Why use stainless steel? Stainless steel is the ideal material choice for a wide range of plant applications due to its resistance to corrosive products such as acids, caustic solutions, oxidizers, and environmental corrosive elements such as salt laden air when found near the sea. However, other gases such as sulfur dioxide, hydrogen sulfide, carbon monoxide, and chlorine also attack stainless steel leading to localized pitting, stress corrosion, and crevice corrosion cracking. Resistance to these gases depends on their temperature and the alloying content material of the stainless steel.
By James (Jim) E. Holden, Technical Director, Energy and Engineered Services, Cortec® Corporation.
The corrosion resistance of stainless steel is primarily a function of a thin surface film of chromium oxide (i.e., the higher the chrome content, the higher the corrosion resistance). Thus, martensitic (~13% Cr) and ferritic (~13% Cr) stainless steels are more susceptible to corrosion than austenitic (~17% Cr) stainless steels.
While not as prone to general corrosion as carbon steels, stainless steels are subject to localized corrosion, pitting, stress corrosion cracking (SCC), and crevice corrosion cracking (CCC) due to surface contamination, corrosive environments, high temperature, and high stress.
Corrosion becomes more of an issue as plants age and the stainless steel is not taken care of properly (i.e., kept clean, sound repairs completed, contact with dissimilar metals minimized).
Asset managers should take several steps to minimize the corrosive impact of age and environment on stainless steel:
1. Ensure that operating conditions have not changed with time and are consistent with the stainless steel material properties.
2. Monitor insulation for evidence of corrosion under insulation (CUI) with Corrosion Under Insulation Probes such as Model ER0600.
3. Ensure that repairs have not inadvertently created potential for galvanic corrosion, stress corrosion cracking, or crevice corrosion cracking.
4. When possible, wash both internal and external surfaces of equipment to remove contaminants. If surfaces are exposed to chlorides such as sea air, wash with a product designed to remove various salts.
5. External surfaces can be protected by coatings having VCI (Vapor phase Corrosion Inhibitor) applied at a dry film thickness (DFT) of approximately 1 to 3 mils (25-75 microns). These coatings supply protection by two methods:
a. The coating prevents contaminants from directly contacting the metal surface.
b. The inhibitor molecule displaces moisture (hydrophobic) and neutralizes the metal surface charge (ionic bonding). There can be no initiation of a corrosion cell with no free electrical charge for corrosive elements to interact with.
6. Depending on the service application, adding VCIs to the process fluid may be possible. The VCI molecule will travel through the liquid to the metal surface, forming a molecular film on the surface to displace moisture (hydrophobic) and neutralize the metal surface charge (ionic bonding). Testing of various VCI additives in freshwater and saltwater applications by Dr. Behzad Bavarian and Lisa Reiner at California State University, Northridge, showed corrosion rates dropped from approximately 10 mpy for salt contaminated solutions to <1.2 mpy. While in freshwater, the corrosion rate dropped to <0.4 mpy.1
Corrosion Under Insulation
A special corrosion phenomenon for both stainless and carbon steel is Corrosion Under Insulation (CUI), which has led to serious damage and failures of equipment. Stainless steel (austenitic or duplex) experiences this due to one or more of the following:2
1. Halogens such as chlorides migrate through the insulation to the hot metal surface and cause corrosion in the presence of water.
2. The halogens become more concentrated as the metal goes through thermal gradients, causing moisture evaporation/condensation cycles on the metal surface.
3. Stainless steel alloys with higher tensile strength experience CI-CSS (chloride-induced stress corrosion cracking) at lower chloride concentrations than stainless steel alloys with low tensile strength. (Chloride concentration is a function of environment [salt air], thermal cycles around 176 °F [80 °C], and time at temperature.)
4. The insulation material may also contribute to CUI in the following ways:
- Water is entrapped between the insulation and the metal.
Depending on the type of insulation, it may hold moisture that
- Some insulation materials contain halogens such as chlorides, which can migrate to the metal surface if moisture is present in the insulation.
