Effect of Dissolved Oxygen Content on Tribo-Corrosion Behavior of Monel 400 Alloy in Seawater

Abstract

When serving in the marine environment, the corrosion of metals is inevitably affected by marine environmental factors, especially dissolved oxygen. This will affect the service life of metal in the marine environment. In this paper, a friction- and wear-testing machine, an electrochemical workstation, and a dissolved oxygen/temperature control system are employed to systematically study the influence of dissolved oxygen (DO) content on the tribo-corrosion characteristics of Monel 400 alloy in seawater. The results show that the DO content has a remarkable effect on the tribo-corrosion behavior of Monel 400 alloy. The corrosion of Monel 400 alloy increases with the increase in DO content, leading to the thickness of corrosion product increasing at higher DO contents. The corrosion product is mainly composed of Ni(OH)₂, Cu₂O, CuO, and Cu(OH)₂. While the corrosion of Monel 400 alloy further affects the wear rate of the alloy, when the corrosion is inhibited, the change of DO content has no effect on the wear rate of Monel 400 alloy. In addition, the inhibition influence of corrosion on mechanical wear is found, which is attributed to the excellent lubrication performance of corrosion products.

1. Introduction

With the decrease in land resources, the dependence of mankind on marine resources is gradually increasing. A “blue revolution” marked by the development of ocean is rising in the world. The exploitation of marine resources is inseparable from marine equipment, meaning that a large number of metal materials need to serve in the marine environment [1,2,3]. As we all know, there are many kinds of inorganic salts in seawater with an average salinity of 35, which makes seawater a natural strong electrolyte [4,5,6,7]. Therefore, metal materials often suffer serious corrosion in the marine environment. In addition, marine equipment contains many friction parts, including sea pumps, transmission shafts, etc. This means that metal materials will not only suffer corrosion but also suffer wear, and they will even suffer coupling damage of corrosion and wear (tribo-corrosion) in the marine environment [7,8,9]. The damage of metal materials greatly affects the safety and stability of marine equipment. At present, special alloys with the advantage of high strength and corrosion-resistance properties (including precipitation-hardened stainless steel, super duplex stainless steel, nickel–aluminum bronze, and nickel base alloys, etc.) are widely used as frictional materials in the marine environment [10,11,12,13,14,15]. In the marine environment, studying the tribo-corrosion characteristics and mechanisms of these typical materials is of great significance to the damage protection of friction parts and even the reliable operation of marine equipment.

The corrosion of metals in the marine environment is affected by many factors, like temperature, dissolved oxygen (DO) content, salinity, pH, and flow rate [16,17]. DO content is the key factor affecting metal corrosion. DO can be used as a cathode depolarizer and can affect the growth of passive film on metal surface [18,19]. The change of DO content has a remarkable effect on the passive behavior of the Ti-6Al-3Nb-2Zr-1Mo alloy. The passive film of the Ti-6Al-3Nb-2Zr-1Mo alloy is an n-type semiconductor, and the main point defect type is oxygen vacancy. Therefore, the de-aerated environment provides a faster growth rate for passive film as compared to the aerated environment [20]. The higher DO content causes an increase in the corrosion rate for brasses, but it makes no difference to the corrosion rate of the Cu-5Ni alloy on account of the barrier effect of corrosion products [21]. For 316L stainless steel, the oxygen content in the passive film formed in the oxygen-free solution is less, and then compared with the aerobic solution, the thickness of passive film is about half [22]. For the passive film of 2507 super duplex steel, although the electric field strength and point defect diffusivity decrease due to the removal of DO, it has no detectable impact on the dissolution of passive film [23]. In addition, the change of DO can affect the wear of the metal material [24]. With the increase in DO content, the wear volume, maximum wear depth, and the microslip region of the 690TT alloy in high-temperature pure water increase [25]. For carbon steel, the wear rate in 0.5% saline solution decreases with the decrease in DO content [26]. Above all, the DO content on the corrosion and wear of metal materials has a remarkable influence. However, few studies have explored the interaction and mechanism of the DO content with respect to the corrosion and wear of metal.

In this paper, the effect of the DO content in seawater on the tribo-corrosion characteristics of Monel 400 alloy is researched. The effects of the DO content on corrosion is remarkable. The DO content significantly affects the thickness of the corrosion product layer on the surface of Monel 400 alloy. While the change of DO content has no effect on wear, the influencing mechanism of the DO content on the tribo-corrosion characteristics of Monel 400 alloy is revealed via scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and a transmission electron microscope (TEM).

