Staying in control of corrosion
Dr A Crossland, senior corrosion engineer and Dr JFD Stott, senior project officer of Capcis offer their explanation to the question 'what is microbiologically induced corrosion (MIC)?'MIC is corrosion that is produced or accelerated by the lifecycle processes of biological organisms. Some micro-organisms can have a direct effect upon corrosion, consuming metal as part of the metabolic process.
Alternatively, and more commonly, it can be an indirect effect as a result of the by-products of bacterial activity, such as H2S generated by sulphate-reducing bacteria or organic acids generated by aerobic bacteria.
Corrosion under organic growth is a major cause for concern because the local environment is inaccessible to corrosion inhibitor and biocide treatments and remediation often requires physical cleaning to be effective - often a costly and time consuming activity. The main types of biological growth implicated in MIC are:
- algae - require air, water and sunlight for photosynthesis. Growth is therefore found in waterways, cooling towers and ponds. Accumulations of dead algae can plug heat exchangers, filters and pipelines,
- bacteria - most common forms found in cooling systems are slime-forming, sulphate-reducing and iron-oxidising bacteria. Most are aerobic but unlike algae do not require sunlight. The group that assist in increasing the corrosion problems are the anaerobic sulphate-reducing bacteria,
- fungi - celluloytic organisms that secrete enzymes to convert cellulose into readily absorbed compounds. Major problem area is the wooden slats within cooling towers and certain fungi can break down paint coatings.
Corrosion problems resulting from fouling include differential aeration cells due to low oxygen levels and under deposit attack. The biomass or fouling produced by aerobic bacteria may provide a habitat for anaerobic sulphate-reducing bacteria (SRB) - desulphovibrio and desulphotomaculum. SRB are the most commonly implicated bacteria in cases of MIC and can result in sever pitting corrosion both internally within pipelines, tanks, externally of marine structures such as piling and in sewage systems in conjunction with the aerobic bacteria Thiobacillus. The current new problem of accelerated low water corrosion is simply revisiting the old problem of MIC resulting from SRB growth in marine sediments. SRB produce hydrogen sulphide, which results in metal sulphide films leading to rapidly increased rates of corrosion of many metals. Copper and its alloys are often found in ships' systems or heat exchangers and suffer dramatic flow enhanced corrosion as a result of the formation of copper sulphide corrosion products. The normally dense, adherent, protective oxide scales are replaced by a loose, poorly-adherent copper sulphide that is easily removed by turbulent flow. Iron and steels form iron sulphide films which, in addition to suffering flow-enhanced corrosion, act as a cathode area giving rise to localised pitting corrosion.
Pitting corrosion rates of over 2mm/year have been observed and flow-assisted corrosion rates of several mm/year. The sulphur cycle does not stop there but is often continued by Thiobacilli, which oxidise sulphide to sulphuric acid - up to 15-20% sulphuric acid may be found beneath organic fouling. The basic requirements for SRB proliferation are:
- presence of the bacteria,
- sulphate source,
- suitable carbon source,
- anaerobic conditions,
- near neutral pH,
- temperature within active range for growth,
- pressure less than 500 atmospheres. Although bacteria can proliferate in the aqueous liquid phase, the bulk of corrosion problems are encountered as a result of sessile bacteria or those bacteria within a biofilm at the metal surface. A schematic example showing the steps of the biofilm formation is shown in Figure 3. The biofilm may compromise outer layers of aerobic bacteria and organic matter that become trapped on the pipe surface. Aerobic bacterial activity depletes the inner regions of the biofilm of oxygen, forming an anaerobic zone near the metal surface.
