Comparison of evaluation methods for hydrogen embrittlement sensitivity of high strength steels

The demand in the automotive industry for steels with strength over 1000 MPa is rapidly increasing. The motivation is to lower the weight of vehicles as much as possible to reduce CO₂ emissions. Thus, material scientists are working at full capacity to develop and design high strength steel grades that can meet the future needs of the automotive industry. However, in addition to the challenge to design steels with excellent properties, these steels should also have acceptable resistance to hydrogen embrittlement. It is commonly accepted that the sensitivity to hydrogen embrittlement of high strength steels increases notably at higher strength levels. Hence, the utilization of these steels in automotive applications is strongly dependent on their susceptibility to hydrogen embrittlement. There are available standardized methods used to evaluate hydrogen embrittlement sensitivity, including laboratory and field exposure tests methods. A commonly employed laboratory test method is the step constant load test (SCLT). This involves loading the sample in an incremental manner until fracture while
the material is simultaneously exposed to corrosive environment. As a complement, field exposure tests can be used to demonstrate the hydrogen embrittlement sensitivity of the steels during real service corrosive environments. Such field tests include e.g., stationary corrosion testing in a marine atmosphere and mobile corrosion testing on a trailer in service.
However, the process of assessment of the hydrogen embrittlement sensitivity is very time-consuming and can delay approval for industrial production. This has a negative impact on the fast implementation of these steels. Moreover, another important consideration is how well these testing methods actually simulate service conditions and result in a similar outcome.
Hence, in this work we present a study aiming to evaluate the available standardized laboratory (SCLT) and field exposure test methods to evaluate the hydrogen embrittlement sensitivity of coated high strength steel grades with strengths over 1000 MPa. The SCLT was performed according to the VDA-238-201 standard for coated samples exposed in 50 g/L NaCl, while the field exposure test methods were carried out using different sample geometries in a marine atmosphere at Bohus Malmön, Sweden according to ISO 9226 corrosion standard for the corrosivity category C5 for carbon steel and C3 for zinc. To evaluate and compare these methods, the fracture surfaces and cross-sections of the specimens were characterized and correlated to the hydrogen level measured by thermal desorption analysis.