Stainless steel flanges are widely used in ship pipeline engineering because of their good corrosion resistance. As an important part of pipeline connection, they have the advantages of convenient connection, easy to maintain pipeline sealing performance, and easy inspection and replacement of certain pipelines. A certain type of ship has recently purchased a batch of 304 flange. The flange is sent to the pickling plant for pickling and passivation treatment before use. The pickling tank is placed in the pickling tank for ten minutes. Corrosion was observed after cleaning. In order to find out the cause of corrosion of the batch of flanges, prevent product quality problems from happening again, and reduce economic losses, we conducted chemical analysis and metallographic examination of the batch flange samples.
1 Physical and chemical testing
1.1 Chemical composition analysis
Chemical analysis samples were taken on the corrosion flange, and the chemical composition was determined by a Baird DV-6 type spark direct reading spectrometer. The results are shown in Table 1. According to the technical requirements of 304 stainless steel chemical composition in ASTM 276-2013 “Standard Specification for Stainless Steel Bars and Shapes”, the content of Cr in the chemical composition of the failed flange is lower than the standard value.
1.2 Metallographic examination
A longitudinal section sample was taken at the corrosion of the failed flange. After polishing, it was not etched. Under the ZEISS metallographic microscope, the non-metallic inclusions were measured according to GB/T 10561-2005. Rating chart microscopic test method: 1.5 grades of sulfides; grade 0 of alumina; grade 0 of silicates; grades of grades of spherical oxides.
The sample was etched by aqueous solution of ferric chloride hydrochloride and observed under a 100 x metallographic microscope. The austenite grains in the material were found to be extremely uneven. The grain size grade was determined according to GB/T6394-2002 “Metal average grain size”. According to the method, the coarse-grained zone can be rated as 1.5 (see Figure 3); the fine-grained zone can be rated as 4.0.
Observing the microstructure of the near-surface corrosion, it can be found that the corrosion starts from the metal surface and concentrates on the austenite grain boundary and extends into the interior of the material. The grain boundary of this region is destroyed by corrosion and the intergranular bond Almost completely lost strength, corroded. Severe metals even form powders that are easily scraped off the surface of the material.
The high-magnification structure of the corrosion flange was observed by a 500x metallographic microscope, and the microstructure was austenite + a small amount of ferrite + the first phase particles precipitated on the grain boundary.
2 Comprehensive analysis
The results of physical and chemical tests show that the content of Cr in the chemical composition of the stainless steel flange is slightly lower than the standard value. The Cr element is the most important element determining the corrosion resistance of stainless steel. It can react with oxygen to produce oxides of Cr and form blunt. The layer acts to prevent corrosion. Moreover, the non-metallic sulfide content in the material is high, and the aggregation of the sulfide in the local region causes the concentration of the Cr element in the surrounding area to decrease, forming a Cr-depleted region, thereby affecting the corrosion resistance of the stainless steel.
Observing the grain of the stainless steel flange, it can be found that the grain size is extremely uneven, and the mixed crystal grains with uneven size in the structure easily form the difference of the electrode potential, resulting in micro-battery, which leads to electrochemical corrosion on the surface of the material.
The coarse-grained mixed grain of the stainless steel flange is mainly related to the hot working deformation process due to the sharp deformation of the grain during forging.
Analysis of the microstructure of the near-surface corrosion of the flange shows that the corrosion starts from the surface of the flange and extends along the austenite grain boundary. The high-magnification microstructure of the material shows the austenite on the grain boundary. There are many first phase precipitates, and the third phase accumulated on the grain boundary is easy to cause the grain boundary to be depleted in chromium, causing intergranular corrosion tendency and greatly reducing its corrosion resistance.
The third phase in stainless steel mainly has fine carbides (M 23C6 ), σ phase and δ ferrite, all of which have a great influence on the corrosion resistance of stainless steel. The formation temperature of the precipitated phase of M23C6 is 450 °C-850 °C, mainly composed of metal chromium. Most of the carbides are distributed on the grain boundaries of the crystals, and some are distributed inside the crystals and crystal defects because the carbides are rich. Chromium, easily lead to chromium deficiency in the region; σ phase formation temperature is 500 ° C -925 ° C, in this temperature zone, ferrite partially or completely decomposes σ phase, 6 phase chromium content is 42% -50% It is a brittle phase with high hardness, which can cause the toughness and corrosion performance of the material to decrease. δ ferrite is a kind of high-temperature ferrite which is formed by cooling from liquid iron to 1538 °C. The phase is brittle and processed. It is easy to cause cracks and is prone to pitting corrosion.
Through a series of failure analysis of corroded stainless steel flanges, the following conclusions can be drawn:
- (1) Corrosion of stainless steel flanges is the result of a combination of factors, in which the first phase precipitated on the grain boundaries of the material is the main cause of flange failure. It is recommended that the heating temperature be strictly controlled during the hot working process, not exceeding the upper limit temperature of the material heating process specification, and cooling rapidly after solid solution to avoid staying in the temperature range of 450 ° C – 925 ° C for a long time to prevent the precipitation of the third phase particles.
- (2) The mixed grains in the material tend to cause electrochemical corrosion on the surface of the material, and the forging ratio should be strictly controlled in the forging process.
- (3) The low content of Cr in the material and the high content of sulfide directly affect the corrosion resistance of the flange. When selecting materials, attention should be paid to the selection of materials with pure metallurgical quality.
Source: China Stainless Steel Flange Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)