Innovation and Science:

Corrosion is one of the most important types of damage for metal materials. This form of negative change of material caused by the environment has probably been a well-known phenomenon for as long as metal has been used. The analysis of such damages has however by no means become a routine, and the examination of the processes connected with it still has a long way to go. New materials are being used in new areas of application, and new damaging mechanisms are being discovered all the time. Probably one of the most fascinating types of corrosion is the so-called microbiologically influenced corrosion (MIC); although it has been known among experts since the beginning of the 20th century, it has only attracted greater attention in the general damage analysis during the last few years.

Corrosion through bacteria

MIC develops through the interaction between microorganisms and the material. One believes nowadays that the ubiquitous organisms, especially bacteria and fungi, settle on the surface of materials and form a so-called biofilm. Certain types of organisms can then under favourable conditions cause chemical changes in this biofilm which have a corrosive effect on the material below. Local forms of attack occur which cannot be explained by the chemical composition of the medium alone.

Different metabolism

MIC can be classified on the basis of the type of metabolism involved. There is for example the group of organisms called sulfate reducers, whose metabolism product of hydrogen sulphate can be aggressive. Others form organic acids or mineral acids (nitric acid, sulphuric acid) as corrosive media. Here we want to take a closer look at the group of manganese-oxidizing microorganisms (MOMOs), which have been studied scientifically for about 15 years at the Technische Versuchs- und Forschungsanstalt of TU Vienna. Their damaging effect concerns mainly stainless steels which show the typical chlorine-induced damage pictures. They can however occur already when the medium contains amounts of chloride which are far below the normal resistance limits. One comes across this type of damage mainly in applications in natural freshwater (drinking, well or river water).

Univ.-Prof. Dipl.-Ing. Dr. Paul Linhardt

EdlSpecial character through passive layer

The class of stainless steels is often referred to colloquially as special steel. The alloy elements are however by no means electro-chemically special. Their special character actually derives from the so-called passive layer which forms invisibly thin through the extremely fast reaction of the metal with the surrounding air and which protects the material from corrosion at least in neutral media like water. In such an environment corrosion generally follows the electro-chemical mechanism, i.e. an oxidant from the medium (usually the dissolved oxygen) tries to react on the surface of the material with electrons of the metal. Electrons are released through the transformation of metal atoms into positively charged ions, the ions pass into the medium and form corrosion products there. The passive layer on stainless steel then prevents the release of metal ions nearly completely and thus prevents the total reaction.

Weak spot of the protective layer

Chloride ions have the adverse quality of settling specifically on weak spots of the passive layer and penetrating it. On such imperfections the metal dissolution can proceed unhindered, resulting in hole corrosion. For this perforation there is a connection between the strength of the oxidant, which is reflected in the electro-chemical potential, and the chloride content of the medium. The stronger the oxidant, the higher the potential will be and the lower the chloride concentration sufficient for the perforation. This connection can be examined through an electro-chemical method. For this test procedure the potential is being varied and at the same time the corrosion current is measured. In this way one can assess the corrosion sensitivity of a material in a medium. In practice there are limits for the acceptable chloride concentration for the various stainless steel alloy categories. For the material 1.4301 for example 200 mg/l of chloride in water is considered as critical. For this it is however implicitly assumed that the oxidant is oxygen. In case of stronger oxidants the limit has to be set lower.

Manganese-oxidizing microorganisms

These microorganisms have the quality of forming an oxidant which has a stronger effect on the metal surface than oxygen. This is the solid manganese dioxide (MnO2, pyrolusite), which moreover speeds up the reaction of oxygen and thus produces a double effect. If such naturally occurring organisms settle on components made of stainless steel, much lower chloride concentrations than normal will already lead to hole corrosion.

Damage on turbines and pipelines

This mechanism was first demonstrated on turbines of a run-of-river power station in the Netherlands. In the meantime several such damages were identified in run-of-river power stations worldwide. But this type of corrosion was also found repeatedly in pipelines, mainly in connection with on-site produced welding seams. Such seams usually do not get any finishing treatment and therefore do not have an optimally formed passive layer, and often there is no passivity at all because of temper colours. Such spots will corrode a bit when they begin to be used, but usually have a high probability of re-passivating. In the presence of MOMOs and a bit of chloride re-passivating is however highly improbable and the result is corrosion to the point of breakage. At present these interactions are the object of detailed investigations. Electro-chemical measurements during such damage analyses have shown that a well passivated material can be completely corrosion-resistant in the respective water, even under stress much higher than that caused by MOMOs. Such observations are another reason for the increasing quality demands for the welding of stainless steel. The resistance of a good passive material can only be achieved for a welding seam through correct formation and through pickling and passivating.

Author: Ao. Univ.-Prof. Dipl.-Ing. Dr. Paul Linhardt