A Guide to Understanding Weld Colours for Students in Welder School

June 25, 2019

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When colours appear in welding, it means that oxidation has occurred. And where there is oxidation, there is increased risk for corrosion. Colours follow a pattern from less to more oxidized, starting with chrome. They then progress though colours like straw, blue, and purple. The darker the colour, the more oxidation has occurred. Different industries have different allowances for weld colours, as in some cases any risk of corrosion is a problem. For example, the pharmaceutical industry does not allow anything past chrome, while sanitary welding in dairy allows for light blues.

Read on to learn about some of the factors that affect and cause weld colours, to prepare you for noticing it in your own work.

The Chemical Reactions Behind Weld Colours, Explained for Students in Welder Training
When we weld, we raise the temperature of the steel. When heated steel comes into contact with the air around it, there is a chemical reaction that occurs, called oxidization. Once there are oxides in the metal, colours appear and indicate that the metal is less resistant to corrosion. Rusting and deterioration can occur, compromising the weld’s function.

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Oxidation occurs when heated steel meets the atmosphere

In a lot of cases, discoloration on a weld can be cleaned off, but it gets more difficult depending on the colour. For lighter colours, sometimes brushing will work, but it’s harder to determine whether the affected layer has been fully removed. You may need to find more aggressive solutions in your career once you have earned your welding diploma. With electrochemical cleaning, both lighter and darker colours can be dealt with. With this method, the weld is immersed in a chemical solution and electric currents are applied, which causes the impurities to dissolve.

Fresh Welds like to Soak up the Atmosphere
A large factor in whether colours will show up on a weld is the atmosphere. Oxygen, nitrogen, and hydrogen in the air are what interact with the molten weld, sometimes causing holes in the weld bead and creating colour.

The amount of oxygen available will cause more or less colour. This is where a shielding gas comes into play. Shielding gas is applied to provide a barrier between the metal and the air around it, briefly covering the weld until it is cold enough to be safe in the atmosphere.

An Introduction to Different Materials and How They Are Affected
After welder training, you will choose materials in your career based on their unique properties. Heat resistance in metals is caused primarily by the chromium content. Chromium is a hard, steel-gray metal that is the main additive to stainless steel. It is used for its corrosion resistance and strength.

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Welders work with a variety of materials

308 stainless steel is often used by artists who intentionally create weld colours for aesthetic purposes. Very vivid colours can be achieved by intentionally oxidizing the material. Titanium is another interesting metal. It is a more brittle metal, and so the weld itself has less integrity the more oxidization occurs. This means that colours on a titanium weld could point to a structurally unsound situation.

The surface finish of steel can also affect oxidation. When surfaces are rougher and more irregular, they can oxidize at a higher rate and are more likely to have darker colours. The irregular surface also reflects light at more angles, increasing the appearance of the colours.

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