Stainless steel in waste water treatment plants

PUBLISHED 9 May 2017

Stainless steel has been used in waste water treatment plants over the last fifty years due to its corrosion resistive properties. This is primarily because they do not experience thinning and a corrosion allowance is not required.


Crevice corrosion at waste water treatment plants

In particular, stainless steel type 304 and 316 have been very successfully used in a range of water and waste treatment systems. Thus, they have a low maintenance requirement throughout their long service life. One doesn not require any chemical additive in order to maintain corrosion resistance. Furthermore, aeration processes do not lead to rates of higher corrosion which maybe the case with carbon steel.

Corrosion in waters normally occurs in crevices where there is a fold or join in a panel. However, crevice corrosion is rare in stainless steel and type 304L and 316L. Chloride levels are under two-hundred parts per million and one-thousand parts per million there. Best practice dictates that crevices are best avoided in the design and fabrication of such plants. Another advantage of stainless steel is that metal leaching rates are also minimal.

Low carbon stainless steel

Using low carbon grades (304L and 316L) when welding and removing surface contamination and heat tint is also good practice when dealing with waste water plants. Stainless steel 316L contains molybdenum at two percent allowing the stainless steel to resist corrosion from chlorides such as seawater. This means that 316L grade steel is a popular choice for chemical processing equipment. It also is for waste water plants in particular where chemical content could increase corrosion risk.

Anti-corrosive qualities

Stainless steel has a higher corrosion resistance than carbon steel and also performs well during maximum flow of water. It also remains anti-corrosive and is high-performance. This is because it has a thin oxide film which forms upon exposure to the air when the steel is removed from pickling at the steel mill. Stainless steel is resistant to microbial action, which can cause corrosion in other metals particularly in crevices. Stainless steel grade 316L performs excellently when exposed to corrosive conditions as a result of highly corrosive pollutants such as chloride, ammonia or sodium hydroxide (soap).


316L stainless steel and anti-corrosive properties

Crevice corrosion or oxygen concentration cell corrosion results from high chloride concentrations and very low pHs. It is known that crevice corrosion of 316L stainless steel is not likely to be found in waters where the rate of chlorine is under one-thousand parts per million. Crevice corrosion of 304 stainless steel is not likely to occur in waters where chlorides are two-thousand parts per million. Therefore, if over two-hundred parts per million of chloride is found to occur, type 316L stainless steel is recommended to be used over 304 type stainless steel. This is due to the fact, that this would provide additional anti-corrosive properties for the waste water industry.

Minimal metal degradation

Stainless steel is capable of handling highly corrosive liquids with minimal loss of metal or degradation. Stainless steel 316L has proved to be particularly resistant to microbiologically influenced corrosion given its higher molybdenum content than that of other stainless steels.


Lower costs when building waste water treatment plants

Biofilms which form on waste water treatment plants can increase manganese oxides. Those can be corrosive to the stainless steel metal causing pitting and corrosion. Controlling corrosion with anodic or cathodic protection is another option for reducing the costs of corrosion. It is worth considering the economic impact of corrosion and choose a higher grade of lower carbon stainless steel. Thus, you can mitigate against the effects in waste water treatment plants. This will prolong their life and also lower costs.

Material availability

At Montanstahl, all different measures of standard structural sections such angle bars, channels, beams and T-sections are available both in 304 and 316L.
For highly corrosive environments, dedicate sections can be produced using special alloys such as duplex or super-duplex grades or high alloy grades like 904L or 254 SMO. Therefore, tap water can be treated without the consumer fearing allergies created by plasticizers like Bisphenol A.

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