What is Pitting Corrosion?
“Pitting Corrosion” is the formation of holes in an otherwise relatively un-attacked surface. It is a form of localised corrosion that proceeds because of local cell action which produces cavities beginning at the surface. The cavities may or may not become filled with corrosion products. Corrosion products may form “caps” over the pit cavities which are described as “nodules” or “tubercules”. While the shapes of pits vary widely, they are usually rough saucer shaped, conical or hemispherical.
Pitting corrosion is quite common. It can occur on a metal surface because of localised differences in composition or surface contamination, deposition or incorporation of metallic and non-metallic particles, or contact with dissimilar metal. This type of attack can by caused by a break in a protective film on the metal surface, such as an inhibitor film, oxide film or break in coating. It also may be caused by the uneven deposition of solids (metallic salts) which in themselves create a local action because of the potential difference between the salts and metal surface. An example is ferric sulphate.
Where does Pitting Occur?
- Generally, pitting corrosion is a small anodic areas surrounded by a cathode
- The pitting phenomena are usually self-propagating, unpredictable and can lead to catastrophic failure.
- Pitting corrosion affects many metallic surfaces, but is prevalent on steels that rely on ‘passive films’ for protection.
- Pit diameter increases with time.
- On many stainless steels, higher concentration of chlorides can lead to a greater concentration of pits in a given area.
- Impurities of unwanted alloying components can great effect pitting rates.
Protective coatings for Pitting Corrosion
The Corrocoat Corrofill range consists of fillers for badly pitted steel, in application, which require a high performance, chemically resistant coating system.
One of our specialist corrosion protection coatings would then be applied to complete the process.
A two-pack organic peroxide, catalysed bis phenol ‘A’ polyester filler or grout.
A general-purpose two or three pack epoxy filler, grout and repair compound.
A two-pack, vinyl ester filler and grouting material containing abrasion resistant fillers.
Contact us to find out more on how we can repair and protect corrosion pitted steel and other pieces of equipment or machinery, suffering from other types of corrosion.