Temper colours on stainless steel: which are harmful, how to pickle and passivate

By Robert Lahnsteiner, International Welding Engineer (IWE) · Updated: November 2026

What temper colours are

Temper colours are thin oxide layers that form next to the weld during welding. They form when hot metal comes into contact with oxygen. The chromium in these layers is then missing in the steel beneath, and the corrosion resistance drops there.

At room temperature, stainless steel protects itself: a passive layer of chromium oxide only a few nanometres thin forms again and again in air, see What makes stainless steel stainless. Under welding heat, by contrast, oxidation proceeds faster and deeper. The layer becomes thicker, changes its composition and loses its protective effect.

The colour comes from light refracted at the oxide layer. The further away from the weld pool, the thinner the layer. From the weld outwards there follow roughly grey, blue, red, yellow and finally bright metal, with smooth transitions. The colour depends not only on the temperature, but also on how long the heat acts and how much oxygen is present.

Which colours are harmful

Yellow reduces corrosion resistance slightly, red and grey considerably. Because every blue discolouration is accompanied by a red one, the practical rule is: anything darker than yellow must go if the weld is exposed to corrosion.

Colour	Effect on corrosion resistance
Straw yellow to yellow	only a slight reduction in resistance; the only colour that also occurs on its own
Dark yellow to red	from darker than reddish yellow, susceptibility to pitting rises sharply
Blue	in itself more resistant again, but never occurs alone: there is always red next to it
Grey, black, scaled	resistance severely impaired; the chromium here is literally burnt

According to DVS 0937 and Warzawa (DVS-Berichte vol. 275). In the salt spray test, the resistance to pitting decreases steadily as the yellow becomes darker.

In addition, with prolonged heating the chromium in the microstructure can redistribute. If it falls locally below about 12 %, the steel is no longer stainless there.

Avoiding temper colours

The simplest approach is not to let temper colours form in the first place. To achieve this, oxygen must be kept away from the hot surface.

Purge the root side: Root protection and purging.
Weld the face side with good gas shielding, if necessary with a trailing gas shield that covers the cooling weld afterwards: Shielding gases.
Keep the heat input low: the shorter the time the metal is hot, the thinner the oxide layer, see Heat input and interpass temperature.
Dry, tight gas lines: moisture in the gas also produces temper colours.

Assessing temper colours

Which temper colour is still acceptable should be specified before welding, for example in the order or in the welding procedure specification. It is then checked by comparison with colour charts or reference samples.

The same oxidation gives the same colour, regardless of the base material. This is why temper colours can be compared with neutral colour charts.
Light and observer influence the judgement: brightness, light colour, experience and colour vision. It is best to record the viewing conditions in the test report.
Through an endoscope colours look different. Here it helps to view reference surfaces first through the endoscope and then directly.
Rough surfaces and printed colour charts render colours differently from smooth steel.

Removing temper colours

Temper colours, scale, slag and spatter impede the passive layer and must be removed from welds exposed to corrosion, mechanically or chemically.

Method	With what	When
Brushing	with stainless steel brushes, used only for this purpose	sufficient if there are no high requirements for corrosion resistance and the surface is bright metal afterwards
Grinding	iron- and sulphur-free discs, final grinding at about grit 180 to 240	over a wide area and with little pressure, so that no new temper colours and stresses form; remove grinding dust immediately
Blasting	glass beads; according to the ISER information sheet also stainless steel grit or other iron-free blasting media	removes light temper colours and gives a uniform surface; for high requirements, pickle afterwards
Pickling	bath, paste or spray pickling, usually with nitric and hydrofluoric acid	removes temper colours, scale and the chromium-depleted layer beneath; the most thorough cleaning
Electropolishing	electrochemical removal, workpiece as anode	particularly smooth, high-quality surfaces; as a hand-held unit also for cleaning welds

According to DVS 0931 and ISER information sheet 823 (2019). For all mechanical methods: use tools and blasting media only for stainless steels, otherwise iron particles remain and rust.

When grinding without subsequent pickling or electropolishing, no high residual stresses or cold deformation may develop in the surface. In austenitic steels they can trigger stress corrosion cracking. Where this risk exists, for example in chloride-containing media, the surface is pickled after grinding.

Pickling in practice

Pickling chemically dissolves the oxide layer and the chromium-depleted zone beneath it. Afterwards the passive layer can form completely anew. The instructions of the pickling agent manufacturer take precedence.

Degrease first: completely remove heavy soiling, grease and oil.
Immersion pickling: at room temperature where possible, exposure time about 30 minutes to 2 hours.
Pickling paste or pickling gel applied to the weld with a brush. Because of the limited acid content, the risk of over-pickling is lower.
Spray pickling for large vessels for which no pickling bath is big enough.
Do not over-pickle: too long, too strong or too warm damages the corrosion resistance.
Rinse thoroughly until no acid can be detected, for example with indicator paper. Do not forget gaps and corners; pickling residues there trigger corrosion.

Health, safety and the environment

Pickling agents for stainless steel usually contain nitric acid and hydrofluoric acid. They require trained personnel, protective clothing, safety goggles, respiratory protection where necessary and occupational health surveillance. The rinse water must not go into the sewer untreated; it can be neutralised. Pickling sludge is hazardous waste.

Passivation

After pickling and rinsing, the passive layer forms by itself in air. Depending on the surface, this can take several hours. Additional passivation is carried out if this time is not available or high requirements apply.

According to the ISER information sheet, a treatment in nitric acid of about 20 % can be used for this. Here too, the part is then rinsed thoroughly with water.

Frequently asked questions

Does straw-yellow temper colour have to be removed?

Straw yellow reduces resistance only slightly and is often tolerated. Whether it may remain depends on the service conditions and should be agreed in advance.

Is brushing enough?

For low requirements, yes, if a bright metal surface without colour residues remains afterwards. Only pickling removes the chromium-depleted zone beneath the temper colour.

Can I blast with quartz sand?

For stainless steels, DVS 0931 mentions only glass beads, the ISER information sheet also other iron-free blasting media. Blasting media that has already been used on carbon steel is off limits in any case.

Do I have to passivate after pickling?

Not necessarily. The passive layer forms by itself in air. Additional passivation is carried out if the component goes into service immediately or particularly high requirements apply.