Analysis of the truth about rusting of galvanized steel plates

The corrosion of galvanized steel plates is the result of the combined effects of coating quality, mechanical damage, and environmental factors. This article starts from the failure mechanism, analyzes the core causes of coating protection failure, and proposes a full cycle solution.

Core causes of failure
1. Congenital defects in the coating: Insufficient purity of the zinc solution or deviation in the immersion process of galvanized steel plates can lead to micro defects such as pinholes and bubbles in the coating. In the high salt spray environment along the coast, the penetration speed of chloride ions increases several times, accelerating the corrosion process.
2. Mechanical damage transmission: Welding spatter, handling scratches directly damage the integrity of the coating. The unrepaired heat affected zone showed red rust within 6 months, while the intact coating area remained rust free for 3 years.
3. Accelerated environmental corrosion: Acidic (pH<4) or high humidity (>85% RH) environments increase the annual corrosion of the zinc layer on galvanized steel plates by 5-10 times, far exceeding the designed protection period.
Analysis of Protection Mechanisms
The galvanized layer achieves anti-corrosion through the synergistic effect of electrochemical sacrificial protection and physical barrier:
Potential difference drive: Zinc (-0.76V) is more reactive than iron (-0.44V) and preferentially dissolves to provide cathodic protection. The 65 μ m coating can maintain effective protection for 5 years, and the protection time decreases sharply when the thickness is less than 45 μ m.
Alloy layer anchoring: Hot dip galvanizing forms a 30-50 μ m zinc iron alloy layer with a shear strength of 200MPa. When the coating is ≥ 80 μ m, the adhesion of the alloy layer is positively correlated with corrosion resistance, but excessive silicon can lead to brittle peeling.
Quantitative control indicators
Coating thickness: The national standard requires ≥ 65 μ m (JGJ/T 251), and ＜ 45 μ m is the failure threshold.
Salt spray test: No rust spots within 72 hours are considered qualified, and white rust within 24 hours is within an acceptable range.
Adhesion: If the peeling area of the grid method is less than 5%, it is considered as level 0, and if it is greater than 15%, it is considered as failure.
Note: White rust (ZnO · Zn (OH)? ）It can delay corrosion, and the measured annual corrosion depth is only 0.5 μ m, far lower than the 10 μ m in the red rust area.
Galvanized steel plate is a welded steel plate with a zinc layer formed on its surface through hot-dip or electroplating processes. It is mainly used in fields such as construction, home appliances, and vehicle manufacturing, where the zinc layer enhances the corrosion resistance of the steel and extends its service life.