**1. Introduction**

Hydrogen assisted cracking (HAC), which is classified into hydrogen induced cracking (HIC) and sulfide stress cracking (SSC) by the presence and absence of applied stress, is one of the most important issues in the petrochemical industries [1–5]. In a recent refining system for low-grade crude oil, the refining temperature and pressure required have increased remarkably. The sensitivity of material deterioration, caused by the inflow of hydrogen atoms in the steel structures, is becoming higher than in the past [6,7]. Accordingly, considerable efforts have been made to develop the steel used as pressure vessel facilities with superior resistance to HAC [7–10]. For this purpose, a variety of technical solutions, such as de-phospho/sulfu-rization of molten steel [11,12], Ca-treatment [13], cast steel soft reduction [14], and thermal mechanical controlled process (TMCP) [15–17], have been proposed. Microstructural modifications by reducing the number of second phases, such as retained austenite (RA), martensite-austenite constituents (MA), and other hard phases, and their role in improving the HAC resistance, have been reported [18,19]. Nevertheless, there are uncertainties regarding the relationship between the alloying elements in the steel and the HAC behaviors, primarily because of the lack of systematic efforts to understand the HAC behaviors, in terms of the hydrogen diffusion/trapping behaviors in the steel matrix and corrosion behaviors on the steel surface.

Theoretically, hydrogen atoms, formed by corrosion reactions on the steel surface, can diffuse easily into the steel matrix with a body-centered cubic (BCC) structure, and become trapped at several metallurgical defects with high binding energy [20–22]. According to the internal pressure and de-cohesion theories, proposed by Zapffe et al. [23], and Troiano [24], respectively, cracks were initiated by the continued accumulation of hydrogen atoms into local areas and their recombination reaction (H + H → H2), and propagated easily through the embrittled regions formed by a weakening of the binding force between Fe-Fe atoms. Therefore, clarification of the physical nature of hydrogen diffusion and trapping phenomena is critically important. Moreover, the other important feature of HAC, occurring in sour environments, is the corrosion behaviors on the steel surface. In particular, the amount of hydrogen infusion and its infusion kinetics can also be controlled by the characteristics of the corrosion products formed on the steel surface [25,26]. Kim et al. [26] investigated the combined addition of Cu and Ni on sour corrosion and the HIC resistance of A516 steel in sour environments. On the other hand, some alloying elements can have both beneficial and detrimental effects on sour corrosion and HAC resistance of steels in sour environments. Hence, a further in-depth study is required.

In this study, the HAC resistance of A516 steel, used in pressure vessel facilities, was investigated systematically, in terms of microstructural modifications, hydrogen diffusion/trapping behaviors, and surface characteristics, which were dependent on the addition of alloying element of C and Mo to the steel. Based on the experimental and analytical results, a desirable alloy design concept for improving the resistance to HAC is suggested.
