Reasons for Surface Rust on 304 Stainless Steel Butterfly Valves
304 austenitic stainless steel (06Cr19Ni10 steel in the new standard and 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-magnetism at room temperature and low temperatures. The reason why 304 austenitic stainless steel has excellent corrosion resistance in the atmospheric environment is due to the formation of a layer of Cr passivation film on its surface. If there are inclusions in the material itself and the environment leads to harsh local corrosion conditions, it is possible to cause material corrosion. Minor corrosion affects the appearance of the product, while major corrosion becomes a potential safety hazard for later use.
A batch of 304 austenitic stainless steel fastener products have a large number of rust spots on the surface. A systematic analysis is conducted from several aspects such as material structure, inclusion level, corrosion resistance at grain boundaries, morphology of rust spots, composition at the location where rust occurs, and composition of the rust layer. By referring to relevant literature, rusting of 304 austenitic stainless steel is generally classified into the following categories: First, grain boundary corrosion caused by local chromium depletion due to insufficient solution treatment or use within the carbide precipitation temperature range, resulting in carbide precipitation at the grain boundaries; Second, local rusting caused by inclusions on the material surface, which破坏 the uniformity of the material surface; Third, stress corrosion that occurs in the presence of ions with strong penetrating ability such as ammonium ions.
Compared with conventional 304 stainless steel, the content of Cr and Ni elements in the rusted samples is normal, but the content of Si element is slightly higher, but it meets the standard requirements. The chemical composition of 304 stainless steel is detected as shown in Table 1.
Table 1 - Chemical composition of 304 stainless steel material samples (W%)
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 Balance
The sample was subjected to sufficient solution treatment, and grain boundary corrosion caused by carbide precipitation along the grain boundaries would not occur. There were rust spots with a diameter of approximately 10 um on the sample surface, and the rusted positions contained elements such as CI, S, Ca, Mg, and AI. It can be inferred that the inclusions present on the sample surface were the direct cause of material corrosion. Test results: Sulfides were present on the material surface within the range of 36 - 125 um, and silicate inclusions were present within the range of 75 - 170 um.
From the results of the salt spray accelerated corrosion test, after the samples with rust spots were 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. This fastener was used in a coastal environment, and the C- concentration was generally in the range of 0.381 - 0.438 mg/m3 of element content, which was much lower than the test concentration. Thus, it can be inferred that the inclusions on the material surface were not the only cause of material rusting, and there were other reasons that accelerated the corrosion of the material.
After cold heading and thread rolling of 304 stainless steel fasteners, pickling and passivation treatment is generally required. The main purpose is to remove various greases, oxide scales, scars, etc. on the surface, and at the same time, rapidly passivate the metal surface exposed to the air to form a new chromium-rich passivation film, thereby playing a role in rust prevention. Therefore, passivation treatment is crucial in the processing of stainless steel fasteners. If the pickling and passivation treatment is improper and the residual acid on the surface is not cleaned thoroughly, it may lead to harsh local corrosion environments on the material surface, such as: high local CI- and low pH value, accelerating material rusting. The presence of C- was also found in the rusted samples, and the high local CI- on the surface accelerated the corrosion of the material.
In response to the causes of rusting, three measures were taken: First, strengthen the process control of raw material procurement to reduce the $ element content in the finished product; Second, the best cleaning before passivation is to rinse with a weakly alkaline solution; Third, for products with rust spots that have occurred, it is necessary to grind the surface layer by about 30 um.
A summary of the reasons for the surface rust of 304 stainless steel is as follows: The 304 stainless steel was subjected to sufficient solution treatment and would not cause grain boundary corrosion due to carbide precipitation. The rust depth at the severely rusted area was 30 um. In the atmospheric environment, the rust would not continue to expand. The inclusions on the material surface were the origin of rusting. However, the inclusions were present in the surface layer. By grinding and removing them, rusting could be avoided.
