The straw, gold, blue, and grey bands around a stainless steel weld are heat tint: a thin oxide layer that forms when hot metal meets air. It isn't just cosmetic, because the metal just under that oxide has lost some of its chromium and rusts more easily than the rest of the sheet.
So a stainless part with untreated weld tint can show brown rust lines along every weld within months, especially outdoors or near the coast. That's the classic "my stainless is rusting" complaint. This post explains what causes the colours, when you need them removed, the main ways to remove them, and what to ask for on your quote.
What causes the colours
Stainless steel resists corrosion because of a very thin, invisible chromium oxide film on its surface. Scratch it in normal air and it repairs itself.
Welding heats the metal far beyond normal conditions. Near the weld, chromium moves to the surface and forms a much thicker oxide layer. That thicker layer is what you see as colour. Roughly speaking, the colours run from pale straw nearest the cool edge, through gold and brown, to purple, blue, and dark grey closest to the weld, where the metal got hottest. Darker usually means thicker oxide.
Here's the catch. Because chromium has been pulled into the oxide, the thin layer of steel directly underneath now has less chromium than it should. Less chromium means weaker corrosion resistance right along the weld.
Does weld discolouration actually matter?
It depends on where the part lives and who sees it.
It matters a lot when:
- The part is outdoors, especially in coastal areas like Durban, Cape Town, or Gqeberha
- It's in food, dairy, brewing, or water contact
- It's in a wet, chloride-heavy, or chemical environment
- It's visible and expected to look like clean stainless
It matters less when:
- The part is hidden inside a dry indoor machine
- It's going to be painted or powder coated anyway
- The part runs hot in service, where it will discolour all over regardless
British and Australian stainless associations both recommend removing heat tint as good practice on any stainless weld where corrosion resistance matters. For drinking water applications, removal is treated as a requirement.
How heat tint is removed
There are a handful of standard methods. Shops often combine them: a mechanical clean followed by a chemical one.
| Method | How it works | Removes the low-chromium layer? | Effect on looks |
|---|---|---|---|
| Wire brushing (stainless brush) | Scrubs the oxide off | Not reliably | Removes colour, leaves a brushed mark |
| Grinding and blending | Takes off oxide and some metal | Yes, if enough metal is removed | Changes finish; needs re-brushing to match |
| Pickling paste | Acid paste dissolves oxide and the depleted layer | Yes | Dulls the surface slightly |
| Pickling bath | Whole part dipped in acid | Yes | Even, matte finish all over |
| Electropolishing | Electrochemical removal of a thin surface layer | Yes | Bright, smooth finish |
A few points worth knowing:
- Wire brushing alone is a cosmetic fix. It can make the weld look clean while leaving weakened metal underneath. And a brush that has touched mild steel will embed iron particles that rust, so stainless needs its own tools.
- Pickling paste is the common way to treat welds on finished fabrications. It uses strong acids, so it has to be applied, timed, and rinsed off properly. Leftover acid can cause pitting.
- Passivation is often mentioned in the same breath. It's a treatment that removes free iron and helps the passive film reform, but on its own it doesn't remove heat tint. Pickling does both jobs.
How to keep discolouration down in the first place
You can't stop heat tint completely, but you can reduce how much there is to clean.
- Lower heat input. Shorter welds, stitch welds instead of long continuous runs, and processes with fine heat control (TIG is the usual pick on thin, visible stainless) all help. Our MIG vs TIG welding post explains the difference.
- Back purging. On tubes and closed sections, shielding gas on the back of the weld stops the inside from oxidising badly. Inside surfaces matter, because that's often where liquid sits.
- Good fit-up. Tight joints need less weld metal and less heat.
- Fewer welds. A bent corner has no weld to clean. If you can design a fold instead of a welded seam, do it.
Matching the finish after cleaning
Cleaning a weld changes the surface around it. On a 2B mill finish sheet, pickling leaves a dull patch that can look different to the rest. On a brushed sheet, the weld area usually needs re-brushing in the same direction so it blends in.
If the finish has to be uniform, say which face is visible and what finish it should have. Our article on stainless steel sheet finishes covers the common options. And if the part is going near the sea, cleaning welds properly is only half the answer: the grade matters too, which we cover in 304 vs 316 stainless steel.
What to ask for on your quote
When you send a stainless fabrication for pricing, tell us:
- The grade (304, 316, or 3CR12) and thickness
- Where the part will be used: indoor, outdoor, coastal, food contact
- Which faces and welds are visible
- Whether welds should be cleaned, pickled, ground flush, or left as welded
- The final finish you want on the visible faces
That lets us price the cleaning properly instead of guessing. Our fabrication service handles welding and assembly, and our metal finishing covers the surface work afterwards. Start a fabrication quote and add your notes in the description.