Heat tint is the thin, colored oxide layer that forms on stainless steel when a weld zone is exposed to oxygen at temperature. The straw, gold, blue and violet coloring around an unpurged weld is not cosmetic. It is a chromium-depleted oxide 20 to 200 nanometres thick sitting over a chromium-poor layer of parent metal, and it is one of the most reliable places for corrosion and rouge to start. ASTM A380 requires heat tint to be removed before passivation — passivation chemistry will not remove it.
What is heat tint and why must it be removed?
When austenitic stainless steel is heated in the presence of oxygen — which is exactly what happens at a weld without adequate inert gas cover — chromium at the surface preferentially oxidizes. The result is a visible interference film whose color depends on its thickness, and beneath it a zone where chromium has been drawn out of the parent metal to feed that oxide.
That chromium-depleted zone is the problem. Stainless steel resists corrosion because it holds enough chromium at the surface to maintain a passive film. Strip chromium out locally and you have created a site that will pit, that will rouge in a hot water loop, and that will fail a ferroxyl test.
The color is therefore a diagnostic, not a defect in itself. Pale straw indicates a thin film over minor depletion. Blue, violet and black indicate progressively thicker oxide over progressively worse depletion.
Heat tint color chart: color, temperature and oxide thickness
The color tells you approximately how hot the surface got and how much oxide formed. This is the reference our field crews work from.
| Color | Approximate temperature | Approximate oxide thickness | Acceptable on product contact? |
|---|---|---|---|
| No visible color | Below 400 °F | Native film, 1–5 nm | Yes |
| Pale straw / light yellow | 400–500 °F | ~20 nm | Sometimes, if specified |
| Gold to tan | 500–600 °F | ~40 nm | No |
| Blue to purple | 600–750 °F | 60–80 nm | No |
| Dark blue to violet | 750–900 °F | ~100 nm | No |
| Brown to grey | 900–1,100 °F | ~150 nm | No |
| Black scale | Above 1,100 °F | Above 200 nm | No — requires descaling |
AWS D18.2 — Guide to Weld Discoloration Levels on the Inside of Austenitic Stainless Steel Tube — is the document most bioprocessing specifications reference when they set an acceptance level for weld discoloration. It provides a graded color scale so that “acceptable tint” is defined by reference to a numbered sample rather than by argument on site. If your specification does not cite a discoloration level, it is worth adding one before fabrication starts.
Does passivation remove heat tint?
No. This is the single most consequential misunderstanding on this subject.
Passivation chemistry — citric acid at 4 to 10 percent, or nitric acid at 20 to 25 percent — is formulated to dissolve free iron and allow the chromium oxide film to reform. It is deliberately not aggressive enough to attack an established oxide scale, because a chemistry that removed heat tint would also attack the base metal.
ASTM A380 is explicit that scale and heat tint are removed first, and passivation follows. A system passivated over visible tint has a documented passivation record and a surface that is still chromium-depleted at every weld. It will typically pass a copper sulfate test on the clean tube and fail a ferroxyl test at the weld.
How is heat tint removed? The three method families
All three work. Which one is correct depends on the tint severity, whether the surface is accessible, and what surface finish must survive the operation.
| Method | Typical parameters | Best for | Effect on finish |
|---|---|---|---|
| Mechanical — abrasive, brushing, blasting | Stainless or non-metallic media only | Accessible exterior welds, heavy scale | Roughens; may violate an Ra requirement |
| Chemical — nitric acid | 20–25% HNO3, 10–30 min at ambient | Light to moderate tint | None measurable |
| Chemical — nitric / hydrofluoric pickling paste or bath | 15–20% HNO3 with 1–3% HF, 5–15 min | Heavy tint and scale | Dulls surface; removes base metal |
| Chemical — citric acid | 4–10%, 140–160 °F | Very light straw only | None |
| Electrochemical weld cleaning | 5–20 V DC, 30–60 s per 6 in of weld | Localized interior and exterior welds | Minimal; often improves appearance |
For product-contact surfaces in pharmaceutical and biotech service, electrochemical cleaning has become the default for localized work: it is fast, it is precise to the weld, it avoids hydrofluoric acid handling, and it does not degrade the surrounding surface finish. Full pickling is reserved for heavy scale, and even then is avoided where an ASME BPE surface finish must be maintained.
Whichever method is used, the surface is neutralized, rinsed with water of specified quality, and then passivated to ASTM A967. Removing tint exposes bare metal; leaving it unpassivated simply moves the problem.
How do you prevent heat tint during welding?
Prevention costs a fraction of removal. Heat tint forms because oxygen reaches hot metal, so every preventive measure is about excluding oxygen until the weld zone has cooled below the tinting threshold.
- Purge the interior. Argon backing gas on the inside of the tube is the primary control. Purge before striking the arc, maintain it through the weld, and hold it until the weld cools.
