Hangbo Alloy Group
Shanghai · Est. 2012
Microbiologically influenced corrosion (MIC) threatens nickel alloys in seawater and process water. A maker explains detection, alloy choice, and mitigation.
Microbiologically influenced corrosion (MIC) is the accelerated attack that takes place when biofilms of bacteria, fungi, or archaea colonize a metal surface and change the local chemistry beneath them. It is a leading cause of unexpected failures in seawater cooling systems, ballast water lines, firewater networks, and process water loops — and it does not spare nickel alloys. While high-alloy materials resist uniform corrosion extremely well, a sessile biofilm can create discrete anaerobic pockets where even a 90/10 copper-nickel or a 6Mo super-austenitic grade can pit and perforate. This guide explains how MIC develops on nickel alloys, where it is most likely to appear, how to confirm it, and how to select and protect materials so it does not take your plant offline.
MIC is not caused by a single organism. It is the outcome of a community. In a mature biofilm, aerobic bacteria at the outer layer consume dissolved oxygen, while sulfate-reducing bacteria (SRB) and other anaerobes thrive in the oxygen-free zone against the metal. The SRB produce sulfide, lowering pH and breaking down the passive film that protects stainless steels and nickel alloys. Iron-oxidizing and manganese-oxidizing bacteria deposit tubercles that trap aggressive species. The result is localized attack — pits, under-deposit corrosion, and sometimes cracking — that conventional coupon testing and thickness monitoring routinely miss because the damage is highly sporadic.
The misconception that “noble” alloys are safe is dangerous. Nickel alloys resist bulk chemistry; they do not resist the micrometer-scale chemistry a biofilm engineers against the surface. A 316L stainless pipe can fail in months in a biofilm-rich seawater line, while a 90/10 copper-nickel in the same service survives for decades — not because copper-nickel is nobler, but because it releases trace copper ions that suppress bacterial attachment. Material choice and biofouling control are inseparable.
The common denominator is warm, nutrient-rich, low-flow or stagnant water with a food source. Typical trouble spots include:
For these services, 70/30 copper-nickel piping and Monel 400 remain the workhorses because their natural biofouling resistance reduces the very condition MIC needs. In higher-strength or more corrosive duty, alloy 625 and 6Mo super-austenitics are specified, but only with an active biofilm-control program.
MIC is diagnosed, not assumed. A pinhole leak is a symptom, not a root cause, so the investigation should combine physical, microbiological, and chemical evidence:
Crucially, thickness-loss trends from routine UT can look benign until a single deep pit penetrates. That is why risk-based inspection should target dead-legs, low-flow branches, and post-weld heat-affected zones, where weld microstructure can be slightly more susceptible.
No alloy eliminates MIC on its own; the winning strategy is material plus environment control. Practical measures that work:
When a failure is already happening, the fastest durable fix is usually to replace the affected segment with a more biofilm-resistant alloy and simultaneously correct the water-treatment regime. Treating only one of the two guarantees a repeat failure.
No single grade is “MIC-proof,” but materials that resist biofouling perform best. 90/10 and 70/30 copper-nickel actively suppress bacterial attachment through trace copper ion release, and Monel 400 behaves similarly. For high chloride plus strength, 6Mo super-austenitics and alloy 625 are used, but always paired with chlorination or another biofilm-control program.
Cathodic protection helps carbon-steel seawater systems but is not a primary MIC defense for nickel alloys. Worse, aggressive overprotection can drive hydrogen into the surface and cause hydrogen damage. The reliable approach is biofilm control (chlorination/bromination), flow design that avoids stagnation, and selecting inherently fouling-resistant alloys — see our galvanic corrosion guide for how dissimilar metals complicate CP design.
In stagnant or low-flow seawater, copper-nickel is generally the safer choice because it resists biofouling, whereas 316L stainless is frequently attacked by MIC under biofilms. In continuously flowing, well-treated systems, super-austenitic and duplex stainless can perform well. For a direct comparison of how these families behave in seawater, review copper-nickel 70/30 in marine service alongside the relevant stainless grades for your chloride level.
MIC leaves characteristic evidence: pits under soft tubercles or slime, iron sulfide deposits, patchy (not uniform) attack, and detectable SRB or high ATP in samples. Erosion shows directional wear; fatigue shows a crack with beach marks. Pair visual pit mapping with microbiological testing and deposit chemistry. For components also seeing cyclic loading, rule out low-cycle fatigue of alloy 625 so you treat the true root cause.
As a dedicated nickel alloy manufacturer serving marine, oil & gas, chemical processing, power generation sectors, Hangbo Alloy Group supplies the grades discussed here across all standard product forms with full ASTM/ASME documentation and EN 10204 3.1 certification. Our material is specified for applications where corrosion resistance, high-temperature strength, and traceable quality are non-negotiable. See our production & testing facilities for more on how every heat is verified.