Stainless steel is used in multiple industries (e.g., food, medical, chemical, oil and gas) and various manufacturing processes for aesthetic purposes and high resistance to corrosion in harsh environments. There are five basic families of stainless steel: austenitic, ferritic, martensitic, duplex, and precipitation hardened. Of these, austenitic is considered the most resistant to corrosion.
By James (Jim) E. Holden, Technical Director, Energy and Engineered Services, Cortec Corporation
With time, all grades of stainless steel will corrode in the right environment unless properly protected. The accepted industry standard is to passivate stainless steel as the last step in the manufacturing process by dipping it in or wiping it with acids such as citric or nitric. The resultant chromium oxide film is a function of metal chromium content, acid used, temperature, and exposure time to the acid. Unfortunately, the oxide layer is not permanent, as it can be damaged during normal plant operations by exposure to harsh chemicals, physical abrasion, or stress due to expansion and contraction.
When the oxide film on the surface is continuously destroyed, it loses its ability to form new oxide films. Once the film is destroyed it exposes the metal to corrosive elements such as organic and inorganic acids, alkalis, and halides. Halides, especially chlorides, are a serious factor causing stainless steel to rust because they destroy passivating films. Chlorides are all around us, in sea and saltwater lakes, household cleaners, and disinfectants. When left exposed to these products for extended periods of time with no adequate cleaning, stainless steel will corrode and tarnish.
The occurrence of corrosion can lead to cracking under the right environmental and physical conditions. One or more of the following types of corrosion may be experienced with varying impact on stainless steel equipment based on the stainless-steel grade, operating environment, and stress loading.
Pitting corrosion is a form of localized corrosion and is usually considered negligible from a design point of view. The exception is deep pitting, which can cause stress Kts and stress multipliers, potentially leading to crack initiation.
Crevice corrosion is a localized form of corrosion normally occurring in crevices or cracks. The ratio of the crevice width to depth has a significant impact on the severity of the corrosion and the rate at which it propagates. Examples of crevices are bolted joints with nuts and washers and screw threads or bolt shanks in close tolerance fit. Crevice corrosion can also occur under deposits on the metal surface and in weld defects.
Galvanic corrosion occurs when two dissimilar metals are joined together—the farther apart they are on the galvanic scale, the higher the potential corrosion rate, although other factors influence the rate of corrosion. Examples of galvanic corrosion come from the use of carbon steel fasteners in stainless steel components and carbon steel tube baffles and tube sheets with stainless steel tubes in heat exchangers. This is of small concern in heat exchangers due to the cathode area (stainless steel tubes) being larger than the anode area (baffle or tube sheet).
- General or uniform corrosion is not an issue with stainless steel if the pH at the surface of the metal is <1.
- Stress corrosion cracking (SCC) needs a material susceptible to corrosion, stress, and a corrosive environment. Care should be taken when using stainless steel in chloride-rich environments to ensure that residual stress from the manufacturing process is eliminated.
The various forms of corrosion can lead or contribute to multiple failure modes. Pitting causes Kts (stress multipliers) such that the failure stress is reduced, causing premature failure. Crevice corrosion cracking can lead to stress corrosion cracking which can be influenced by low cycle and high cycle fatigue. Stress corrosion cracking is the most severe form of corrosion cracking and can lead to sudden and unexpected failures of ductile material.
Manufacturers and operators should take the necessary precautions to prevent stainless steel corrosion. These precautions may include proper handling and cleaning, removing the potential for galvanic cells, removing the potential for carbon steel (iron) contamination, and using protective coatings. Exposed surfaces should be periodically cleaned with soap, mild detergents, ammonia, or organic solvents. Another choice is to clean with a mild alkaline cleaner containing a volatile corrosion inhibitor that supplies protection through hydrophobic action (displaces moisture) and adsorption (ionic, covalent, or metallic) to neutralize the surface charge on the metal.
After cleaning, the surface should be rinsed and dried. Another approach is to coat the exterior of a stainless-steel part with a coating containing a volatile corrosion inhibitor (vapor phase or migrating). The coating supplies corrosion protection in three ways. (1) The coating itself acts as a barrier, keeping moisture and contaminants from contacting the metal. (2) The corrosion inhibitor molecule travels through the coating to the metal surface, creating a molecular level barrier. (3) This barrier supplies protection by adsorption (ionic, covalent, or metallic) to neutralize the surface charge on the metal. With no free electrical charge (electron or proton) to combine with corrosive elements, corrosion cells cannot occur.
Dr. Behzad Bavarian, California State University, Northridge, has conducted multiple tests evaluating the ability of volatile corrosion inhibitors to minimize corrosion and corrosion related cracking in various steels and aluminum. The testing evaluated the impact of alkaline cleaners and volatile corrosion inhibitors (which are fogged into a volume) on pitting, general corrosion, crevice corrosion cracking, and stress corrosion cracking. In all instances, testing showed a significant reduction in corrosion rate.
References:
- Bavarian, Behzad, Jia Zhang, and Lisa Reiner. “SCC and Crevice Corrosion Inhibition of Steam Turbine ASTM A470 Steel.” Paper No. 01070, CORROSION 2012 Conference & Expo, NACE International, <https://store.ampp.org/scc-and-crevice-corrosion-inhibition-of-steam-turbine-astm-a470-steel-and-7050-t74-al-alloys-using-v>.
- Bavarian, Behzad, Jia Zhang, and Lisa Reiner. “Electrochemical and SCC Inhibition of Multi-alloy Systems using Vapor Corrosion Inhibitors.” Paper No. 2130, CORROSION 2013 Conference & Expo, NACE International, <https://store.ampp.org/electrochemical-and-scc-inhibition-of-multi-alloy-systems-using-vapor-corrosion-inhibitors>.
- Bavarian, Behzad, Lisa Reiner, and Hamed Youssefpour. “Vapor Phase Inhibitors to Extend the Life of Aging Aircraft.” Paper No. 05329, CORROSION 2005 Conference & Expo, NACE International, <https://store.ampp.org/05329-vapor-phase-inhibitors-to-extend-the>.
- Dawe, Graham. “Does Stainless Steel Rust? The Truth About Stainless Steel Corrosion.” Kanyana Engineering Blog, 21 Dec 2022, <https://kanyanaengineering.com.au/does-stainless-steel-rust/>.
- MGNewell. “Passivation of Stainless Steel.” White Papers – Sanitary Principles, <https://mgnewell.com/wp-content/uploads/2016/11/Passivation-of-stainless-steel.pdf>.
6. World Material. “Does Stainless Steel Rust or Tarnish, and Why?” <https://www.theworldmaterial.com/does-stainless-steel-rust/>.
- British Stainless-Steel Association. “Corrosion Mechanisms in Stainless Steel.” <https://bssa.org.uk/bssa_articles/3-corrosion-mechanisms-in-stainless-steel/>.
- OneMonroe. “How to Protect Stainless Steel from Rust and Corrosion.” 26 Oct 2015, <https://monroeengineering.com/blog/how-to-protect-stainless-steel-from-rust-and-corrosion/>.