Why does my Stainless Steel have an Unsightly Brown Discolouration?

Don’t panic this is known as “tea staining”, a common cause of the deterioration of Stainless Steel surfaces.

Tea staining is the 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 material – ASSDA.

Although a cosmetic issue only and tea staining does not affect the structural integrity or lifetime of the material, it makes the appearance of initially pristine stainless steel, very unattractive.

Stainless Steel Tea Staining Discolouration
Stainless Steel Discolouration known as Tea Staining

Tea staining is also known as or often referred to as:

  • Coastal corrosion
  • Brown spots or marks on stainless steel

Stainless steel wire & mesh is available in grades 304 and 316. Generally, grade 316 should be selected for applications within 5km of the surf, or 1km from sheltered coastal waters, however, tea staining can occur up to 20km from the coast, depending on environmental factors such as:

  • Pollution levels
  • Wind exposure
  • Rain sheltered areas
  • Higher temperatures
Stainless steel woven mesh- displaying the brown discolouration on the surface known as tea staining
Stainless steel woven mesh- displaying the brown discolouration on the surface known as tea staining

Why is it important to understand your application & environment?
Stainless steel is specified for applications that require good visual appeal and structural integrity in a wide range of environments. With minimal care, your stainless steel wire or mesh will continue to look good for many, many years. However, stainless steel is “LOW maintenance”, not “NO maintenance”.

The presence of sea salt on the surface of stainless steel is the major cause. Sea salt stays “wet” (and corrosive) until a very low relative humidity. Tea staining also occurs in polluted urban environments. Tea staining generally does not occur indoors because of the clean environment and low humidity.

Factors that can increase tea staining are:

  • High temperatures and high relative humidity, as found in tropical areas. The moisture in the air dissolves the salt deposits on the surface and keeps them wet
  • Rain sheltered areas, such as the underside of a sloping roof or a building rain shadow
  • Poor drainage or designs with corners, grooves, folds, and crevices which can trap water. These can lead to more serious corrosion than tea staining

Tea staining can occur up to 20 kilometres from the surf, due to wind exposure, pollution levels, lack of sheltering and higher temperatures, so it is important for you to be aware and understand where your stainless steel is being used, so you can use the best grade for your project.

 

How can I tackle tea staining?
To avoid tea staining, you should clean your material regularly with a detergent  & warm water mixture. Dry with a microfibre cloth to keep your stainless steel in a lasting condition. This mixture can also be used to remove any tea staining from your metal – however proactive steps can be taken to ensure your stainless steel does not develop any brown discolouration.

Stainless steel wire and mesh care tea staining

A guide to specifying and cleaning external stainless steel

Environment Grade 304 Grade 316
Clean Inland 3 – 6 months  6 – 12 months
Polluted urban/Industrial Not suitable 6 – 12 months
Coastal/Marine (not splashed)  Not suitable 3 – 6 months

Source: adapted from ASSDA

Or keeping it simple when you wash your windows wash your stainless steel! The washing frequency may be reduced for surfaces subject to regular flushing by natural rainfall. However, if your application is for architectural or external use and you want to maintain the stainless steel- silver appearance then it’s time to consider ‘The Lasting Look’.

 

What other options are there to achieve the “Lasting Look”?
Pickling, passivation, and electro polishing are treatments that can be applied to the surface of stainless steel to remove contaminants and help with the creation of a constant chromium-oxide, passive film. Using these finishes will reduce the risk of tea staining.

Picking removes the heat affected layer of stainless steel and prepares the surface for passivation. Passivation is a process which can be performed on its own, or after pickling. The surface is treated with an oxidizing acid, to dissolve carbon steel & sulphide, and remove iron and other contaminants from the surface. When done correctly, passivation does not affect the appearance of the stainless steel.

Electropolishing is a finishing process that can be used on all types of stainless steel, and increases the corrosion resistance, leaving the stainless steel with a lasting bright finish. It is an electrochemical solution to remove the outer slayer of the surface material, to remove contaminants, heat tint, oxide scale, burrs, micro abrasions, pits, and other imperfections.

 

Fabricating with Stainless Steel
When fabricating with stainless steel wire or mesh, be aware that other causes of staining that are not “tea staining”, include carbon steel contamination, uncleaned welds and chemical fumes, such as hydrochloric acid or bleach.

 

For further information on tea staining:
Australian Stainless Steel Development Association (ASSDA)

ASSDA’s Technical FAQ No 6 – Preventing Coastal Corrosion (tea staining)

 

How can Stainless Steel Wire and Mesh (SSWM) assist you with your next project?

SSWM adds value to our Customer’s with our service and availability. It is via our industry knowledge that can provide you with the options that that you can then select the most appropriate product for your application and environment.

For further information please visit our Cleaning, Care & Maintenance page.

Our technical sales team is here to help and with many years of industry experience they can provide solutions to your problems whether it’s stainless steel:

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