The rising cost of copper and global supply chain headaches have led more municipal water system owners to switch to using stainless steel piping. On the surface, stainless steel is a straightforward choice; it’s less expensive, has similar corrosion-resistant properties, and is lighter than copper. However, this analysis overlooks copper’s unique antimicrobial property. Stainless steel doesn’t have the same contact-killing effect on microorganisms, such as bacteria, algae, fungi, etc. As a result, stainless steel piping can prematurely fail due to microbiologically influenced corrosion (MIC).
Although 316 austenitic stainless steel has comparable, albeit lower, chloride resistance as the copper alloys used in water systems, the same cannot be said about the leaner, less expensive austenitic grades. This too, has led to early failures.
Naturally, these setbacks come as unwelcome surprises to many operators and installers with plenty of finger-pointing as a result. However, this doesn’t mean that stainless steel is the wrong choice. It means the differences between alloys must be recognized and considered when building the system.
Two Corrosion Causes, Two Corrosion Types
As mentioned above, the two most typical causes for these early failures are MIC and chlorine. With MIC, commonly present microbes will feed on nutrients present in the water. One particular microbe family, referred to as sulfate-reducing bacteria (SRB), is a significant cause of MIC. While they do not themselves attack the metal, they produce enzymes that accelerate the reduction (conversion) of sulfate compounds in the water into corrosive hydrogen sulfide (H2S). Both copper alloys and the austenitic stainless steel are susceptible to attack with even the smallest amount of H2S present. This is the danger of MIC, an otherwise harmless chemical environment can become damaging to a resistant metal alloy if certain bacteria colonies are allowed to grow on its surface.
Under normal conditions, 316 stainless steel can resist up to concentrations of up to 1000 parts per million (ppm) of chlorine, but the design of the system can create conditions where it only requires around 5ppm to begin the corrosion process. The chlorine attacks the chromium oxide (Cr2O3) that makes up the passive layer and once it penetrates, the bare metal surface is free to corrode.
Whether it’s caused by bacteria or chlorine, localized corrosion takes two primary forms: pitting and crevice corrosion. It occurs when the passive layer is weakened by the acidic environment – with compounds like H2S or Cl–. Once the passive layer is penetrated, it allows contact between the acid and the easily corroding iron in the base metal. Once the corrosion starts, it continues further into the metal, forming pits. The local environment inside of a pit becomes far more corrosive than outside the pit because the aggressive chemicals inside the pit are typically not flushed out by the flow in normal operation. If not detected, the pits will eventually penetrate fully through the pipe, causing it to leak and require replacement.
Crevice corrosion is a geometry-based corrosion mechanism stemming from a stagnation of the process fluid. Features such as: rough weld beads, improperly sealed fittings, and loose/damaged gaskets are crevice formers that do not allow for proper mixing, or homogenization, of the local chemistry. When mixing isn’t possible, a differential concentration cell occurs, with oxygen being the major element imbalanced. The oxygen-rich region behaves cathodically compared to the oxygen-poor, stagnant region, and corrosion begins in the oxygen-poor region. MIC and crevice corrosion go hand-in-hand because the biofilm in which MIC-inducing bacteria may exist can provide the detrimental geometry in which a crevice develops. The adverse geometry plus aggressive chemistry formed from bacterial processes can lead to a severely exacerbated crevice corrosion issue in an environment featuring process fluid and resistant metal that would otherwise be safe from chemical attack.
Designing for the Difference
A common factor leading to these two types of corrosion is that certain areas of the system set up conditions to cause the failure. A combination of heat, stagnant or slow-moving water, accompanied by the presence of aggressive chemicals makes the perfect recipe for the corrosion of stainless steel. A “dead leg” where water can stagnate allows microorganisms to settle to the bottom of the pipe and build up. Similarly, tight curves, which are characteristic with copper pipe, slow the flow of water enough to induce corrosion. Copper’s antimicrobial characteristics allow for leniency in the amount of stagnation in system design with respect to MIC susceptibility, but these same design features are not suitable in stainless steel systems.
The answer is to design the system so that water does not stand within the system and can constantly flow at a determined rate. Bends in the piping need larger radii to accomplish this same goal, and 316 stainless steel must be chosen over 304, 306 or other grades.
Even with these steps, more can be done to ensure the integrity of the system. After the piping has been assembled and welded, it should be treated to enhance the passive layer. Flushing the system is good for removing any construction debris which might collect and form a starting point for corrosion, but it does not enhance the chromium oxide layer. The inner surfaces of the piping may need to be cleaned, pickled, electropolished, or some combination of all three processes before a chemical passivation process is performed. Chemical passivation selectively removes free iron from the stainless steel surface in favor of the more resistant element chromium. As the amount of chromium relative to iron on the surface increases, so does the general corrosion resistance.
After the system is commissioned, the passive layer can be restored to “like new” strength with regularly scheduled cleaning and re-passivation. Along with regular chemical analysis of the water within the system to detect signs of microbes or corrosion product, passivation can fulfill cost-saving and reliability promises of using stainless steel in municipal water systems.