Grade 304 Stainless Steel
Grade 304 is the standard “18/8” austenitic chromium-nickel stainless steel. It is the most versatile and most widely used stainless steel, available in the widest range of products, forms and finishes. It has excellent forming and welding characteristics. The austenitic structure also gives these grades excellent toughness, even down to cryogenic temperatures.
304 and its variant are general purpose stainless steel grade with good atmospheric corrosion resistance and many corrosive media. It subjects to pitting and crevice corrosion in warm chloride environments, and to stress corrosion cracking above about 60°C. Considered resistant to pitting corrosion in potable water with up to about 200mg/L chlorides at ambient temperatures. This grade is suitable in drinking water industry in general.
Chloride is an enemy for any austenitic stainless steel since it is a strong deploriser. The risk of pitting and crevice corrosion of 304 Stainless steel increases with the chloride concentration in the water. Although most of drinking water standards regulate the maximum chloride level, the chemical injection may lead to the increase of chloride content locally. This brings the risk of crevice and pitting corrosion locally. The water treatment chemicals may increase the overall chloride content, e.g. chlorine, chloramine, ferric chloride etc. Since 304 stainless steel’s allowable chloride level is relatively low, any contribution for chloride needs to be balanced carefully.
Stagnant water is usually the biggest enemy for 304 stainless steel in drinking water service.
The depleted oxygen in water due to long term stagnant is the main cause of pitting corrosion
- This is particularly serious in circumferential weld with unremoved heat tint.
- Flourish microbial growth during long term stagnant is almost inevitable. The MIC will change the local corrosion environment and lead to under deposit corrosion, crevice corrosion, mostly in pitting forms
Iron and cupper ions are two major corrodants. The Fe(II) can be converted to Fe(III) in the presence of oxygen. It will cause pitting, which is commonly seen in the workshop conditions as iron contamination. Drop of copper is the source of galvanic corrosion for stainless steel. Although the passivation layer of stainless steel is usually effective to prevent the general corrosion, localised pitting corrosion is still the real concern.
Chlorine, chloramine, ozone, permanganate is widely used in drinking water treatment process to remove NOM, colour, odour and taste in the water. For example red water, which is contamination of iron and manganese ion in the water, is largely reduced by permanganate treatment. It oxidised the ferrous ion and Mn(II) to more insoluble Ferric and MnO2, which are easily precipitate from the water. The main corrosion risk of these oxidants resides on the remaining free oxidant in the water which is usually very low and closely monitored for health and sanity purpose. The major corrosion concern will be the near the oxidants injection zone and mixing area.
It is widely accepted that activate carbon leaching from carbon filter can cause grieve to downstream 304 stainless steel vessel and piping. A granular carbon filter is a much preferred option. Active carbon is a strong depolariser and cause galvanic corrosion.
Biological process to remove NOM
304 stainless steel may suffer pitting corrosion, therefore it is not a suitable materials for such process.
For 304 grade stainless steel, a Swagelok type of compression joint is widely utilised for the joint. The Teflon or other non-metallic sealant is used in such fittings. However, due to the inevitable crevice in such service, very low chloride limit (<50 ppm) should be observed for 304 stainless steel.
If brazing is selected for the piping joint, then the risk of galvanic corrosion has to be considered and proper brazing procedure has to be followed.