Tea staining of stainless steel is a relatively common occurrence, where the Australian Stainless Steel Development Association (ASSDA) has defined tea staining as the “discolouration of the surface of stainless steel which tends to follow the ‘grain’ of any surface finish”. The relationships between the contributing factors are complex, but generally become increasingly prevalent the closer the structure is to salty water or the local conditions are too harsh for the stainless steel grade used.
Although initially just a cosmetic issue, having no effect whatsoever on the structural integrity of the material, tea staining indicates corrosion. So, what are some of the contributing factors that cause this and how do you treat tea staining?
Why Does Tea Staining Occur?
Industry bodies have undertaken extensive research to understand the causes of this and have identified that tea staining can occur when not only the structure is in close proximity to the ocean, but also it is evident in structures located more than twenty kilometres from the ocean. Some alternative factors that causes the discolouration includes elements such as wind, presence of corrosives on the surface, weather and pollution.
Detailed below are some important factors that promote the occurrence of tea staining.
Presence of Corrosive Substances
The presence of sea salt on the surface of the stainless steel is one of the major factors that causes tea staining. The underlying characteristics of sea salt is the ability to stay wet until a very low relative humidity (RH). The consequence of this is that the surface stays wet (and is corroding) longer with sea salt compared with sodium chloride. On top of this, the presence of industrial pollutants can also aid in making the conditions more aggressive.
Location and Atmospheric Conditions
A combination of atmospheric conditions with high humidity and high temperatures creates optimal conditions for the occurrence of tea staining, as does intermittent exposure (for example, spray from rough seas). High temperatures increase the rate of corrosion, but elevated humidity will increase the time taken for the water on the surface to evaporate creating a higher concentrated chloride solution. It is this contact with the solution that causes corrosion rather than with the subsequent solids.
Tea staining is rarely a problem indoors as these locations do not present the above conditions, experience low humidity and the absence of corrosive deposits.
There are hundreds of grades of stainless steel with different chemical compositions but only about 10 of these are commonly used. Every stainless steel grade owes their corrosion resistance to the thin chromium oxide film on the surface.
A formula based on the content of these three elements is useful to rank the corrosion resistance of different grades. The number (or Pitting Resistance Equivalent [PRE]) is calculated and the PRE ranges from 10.5 for the grades with the lowest corrosion resistance to more than 40.
For acceptable corrosion resistance, typically, a PRE of ~18 is adequate away from marine influences; PRE ~24 is required for marine atmospheres while severe marine atmospheres may require PRE ~34. The higher the PRE, the greater the corrosion resistance.
Recommendation For Minimising Tea Staining
Stainless steel is a low maintenance material but it is not generally maintenance free. A light and regular wash is best and natural rain washing may be sufficient. Lower grades will require maintenance that is more regular and if the environment causes sticky deposits. Application of oils or waxes will temporarily restrict chloride access to the stainless steel but they need regular renewal. These temporary protectives also tend to attract debris and dull the surface.
The rule of thumb for surface finishes is that the smoother the finish, the better will be the resistance to tea staining.
Initial material selection for the enclosure is of critical importance to the longevity of the equipment. Not all metals are created equal and understanding the characteristics of each metal will allow you to understand how to best care for it.
Emery Industries designed our Teastain Remover to remove tea staining from stainless steel stanchions, furniture, rails and medical equipment. Emery Teastain Remover will not corrode stainless nor will it turn stainless green or brown, as hydrochloride acid based cleaners will.
To be used in conjunction with our scourer pads and handle.
Conclusion and Recommendations
Tea staining of stainless steel surfaces can be a problem when a significant level of chloride is exposed to the metal surface. Although it does not look very pleasant, it tends to be a surface phenomenon only, and does not affect the structural integrity or longevity of the material.
The cause of tea staining has been examined in some details to help evaluate the potential remedies for the condition. Recommendations to help prevent tea staining include correct specification of the grade of stainless steel to be used, surface finishes to help reduce the problem, as well as maintenance procedures, and preventative coating treatments.
Emery Industries recommends the purchase of our Emery Tea Stain remover when there are signs of tea staining and apply it using a Scotch-Brite 75 Green Heavy-duty Scourer Pad (contains no metal in the scourer).