How to Passivate Stainless Steel Welds (ASTM A380)

Passivation of stainless steel welds is a chemical process that restores the stainless steel’s corrosion-resistant properties following welding. Passivation is necessary because the heat generated during welding burns off the stainless steel’s protective chromium oxide layer. Welding may also cause oxidation and contamination with iron.

Passivation removes contaminants and restores corrosion resistance. This guide explains how to passivate stainless steel welds to the ASTM A380 standard, which is ‘Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems.’ Read on to learn more.

Stainless steel materials are notable for their corrosion resistance, which is made possible by a very thin, invisible layer of chromium oxide on the surface. When stainless steel is welded, that layer is disrupted by the head of the welding process and exposure of the steel to oxygen. Welding reaches very high temperatures that burn off the protective chromium coating, cause oxidation (known as ‘heat tint’), and even create changes in the steel grain structure.

The welding process also reduces the level of chromium in the stainless steel, which should be at around 10.5% if the steel is to retain its stainless properties. The process by which chromium is reduced is called ‘sensitisation.’ Welding may also introduce contaminants such as grinding dust or iron particles. If iron is left at the weld site, the component may end up rusting. Finally, welding may increase the susceptibility of the steel to cracking or fatigue.

Citric acid and nitric acid passivation help restore corrosion resistance after welding by allowing the chromium oxide layer to reform. Both methods remove free iron. Nitric acid passivation is a more aggressive treatment, while citric acid treatment is notable for not attacking the base metal. Nitric acid has risks, such as creating toxic fumes, and has to be carefully handled. By contrast, citric acid is biodegradable, doesn’t generate toxic fumes, and can be more easily disposed of. For these reasons, citric acid is increasingly finding favour over nitric acid, which was the traditional choice for passivating stainless steel.

Let’s consider the qualities of each of these passivation methods:

If you’re looking for a guide to best practices when it comes to cleaning and passivation of stainless steel welds, look no further than the international standard ASTM A380. This standard explains how to correctly clean, descale, and passivate stainless steel parts following fabrication. ASTM A380 is often used as the standard when manufacturing pipework, tanks, and other equipment where corrosion resistance is paramount.

It’s important to recognise that ASTM A380 is a guide and not a mandatory standard (unless it has been specified as such in a contract). It provides a detailed description of best practices for cleaning and passivation, and a reference guide to inform fabrication quality. There is another standard relevant to passivation, ASTM A967, which concerns how to verify passivation performance. It sets out approved chemical treatments to remove contamination. This helps to ensure regulatory and customer requirements are met. These standards are often used together in engineering projects.

ASTM A380 covers:

• Cleaning of stainless steel. This includes degreasing, cleaning with solvents, cleaning with detergent, and water break testing.
• Passivation. The standard includes the best procedures for both nitric acid and citric acid passivation (see above), along with rinsing and drying requirements.
• Descaling and pickling. ASTM A380 has guidelines for mechanical cleaning of components, removing weld oxidation, and the use of acid treatment.
• Inspection and testing. This includes guidelines for visual inspection, copper sulphate testing, water immersion tests, and tests of exposure to high humidity.

The goal of passivation, as outlined in ASTM A380, is to remove iron impurities from the stainless steel and allow a fresh layer of chromium oxide to form to protect against corrosion.

The passivation process can be broken down as follows:

1. Pre-cleaning and degreasing. Stainless steel weld cleaning is vital. Because dirt or oil on the component can impede passivation, it’s important to start by cleaning the stainless steel component with an alkaline cleaner or approved solvent. The component can then be rinsed with deionised water. A water break test may be used to indicate successful cleaning. (Water should sheet evenly over the component, rather than form beading)
2. Removing heat tint. If the weld is discoloured, you will want to remove heat tint. This can be done mechanically, by using a pickling paste or electrochemical cleaning. Don’t be tempted to skip this step – it removes heavy oxidation that passivation alone may not touch
3. Rinsing the component. High-quality water should be used to rinse the stainless steel. This helps to remove residues from pickling. Non-chlorinated water should be used
4. Applying the passivation solution. This will be either nitric or citric acid. Components that are small enough can be immersed in the acid solution. For pipework, the solution can be circulated through the system. To treat individual welds, spraying or wiping on the acid solution is also an option. The solution should be applied for between 20 and 60 minutes
5. Final rinsing. Once the component is ready, give it a final rinse with clean water. Again, deionised water is preferred. All chemical residues should be removed at this stage. Residual acid may stain the component later on
6. Drying. The passivated stainless steel component should then be left to dry in the air. This helps to create the new protective layer as the chromium in the steel reacts with oxygen to create the new chromium oxide skin for the component. It’s a process that takes place naturally once contaminants have been removed
7. Inspection and testing. Inspection methods vary, from visual inspection to water break tests and copper sulphate tests (which can detect any remaining free iron). High humidity testing may also be used. The precise testing requirements also vary. For stainless steel used in food, pharmaceuticals, and water treatment, documentation proving successful testing has taken place may be required

Whether you use nitric acid or citric acid for the passivation process, both require careful handling. ASTM A380 and ASTM A967 provide guidelines on how to do this safely. Workers should wear personal protective equipment such as chemical-resistant gloves, goggles, face shields, acid-resistant aprons, long sleeves and trousers, and safety boots while handling acid. That’s because nitric acid is corrosive to skin and eyes. Citric acid is milder but is still an irritant to the eyes and skin.

The hydrofluoric acid used in pickling is extremely hazardous. If it penetrates the skin, emergency treatment is required. For all these reasons, effective PPE is a must during passivation. For handling nitric acid, respiratory protection may also be needed if ventilation is limited on the site.

It’s best to apply the nitric acid solution outside if possible because doing so can release nitrogen dioxide fumes. If working indoors, mechanical ventilation is required. It’s also important to remember to always add acid to water, not water to acid, and to use chemical-resistant containers when handling passivation materials. In the event of a spill, there should be emergency procedures in place. A large spill may necessitate the evacuation of the site. For smaller spills, there should be neutralising agents such as sodium bicarbonate on site. Ensure employees also have rapid access to an eyewash station and shower.

When it comes to disposal, acid should be neutralised. All acid solutions should be disposed of in accordance with environmental regulations. Disposal should be documented correctly. Untreated nitric acid solution must never be discharged without being treated first. Remember too that nitric acid can intensify a fire and should be stored separately from flammable materials, oil, and solvents.

Finally, ensure all employees involved in passivating stainless steel are trained in how to do it safely and correctly.

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