Causes and Solutions of Corrosion of Stainless Steel Butterfly Valves

Stainless steel butterfly valve appears rusty during use. After metallographic analysis, dyeing test, heat treatment test, SEM and other test analysis, the key factor to find the material corrosion is that the carbides along the grain boundary precipitate in the material to form a chromium-depleted area, resulting in stainless steel butterfly valve corrosion.

Stainless steel butterfly valve made of CF8M rusted during use. After austenitic stainless steels are normally heat treated, the microstructure at room temperature should be austenite, and the corrosion resistance is very good. In order to analyze the causes of corrosion of the butterfly valve, samples were taken for analysis.

1 Test methods Samples were taken for chemical composition analysis (to determine whether it meets the standard requirements), metallographic examination, heat treatment process test and SEM analysis.

2 Test results and analysis 2.1 Chemical composition Chemical composition analysis results and standard components.

2.2 Metallographic analysis Metallographic specimens were cut from a butterfly valve where corrosion was observed. After grinding and polishing, they were etched with aqueous ferric chloride solution and observed on a Neophot-32 metallographic microscope. The body is composed of another kind of precipitate. Theoretically, after austenite stainless steels undergo normal heat treatment, homogeneous austenite structure should be obtained. The other precipitate that appears in the organization is exactly what organization. There are two kinds of judgments: one is the sigma phase and the other is carbide. The σ phase and the carbide formation conditions are different, but all have a common feature, that is, the susceptibility of austenitic stainless steel to intergranular corrosion.

First of all, a varicolor method was used to identify the σ phase. Alkaline red blood salt solution (red blood salt 10g + potassium hydroxide 10g + water 100ml), the sample boiled in the reagent after 2 ~ 4min, ferritic yellow, carbide corrosion, austenitic bright color, The σ phase turns from brown to black. Using the method described above, the sample cut from the butterfly valve was boiled in an aqueous solution of alkaline hemi-blood salt for 4 min. Observed under the microscope, the precipitate maintained its original morphology and no significant changes were found. Therefore, it was decided to use a heat treatment method to further test the face.