ABSTRACT
While we know that the presence and activity of microorganisms can directly or indirectly influence corrosion of metals in many different ways, the common view is often oversimplified, for example, assuming that bacteria “prefer” growth on weld metal as compared to pipe parent metal. When viewed from the scale of a microorganism, the microstructure of most engineering alloys is heterogeneous in composition, mechanical properties, residual stresses, local potentials and surface topography. Each of these characteristics can influence both abiotic and biotic corrosion initiation, propagation, and growth. Features that are visually observable (welds, crevices, pits, etc.) are enormous when considered from the microbiological scale and may have more macroscopic effects on corrosion than microscopic features.
Metallurgy is an important consideration in both the design phase and in the operations phase of oil and gas asset life. In the design phase, engineers consider corrosion threats, including microbiologically influenced corrosion (MIC), and weigh the lifecycle costs of using carbon steel with chemical treatment (with lower capital expense [CAPEX] and higher operating expense [OPEX]) or corrosion-resistant alloys (CRAs) with higher CAPEX and lower OPEX. CRAs offer many advantages in corrosion resistance as compared to carbon steels; however, they are also susceptible to localized corrosion or cracking mechanisms under specific physiochemical conditions. MIC is often a localized corrosion mechanism, and it occurs under biofilms that form occlusions 36on the metal surface and in crevices that challenge the pitting resistance of many CRAs, particularly in chloride-containing environments. Thus, when using CRAs, it is important to consider whether deposits and biofilms will form on the surface at some point in the operational life of the asset because alloys with good resistance to abiotic corrosion in a specific service environment may offer minimal improvement in performance over carbon steel if biofilms are present.
Metallurgy is also an important consideration when assessing potential MIC problems in an asset, such as when conducting a basic or root-cause analysis (RCA). RCA has proven to be a useful tool when MIC investigations are conducted properly and thoroughly because the end result should provide the operator a means to prevent similar incidents in the future. When assessing corrosion damage or performing a corrosion root-cause analysis, metallurgy must be considered both in terms of abiotic effects and biotic effects. Some manufacturing and fabrication processes can have a detrimental effect on biotic or abiotic corrosion resistance, resulting in performance that can be significantly different from what was predicted based upon common electrochemical or corrosion coupon testing. Understanding the effects of various manufacturing and fabrication practices on corrosion resistance can allow users to optimize those practices to reduce the likelihood of, or severity of, corrosion.
