Why does the surface of stainless steel valves rust?
Overview of 304 austenitic stainless steel
304 austenitic stainless steel (06Cr19Ni10 steel in the new standard, 0Cr18Ni9 steel in the old standard) is widely used in petrochemical, metallurgical machinery, aerospace, food processing, instrumentation, household appliances, and hardware manufacturing industries due to its good toughness, plasticity, weldability, corrosion resistance, and non-magnetic properties at room and low temperatures.
The reason why 304 austenitic stainless steel has excellent corrosion resistance in the atmospheric environment is due to the Cr passivation film formed on its surface. If the material itself has inclusions and the environment causes harsh local corrosion conditions, it may cause material corrosion, which may affect the appearance of the product or become a safety hazard in later use.
A batch of 304 austenitic stainless steel fasteners have a large number of rust spots on their surfaces. A systematic analysis was conducted from the aspects of material organization, inclusion level, intergranular corrosion resistance, rust spot morphology, rust location composition, and rust layer composition. According to relevant literature, 304 austenitic stainless steel corrosion is generally divided into the following categories: First, due to insufficient solution treatment or use within the carbide precipitation temperature range, carbides precipitate at the grain boundary, and intergranular corrosion caused by local chromium depletion of the material; second, due to inclusions on the surface of the material, the surface uniformity of the material is destroyed, thereby causing local corrosion; third, stress corrosion occurs in the presence of ions with strong penetrating power such as chloride ions.
Compared with conventional 304 stainless steel, the rusted sample has normal Cr and Ni content, but the Si content is slightly higher, but it meets the standard requirements. The chemical composition of 304 stainless steel is shown in Table 1.
| C | Si | Mn | S | P | Ni | Cr | N | Fe |
| 0.039 | 0.35 | 1.26 | 0.025 | 0.018 | 18.12 | 8.10 | 0.055 | margin |
Table 1 Chemical composition of 304 stainless steel material sample (W%)
The sample is fully solution-treated, and intergranular corrosion caused by carbide precipitation along the grain boundary will not occur. There are rust spots with a diameter of about 10μm on the surface of the sample, and the rusted position contains elements such as CI, S, Ca, Mg, and Al. It can be inferred that the inclusions on the surface of the sample are the direct cause of the corrosion of the material. Test results: Sulfides exist on the surface of the material in the range of 36-125μm, and silicate inclusions exist in the range of 75-170μm.
From the results of the salt spray accelerated corrosion test, after the rust-spotted sample was placed in the salt spray box for accelerated corrosion for 14 days, the rust did not expand significantly, and no new rust spots were formed. The fastener is used in a coastal environment, and the CI- concentration is generally 0.381-0.438mg/m3 of element content, which is far less than the test concentration. It can be inferred that the inclusions on the surface of the material are not the only cause of the rust of the material, and other reasons accelerate the corrosion of the material.
After cold heading and thread rolling, 304 stainless steel fasteners are generally pickled and passivated. The main purpose is to remove various greases, scales, scars, etc. on the surface, and at the same time, the metal surface exposed to the air is quickly passivated to form a new chromium-rich passivation film, thereby playing a rust-proof role. Therefore, passivation treatment is crucial in the processing of stainless steel fasteners. If the pickling and passivation treatment is not done properly and the residual acid on the surface is not cleaned up, it may lead to a harsh local corrosion environment on the surface of the material, such as local CI- is high, PH value is low, etc., which accelerates the corrosion of the material. The presence of CI- was also found in the rusted samples. The high local CI- on the surface accelerated the corrosion of the material.
Three measures are taken to address the causes of rust:
1. Strengthen the control of the raw material procurement process and reduce the S content in the finished product;
2. It is best to rinse with a weak alkaline solution before passivation;
3. For products with rusted points, the surface layer needs to be polished by about 30μm.
Summary of The causes of rust on the surface of 304 stainless steel
304 stainless steel is fully solution-treated and will not cause intergranular corrosion due to the precipitation of carbides. The rust depth of the severely rusted area is 30μm, and the rust will not continue to expand in the atmospheric environment. Inclusions on the surface of the material are the origin of rust, but inclusions exist in the surface layer. After polishing and removal, rust can be avoided.
