The escalating cost of copper and challenges in the global supply chain have prompted more owners of municipal water systems to transition to the use of stainless steel piping. At first glance, selecting stainless steel appears straightforward: it is less expensive, has comparable corrosion-resistant qualities, and is lighter than copper.
However, this assessment overlooks copper's distinctive antimicrobial characteristics. Unlike stainless steel, copper boasts a contact-killing impact on microorganisms like bacteria, algae, and fungi. Consequently, stainless steel piping can encounter premature failure due to microbiologically influenced corrosion (MIC).
Though 316 austenitic stainless steel demonstrates similar, albeit somewhat reduced, chloride resistance compared to the copper alloys employed in water systems, the same cannot be said for the leaner, more economical austenitic grades. This factor has also contributed to premature breakdowns.
Naturally, these setbacks come as unwelcome surprises for many operators and installers, often resulting in finger-pointing. Nevertheless, this does not imply that opting for stainless steel is incorrect. Instead, it underscores the need to recognize and account for variations between different alloys when constructing the system.
Two Corrosion Causes, Two Corrosion Types
As noted earlier, the two most common reasons behind these premature failures are MIC and chlorine. In the case of MIC, prevalent microorganisms in the water consume available nutrients. Notably, a group of microorganisms known as sulfate-reducing bacteria (SRB) significantly contribute to MIC.
While these microbes do not directly attack the metal, they produce enzymes that expedite the transformation of sulfate compounds in the water into corrosive hydrogen sulfide (H2S). Both copper alloys and austenitic stainless steel are vulnerable to corrosion even in minimal H2S.
This illustrates the threat of MIC; an otherwise innocuous chemical environment can turn detrimental to a resilient metal alloy if specific bacteria colonies are permitted to thrive on its surface.
Under standard circumstances, 316 stainless steel can withstand chlorine concentrations of up to 1000 parts per million (ppm). However, the system's design can create conditions where corrosion initiation requires merely around 5 ppm of chlorine.
The chlorine targets the chromium oxide (Cr2O3) constituting the passive layer, and once breached, the exposed metal surface becomes susceptible to corrosion.
Whether caused by bacteria or chlorine, localized corrosion takes two primary forms: pitting and crevice corrosion. This phenomenon arises when the passive layer succumbs to the acidic environment due to compounds like H2S or Cl-. Once this layer is breached, it permits interaction between the acid and the readily corroded iron in the base metal.
Once corrosion initiates, it progressively penetrates the metal, giving rise to pits. The interior of these pits becomes significantly more corrosive than the surroundings, as the potent chemicals within them are typically not flushed out by the normal operational flow.
If left undetected, the pits will eventually perforate the pipe entirely, resulting in leakage and necessitating replacement.
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Crevice corrosion, a corrosion mechanism influenced by geometry, emerges from fluid stagnation. Features like rough weld beads, inadequately sealed fittings, and damaged gaskets create crevices that hinder proper mixing or homogenization of the local chemistry.
When adequate mixing is unachievable, a differential concentration cell forms, primarily imbalanced in terms of oxygen. The oxygen-rich area behaves cathodically compared to the oxygen-poor, stagnant region, prompting corrosion to commence in the oxygen-depleted zone.
MIC and crevice corrosion are closely related since the biofilm housing MIC-inducing bacteria can foster unfavorable conditions for crevice development.
The adverse geometry, coupled with the aggressive chemistry resulting from bacterial processes, can exacerbate crevice corrosion significantly in environments where process fluid and resistant metal would otherwise be impervious to chemical attacks.
Designing for the Difference
A shared factor contributing to these two forms of corrosion is that particular system sections create conditions that lead to failure. A combination of heat, stagnant or slow-moving water, coupled with aggressive chemicals, forms the ideal recipe for the corrosion of stainless steel.
A section known as a "dead leg," where water can gather and stagnate, allows microorganisms to settle at the pipe's base and accumulate. Likewise, tight bends, often found in copper pipes, decrease the water flow speed enough to induce corrosion.
Copper possesses antimicrobial properties that allow some leniency in terms of stagnation in system design concerning susceptibility to MIC. However, these same design traits are not appropriate for 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 should have larger radii to achieve the same objective, and the selection should favor 316 stainless steel over 304, 306, or other grades.
Despite these measures, additional steps can be taken to ensure system integrity. Following the assembly and welding of the piping, it should undergo treatment to enhance the passive layer.
Flushing the system effectively removes construction debris that could gather and become a starting point for corrosion but does not strengthen the chromium oxide layer.
The inner surfaces of the pipes might require cleaning, pickling, electro-polishing, or a combination of these processes before applying a chemical passivation method. Chemical passivation aims to remove excess iron from the stainless steel surface, promoting the presence of the more resilient chromium element.
As the chromium content relative to iron increases on the surface, the overall corrosion resistance also rises.
Once the system has been put into operation, the passive layer's effectiveness can be restored to a "like new" state through regular cleaning and re-passivation routines.
In addition to routinely analyzing the water within the system to identify indications of microbes or corrosion byproducts, passivation helps uphold the cost-saving and reliability benefits associated with utilizing stainless steel in municipal water systems.
This information has been sourced, reviewed and adapted from materials provided by Astro Pak Corporation.
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