What chemical melts stainless steel
Annealing colors on stainless steel
"How to prevent metal discoloration"
Some of the minerals in nature impress with their beautiful, iridescent surface. This effect can also be observed on metal surfaces - which in industrial metalworking is desirable in some cases and undesirable in others. But how do these bright colors come about and how can you prevent the phenomenon of metals, also known colloquially as “tarnishing”?
What are temper colors?
In nature you can admire brightly colored shimmering minerals such as colored pebbles or copper pebbles. Depending on the incidence of light, these stones shimmer in all sorts of rainbow colors and are reminiscent of the iridescent film that also forms on an oil puddle or on soap bubbles. This coloring is created by very thin, transparent oxide layers that can form on the surface of minerals and metals. These so-called interference colors are created by superimposing light rays that are refracted or reflected on the thin layers. Depending on the thickness of the layer, they are broken to different degrees and perceived as different colors by the human eye.
How are temper colors created?
In nature, tempering or tarnishing colors develop on minerals, the surface of which can be easily oxidized by atmospheric oxygen. These include minerals containing copper or iron, such as colored copper gravel or hematite. In metal processing, tempering colors are sometimes desirable when a metal is to be given a certain color. Some metal surfaces are refined in this way and only receive a certain color. Titanium is given the desired color tone through anodizing, the targeted creation of an oxide layer. This can range from gold to blue and purple to green, which is used in the watch and jewelry industry. Tempering is used in steel processing to change the properties of the steel, such as hardness and toughness, and to reduce internal stresses. In other metalworking processes, such as welding, the formation of an oxide layer is undesirable and attempts are made to prevent its formation.
Temperature and exposure time influence the tempering color
With titanium, stainless steel and any other metal, the thickness of the oxide layer determines the color of the surface. When heated, oxygen diffuses into the upper metal layers and the color depends on how deep it penetrates into the surface layer. The depth of penetration depends on the temperature and the duration of exposure. Conversely, one can deduce the thickness of the oxide layer from the color of the metal. For example, a pale yellow color on stainless steel indicates a thin oxide layer and the processing took place at around 200 ° C. In general, the color palette runs from light yellow to red to blue and finally black, with black layers being referred to as tinder. Several colors can also appear next to each other, as can be seen, for example, with weld seams.
Which materials are particularly affected?
In general, an oxide layer can develop on all metals. For some only when heated, for others in the air. The "tarnishing" of silver or metal shows that the process itself is well known in everyday life. In the case of silver, the black layer is not created by atmospheric oxygen, but by hydrogen sulfide, which reacts with silver to form silver sulfide. But even this thin coating shows a slightly iridescent color on closer inspection. In some cases, such as titanium, the color that results from the oxidation is desirable, in other cases it is not. Annealing colors on stainless steel are generally undesirable, especially if they occur on weld seams, as they have a negative effect on the corrosion resistance, one of the most important properties of stainless steel.
Where does the corrosion resistance come from and why should tempering colors on stainless steel be avoided?
Corrosion-resistant steels usually contain a minimum of 11% chromium, which reacts with atmospheric oxygen and forms a thin, firmly adhering layer of chromium oxide. This process, known as passivation, prevents further oxidation of the steel underneath. In addition, the passive layer is renewed even if the surface is damaged or destroyed, provided there is enough oxygen available and there are no other deposits on the surface that would interfere with the formation of the layer.
Annealing colors that form on stainless steel can disturb this delicate balance. Chromium reacts more easily with the oxygen diffusing into the deeper layers. The surface “depletes” of chromium and the formation of the protective chromium oxide layer is no longer guaranteed. At this point, pitting corrosion can occur and continue inside the material to form cavernous cavities. The material is irreparably damaged. If tempering colors have formed on stainless steel, these must be removed; only very light discolorations can be tolerated in most cases.
Various options are available for removing annealing colors on stainless steel
The simplest option - mechanically
The mechanical removal of tarnish on stainless steel is done by brushing, grinding or blasting. As with all treatment methods, it is important to clean and degrease thoroughly beforehand. If a small layer is to be removed from the metal, brushing is generally sufficient. However, it should be noted that, depending on the workpiece and geometry, not all points can be reached. Alternatively, temper colors can be removed from metal by grinding. Here one should make sure that local overheating does not occur during processing and that new oxide layers are formed, which prevent the formation of the passive layer. The grinding process should also be carried out in several steps as well as with different grain sizes - from coarse to fine. Blasting has established itself among the mechanical processing methods, especially on an industrial scale. It also removes stubborn temper colors and at the same time ensures an advantageous, because uniform, surface quality.
Removing with chemistry
Pickling is the chemical variant to remove annealing colors. It is treated with an acid, in the case of stainless steels for example a mixture of nitric acid and hydrofluoric acid. In addition to immersion pickling, in which the workpiece is immersed in a bath and which is particularly suitable for small to medium-sized components, pickling pastes or gels can be applied locally to remove any tarnishing. Spray pickling is used for very large or complex components.
Often both processes are used one after the other and a mechanical treatment is followed by pickling.
Smooth surfaces on stainless steel - the third variant
A third option is electrochemical treatment: “electropolishing” or “anodic pickling”. The principle corresponds to the reverse electroplating: the workpiece is immersed in an electropolishing bath, connected in direct current as an anode and the metal on the surface is electrochemically dissolved. This process has the advantage that a very clean and tension-free surface is created.
Avoidance is the best strategy
The best way to remove tarnish is not to let it develop in the first place. Welding in a protective gas atmosphere is known as forming and can effectively prevent the formation of tempering colors. Either inert gases such as argon or nitrogen or mixtures of argon-hydrogen or nitrogen-hydrogen are used. In these mixtures, the reducing effect of hydrogen is used, which at correspondingly high temperatures can reduce metal oxides that have already formed and, secondly, can react with any residual oxygen to form water. The selection of the suitable gas mixture depends above all on the material to be processed.
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