My Web Stats

TYPES OF CORROSION

Uniform Corrosion: Uniform corrosion is a form of corrosion where the surface is removed almost evenly. The partial reactions (metal dissolution and oxygen reduction) are statistically distributed over the surface, leading to more or less homogenous dissolution of the metal and uniform formation of corrosion products (e.g. red rust on steel). The extent of this form of corrosion can usually be well estimated on the basis of previous experience. The rate of corrosion is usually given in micrometres per year (m/a). Using these average values, it is possible to calculate the life expectancy of a component, and thus to enhance its life expectancy by increasing its thickness. Uniform corrosion takes place, for example, on unprotected carbon steel and on zinc-coated steel under atmospheric conditions.

Galvanic Corrosion: refers to corrosion damage where two dissimilar metals have an electrically conducting connection and are in contact with a common corrosive electrolyte. In the electrochemical model of corrosion, one of the two partial reactions (anodic metal dissolution and cathodic oxygen reduction) takes place almost exclusively on one metal. Generally, the less noble metal will be dissolved (anodic metal dissolution), whereas the more noble part is not attacked by corrosion (serves only as the cathode for oxygen reduction). Where galvanic corrosion takes place, the rate of corrosion of the less noble metal is higher than it would be in a free corroding environment without contact with another metal. Using thermodynamic data and taking common experience gained in typical applications into account, it is possible to predict which material combinations will be affected by galvanic corrosion.

Crevice Corrosion: Crevice corrosion refers to corrosion occurring in cracks or crevices formed between two surfaces (made from the same metal, different metals or even a metal and a non-metal). This type of corrosion is initiated by the restricted entrance of oxygen from the air by diffusion into the crevice area leading to different concentrations of dissolved oxygen in the common electrolyte (the so-called aeration cell). Again, the two partial reactions will take place on different parts of the surface. Oxygen reduction takes place in the outer areas with higher oxygen concentrations easily accessible by the surrounding air, whereas the anodic metal dissolution occurs in the crevice area resulting in localized attack (e.g. pitting). It may also occur under washers or gaskets when the entry of water underneath is not prevented.. There are lower and upper limits to the size of a crevice in which corrosion may be induced. If the crevice is too tight, no electrolyte for corrosion will be introduced. If the crevice is too wide to reduce oxygen entrance, the aeration cell and consequently different concentrations of oxygen cannot develop. However, the critical crevice width depends on several factors such as the type of metals involved, the corroding environment and wet/dry cycles.

Hydrogen-assisted cracking: Hydrogen-assisted cracking is caused by the diffusion of hydrogen atoms into the metal. The presence of hydrogen in the lattice weakens the mechanical integrity of the metal and leads to crack growth and brittle fracture at stress levels below the yield strength. Like stress corrosion cracking, it can lead to sudden failure of metal parts without any detectable warning signs. In common applications, hydrogen damage is usually only relevant for high-strength steel with a tensile strength of approximately 1000N/mm2 or higher. As for SCC, three different conditions must be present at the same time;

  • Mechanical (load, stress)
  • Material (hardness structure)
  • Environmental (external, internal hydrogen)

The sources of hydrogen can be the production process such as steel making, pickling, and electro galvanizing (primary hydrogen). A secondary source can be the hydrogen formed during a corrosion process. During the corrosion process, hydrogen is formed and diffuses into the material. This hydrogen intake leads to a decrease in the toughness or ductility of a metal.

Stress Corrosion Cracking (SCC): Stress corrosion cracking (SCC) is a combined mechanical and electrochemical corrosion process that results in cracking of certain materials. It can lead to unexpected sudden brittle failure of normally ductile metals subjected to stress levels well below their yield strength. Internal stresses in a material can be sufficient to initiate an attack of stress corrosion cracking. Stress corrosion cracking is not simply an overlapping of corrosion and mechanical stresses but can be understood as an autocatalytic, self-accelerating process leading to high metal dissolution rates (anodic reaction). Initially, a small pit is formed and develops into a crack due to applied or residual stress in the material. The crack formation opens up a new active (non-passive) metal surface, which again will corrode very easily. This leads to further crack propagation and again to the exposure of new highly active metal surfaces in the crack. Metal dissolution in the crack will advance rapidly until mechanical failure occurs. SCC is a highly specific form of corrosion that occurs only when the following three different requirements are fulfilled at the same time,

  • Mechanical (load, stress)
  • Material (susceptible alloy e.g.: austenitic stainless steel)
  • Environment (highly corrosive, chlorides)

Pitting Corrosion: Pitting corrosion is a localized form of corrosion that leads to the creation of small holes or nepits in the metal. This form of corrosion is mainly found on passive metals. Passive metals and alloys, such as aluminium, titanium and stainless steel owe their corrosion resistance to a thin oxide layer on the surface with a thickness of only a few nanometers. The corrosion initiating process starts with a local breakdown of the passive layer. The local corrosive attack can be initiated on stainless steels, for example, by chloride ions.

Formicary Corrosion or Ants Nest Corrosion: It is corrosion that occurs in Copper Based alloys. Due primarily to appearance, and the fact that it appears like ant nests in the copper under magnification it is often referred to as ants nest corrosion. If you see corrosion it is probably not formicary corrosion, because it is not generally visible without magnification. A second nickname for formicary corrosion is pinhole corrosion due to the small size of the actual holes in the copper. However, you may see some grey, black, or blue discolouration on the copper surface where the corrosion is found.

ALL types of marine corrosion:

Crevice corrosion
Uniform corrosion
Microbiological corrosion
Hydrogen embrittlement
Pitting corrosion
Erosion corrosion
Galvanic corrosion
High-temperature corrosion
Stress corrosion cracking
Stress assisted corrosion.
Stray current corrosion.
Waterline corrosion.
Weld corrosion.
Coating related corrosion.
Corrosion under lagging.
Intercooler & heat exchanger corrosion.