Worst of the Elements
Chemicals and Pollution
Chemicals and Pollutants

Even the most remote corners of the earth aren’t immune to the effects of chemicals and pollutions — their presence has been found in the polar ice caps, for example. Salts (chlorides) and acids (sulfurs) enhance condensation as a result of changes in vapor pressure, which leads to an increase in corrosion. Similarly, the nitrogen and sulfur compounds discharged by some industries can form acids, accelerating the corrosion process on the metal these substances encounter.

Salt

Airborne salinity refers to the content of gaseous and suspended salt in the atmosphere. In general, high salt deposition rate tends to result in high rates of corrosion. Airborne salinity is predominantly related to a marine climate.

At locations very close to the sea, the corrosion rate under a cover can be higher than being openly exposed because of the increased time of wetness under the sheltered condition.

Steel Corrosion Chart

Sulfer

Sulfur dioxide (SO2), a gaseous pollutant in the atmosphere resulting predominantly from the combustion of most fossil fuels, plays an important role in atmospheric corrosion of most metals in urban and industrial type atmospheres. It is adsorbed on metal surfaces, has a high solubility in water, and tends to form sulfuric acid (acid rain) in the presence of moisture films.

Sulfate ions are formed in the surface moisture layer by the oxidation of sulfur dioxide and their formation is considered to be the main corrosion accelerating effect from sulfur dioxide.

Critical RH (Relative Humidity)

The combined effect of the physical and chemical properties of a metal surface means that there is a critical relative humidity above which the corrosion rate “becomes rapid.” The actual value depends upon the nature and composition of the surface. Clean iron in pure air does not corrode until the air is practically saturated. However, if the air contains even a trace (0.01%) of sulphur dioxide, the critical relative humidity drops to 70%.

In general, high salt deposition rate tends to result in high rates of corrosion. Airborne salinity is predominantly related to a marine climate.