In the Full Threaded Stud industry, screws and nuts play the role of connection and fixation in various industries. Many fasteners used outdoors, such as bridges and nuts on guardrails, are exposed to the sun and rain for a long time. Worse, the ocean is more prone to rust. Rusty screws not only affect the appearance, but also have safety hazards. Then let me talk about why the screws rust? What kind of screws are not easy to rust? How to improve the screw performance of fasteners?
The principle of fastener rust
Metal fasteners will come into contact with chemical components in humid air during long-term use. There has been a chemical change-electrochemical corrosion. This corrosion is also the most common. This is also a problem frequently encountered by fasteners.
There are many corrosion principles of metals, among which electrochemical corrosion is the most widespread. When metal is placed in an aqueous solution or humid environment, micro-batteries are formed on the metal surface, which is also called electrochemical corrosion.
The anode undergoes an oxidation reaction to dissolve the anode, and the cathode undergoes a reduction reaction, which generally only serves to transfer electrons. The formation of corrosive cells is mainly due to the adsorption of moisture in the air on the metal surface to form a water film. This water film contains a small amount of hydrogen ions and hydroxide ions, and also dissolves oxygen and other gases. The formed electrolyte solution and the iron and a small amount of carbon in the steel form countless tiny galvanic cells.
In these primary batteries, iron is the negative electrode and carbon is the positive electrode. Iron loses electrons and is oxidized. Electrochemical corrosion is the main cause of corrosion
Galvanic corrosion is a common type of corrosion. Galvanic corrosion is a combination of two electrodes, metal and electrolyte, which constitute a corrosive galvanic cell. For example, iron and oxygen, because the electrode potential of iron is always lower than that of oxygen, iron is the negative electrode and is corroded. It is characterized by the formation of many small drums of different diameters on the surface where oxygen corrosion occurs, and the second layer is black powdery ulcer corrosion pits.
Pitting corrosion is a very local form of corrosion in which small holes are formed on the metal surface in corrosive media. Pitting corrosion is one of the most destructive types of corrosion. Corrosive pores exist in isolation, and some are tightly combined. Pitting corrosion causes a small weight loss of the metal, but it can cause perforation damage.
Pitting corrosion is a unique form of anode reaction in electrochemical reaction. If the metal is passivated in this medium, the active anions (such as chloride ions) contained in the medium can be selectively and selectively adsorbed on specific points (such as defects and impurities) of the passivation film. Compounds that combine with cations in the passivation film to form soluble substances, where small corrosion spots appear and become pitting nuclei (about 20-30pm). Continue to develop can be turned into an etched hole. This corrosion is dangerous because it can cause the entire structure to fail.
Crevice corrosion refers to the local corrosion of cracks and other hidden areas that occur on the metal surface in corrosive media. Holes, gasket contact surfaces, lap joints, lower deposits and China Fasteners gaps are places where crevice corrosion often occurs. Metals that are resistant to corrosion by oxidation or passivation layers are particularly susceptible to corrosion. Crevice corrosion occurs in many media, especially in oxygen-containing media. Crevice corrosion is also an electrochemical corrosion
Intergranular corrosion is a localized corrosion. Corrosion extends inward along the interface between the metal grains. It is mainly due to the difference in chemical composition on the surface and internal of the crystal grain and the presence of impurities or internal stress at the grain boundary. Intergranular corrosion destroys the bond between the crystal grains and greatly reduces the mechanical strength of the metal. Moreover, after the corrosion of metals and alloys, the surface still maintains a certain metallic luster, without signs of damage, but the intercrystalline bonding force is significantly weakened, the mechanical properties are deteriorated, and it cannot withstand tapping, so this is very dangerous corrosion. Usually found in brass, hard aluminum alloy and some stainless steel, nickel-based alloys
Stress Corrosion Cracking (SCC)
Stress corrosion cracking (SCC) is the result of cracking caused by the interaction of materials or parts under stress and corrosive environments (usually at elevated temperatures). Stress corrosion may be caused by external stress. The cracking caused by the interaction of materials or parts with the corrosive environment under the action of stress is called stress corrosion cracking, which is the result of the combined action of stress and corrosion. There are many reasons for SCC, including stress caused by cold working, welding, and heat treatment. These factors, coupled with exposure to environments that often increase and exacerbate stress cracking, may mean that some people will suffer minor stress corrosion that can lead to failure or irreparable damage. If there is only one aspect, stress or the action of the medium, damage will not occur, but when the two work together, they will quickly rupture. Therefore, when stress corrosion occurs, the stress is very low and the corrosiveness of the medium is also very weak.