Passivation usually occurs naturally on the surfaces of stainless steels, but it may sometimes be necessary to assist the process with oxidising acid treatments. Unlike pickling,no metal is removed from the surface during acid assisted passivation. The quality and thickness of the passive layer is however quickly developed during acid passivation treatments. There may be circumstances when the pickling and passivation processes occur sequentially (not simultaneously), during acid treatments involving nitric acid. Nitric acid alone will only passivate stainless steel surfaces. It is not an effective acid for pickling stainless steels
There is a range of pickling methods that can be used on stainless steel fabrications, buildings components and architectural metalwork. The most important constituents of stainless steel pickling products are nitric and hydrofluoric acids. The main methods, used by pickling pecialists,
for pickling complete fabrications or large areas are :
- Tank immersion pickling
- Spray pickling
- Circulation pickling
Tank immersion usually involves off-site pickling at the fabricator or pickling specialists’ plant. Spray pickling can be done on-site, but should be done by specialists with the appropriate safety and acid disposal procedures and equipment. Tank immersion has the advantage of treating all the fabrication surfaces for optimum corrosion resistance and uniformity of pickled finish. It is also the best option on health and safety grounds as it is always done “off-site”. Pickling done at a specialist stainless steel fabricator or finisher’s plant, where the process can be carefully controlled, also minimises the environmental impact of the process
Smaller areas, especially around welded areas, can be pickled by
- Brush on pastes or gels
- Electrochemical cleaning
These methods can be used on-site and do not require specialist knowledge for effective and safe execution of the operation. It is important that adequate expertise and super vision are available to minimise health, safety and environmental risk s whilst providing a correctly pickled surface. Corrosion can occur to the areas treated if acid contact times and final rinsing procedures are not properly controlled to the supplier’s instructions. The contact times for different grades (types) of stainless steels can vary. It is important that the operators are aware of the specific grade of steel being pickled and the hazards of the products being used, so that safe, satisfactory results can be obtained. It is important that all traces of pickling products, pickle residues and contamination are completely rinsed from the surface of the steel parts to achieve a fully corrosion resistant and stain free surface. Competent stainless steel cleaning and restoration specialists normally use de- ionised (distilled) water for final rinsing to get the best results on architectural metalwork. Your nearest national stainless steel develop- ment association should be able to advise on pickling product and pickling service suppliers locally available.
The passive layer on stainless steels is not a simple oxide or ‘scale’, which would form by heating the steel. During heating the natural transparent passive layer grows in thickness forming ‘heat tint’ colours and eventually a grey oxide scale. The result of these visible oxide layers is usually a reduction in ambient temperature corrosion resistance. Stainless steel components, such as furnace parts, designed for high temperature service, make use of these thicker, but tenacious, oxide scale coatings for their high temperature oxidation protection. In contrast components intended for ‘ambient temperature service environments rely on the thin transparent ‘passive layer’ for their corrosion protection. Although this passivation process normally occurs naturally, the process of forming the chromium-rich oxide passive layer can be promoted by powerful oxidising conditions. Nitric acid is extremely useful for this and is widely used in commercially available stainless steel passivation treatments. Weaker oxidising acids, such as citric acid, can also help in the form ation of the passive layer. Acid passivation should be regarded as the exception, rather than the rule for stainless steel components and fabrications. Steel delivered from manuf acturing mills and reputable stockholders will be fully passive. The treatment may be needed however on intricately shaped machined parts. In these special cases the supply of oxygen to all the newly formed surfaces may be restricted, resulting in the natural passivation process taking longer to complete, than on the open,exposed surfaces. There is a danger that if parts like these are put straight into service in an environment normally considered suitable for the particular steel grade used, they might not be fully passive and suffer unpredictable corrosion. Passiv ation treatments done under these circumstances eliminate these unnecessary corrosion risks.
Tank immersion, spray pickling and nitric acid passivation treatments should be entrusted to competent fabricators or stainless steel finishing specialists. The selection and control of these potentially hazardous processes is critical for ensuring that satisfactory corrosion resistant finishes are obtained Specialist operators should be carefully selected, ensuring they work to all current national and European health, safety and environmental rules, codes and laws pertaining to these processes. Where appropriate, the process and final surface finish should be agreed and specified. Contractual agreement of surface finish can be reached using surface measurement parameters including surface roughness (Ra),reflectivity and gloss, but should always be confirmed by representative swatch samples, produced by the surface finishing contractor
EN 2516:1997 Passivation of corrosion resisting steels and decontamination of nickel base alloys.
The various stainless steel families are allocated Process Classes that define either one or two stage passivation treatments using nitric acid or sodium dichromate solutions. American Standards cover a wider scope of processes including cleaning, pickling and passivation.
- ASTM A380 – Practice for Cleaning, Descaling and Passivating of Stainless Steel Parts, Equipment and Systems
- ASTM A967 – Specification for Chemical Passivation Treatments for Stainless Steel Parts
Pickling – the process of removing discoloration of stainless steel after welding which caused reduction of the chromium content on the steel surface; the pickling effect is achieved in a pickling bath treatment or locally with pastes and gels
|for austenitic steel||for ferritic steel|
|52% nitric acid (36° Baumégo)||100 litres||100 litres|
|65% hydrofluoric acid or sodium fluoride||20 litres 30 kg||100 litres 15 kg|
|water||900 litres||900 litres|
|bath time at 20ºC||approximately between 15 min and 3 h|
Passivation – Restoring corrosion resistance of the steel through the oxidising effects of acids; passivation is achieved in a passivation bath, or by local application of pastes and gels
|for ferritic steel|
|52% nitric acid (36° Baumégo)||250 litres|
|bath time at 20ºC||between from 15 min and 1h|
|compiled based on the Euro Inox: ‘Welding of Stainless Steel’|