Microbially induced corrosion requires two elements to be present:Microbes which can initiate the attack
Exact microbes -- and the resulting damage they cause -- vary. However, bacteria are often a common source of microbial corrosion.
Most bacteria -- both aerobic and anaerobic -- fit into one of four categories:
Sulphate-reducing bacteria: Includes Desulfovibrio and Desulfotomaculum
Sulphur-oxidizing bacteria: Includes Thiobacillus, Beggiatoa, Chlorobium, and Chromatium
Slime-forming bacteria: Includes Pseudomonadaceae, Enterobacteriaceae, Micrococcaceae, and Bacillaceae
Iron-oxidizing bacteria: Includes Siderocapsa, Gallionella, Sphaerotilus, sheathed iron-oxidizing bacteria, and Hyphomicrobium
Each group of bacteria will exhibit different responses and encourage or accelerate different forms of corrosion.
Regardless of the bacteria, environment, or materials involved, microbial corrosion progresses across three phases:
Microbe attachment
Growth of initial pit and tubercle
Maturation of tubercle and pit
In most cases, this growth and progression will appear in localized patches as microbes tend to settle and colonize in distinct regions or patches.
These then form tubercles which help to further encourage concentration and protect the corrosive region from treatment and removal.
While corrosion could spread to cover an entire surface given enough time, symptoms are often detected -- or failures created -- before reaching this point.