Reasons why 304 stainless steel materials rust

The best aesthetic effect in outdoor environments is nickel-containing stainless steel. The representative 304 stainless steel is widely used in curtain walls, side walls, roofs and other construction purposes. However, in severely corrosive industrial or marine environments, it is better to use a higher grade 316 stainless steel. When it comes to stainless steel sliding doors, people have fully understood the advantages of using stainless steel in structural applications. There are several design guidelines that include 304 and 316 stainless steel. In actual use, stainless steel is produced in fully standard metal shapes and sizes, and there are also many special shapes. Commonly used products are made of thin plates and strip steel, and medium and thick plates can also be used to produce special products. For example, the production of hot-rolled structural steel and extruded structural steel. In addition, there are round, oval, square, rectangular and hexagonal welded pipes or seamless steel pipes and other products in various forms, including profiles, bars, wires and castings.
Since stainless steel is used in a wide range of forms, it is natural that different corrosion phenomena will occur in different usage environments, which are generally summarized as the following four points:
1. Chloride ion corrosion
Chloride ions are widely present in the atmosphere, such as table salt, sweat stains, sea water, sea breeze, soil, etc. Stainless steel corrodes faster in the medium where chloride ions exist, even more than ordinary low carbon steel. Therefore, there are special requirements for the environment in which stainless steel is used. It needs to be wiped frequently, dusted, and kept clean and dry. If it rusts under such circumstances, it can be characterized as "improper use". ) For example: There is an oak container containing a solution containing chloride ions. The container has been used for nearly a hundred years. It was planned to be replaced in the 1990s. Because the oak material was not suitable for the needs of modern life, the container was replaced with stainless steel after two weeks. Leakage due to corrosion.
2. Solid solution treatment
The alloy elements are not dissolved into the matrix, resulting in a low alloy content in the matrix structure and poor corrosion resistance.
3. Intergranular corrosion
Materials without the addition of titanium and niobium tend to have intergranular corrosion. Adding titanium and niobium, coupled with structural stabilization treatment, can reduce intergranular corrosion. From a metallographic point of view, because stainless steel contains chromium, a very thin chromium film is formed on the surface. This film isolates the oxygen that intrudes into the steel and plays a role in corrosion resistance. In order to maintain the inherent corrosion resistance of stainless steel, the steel contains more than 12% chromium. Used where welding is required. The lower carbon content minimizes the precipitation of carbides in the heat-affected zone close to the weld, and the precipitation of carbides may cause intergranular corrosion (welding corrosion) of stainless steel in certain environments.
4. Surface mark phenomenon
The phenomenon of surface marks refers to the occurrence of line or point-like pit mark defects on the surface of the product after deep processing of 304 stainless steel or grinding of processed products.
Reason: There are foreign objects on the surface and mold surface, and there are scars on the surface of the edge pad.
Countermeasures: Remove foreign matter and scars before processing, and also remove orange peel by sufficient grinding.
The orange peel phenomenon refers to the phenomenon that the surface of 304 stainless steel molded products develops a shape like orange peel during grinding or other circumstances.
Reasons: Insufficient grinding, the larger the processing, the more serious the orange peel, the raw material grains are coarse, and the heat treatment softens it.
Countermeasures: Fully grind deeply processed products, select suitable heat treatment conditions, and control the grain size of raw materials.