Introduce the reasons and solutions for corrosion of stainless steel BURKERT solenoid valves separately
Metallographic samples were cut from the BURKERT solenoid valve that showed signs of corrosion. After grinding and polishing, they were corroded with an aqueous solution of ferric chloride and observed and analyzed on a Neophod-32 metallographic microscope. The metallographic structure consisted of austenite and another precipitate. In theory, austenitic stainless steel should obtain a uniform austenite structure after normal heat treatment. There are two ways to determine which structure the other precipitate appearing in the organization is: one is the σ phase, and the other is the carbide. The conditions for the formation of σ phases and carbides are different, but they all share a common characteristic, which is the sensitivity of austenitic stainless steel to intergranular corrosion.
The mixed color method was used for the identification of the σ phase. Using an alkaline red blood salt solution (10g red blood salt+10g potassium hydroxide+100ml water), the sample was boiled in this reagent for 2-4 minutes. The ferrite turned yellow, the carbides were corroded, the austenite appeared bright, and the σ phase changed from brown to black. The sample cut from the stainless steel butterfly valve was boiled in alkaline red blood saline solution for 4 minutes using the above method, and observed under a microscope. The precipitate maintained its original shape and no significant changes were found. Therefore, it was decided to use heat treatment method for further facial analysis.
Corrosion occurred during the process. Through metallographic analysis, staining test face, heat treatment test face, SEM and other experimental analyses, it was found that the key factor causing material corrosion is the precipitation of carbides along grain boundaries in the material, which forms a chromium deficient zone, resulting in the corrosion of stainless steel butterfly valves.
The stainless steel butterfly valve made of CF8M material experienced rusting during the process. After normal heat treatment, the microstructure of austenitic stainless steel should be austenitic at room temperature, with good corrosion resistance. In order to analyze the cause of rust on the butterfly valve, samples were taken for analysis.
There is no single grade of stainless steel that can use all acid-base media. The selection of stainless steel butterfly valves varies depending on the concentration and temperature of the acid-base medium.
The widely used material for manufacturing BURKERT solenoid valves is austenitic stainless steel, with the majority being the 18-8 Cr Ni series. Austenitic stainless steel is a type of stainless steel with multiple grades and an important range of applications. The characteristics of austenitic stainless steel are: non-magnetic, good plasticity, and excellent cold and hot processing. Its composition generally contains high levels of chromium, nickel, and other corrosion-resistant elements.
The commonly used BURKERT solenoid valve is corrosion-resistant to oxidizing media such as atmosphere, dilute and medium concentrations of nitric acid, and concentrated sulfuric acid, but it is not corrosion-resistant to reducing media such as hydrochloric acid, sulfurous acid, boiling glacial acetic acid, etc. When corrosion occurs under over passivation conditions, 18-8 type Cr Ni austenitic stainless steel is not corrosion-resistant. In this case, austenitic stainless steel with high silicon content should be selected for manufacturing.
If the medium is nitric acid, austenitic stainless steel containing molybdenum is not corrosion-resistant, especially in boiling nitric acid. Therefore, austenitic stainless steel butterfly valves containing molybdenum are usually not in nitric acid.
Stainless steel containing molybdenum has better corrosion resistance in organic acids and certain reducing acids, while austenitic stainless steel containing molybdenum, copper, and silicon has better corrosion resistance in sulfuric acid media, such as ZG1Cr24Ni20Mo2Cu3 and CN-7M (AC1 grade) austenitic stainless steel, which are widely used in sulfuric acid resistant butterfly valves. Some corrosion-resistant alloys with a mass fraction greater than 50%, such as BURKERT solenoid valves, are also austenitic matrices and are used in high-temperature, high concentration sulfuric acid, which is very good.
BURKERT solenoid valves are generally not corrosion-resistant in hydrochloric acid and can only be passivated in extremely dilute hydrochloric acid. 18-8 type Cr Ni austenitic stainless steel has a wide concentration and temperature range in alkaline media such as NaOH, KOH, etc., and has good corrosion resistance.
