The given topic is about the hot chemical oxidation of steel, where the surface layer of the parts is made chemically passive to a depth of a few micrometers.
The oxidation technology itself is simple: the steel, cleaned of corrosion, oils, fats, and other impurities, is kept in a solution at a temperature of 120-150°C, which consists of the following chemicals:
• 1L of soft water
• 750-800g NaOH (caustic soda)
• 220-240g NaNO3 (sodium nitrate)
• 40-80g NaNO2 (sodium nitrite)
All these chemicals are available from Keemiakaubanduse OÜ. Additionally, 96% sulfuric acid can be used for cleaning.
The steel items are kept in the solution for 15-45 minutes as needed, and visual inspection can be performed in between. They are then removed, washed clean, dried, usually coated with raw linseed oil, and later slightly heated.
The gloss of the oxide layer can be adjusted by the amount of NaNO2 in the solution. Without NaNO2, a matte black oxide layer forms, and by increasing its amount, a perfect glossy oxide layer can be created.
The oxide layer essentially provides corrosion protection, with a visual effect as an added benefit. Generally, the oxide layer created in this way protects against the action of organic acids commonly found in nature. It does not protect against inorganic acids (such as hydrochloric, sulfuric, phosphoric, etc.). The oxide layer is quite weak and wears off easily. For this reason, many military weapons are not black but rather brownish, with a layer significantly weaker than a black oxide layer. On military mass production items, only the oxide layer is present. Smaller items may have the oxide layer covered with various substances, ranging from linseed oil and beeswax to lacquers.
Warnings:
• Caustic soda is corrosive and extremely dangerous when hot!
• Never add water to hot caustic soda solution! Adding water to an environment over 100 degrees Celsius will cause the added water to boil instantly and can splash the hot caustic soda solution up to 6-7 meters away!
• Sodium nitrite (NaNO2) is toxic!
• Only steel can be oxidized! Items made of aluminum, brass, copper, or other metals-alloys should not be placed in the solution; they will corrode and dissolve completely, becoming non-existent!
Regarding the colors of the oxide layer, using manganese oxide or some other salts, various shades can be achieved—golden, brownish, bluish, or any other. A nice brown oxide layer can also be obtained as a technological defect when the active substances in the solution are depleted during the process.
The best container for oxidation is a stainless steel one. It does not break, withstands heat, and does not participate in the reaction.
Before oxidation, the parts must be pickled, i.e., briefly held in a 5% sulfuric acid solution, rinsed with cold water, washed in hot household soap solution, and then placed in the preheated solution (heating the solution from scratch is not good, a new corrosion layer forms on the surface of the item—the same one that pickling was used to remove!).
Rust (Fe2O3xnH2O) does not oxidize! This means any untreated and deep-seated rust spots will remain unchanged on the surface and at the base, continuing to corrode vigorously! Only a clean steel surface can be oxidized!