The principle of Dalian blackening surface treatment is to heat steel parts in a chemical solution to an appropriate temperature to oxidize their surface to form a smooth and dense oxide film. Depending on the composition of the chemical solution, two methods can be used to obtain oxide films: alkaline oxidation and acidic oxidation. This oxide film, also known as Fe3O4, can effectively protect the interior of steel parts from oxidation and achieve rust prevention.

The maintenance film formed by steel blackening agent is not affected by the alloy elements in steel parts, and has good adaptability. It can be used for tempering and blackening of cast iron, spring steel, and various alloy steels after heat treatment processes such as carburization, carbon nitrogen co infiltration quenching, and high-frequency local quenching. Therefore, it has wide applicability and has been successfully applied to materials such as 50CrV 60Si2Mn 65Mn 35CrMo 70 # steel wire, 12Cr 20CrNiMn, etc.

Corrosion resistance of Dalian blackened surface treatment:

① Using analytical pure reagents and distilled water, a 13ml mixed solution of copper sulfate 41g/l, sodium chloride 35g/l, and 0.1mol/l hydrochloric acid was dripped at 15 ℃ -20 ℃, with a discoloration time greater than 3 minutes;

② Using 3% chemically pure copper sulfate, drop at 15 ℃ -20 ℃, with a discoloration time greater than 5 minutes;

③ Drip with a 5% sulfuric acid solution, exposing the substrate for more than 6 minutes;

④ Soak in a 3% sodium chloride solution for more than 80 minutes before rusting.

When self checking the blackening quality of the workpiece, it should be carried out before the oiling process. The 2% copper sulfate solution immersion method for 30 seconds without copper precipitation is considered qualified. 3% copper sulfate drop test. Dip the workpiece with copper sulfate or 2% copper sulfate for etching test, immerse the workpiece completely in this copper sulfate solution, and observe with the naked eye that the oxide film does not change color after 30 seconds, which is considered qualified. The drop test and etching test are both conducted before the workpiece is oiled. When inspecting the finished product, the oil stains should be removed with alcohol. After blackening and oxidation, the workpiece will not rust within one year of normal storage.

There are two commonly used methods for blackening treatment: traditional alkaline heating blackening and later room temperature blackening. But the room temperature blackening process is not very effective for low-carbon steel. Alkaline blackening is subdivided, and there is a difference between once blackening and twice blackening. The main components of black liquor are sodium hydroxide and sodium nitrite. The temperature difference required for blackening is not significant, and a good surface can be obtained between 135 and 155 ℃, but the required time is only a bit short.