Forms of corrosion
There are many different types of corrosion, each of which can be classified by the cause of the metal's chemical deterioration. This article explains 10 common types of corrosion, including galvanic corrosion and flow-assisted corrosion.
Some of the below forms of corrosion are unique, but all of them are more or less interrelated.
Uniform or general corrosion are defined as a type of corrosion attack that is more or less distributed over the entire exposed surface of a metal. Uniform corrosion is a common form found in ferrous metals and alloys not protected by surface coatings and/or cathodic protection.
Galvanic corrosion is the accelerated corrosion of a metal because of contact with a more noble metal in an electrolyte, Galvanic series of metal and alloys is used for predictions of this type of corrosion.
Corrosion has been an important topic in construction history since mankind started to use rivets to assemble ships.
Concentration cell is a form of galvanic corrosion. Just as two dissimilar metals joined cause corrosion, so do dissimilar condition within the electrolyte. The corrosion occurs when two or more areas of the same metal surface are in contact with electrolytic solutions of different concentrations. The same metal has different electrical properties in the presence of different concentrations of the same electrolyte.
Differential aeration (oxygen concentration cell) and ion concentration (salt concentration cells) create dissimilar polarities (anodic and cathodic areas). Differences in dissolved oxygen concentration lead to localized corrosion of metal in hidden areas as under deposits or in crevices.
Crevice corrosion is a localized attack on a metal adjacent to a crevice between two joining surfaces (two metals or metal-nonmetal crevices). It is considered good design practice to avoid, if possible, all shapes and joints that form crevices into which oxygen does not have ready access. Various factors influence crevice corrosion are:
- Metal alloy composition, metallographic structure.
- Environmental conditions such as pH, oxygen concentration, chloride concentrations and temperature.
- Geometrical features of crevices, surface roughness
Pitting corrosion is a localized phenomenon confined to smaller areas. Pitting corrosion are normally found on passive metals and alloys such as aluminium alloys, stainless steel and alloys when the oxide film is chemically or mechanically damaged and does not re-passivate. The resulting pits can be narrow and deep which can rapidly perforate the wall thickness of a metal. Pitting tendency can be predicted through measurement of pitting potentials, and be prevented by proper selection of materials, control chloride concentration and cathodic protection.
The internal structure of a metal or alloy sometimes influences corrosion and lead to selective corrosion attack. Examples are dezincification, dealuminification and intergranular corrosion. Localised attack at or nearer to grain boundaries in a metal or alloy can be termed as intergranular corrosion.
Erosion corrosion is the deterioration of metals and alloys due to relative movement between metal surfaces and corrosive fluids. Depending on the rate of this movement, abrasion takes place. This type of corrosion are characterized by grooves and surface irregularities. Means of limiting erosion-corrosion and abrasion-corrosion are to choose a more resistant material and improve the design.
Cavitation corrosion is a particular form of erosion caused by the “implosion” of gas bubbles on a metal surface which cause pits on the metal surface. It is often associated with sudden variations in pressure related to the hydrodynamic parameters of the water (e.g. propeller blades). Although the collapse of a small cavity is a relatively low-energy event, highly localized collapses can erode metals over time. The pitting caused by the collapse of cavities produces great wear on components and can dramatically shorten a propeller’s or pump’s lifetime. Even though cavitation is slightly reduced by cathodic protection the main measures to eliminate cavitation erosion is to ensure good surface conditions and proper water flow.
Flow-assisted corrosion, or flow-accelerated corrosion, occurs when a protective layer of oxide on a metal surface is dissolved or removed by wind or water, exposing the underlying metal to further corroding and deteriorate.
Stress corrosion cracking (SCC) refers to failure under simultaneous presence of a corrosive medium and a tensile stress.