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Analysis of Blackening and Blueing Corrosion Protection

Which one can better improve the corrosion resistance of metals?

The corrosion resistance of metals that have been exposed to blackening and bluing originates from the use of bolts. When the company's products are actually purchased, bluing treatment is required, but when they arrive, they are blackened products. Although in reality, due to various unpredictable factors, some manufacturers claim that the corrosion resistance of products treated with blackening technology is stronger than those treated with bluing on the market, from the perspective of quality management, this is not the case. So this article will first share the theoretical differences in corrosion prevention between blackening and bluing for reference.

1、 The process principle of blackening and bluing

Blackening and bluing are both caused by chemical reactions that form a protective film on the surface of steel parts. This film can effectively isolate air and prevent further oxidation inside the steel parts, thereby achieving the purpose of rust prevention.

Blackening treatment:

Mainly carried out in alkaline oxidation solution for chemical oxidation. Usually, solutions containing sodium hydroxide, sodium nitrite, and other components are used. At relatively low temperatures (usually around 135-145 ℃), iron reacts with oxidants in the solution to form a black oxide film on the surface of the bolt. The main reaction process is the reaction between iron atoms and oxidants in the solution to generate iron oxide. This oxide film is the source of the black surface of the bolt after blackening treatment.

Although the oxide film formed by blackening treatment itself has a certain rust prevention effect, its susceptibility to damage and high porosity may result in poor rust and corrosion prevention when used directly. Based on a comprehensive evaluation of the usage environment, if the overall performance of this oxide film is improved, additional saponification and oil immersion treatments can be carried out after blackening treatment. Saponification treatment can fill the micro pores on the surface of the oxide film, forming a closed structure; And oil immersion treatment can form a layer of oil film on the surface of the parts, further enhancing rust prevention and corrosion resistance.

Blue processing:

Then the bolts and other steel parts are heated at a higher temperature (usually around 550 ℃), using a chemical solution containing a strong oxidant to form a sky blue oxide film on their surface. The main component of this oxide film is iron tetroxide, which is relatively denser and harder to blacken. It can effectively isolate air and prevent further oxidation inside the steel parts, thus having better rust and corrosion resistance.

2、 Differences in appearance and application scenarios between blackening and bluing

2.1. Appearance differences:

The appearance color of the bolt after blackening treatment is black, with a relatively dark and uniform color. This black color is relatively pure, with no obvious color changes or gloss.

The bolt color after blueing treatment is mainly deep blue or blue black, which may have a certain metallic luster. Compared to blackening treatment, bolts treated with bluing have a more layered color and may look more aesthetically pleasing.

2. 2 Application scenarios: 

Due to the relatively low cost and simple process of blackening treatment, it is suitable for situations where the appearance requirements of bolts are not particularly high and it is widely used in the processing of ordinary mechanical parts in general environments. It has certain concealment and practicality. For example, for bolts used in ordinary outdoor environments and mildly corrosive indoor environments, blackening treatment can meet basic anti-corrosion and appearance requirements for turning boxes into sleeves.

The appearance of the bolts after blueing treatment is more beautiful and has good corrosion resistance, so they are commonly used in situations where high requirements are placed on appearance and corrosion resistance. For example, in some fields such as precision machinery, automotive parts, and high-end equipment, bolts that have undergone bluing treatment can not only effectively prevent corrosion, but also enhance the overall aesthetics of the product.

In practical applications, the choice between blackening or bluing depends on the specific usage environment, requirements, and supplier's level of craftsmanship. Whether treated with blackening or bluing, the oxide film formed has a certain lifespan. During use, due to factors such as friction, impact, scratches, etc., the oxide film may be damaged, thereby affecting its rust prevention performance. Therefore, it is necessary to pay attention to protecting the oxide film on the surface of the steel parts to avoid damage.