Introduction
Due to their durability and resistance to corrosion, stainless steel products have become increasingly popular in a wide array of industries and applications. While the greatest benefits of using stainless steel are easy maintenance and durability, the ability to resist localized corrosion is also a major advantage over other materials.Selecting the right stainless steel for an application requires thoroughly understanding the properties of the different grades and comparing them to the application requirements.
Types of Stainless Steel
Stainless steel is a broad term for steel alloys that contain chromium. The addition of chromium improves the steel’s corrosion resistance and increases its strength. There are several types of stainless steel, which include austenitic, ferritic, martensitic, and duplex. Each type of stainless steel has distinct properties, which must be considered when selecting the grade for an application.
Austenitic Stainless Steel
Austenitic stainless steel is stainless steel with austenite as its primary microstructure. It offers the best corrosion resistance of all the stainless steel alloys, allowing it to withstand exposure to even the most corrosive chemistries.Austenitic stainless steel is further divided into subcategories, depending on its composition. The most common type is 302 stainless steel, which contains 17-19% chromium, 8-10% nickel, and 0.15% carbon. 302 steel offers a higher level of corrosion resistance and is typically used in applications that are exposed to corrosive chemicals. Other common types of austenitic stainless steel include 304, 316, and 321.
Ferritic Stainless Steel
Ferritic stainless steel is stainless steel with ferrite as its primary microstructure. It is an alloy of chromium, molybdenum, and other elements. Ferritic stainless steel provides excellent corrosion resistance in mildly corrosive environments, such as fresh water and low-level chlorides. It is typically used in applications that deal with fresh water and low-chloride solutions, such as outdoor tanks and pumps. There are several different types of ferritic stainless steel, including 409, 441, and 444.
Martensitic Stainless Steel
Martensitic stainless steel is stainless steel with martensite as its primary microstructure. It is an alloy of chromium and other elements, such as molybdenum and nickel. Martensitic stainless steel is known for its high strength and toughness. While it offers effective corrosion resistance, it is not as corrosion resistant as austenitic or ferritic stainless steel. It is typically used in applications that involve much physical stress, such as heavy metal machining and mining operations. Common types of martensitic stainless steel include 410 and 420.
Duplex Stainless Steel
Duplex stainless steel is stainless steel with both ferrite and austenite as its primary microstructure. It is an alloy of chromium and other elements, such as molybdenum and nitrogen. Duplex stainless steel offers excellent corrosion resistance and high strength, making it a popular choice for many industrial applications. Common types of duplex stainless steel include 2205 and 2304.
Localized Corrosion and Selection Criteria
Localized corrosion is a form of corrosion that occurs in a localized area. It occurs when the environment is different in one area from another. This form of corrosion is particularly damaging, as it can affect the structural integrity of a part or product. To select the right grade of stainless steel for an application, a thorough understanding of localized corrosion is needed.
Test Standards and Grading Systems
When selecting stainless steel grades for corrosion resistance, certain test standards and grading systems must be taken into consideration. For example, the ASTM A955 test standard is used to evaluate the localized corrosion resistance of stainless steel. This standard is divided into 10 different grades, ranging from Grade 1 to Grade 10. A grade 1 grade indicates the best corrosion resistance, while a grade 10 grade indicates the worst. Other test standards, such as ASTM A262, are also used to evaluate the corrosion resistance of stainless steel.
Conclusion
Selecting the right grade of stainless steel for an application requires a thorough understanding of the localized corrosion and the test standards used to evaluate the corrosion resistance of the material. It is important to compare the requirements of the application, such as exposure to corrosive chemicals and physical stress, to the properties of different stainless steel grades. When selecting the right grade, it is important to consider the ASTM A955 test standard and the grades it consists of, as well as other test standards.
