Selection of stainless steel in sulfuric acid medium

The Selection of Stainless Steel in Sulfuric Acid Environment

Stainless steel has a wide range of applications in the field of corrosion protection due to its super corrosion resistance. Its corrosion resistance can be divided into three categories: common corrosion resistance (pitting and crevice corrosion, corrosion fatigue corrosion and uniform corrosion); stress corrosion resistance; high corrosion resistance (e.g. oxidation corrosion and polythionic acid corrosion). However, in the sulfuric acid environment, the corrosion resistance of common stainless steel will be weakened severely, and the average corrosion rate is often 1/10, 1/100 and even 1/1000 of that in the environment without sulfuric acid medium. Therefore, it is important for us to know the selection of stainless steel in sulfuric acid environment.

There are different types of stainless steel for different environment, such as general purpose stainless steel, special stainless steel for sulfuric acid, and nickel-based stainless steel (specifically in sulfuric acid).

General-purpose stainless steel means unsaturated austenitic stainless steel 300 series, such as 304 and 316, which are widely used in freshwater and weak-medium acid medium. They can also be used in some sulfuric acid solutions with low concentrations and low temperatures. In addition, when the sulfuric acid concentration is lower than 50%, the temperature is lower than 30℃, and the mixed gas contains no oxidant, the 304 and 316 stainless steels may be selected.

In the sulphuric acid environment of medium concentration, the 304 and 316 stainless steels will undergo surface passivation, which will cause serious corrosion. Generally, the 309 and 310 austenitic stainless steels are selected. The 309 stainless steel contains Ti with sufficient resistance to medium corrosion, and the addition of Ni to the 310 stainless steel further improves its corrosion resistance.

In the environment of high concentration sulfuric acid or high temperature sulfuric acid, it is necessary to consider the selection of super austenitic stainless steel or nickel-based stainless steel such as 904L and 625.

When selecting a stainless steel for sulfuric acid environment, the following tips should be noted. First, the temperature of sulfuric acid strongly affects its corrosiveness. Generally, the corrosive medium will become more aggressive as the temperature rises. Second, the concentration and flow rate of sulfuric acid must be considered, as well as the presence of other oxidizing agents. Last, the structure of the components should be considered. Generally, the surfaces of components should be smooth, round and well slotted to reduce the corrosive attack.

In conclusion, several types of stainless steels can be selected for sulfuric acid according to different environment requirements, such as unsaturated austenitic stainless steel 300 series, 309 and 310 austenitic stainless steel, and super austenitic stainless steel and nickel-based stainless steel. When selecting a stainless steel for sulfuric acid environment, the temperature, concentration and flow rate of sulfuric acid, as well as the presence of oxidizing agents, should be taken into account. Moreover, the structure of the components should also be considered to reduce corrosion attack.

When working in a sulfuric acid medium, stainless steel is a commonly used material. It is an alloy of chromium and molybdenum mixed with other elements such as nickel and zinc; the steel should have at least 12% chromium content. The chromium adds a film of chromium oxide to the surface, which makes the steel acid-resistant.

Stainless steel can provide good acid resistance, but there are some issues that need to be considered before its use. These include the temperature at which the acid will be used, its concentration, and how aggressive the acid is. For instance, with more aggressive acids, more durable stainless steel may be necessary to prevent corrosion. It is also important to select the grade of stainless steel that is appropriate for the application. For acidic environments, grade 316 and 316L are common choices.

In addition to the grade of stainless steel, there are also considerations such as fabrication techniques and finishing techniques. The fabrication techniques should be chosen according to the exact application, while the finishing techniques should be chosen to provide the best possible corrosion resistance.

The installation of stainless steel also requires proper care; improper installation can lead to corrosion even when stainless steel is used. The installation should be done in a way that eliminates any gaps or crevices that could trap water and cause corrosion. Additionally, proper protective coatings should be used to ensure the surface is properly sealed.

Overall, stainless steel is a great choice for use in sulfuric acid mediums, but it is important to carefully select the grade and properly prepare the environment and installation. By selecting the right grade, fabrication, and finishing techniques, and properly preparing the environment and installation, stainless steel can be used safely and effectively in sulfuric acid mediums.