Corrosion occurred during the process. After metallographic analysis, staining test face, heat treatment test face, SEM and other experimental analyses, it was found that the key factor causing material corrosion is the precipitation of carbides along grain boundaries in the material, which forms a chromium deficient zone, resulting in the corrosion of stainless steel butterfly valves. The stainless steel butterfly valve made of CF8M material experienced rusting during the process. After normal heat treatment, the microstructure of austenitic stainless steel should be austenitic at room temperature, with good corrosion resistance. In order to analyze the cause of rust on the butterfly valve, samples were taken for analysis.
The material of BURKERT solenoid valve is nickel chromium austenitic stainless steel, which is generally in a solid solution state. At room temperature, its structure is austenitic, and austenitic stainless steel has good corrosion resistance in a wide range of corrosive media, especially in the atmosphere. Analysis of the causes of corrosion of stainless steel butterfly valves: Based on the comprehensive test results, it can be determined that the precipitated phase in the material structure of the butterfly valve is not the σ phase, so the corrosion phenomenon of the butterfly valve is not caused by the σ phase; Through SEM observation, it was confirmed that the precipitated phase in the structure of the butterfly valve is mainly composed of chromium carbides, and this eutectic structure is distributed along the grain boundaries.
The analysis results of BURKERT solenoid valve indicate that the chromium content of carbides distributed on grain boundaries is significantly higher than that of the matrix. This type of carbide is M23C6. With the precipitation of carbides and the absence of chromium diffusion supplementation, chromium carbide precipitates along the austenite grain boundaries in the form of chromium carbide, forming a chromium poor zone around the carbides, making the austenite stainless steel grain boundaries susceptible to corrosion.
The carbides precipitated along the grain boundaries of BURKERT solenoid valves are the main cause of butterfly valve corrosion; After solid solution treatment, austenitic stainless steel is saturated with a large amount of carbon and chromium due to the dissolution of most carbides during high temperature heating, and is fixed due to subsequent rapid cooling, making the material highly corrosion-resistant. Therefore, the heat treatment process should be strictly controlled. During solid solution treatment, the workpiece should be heated to a high temperature to fully dissolve the carbides, and then rapidly cooled to obtain a uniform austenite structure. After solid solution treatment, if slow cooling is used, chromium carbide will precipitate along the grain boundaries during the cooling process, resulting in a decrease in the material's corrosion resistance.
Speaking of BURKERT solenoid valves, I believe everyone is very familiar with them. They are a common type of pipeline valve in our daily lives and are widely used in industries such as petroleum and gas. Below, the editor will introduce the practical scope and installation points of groove butterfly valves.
1. Practical range of BURKERT solenoid valve: diameter of 50-400mm, pressure of 2.5-4.0MPa, various room temperature liquids below 200 ℃.
2. The installation site of BURKERT solenoid valve should be cleaned up, and the lighting, water supply and drainage should be unobstructed. The vulnerable parts on the outer surface of the equipment should be protected with protective covers. No items should be stored on the equipment, and it should not bear any weight.
3. To prevent electrochemical corrosion, the BURKERT solenoid valve should avoid scratching the surface during transportation and installation of stainless steel and aluminum equipment and accessories. During construction, contact with other metals should be minimized and wrapped with plastic sheeting or other materials.
4. The binding points on the accessories of BURKERT solenoid valve equipment should be cushioned to prevent equipment damage. Lifting should be carried out in order to prevent rework. When installing accessories, they should be handled gently, and the lifting position for heavy objects should be selected reasonably.
5. After unpacking the BURKERT solenoid valve equipment, the installation site should be sealed off and unauthorized personnel are strictly prohibited from entering. The installation site should be spacious, bright, clean, dry, and equipped with measures to prevent wind, rain, and snow. Equipment and accessories should be classified and stored, and their storage areas should have moisture-proof measures: surface scratches and damage caused by collisions should be avoided.