**1. Introduction**

Due to the high bearing capacity and sti ffness, the prestressed concrete (PC) structures are widely used in large-span building, bridge and hydraulic structures [1]. The PC structures adopt either bonded or unbounded systems and are usually considered to acquire su fficient durability due to the limitation of concrete cracking and appropriate protection measures for the high-strength wires and strands [2,3]. However, prestress failures in PC structures have been reported in literature. Schupack and Suarez [4] showed, through an investigation from 1978 to 1982 in USA, about 50 PC structures had undergone di fferent degrees of corrosion, among which 10 cases were caused by stress corrosion or hydrogen brittleness. Hydrogen is introduced by the improperly pickling process in general. As for the hydrogen-induced stress corrosion cracking mechanism, Vehovar et al. [5] reported its cause as the embrittlement of the prestressing tendons due to the penetration of hydrogen atoms, generated by hydrogen reduction at the alloy's surface. Enos and Scully et al. [6] used a simulated steel-concrete interface and laboratory-scale prestressed concrete pilings to study the safe cathodic protection limits for prestressing steel in concrete about hydrogen embrittlement. Moreover, the dislocations in the prestressing tendons relative to the production process and heat treatment are sensitive to hydrogen embrittlement. On this basis, Gertsman [7] and Jaka Kovac et al. [8] studied and characterized the intergranular SCC (IGSCC) processes. The results show that geometrical parameters and chemical parameters play an important role in the intergranular stress corrosion cracking of materials.

Walter [9] reported 242 cases of prestress failure, from 1951 to 1979, among which a large portion were attributed to the environmental actions. Among these failure causes, the stress corrosion cracking (SCC) is always a main engineering concern as it reduces the fracture strength of high-strength steel wires, leading to unexpected brittle failures of PC structures [10,11]. Moreover, the SCC can be sensitive to the aggressive agents present in the corrosive environments [12].

The SCC is caused by the combined action of stress, environmental agents and metal components, and the widely accepted mechanisms include the surface (depassivation) film of wires is broken jointly by the tensile stress and the environmental actions; the freshly exposed steel surface serves as the anode dissolved at the crack tip, leading to the steel fracture [13,14]. The conventional test methods include the deformation, sustained loading and slow strain rate testing [15]. During the mechanical loading, the electrochemical methods have been used to characterize the SCC process, identifying the susceptibility potential range for prestress wires [8,16] and detecting the stress corrosion fracture of a stainless steel through the phase shift of electrochemical impedance spectroscopy (EIS) [17]. As for the corrosive environment, a SCC test method was developed by the International Prestressed Concrete Federation (FIP, now merged into *fib*), using ammonium thiocyanate (NH4SCN) solution as the corrosive environment for SCC test. This method has been standardized by ISO 15630 [18] and GB/T 21839 [19]. Perrin et al. [20] used this method to investigate the SCC mechanisms together with detection of EIS, electrochemical noise and metallographic analysis, and confirmed the validity of this method for SCC investigation.

Nowadays, high-strength steel wires for civil engineering use are usually classified following the mechanical properties including the rupture strength and the elongation, with a rough account for chemical compositions [21–23]. However, the SCC risk of high-strength steel is rather sensitive to the chemical composition. Thus, the SCC performance should always be investigated with respect to the environmental actions and steel chemical compositions. Accordingly, it is of interest to compare the SCC performance for streel wires with similar chemical compositions from di fferent standards. To this purpose, this study retains three steel wires, conforming to BS 5896 [21] and GB/T 5224 [22], and investigates the SCC resistance of these wires through stress-immersion tests and the role of stress on the electrochemical behaviors.
