What Does Spot Corrosion Mean?
Spot corrosion is a type of non-uniform corrosion which is localized in nature, such as pitting and crevice corrosion. Spot corrosion or rusting appears beneath the coating surface. This type of attack could lead to failure of the material. Hot spots can be discovered using a half cell method.
Spot corrosion is also known as soft spot corrosion or black spot corrosion.
Corrosionpedia Explains Spot Corrosion
Spot corrosion is a localized form of non-uniform corrosion that takes place at "hot spots" due to low water velocity and/or high heat flux, for example. These conditions can be caused by poor steam distribution or the absence of water on the cooling-water side of a tube. This form of corrosion rarely takes place in main steam condensers.
This type of corrosion is characterized by pitting on the cooling brine side of tubes at localized sites of high temperature. It can occur in clean as well as polluted seawater. The attack could occur in clean seawater where the temperature on steam side is equal or greater than 265 °F (130 °C). In polluted seawater the rate of attack is very low, up to the temperature of 165 °F (75 °C), then it increases as the temperature goes up to 210 °F (100 °C).
Sometimes the surface of a bronze object may look intact, but has a pimply appearance — upon removing the surface over the spot, a corrosion pit containing light green powder may be found. If left untouched, the pit gradually gets deeper and larger, and may eventually burst, revealing the corrosion below.