Frequent readers of this blog are familiar with stainless steel being highly corrosion-resistant, yet not corrosion-proof. Chlorine-based compounds are just one of the chemicals that can damage the protective chromium oxide (Cr2O3) layer. Once this passive film is breached, the underlying metal can begin to corrode. In most cases, this corrosion can be detected during regular inspections, including checking filters and looking for evidence of rouge or corrosion. However, another corrosion phenomenon exists other than rouge that frequently takes operators by surprise. This is known as Microbiologically Influenced Corrosion, or MIC.
Microbiologically influenced corrosion, as the name suggests, is caused by the presence of microorganisms. These can be bacteria, fungi or microalgae. These organisms don’t corrode the metal themselves, but they accelerate existing corrosion reactions. The existence of MIC was first reported in the nineteenth century. It affects many types of structures – including vessels and pipelines – in a wide variety of industries. Systems transporting or storing crude oil and hydrocarbon fuels are particularly hard hit. However, those which are exposed to soils, seawater or process chemistry can also experience MIC. Most notably, water distillation, sewage treatment and municipal freshwater systems can be especially vulnerable because MIC corrosion is not expected.
What is Microbiologically Influenced Corrosion (MIC)?
MIC occurs when microorganisms are present along with a combination of water, nutrients, a metallic host location and oxygen. Heat and stagnant or slow-moving water create ideal situations for MIC as it allows the microbes to clump together. A biofilm develops and adheres to the metal surface, disrupting the smoothness thereby allowing other organisms or contaminants to collect.
Rouging of stainless steel typically requires the presence of free iron on the metal surface. MIC is such a concern because it doesn’t need free iron to begin corrosion. Even without iron contaminants, the presence of the biofilm changes the chemistry of the immediate environment to something far more aggressive.
A common MIC mechanism is the formation of corrosive chemicals through anaerobic (oxygen-absent) bacteria metabolism. Some anaerobic bacteria can use any sulfur in the water to sustain respiration when oxygen is scarce. These Sulfur-Reducing Bacteria (SRB) reduce elemental sulfur to highly acidic hydrogen sulfide (H2S). Like chlorine, hydrogen sulfide will break down the passive layer and begin to form pits in the metal’s surface as it chemically reacts. Once formed, the growth of a corrosion pit is autocatalytic. This means that not only is the corrosion reaction self-sustaining, it is also compounding and speeds up the reaction rate over time. This is due to the pits themselves becoming pockets where ions generated by the ongoing reaction cannot be flushed out of the area by any flow of the surrounding solution. The presence of these ions decreases the pH of the local liquid, making it more acidic, and therefore, a more corrosive environment.
What makes MIC such an unpleasant surprise is that system operators often don’t know it’s taking place until the corrosion results in what looks like a weld failure, but it’s the MIC creating a pinhole leak. In most cases, MIC appears in heated affected zones (HAZ). These can appear to result from bad welds or defective material because it takes careful analysis of the metal and the liquid to determine that the cause was MIC and not another source. As a result, the situation that caused the failure can persist even after the damaged section is repaired or replaced.
Preventing MIC Before It Begins
In many cases, system design sets the stage for conditions favorable to MIC. “Dead legs” or tight bends in piping where the water does not flow regularly or can flow only slowly, promote biofilm growth. In addition, crevices in the surface of the vessel or piping, such as joints, can serve as an anchor point for biofilm. Crevices, like pits, may corrode autocatalytically when the aggressive chemistry starting the corrosion cannot be flushed out.
The first condition can be prevented – or at least significantly reduced – by engineering piping systems to avoid slow-flowing or stagnant water. For systems that constantly flow or have an idle flow setting, piping should be designed to prevent areas where the velocity is greatly reduced, such as increasing the radii of all bends. For lines that are not sloped to remove water, the process of dewatering pigs can assist in removing the residual water.
After a system is properly designed, properly treating the stainless steel system is crucial. The wetted surface should have minimum roughness, as measured by the roughness average, or Ra. This minimizes the ability of biofilms to find a place to anchor. All joints must be made smooth to avoid the creation of crevices as mentioned above. These goals can be achieved with chemical pickling or electropolishing as the system is constructed or prior to commissioning. The final step before commissioning is to chemically enhance the stainless steel’s natural passive layer with a passivation treatment. This ensures that the entire surface of the stainless steel – including weld and joint locations – is maximally resistant to chemical interactions.
Once the system is operational, it should be regularly inspected. This inspection should include water analysis, looking specifically for biocontamination. Additionally, re-passivation treatments should be regularly scheduled to renew the passive layer. The frequency of these treatments should be based on the operation and operating conditions of the system. Taken together, these steps should reduce the likelihood of any unpleasant surprises courtesy of MIC.