Whether for consumer products or industrial applications, one of the primary reasons stainless steel is chosen is for its corrosion-resistant properties.
However, the level of resistance and potential vulnerabilities of a stainless steel product—whether sheet or bar—depend on both its metallurgical composition and manufacturing process.
This guide explores the different types of corrosion to watch for when working with stainless steel, as well as other factors that may affect the appearance or performance of common stainless steel grades.
But before diving into the specifics, let’s first examine what makes stainless steel so resistant to corrosion…
While different grades of stainless steel vary in metallurgical composition, alloying, and manufacturing processes, they all share one crucial feature: a passivation layer.
This protective layer of chromium oxide forms when the metal’s surface is exposed to oxygen. Although too thin to be seen, the passivation layer is typically clear, allowing the stainless steel’s natural finish to show through, whether polished to a high gloss or left matte.
Water-resistant and self-regenerating, this layer protects the metal from corrosion. Even if it discolors under extreme temperatures, it continues to function effectively.
However, stainless steel isn’t immune to all forms of corrosion and can still be vulnerable under certain conditions.
One of the most common scenarios in which stainless steel corrodes is through sustained contact with liquids or chemicals.
Known as general corrosion, this type is typically uniform across the surface of the steel. Fortunately, most stainless steels respond in a predictable manner to a range of chemicals.
If you’re planning to use stainless steel, consider exposure to the following to gauge general corrosion risks.
Stainless steel is generally resistant to acidic corrosion, but its exact resistance depends on the grade of steel, acid concentration, type of acid, and environmental temperature. For example:
- Grades 316 and 317 resist phosphoric acid at most concentrations and sulfuric acid at concentrations below 10%.
- Grade 904 is effective against high concentrations of sulfuric acid.
- Types 304L and 430 resist nitric acid.
- Hydrochloric acid is an exception and should be avoided due to its corrosive effects on stainless steel.
Stainless steel offers strong resistance to weak bases, even at high concentrations and temperatures. However, stronger bases can lead to cracking or etching corrosion. Special care must be taken with chloride solutions, such as sodium hypochlorite.
Resistance to organic compounds varies. Generally, 300-series stainless steels perform best with organic solutions, though exact performance depends on environmental factors like temperature and oxygen availability.
In addition to general corrosion, stainless steel can suffer from several specialized forms of damage. Fortunately, most of these can be prevented or mitigated. Below are some common types of specialized corrosion:
Galling occurs when stainless steel parts are forced together, often in fasteners like nuts and bolts. The corrosion is typically not visible until the parts are separated. It happens due to friction and pressure, which causes surface material to degrade over time.
Also known as bimetallic corrosion, this occurs when stainless steel is used in combination with other metals in a moist environment. Moisture, such as rain or condensation, acts as an electrolyte, facilitating electrical currents that cause one metal to corrode faster than the other.
Pitting corrosion occurs due to chemical exposure or poor aeration, and is more common in steels with unwanted inclusions or manufacturing defects, such as Manganese (II) Sulfide, which can initiate pitting.
Ensuring adequate oxygen levels can reduce or halt pitting corrosion, depending on the cause. Steels containing molybdenum offer enhanced resistance to pitting and slow its progression. Factors such as pH, chloride concentration, and temperature of the environment also play a role in the likelihood and severity of pitting.
Crevice corrosion arises when a gap between stainless steel and another material traps chlorides or restricts oxygen, preventing the regeneration of the protective oxide layer. This type of corrosion is commonly found in the gaps between metals but can also occur between metallic and non-metallic surfaces.
Preventing crevice corrosion involves proper welding, ensuring adequate drainage, and using appropriate gaskets, all of which help reduce or eliminate crevices where corrosion can develop.
Also known as intergranular corrosion, weld decay occurs when stainless steel is heated to a specific temperature range, typically between 550°C and 850°C. This causes chromium to precipitate out of the steel, reducing its ability to regenerate the protective passivation layer.
To mitigate weld decay, low-carbon steels like 304L and 316L can be used during welding. Additionally, stabilizing additives such as titanium or niobium can help reduce chromium precipitation. Applying a post-weld heat treatment is also recommended to further minimize the risk of corrosion.
Maintaining the appearance and performance of stainless steel hinges on protecting the chromium layer that gives it its corrosion-resistant properties. Doing so also enhances safety, ensuring that stainless steel structures, containers, and equipment remain structurally sound over time.
By carefully considering the environment in which the stainless steel will be used and selecting the appropriate grade for your application, you can optimize corrosion resistance and maximize the longevity of your investment.
While this guide provides a useful overview of corrosion types and prevention strategies, consulting with a professional is recommended to ensure the best product fit and safety.