- Verify residual oxygen, do not assume it. An oxygen analyzer on the purge exhaust is the only way to know. Typical practice for hygienic tube is to weld below 50 ppm residual oxygen; critical and high-purity work is commonly specified below 20 ppm, and semiconductor service tighter still.
- Control purge dwell. Most tint we are called to remove is not from a bad weld but from a purge that was stopped too early while the metal was still above 400 °F.
- Use trailing shields on exterior welds where the torch gas alone will not cover the cooling bead.
- Manage heat input. Excessive amperage or slow travel puts more of the tube above the tinting temperature for longer.
Automated orbital welding helps considerably here, because the weld schedule — current, travel speed, purge timing — is repeatable rather than dependent on operator technique on the day.
What surface finish is required after heat tint removal?
Removing tint is only half the acceptance criterion. The finish left behind has to meet the service requirement, which is where aggressive mechanical methods get projects into trouble.
| Service | Typical Ra requirement | Equivalent | Practical consequence |
|---|---|---|---|
| Pharmaceutical product contact | ≤0.4 µm | ≤16 µin — ASME BPE SF4 | Rules out abrasive blasting; favors electrochemical |
| Biotech and bioprocess | ≤0.5 µm | ≤20 µin, often electropolished | Electropolish after tint removal |
| Food and dairy, 3-A | ≤0.8 µm | ≤32 µin — #4 finish | Fine mechanical methods acceptable |
| Aerospace and general industrial | 1.6–3.2 µm | 63–125 µin | Most methods acceptable |
Is heat tint removal required for food-grade equipment?
Yes, in practice. 3-A Sanitary Standards and EHEDG hygienic design guidance both require product-contact surfaces to be corrosion resistant, cleanable, and free from crevices and imperfections that harbour soil. A chromium-depleted, oxide-covered weld fails all three tests, and in wet food and dairy service it will corrode noticeably faster than in a dry environment.
Why you must never use carbon steel tools on stainless
Using a carbon-steel wire brush, a previously used grinding wheel, or shared abrasive media on stainless embeds iron particles in the surface. Those particles rust within days and become the seed for pitting. ASTM A380 requires that only stainless steel or non-metallic media contact the surface, and that tooling be dedicated to stainless.
It is a genuinely common cause of “the new system is rusting” calls. The steel is fine; the brush was not.
What are the safety hazards?
Heat tint removal chemistry is not benign and belongs in a written procedure with the corresponding controls.
- Hydrofluoric acid in pickling pastes and baths causes deep, delayed-onset burns and systemic toxicity, and requires calcium gluconate on site, dedicated PPE and trained personnel.
- Nitric acid is a strong oxidizer and generates nitrogen oxide fumes; ventilation and eye protection are mandatory.
- Confined space entry applies to work inside vessels and large-diameter piping, under OSHA 29 CFR 1910.146.
- Waste disposal of spent fluoride-bearing pickling solutions is regulated and materially more expensive than citric-based waste streams.
Weld discoloration on a system that has to pass inspection?
Paul Industries removes heat tint and weld discoloration, re-passivates to ASTM A967, and documents the result — on new fabrication and on systems already in service, nationwide. We match the method to the surface finish requirement rather than reaching for the most aggressive option.
Frequently asked questions about heat tint removal
What is heat tint on stainless steel?
Heat tint is the colored oxide film that forms when stainless steel is heated in the presence of oxygen, typically at a weld without adequate inert gas purge. It is 20 to 200 nanometres thick depending on color, and it sits over a chromium-depleted layer of parent metal, which is why it is a corrosion initiation site rather than a cosmetic issue.
Does passivation remove heat tint?
No. Passivation chemistry, whether 4 to 10 percent citric or 20 to 25 percent nitric acid, is formulated to dissolve free iron and is not aggressive enough to remove an established oxide scale. ASTM A380 requires heat tint to be removed before passivation is carried out.
What do the heat tint colors mean?
Color indicates approximate temperature and oxide thickness. Pale straw is roughly 400 to 500 degrees F at about 20 nanometres, gold to tan 500 to 600 degrees F at about 40 nanometres, blue to purple 600 to 750 degrees F at 60 to 80 nanometres, dark blue to violet 750 to 900 degrees F at about 100 nanometres, brown to grey 900 to 1,100 degrees F, and black scale above 1,100 degrees F and above 200 nanometres.
How do you remove heat tint from stainless steel?
There are three method families: mechanical removal using stainless or non-metallic media only; chemical removal using nitric acid, nitric with hydrofluoric acid for heavy tint, or citric acid for very light straw; and electrochemical weld cleaning at 5 to 20 volts DC for roughly 30 to 60 seconds per 6 inches of weld. All must be followed by rinsing and passivation to ASTM A967.
What is AWS D18.2?
AWS D18.2 is the Guide to Weld Discoloration Levels on the Inside of Austenitic Stainless Steel Tube. It provides a graded color scale so that an acceptable level of weld discoloration can be specified by reference to a numbered sample rather than being argued on site. Most bioprocessing specifications cite a discoloration level from it.
