Uniform corrosion:

As the name implies, uniform corrosion occurs uniformly over the whole metal surface with an almost uniform rate of metal loss. In general, it only occurs when acids or alkalis are involved. Alkaline lyes, (, such as caustic potash and caustic soda, in particular, are especially good at dissolving the oxide layer, as are halogen hydracids, such as hydrochloric and hydrofluoric acids, to a lesser extent.

Corrosion with shallow pit formation:

Shallow pit corrosion is a form of corrosion with a localised increase in metal loss. The diameters of the shallow pits are significantly larger than their depths. Alkaline media cause this type of attack.

Pitting corrosion:

One uses the term ‘pitting corrosion’ to describe localised electrolytic removal of metal. The depth of a pit is usually much greater than its diameter. The corrosive attack is acicular (needle-like) in appearance or is in the form of craters that hollow out beneath the surface. Individual pits might coalesce. The so-called ‘pitting potential’ can be used to estimate the likelihood of a particular alloy undergoing pitting corrosion.

Pitting corrosion can result from a deposition of a heavy metal on the surface or iron particles embedded in the surface.

Crevice corrosion:

Crevice corrosion is a localised form of accelerated corrosion in crevices caused by differences in concentration of the corrosive medium. Even differential aeration of the corrosive medium can lead to more rapid metal loss in the areas of the surface that are more poorly aerated. The critical crevice width below which crevice corrosion can occur is typically less than 0.5 mm.

Galvanic corrosion:

This form of corrosion can occur if two different metals are electrically connected via an electrolyte. The metal part that undergoes accelerated corrosion forms the anode of the corrosion cell. For example, if an aluminium alloy is joined together with another metal, e.g. steel, using rivets or screws, pitting or shallow pit corrosion can occur in the aluminium alloy part after prolonged exposure to an aggressive corrosive medium (which forms the electrolyte). The steel part is then the more noble metal and enjoys cathodic protection, whereas the aluminium part acts as an anode. However, it is usually not possible to predict with certainty which of the metals in a corrosive medium will be the anode and which will be the cathode. In practice, there are too many factors affecting corrosion behaviour: the relative sizes of the anodic and cathodic areas, the nature of the corrosive medium and its conductivity as an electrolyte, passivation effects, the duration of the exposure to the electrolyte etc.

Deposit attack:

Permanent contact with a foreign body can lead to localised attack, either in the form of crevice corrosion (where there is contact with a non-conducting foreign body) or contact corrosion when the foreign body conducts electrons.

Corrosion by condensed water:

This type of corrosion occurs when water condenses out onto the metal surface. It poses a particular risk, for example, to semi-finished products piled up on top of each other or coiled up if they are moved from a cold environment into a heated space. The same applies to flat structures that are poorly aerated.
 


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