Weld Heat Tint and Discoloration: Acceptance Criteria and Cleaning (AWS D18.2)

Weld heat tint is the visible oxide discoloration that forms on stainless steel next to a weld, and its color tells you directly how much heat and oxygen damaged the protective chromium-oxide film. AWS D18.2 provides a weld discoloration sample chart that ranks these tint colors from acceptable light straw shades to unacceptable dark blue, grey, and black, giving fabricators an objective way to judge sanitary and corrosion-critical welds. Darker tint means chromium depletion under the oxide and reduced corrosion resistance, so acceptance criteria and proper cleaning count as much as the weld itself. At Zhejiang Wenqiang Stainless Steel Co., Ltd. we treat post-weld condition as part of the deliverable, not a cosmetic afterthought.

Why heat tint forms and what the colors mean
Stainless steel resists corrosion thanks to a thin, self-healing chromium-oxide passive film. Heat the weld zone in the presence of oxygen and that film thickens into an interference oxide whose color depends on temperature and oxygen exposure, much like temper colors on carbon steel. Light straw and pale gold point to a thin oxide formed at modest temperature with little chromium loss. Deeper blue, then grey and black, point to hotter, more oxidizing conditions and a thicker oxide.
The trouble is what sits beneath the color. To build a thick oxide, the surface draws chromium from the metal just below, leaving a chromium-depleted layer. Corrosion resistance depends on keeping roughly 10.5% or more chromium in the passive film, so a depleted layer under dark heat tint corrodes far more readily. Field experience and lab work agree that darker tint tracks with lower pitting and crevice-corrosion resistance, which is why sanitary, pharmaceutical, and chloride-exposed systems keep tint tight.

The AWS D18.2 concept
AWS D18.2 is a visual acceptance standard built around a physical sample chart of weld discoloration on the inside of austenitic tube and pipe welds. It lays out a graded series of samples, each representing a level of discoloration from none through increasingly dark tint, and puts a number on each. A fabricator or inspector compares the actual weld to the chart under defined lighting and records the matching level. Higher number, darker tint, and more lost corrosion resistance.
The chart is most at home with hygienic tubing, where the inside surface contacts product and where back-purging quality decides the result. Its value is turning a subjective call about "how dark is too dark" into a repeatable, communicable level you can write into a specification and check on receipt.

The level you allow depends on the governing specification and how corrosive the service is; a potable-water line may tolerate more tint than a high-purity or high-chloride system.

Prevention with back-purging and heat control
The most effective way to control heat tint is to keep oxygen out of the weld zone during welding and cooling. Back-purge the inside of the pipe with argon, shield the outside properly, and you hold oxygen below the level that forms heavy oxide, so a well-purged weld comes out bright or only lightly strawed. Keep the purge going until the weld cools below the oxidation threshold, because tint keeps developing while the metal is hot and exposed.
Controlling heat input, travel speed, and interpass temperature limits the total oxidation and the width of the affected zone. Too much heat gives broader, darker tint and, in duplex grades, risks metallurgical damage on top of that. Good fit-up and consistent technique cut down the rework that repeated heating would otherwise force.

Removing heat tint
When tint runs past the acceptance level, it has to come off along with the chromium-depleted layer underneath, and the surface has to be repassivated. Which method you use depends on access and service.
Pickling paste, a gel of nitric and hydrofluoric acids, is brushed onto the weld to dissolve the oxide and the depleted layer chemically, then rinsed thoroughly; it restores corrosion resistance but demands careful handling and neutralization of the waste. Electropolishing takes off a thin surface layer electrochemically, leaving a bright, smooth, chromium-enriched finish that high-purity systems prefer. Mechanical methods such as stainless-wire brushing, abrasive blasting with clean media, or grinding remove tint physically, but they must use stainless-only tooling to avoid embedding carbon-steel iron, and on their own they may leave a less corrosion-resistant surface unless followed by passivation.
After any removal, passivation with nitric or citric acid re-forms a clean, chromium-rich oxide and restores the corrosion resistance the heat tint had cost.

When tint is acceptable
Not every trace of tint has to go. Light straw discoloration on a well-purged weld in mild service often meets the specification and leaves corrosion resistance essentially intact. Let the governing standard and the environment decide: hygienic, high-purity, and chloride-bearing services want near-bright welds, while less aggressive services may accept a low D18.2 level without cleaning. What matters is fixing the acceptable level up front and verifying it, rather than eyeballing it without a reference.

Practical guidance
Specify a maximum AWS D18.2 discoloration level suited to the service, prevent tint through disciplined back-purging and heat control, and remove and repassivate any weld that goes over the limit. Use stainless-only tooling for mechanical cleaning to keep iron out, and rinse pickled surfaces thoroughly. Wenqiang produces welded stainless pipe and tube with controlled purging and post-weld cleaning, and our in-house laboratory verifies weld quality and surface condition so the delivered product meets both the discoloration and corrosion-resistance requirements of your application.

Frequently Asked Questions
Q: Does weld heat tint really reduce corrosion resistance?
A: Yes. Darker heat tint forms by drawing chromium from the metal just below the surface, leaving a chromium-depleted layer. That layer pits and crevice-corrodes more readily, which is why AWS D18.2 grades tint and why demanding services limit it.
Q: What is the best way to prevent weld discoloration?
A: Back-purging the inside of the joint with argon and shielding the outside excludes oxygen during welding and cooling, keeping the weld bright or lightly strawed. Controlling heat input and holding the purge until the weld cools further limit tint.

For welded stainless pipe with controlled purging, post-weld cleaning, and verified weld quality, contact Zhejiang Wenqiang Stainless Steel Co., Ltd. at +86 577 8922 2595 / https://www.chinawqsteel.com/