What is Tea Staining in Stainless Steel?
Tea staining is discolouration of the surface of stainless steel by corrosion. It is a cosmetic issue that does not affect the structural integrity or the lifetime of the materials. Tea staining occurs most commonly in coastal areas and becomes progressively worse closer to the marine source. However, wind exposure, pollution levels, local sheltering and higher temperatures can create environments where similar corrosion might occur further from the sea. These same factors also increase corrosion rates of alternative materials. Other causes of staining that are not tea staining include carbon steel contamination, uncleaned welds and chemical fumes such as hydrochloric acid or bleach.
Marine sites are defined as areas where windborne sea spray or mist may be present. These contain chlorides which can also concentrate in condensation or as surface moisture evaporates.
Why Does Tea Staining Occur?
The relationships between the contributing factors are complex, but generally become increasingly critical closer to salty water. Tea staining occurs as a result of the local conditions (such as temperature, relative humidity and presence of corrosives on the surface) being too aggressive for that stainless steel grade in its installed condition.
Another significant marine area to remember is swimming pools. Whilst not strictly a ‘marine’ area, the chlorine used to purify the water in swimming pools acts in a similar way to sea salt, corroding the stainless steel in a pool environment.
Swimming pool environments have changed significantly in recent years, most markedly in leisure pools. Higher water temperatures combined with an increase in the number of bathers, has led to higher levels of chemical disinfection. Chlorine-based disinfectants are the norm, which together with contaminants introduced by bathers, produce chloramines. These are thought to be the most important factor in the corrosion of stainless steel in a swimming pool environment.
How Can You Minimise Tea Staining?
The effects of corrosion can be lessened by choosing the correct grade and finish of stainless steel for the application. Whilst stainless steel is low maintenance, it is rarely maintenance free. Therefore, caring for your steel correctly goes along way toward ensuring it stays in perfect condition for as long as possible.
Conditions Reducing the Risk of Tea Staining
- Absence of corrosives – especially salt.
- Atmospheric conditions – lower temperatures and low relative humidity (RH) are better.
- Surface orientation and design – free drainage and avoidance of traps which can concentrate corrosives. This includes open exposure to allow rain washing.
- Surface roughness – smoother is better.
- Chemical cleanliness or passivation of the surface improves the corrosion resistance.
- Appropriate grade for exposure conditions – increasing PRE increases corrosion resistance.
- Maintenance – or corrosives will accumulate
View our range of cleaning products here. These have been specifically designed for use on stainless steel, and will combat any early signs of corrosion.