LITERATURE REVIEW -3

The presence of a network of ferrite particles in an austenitic matrix acts as a barrier to crack propagation, thus enhancing the resistance to stress corrosion of the material. This effects associated to the morphology and distribution of the phase rather than its contents welding in a direction parallel to the stress using reductively high heat input improved,the stress corrosion of the material even further. The HAZ ( heat affected Zone) of AISI 304 Steel was highly susceptible to stress corrosion in chloride solution. The presence of carbide precipitates in the austenite grain boundary deteriorated the corrosion resistance of the steel, as they promoted anodic dissolution and the development of the stress corrosion cracks .
A recent literature review by KOVACH: and others (1993) identified thirty seven publications involving MIC - micro biologically influence corrosion) and stainless steel focus on those which involved the high performance stainless and stainless steels in an effort to determined whether or not MIC poses a threat to the engineering application of these materials .It is easy to summarize the characteristics which are generally regarded as representative of MIC  and help in distinguishing MIC activity. These characteristics have been identified based on service experience and were first detailed by Tatnall (3) MIC which is characterized by:
        (a) The pressure of the distinctively shaped coloured deposits that are usually soft and easily                     deformed.
        (b) Large cavities under pinhole surface penetrations or open pitting,gouging, and stunningly.
        (c) Frequent association with welds.
        (d) Rapid rate of penetration.
However, it is important to know that these corrosion characteristics are not limited or attack by MIC.
It is generally accepted that opposite identification of MIC requires confirmation of the presence of aggressive microbial species in substantial quantities and the elimination of other possible corrosion mechanism.