Q:How to distinguish between slight white rust and severe corrosion?
A:Slight white rust: It is in the form of white powder or flakes, only covers the surface of the coating, does not penetrate the substrate, and can be removed by lightly scraping with fingernails, and the underlying zinc layer is still bright.
Severe corrosion: Reddish-brown rust (rust on the substrate), peeling of the coating or large areas of black spots appear. At this time, the plate needs to be scrapped directly and cannot be repaired.
Q:What is the process for treating slight white rust?
A:Step 1: Dry physical removal (avoid water)
Fine sandpaper: 800-1000 mesh (made of silicon carbide or aluminum oxide, steel wool is prohibited), used to gently polish the white rust area until the surface restores its metallic luster.
Step 2: Degreasing and cleaning (removing grease and residual impurities)
Dip a pure cotton cloth or sponge in a diluted degreaser (according to the manufacturer's ratio, usually 1:10 with water), and gently wipe the polished area to remove grease, fingerprints and sandpaper debris.
Immediately blow dry with dry compressed air after wiping to ensure that there is no liquid residue on the surface (residual water may cause secondary rust).
Step 3: Neutralization treatment (optional, for alkaline environment)
If the storage environment is alkaline (such as contact with cement floors, alkaline dust), use 0.5% dilute acetic acid solution (food-grade acetic acid mixed with water) to gently wipe the white rust area to neutralize the residual alkali.
Step 4: Rust prevention and timely processing
Apply a silicone-free rust preventive (such as water-based rust preventive paint, film thickness 1-3μm) to ensure that the polished area is covered to prevent secondary oxidation.
If the stamping process can be started within 2 hours, the rust preventive can be omitted and the product can be directly stored dry.
Q:What should we pay attention to during dry physical cleaning?
A:Grind in one direction along the rolling direction (along the grain) of the plate to avoid scratches on the coating caused by circular or cross grinding.
The grinding range should exceed the edge of the white rust area by 10-15mm to ensure that the surrounding hidden oxide layer is completely removed.
Q:How to choose cleaning agent for degreasing cleaning?
A:Neutral water-based degreaser (pH 6.5-7.5), which must pass the automobile factory coating process certification (such as not containing silicon, phosphorus, sulfur and other components).
It is prohibited to use solvent-based cleaning agents (such as alcohol, gasoline), which may damage the passivation film of the zinc coating.
Q:What are the absolutely prohibited operations when dealing with slight white rust on galvanized sheets for automobiles?
A:Grinding with steel wool or coarse sandpaper (<600 mesh): It will destroy the passivation film on the surface of the coating and accelerate corrosion.
Rinse with water directly and dry naturally: Residual water marks may form an oxygen concentration cell, causing local corrosion to worsen.
Treating GA plates (alloyed coating) with strong acid/alkali: It will dissolve the Fe-Zn alloy layer and affect welding and coating performance.