Fig. 5 shows SEM micrographs of wear scars on the disk surface under different conditions. The sliding grooves of the wear scar on the disk surface are deeper in the sliding direction. The adhesive area and abrasion scars are more evident and larger with the loading force of 300 N, as shown in the SEM image in Fig. 5(a); however, fewer microvoids and microcracks are observed because the amount of salt water with sand between the sliding grooves of the disk and ball decreases with a high loading force between the contact surface of the ball and disk. This decrease in the amount of salt water with sand substantially increases the friction and wear and adversely influences the ball-bearing effect between the ball and disk. Furthermore, the dominant wear mechanisms are the abrasive wear mechanism and the adhesion wear mechanism under high loading forces, and the fatigue wear mechanism does not substantially influence the wear mechanism.
Figure 5. SEM micrographs under different loading conditions at 200 r/min for 60 min: (a) 300 N (with sand); (b) 200 N (with sand); (c) 100 N (with sand); (d) 50 N (with sand); (e) 100 N (without sand); (f) 50 N (without sand)
While the wear scars of the disk surface under 200 and 100 N of loading force were severely plowed, microvoids and isolated pits were much larger than those observed under other loads, consistent with the COF results in Figs. 2(a) and 2(b). The primary wear mechanism is abrasion because the action of sand particles and plowing dominates the whole abrasion process, which leads to high friction and wear during the ball-on-disk test under a high loading force. Meanwhile, the microvoids and isolated pits indicate that surface fatigue strongly influences the wear of the disk. Therefore, the wear mechanisms are the abrasive wear mechanism and the fatigue wear mechanism at relatively high loadings such as at 200 and 100 N, as shown in Figs. 5(b)–5(d). However, the adhesive effect between the disk surface and ball only slightly influences the wear mechanism.
In particular, the worn disk surface exhibited slight abrasion and fewer pits and microvoids were observed on the worn disk under a lower loading force of 50 N (Figs. 5(e) and 5(f)). The loading force plays an important role in the friction and wear behavior because the amount of salt water with sand between the contact surface of the ball and disk changes dramatically with loading force. These changes will affect the friction and wear substantially and will further influence the ball-bearing effect between the ball and disk.
Fig. 6 shows the corrosion product morphology of the worn track after 7 d of the specimen being immersed in 3.5wt% NaCl solution. Figs. 6(a)–6(c) indicate that the corrosion rust layer on the worn tracks of the disk is a loose structure of a block cluster. Figs. 6(d) and 6(e) shows two types of corrosion products on the worn tracks of the disk: one like block clusters and another with a needle-and-sheet appearance. Fig. 6(f) shows that the corrosion products on the worn tracks of the disk are denser and exhibit the block and layer structure. Other authors  have noted that the continuous compact needle or sheet formed under the outer rust layer is the most stable corrosion product and can effectively prevent further corrosion of the matrix.
Figure 6. Corrosion product morphology of the worn track after 7 d of immersion in 3.5wt% NaCl solution: (a) 300 N (with sand); (b) 200 N (with sand); (c) 100 N (with sand); (d) 50 N (with sand); (e) 100 N (without sand); (f) 50 N (without sand)
Even if the corrosion products cover the disk surface of the worn track, the salt water can still contact the disk surface through the microcracks between the corrosion products, eventually causing the rust layer to fall off. After loose corrosion products fall off, they cannot prevent further corrosion of the surface of the sample; the rust layer will then be reformed on the sample surface [26–27]. In addition, the wear action between the ship hull surface and seawater sand can easily remove the corrosion products of the contact surface of the steel and expose a fresh metal surface, thereby further accelerating corrosion [29–30].
Figs. 7(a) and 7(b) show the corrosion product morphology and EDS analysis results of the worn track after 7 d of immersion in 3.5wt% NaCl solution. The corresponding EDS analysis results revealed the presence of C, O, Na, Mg, Si, S, Cl, and Fe at point A in Fig. 7(a) and C, O, Na, Si, Mo, Cl, and Fe at point B in Fig. 7(b). The results show that the poor Mo area and the rich Mo area are represented by point A in the matrix and point B in Fe–Mo oxides, respectively. In Fig. 7(b), the Mo content at point B is 1wt%; by comparison, no Mo is observed at point A in Fig. 7(a), but the Fe and O contents are high, indicating that it is Fe oxide.
Fig. 8 shows an SEM image and EDS analysis results for the cross section of worn track after 7 d of immersion in 3.5wt% NaCl solution under a loading force of 300 N. It indicates that the distributions of Fe, Na, and Mn are homogeneous. However, as evident from Fig. 8, the O, which is the main element combined with Fe, is mainly clustered in the grooves, indicating that wear strongly influences iron oxidation. The main corrosive element in the environment, Cl, exhibits obvious segregation in the groove. In the corrosion process, Cl, as the most important corrosive element, functions as a catalyst. Balusamy and Nishimura  as well as other authors [36–37] have suggested that higher Cl-ion concentrations results in the formation of a larger amount of porous β-FeOOH in the corrosion products. The greater proportion of such a porous structure in the corrosive medium and the exchange of oxygen in the environment provide a transmission channel, accelerating the corrosion of the metal. This effects especially promotes early corrosion of the metal. Some researchers have reported that the corrosion products expected in a NaCl solution are chloride green rust (i.e., Fe4(OH)8Cl·2H2O), Fe3O4, Fe2O3, and Fe(III) oxyhydroxides, mainly γ-FeOOH [38–40]. Fe(OH)2 can form, whereas Fe(OH)3 does not form (FeOOH phases are substantially more stable).
