Analysis and Countermeasures of corrosion failure of 316L stainless steel flange

WAN G Fengping ,L I Xiaogang ,L IN Cui ,L I Dechao , YAN G Genzhu ,WAN G Bo

  1. Corrosion and protection Center , U niversity of Science and Technology Beijing , Beijing 100083;
  2. The Corporation of S hijiazhang Chemical Fibre , S hijiazhang 050032
  • ABSTRACT: An analysis was conducted on co rro sio n failure of 316L stainless steel flange in benzoic acid by diverse laboratory examinations. It has been found that intergranular co rro sio n caused by the joint of stainless steel is the immanent cause of co rro sio n failure of 316L stainless steel flange. Besides, the difference between jointing materials and base metal can give rise to dissimilar metal co rro sio n ,so do the use of conductive material – plumbago served as a padding. The crevice between stainless steel flanges can cause crevice corrosio n. So me pertinent preventive met hods are put forward to on the basis of the analysis of co rro sio n failure and simulated tests in laboratory.

316L stainless steel flange in a factory has serious local corrosion during the process of transporting benzoic acid at 283 C. The annual corrosion depth is about 10mm/a, resulting in premature failure of stainless steel flanges. 316L (00Cr17Ni14Mo2) stainless steel is ultra low carbon steel, the main component (mass%): 0. 020C, 16. 2Cr, 13. 2Ni, 2. 20Mo. stainless steel service medium benzene Formic acid (Benzoicacid) is a white crystal at room temperature, with a relative density of 1.2659, slightly soluble in cold water, soluble in hot water, easily soluble in ethanol, ether and other organic solvents, melting point at 122 and boiling point 249. The corrosion failure of 316L stainless steel flange is analyzed in this paper, and the cause of corrosion failure is accurately analyzed, and the aim is put forward. A sexual solution.
1 macro microcosmic analysis
1.11 macro analysis
The macroscopic observation of the damaged components shows that the corrosion area is located on the inner surface of the butt surface of the flange plate, the corrosion area is located at the weld seam, and the corrosion destroys a little matrix metal (as shown in Figure 1). The surface of the corroded site is attached with more black powder, the black powder is removed by the brush and the corrosion observed by the naked eye. The morphology is a trench shaped ring along the butt face. The width of the corrosion ditch is about 2. 4mm to 4. 7mm, and the depth of the corrosion ditch is about 2. 5mm.. According to the macroscopic observation of corrosion, the corrosion of the 316L stainless steel can be preliminarily judged to be local corrosion, which is due to the electrochemical corrosion of the interaction of metal under high temperature and corrosive medium.
1.12 microanalysis
5 samples were intercepted from different parts of the stainless steel (see Fig. 2). The cross section of the specimen was made from SiC water resistant sandpaper from 200#.
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Fig. 1 Corrosio n site of failed stainless steel flange. (a)original corroded surface of stainless steel flange (b) corroded flange surface afterremoving co rro sio n product

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Fig. 2 Samples cut from failed stainless steel flange

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Fig. 3 Five different sites analyzed on surface of failed sa mplewith scanning electron microscope

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Polished to 1500 in succession, polishing with 2. 5 diamond polishing pastes, respectively, metallographic microscope and scanning electron microscope (SEM) for microstructure observation, observation shows that 316L stainless steel flange base is austenitic stainless steel, welded metal The metallographic structure is a highly oriented columnar crystal structure. Microstructural analysis was performed on five different areas of the corrosion surface of the stainless steel sample retrieved from the site (as shown in Figure 3). The results are shown in Table 1. Analysis of on-site stainless steel microstructures and microstructure analysis showed that the microstructure of the stainless steel substrate and the weld metal was significantly different, but the composition of the two steels was not essentially different. Figure 4 shows the stainless steel surface removed from the site. Corrosion morphology. It can be seen that the corrosion at the grain boundary of stainless steel is very serious, and the corrosion morphology at the surface is a typical morphology of intergranular corrosion. Therefore, the corrosion of the stainless steel flange exhibits severe intergranular corrosion.

