Causes and countermeasures of pickling corrosion of 304 stainless steel flanges

Abstract: The customer recently purchased a batch of 304 stainless steel flanges, which were to be pickled and passivated before use. As a result, bubbles appeared on the surface of the stainless steel flanges after being placed in the pickling tank for more than ten minutes. After the flanges were taken out and cleaned, corrosion was found. In order to find out the cause of the corrosion of the stainless steel flanges, prevent quality problems from occurring again, and reduce economic losses. The customer specially invited us to help him with sampling analysis and metallographic inspection.

First, let me introduce the 304 stainless steel flange. It has good corrosion resistance, heat resistance, and low-temperature mechanical properties. It is corrosion-resistant in the atmosphere and acid-resistant. It is widely used in fluid pipeline projects such as petroleum and chemical industry. As an important part of pipeline connection, it has the advantages of easy connection and use, maintaining pipeline sealing performance, and facilitating the inspection and replacement of a certain section of the pipeline.

Inspection process

1. Check the chemical composition: First, sample the corroded flange and use a spectrometer to directly determine its chemical composition. The results are shown in the figure below. Compared with the technical requirements of 304 stainless steel chemical composition in ASTMA276-2013, the Cr content in the chemical composition of the failed flange is lower than the standard value.

2. Metallographic inspection: A longitudinal cross-section sample was cut at the corrosion site of the failed flange. After polishing, no corrosion was found. Non-metallic inclusions were observed under a metallographic microscope and the sulfide category was rated as 1.5, the alumina category was rated as 0, the acid salt category was rated as 0, and the spherical oxide category was rated as 1.5; the sample was etched by ferric chloride hydrochloric acid aqueous solution and observed under a 100x metallographic microscope. It was found that the austenite grains in the material were extremely uneven. The grain size grade was evaluated according to GB/T6394-2002. The coarse grain area can be rated as 1.5 and the fine grain area can be rated as 4.0. By observing the microstructure of the near-surface corrosion, it can be found that the corrosion starts from the metal surface, concentrates on the austenite grain boundaries and extends to the inside of the material. The grain boundaries in this area are destroyed by corrosion, and the bonding strength between the grains is almost completely lost. The severely corroded metal even forms powder, which is easily scraped off the surface of the material.

3. Comprehensive analysis: The results of physical and chemical tests show that the Cr content in the chemical composition of the stainless steel flange is slightly lower than the standard value. The Cr element is the most important element that determines the corrosion resistance of stainless steel. It can react with oxygen to produce Cr oxides, forming a passivation layer to prevent corrosion; the non-metallic sulfide content in the material is high, and the aggregation of sulfides in local areas will lead to a decrease in the Cr concentration in the surrounding area, forming a Cr-poor area, thereby affecting the corrosion resistance of stainless steel; observing the grains of the stainless steel flange, it can be found that its grain size is extremely uneven, and the uneven mixed grains in the organization are prone to form differences in electrode potential, resulting in micro-batteries, which lead to electrochemical corrosion on the surface of the material. The coarse and fine mixed grains of the stainless steel flange are mainly related to the hot working deformation process, which is caused by the rapid deformation of the grains during forging. Analysis of the microstructure of the near-surface corrosion of the flange shows that the corrosion starts from the flange surface and extends to the inside along the austenite grain boundary. The high-magnification microstructure of the material shows that there are more third phases precipitated on the austenite grain boundary of the material. The third phases gathered on the grain boundary are prone to cause chromium depletion at the grain boundary, causing intergranular corrosion tendency and greatly reducing its corrosion resistance.

Conclusion

The following conclusions can be drawn from the causes of pickling corrosion of 304 stainless steel flanges:

1. The corrosion of stainless steel flanges is the result of the combined action of multiple factors, among which the third phase precipitated on the grain boundary of the material is the main cause of flange failure. It is recommended to strictly control the heating temperature during hot working, not to exceed the upper limit temperature of the material heating process specification, and to cool quickly after solid solution to avoid staying in the temperature range of 450℃-925℃ for too long to prevent the precipitation of third phase particles.
2. The mixed grains in the material are prone to electrochemical corrosion on the surface of the material, and the forging ratio should be strictly controlled during the forging process.
3. The low Cr content and high sulfide content in the material directly affect the corrosion resistance of the flange. When selecting materials, attention should be paid to selecting materials with pure metallurgical quality.