Causes of rust on stainless steel surface
The reasons for the large-scale corrosion of common 304 stainless steel in the atmospheric environment are systematically analyzed from several aspects such as material structure, inclusion level, intergranular corrosion resistance, rust point morphology, and rust composition. At the same time, the salt spray accelerated corrosion test method is used to test the corrosion resistance of the material. The results show that the surface inclusions cause the local corrosion potential of the material to decrease, and the material is not cleaned with the pickling solution after the local pickling, which causes the material to corrode. The corrosion only occurs at the depth of 30 um on the surface. Grinding can avoid further corrosion of the material.
304 austenitic stainless steel has good toughness, plasticity, weldability, corrosion resistance and non-magnetic properties at room temperature and low temperature. It is widely used in petroleum, chemical industry, metallurgy, machinery, aviation, marine food, processing instruments, and household appliances. In industries such as hardware manufacturing, the reason why 304 stainless steel has excellent corrosion resistance in atmospheric environments is due to the formation of a Cr passivation film on its surface. If the material itself contains inclusions and the environment causes severe local corrosion conditions, it may be possible If the corrosion of the material is small, it will affect the appearance of the product, and if it is large, it will become a potential safety hazard for later use.
In 2008, a food processing user reported that the 304 (7mmX1500mm) size product he purchased was placed outdoors after being formed, welded, and pickled with pickling paste, and a large number of rust spots appeared on the surface of the product (see Figure 1). In order to find out the cause of material corrosion and avoid this kind of phenomenon from happening again, take samples from the corrosion products on site and conduct salt spray test
Figure one
1. Salt spray test results
After cleaning the surface of the rusted sample, it was put into the salt spray test box for salt spray test. The test lasted for 14 days. The macroscopic appearance after the test is shown in Figure 7.
(a) Keep rusty spots (b) Remove rusty spots
Figure 7 Macro morphology after salt spray test
Comparing the morphology before the test, the rust condition is basically the same as the original one, and there is no obvious expansion. However, after removing the rust spots, 14 days later, the samples did not undergo any rust. From this, it can be judged that the rust only occurs on the surface and will not extend to the substrate. After polishing, it can prevent the rust from continuing to occur.
2. Analysis and discussion
304 stainless steel has superior corrosion resistance. Under ordinary atmospheric conditions, such corrosion will not occur for more than ten days. Consult the relevant literature, 304 steel rust is generally divided into the following categories:
One is due to insufficient solution treatment or use in the carbide precipitation temperature range, resulting in the precipitation of carbides at the grain boundaries, and intergranular corrosion caused by local chromium depletion in the material;
Second, because of inclusions on the surface of the material, the passivation film formed on the surface of the material is destroyed, and because of the serious inclusions, the uniformity of the surface of the material is destroyed, causing local corrosion;
The third is stress corrosion that occurs in the presence of strong penetrating ions such as chloride ions.
From the above test results, compared with the conventional 304, the content of Cr and Ni elements in the rusted sample is normal, but the content of S element is slightly higher, but it still meets the standard requirements. The sample solution treatment is sufficient, and the intergranular corrosion caused by the precipitation of carbides along the grain boundary will not occur. There are rust spots with a diameter of about 10 μm on the sample surface, and the rust position contains elements such as Cl, S, Ca, Mg and Al. It can be inferred that the inclusions on the sample surface are the direct cause of the corrosion of the material. However, from the results of the salt spray accelerated corrosion test, after 14 days of accelerated corrosion with the preserved rust sample placed in the salt spray box, the rust did not expand significantly, and no new rust spots were formed. In the coastal environment where the material is used, the CI concentration is generally 0.381 ~ 0.438 mg/m3, which is far less than the test concentration. From this, it can be inferred that the inclusions on the surface of the material are not the only cause of material corrosion, and there are other reasons that accelerate the material. Rust.
The investigation found that after 304 stainless steel is welded, it is generally treated with pickling paste for partial pickling and passivation. The main purpose is to remove all kinds of grease on the surface, welding spatter, weld scars, oxide scale, etc. The metal surface exposed to the air is quickly passivated, forming a new chromium-rich passivation film, which then plays a role in preventing rust. Therefore, passivation treatment is very important in the process of stainless steel processing. If the pickling and passivation treatment is improper and the residual acid on the surface is not cleaned, it may cause serious local corrosion on the surface of the material, such as high local CI, low pH, etc. , Accelerate the corrosion of materials. The high level of CI on the surface accelerates the corrosion of the material.
In view of the causes of corrosion, the following three measures are recommended:
(1) Strengthen the control of the steelmaking process and further reduce the content of S in the finished product;
(2) After using the pickling cream, the user should rinse with clean water immediately, preferably with a weak alkaline solution;
(3) For rusted products, the surface layer needs to be polished about 30 μm.
If you want to know How to clean the stainless steel surface? You can click How to clean the stainless steel surface?