Aerobic and anaerobic bacteria develop where the conditions are most favourable for them and SRB may proliferate rapidly in the anaerobic region. As the biofilm thickens, differential aeration cells can be established resulting in localised corrosion, proliferation of SRB then results in H2S generation and accelerated pitting corrosion A corrosion cell requires a number of basic components, an anode, a cathode and a conductive electrolyte. The most basic corrosion effect of biological fouling is the establishment of a differential aeration cell, shown schematically in Figure 4. SRB utilises sulphate ions to generate energy in conjunction with organic carbon compounds as follows:
SO42 + organic carbon ? H2S + H2O + CO2 + energy Alternatively, the SRB can utilise hydrogen generated from the cathodic reaction during corrosion: 4H2 + SO42- ? 4H2O + S2- Sulphides in the form S2-, HS- or H2S react with the ferrous substrates to form FeS corrosion products.
The FeS scales are crystalline and as such have a relatively large surface area. They are also strongly cathodic and stoichometrically unstable, changing form over time from mackinawite to other forms of iron sulphide, such as pyrite or greigite. Sulphide films are often considered to be semi-protective, however, as the film changes chemically, its mechanical properties also change and forms such as pyrite are brittle and spall easily or are damaged by turbulent flow or particle impingement. As the films are damaged, areas of the underlying steel are exposed. A galvanic corrosion cell is also established, which can drive localised pitting corrosion at more than 2mm/year. Figure 4 shows a typical corrosion cell. A sulphur cycle is often found where sulphide generate by SRB is metabolised by the aerobic bacteria Thiobacclius to produce sulphuric acid, viz:
H2S + 202 ? H2SO4 + energy
Such activity has been found in sewage pipes, WTWs, ventilation systems and on the underside of deck plates in vessels carrying sour crude oil. These are typical locations where fluids may be partly aerated, which may stimulate aerobic bacteria, producing sulphuric acid from sulphides or sulphur present from oxidised sulphide scales.
The typical overall cell is shown in Figure 5 and an example of a failed sewer pipe as a result of Thiobacillus is shown in Figure 6. Once MIC has been initiated, it is difficult to control simply by adding biocides. Chemical treatments have poor penetration through corrosion products or biofilms and are much less effective on established sessile bacterial colonies. Effective control can only be achieved in conjunction with thorough physical cleaning. Biocides fall into two categories, organic and oxidising.
Oxidising biocides, such as chlorine or ozone, are commonly used in potable water systems or oilfield waters. They are consumed by reaction with organic compounds in the fluids and have a relatively short lifetime.
Organic biocides are based on a number of compounds - aldehydes, such as gluteraldehyde, quaternary amine compounds and organo-bromides. The use of organic biocides is limited by the requirements for shipping and handling of such toxic substances and discharge illuminations.
The key to effective control with biocides is product selection and the dosing regime to prevent fouling reaching a level where chemical treatment alone is insufficient. Biocide dosing in pipelines is often combined with pigging. Pigging acts to disrupt the biofilm, remove corrosion scales or precipitates and improve biocide penetration.
Figure 7 shows the corrosion products removed from a SRB infected 200m long water flowline by pigging.
Note the extensive iron sulphide corrosion products. CP is widely used to control corrosion within tankers and can be effective at preventing MIC as a result of SRB proliferation by achieving a polarisation of -900mV. Barrier coatings also provide an effective control against MIC.
However, both methods have limitations. CP controls corrosion by cathodically polarising the metal surface to stifle the anodic corrosion reaction and does not actually control the bacterial population. Coatings simply provide a physical barrier to prevent contact of the corrosive species (organic acids, H2S, etc) with the metal surface. In both cases, bacterial growth can still continue and in the event CP was lost or coating damage or breakdown occurred, corrosion would initiate rapidly. The best approach to the control of MIC is prevention and good housekeeping to prevent sessile films becoming established. Regular pigging of pipelines, development of a biocide treatment regime and flushing of stagnant areas where possible help to mitigate MIC. Microbial control can be improved at the design stage, a minimum fluid velocity of 2m/s tends to inhibit biological fouling or cupronickel and titanium surfaces such as heat exchanger tubes.