydrogen generation zinc oxide, zinc (01),), zinc oxide (Zno), zinc carbonate (ZnCo.) insoluble, such as compound.These corrosion products are precipitated in the form of precipitation, and merged into a relatively dense thin joint, which has a blocking effect on the development of corrosion to the deep part. Therefore, the corrosion loss of the galvanizing layer is relatively slow. As long as the galvanizing layer is not penetrated by corrosion, it will continue to play an isolating and protective role on the steel base.When part of the corrosion of the galvanizing layer passes through, the isolation and protection is lost here.
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, the formation of micro batteries, zinc coating as anode, base steel as the cathode and anode oxidation reaction, to produce zinc corrosion: cathodic reduction reaction occurs, the corrosion of iron is restrained, this is the "sacrificial anode protection" zinc, zinc as the anode offer to turn themselves, by its own corrosive wear, as cathode iron, made from corrosion...
Experimental studies show that the electrochemical protective effect of the galvanizing layer on the steel surface without coating beside it decreases with distance.In the neighboring area of zinc coating for reserve, completely in the region, steel corrosion does not occur: completely protected areas, as part of the reserve, the region is not serious corrosion steel: part of the reserves, for without reserve, this area cause serious corrosion of steel, the same as the bare steel corrosion degree, and the galvanized layer in this area, not far from abroad made from zinc/steel bimetallic sample of atmospheric exposure test results show that the complete reserve ~ 1 mm, the width of the width of the part of the reserves to 3.5 m. that is to say, the galvanized layer has a complete protection from ~ 1 m, a part of the distance protection ~ 4. 5 mm. 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 the beyond "thoroughly reserve + part of the reserve" size.Beyond that, even if the water film is completely covered, the steel base corrodes.
The second case.From metal coating such as tin plating layer (both hot tin and tin plating), for the steel base is Yin polarity, with isolation protective coating on the steel base only, do not have offer d anodic protection effect, if the tin plating layer and local breakage, electrolyte cover, tin as cathode, iron as anode, will add to the steel substrate corrosion perforation, so, this kind of coating is only before its corrosion penetration, the isolation function to protect the steel base.
Whether the coating is anodic to the steel base or negative to the steel base, its working life mainly depends on the thickness of the coating (the weight of the coating per unit area) and the environmental atmosphere. The thicker the coating, the heavier the coating, the longer the service life of the coating.