While the various stainless steel grades may differ in their metallurgical composition, alloys or manufacturing processes, they all share a common characteristic, the passive layer. This layer of chromium oxide forms when the metal surface is exposed to oxygen.
The layer is too thin to be directly observed and remains transparent in most cases. This allows the finished texture of stainless steel products to show through and provides a variety of appearance options from high gloss to matte.
The passive layer is impermeable to water and is self-generated, protecting the metal. Although it may change color in extreme temperatures, it still performs its function. However, stainless steel is not indestructible. It is still susceptible to corrosion under the right circumstances.
If you plan to use stainless steel, consider the following to assess the general corrosion risk.
Acids
Stainless steel is generally resistant to acid corrosion. However, the exact level of corrosion resistance depends on the steel used, the concentration and type of acid, and the ambient temperature.
For example:
Grades 316 and 317 resist most concentrations of phosphoric acid and sulfuric acid concentrations below 10%.
Grade 904 resists high concentrations of sulfuric acid.
Types 304L and 430 are resistant to nitric acid.
Hydrochloric acid is an exception to stainless steel’s general acid resistance and should be avoided.
Bases
Stainless steel is very resistant to weak bases in high concentrations and at high temperatures. However, strong bases can cause cracking or corrosion. Special care must be taken with chloride solutions such as sodium hypochlorite.
Organics
Resistance to organics varies. Generally, 300 series steels perform best in these solutions. However, exact recommendations vary depending on environmental factors such as temperature and oxygen availability.