Don't discount small critters and biological growths.
Microbial Induced Corrosion can cause rapid localised attack and degradation of many metals including stainless steels.
Microbiologically Influenced Corrosion (MIC) refers to the corrosion damage of metal surfaces through depolarization by bacteria/micro-organisms resulting in pitting on the surface. Many of these micro-organisms can move/migrate and reproduce rapidly under the right conditions (where their numbers can double every twenty minutes). When favourable conditions (water, a source of nutrients and appropriate range of temperatures) are met MIC can lead to rapid corrosion damage and surface pitting.
The micro-organisms causing MIC have been reported in extreme conditions with pH levels ranging from 0-11, temperatures up to 100℃ and pressures as high as 1000bar, low to high oxygen concentrations and salinity levels up to 30%. Fortunately, ideal conditions for bacteria causing corrosion are generally more specific with temperatures ranging from 15-45℃ and pH ranging from 6-8. However, spores produced by some of these micro-organisms can survive extreme conditions for extended periods until conditions are suitable.
Typical environments where MIC occurs includes soils, fresh water and seawater, raw petroleum products, hydraulic and lubricating oils, and fuels where chemicals in these environments provide the energy source/food for the micro-organisms. The interaction of the micro-organism in the environment can change the anodic/cathodic reaction, damage protective films, create local environments that are corrosive and/or produce deposits that help initiate and maintain corrosive conditions. Although not clearly defined, such micro-organisms are generally divided into either i) aerobic (survival is oxygen dependent) or ii) anaerobic (survives independent of oxygen) groupings.
Some bacteria are able to influence the oxidation or reduction of metals such as iron or manganese and influence the rate of corrosion directly. Other micro-organisms produce bio films (slime or similar deposits) which, due to differences in thickness, affect the concentration of oxygen and or metal ions on the surface causing anodic and cathodic regions on the material.
A number of anaerobic sulphate reducing bacteria (SRBs) produce hydrogen sulphide from any sulphate containing compounds (sea water, oil, fuels) and hydrogen released during the corrosion process (or that from the reduction of cellulose, sugars or other organic products). These sulphides alter the anodic cathodic reaction occurring on iron based products significantly. Although SRB are generally anaerobic some of the bacteria are able to produce a protective film/layer around themselves in which a controlled environment suitable for their survival is established, allowing them to tolerate adverse oxygen concentrations and multiply in hostile environments. These films or tubercles can be highly tenacious, allowing the bacteria to survive changes to the global environment (such as those that occur during flushing or commissioning and operation). Not only do some of these bacteria cause MIC some are also responsible for some forms of pneumonia in humans.
Some aerobic bacteria are capable of oxidising elemental sulphur into sulphuric acid creating localised highly acidic environments that cause rapid dissolution of metals. These bacteria require a source elemental sulphur that is often found in raw petroleum bearing products and where sulphur bearing organic products are common.
These mechanisms generally increase the rate of corrosion and MIC can cause very rapid corrosion. However, it is often not easy to identify conclusively and requires a combination of microbiological, metallurgical and chemical analyses. Often MIC is identified as a potential mechanism by the shape/form of the areas of degradation, rates of corrosion, knowledge of the environment and analysis of the corrosion product but can only really be confirmed by culturing samples and analysing the cultures. Once identified steps to mitigate include the use of inhibitors and germicides. Controlling the environment (e.g. removing the sulphur content by suitable aeration (e.g. the aeration of sewage), removing water from oils/fuels, preventing the build-up of organic material in tanks) may be options to prevent MIC in certain cases. Alternatively coatings and/or use of cathodic protection can also be highly effective.
Published in Technical Tips by Origen Engineering Solutions on 1 June 2017