The 304 stainless steel pipe is used in the environment and the production process defects, the surface of the steel pipe has rust spots, and the stainless steel will not rust. This is only a theoretical concept.
Stainless steel rust caused by chloride ion corrosion
Chloride ions are widely used in environments such as salt/sweat/seawater/sea breeze/soil. The corrosion rate of chloride ion to stainless steel is very fast, even higher than that of ordinary low carbon steel. Therefore, the use environment of stainless steel is relatively high, and it is necessary to wipe and remove dust frequently to keep it clean and dry.
Especially in areas that rely on the ocean, due to the humid climate, the air contains a lot of chloride ions. If 304 stainless steel pipes are used in this environment, the surface of the steel pipes will rust.
How to solve it?
In this case, we generally choose high-grade 316 or 316L stainless steel pipes. The nickel content of this material is higher than that of 304 stainless steel, and the 316 or 316L material contains 2-3% molybdenum, so this material Its corrosion resistance is much better than 304 stainless steel.
Stainless steel rust caused by improper solution treatment
Alloying elements are insoluble in the matrix, the alloy content is low, and the corrosion resistance is poor.
The solution is to control the temperature and time during the solid solution, and water cooling in time.
Materials without Ti and Nb have a tendency of intergranular corrosion. The addition of titanium and niobium, coupled with stabilization treatment, can reduce intergranular corrosion. Stainless steel is a kind of high alloy steel resistant to air or chemical corrosive media. The surface is beautiful and corrosion-resistant. It does not require color plating but has its inherent surface characteristics. It is used in many aspects of steel. It is often referred to as stainless steel.
From a metallographic point of view, due to the chromium in the stainless steel, a thin chromium film is formed on the surface of the stainless steel, which can isolate the oxygen in the steel and play an anti-corrosion effect. In order to maintain the inherent corrosion resistance of stainless steel, the chromium content in the steel must exceed 12%. Used for welding. The low carbon content minimizes carbide precipitation in the heat-affected zone near the weld. In some cases, the precipitation of carbides may cause intergranular corrosion (welding corrosion) of stainless steel.
Surface trace phenomenon
Reason: There are foreign bodies on the surface and the surface of the mold, and there are scars on the surface of the edge pressure pad.
Countermeasures: Remove foreign bodies and scars before processing, and fully grind to remove oxide scale.
Orange peel phenomenon refers to the phenomenon of orange peel on the surface of 304 stainless steel during grinding and molding.
Reason: the less the number of grinding, the greater the processing volume, the more serious the orange peel, the coarse grain of the raw material, and the softening after heat treatment.
Countermeasures: Deep-processed products should be fully ground, suitable heat treatment conditions should be selected, and the particle size of raw materials should be controlled
Stainless steel rust after cutting steel pipe
There is another situation, for example, there are traces of rust on the nozzle of the stainless steel pipe. The main reason is that the factory cuts the steel pipe after pickling, and the pipe mouth of the steel pipe will be stuck with grinding wheel dust or other impurities. These impurities will adhere to the surface of the steel pipe for a long time, otherwise, it will rust under humid conditions. If you do not pay attention to this situation, the corrosion on the surface of the steel pipe will increase due to the rust condition, the passivation layer on the surface of the steel pipe will be destroyed, and the service life of the steel pipe will be reduced.
Stainless steel rust caused by insufficient pickling time
The pickling of stainless steel pipes or stainless steel plates is an important production process. Due to the rapid production in some factories, the pickling time has been shortened. After pickling, the inner and outer surfaces of stainless steel pipes cannot be cleaned easily and thoroughly. Because after annealing, the steel pipe forms an oxide coating on the surface of the steel pipe. If the oxide scale cannot be completely removed with acid, the corrosion of stainless steel will be accelerated during use and pitting or agglomeration corrosion will be observed. As with small diameter stainless steel pipes, if the pickling solution cannot be filled, no oxide scale will be found on the inner surface at all.