Application of Stainless Steel Hygienic-Grade Pipes in Wastewater Treatment

Jan 17,2022

304/304L and 316/316L stainless steels are currently the primary materials used in urban wastewater treatment equipment, including screens, grit chambers, overflow weirs, bolts, movable bridges, aeration tanks, digestion tanks, and sludge pipelines. The weight-loss corrosion rate of sanitary-grade 304 and 316 stainless steel pipes is less than 0.1 mil per year. With a corrosion rate below this threshold, these materials demonstrate a promising service life exceeding 20 years.
These very low corrosion rates mean that when designing sanitary-grade stainless steel piping systems, there is no need to factor in a “corrosion allowance” for wall thickness. However, carbon steel and ductile iron pipes must include a corrosion allowance. Consequently, using stainless steel sanitary-grade piping can result in material savings by weight.
It is reported that crevice corrosion did occur on some samples of sanitary-grade stainless steel pipes. This type of corrosion is localized and typically develops in areas that are “shaded” or “creviced,” such as bolted joints, welds that are not fully penetrated, or beneath sludge deposits. To minimize this kind of corrosion as much as possible, crevices should be eliminated wherever feasible, all welds must be fully penetrated, and deposits should be removed regularly. In aeration tanks, sediment is generally cleared during periods when the tank is shut down for odor control and cleaning. After taking these measures, long-term continuous use should not pose significant problems. During operation, agitating or aerating the aeration tank can further enhance the performance of the sanitary-grade stainless steel pipes.
In particular, 316L stainless steel pipes contain molybdenum, giving them even stronger corrosion resistance.
(1) Atmospheric corrosion resistance
The addition of chromium to 304 and 316L stainless steel tubes enhances their corrosion resistance. The expected weight loss of chromium, which is “rust,” can be determined based on its concentration in the steel and how it changes over time. Data indicate that with higher chromium content, the weight loss after 25 years of exposure to marine atmospheric conditions is only about 12.5% of that observed in plain carbon steel—a relatively small amount. However, these “pure chromium” stainless steels develop a surface layer of rust, making them unsuitable for use in many atmospheric environments where appearance is particularly important. Instead, austenitic steels such as 304 and 316, containing 8% to 10% nickel, offer superior corrosion resistance; their exposed surfaces retain their original luster for many years when exposed to atmospheric conditions.
(2) Other chemical additives
Green vitriol is a chemical additive frequently added to wastewater treatment plants. According to Delenzo, sanitary-grade stainless steel pipe samples of grades 304 and 316 exhibit excellent corrosion resistance in green vitriol solutions with pH values ranging from 1 to 2.5, with annual corrosion rates of less than 0.8 mil. In typical wastewater treatment plants that do not contain sulfuric acid—and therefore in environments where green vitriol is present in low-acid conditions—the corrosion rate of sanitary-grade stainless steel pipes can be considered negligible.