Stainless steel is used in waste water treatment plants for its corrosion resistive properties. Stainless steel does not experience thinning like milder steels and a corrosion allowance is not required for stainless. Stainless steels in this industry have a low maintenance requirement and a long service life. Chemical additives are not required to maintain corrosion resistance once the stainless has undergone a proper passivation process. Furthermore, aeration processes do not lead to rates of higher corrosion with stainless as they do with carbon steel.
While choosing the right grade of stainless steel is important, there may be some limitations. In environments where there are changing pressures, stainless steel can risk fatigue over time. Several grades of stainless steel (such as 303 and 304) are susceptible to certain types of corrosion, including crevice corrosion, chloride pitting, pinhole leaks, and external corrosion. Some ways to ensure the longest life of stainless steel used for wastewater applications include:
- Drain pipelines during periods of non-use to prevent stagnant water, which encourages microbial activity that causes corrosion to occur.
- Pickling stainless steel pipes to help control surface corrosion. Heat tints near welded areas can make them susceptible to biocorrosion and the pickling process promotes a corrosion-resistant surface. In the pickling process, scale is removed from prior annealing treatments that can cause a depleted chromium layer at the annealing or inclusion points. When the pickled stainless steel part is exposed to the oxygen in the air, a passive layer forms which is the source of the corrosion resistance.
- Follow proper operation guidelines to prevent the beginning of corrosion caused by scaling, excessive chlorination, poor fabrication, or MIC (Microbiological Influenced Corrosion).
The corrosion resistance of stainless steel is primarily due to the presence of a stable passive film that forms on the surface. However, the presence of microorganisms, chloride ions, and reduced sulfur compounds are potential causes of localized pitting and corrosion attacks in stainless steels. An electropolished surface both prohibits microbes from attaching to the underlying steel creates a surface that is easier to clean and less prone to developing biofilms. Certain grades of stainless steel — including duplex and austenitic alloys — can offer enhanced resistance to microbial corrosion when electropolished. Of course, stagnant water or damaged surfaces can diminish the effectiveness and with enough exposure or abuse, even stainless steel will succumb to the attack of microbes. This makes proper care and maintenance of stainless steel components a critical aspect in fending off microbial corrosion. Electropolishing removes surface imperfections and restores the corrosion-resistant layer to the stainless steel surface.
Electropolishing offers the following benefits:
- It greatly reduces the potential for MIC because it is more difficult for bacteria to adhere well to the smooth surface.
- It provides a higher level of corrosion resistance in high-chloride (and high-chlorine) environments such as brackish and seawater RO applications.