2. Experimental Section

2.1. Materials and Machine

The Monel 400 ring (outer diameter: 54 mm, inner diameter: 38 mm, height: 13 mm), as a study object, is offered by Shanxi Qingye special materials Co., Ltd (Xian, China). Its chemical composition includes nickel (67.1, wt%), copper (29.65, wt%), iron (1.85, wt%), manganese (1.09, wt%), silicon (0.22, wt%), carbon (0.08, wt%), sulfur (0.003, wt%), and phosphorus (0.003, wt%). The Al₂O₃ spherical pin, as a counterpart, is supplied by Dehe special porcelain factory, its dimension is φ 4.8 mm × 13 mm, and the sliding surface is hemisphere-shaped with a spherical radius of 4.75 mm. Artificial seawater, as a corrosion medium, is prepared according to ASTM D1141-98 [27], and its chemical composition is shown in

Table 1. The chemical composition of artificial seawater.

Components	NaCl	MgCl2	Na2SO4	CaCl2	KCl	NaHCO3	KBr	SrCl2	H3BO3	NaF
Contents (g/L)	24.53	5.20	4.09	1.16	0.695	0.201	0.101	0.025	0.027	0.003

. Monel 400 rings are enclosed with insulating paint before testing. Except the working surface of the Monel 400 ring, the other surfaces are covered with insulating paint to avoid errors caused by corrosion. Then, Monel 400 rings are ultrasonically cleaned with ethanol and dried for later use.

All wear tests in this paper are performed on the MMW-1 frictional machine. By combining the frictional machine with a dissolved oxygen/temperature control system (Yianda, Guangzhou, China), the temperature and dissolved oxygen are controlled during testing. PID instrumentations are used to control the temperature and dissolved oxygen of the water environment accurately. By a built-in heating system (ceramic heaters), the external Hastelloy heat exchanger can achieve the effect of controlling temperature. Dissolved oxygen content is measured by using the galvanic membrane method. Precise flow control for oxygen and nitrogen is combined with the PID learning algorithm, which can achieve precise control of dissolved oxygen content. Moreover, the Princeton electrochemical workstation (PARTAT 3000A, Ametek, Berwyn, PA, USA) and frictional machine are combined to simultaneously monitor the electrochemical signal during the test. Please refer to our previous article for the test device diagram [28].

2.2. Test Process

With the temperature of 25 °C, the applied load of 75 N, and the rotational velocity of 0.25 m/s, the tribo-corrosion characteristics of Monel 400 alloy are investigated at different DO contents, including 2 ppm, 5 ppm, 8 ppm, and 11 ppm. A three-electrode systema consisting of a working electrode (Monel 400 alloy), a reference electrode (Ag/AgCl electrode), and a counter electrode (Pt loop) is used to record the electrochemical signal. During the frictional process, the Al₂O₃ pin moves in a unidirectional circular motion around the Monel 400 ring. Open circuit potential (OCP) is monitored before friction for 20 min, during friction for 60 min, and after friction for 20 min. The wear rate obtained from the OCP process is the total material loss rate (T). By applying the cathodic potential of −0.9 V, the potentiostatic curve is measured to evaluate the mechanical wear rate (W₀) with the duration of 60 min, in which the electrochemical corrosion is inhibited. For the ratio of corrosion, the Tafel curves are measured under the conditions of no-sliding (C₀) and sliding (C_w). The scan range is changing from −0.8 V to 0.2 V, with a scan rate of 1 mV/s.

2.3. The Calculation of Each Component in the Process of Tribo-Corrosion

The total material loss rate (T) consists of mechanical wear rate (W₀), pure corrosion rate (C₀), and the interaction of corrosion and wear (S), as shown in Equation (1). Therein, the interaction includes the influence of corrosion on wear (ΔW_c) and the influence of wear on corrosion (ΔC_w). According to the standard of ASTM G119-09 [29], each component in the process of tribo-corrosion is calculated as follows:

$$T=C_w+W_c=C_0+{\Delta C}_w+W_0+{\Delta W}_c=C_0+W_0+S.$$

(1)

T, W₀, C₀ and C_w can be calculated via the following equations:

$$T=\frac{(m_0-m_1)}{S\rho t},$$

(2)

$$W_0=\frac{\left(m_0-m_2\right)}{S\rho t},$$

(3)

$$C_0=\frac{3.27\times {10}^{-3}\times i_0\times E_w}{\rho },$$

(4)

$$C_w=\frac{3.27\times {10}^{-3}\times i\times E_w}{\rho },$$

(5)

where m₀ is the initial mass of Monel 400 ring, g; m₁ is the final mass of Monel 400 ring after OCP test, g; m₂ is the final mass of the Monel 400 ring after the cathodic protection test, g; S is the area of the wear scar, mm²; $\rho$ is the density of Monel 400 alloy, g/mm³; t is the duration of test, h; i₀ and i are the corrosion current density obtained from Tafel curves of the static corrosion and tribo-corrosion process, respectively, μA/cm²; abd E_w is the equivalent weight of Monel 400 alloy, which is 34.60.