The electrochemical corrosion process can be analyzed through measurement of polarization curves. Fig. 9(a) shows the polarization curve of the worn surface of the EH47 steel disk sample fully immersed in seawater for 0 d under low load (without sand). No significant difference is observed in the polarization curve of the worn surface of EH47 steel samples under loads of 50 and 100 N. The anode curve shows obvious passivation characteristics, and the passivation zone is ~0.7 V, starting from −0.9 V into the passivation zone and ending at −0.2 V.
Figure 9. Polarization curves of the worn surfaces of an EH47 steel disk sample under low loads (without sand, 200 r/min, 60 min)
Table 1 shows the results of fitting the polarization curve of the worn surface of the EH47 steel disk sample fully immersed in seawater for 0 d under low load (without sand). According to the data in Table 1, the minimum corrosion current density (Icorr) on the wear surface of the EH47 steel sample was 2.294 × 10−5 A/cm2 when the load was 50 N; this specimen exhibited a lower corrosion rate and better corrosion resistance than that subjected to a load of 100 N. These results indicate that a large load can accelerate corrosion of the wear surface of the EH47 steel sample.
Load βa /
V vs. SCE
100 3.794 6.521 −0.966 2.731 × 10−5 50 3.569 6.059 −0.976 2.294 × 10−5 Note: βa——Anodic Tafel constants, βc——Cathodic Tafel constants, Ecorr——Corrosion potential.
Table 1. Results of fitting polarization curves of the worn surfaces of an EH47 steel disk sample at low loads (without sand, 200 r/min, 60 min)
Fig. 10 shows the Nyquist impedance spectra of the worn surface of the EH47 steel disk sample immersed in seawater for 0 d under low load (without sand). As evident from the figure, the Nyquist plot shows an obvious capacitive arc, all of which are complete semicircles. The arc radius and impedance of the EH47 steel sample subjected to a load of 50 N are larger than those of the sample subjected to a load of 100 N, and the corrosion rate is lower than that of the sample subjected to 100 N, consistent with the polarization curves.
Figure 10. Nyquist diagram of the worn surfaces of an EH47 steel disk sample under low loads (without sand, 200 r/min, 60 min)
Fig. 11 shows that the material spontaneously forms a macroscopic galvanic corrosion system and that electrons flow from the anode to the cathode during the corrosion process, resulting in oxygen adsorbed onto the surface of the steel being reduced and forming OH−. The wear process subsequently promotes the formation of OH− at the interface between the electrochemical anode and seawater, and the OH− reacts with Fe2+ to form Fe(OH)2. The unstable Fe(OH)2 easily reacts with O2 to form stable Fe(OH)3, which is a reddish-brown powder with low bonding strength to steel substrates. It does not form a dense corrosion film to protect the metal substrate.
The wear and corrosion mechanism in 3.5wt% NaCl solution with sand between the contact surface of the ball and the EH47 steel disk under a high loading force is explained in Fig. 11. The worn scar and deep groove are caused by the large loading force. In the first stage, the dominant wear mechanisms were the abrasive wear mechanism and the adhesion wear mechanism under the large loading force. In this stage, the saltwater could enter the contact surface between the worn track and the ball through microcracks and pores of the rust layer (which may finally peel off). However, the corrosion products on the contact surface of the steel can be removed easily by the wear action between the ship hull surface and seawater, resulting in exposure of a fresh metal surface and further accelerating corrosion. Thus, the experimental results show that the wear mechanism was the predominant wear and corrosion mechanism. Although the impact of corrosion was light, the corrosion could still occur under abrasion friction, which exposes fresh surfaces and accelerates wear.
Effect of large load on the wear and corrosion behavior of high-strength EH47 hull steel in 3.5wt% NaCl solution with sand
4 November 2019
Revised: 23 December 2019
Accepted: 25 December 2019
Available online: 22 October 2020
Abstract: To simulate the wear and corrosion behavior of high-strength EH47 hull steel in a complicated marine environment in which seawater, sea ice, and sea sand coexist, accelerated wear and corrosion tests were performed in a laboratory setting using a tribometer. The effect of large loads on the behavior of abrasion and corrosion in a 3.5wt% NaCl solution with ice and sand to simulate a marine environment were investigated. The experimental results showed that the coefficient of friction (COF) decreases with increasing working load; meanwhile, the loading force and sand on the disk strongly influence the COF. The mechanisms of friction and the coupling effect of abrasion and corrosion in the 3.5wt% NaCl solution with sand were the wear and corrosion mechanisms; furthermore, the wear mechanism exerted the predominant effect.