Exfoliation Corrosion

This type of corrosion progresses laterally from the corrosion initiation sites on the surface and usually continues intergranularly along planes parallel to the surface. Corrosion products produced at the grain boundaries force the metal out of the underlying layer, which ultimately leads to delamination or flaking of the metal. This type of corrosion mainly affects aluminum alloys.

This type of corrosion is usually related to the method of alloying, annealing of impurities in the alloy matrix, and the distribution of intermetallic compounds on the surface and grain boundaries. An example of this type of corrosion is shown in the figure below.

Copper-magnesium alloys of the 2000 series, zinc-copper-magnesium alloys of the 7000 series are susceptible to this type of corrosion (flaking) in industrial and marine environments.

Subprecipitation Corrosion

Subprecipitation corrosion occurs when the metal surface is covered by deposits or any other foreign body. This can lead to crevice corrosion. If the deposit formed on the metal surface is also conductive, galvanic corrosion is also possible.

Corrosion by condensed acid

During the start-up and shutdown of a plant, conditions may be such that the temperature and pressure are below the dew point of the vapors in the environment, which can cause these gases to condense on metal surfaces and cause corrosion. For example, in combustion chambers where sulfur is burned, sulfur may combine with water vapor to produce sulfuric acid, which, after condensing, can cause various types of corrosion on metal surfaces.

Corrosion in a non-operating state

As the name suggests, this type of corrosion occurs when the plant is not operating. Usually, in this case, moisture in the air or chemical vapors cause corrosion on metal surfaces.

Corrosion at high temperatures

If there are liquid or solid impurities on the metal surface, these impurities can react with the protective oxide layer and destroy this layer. These reactions usually occur at temperatures above 600 degrees Celsius, in which case the protective layer is destroyed and the metal undergoes hot corrosion or high-temperature corrosion.

As a result of the reaction of the metal with the environment containing hot gas, an oxide layer is formed on the surface of the metal. This oxide layer acts as a protector on the surface and prevents the continuation of corrosion reactions. However, in this layer, metal ions, electrons and oxygen ions can move, and in this way this oxide layer grows.