When stainless steel is exposed to high temperatures, its surface develops a spectrum of colors, known as heat tint or temper coloring. This iridescent effect indicates the metal has undergone a chemical transformation. The rainbow appearance is not a stain but results from a two-part process: oxidation combined with a physical effect related to how light interacts with the resulting surface layer. This discoloration provides a record of the metal’s thermal history.
The Chemical Process of Oxidation
The corrosion resistance of stainless steel relies on a microscopic, self-repairing layer of chromium oxide that naturally forms on its surface. When the metal is heated, particularly between \(400^circtext{F}\) and \(1300^circtext{F}\) (\(204^circtext{C}\) to \(704^circtext{C}\)), the oxidation process accelerates significantly. Chromium atoms within the alloy migrate to the surface and react with oxygen, creating an increasingly thick, transparent layer of chromium oxide (\(text{Cr}_2text{O}_3\)).
Excessive heat draws chromium from the base metal to form this surface layer. This migration depletes the metal just beneath the oxide film, creating a chromium-reduced layer. As the oxide film thickens, the underlying metal loses the chromium concentration necessary to maintain its corrosion-resistant properties.
Creating the Rainbow Effect
The colors observed on the heated stainless steel are not inherent to the oxide layer’s composition but are generated by thin-film interference. This is the same physical principle that causes iridescence in a soap bubble or oil slick. Light waves reflecting off the surface of the oxide layer interfere with light waves that penetrate the layer and reflect off the metal surface underneath.
Because the oxide film is transparent, light reflects from two boundaries: the air-oxide interface and the oxide-metal interface. As the oxide layer thickens with continued heat exposure, the distance light travels within the film changes. At specific thicknesses, certain wavelengths of light are canceled out while others are reinforced, creating the distinct colors. A slight change in the thickness of this nanometer-scale film is enough to shift the visible color from yellow to blue.
Color Sequence and Temperature
The progression of color correlates directly to the thickness of the chromium oxide layer, which is determined by the temperature and duration of heating. As heat is applied, the film begins to form, creating a pale straw or light yellow color around \(550^circtext{F}\) (\(288^circtext{C}\)). Continued heating drives the color through straw yellow, dark yellow, and then brown, which appears around \(750^circtext{F}\) (\(400^circtext{C}\)).
The spectrum progresses into purple-brown, dark purple, and finally blue, registering near \(1000^circtext{F}\) (\(538^circtext{C}\)). These colors provide an indicator of the maximum temperature reached, as the oxide layer grows thicker sequentially. If the temperature rises substantially higher, the color shifts to dark gray or black, indicating a very thick and less protective oxide scale.
Addressing the Discoloration
While aesthetically displeasing, the heat tint itself is not a direct hazard. The concern arises because chromium depletion beneath the colored film significantly lowers the steel’s resistance to corrosion, especially in moist or aggressive environments. Darker colors, like blue or gray, indicate a thicker oxide and a more severely depleted layer, making the metal susceptible to localized corrosion such as pitting.
To restore the original corrosion resistance, the damaged oxide layer and the underlying chromium-depleted layer must be removed. This can be achieved through mechanical methods, such as grinding or polishing, which physically abrade the surface. Chemical pickling is also effective, using acidic solutions like nitric and hydrofluoric acid to dissolve the oxide film and the compromised metal underneath. Preventing discoloration is often preferred, which involves limiting heat input during processes like welding or using inert gas shielding, such as argon, to prevent oxygen from reacting with the hot metal surface.