Selection of Stainless Steel in Phosphoric Acid Media

Selecting Stainless Steel in Phosphoric Acid Media

Introduction

In many industries, phosphoric acid is widely used in cleaning, paper production, and as a part of food and beverage processing. As with any industrial setting, steel components are essential. Within phosphoric acid media, stainless steel is known for its superior corrosion resistance; however, not all grades of stainless steel provide the same corrosion resistance, making it important to select the one best suited for the application at hand. This article explores the corrosion properties of stainless steel and examines the common grades of stainless steel used in phosphoric acid media.

Corrosion Properties of Stainless Steel

Stainless steel’s superior corrosion resistance is derived from the addition of chromium to its alloy composition. Chromium forms a protective layer of chromium oxide on the surface, which acts as a barrier against corrosive chemicals. The chromium content is typically 12-13%, which is what allows stainless steel to provide superior corrosion protection. The ability of stainless steel to form this protective layer increases with higher amounts of chromium and other alloying elements like nickel, molybdenum, and nitrogen.

Common Grades of Stainless Steel Used in Phosphoric Acid Media

Due to its superior corrosion protection, stainless steel is often used in applications involving phosphoric acid. Several grades of stainless steel are suitable for use in this media, including grade 304, grade 316L, grade 904L, and duplex 2205. The most common grade used is type 304, followed by type 316L for its improved corrosion resistance. Grade 904L is typically used for higher concentrations of phosphoric acid and offers superior corrosion protection over grade 316L. Finally, duplex 2205 is highly recommended for applications involving low-to-moderate concentrations of phosphoric acid.

Conclusion

In summary, selecting the appropriate grade of stainless steel for an application involving phosphoric acid is critical for ensuring long-term performance. Grade 304 is the most commonly used grade for moderate concentrations, while grade 316L is often used for higher concentrations. For higher concentrations of phosphoric acid, grade 904L or duplex 2205 are recommended. While the cost of the grades mentioned can be a consideration, the performance benefits of selecting the correct grade of stainless steel far outweigh any cost.

When selecting stainless steel for phosphoric acid medium environment, attention should be paid to the following aspects:

1.The content of Cr, Mo, N and other alloying elements in stainless steel should be high. Chromium and molybdenum can increase the strength and corrosion resistance of stainless steel, so the content of Cr and Mo elements should be higher, and the content of N element should also be adjusted higher, the optimum range of Cr content is at least 16~20%, the optimum range of Mo content is 2%-6%, and the optimum range of N element is 0.1%-0.4%.

2. Stainless steel should contain ferrite, austenite and martensite. In order to make stainless steel have better corrosion resistance and higher strength, it should contain as much austenite and martensite as possible, and the structure of ferrite should be controlled below 20%.

3. The corrosion resistance of stainless steel mainly depends on the stable and continuous passive film on the steel surface, so the selective precipitation of Cr, Mo and N elements should be avoided.

4.Stainless steel should have enough ductility and toughness, that is, it should have good stamping and cold heading, welding performance and heat treatment performance.

Therefore, when selecting stainless steel for phosphoric acid medium environment, we should pay attention to the Cr, Mo, N content, ferrite structure and ductility of stainless steel. 321 stainless steel is usually used, which has good corrosion resistance and high temperature strength, and can meet the requirements of phosphoric acid medium environment.