For the production of fastener screws, surface treatment is an inevitable process, many manufacturers are asking about the surface treatment of fastener screws, we have summarized the common surface treatment of fastener screws in eight forms, such as: black (blue), phosphating, hot dipping zinc, dacromet, electrogalvanizing, chrome plating, nickel plating and zinc plating. The fastener screw surface treatment is the process of forming a covering layer on the surface of the workpiece by a certain method, and its purpose is to give the product surface beauty and anti-corrosion effect.

In order to meet the needs of products, screw fasteners generally use surface treatment processes to meet customer requirements, some are changes in the surface treatment of hardware screws, some are changes in the performance of screws, so what material is suitable for what surface treatment, what kind of surface treatment can achieve rust prevention performance? Small series to briefly introduce the commonly used screw fastener surface treatment process includes what and their respective characteristics:

1. Blackening (bluing) The fasteners to be blackened are placed in the solution tank (145±5℃) of sodium hydroxide (NaOH) and sodium nitrite (NaNO2) oxidizing agents to heat and oxidize, so that the surface of the metal fastener generates a layer of magnetic ferric oxide (Fe3O4) film, the thickness is generally 0.6-0.8μm black or blue-black.

Pressure vessel fasteners HG/20613-2009 and HG/T20634-2009 standards require blue treatment.

2. Phosphating Phosphating is a chemical and electrochemical reaction to form a phosphate chemical conversion film process, the phosphate conversion film formed is called phosphating film.

The main purpose of phosphating is to provide protection for the base metal and prevent the metal from being corroded to a certain extent; It is used for primer before painting to improve the adhesion and anti-corrosion ability of the paint film. It is used for antifriction lubrication in metal cold working process.

The standard for large diameter double-ended studs for pressure vessels requires phosphating.

3. Hot dip zinc Hot dip zinc is to immerse the steel member after rust removal into the zinc liquid melted at a high temperature of about 600℃, so that the surface of the steel member is attached to the zinc layer.

The thickness of the zinc layer shall not be less than 65μm for the thin plate less than 5mm, and not less than 86μm for the thick plate of 5mm and above. So as to play the purpose of corrosion prevention.

4. DACROMET is the translation and abbreviation of Dacromet, referred to as Dacromet, Dacromet rust, Dicron. The domestic name is zinc-chromium coating,

It is a new type of anti-corrosion coating with zinc powder, aluminum powder, chromic acid and deionized water as the main components. There is no hydrogen embrittlement problem,

And the torque-preload consistency is very good. If you do not consider the environmental problems of hexavalent chromium, it is actually most suitable for high-strength fasteners with high corrosion protection requirements.

5. Chrome plating Chromium plating is mainly to improve the surface hardness, beauty, rust prevention. Chromium coating has good chemical stability in alkali, sulfide,

Nitric acid does not interact with most organic acids, but is soluble in halogenic acids (such as hydrochloric acid) and hot sulfuric acid.

Because chromium does not change color, it can maintain its reflection ability for a long time when used, which is better than silver and nickel.

6. Electrogalvanizing Electrogalvanizing, also known as cold galvanizing in the industry, is the process of forming a uniform, dense and well-combined metal or alloy deposition layer on the surface of the parts by electrolysis.

Compared with other metals, zinc is a relatively cheap and easy to coat a metal, is a low value corrosion protection coating, is widely used to protect steel parts, especially to prevent atmospheric corrosion,

And used for decoration. Plating technologies include groove plating (or hanging plating), roll plating (suitable for small parts), blue plating, automatic plating and continuous plating (suitable for wire and strip).

Electrogalvanizing is the most commonly used coating for commercial fasteners. It is cheaper and looks better. It can be black or army green. However, its anti-corrosion properties are mediocre,

Its corrosion resistance is the lowest in zinc plating (coating) layer. General electrogalvanizing neutral salt spray test within 72 hours, there are also special sealers,

It makes the neutral salt spray test more than 200 hours, but the price is expensive, which is 5-8 times that of general galvanized. Fasteners for structural parts generally have colored zinc and white zinc,

For example, 8.8 and other commercial grade bolts.

7. Nickel plating Nickel plating is mainly wear-resistant, anti-corrosion, anti-rust, generally thin thickness of the process is divided into electroplating and chemical two categories.

8. The principle of zincizing powder zincizing technology is that the zincizing agent and steel parts are placed in the zincizing furnace and heated to about 400 ℃.

The active zinc atom penetrates into the steel parts from the surface to the inside. At the same time, iron atoms spread from the inside out,

This forms a zinc-iron intermetallic compound on the surface of the steel, that is, the galvanizing layer.