The colors next to a stainless weld aren’t decoration. Weld heat tint is a chromium-depleted oxide layer, and where it forms, the steel resists corrosion less well than the metal beside it. On sanitary, pharmaceutical and chemical equipment, that’s a failure waiting to happen. This guide explains what the colors mean, how back purging prevents them, and how to remove discoloration properly.
Need stainless welded clean inside and out? Weld Creations back purges full-penetration joints as standard. See sanitary welding or request a quote.
What heat tint actually is
Stainless steel resists corrosion because chromium reacts with oxygen to form a thin, invisible passive film. During welding, the metal gets hot enough to react much faster, and if oxygen is present, a thicker oxide grows. That oxide:
- Is visible, as straw, bronze, blue or gray bands, because thin oxide layers produce interference colors
- Is chromium-rich itself, which means the metal immediately beneath it is left chromium-depleted
- Reduces corrosion resistance locally, which is where pitting starts
The darker the color, the thicker the oxide and the more depleted the layer beneath.
Acceptance limits come from your specification. Sanitary work often references a stainless tube discoloration chart such as AWS D18.2, and ASME BPE sets requirements for bioprocess equipment. Put the acceptance criteria on the drawing so the shop and the inspector judge against the same standard.
Back purging: preventing it in the first place
Removing heat tint costs time and can’t fully undo depletion. Preventing it is the real answer, and on tube, pipe and closed sections that means purging the back side of the joint with argon.
How it’s done properly:
- Displace the air before striking an arc. The joint volume is flooded with argon, typically several volume changes.
- Verify, don’t assume. An oxygen analyzer on the purge outlet shows when the atmosphere is clean enough. Sanitary work commonly targets low parts-per-million before welding.
- Use purge dams to limit the volume on long runs, so purging is fast and gas isn’t wasted.
- Keep flow gentle. Too much flow creates turbulence that pulls in air, and can push the root bead out of shape.
- Maintain the purge until the weld cools below the temperature where it reacts with air.
- Shield the face too, with an appropriately sized gas lens and adequate post-flow.
The same discipline, in a stricter form, applies to titanium, where contamination embrittles the metal rather than just reducing corrosion resistance.
Removing heat tint the right way
When tint exceeds the limit, there are three accepted routes, and one common mistake.
1. Mechanical. Dedicated stainless brushes, abrasive pads or blasting media used only on stainless. Effective on accessible surfaces, but it changes the surface finish, and aggressive grinding can smear the surface and leave a worse texture than it fixes.
2. Chemical pickling. Acid pastes or baths remove the oxide and the depleted layer. This is the most thorough route for heavy tint, and it’s what ASTM A380 covers for cleaning and descaling. It needs proper handling, rinsing and neutralization.
3. Electropolishing. Removes a thin layer of metal and leaves a smooth, passive surface. Common on pharmaceutical product contact. See passivation vs electropolishing.
After any of these, passivate. Passivation restores the passive layer and removes free iron. It does not remove heat tint by itself, so the order matters: remove the oxide, then passivate.
The common mistake: cleaning stainless with a carbon-steel wire brush or a grinding disc that has been used on steel. That embeds free iron, which rusts and looks exactly like the stainless failing. Dedicated stainless-only tooling is not optional.
What this means when you’re buying welded stainless
Ask a prospective supplier:
- Do you back purge every full-penetration joint? The answer should be yes without hesitation.
- How do you verify the purge? Look for an oxygen analyzer, not “we count to thirty”.
- What discoloration limit do you work to? They should ask which standard applies to your part.
- How do you remove tint when it exceeds the limit? Mechanical, pickling or electropolishing, then passivation.
- Do you keep stainless tooling separate from carbon steel? This is the simplest tell of a shop that understands stainless.
Those five questions separate shops that weld stainless from shops that weld stainless properly. More in how to choose a sanitary welding company.
How we handle it
We back purge full-penetration joints with argon, control heat input to limit tint in the first place, use stainless-only brushes and media, and coordinate pickling, passivation and electropolishing through specialist partners with certificates delivered alongside the parts. See stainless steel welding or our capabilities.