Fastener processing methods are commonly used: blackening, phosphating, and galvanizing.
Blackening treatment is also called blue treatment, which is a method of metal surface treatment, and is also a widely used metal anti-rust treatment technology. The blackened metal surface forms a black hard film, and the hard coat layer protects the metal surface from oxidation, thereby achieving rust prevention.
The commonly used methods for blackening are traditional alkaline warming and blackening, and late blackening at room temperature. The main components of the blackening solution are sodium hydroxide and sodium nitrite.
Alkaline heating black
Alkaline heating blackening is the most typical blackening method for steel. The process is relatively mature and the blackening quality is relatively stable. It is the most widely used blackening process. In this method, a concentrated solution of NaNO2 and NaOH is used, and the workpiece to be blacked is placed in the solution and boiled at 140 ° C for about 40 minutes. X-ray diffraction analysis showed that a dense crystalline Fe3O4 film was formed on the surface of the steel. The oxide film has good contact with the surface of the steel substrate and has good adhesion and corrosion resistance. The blackening process is not high in the pre-treatment of the workpiece due to the high temperature and strong alkali medium.
Disadvantages: Alkaline heating and blackening are carried out under heating. This processing technology has major disadvantages, such as labor, time-consuming and high energy consumption, especially when the blackening agent evaporates harmful gases under heating. It is harmful to health and is currently being phased out.
Normal temperature blackening technology
The steel blackening agent at room temperature has the advantages of stable solution, short blackening time, convenient and simple use process, sufficient raw materials and low cost, especially the adhesion of the black film layer, and the corrosion resistance is obviously superior to the original product. The most critical part of the use of blackening liquid at room temperature is the pre-cleaning of the workpiece. Before the blackening of the workpiece, the degreasing cleaning agent must be used to remove the oil, otherwise the blackening effect will be affected.
Common blackheading defects in normal temperature blackening agent, uneven blackening film, light blackening film, poor gloss, rust spots, and firmness than alkaline blue. Each of the shortcomings has a corresponding exclusion method in the manufacturing process.
Phosphating
It is a process of chemically and electrochemically reacting to form a phosphate chemical conversion film, and the formed phosphate conversion film is called a phosphate film.
The purpose of phosphating is mainly to provide protection to the base metal, to prevent the metal from being corroded to a certain extent; to base the paint before painting, to improve the adhesion and corrosion resistance of the paint layer; to reduce friction in the metal cold working process Lubrication
Commonly used process: pre-degumming → degreasing → derusting → water washing → (table adjustment) → phosphating → water washing → phosphating post-treatment
Galvanized
It refers to the surface treatment technology that coats the surface of metal, alloy or other materials with zinc for aesthetics and rust prevention.
Zinc hardly changes in dry air. Zinc is easily soluble in acid and soluble in alkali, so it is called an amphoteric metal. Therefore, galvanized fasteners are rarely used in corrosive environments of pressure vessels.
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