1.What is the formation mechanism of galvanic corrosion?
The core reason is the difference in electrode potential. The standard electrode potential of zinc is about -0.76V (more active), while the potential of common bolt materials (such as carbon steel and stainless steel) is higher.
When the two are in contact in an electrolyte environment, zinc is preferentially oxidized and dissolved as the anode, and the bolt is protected as the cathode, forming a "sacrificial anode protecting the cathode" galvanic couple.
2.What are the manifestations of corrosion?
White corrosion products appear on the surface of the zinc layer. In severe cases, the zinc layer partially falls off, exposing the base metal, which in turn causes base corrosion.
If the bolt material is a metal with low activity, it may reversely cause the bolt to become an anode and be corroded.
3.What are the key factors affecting the rate of galvanic corrosion?
Material potential difference
The larger the potential difference, the stronger the galvanic current and the faster the corrosion rate. For example: the potential difference between a carbon steel bolt and a galvanized part is smaller than that between a stainless steel bolt and a galvanized part, so the zinc layer corrodes faster when the stainless steel bolt is connected.
Area ratio
If the surface area of the bolt is much larger than the galvanized layer, the zinc layer will be consumed faster. On the contrary, if the area of the galvanized layer is much larger than the bolt, the corrosion rate will slow down.
Environmental factors
Humidity: When the humidity is greater than 60%, water vapor forms an electrolyte film, which accelerates the galvanic reaction; high humidity (such as coastal areas and rainy seasons) will significantly aggravate corrosion.
Pollutants: Chloride ions and sulfur dioxide will destroy the passivation film on the surface of the zinc layer, increase the conductivity of the electrolyte, and accelerate corrosion.
4.How to enhance the protective capability of galvanized layer?
Increase the thickness of the zinc coating: The conventional zinc coating thickness is 5-15μm, which can be increased to 20-30μm, extending the time for the zinc coating to be consumed.
Post-treatment passivation: After zinc plating, chromate passivation or chromium-free passivation is performed to form a dense passivation film on the surface of the zinc coating, reducing the dissolution rate of zinc.
Coating sealant: A transparent organic sealant is applied to the surface of the zinc coating to enhance corrosion resistance and isolate direct electrical contact with the bolts.
5.How to improve the connection structure and environment?
Ensure sufficient preload: Use torque control to make the bolts fit tightly with the galvanized parts, reduce the gap, and avoid crevice corrosion.
Install drainage structure: Design drainage holes at the connection parts to avoid water accumulation; outdoor equipment can be equipped with rain covers to reduce direct rain erosion.
Environmental control: In high humidity and high salt spray environments, regularly spray anti-rust oil or vapor phase rust inhibitors on the connection parts to inhibit the formation of electrolytes.