Application of 20MnMo materials

20MnMo alloy steel is forged steel for low-alloy pressure vessels. It is used to manufacture pressure vessel components and important forgings at -40℃~470℃. For example, components such as high-neck flanges and flat covers in high-pressure vessels.

Recently, I came across a container made of this material, which contains a trace amount of H2 and the rest is water and products. It is non-corrosive and has an operating temperature of 200 degrees. On the flange sealing surface of the top nozzle (the nozzle where the stirring shaft is installed), pitting corrosion was found on the entire ring, resulting in leakage. I don’t know why this material was chosen for this working condition. It is rarely encountered. Crevice corrosion? Galvanic corrosion? In doubt...

This post was last edited by nj1952 on 2017-8-8 16:29 Based on your description, crevice corrosion may occur on the sealing surface. The conditions for crevice corrosion are: first, there is an electrolyte solution, and second, there is a narrow gap in the structure. The gap width (generally 0.025 ~ 0.1 mm) is enough to allow the electrolyte solution to enter, causing the metal inside the gap and the metal outside the gap to form a short-circuit galvanic cell, and strong corrosion and localized corrosion occur within the gap. I have encountered galvanic corrosion. A high-pressure hydrogen storage tank in a chemical plant (horizontal container, multi-layered structure 16MnR, 20MnMo bowl-shaped heads welded on both ends, working pressure 15MPa, normal temperature, hydrogen), on the left head of the container, a DN25 gas inlet and outlet pipe (20 steel, L=150mm) is welded horizontally, and it is welded to the system 304 stainless steel pipe. After running for a period of time, the carbon steel side close to the CS-SS weld of the gas outlet pipe was corroded and perforated, and hydrogen leaked. Cut the pipe and dissect it for inspection. Corrosion occurs on the inside of the pipe, at the lower edge of the horizontal position. The corrosion is most severe at the fusion line on the CS side of the weld (it has penetrated). ； The inspection found that as the distance SS increases, the corrosion gradually weakens (the corrosion range of this section is about 25mm away from the fusion line, and the lower edge of the inner wall of the pipe is prone to liquid accumulation), and the inner surface of the container shell is uniformly corroded, within a controlled range. After analysis, it was determined to be galvanic corrosion (bimetal corrosion). After updating the air inlet and outlet pipes on the container, the connection with the system was changed from welding to flange connection, and a section of easy-to-replace pipe (20 steel, L=200mm) was added between the air inlet and outlet pipes and the stainless steel pipe for regular replacement. This method has been used to this day, and there has been no hydrogen leakage incident. The nozzle flange sealing gasket of the high-pressure hydrogen storage tank of this device uses a lens gasket (20 steel), and no crevice corrosion has occurred.

This post was last edited by Pingxin HAPPY at 2017-8-9 08:48. The corroded flange is connected to the stainless steel flange through a graphite pad. Is it also galvanic corrosion? What is the electrolyte for galvanic corrosion you mentioned above?

This post was last edited by nj1952 on 2017-8-9 15:03 1. Graphite is conductive and has the possibility of galvanic corrosion, but it is prone to crevice corrosion.； In actual work, crevice corrosion often occurs in this node. 2. Electrolyte Electrolyte is a compound that can conduct electricity when dissolved in an aqueous solution or in a molten state. According to the strength of its electrical conductivity in aqueous solution or molten state, it can be divided into strong electrolytes and weak electrolytes. Strong electrolytes generally have: Strong acids, bases, reactive metal oxides and most salts, e.g.: Sulfuric acid, hydrochloric acid, calcium carbonate, copper sulfate, etc. Weak electrolytes generally have: Weak acids, weak bases, a small amount of salts, such as: Acetic acid, ammonia monohydrate (NH3·H2O), lead acetate, mercury chloride. Additionally, water is a very weak electrolyte. The difference between electrolytes and non-electrolytes: Electrolytes are compounds that conduct electricity in an aqueous solution or molten state ； Nonelectrolytes are compounds that do not conduct electricity either in aqueous solution or in the molten state. Elements and mixtures are not electrolytes or non-electrolytes, regardless of whether they can conduct electricity in an aqueous solution or in a molten state. For example, all metals are neither electrolytes nor non-electrolytes. Because they are not compounds, they do not meet the definition of electrolytes. (1) Whether it can be ionized (essential difference): Electrolytes are compounds that can ionize under certain conditions, while non-electrolytes cannot ionize. (2) Common substance categories: Electrolytes are generally acids, bases, salts, typical metal oxides and certain non-metal hydrides. Non-electrolytes are usually non-metal oxides, some non-metal hydrides and most organic compounds. (3) Compound category: Electrolytes are ionic compounds and some covalent compounds, while non-electrolytes are all covalent compounds.

I just got the latest news. There is also corrosion on the inner wall and weld of the pipe. It is a large area of ​​locally dispersed pit-like corrosion. The cause is under investigation...

I just got the latest news. There is also corrosion on the inner wall and weld of the pipe. It is a large area of ​​locally dispersed pit-like corrosion. The cause is under investigation...

I just got the latest news. There is also corrosion on the inner wall and weld of the pipe. It is a large area of ​​locally dispersed pit-like corrosion. The cause is under investigation...

This post was finally edited by nj1952 at 2017-8-9 15:35 to recommend you the case literature on the 4th floor.: Analysis of the corrosion causes of the outlet pipe of the high-pressure hydrogen storage tank of the 4010NA system/China Boiler and Pressure Vessel Technology/Issue 4, 1998

I have another question to ask. Many heat exchangers that have been inspected have carbon steel shells and often have a stainless steel expansion joint welded to them. This should be similar to this situation, but such corrosion is rarely found. Are the conditions not met?