Alkaline heating and blackening treatment of fasteners

Blackening treatment is a commonly used surface treatment method for fasteners, which forms an oxide film on the surface of metal fasteners to isolate air and achieve rust prevention.

There are two main ways to blacken fasteners: traditional alkaline heating (high temperature) blackening treatment and room temperature blackening treatment. Today, we will discuss in detail the traditional alkaline heating blackening treatment.

The main components of the blackening solution for alkaline high-temperature blackening treatment are sodium hydroxide and Sodium nitrite. The temperature required for blackening is 135 ℃ -155 ℃, and the time required is 30min -90min. Before treatment, fasteners and other workpieces need to be cleaned, such as oil removal, rust removal, and post treatment after the completion of blackening process, such as saponification, oil immersion, etc.

The principle of alkaline high temperature blackening treatment is that under high temperature, metal iron interacts with oxidants (Sodium nitrite, Sodium nitrate, etc.) and alkali (sodium hydroxide) to produce a series of chemical reactions, and finally forms a black alkaline iron oxide film. This type of film is relatively dense, with high hardness, and stable rust resistance in high-temperature air. However, its rust resistance is poor in water or humid environments, so it is necessary to undergo post-treatment such as oil immersion after oxidation treatment.

The advantages of alkaline high-temperature blackening treatment are: the blackening film layer is beautiful and bright, with good wear resistance, simple equipment, low cost, and the accuracy and inner and outer diameter specifications of the treated fasteners will not change.

However, there are nitrite harmful to human body in the treatment solution of alkaline high temperature blackening treatment, and the treatment temperature is high, energy consumption is large, and a large amount of acidic and alkaline wastewater will be generated, which will pollute the environment.