![]() The combination of the salts and the elevated temperature of the slats causes the formation of magnetite (Fe2O3) instead of rust (another iron compound). This is because lower temperatures elongate the process, while higher temperatures would cause rust on the finished part’s surface. It is vital that the temperature of the black oxide coating process stays in this range. The temperature range of this process is usually around 275☏ to 295☏. The various salt baths used include caustic soda (NaOH), nitrates, and/or nitrites salt baths.Īutomated transporters move the parts from one bath to another to fully affect the process. It involves dipping the part in salt baths at high temperatures. The hot black finish is the most popular type of black oxide finish. ![]() These temperatures also different the various types of black oxide finish. Lacquer is also a good choice, especially if you need to finish the parts quickly, as it dries very quickly.Īs mentioned earlier, there are various operating temperatures at which machinists carry out the black oxide finish process. Wax gives a duller surface, but its lubrication and corrosion-augmenting properties are even higher. It gives a glossy surface and also increases the lubrication and corrosion properties. Oil is the commonly used after-finish among machinists. The common after-finish candidates are oil, wax, and lacquer. However, for other metal parts, the after-finish is quite vital. This is because these metals have inherent anti-corrosion properties. However, this rule does not apply to black oxide stainless steel or brass surface. Without the after-finish, the finished part is still highly prone to corrosion. However, at the cold temperature operating level, the process deposits a copper selenium coat (Cu2Se) on the part surface. This stage has three different temperatures, determining the coating material on the part’s surface.Īt the hot (141☌) and mild temperatures (90 – 120☌), the process leaves a magnetite coat (Fe2O3) on the part. The machinist dips the finished metal parts into tanks containing various alkaline solutions such as sodium hydroxide, nitrates, and/or nitrates. This is where the machinist finally applies the finish. The machinist must de-plate the part as the black oxide finish cannot stay on plated parts. This conditional stage occurs only when the finished part is plated with another material (e.g., chromium, zinc, cadmium, etc.). This will help prevent bleed-outs which could impair the surface of the part. This involves rewashing the part with an alkaline solution. This process is only necessary when the finished parts have blind holes, or it is an assembly of parts. It involves removing all the rusts and oxide stains on the finished parts by soaking them in acid. This stage is only necessary if the parts have rust or scales. This process involves washing off the alkaline solution used in cleaning with distilled water. ![]() The machinists degrease and clean the part using an alkaline solution. ![]() This process involves preparing the part for the black oxide finish coating. This section will explain each of these steps in detail. The back oxide finish process generally involves five steps which include: ![]()
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