Why is 316 stainless steel thick-walled tube more corrosion-resistant than 304 stainless steel thick-walled tube

Stainless steel has three common materials: 201, 304, and 316L. 201 has poor corrosion resistance and is cheap; 304 is a general material with strong corrosion resistance and the widest application range; 316 stainless steel thick-walled tube is more corrosion-resistant than 304 stainless steel thick-walled tube. To analyze the performance of the material, we must first look at the composition of the material. In addition, we need to find out the type of corrosion.

The types of corrosion of stainless steel materials include electrolyte corrosion and high-temperature corrosion. Let's compare the composition differences of the two materials: 304 stainless steel has a nickel content of 8.0%-10.5% and a chromium content of 18.0%-20.0%; 316 stainless steel has a nickel content of 10.0%~14.0%, a chromium content of 16.0%~18.0%, and a molybdenum content of 2.00%-3.00%. The nickel and chromium content is an important factor in determining the material's resistance to electrolyte corrosion. 316 has a significantly higher content than 304, so it has stronger corrosion resistance; and 316 contains a high-temperature resistant element, molybdenum, which can prevent high-temperature fatigue and embrittlement.

Theoretically, the electrode potential of the matrix (ferrite) of general steel and the carbides (cementite) or non-metallic inclusions dispersed on the matrix are different. The matrix is the negative electrode; the carbides and non-metallic inclusions are the positive electrode. Therefore, micro-batteries are formed in the electrolyte solution, causing the matrix to be continuously corroded.

If the carbides are very dispersed in the 316 stainless steel thick-walled tube, a large number of micro-batteries will be formed, accelerating corrosion. It can be said that the multiphase structure of steel and the negative charge of the matrix are the two fundamental reasons why general steel is prone to corrosion in electrolyte solutions. Therefore, changing the negative charge of steel to increase the electrode potential, making the steel structure present a single-phase structure, and forming a dense and stable passivation film on the surface of the steel are three basic aspects to improve the corrosion resistance of steel.

316 stainless steel can produce passivation in the oxidizing medium of corrosion resistance. This is because the stainless steel and the oxidizing medium form a thin oxide film that is tightly attached to the surface of the stainless steel. The Cr: Fe content in the film varies between 0.7% and 9%. The Cr: Fe content in steel is only 0.24%, which shows that chromium is enriched in stainless steel.

316 stainless steel thick-walled tubes add Mo, which has excellent corrosion resistance, atmospheric corrosion resistance and high temperature strength. Due to the different expansion coefficients, it will produce large thermal stress when heated, especially when the temperature changes rapidly or after multiple heating. Fatigue cracks will occur. Fatigue cracks are different from general high-temperature fatigue. The former is a kind of thermal embrittlement caused by internal thermal stress as the temperature changes, and the latter is caused by changes in external loads at high temperatures.

Why is 316 stainless steel thick-walled tube more corrosion-resistant than 304 stainless steel thick-walled tube? From the basic corrosion resistance, the chromium and nickel content of 316 stainless steel thick wall tube is higher than that of 304 stainless steel thick wall tube. Its corrosion resistance in various organic acids, inorganic acids, alkalis, salts, and seawater is much better than that of 304 stainless steel thick wall tubes. In addition, 316 stainless steel thick wall tubes contain unique molybdenum elements, have good heat fatigue resistance, and can effectively prevent embrittlement.