Why Do Air Receiver Tanks Rust or Corrode?

After long-term or years of service, air receiver tanks are inevitably subject to corrosion caused by rainwater, air or humid atmosphere. 

Such corrosion arises from the action of moisture, oxygen, acidic pollutants and other substances in the air. 

Rust formation on steel exposed to natural atmospheric conditions is the most common form of atmospheric corrosion.

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1 Three Types of Atmospheric Corrosion
Dry atmospheric corrosion
There is almost no water vapor in the atmosphere. Ordinary metals form an invisible oxide film at room temperature, and the steel surface retains its metallic luster.
Humid atmospheric corrosion

It refers to metal corrosion occurring beneath a thin water film invisible to human eyes. Water vapor exists in the atmosphere. 

When vapor concentration reaches the critical humidity (65% for iron, nearly 100% for copper), a thin water film forms on the metal surface and triggers uniform corrosion.

If pollutants such as CO₂, H₂S and SO₂ exist in the atmosphere, the corrosion rate increases significantly. 

Steel corrosion under atmospheric conditions is essentially electrochemical corrosion occurring under a water film.

Corrosion under visible liquid film conditions for air receiver tanks

This type of corrosion takes place when relative humidity approaches 100%, under rainy conditions or in contact with other aqueous solutions. 

Moisture accumulates into droplets and forms a visible water film on the metal surface.

2 Influencing Factors of Atmospheric Corrosion
Effect of water
Water is the primary corrosive agent acting on steel in atmospheric environments (generally, higher humidity leads to stronger corrosiveness). Its corrosion mechanism is shown below:
a. Water acts as an electrolyte and can dissolve a large quantity of ions, triggering metal corrosion.
b. Water dissociates into H⁺ and OH⁻. Variations in pH value have a remarkable influence on the dissolution and corrosion of metals and their oxides.
Effect of sulfur dioxide (SO₂)

In regions polluted by industrial exhaust gas, SO₂ imposes the most severe corrosive impact on steel. Exhaust gas generated by burning petroleum, 

natural gas and coal contains abundant SO₂. The corrosion rate of steel plates rises along with increasing SO₂ concentration in the atmosphere.

Influence of marine atmosphere

Atmosphere near the ocean contains salt mainly in the form of NaCl. Chloride ions (Cl⁻) feature strong aggressiveness and accelerate corrosion. 

The farther away from the ocean, the lower the salt content in the air and the milder the corrosion.

Other influences

Atmospheric environments in petroleum production areas may contain high levels of harmful substances including Cl₂, NH₃, H₂S and solid dust particles.

Corrosion on the steel of air receiver tanks intensifies as the concentration of these substances rises. The synergistic effect of multiple substances will further aggravate steel corrosion.