*4.1. Metallography*

The microstructure of high strength wires was observed by metallography microscope (Olympus GX51, Tokyo, Japan) (Magnification factor: 50×–1000×, Light source: 6V30WHAL halogen lamp) before the corrosion test. Figure 6 shows the metallographic microstructures of Wires A, B and C: All the three wires are composed of sorbite, product of austenite isothermal transformation, having eutectoid structures of alternate thin layers of ferrite and cementite. The sorbite lamellae size of B and C is finer; the grain boundary defects are accordingly reduced, and the corrosion resistance is expected to enhance. The results are consistent with Ren's study [36].

The ferrite and cementite were grey-white in Figure 6, and the interface between them was black. The three wires all had sorbitization treatment applied to ensure a high degree of strength and toughness. When the wires are produced, chemical composition adjusting is often used to reduce the lower limit temperature of sorbite transformation and avoid the occurrence of bainite and martensite. Generally, the alloying elements Mn, Cr, Ni and Cu will improve the stability of austenite and delay the decomposition of austenite. As shown in Table 1, the carbon content of Samples A, B and C is

similar while the Mn, Cr and Cu contents of Wires B and C are higher. Therefore, the completion of the sorbite transformation is prolonged, the precipitation of eutectoid ferrite and pearlite is reduced, and the termination temperature of the sorbite transformation is also significantly decreased; all these factors give Wires B and C higher tensile strength. In addition, the surface film of wires B and C formed by the corrosion was more compact due to the higher content of Cr and Ni. The corrosion resistance was improved, which was consistent with the anodic polarization curve and EIS conclusion.

**Figure 6.** Metallographic microstructures of three high strength wires: Wire A ×500 (**a**) and ×1000 (**b**), Wire B ×500 (**c**) and ×1000 (**d**), Wire C ×500 (**e**) and ×1000 (**f**).

## *4.2. Fracture Surface*

The fracture cross sections of the three wires (A, B, C—10 min) under 70% *ft* tensile stress were observed by Scanning Electronic Microscopy (SEM, Quanta 250 FEG, Hillsboro, OR, USA). The acceleration voltage was 20 kV, and high vacuum mode was <6 × 10−<sup>4</sup> Pa, resolution was 1.2 nm; see Figure 7.

**Figure 7.** SEM images of rupture surface of wires: (**<sup>a</sup>**,**b**) wire A, (**<sup>c</sup>**,**d**) wire B, (**<sup>e</sup>**,**f**) wire C, (**g**) corrosion pits.

The fractographic and SEM images in Figure 7 confirmed that the macroscopic fracture of all three wires under stress corrosion is of brittle nature. These fracture surfaces present radial pattern without plastic deformation. The splitting surface is strip-like, and the end of the fracture is radial from the section center to the periphery, which was consistent with the literature observations [37,38]. Under higher magnifications, all sections showed more characteristics related to corrosion: the corrosion pits on the sections are generally deep and narrow, and the surface fractures develop into the solid matrix of steel. These fractures were developed from the microcracks on the wire surface extended along the wire length and tension, results of tensile stress and NH4SCN solution. Some cracks, torn edges and quasi-cleavage fractures are found on the wire surface in the crack growth area. The fractography analysis of these fractures attributes the major pattern to be trans-granular.

Energy spectrum (EDX) was used to analyze the chemical composition of inclusions in the fractures in Figure 8: Wire A has carbides non-metallic inclusions while Wires B and C had oxide non-metallic inclusions. The existence of non-metallic inclusions in the samples destroyed the continuity of the metal matrix structure, which deteriorates the mechanical properties of all the wires. These inclusions can be the crack nucleation sites, which promotes the fracture propagation in solid matrix of granular nature, especially under the high level of tensile stress.

**Figure 8.** EDX analysis of the fracture inclusions on cross sections of high-strength wires: (**a**) A, (**b**) B and (**c**) C.

#### *4.3. Corrosion Products*

The original oxide film defects of steel wire were split to expose the fresh metal under tensile loading. The exposed substrate and the oxide film around the defects formed a corrosion source in NH4SCN solution. The exposed fresh metal acted as anode while oxide film acted as cathode, which led to an increase in the corrosion rate. The corrosion products film is formed in the anodic dissolution process, and the defects trigger continuously under tensile loading leading the film to be deteriorated.

Some ferric iron formed in the anodic dissolution process, and the reaction of ferric iron in solution containing thiocyanate produces ferro-thiocyanate compound, i.e., Fe3<sup>+</sup> + xSCN−→[Fe(SCN)x] <sup>3</sup>−x. The ferro-thiocyanate compounds form on the steel surface and will promote the anodic dissolution of steel [30]. From the available EIS and other results in this study, the film becomes loose, and the surface of the steel wire becomes rougher under tensile loading. The decreased transfer resistance and the sharp increase of corrosion current under tensile stress indicate that the film was rapidly deteriorated, which is confirmed also by potentiodynamic polarization curves.

Figures 9 and 10 showed the SEM microstructure and the energy spectrum analysis of prestressed steel wire surface after corrosion in NH4SCN solution, respectively. The surface film consisted of the original iron oxide film and the corrosion products film whose components are mainly iron thiocyanate and iron sulphide. The original iron oxide film was dense while the corrosion products film was rough and loose.

**Figure 9.** SEM microstructure of prestressed steel wire surface in NH4SCN solution.

The steel wires with similar chemical composition were selected to study the stress corrosion behavior in this paper. The results showed that they had di fferent corrosion resistance in the same corrosion environment. It indicated that even if the contents of Cr, Ni and Cu increased slightly, the performance of corrosion product film on the wires surface would be a ffected. Meanwhile, stress corrosion is the result of multiple factors synergy, such as composition and microstructure. Thus, in order to improve the stress corrosion resistance of steel, it is necessary to improve many aspects.

**Figure 10.** Energy spectrum of prestressed steel wire surface in NH4SCN solution: (**a**) corrosion products film, (**b**) original iron oxide film.
