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Pitting corrosion

Pitting corrosion

Pitting corrosion can occur when the passive layer is breached locally. This is initiated by an interaction between the halogenide ions and the passive layer. Pinhole-like depressions are formed and grow into pitting corrosion sites. In the building industry, it is usually the chloride ions from seawater, aerosols or de-icing salt nebula that cause pitting. The risk of corrosion of stainless steels increases with decreasing chloride content, temperature and increasing pH value. Chloride enriched acidic media are therefore particularly critical. Thanks to deposits, external rust, slag residues and discoloration on the surface the danger of pitting corrosion is enhanced. By further increasing the content of chromium, in particular by addition of molybdenum and, partially, nitrogen, the resistance of stainless steels against pitting is increased. The corrosion resistance in pitting corrosion generating media therefore depends to a significant extent on the surface quality of stainless steels and thus may also be affected by the manufacturing process of the steels.

Crevice corrosion

Crevice corrosion in the screw attachment area between nut and hole edge area on the sheet

Crevice corrosion is - as the name suggests - linked to the presence of crevices. This may have arisen structurally or operationally (for example, by deposits). Ferritic stainless steels react particularly sensitive to crevice corrosion since the passive layer is more sensitive than that of austenitic and duplex stainless steels. This results in the corrosion of these critical surfaces progressing faster once corrosion is initiated. The statements on pitting corrosion apply here since crevice corrosion is essentially the same mechanism as pitting corrosion.

Corrosion fatigue

Corrosion fatigue

For pure vibration stress (without corrosion load) there is a lower AC voltage, below which breaks are no longer observed.  The so-called „fatigue strength“. Fatigue strength is usually missing in corrosion fatigue and the steel can still break below this limit. In contrast to stress corrosion cracking which act occurs in specific media, corrosion fatigue can basically occur in all corrosive media in conjunction with alternating loads, often then in the form of transgranular corrosion. Therefore, corrosion fatigue does not usually occur in the construction or the consumer goods sectors.

Selective corrosion

Intergranular corrosion
Selective corrosion
Selective corrosion + pitting

In selective corrosion certain structure constituents, grain boundary areas or alloy elements are preferably dissolved. A distinction is made according to the area of the destroyed structure:

Transgranular corrosion (TC) when it passes through the grains or comes into connection therewith Intergranular corrosion (IC), if the destruction runs along the grain boundaries.

Since selective corrosion occurs in grain boundary areas, it is not easily recognizable with the naked eye and, therefore, especially dangerous. The intergranular corrosion can occur in acidic media, when chromium carbides at the grain boundaries are eliminated by heat (between 450-850°C in the austenitic steels, above 900°C in ferritic steels). This kind of heat development occurs near welding areas (heat-affected zones). It causes local chromium depletion in the area of the eliminated chromium carbides (sigma phase). In previous practice, therefore, the carbon content of austenitic steels had to be thickened by adding titanium (Ti) or niobium (Nb). This stabilization is known in obsolete material grades 1.4571 or 1.4541. Materials such as 1.4404 or 1.4307 need not be treated thanks to their lower carbon content.

Surface corrosion

Surface corrosion is characterized by a uniform or approximately uniform material removal. In general, an removal of less than 0.1 mm/year is approved as sufficient resistance to surface corrosion. If, instead of the removal rate, the mass loss rate per area unit is used as a measure, then the relationship for stainless steels for conversion is 1 g/h*m² = 1.1 mm/a. Uneven surface corrosion is referred to as "shallow pit corrosion". According to experience stainless steels can only be effected by more or less uniform corrosion in acidic aqueous media. For the atmospheric corrosion is important to note that when the pH increases (>4) the corrosion rate active state decreases greatly.

Stress corrosion cracking

In this type of corrosion, cracks form which are generally transgranular on stainless steels (transgranular stress corrosion). Due to this, the often poorly visually perceptible construction component weakening is dangerous!

Stress corrosion cracking is only possible if the following three conditions are present simultaneously: 

  • The surface of the component is under tensile stress
  • Impact of a specific acting medium (usually chloride ions, possibly hydrogen e.g. hydrogen embrittlement by humidity in the welding or pickling processes
  • Tendency of the material to stress corrosion cracking

Stress corrosion cracking is to be expected primarily in chloride media, whereby the risk generally increases with increasing chloride content, decreasing pH value and increasing temperature. The stability of stainless steels against chloride-induced stress corrosion cracking decreases in the following order: ferritic chromium steels, ferritic-austenitic steels, austenitic chromium-nickel (molybdenum) steels. Experiments have shown that the materials 1.4301 / 1.4307, 1.4401 / 1.4404 or 1.4571 are vulnerable to stress cracking up to 50°C, whereas the lean duplex steels show virtually no stress corrosion cracking up to this temperature!

Bimetallic corrosion (contact corrosion / galvanic corrosion)

Bimetallic corrosion

This type of corrosion occurs when two different metallic materials are in contact in the presence of a liquid medium, which acts as an electrolyte. The less noble material (anode) is attacked at the contact point and goes into solution. The nobler material (cathode) is not attacked. The stainless steels are generally the more noble materials in comparison to many other unalloyed and low-alloy steels and aluminum. Bimetallic corrosion is particularly critical when the surface of the noble material is large in comparison to the surface of the less noble material. The greater the potential difference of the two materials, the higher the risk of galvanic corrosion. Damage can be avoided by insulated the two materials from each other. When significant differences in size of the workpiece surfaces of paired materials, the smaller area should be the more noble material and the larger area the less noble material.