Why zinc protects steel against corrosion
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Why zinc protects steel against corrosion
Galvanic series of metals in sea water electrolyte
The table below shows a series of metals arranged in order of electrochemical activity in a sea water electrolyte. Metals high in the scale provide cathodic or sacrificial protection to the metals below them. Zine therefore protects steel.
The scale indicates that magnesium, aluminium and cadmium should also protect steel.
Magnesium is highly reactive and is too rapidly consumed.
Aluminium forms a resistant oxide coating and its effectiveness in providing cathodic protection is limited.
Cadmium provides the same cathodic protection for steel as zinc but its applications are limited for technical and economic reasons to small electroplated parts such as fasteners.
When zinc and steel are in contact in an electrolyte differences in electrical potential develop and an electrolytic cell is formed. Zinc is more electrochemically active than steel, as shown in the galvanic series. The zinc therefore becomes the anode for all the steel, preventing the formation of small anodic and cathodic areas on the steel surface.
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