Stainless steel is known for its corrosion resistance in many environments, with different alloys having different levels of corrosion resistance. Also, stainless steels are available with a wide range of strengths. Understanding the reasons for the corrosion resistance is helpful for selecting alloys based on the required strength and environment to which the steel will be exposed.
Stainless steel corrosion resistance is a result of a very thin (about 30 - 50 nanometers) oxide layer on the steel’s surface. This oxide layer is referred to as a passive layer because it renders the surface electrochemically passive in the presence of a corrosive environment.
The passive layer forms because of the chromium added to stainless steel. Stainless steel must contain at least 10.5% chromium for the passive layer to form. The more chromium that is added, the more stable the passive layer becomes. Other elements such as nickel, manganese, and molybdenum can be added to improve the passive layer and enhance stainless steel corrosion resistance.
For mildly corrosive environments, the low end of chromium content may be sufficient, without the need for additional alloying elements. For more aggressive environments such as seawater and sodium chloride solutions, more chromium and some nickel are required, and perhaps even some molybdenum. And for even more aggressive environments, more chromium, more nickel, and more molybdenum are needed.
Another requirement for the formation and maintenance of the passive layer is the steel surface must be exposed to oxygen. Corrosion resistance is greatest when the steel is boldly exposed and the surface is maintained free of deposits.
Under certain circumstances, the passive layer can break down at localized spots. When this happens, the metal corrodes in the localized spots. This is called pitting corrosion. One common cause of pitting corrosion is exposure to aqueous environments that contain chloride, such as coastal atmospheres, road salt combined with rainwater, and even tap water containing high levels of chloride.
The tricky part of pitting corrosion is that stainless steel is selected because of its corrosion resistance. People can be caught off guard when selecting an alloy and not considering the pitting susceptibility. For example, in an environment that contains a high chloride content, 316 may be a better choice than 304. 316 contains about the same amount of chromium and nickel as 304, but also contains molybdenum, which improves pitting resistance.
During the fabrication of stainless steel components or structures it is possible to cause changes to a stainless steel's microstructure that result in degraded corrosion resistance. This occurs when austenitic stainless steels (e.g. 304 and 316) are exposed to temperatures between about 425 °C (797 °F) and 870 °C (1598 °F).
When the exposure time is too long, the chromium near the metal’s grain boundaries diffuse to the grain boundaries and react with carbon to form chromium carbide particles on the grain boundaries. As a result, the areas near the grain boundaries become depleted of chromium and lose their corrosion resistance. These areas will be preferentially attacked when the steel is exposed to a corrosive environment, and the grains fall out and the metal loses strength. This is called intergranular corrosion.
Welding is a common cause of forming chromium carbide particles in austenitic stainless steel. During welding, the metal in the heat affected zone is heated to the temperatures required to form the chromium carbide particles on the grain boundaries. One way around this problem is to use an alloy with lower carbon content, such as 304L or 316L. Both alloys contain less carbon than 304 and 316, which reduces the likelihood of forming the chromium carbide particles.
When selecting a stainless steel alloy, it’s important to consider the required strength, specific corrosion conditions, and how components will be fabricated and joined. There are many different alloys from the five alloy families – ferritic, martensite, austenitic, precipitation hardened (PH), and duplex. These different alloy families offer different sets of properties, and the alloy variations within a family – 420 vs 410 and 304 vs. 316 - offer the ability to fine tune a design to meet performance, reliability, and cost requirements.
Want to learn more about stainless steel metallurgy? Check out our recording of a past webinar on Stainless Steel Metallurgy