Brownish discolouration on stainless steel material surface, observed on the Coast line and also in-land (as far as about 20 km from the marine source) and caused by corrosion, is known as Tea Staining.

Tea staining is a “make-up” issue that does not damage the stainless steel integrity and does not really influence the useful lifetime of the material.

Closer to the Sea – worse the tea staining. Usually it is detected in radius of about 5 km from the Sea, in some cases expanded as far as 20 km (in combination with other negatively influencing factors, such as: high pollution levels, high environmental temperatures, exposure to windy climate, etc.).

As a rule of a thumb, salty water is the worst enemy of the stainless steel; closer to salty waters – more critical the tea staining.

Factors that may promote the occurrence of tea-staining could be grouped as follows:

1 – Atmospheric conditions. Worst scenario is given when high temperature is combined with high level of humidity. Tropical climates are „best“ climate for appearance of tea staining.

Humid air carries salty particles that get deposited onto the stainless steel surface thus creating a corrosive solution. Tea staining is rarely observed in-land and/or in-doors.

2 – Corrosive agents in the air. Closeness to marine source or to any other aggressive industrial pollutants (chemical factories) may cause tea staining onto stainless steel and accelerate the process of corrosion.

3 – Stainless steel grade chosen. There is huge variety of stainless steel grades although about less of a dozen are commonly used. Each grade has different PRE (Pitting Resistance Equivalent).

Higher the PRE, greater the corrosion resistance of the material (chemical composition that includes Chromium, Molybdenum and Nitrogen would definitely contribute to increase the corrosion resistance too).

It is considered an appropriate PRE for marine exposure, around 25. For in-land (far from marine source) – PRE of 18 is acceptable. Finally, for severe sea exposure PRE of about 34 is to be considered.

Here it is a formula for calculating PRE for different grades of stainless steel material,

PRE = % Chromium + 3,3 % Molybdenum + 16% Nitrogen

Grade 316L or similar shall be considered as minimum within 5 km of the marine source.

4 – Material Surface characteristics. Smoother the surface, less probability of corrosion (tea-staining). Mirror polished material is a must for areas heavily exposed to marine environment. For general use, mill finishes like 2B and BA (bright annealed) are recommended.

If there are folds or grooves onto the metal surface – more probability of trapping salty precipitants (chlorides), hence – facilitates the corrosion.

In addition, the metal surface should be clean and free of contaminants normally coming from industrial environments (factories and machining / processing) – such as swarf (particularly carbon steel), manganese sulphide inclusions, steel smears, weld spatter, etc.

Passive layer of the stainless steel may result partly destroyed by welding, because of the heat input (underlying chromium gets depleted in welded areas).

To restore the passive layer of the stainless steel thus ensuring maximum resistance to corrosion, different actions could be undertaken, such as acid passivation, electropolishing, acid pickling, etc.

5 – Proper design of the installation. Drainage should be secured at all time. Horizontal surface and/or “texturized” metal surface (not smooth) are inappropriate for avoiding appearance of tea-staining or other more severe forms of corrosion.

Horizontal surface cause ponding, grain should be vertical to avoid it. Other “dangerous” zones within any design are corners and intermittent welds, where aggressive particles (water/chlorides/chemicals) could be easily trapped and remain, causing different forms of corrosion. Metal surface should be designed for free draining and a proper exposure to rain washing. Surface channelling (run-off) should be avoided. Last but not least, sheltered areas for not being exposed to rain washing, are particularly susceptible to tea-staining and corrosion of other types, in general. After installation, a visual inspection must be run for discovering surface damages or rests of contaminants. Imperfections should be removed if any detected, and the chemical resistance (passivation „self-healing“ layer) should be recovered usually by chemical means (pickling, passivating, electropolishing).

6 – Regular maintenance. It is a must condition for avoiding depositing of an aggressive agents that would lead to corrosion. Measures taken may vary from a rain washing, to regular wash (tap water), using solvents or detergents (or mixed), even using waxes (lattes do prevent the access of chloride to the material surface, but this effect is temporarily, so renewal of the wax is requested periodically). As an average maintenance, wash with soap or mild detergent and warm water, then rinse with clean cold water. Best would be if the washed surface is then wiped dry. Do NOT use bleach, chloride-containing or abrasive cleaners, as these will damage the stainless steel surface. Do NOT use hydrochloric acid for removing residues on stainless steel – it will stain the material surface and normally would lead to a more severe corrosion.

As mentioned, restoring the passivating layer of the stainless steel material (guaranteeing its best performance and withstanding to corrosion), may take place via pickling, passivating, or electropolishing. All these methods are chemical treatments, safety precautions should be taken under consideration due to the use of strong acids. Acids are harmful for the workers/operators and also for the environment, if not properly handled. After finalizing the chemical treatment the acids must be removed by rinsing the components / material. Residual acids may initiate pitting corrosion. Before rinsing some acids need to be neutralized by an alkali.

None of the above methods would remove stains, oily rests, or dirt. If material outcomes dirty from a factory / production mill, an alkaline or detergent cleaning is recommended before applying pickling or passivation treatment.

Pickling – If coloured oxide layer can be visually observed, it means the chromium underneath has been depleted due to heat treatments (usually by welding). This area then is more susceptible to corrosion (less corrosion resistant). The damaged layer has to be removed and fully-alloyed stainless steel surface is to be reached-out again. Pickling also removes ferrous particles. Pickling involves the use of nitric acid (HNO3) and hydrofluoric acid (HF) solutions. For welding areas there are also pickling pastes and gels on the market. After pickling the visual brightness of the metal surface may change, it gets slightly “dullish”.

Passivation – it is a chemical treatment of the surface of the stainless steel, by acid solutions or pastes or gel, for removing free iron contaminants and also for forming the passive layer on a mechanically polished surfaces / freshly created areas – e.g. through machining, grinding, mechanical damages. Passivation usually involves nitric acid (HNO3) solution. Visual changes in the appearance of the stainless steel surface are not normally observed after passivation, although mirror polishing would become a bit “clouded”. If it is a case of a severe contamination by grinding debris for example, then pickling may be preferred option instead of passivation. Pickling uses more aggressive acids than passivation, pickling acids go and act below the surface level of the metal, whilst passivation acids are less „harmful“ acting on superficial level of the metal, usually passivation takes place after pickling is accomplished.

Electropolishing – it is an alternative to pickling treatment. It takes place in baths in a factory. Electropolishing is very effective in metal contaminants removal and passivation of material, resulting in smooth, bright and highly-corrosion-resistant finish. Sharp edges get rounded and after-polishing-operations-peaks get removed. Mirror polished surface would lose it mirror reflectance after being electropolished.

Sources:

ASSDA – Australian Stainless Steel Development Association
Nickel Institute

Written by Nedko Nedev
ISOTUBI, S.L., Export Dept.