Corrosion of Zinc

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Understanding Zinc Corrosion in Atmospheric Conditions

Zinc demonstrates superior corrosion resistance compared to steel in most natural atmospheric environments, with the rate of corrosion varying significantly based on environmental conditions. In seacoast atmospheres, for example, zinc's corrosion rate is approximately 1/25 that of steel. This exceptional resistance stems from zinc's ability to form insoluble basic carbonate films on its surface.

Three critical factors determine zinc's atmospheric corrosion rate:

• The duration and frequency of moisture exposure
• The surface drying rate
• The level of industrial pollutants present in the atmosphere

In dry air conditions, zinc undergoes a slow oxidation process where a protective double-layer forms. The inner layer consists of dense oxides that adhere tightly to the zinc surface, while the outer layer may periodically break away. This protective mechanism is particularly effective in tropical climates, where zinc exhibits remarkably slow oxidation rates.

Surface drying rates play a crucial role in the corrosion process, as thin moisture films with high oxygen concentration can accelerate corrosion. Atmospheric corrosion encompasses temperatures ranging from -18 to 70°C, both in open and enclosed spaces, with corrosivity influenced by factors including industrial pollution, marine conditions, humidity, temperature fluctuations, and rainfall patterns.

Zinc Corrosion Behavior in Aqueous Environments

The corrosion behavior of zinc in water environments is primarily determined by the water's impurity content. Natural water sources, including rainwater, contain various substances that can affect zinc's corrosion resistance:

• Dissolved gases (nitrogen, oxygen, CO₂)
• Environmental particles (dust, smoke)
• Organic matter (decaying vegetation, microorganisms)
• Dissolved minerals and salts
• Colloidal and suspended materials

The formation of protective surface layers, crucial for zinc's corrosion resistance, depends heavily on water composition. Interestingly, zinc exhibits higher corrosion rates in distilled water compared to domestic or river water, as the latter contains beneficial scale-forming salts. This protective scale formation is influenced by three key factors:

1. Hydrogen ion concentration (pH value)
2. Total calcium content
3. Overall alkalinity

Waters with high CO₂ content typically show aggressive behavior toward zinc. When the pH falls below the calcium carbonate equilibrium point, the water tends to dissolve rather than deposit protective scale.

Temperature, motion, fluid agitation, and exposure duration also significantly impact zinc's corrosion behavior in aqueous environments. The protective layer's initial formation ability largely determines the long-term corrosion resistance of zinc coatings in water.

Zinc Behavior in Chemical Environments

The performance of zinc in chemical environments varies significantly depending on the specific chemical composition it encounters. Zinc exhibits particular vulnerability to acids and strong alkaline solutions, making it unsuitable for applications involving these substances.

Acid and Alkaline Environments

Even highly diluted acid concentrations can accelerate zinc corrosion beyond practical usefulness. While alkaline solutions of moderate strength show less aggressive behavior compared to acids, they still pose significant corrosion risks that limit zinc's applications. However, certain industrial applications have found ways to manage these limitations. For example, in refrigeration systems using calcium chloride (CaCl₂) brines, corrosion control is achieved by:

• Maintaining mildly alkaline pH levels
• Adding inhibitors such as sodium chromate (Na₂CrO₄)
• Utilizing protective compounds like dichromates, borates, and silicates

Organic Compound Interactions

Zinc shows excellent compatibility with many organic liquids that are:

• Nearly neutral in pH
• Substantially water-free
• Common examples include gasoline, glycerine, and inhibited trichlorethylene

However, the presence of water in these organic environments can lead to localized corrosion due to oxygen access limitations. Additionally, zinc may catalyze the decomposition of certain solutions like trichlorethylene, potentially resulting in acid attack.

Gaseous Environment Effects

In gas exposure scenarios, zinc's behavior follows distinct patterns:

• Most common gases at normal temperatures don't affect zinc when moisture is absent
• Dry chlorine has minimal impact
• Hydrogen sulfide (H₂S) exposure results in the formation of protective zinc sulfide (ZnS)
• Sulfur dioxide (SO₂) and chlorides can be corrosive due to the formation of water-soluble and hygroscopic salts

Indoor Exposure and Protection Methods

Indoor Atmospheric Effects

In typical indoor environments with moderate relative humidity, zinc exhibits minimal corrosion tendencies. The corrosion process typically begins at localized points where dust particles accumulate on the surface. Temperature fluctuations significantly impact indoor corrosion, particularly in areas where condensation occurs and moisture dries slowly. Thin moisture films pose a unique challenge due to their high oxygen content maintenance, resulting from the large water/air interface areas.

Protective Zinc Coating Methods

Seven primary methods protect iron and steel surfaces through zinc coating:

• Hot dip galvanizing
• Continuous-line galvanizing
• Electro-galvanizing
• Zinc plating
• Mechanical plating
• Zinc spraying
• Zinc-bearing paints

Hot dip galvanizing, the most established method, has served industry for nearly two centuries. This process involves complete immersion of steel or iron in molten zinc, with modern applications utilizing carefully controlled conditions and scientific research to optimize results.

The Continuous Galvanizing process (Sendzimir Process) incorporates small amounts of aluminum in the zinc bath, producing highly ductile coatings suitable for deep drawing and folding applications without coating damage.

The Galvanizing Process in Detail

Successful galvanizing requires thorough surface preparation. All scale and rust must be removed through inhibited acid pickling, while iron castings undergo shot- or grit-blasting to remove molding sand and surface graphite. Two primary application methods exist:

• Dry process: Uses pre-fluxing with zinc ammonium chloride followed by low-temperature drying
• Wet process: Employs direct dipping through a flux blanket

Electrogalvanizing represents a significant advancement in coating technology. This process creates very thin, formable coatings ideal for deep drawing or painting applications. The resulting uniform, pure zinc coatings demonstrate excellent adherence characteristics, though thinner than hot-dip galvanized layers. The process particularly suits milled products such as sheet metal, wire, and certain pipe applications.