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Fig. 4 Surface morphologies of failed flange. (a) surface mo rp hology of the basemetal (b) surface mo rp hology of welded seam
2. Corrosion analysis
According to the above analysis, it can be concluded that the corrosion type of 316L stainless steel is local corrosion. The reason for the failure is the combined action of 316L stainless steel subjected to several kinds of corrosion during service. These corrosion effects include: 1) Intergranular corrosion of stainless steel welds, 2) Flange connection stainless steel
Galvanic corrosion caused by the connection of conductive non-metallic materials with graphite, 3) crevice corrosion due to the presence of gaps at the flange joints, the combined effect of these three corrosions leads to severe localized corrosion of 316L stainless steel. Intercrystalline corrosion plays a leading role.
2.1 Intergranular corrosion of stainless steel welds
The intergranular corrosion always starts from the metal surface, and concentrates on the grain boundary of the metal microstructure and penetrates deep into the metal material. This kind of corrosion is a kind of selective corrosion damage. It differs from general selective corrosion in that The locality of corrosion is microscopic, but not necessarily macroscopically. After intergranular corrosion occurs, sometimes it is not easily apparent from the appearance, but due to the destruction of the grain boundary region due to corrosion, the intergranular bonding The strength is almost completely lost. Those with larger corrosion depths can lose metal sounds, and metals that are heavily corroded can even become powders, which can fall off the components and eventually lead to equipment failure.
The weld metal of the 316L stainless steel flange provided by a factory completely formed a carbon-like powder in the range of 2.4mm to 4.7mm in width and 2.5mm in depth, which is a serious intergranular corrosion. Therefore, the corrosion damage to the surface Macroscopic observation and microscopic examination of corrosion morphology can determine that the intergranular corrosion of 316L stainless steel flange mainly occurs on the weld seam, and at the same time, there is a knife-like corrosion on the fusion line of the weld seam, so all the weld metal is damaged. The stainless steel substrate near the weld is also partially lost (as shown in Figure 4b).
The intergranular corrosion on the weld seam usually occurs in the case of multi-pass multi-layer welding. The previous weld metal is affected by the heat of the rear weld bead and is in the sensitization temperature range, so that the intercrystalline lean chromium does not resist corrosion. From Figure 2, we can see that the 316L stainless steel flange of a factory has two welding seams, one is on the butt surface of the flange and the other is on the back side of the butt surface. The welding of the next weld is for the previous weld. Metals have a thermal effect and intergranular corrosion occurs.
2.2 Crevice corrosion
Crevice corrosion is due to the presence of gaps between metal and metal, metal and non-metal surfaces, and localized corrosion that occurs when the medium is present. Crevice corrosion occurs. First, there should be a gap in the etching conditions. The gap width must enable the etching solution to Into the gap, while the width of the gap must be so narrow that the liquid can stay in the gap, the crevice corrosion is generally the most sensitive gap width of 0.025mm ~ 0.1mm.
Crevice corrosion can occur on all metals and alloys, and is particularly prone to corrosion on metals and alloys that are passivated. 316L stainless steels are austenitic stainless steels. In general, the crevice corrosion resistance of austenitic stainless steels is not ideal. Crevice corrosion media can be any aggressive solution, acidic or neutral, and solutions containing chloride ions are the most susceptible to crevice corrosion. Crevice corrosion is more likely to occur for the same alloy than pitting because of cracks. The critical potential of corrosion is lower than that of pitting corrosion.
The 316L stainless steel flange of the plant uses about 0.5mm graphite gasket, which meets the conditions of crevice corrosion. Therefore, stainless steel crevice corrosion will occur at the flange joint.
2.3 Galvanic corrosion
An accelerated corrosion caused by a negative metal when the metal is in electrical contact with another metal (including the same metal in another environment or different use state) or a non-metal electronic conductor in a corrosive electrolyte Galvanic corrosion. The cause of galvanic corrosion is that the two metals that make up the galvanic couple or the same metal in different states of use have a potential difference in the corrosive medium. When they are in an electrically connected state, they are At the anode metal/electrolyte interface, metal anodic dissolution occurs and electrons are released. The latter flows through the electron conductor to the positive electrode material, and a cathodic reduction reaction occurs at its interface with the electrolyte. Under the continuous action of this electric field, a negative electrode occurs. The metal is continuously accelerated dissolution, and the original self-corrosion process of the cathode material is thus inhibited.
The 316L stainless steel flange connection uses conductive graphite material. The flange is a non-insulating flange. Under the condition of electrolyte benzoate in the pipeline, it is not difficult to find the concept of galvanic corrosion. Galvanic corrosion necessarily occurs.
In addition, the difference between the microstructure of the stainless steel substrate and the welding material is also easy to form corrosion micro-cells, which is an inherent factor of galvanic corrosion.
2.14 Conclusion
Through a series of failure analysis of stainless steel flanges in the field, the following conclusions can be drawn:
1 The corrosion of stainless steel flanges is a typical localized corrosion, which is the result of a combination of various factors. Among them, the intergranular corrosion caused by stainless steel welding is the main cause of metal failure.
2 In the benzoic acid melt, galvanic corrosion is caused by the use of a conductive graphite gasket and the difference in microstructure between the welding material and the matrix material.
3 Crevice corrosion due to the gap between stainless steel flanges is one of the factors that promote the failure of stainless steel components.
4 The interaction of various corrosion factors makes the corrosion rate of the stainless steel flange very large, up to 10mm/
3. Protective measures
Based on the analysis of the cause of corrosion failure of the 316L stainless steel flange, the following protective measures are proposed:
1 Select the integral flange to avoid the welding of stainless steel and prevent the intergranular corrosion of 316L stainless steel.
2 Select high-quality welding materials and avoid local corrosion damage caused by large cathodes and small anodes.
3 Choose the right welding process:
1) Heat treatment before welding, mainly degreasing the stainless steel surface;
2) Must be welded under argon protection environment, ie argon arc welding, to prevent air from invading the arc column and the weld pool;
3) Post-weld heat treatment, that is, after welding for more than 1100 °C temperature to maintain 2h, and then quenching treatment, making it austenitic stainless steel, in order to avoid martensitic transformation.
4 The entire stainless steel is welded on the flange to avoid direct contact between the medium and the weld.
5 Surface coating. May be inorganic or organic. Surfacing of Ni235 on the flange surface can effectively prevent benzoic acid from corroding the stainless steel. It is also possible to choose PTFE (Polytetrafluoroethylene) (PTFE) for safe use. 290°C.
6 Select a suitable gasket to avoid galvanic corrosion. Therefore, it is best to use insulating material between the flanges (ie, insulating flanges), optional high temperature resistant insulating materials include Teflon, or in Lan’s docking surface is sprayed on the surface.

Source: Network Arrangement – China Stainless Steel Flange Manufacturer – Yaang Pipe Industry (www.metallicsteel.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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References

  • ANALYSIS ON CORROSION FAILURES OF 316L STAINLESS STEEL AND PREVENTION METHODS DOI: 2003, Vol. 15  Issue (3): 180-183
  • https://www.yaang.com/what-is-flange-connection.html
  • https://www.yaang.com/316-316l-stainless-steel-unss31600-unss31603.html

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