How do you prevent heat tint when welding stainless?
Purge the interior with argon before striking the arc, maintain it throughout, and hold the purge until the weld cools below about 400 degrees F. Verify residual oxygen with an analyzer rather than assuming: hygienic tube is commonly welded below 50 ppm oxygen and critical high-purity work below 20 ppm. Use trailing shields on exterior welds and control heat input.
What residual oxygen level is acceptable for purging?
Common practice for hygienic and sanitary tube is to weld with residual oxygen below 50 ppm in the purge, with critical high-purity and bioprocessing work frequently specified below 20 ppm. Semiconductor service is tighter again. The level should be stated in the weld procedure and verified with an oxygen analyzer.
Can citric acid remove heat tint?
Only the very lightest straw coloring. Citric acid at 4 to 10 percent is a passivation chemistry, not a descaling chemistry, and it will not remove gold, blue, violet or black oxide. Those require nitric acid, nitric with hydrofluoric acid, mechanical removal, or electrochemical cleaning.
What surface finish is required after heat tint removal?
It depends on service. Pharmaceutical product-contact surfaces typically require Ra of 0.4 micrometers or better, equivalent to 16 microinches and ASME BPE SF4. Food and dairy work to 3-A generally requires 0.8 micrometers or 32 microinches. Aerospace and general industrial commonly accept 1.6 to 3.2 micrometers.
Why can’t you use a carbon steel brush on stainless steel?
Carbon steel tooling and previously used abrasive media transfer iron particles into the stainless surface. Those particles rust within days and become pitting initiation sites. ASTM A380 requires that only stainless steel or non-metallic media contact the surface, and that tooling be dedicated to stainless.
Is heat tint removal required for food-grade equipment?
In practice, yes. 3-A Sanitary Standards and EHEDG guidance require product-contact surfaces to be corrosion resistant, cleanable and free of imperfections that harbour soil. A chromium-depleted, oxide-covered weld does not meet those requirements and corrodes faster in wet food and dairy service.
What are the safety hazards of heat tint removal?
Hydrofluoric acid in pickling products causes deep, delayed-onset burns and systemic toxicity and requires calcium gluconate on site with trained personnel. Nitric acid is a strong oxidizer producing nitrogen oxide fumes. Work inside vessels or large piping triggers confined space entry requirements under OSHA 29 CFR 1910.146, and fluoride-bearing waste disposal is regulated and costly.
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More questions we are asked
How to remove weld discoloration from stainless steel tubing
Method follows severity, and severity is judged by color. Light straw tint can often be removed by mechanical means, using non-woven abrasive or a stainless wire brush reserved exclusively for stainless, never one previously used on carbon steel. Moderate to heavy blue, purple or grey tint needs chemical treatment: pickling with a nitric-hydrofluoric paste or gel dissolves the oxide and the chromium-depleted layer beneath it, which mechanical methods cannot reach. Electrochemical weld cleaning is an alternative for accessible external welds. In every case the treated area must then be passivated, because all three methods strip the passive film. Simply polishing tint away and stopping there leaves the depleted layer in place and the joint corrodes anyway.
What causes rainbow colors on stainless steel tube welds?
The colors are interference effects from a chromium oxide layer that thickens as the metal heats, and the sequence from straw through gold, blue, purple to grey corresponds to increasing oxide thickness and therefore to increasing heat input and oxygen exposure during welding. Two conditions produce them: too much heat, and inadequate inert gas shielding on the weld root. On tube, the usual cause is insufficient internal purge, either a purge gas flow too low, a purge started too late or ended too early, or oxygen ingress through a leaking purge dam. The visual attractiveness is misleading. Thicker tint means a more chromium-depleted layer directly beneath it, which is precisely where corrosion starts.
How to prevent heat tint on stainless steel tubing welds?
Purge properly and control heat. Establish the internal purge before striking the arc and maintain it until the weld has cooled below the oxidation threshold, which is later than most operators assume. Verify purge quality with an oxygen analyzer rather than by timing; on high-purity work, oxygen below around 50 parts per million at the weld is the usual target and lower is better. Use purge dams to reduce the volume being displaced on long runs. Keep heat input to the minimum that achieves full penetration, which is where orbital welding has a real advantage since the parameters are programmed and repeatable. Finally, protect the outside with trailing shield gas where the joint geometry allows.
Companies specializing in stainless steel weld discoloration removal services
This is normally part of a passivation or fabrication scope rather than a standalone service, and treating it separately is usually the wrong procurement. Whoever removes the tint must also passivate the treated area, because pickling and mechanical cleaning both strip the passive film, and on a hygienic system the work has to be documented to feed the turnover package. Ask any candidate which method they propose for the tint severity present, how they protect surrounding surfaces from the chemistry, how spent pickling paste is neutralized and disposed of given it contains hydrofluoric acid, and what acceptance test follows. Paul Industries delivers welding, tint removal, passivation and documentation under one contract nationwide.