Stainless steel flanges are widely used in ship pipeline engineering due to their good corrosion resistance. As an important part of pipeline connections, they have the advantages of easy connection and use, maintaining pipeline sealing performance, and facilitating the inspection and replacement of a certain section of pipeline.
Stainless steel flanges are widely used in ship pipeline engineering due to their good corrosion resistance. As an important part of pipeline connections, they have the advantages of easy connection and use, maintaining pipeline sealing performance, and facilitating the inspection and replacement of a certain section of pipeline. A certain type of ship of the company recently purchased a batch of 304 flanges. The flanges were sent to the pickling plant for pickling and passivation treatment before use. After being placed in the pickling tank for more than ten minutes, bubbles began to appear on the surface of some flanges. Corrosion was found after cleaning. In order to find out the reasons for the corrosion of this batch of flanges, prevent the recurrence of product quality problems, and reduce economic losses, we conducted chemical analysis and metallographic inspection of flange samples from this batch.
1 Physical and chemical inspection
1.1 Chemical composition analysis
A chemical analysis sample was cut from the corroded flange, and its chemical composition was measured using an American Baird DV-6 spark direct reading spectrometer. The results are shown in Table I. Comparing the technical requirements for the chemical composition of 304 stainless steel in ASTMA276-2013 “Standard Specification for Stainless Steel Bars and Shapes”, the Cr element content in the chemical composition of the failed flange was lower than the standard value.
1.2 Metallographic inspection
Take a longitudinal cross-section sample from the corrosion area of the failed flange. After polishing, it is not etched and observed under a Zeiss metallographic microscope. Non-metallic inclusions are determined according to GB/T 10561-2005 “Standard for determination of non-metallic inclusion content in steel.” According to the “Grading Chart Microscopic Examination Method” assessment: sulfide type is level 1.5; alumina type is level 0; silicate type is level 0; spherical oxide is level 1.5.
The sample was etched with 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 level was determined according to GB/T6394-2002 “Metal average grain size” According to the “Method” assessment, the coarse-grained area can be rated as level 1.5 (see Figure 3); the fine-grained area can be rated as level 4.0.
Observing the microstructure of the near-surface corrosion area, it can be found that corrosion starts from the metal surface, concentrates on the austenite grain boundaries, and extends into the material. The grain boundaries in this area are destroyed due to corrosion, and the bonding between grains The strength is almost completely lost, and severely corroded metals may even form powder that can be easily scraped off the surface of the material.
Observe the high-magnification structure of the corroded flange through a 500x metallographic microscope. The microstructure is austenite + a small amount of ferrite + the third phase particles precipitated on the grain boundaries.
2 Comprehensive analysis
The physical and chemical test results show that the Cr element content in the chemical composition of the stainless steel flange is slightly lower than the standard value. Cr element is the most important element that determines the corrosion resistance of stainless steel. It can react with oxygen to produce Cr oxides to form passivation. chemical layer to prevent corrosion. Moreover, the content of non-metallic sulfides in this material is high. The accumulation of sulfides in local areas will cause the concentration of Cr elements to decrease in the surrounding areas, forming Cr-poor areas, thus affecting the corrosion resistance of stainless steel.
Observing the grains of the stainless steel flange, it can be found that the grain size is extremely uneven. The mixed grains of uneven sizes in the structure can easily form differences in electrode potentials, resulting in micro-batteries, which can lead to electrochemical corrosion on the material surface. The coarse and fine mixed grains of stainless steel flanges 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 interior along the austenite grain boundaries. The high-magnification microstructure of the material shows that the corrosion occurs on the austenite grain boundaries of the material. There are more third phases precipitating, and the third phase accumulated on the grain boundaries can easily lead to chromium deficiency in the grain boundaries, causing intergranular corrosion tendencies and greatly reducing its corrosion resistance.
The third phase in stainless steel mainly includes fine carbides (M 23C6), σ phase and δ ferrite, etc., which all have a great influence on the corrosion resistance of stainless steel. The formation temperature of the M23C6 precipitated phase is 450°C-850°C. It is mainly a carbide composed of metallic chromium. Most of it is distributed on the grain boundaries of the crystal, and some is also distributed inside the crystal and at crystal defects, because the carbide is rich in Chromium can easily lead to chromium deficiency in this area; the formation temperature of σ phase is 500℃-925℃. When staying in this temperature zone, ferrite will partially or completely decompose into σ phase. The chromium content of 6 phases is 42%-50%. , is a brittle phase with high hardness, which can cause a decrease in material toughness and corrosion properties; delta ferrite is a high-temperature ferrite, which is formed by crystallization when liquid iron is cooled to 1538°C. This phase is relatively brittle and cannot be processed during processing. It is easy to cause cracks and pitting corrosion.
3Conclusion
Through a series of failure analyzes of corroded stainless steel flanges, the following conclusions can be drawn:
(1) The corrosion of stainless steel flanges is the result of a combination of factors, among which the first phase precipitated on the material grain boundary is the main cause of flange failure. It is recommended to strictly control the heating temperature during thermal processing and not exceed the upper limit temperature of the material heating process specifications. At the same time, cool quickly after solid solution to avoid staying in the 450℃-925℃ temperature range for too long to prevent the precipitation of third phase particles.
(2) Mixed grains in the material can easily cause electrochemical corrosion on the material surface, and the forging ratio should be strictly controlled during the forging process.
(3) The low Cr element 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.
Author: Maria Yang
Post time: Nov-03-2023