Protection against corrosion

Rust is the most important degradation phenomenon of metal substances. At the beginning of the degradation process, the beauty of the metal fastener may be lost, and then the function of the parts may be lost, and the fastening coupling must be replaced. In fact, the appearance and functionality of bolts are very important, because in addition to the mechanical properties required by the application, general bolts must also have good corrosion resistance, and must be beautiful in most mechanical components. Appearance.
1. The rust phenomenon appears in different forms.
The corrosion of fasteners in general is the same degree of damage to the metal surface. Other cases may be local rusting. In this case, we observe that the metal is oxidized in the small anode area, but there is no change in the metal surface in the large cathode area.
2. Types of corrosion
The first is uniform rust. This kind of rust phenomenon spreads over the entire fastener tissue surface and continues to produce metal reduction. This is the most common corrosion process. This corrosion does not cause too much danger and is easy to detect in advance.
Secondly, it is the phenomenon of local corrosion that is dangerous. Pinhole corrosion. In this case, it can be observed that local erosion will lead to material perforation. The anode area is very small, and the cathode area without any changes on the surface is rather large. This phenomenon often occurs in stainless steel, nickel steel and aluminum fasteners. Electric rotary rust. As far as fasteners are used, the damage caused by electro-rotating corrosion is very serious. Electro-rotation corrosion may occur in the joining of any two types of fasteners of different metal materials; the reason is that one of the metals has high activity and is easier to cause chemical reactions, so the corrosion rate increases, while the other metal has low activity and it is difficult to cause reactions. While the corrosion rate is reduced, the connection structure causes a reaction in the cathode direction. This phenomenon is even more pronounced when the electrochemical reactions of the two metals are quite different. If carbon steel is matched with stainless steel fasteners, the corrosion rate of steel parts will increase. When screws are connected, they are actually used to join two or more components. In order to avoid electro-rotating corrosion, when joining parts, the same joining materials or materials with similar electrochemical reactions should be specified.
Electro-rotation corrosion form or reaction of various metals in seawater
Form Metal Material
Inert or reactive toward the cathode
Stainless steel 18-8 type grades
Stainless steel (including 11%-30% chromium) series
nickel
Bronze (copper tin alloy)
Brass (copper-zinc alloy)
cast iron
steel
Active or react towards the anode
cadmium
aluminum
Zinc
Magnesium or magnesium alloy
Generally, fastener coatings are divided into two types. With the phosphate layer as the base, painting can increase the corrosion resistance of the fastener surface. The electrolytic galvanized layer is used as a barrier protection, and when the paint is bad and the electro-rotating rust phenomenon starts, the zinc material is used as the anode material, and the steel is used as the cathode material, which can avoid the danger of rust. Type of protection a. Electro-galvanized layer
The electro-galvanized layer is a typical anode plating layer. When corrosion occurs, the anode is preferentially dissolved to protect the base material. Limited by the thickness of its coating of 8-15μm, after chromate passivation treatment, its anti-corrosion performance is greatly improved, can withstand the 48-96h neutral salt spray test, and can be used for neutral outdoor protection. Zn-Ni, Zn-Fe, Zn-Co electroplated layers developed in recent years, especially Zn-Ni, Zn-Co electroplated layers, when the Ni content in the coating is 6%-10% or the Co content is 0.4%-1.0 %, after passivation of the 5-8μm coating, excellent anti-corrosion effect can be obtained, and it can resist the neutral salt spray test for more than 720h.
b. Non-electroplated zinc layer
The non-electrogalvanized layer mainly includes hot-dip galvanized layer, mechanical galvanized layer and Dacromet coating.
Hot-dip plating is formed by the natural adhesion of molten metal on the surface of metal fasteners to form a thin film, which is characterized by a thick coating. Mechanical plating uses the mechanical action of the small spherical object and the coating metal powder to rotate and collide in the aqueous solution to make the coating metal form a thin film. Its characteristic is that the coating is generally a composite film with good corrosion resistance. Dacromet coating is formed by sintering flake metal powder, organic matter and additives at about 300℃ to form a film. Its characteristic is that the film has good corrosion resistance.