The corrosion behavior of zinc coatings in outdoor settings primarily depends on air composition, humidity, rainfall patterns, and other environmental factors. Outdoor atmospheres can be categorized as follows:
(1) Rural Environments
Clean rural air typically contains minimal pollutant gases, with rainfall being the primary corrosion agent. However, thunderstorms generate nitrogen oxides that dissolve in rainwater to form nitrate/nitrite ions and nitric/nitrous acids. These acidic components accelerate zinc corrosion.
(2) Urban Environments
Characterized by elevated CO₂ levels (0.03-0.07% by volume) forming carbonic acid, along with SO₂ emissions from domestic coal combustion. These pollutants create corrosive conditions for zinc coatings.
(3) Industrial Environments
Contain significant SO₂ concentrations, sulfur-bearing compounds (sodium sulfate, calcium sulfate), and particulate matter (soot, dust). These elements synergistically accelerate zinc corrosion rates.
(4) Coastal Environments
Airborne chloride ions from marine aerosols and NaCl/iodide ions in rainwater (from sea spray) create a chloride-rich environment. Sulfate contamination is also commonly detected, all contributing to enhanced corrosion.
(5) Tropical Environments
High temperatures and alternating wet/dry periods promote formation of a relatively thick zinc oxide (ZnO) protective layer. However, prolonged rainy seasons with significant temperature fluctuations compromise this layer's effectiveness.
Major corrosion products for outdoor zinc coatings include:
Primary products: Zinc oxide (ZnO), zinc hydroxide [Zn(OH)₂]
Secondary products: Basic zinc carbonate [Zn₅(OH)₆(CO₃)₂], zinc sulfate (ZnSO₄)
