The corrosion resistance of Q355NH weathering steel exhibits regular variations with temperature, which is directly related to temperature's impact on the "formation rate and compactness of the rust layer" and the "activity of corrosion electrochemical reactions". The specific changes are as follows:
1. Low-temperature environment (≤0°C, e.g., cold regions, winter): Relatively stable corrosion resistance, slow rust layer formation
Low temperatures significantly reduce the electrochemical corrosion rate on the steel surface (due to weakened ion migration and oxidation reaction activity). As a result, the corrosion process of Q355NH slows down-over the short term (e.g., 1–2 years), the thickness of the surface rust layer increases slowly, and large areas of loose rust flakes rarely form.
However, note that if low temperatures are accompanied by high humidity and freeze-thaw cycles (e.g., freezing and melting of rain/snow), moisture repeatedly freezes and expands in the gaps of the rust layer, which may damage the integrity of the initial rust layer and intensify local "micro-corrosion". Nevertheless, compared with ordinary carbon steel, the Cu and Cr elements in Q355NH still promote the formation of a locally compact rust layer, ensuring its overall corrosion resistance remains superior to that of ordinary steel.
2. Medium-temperature environment (10–30°C, e.g., temperate normal temperature): Optimal corrosion resistance, highest efficiency of rust layer densification
This temperature range is the best condition for Q355NH to form a "stable protective rust layer": the temperature is high enough to promote the uniform diffusion of Cu and Cr elements in the rust layer, accelerating the formation of a compact rust layer (typically 20–50 μm thick) rich in Cu₂O and Cr₂O₃, while not causing the corrosion reaction to run out of control due to excessively high temperatures.
At this stage, the rust layer can effectively block the contact between oxygen, moisture and the base metal. The corrosion rate can be reduced to 0.01–0.03 mm/year (only 1/5–1/3 of that of ordinary Q355 steel), making this the "optimal performance range" for Q355NH's weathering resistance.
3. High-temperature environment (≥35°C, e.g., tropical regions, summer exposure areas): Accelerated short-term corrosion, decreased long-term rust layer stability
High temperatures greatly enhance the activity of electrochemical corrosion. The oxidation and hydrogen evolution reaction rates on the surface of Q355NH accelerate-over the short term (e.g., 1–3 months in summer), the thickness of the rust layer may increase rapidly to 60–80 μm. However, the rust layer tends to become loose and porous due to "rapid formation", and Cu and Cr elements cannot be fully enriched to form a compact structure, leading to the risk of local "rust layer peeling".
If high temperatures are accompanied by high humidity (e.g., tropical rainy seasons), corrosion will further intensify. At this point, the annual corrosion rate of Q355NH may rise to 0.04–0.06 mm/year (still better than ordinary steel, but a decrease of approximately 30% compared to the medium-temperature environment). It is necessary to maintain corrosion resistance by regularly removing loose surface rust (to avoid water accumulation).
