Comparison of Stress Corrosion Cracking of High-Chromium Stainless Steels
Authors: Carpenter Chris 
Published: August 2021

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper NACE 2020-14695, “Comparison of Stress Corrosion Cracking Behavior of Fe13Cr5Ni- and Fe17Cr5.5Ni-Based High-Chromium Stainless Steels in High-Pressure/High-Temperature CO2 Environments,” by Yameng Qi, Zhonghua Zhang, and Chunxia Zhang, Baoshan Iron and Steel, prepared for the 2020 NACE International Corrosion Conference and Exposition, Houston, 14–18 June. The paper has not been peer reviewed.
Stress corrosion cracking (SCC) of Fe13Cr5Ni- and Fe17Cr5.5Ni-based alloys in high-pressure/high-temperature (HP/HT) carbon dioxide (CO2) environments was investigated through slow-strain-rate tests (SSRTs) and electrochemical methods. The results show that a remarkable decrease in tensile strength and elongation to failure was observed when testing in a CO2 environment compared with that of air. Fe17Cr5.5Ni-based alloys possessed better SCC resistance than Fe13Cr5Ni-based alloys. The better SCC resistance of the former could be attributed to good repassivation capacity and pitting-corrosion resistance induced by the increase in chromium (Cr) and nickel (Ni) content.

Introduction
When service temperature exceeds 150°C, SCC resistance of Fe13Cr5Ni-based alloys could become an issue. Compared with Fe13Cr5Ni-based alloys, 22Cr duplex stainless steel has an excel-lent performance when exposed to temperatures over 150°C and stable SCC resistance in HP/HT CO2 environments. However, the cost of 22Cr duplex stainless steel is extremely high.

Experimental Procedure
Fe13Cr5Ni- and Fe17Cr5.5Ni-based alloys were produced by the authors’ research institute. The materials were in a quenched and tempered state. For micrographic observation, each specimen was ground with 2,000-grit carbide silicon paper and polished with 1.2-µm diamond paste. They were then degreased with acetone and etched with hydrochloric ferric chloride solution (a mixture of 5-g ferric chloride, 25-mL hydrochloric acid, and 25-mL ethanol).
The steel microstructures were characterized using an optical micro-scope. The micrograph in Fig. 1a for the F-13Cr5Ni-based alloys shows a martensite phase with no notable second phases. Fe17Cr5.5Ni alloys possess long strip ferrite and martensite phases (Fig. 1b).
For SSRTs, smooth tensile specimens with a gauge length of 25.4 mm and a diameter of 3.81 mm were prepared. The specimens were cut from the Fe13Cr5Ni- and Fe17Cr5.5Ni-based alloys into an 8-mm-thick, 12-mm-outer- diameter disc for electrochemical measurement. All specimens were polished to a 1,200-grit surface finish, degreased with acetone, cleansed with distilled water, and dried in air. SSRT and electrochemical-measurement procedures are detailed in the complete paper.

Results
SCC Susceptibility. As expected, tensile strength and elongation to failure of Fe13Cr5Ni- and Fe17Cr5.5Ni-based alloys deteriorated in HP/HT CO2 environments. Compared with an environment of air, the elongation to failure of Fe13Cr5Ni- and Fe17Cr5.5Ni-based alloys in HP/HT CO2 environments decreased by approximately 30 and 25%, respectively. In addition, tensile strength and elongation to failure of Fe17Cr5.5Ni-based alloys were greater than those of Fe13Cr5Ni-based alloys. Elongation, reduction in area, and time to failure of Fe17Cr5.5Ni-based alloys were found to be much higher than that of Fe13Cr5Ni-based alloys in HP/HT CO2 environments. It can be concluded that Fe17Cr5.5Ni alloys possess better SCC resistance than Fe13Cr5Ni alloys in these environments.