Session: MF-02-04 Materials for Hydrogen Service-High Alloy Materials 1

Paper Number: 121921

121921 - The Influence of Electrochemical and Gaseous Hydrogen Environments on the Mechanical and Fracture Behavior of Duplex and Austenitic Steels 

Both electrochemical and gaseous hydrogen environments have been utilized to evaluate hydrogen embrittlement (HE) characteristics of a metallic material. Although gaseous environments best represent gaseous hydrogen service in practice, this method requires stricter safety protocols than electrochemical charging, and the relevant facilities and capabilities are limited globally. Therefore, there is a critical need to develop an accelerated HE testing methodology utilizing electrochemical charging as a simpler and cost-effective alternative to the gaseous exposure method. Understanding correlations and differences between the two methods is key to the development of an accelerated HE test method.

In this study, the influence of electrochemical and gaseous hydrogen environments on the mechanical and fracture behavior of duplex and austenitic steels was investigated by means of rising displacement testing using circumferential notch tensile (CNT) and arc-shaped fracture mechanics specimens during hydrogen exposure, followed by fractographic analyses. The current study explored high manganese (Mn) austenitic and duplex (ferrite-austenite) steels, produced through alternative alloying approaches compared to stainless steels. Through the comparative HE studies using a 255 duplex stainless steel as a reference material, the electrochemical charging parameters that produce notch tensile results similar to those obtained from gaseous environments were identified, i.e. 0.05M NaOH aqueous solution and 1.65 mA/cm² current density. Regardless of the notch tensile results, the fractographic appearances for the CNT specimens were significantly different between the two environments. The electrochemical charging environment resulted in a more symmetrical fractographic appearance, i.e. the ductile fracture zone near the center of the fracture surface was surrounded by hydrogen-affected brittle fracture zone near the notch root. In contrast, the specimens embrittled in the gaseous environment showed the ductile fracture zone located away from the center of the fracture surface. In all cases, the hydrogen-affected brittle fracture zones involved transgranular fracture features, e.g. cleavage facets.

The sensitivity of hydrogen-assisted fracture to the environmental exposure condition (i.e. gaseous versus electrochemical) depends on the alloy conditions and/or microstructures. For example, both high Mn austenitic and duplex steels were more severely embrittled in a gaseous hydrogen environment at a pressure of approximately 100 MPa than in an electrochemical environment utilizing a 0.05M NaOH solution and a 1.65 mA/cm² current density, whereas the duplex stainless steel exhibited similar notch tensile results for the equivalent hydrogen environments. In addition, the fracture surfaces of the high Mn steel specimens exhibited large secondary (delamination) cracks parallel to the rolling direction in both environments, whereas such features were not noted in the duplex stainless steel. These results indicate that the HE mechanisms can fundamentally depend on the alloy conditions as well as the hydrogen environment.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Presenting Author: Lawrence Cho Colorado School of Mines

Presenting Author Biography: Dr. Lawrence Cho is a Research Assistant Professor with the Advanced Steel Processing and Products Research Center (ASPPRC) within the Department of Metallurgical and Materials Engineering at Colorado School of Mines. He has been involved in University-Industry collaborative research programs with a strong focus on the application of fundamental steel science to practical challenges related to the development of new and advanced materials for the automotive industry. His previous research focused on the physical metallurgy of coated, advanced high to ultra-high strength steels, specifically related to structure/property relationships, surface science and the coating technology, and liquid metal embrittlement. More recently, his research interest has been in the area of the development and evaluation of materials for the storage and transport of hydrogen in general and the problem of hydrogen embrittlement of high strength steels.

Authors:

Lawrence Cho Colorado School of Mines
Yuran Kong Colorado School of Mines
John Speer Colorado School of Mines
Kip Findley Colorado School of Mines
Milan Agnani Sandia National Laboratories-Livermore
Joseph Ronevich Sandia National Laboratories-Livermore
Chris San Marchi Sandia National Laboratories-Livermore