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Like hot black oxide, mid-temperature black oxide converts the surface of the metal to magnetite (Fe 3 O 4). However, mid-temperature black oxide blackens at a temperature of 90–120 °C (194–248 °F), [6] significantly less than hot black oxide. This is advantageous because it is below the solution's boiling point, meaning there are no ...
The oxide is converted into its hydrated form and the resulting swelling reduces the porosity of the surface. Mid-temperature sealing process which works at 160–180 °F (70–80 °C) in solutions containing organic additives and metal salts. However, this process will likely leach the colors.
Sometimes, temperature and gas conditions can be maintained such that a mixed sulfide feed (for instance a feed containing both copper sulfide and iron sulfide) reacts such that one metal forms a sulfate and the other forms an oxide, the process is known as "selective roasting" or "selective sulfation".
The "hot" process is an alkali salt solution using potassium nitrite or sodium nitrate and sodium hydroxide, referred to as "traditional caustic black", that is typically done at an elevated temperature, 135 to 155 °C (275 to 311 °F).
An Ellingham diagram is a graph showing the temperature dependence of the stability of compounds. This analysis is usually used to evaluate the ease of reduction of metal oxides and sulfides . These diagrams were first constructed by Harold Ellingham in 1944. [ 1 ]
Chemical coloring of metals is the process of changing the color of metal surfaces with different chemical solutions. The chemical coloring of metals can be split into three types: electroplating – coating the metal surface with another metal using electrolysis. patination – chemically reacting the metal surface to form a colored oxide or ...
An additional step can be added to the nitrocarburizing process called post-oxidation. When properly performed, post-oxidation creates a layer of black oxide (Fe 3 O 4), that greatly increases the corrosion resistance of the treated substrate while leaving an aesthetically attractive black color. [25]
Iron(II,III) oxide, or black iron oxide, is the chemical compound with formula Fe 3 O 4. It occurs in nature as the mineral magnetite . It is one of a number of iron oxides , the others being iron(II) oxide (FeO), which is rare, and iron(III) oxide (Fe 2 O 3 ) which also occurs naturally as the mineral hematite .