Zinc is a metal widely used in outdoor applications, most commonly as a protective coating on steel through a process called galvanizing. Zinc does corrode when exposed to the elements, but it does not “rust” in the familiar way that iron-based metals do. Understanding the specific chemical reaction zinc undergoes is key to appreciating why it is a preferred material for long-term outdoor durability.
The Chemistry of Zinc Corrosion
The fundamental difference between iron and zinc corrosion lies in the material formed when the metal reacts with oxygen and moisture. Iron forms hydrated iron(III) oxide (rust), which is flaky and porous. This non-adherent rust layer constantly exposes fresh iron to the corrosive environment, allowing degradation to continue unchecked.
Zinc undergoes passivation when exposed to the atmosphere, forming a dense, self-healing surface layer called a patina. This patina develops in stages: atmospheric oxygen converts zinc into zinc oxide, which reacts with moisture to form zinc hydroxide. The zinc hydroxide then reacts with carbon dioxide in the air to form basic zinc carbonate.
The resulting basic zinc carbonate layer is thin, tightly adherent, and highly insoluble in rainwater. This stable film creates an effective barrier that separates the underlying zinc metal from the environment. This protective behavior dramatically slows the rate of further corrosion, allowing zinc structures to last for decades in harsh outdoor settings.
Key Environmental Factors Affecting Zinc’s Lifespan
While the zinc patina is protective, its stability is significantly influenced by local environmental conditions. The rate at which the coating is consumed varies widely depending on the type of atmosphere.
Atmospheric Pollution
High levels of atmospheric pollution, particularly sulfur dioxide from industrial emissions, accelerate corrosion. Sulfur dioxide reacts with the protective layer to form water-soluble zinc sulfates, which are easily washed away by rain. This continuous cycle exposes new zinc to the atmosphere, increasing the overall corrosion rate.
Coastal Environments
Coastal environments also present a challenge due to airborne chlorides from sea spray. Chlorides react with the zinc patina, forming zinc chloride-based compounds. Since these compounds are more soluble than the protective carbonates, the patina degrades faster, leading to higher corrosion rates.
Time of Wetness
The time the zinc surface remains wet is a major factor, as moisture is necessary for the initial chemical reactions. In sheltered areas where moisture cannot dry quickly, the increased time of wetness can lead to a higher corrosion rate than in fully exposed areas. Conversely, in clean rural areas with minimal pollution, galvanized coatings can last for over 100 years.
How Zinc Protects Other Metals Outdoors
Zinc’s primary function in outdoor construction is to protect steel and iron from corrosion through galvanization. Galvanized steel uses the zinc coating both as a physical barrier and for active electrochemical defense.
This active defense is called sacrificial protection, or cathodic protection, because zinc is more electrochemically active than iron. When zinc and steel are in contact with an electrolyte, such as moisture, the zinc acts as the anode in the resulting electrochemical cell.
If the zinc coating is scratched, exposing the underlying steel, the zinc will preferentially corrode, sacrificing itself to protect the steel. The zinc supplies electrons to the exposed steel, preventing the iron from oxidizing and forming rust. This mechanism ensures the steel remains intact until the adjacent zinc coating is entirely consumed.