Surface finish is the last step of metal injection molding, it is not a necessary but very important processing. Surface finish includes electroplating, anodize, electroforming, sandblasting, polishing, electrophoresis etc. In this article we are going to introduce Black oxide, which is a very common surface treatment for gun parts.

Black oxide is a kind of commonly used method of chemical surface treatment. The principle is to make the metal surface to produce a layer of oxide film, to isolate the air and attain purpose of rust protection. Black oxide can be adopted when cosmetic requirements is not high, the black finish steel parts is sometimes called blue.

　　The process of black oxide is as following:

　　1) Rinse

　　2) Degreasing: Workpiece must be completely immersed in the degreaser; Degreasing solution concentration PH value 12-14, processing time is 10 to 30 min, every 3-5 minutes shaking up and down several times, when the solution concentration is lower than PH 12 replenish the degreasing powder;

　　3) Water cleaning

　　4) Pickling：Concentration of the pickling solution PH 2 to 4, the handling time is 5-10 min

　　5) Water cleaning

　　6) Black oxide: pool solution concentration PH 2.5 ~ 3.5, processing time 10-12 min;

　　7) Water cleaning

　　8) Blow-dry

　　9) Oiling

　　The characteristic of black oxide:

　　1. Safe and no electric power. High temperature alkaline black oxide needs electric power.

　　2. Improve working efficiency, needs 1 to 2 hours.

　　3. With low cost, simple equipment, easy to operate; Black time is controlled strictly.

　　4. Has srong adaptability.

The commonly used black finish method is traditional alkaline heating black and normal temperature black that appeared relatively late. But the effect of normal temperature black is not very good for low carbon steel. A3 steel is better to use alkaline black. Alkaline black can be divided into once black and twice black.

The main components of the black oxide solution are sodium hydroxide and sodium nitrite.

The temperature tolerance of black oxide is pretty big, we can get pretty good surface at the temperature 135-155℃, only the needed time is different.

In practice, it is important to note the quality of the workpiece cleaning and degreasing before black oxide, and the passivation oiled after black. Black quality often determined by these processes.