Localized corrosion such as pitting and crevice corrosion is a concern for natural seawater service, where fouling and/or microbiological influenced corrosion (MIC) may occur.
Macro-fouling is built up of oysters, barnacles, mussels, tube worms and the like. Microbiologically influenced corrosion (MIC) is caused by an active biofilm which develops after some time in a natural water. The biofilm is attached to any material in, for example, seawater.
Once fouling has started, other factors make the conditions worse; amount of inorganic particles – may settle on top of the fouling (sediment) – as well as chloride content and sulphide ions. Pitting or crevice corrosion is a risk under the fouling.
The immersion of stainless steels, as well as any kind of material, in natural seawater induces the development of a microbial film called biofilm which is established after 1-3 weeks. The activity of the micro-organisms in the biofilm causes the electrochemical open circuit potential (OCP) of the stainless steel to increase. This increase is called potential ennoblement, that will increase the risk of crevice corrosion or pitting if the resistance to pitting and crevice corrosion of the steel grade is exceeded. At 35–40ºC (95–104ºF) the biofilm is however killed and the effect vanishes.
The presence of an active biofilm on the stainless steel surface may result in that the seawater is more corrosive at temperatures below 35–40ºC (95–104ºF) than at a few degrees in temperatures above, and the seawater becomes more aggressive as the temperature dependence in corrosivity develops.
Microbiologically influenced corrosion (MIC) does not involve new corrosion mechanisms. Thus, the resistance of stainless steels to this corrosion type increases with increasing content of the alloying elements, which are beneficial for resistance to pitting corrosion and crevice corrosion.
This means that standard austenitic steels of the ASTM 304L and 316L types are vulnerable to both types of microbiologically influenced corrosion (MIC). Sandvik SAF 2205 and Sandvik 2RK65 shall be seen as borderline materials, whereas hyper-duplex, super-duplex and high-alloy austenitic stainless steel grades can be regarded as immune to microbiologically influenced corrosion (MIC) in seawater.
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