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

There are three commonly used stainless steel materials: 201, 304, and 316L. 201 has poor corrosion resistance but is inexpensive; 304 stainless steel pipe is a general-purpose material with strong corrosion resistance and the widest range of applications; 316 stainless steel thick-walled pipe has stronger corrosion resistance than 304 stainless steel pipe. Analyzing the material properties of stainless steel pipes first requires looking at the material composition. Additionally, we need to identify the types of corrosion that occur. Today, we will explain this in detail:

The types of corrosion that stainless steel materials are susceptible to include electrolyte corrosion and high-temperature corrosion. Let's compare the compositional differences between the two materials: 304 stainless steel pipe has a nickel content of 8.0%-10.5% and a chromium content of 18.0%-20.0%; 316 stainless steel thick-walled pipe 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%. Nickel and chromium content are crucial factors determining a material's resistance to electrolyte corrosion. 316 stainless steel thick-walled pipes have significantly higher nickel and chromium content than 304 stainless steel pipes, resulting in stronger corrosion resistance. Furthermore, 316 stainless steel thick-walled pipes contain molybdenum, a high-temperature resistant element, which helps prevent high-temperature fatigue and embrittlement.

Theoretically, the electrode potentials of the matrix (ferrite) and the carbides (cementite) or non-metallic inclusions dispersed in the matrix of general steel are different; the matrix is ​​the negative electrode, while the carbides and non-metallic inclusions are the positive electrode. Therefore, micro-cells are formed in electrolyte solutions, causing continuous corrosion of the matrix.

If the carbides are highly dispersed in 316 stainless steel thick-walled pipes, a greater number of micro-cells will form, 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 the steel to increase the electrode potential, making the steel structure a single-phase structure, and forming a dense and stable passivation film on the steel surface are several basic aspects of improving the corrosion resistance of steel. 1. 316 stainless steel thick-walled pipes can undergo passivation in corrosive oxidizing media. This is due to the reaction between the stainless steel and the oxidizing medium, forming a thin oxide film tightly adhering to the stainless steel surface. The Cr: Fe content in this film varies between 0.7% and 9%, while the Cr: Fe content in the steel itself is only 0.24%, indicating that chromium is enriched in stainless steel.

2. 316 stainless steel thick-walled pipes contain added Mo, resulting in exceptionally good corrosion resistance, atmospheric corrosion resistance, and high-temperature strength. Due to the difference in expansion coefficients, significant thermal stress is generated during heating. Especially under rapid temperature changes or after repeated heating, thermal fatigue cracks may occur. This differs from general high-temperature fatigue; the former is thermal embrittlement caused by internal thermal stress with temperature changes, while the latter is caused by changes in external load at high temperatures.

Why are 316 stainless steel thick-walled pipes more corrosion-resistant than 304 stainless steel pipes? From a basic corrosion resistance perspective, 316 stainless steel thick-walled pipes have higher chromium and nickel content than 304 stainless steel pipes, resulting in significantly better corrosion resistance in various organic acids, inorganic acids, alkalis, salts, and seawater. Furthermore, 316 stainless steel thick-walled pipes contain a unique molybdenum element, which provides excellent resistance to thermal fatigue and effectively prevents embrittlement.