TEA STAINING

TEA STAINING

What is tea staining?

Tea staining is discoloration of stainless steel caused by corrosion. It is not detrimental to the structural integrity. Tea staining is most common in areas that are near a marine environment. Other probable causes of staining are carbon steel contamination, uncleaned welding, or chemicals such as Chlorine. 

Why does tea staining occur and how do I stop it?

One of the major factors that contribute to tea staining is the presence of sea salt on stainless steel surfaces. Sea salt, sodium chloride (NaCl), can slit into Na and Cl. Chlorine will break down the protective oxide film on the surface of passive stainless steel and will start tea staining.

Chemical passivation helps to increase the thickness of the protective oxide film and will help to protect the stainless steel. AE Stainless Direct does offer a service to apply a Citric Acid solution to enhance the passivation layer. There are commercial meters available to measure and evaluate the surface of passivated parts.

Other factors:

1) Carbon steel contamination 

Typically, this happens when mild steel is cut or grinded in close proximity to stainless steel. The free iron (ferric material) left on the surface will corrode. This can happen in the factory during production or during on-site installation while installing other services.It is critical to separate stainless steel works from mild steel works. If carbon steel contamination has occurred, the stainless steel part must be cleaned to remove all the carbon steel, either mechanically or chemically. Once removed, natural passivation occurs 24 - 48 hours or chemical passivation can be done within 30 minutes.

Carbon steel contamination

2) Chlorine

Chlorine, commonly used at waste water treatment plants, will attack even grade 316 Stainless Steel. The only solution is separation – either physically removing the Chlorine source away from the stainless steel or sealing the stainless steel. Note sealing can be dangerous, since it will also remove the oxygen required for the “self-healing” of a damaged passivation layer. Powder coating is too porous. A clear polymeric resin coating is recommended. AE Stainless Direct does offer a service to apply this coating onto an enclosure.

Exposed to Chlorine

Cleaned and treated with Citric Acid

3) Cleaning 

Though stainless steel is generally low maintenance; it isn't maintenance free.A light and regular wash is best and natural rain washing may be sufficient. But there are factors which will increase the requirement for maintenance:
    • Surface orientation
    • Surface design
    • Surface roughness

Corrosion is highly accelerated by poor drainage and is more likely to occur where there are deep grooves or folds in a surface because salts (chlorides) can be trapped in them. A deep groove has more trapped water (and salts), which means it is exposed to higher salt concentrations over a longer period of time, which can initiate corrosion. As the surface dries, salts become concentrated, making the conditions more aggressive. Approximately 0.5 m Ra is the critical surface roughness for surfaces that have been cut or polished. Surfaces that have been polished smoother than approximately 0.5 m Ra, are much less susceptible to corrosion. 

Summary

Start by choosing the correct grade of stainless steel. If the enclosure is near a marine environment, it must be 316L stainless steel. All AE Stainless Direct’s stock stainless steel range are 316L stainless steel.

After installation, clean/treat the enclosure with Citric Acid. This is 100% safe and environmentally friendly, unlike Nitric Acid which is very toxic.

If the enclosures are installed within a few hundred meters of the marine environment, consider coating the enclosure with a clear polymeric coating or implementing a quarterly washdown maintenance service.