A perfect example of CI-SCC took place on an insulated buffer vessel made from ASME AS-240 Grade 321 steel holding 176 oF (80 oC) alkaline polymer. The vessel had been in service for 15 years, exposed to salt laden sea air from the Gulf of Mexico. A leak was detected in a section of the vessel and insulation was removed to repair it. During the weld repair, additional undetected cracks opened and needed to be fixed as well. This phenomenon is typical for equipment that has been in service for lengthy periods of time and exposed to corrosive environments, especially those with high chlorides.3
Figure 4 shows a typical picture of localized corrosion under insulation that ultimately leads to crevice corrosion and/or stress corrosion cracking.
Vapor Phase Corrosion Inhibitors (VCI)
VCI products help minimize CUI related issues in a plant by disrupting the mechanism causing corrosion. These products can be applied by painting them onto the surface being insulated or onto the insulation itself (for new or repair applications), or by injecting them into the insulation of operational equipment. The inhibitor molecule displaces moisture (hydrophobic) and neutralizes the metal surface charge (ionic bonding). As a result, there can be no initiation of a corrosion cell because there is no free electrical charge for corrosive elements to interact with.
Dr. Behzad Bavarian, California State University, Northridge, evaluated the efficacy of VCI products to minimize corrosion under insulation. He evaluated two different application methods: direct application to the insulation4 and injection into the insulation.5
When applied directly to the insulation prior to wrapping the pipe, testing showed a decrease in corrosion rate by a factor of 15. The test was conducted for 240 hours, cycled between 170.6 °F (77 °C) and 338 °F (170 °C) with 200 ppm sodium chloride solution injected every 48 hours.4 Similar testing was conducted by injecting VCI into the insulation blanket of a pipe. The test demonstrated a reduction in corrosion rate by a factor of 30.5
Although the referenced tests were performed on API 5L X65 piping, the results are applicable to stainless steel piping due to the mechanism by which VCI provides corrosion protection by the dual mechanisms of pH neutralization and hydrophobic film formation. The inhibitor molecule forms a stable strong bond at the interface of the metal, preventing corrosive elements from attacking the metal surface. A hydrophobic film approximately 0.25 mils (6.4 microns) thick is formed by adsorption onto the metal surface and is stable to 348.8 °F (176 °C). The presence of this hydrophobic film does not allow the metal surface to be wet by moisture or any aqueous solution, resulting in retardation of corrosion reactions. The principle of localized corrosion involves the same chemical reaction, regardless of material type: hydrolysis reactions lead to an acidic solution (pH 2-3) inside restricted geometries (cracks, pits, under deposits, and crevices) that break down the protective passive film. VCI can neutralize the pH in those locations to maintain the passive film stability.5
1. Bavarian, Behzad and Lisa Reiner. “Application of Vapor Phase Corrosion Inhibitors for the Contaminated Environments.” Report for Cortec® Corporation by California State University, Northridge, August 2018, <https://www.cortecvci.com/Publications/Papers/Vapor- Phase-Corrosion-Inhibitors-Report-2018.pdf>. Last accessed 3 May 2022.
2. Kumar, Anup. “Overview of Corrosion Under Insulation (CUI).” What Is Piping: A Blog for Engineers, <https://whatispiping.com/corrosion-under-insulation-cui/>. Last accessed 3 May 2022.
3. Turcott, Shane. “Corrosion Under Insulation (Stainless Steel).” Accendo Reliability blog, <https://accendoreliability. com/corrosion-insulation-stainless-steel/>. Last accessed 3 May 2022.
4. Bavarian, Behzad. “Protection Effectiveness of Vapor Corrosion Inhibitor VpCI 619 for Corrosion Under Insulation at Elevated Temperatures.” Report for Cortec® Corporation by California State University, Northridge, February 2018, <https://www.cortecvci.com/Publications/Papers/CUI-report-on-VCI-619.pdf>. Last accessed 3 May 2022.
5. Bavarian, Behzad et al. “Protection of Corrosion under Insulation using Vapor Phase Corrosion Inhibitors, Corrologic VpCI-658.” Report for Cortec® Corporation. Dept. of Manufacturing Systems Engineering & Management, College of Engineering and Computer Science, California State University, Northridge, April 2015, <https://www.cortecvci.com/Publications/Papers/Corrologic-VpCI-658-inhibitor-effects-on-CUI-final-report.pdf>. Last accessed 3 May 2022.