Sinopmedicine Group Animal Health Co., Ltd.

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Application of stainless steel sanitary pipe in sewage treatment

Jan 17,2022

304/304L and 316/316L stainless steel are the main materials for screens, grit basins, overflow weirs, bolts, moving bridges, aeration tanks, digesters and sludge pipelines in current urban wastewater treatment equipment. The weight loss corrosion rate of 304 and 316 stainless steel sanitary grade pipes is <0.1 mils/year. The corrosion rate of this material is <0.1 mil/year and the service life shows promising over 20 years.
These very low corrosion rates mean that there is no need to have a "corrosion allowance" for wall thickness when designing stainless steel sanitary grade pipe equipment, but there must be a corrosion allowance for carbon steel and ductile iron. Therefore, the use of sanitary pipes in stainless steel can be saved more by weight/material.
It is said that crevice corrosion did occur on some samples of stainless steel sanitary pipe. This corrosion is localized and generally occurs in "shielded" or "crevice" areas, such as bolted joints, incomplete welds, or sludge deposits. To minimize this erosion, crevices should be eliminated wherever possible, all welds must be fully penetrated, and deposits must be cleaned regularly. The sediment in the aeration tank is generally removed when the aeration tank is deactivated for deodorization. After taking these measures, long-term continuous use will not cause any major problems. Stirring or oxygenating the aeration tank during operation can further play the role of stainless steel sanitary grade pipe.
In particular, 316L stainless steel pipe contains molybdenum element, corrosion resistance is stronger.
(1) Atmospheric corrosion resistance
The addition of chromium to 304 stainless steel pipe and 316L stainless steel pipe improves corrosion resistance. The amount of chromium expected to lose weight for stainless steel to "rust" can be determined according to the variation with the content in the steel. The data indicate that the greater the chromium content ratio and the relative small weight loss after 25 years of marine atmospheric exposure equal to 12.5 percent of the steel. These "pure chromium" stainless steel and the surface layer has a layer of rust, which is not suitable for use in many atmospheric environments where appearance is particularly important. Instead of containing 8% to 10% Ni austenitic steel 304 and 316 more corrosion resistant, the exposed surface remains shiny as first for many years in the atmosphere.
(2) Other chemical additives
Copperan is a chemical additive often added to wastewater treatment plants: Delenzo said that 304 and 316 stainless steel sanitary pipe samples have good corrosion resistance in a solution of copperan with a pH of 1 to 2.5, and the corrosion rate is less than 0.8 mils per year. The corrosion rate of sanitary stainless steel pipes in general sulphuric acid-free wastewater treatment plants and, therefore, in acid-free chlorite environments is negligible.