Moist air can cause oxidation and corrosion of metal materials such as tinplate, causing rust, looseness, delamination, and pulverization on the metal surface, causing it to lose its original characteristics and luster. The critical relative humidity of air for metal oxidation corrosion is 70% for steel, 63% for iron, 60% for zinc, and 76% for aluminum. In humid air above the critical value, the oxidation, corrosion, and rust of metals increase with increasing humidity.

When the air humidity reaches around 85%, due to the exposed iron base at the cut points on both sides of the sheet after slitting, the sheet that is directly exposed to the air or not tightly packaged will generally show rust after 3 days. When the air humidity reaches around 90%, the tinplate directly exposed to the air will show rust after 3 hours, and rust will become very serious after 5 hours.