Whether for the consumer market or industrial use, one of the most popular reasons for choosing stainless steel is its corrosion-resistant properties.
However, the exact resistance levels and weaknesses of a stainless product, sheet or bar will depend on both it’s metallurgic composition and how it was made.
This guide covers the various types of corrosion to be wary of while working with stainless steel and other concerns that might impact either the visual appearance or performance of popular stainless steel grades.
But before we dig into the details, let’s take a look at what makes stainless so resistant in the first place…
How Does Stainless Steel Resist Corrosion?
While various stainless steel grades might differ in their metallurgic composition and alloying or manufacturing processes, one trait they all share is a passivation layer.
This layer of chromium oxide forms when the surface of the metal is exposed to oxygen.
The layer is too thin to observe directly and under most circumstances remains clear. This allows the finished texture of a stainless steel product to show through and provides a variety of appearance options from high gloss to matte.
Impervious to water, the passivation layer is self-generating and serves to protect the metal. While it might discolor under extreme temperatures, it’s still working.
However, stainless steel is not invincible. It’s still susceptible to corrosion under the right situations.
Stainless Steel and Chemical Compounds: What to Consider
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. However, exact resistance levels will depend on the steel in use, concentration, and types of acid, and environmental temperature.
- Grades 316 and 317 offer resistance to phosphoric acid at most concentrations and sulfuric acid at concentrations below 10 percent.
- Grade 904 offers resistance to high concentrations of sulfuric acid.
- Types 304L and 430 offer resistance to nitric acid.
Hydrochloric acid is an exception to the general acid resistance of stainless steel and should be avoided.
Stainless steel offers strong resistance to weak bases in both high concentrations and high-temperature environments. However, stronger bases will cause cracking or etching corrosion. Particular care must be taken with chloride solutions such as sodium hypochlorite.
Resistance to organics differs. In general, 300-series steels perform best with these solutions. However, exact recommendations vary based on environmental factors such as temperature and oxygen availability.
If you’re planning to use stainless steel to store organics, consulting a professional, such as Unified Alloys, can help to both highlight concerns and identify options to create a safe, long-lasting stainless steel solution.
Specialized Types of Stainless Steel Corrosion and Damage
Apart from general corrosion, stainless steel corrosion can take on several specialized forms.
Fortunately, there are ways to address most forms or avoid them entirely. The following are common forms of specialized corrosion.
This corrosion occurs when stainless steel parts are forced together. Commonly seen with nuts, bolts, and other fasteners, corrosion isn’t visible until the parts are separated or disassembled.
If stainless steel parts must be forced together, consider using multiple metal types–even using multiple types of stainless steel might help. With threaded parts, lubrication and reduced force can reduce galling. You might also consider nitronic alloys or if your application and budget allow.
Also known as bimetallic corrosion, this corrosion occurs when stainless steel is used with other metals in a moist environment. Rain, condensation or other moisture sources can act as an electrolyte in the presence of an electrical current causing one metal to corrode at an accelerated rate.
Exact characteristics of galvanic corrosion vary based on metals used, temperatures, areas of contact and other factors. For more information on the topic, consulting the British Standards Institution’s PD 6484 is recommended.
This corrosion is typically a result of exposure to chemicals in the environment or poor aeration. It is also more common in steels with unwanted inclusions or manufacturing defects. For example, Manganese (II) Sulfide often initiates pitting.
Adequate oxygen availability can help to reduce the rate of pitting or halt it entirely depending on the cause. Steels with molybdenum also have an increased ability to both resist pitting and slow its progress. Other considerations include pH, chloride concentration and temperature of the environment in which the steel is used.
4. Crevice Corrosion
This corrosion occurs when a crevice between the stainless steel and another material allows chlorides to concentrate or prevents proper oxygen levels to regenerate the steel’s oxide layer. While typically found near or within the gap between two metals, crevice corrosion can also occur between metallic and non-metallic surfaces.
Welding, ensuring proper drainage and using proper gaskets can also help to prevent crevice corrosion as these all help to decrease access to crevices or eliminate them altogether.
5. Weld Decay
Also known as intergranular corrosion, this corrosion results from heating stainless steel to a specific temperature range—often between 550C and 850C. This causes chromium to precipitate from the stainless steel and reduces the ability for the passivation layer to regenerate.
Using low carbon steels—such as 304L and 316L—when welding can help to reduce the impact of weld decay. Additives such as titanium or niobium can further stabilize the steel and reduce precipitation while welding. In all cases, using a post-weld heat treatment is recommended to minimize corrosion.
Keeping stainless steel looking and performing great is all about protecting the chromium layer that gives it stainless properties.
This also improves safety by ensuring that your stainless steel structures, containers and equipment stay structurally sound for as long as possible.
By considering the environment in which your stainless steel is used—and matching the grade to your intended application—you can optimize corrosion resistance and ensure long-lasting results for your investment.
Though this guide offers a good starting point on different corrosion types and prevention measures, consulting with a professional is always recommended to ensure an ideal product fit and safety.
Unified Alloys offers more than 40 years of expertise serving some of the biggest industries in Canada and North America. Have a question about stainless steel corrosion or applications? Call us and we’d be happy to discuss your needs and find the perfect solution.