2.4. Characterization

After tribo-corrosion and cathodic protection tests, the worn morphologies of the damaged surface, cross-section, and corrosion product are observed by FESEM (Quanta 650 FEG, FEI Company, Hillsboro, OR, USA). Then, a Helios G4 FIB (FEI Company, Hillsboro, OR, USA) is used to prepare the thin film surrounding the selected area for sample preparation of TEM. Next, the thickness of the corrosion product is observed by TEM (Talos F200x G2, FEI Company, Hillsboro, OR, USA) and EDS (Super-x). The chemical composition of corrosion product is analyzed via X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA USA).

3. Result

The open circuit potential (OCP) can reflex the corrosive susceptibility of the alloy to some extent. Generally, when OCP is more positive, the corrosive susceptibility of the alloy is weaker [30]. Figure 1a shows the change of OCP with time during the static corrosion process of the Monel 400 alloy under various DO contents. The OCP of the Monel 400 alloy becomes more positive with the increase in DO content, indicating that the corrosive susceptibility of Monel 400 alloy is weaker and the passivated rate is faster when the DO content is higher. Combined with Tafel curves (Figure 1b), with the DO content increasing, the corrosion potential shifts to a more positive position. The corrosion current in the anodic zone decreases, while the corrosion current in the cathodic zone increases. Because the oxygen consumption reaction mainly occurs in the cathodic region [20], the cathodic reaction rate accelerates with the increase in the DO content in seawater, which, in turn, affects the anodic reaction rate, resulting in an increase in the coverage area of corrosion products on the surface of Monel 400 alloy. Thereby, the anodic current reduces. When Monel 400 alloy slides against the Al₂O₃ pin, the OCP drops sharply (Figure 2a). Compared with other DO contents, the difference in the OCP between the static corrosion and the tribo-corrosion process is larger when the DO content is 2 ppm. Moreover, when the friction stops, OCP does not return to the initial value within 20 min. This means that the surface of Monel 400 alloy is severely damaged at 2 ppm, and the passive film recovers slowly due to the low oxygen content. On the contrary, at higher DO contents, the OCP of Monel 400 alloy returns to the initial value instantaneously when the friction stops. As can be seen from Figure 2b, compared with the no-sliding condition, the corrosion potential (E_corr) of Monel 400 alloy after sliding moves towards the cathodic direction, and the corrosion current increases to nearly one order of magnitude. It shows that mechanical wear promotes electrochemical corrosion.

After tribo-corrosion and cathodic protection tests, the total material loss rate (T) and frictional coefficient (COF) of Monel 400 alloy under various DO contents are shown in Figure 3. Under the same DO content, T and COF of Monel 400 alloy after cathodic protection tests are higher than after tribo-corrosion tests. This indicates that electrochemical corrosion has an inhibiting effect on mechanical wear, which may be attributed to the corrosion products with excellent lubrication properties formed on the surface of Monel 400 alloy during tribo-corrosion conditions. After tribo-corrosion tests, the T and COF of Monel 400 alloy decrease first then increase with the increase in DO content. T reaches the lowest value at 5 ppm and 8 ppm, while COF reaches the lowest value at 8 ppm. After cathodic protection tests, with the DO content increases from 2 ppm to 11 ppm, the mechanical wear rate of Monel 400 alloy basically does not change, and COF fluctuates around 0.38.

In order to evaluate the corrosion and wear interaction of Monel 400 alloy under various DO contents, the proportion of each component in the tribo-corrosion process is calculated, respectively. The relevant data are exhibited in Figure 4. The influence of DO content on the corrosion rate of Monel 400 alloy is shown in Figure 4a. With the DO content increasing from 2 ppm to 11 ppm, the corrosion rate (C₀) during static corrosion conditions increases from 0.01 mm/y to 0.09 mm/y. The corrosion rate (C_w) during tribo-corrosion conditions increases from 0.15 mm/y to 0.56 mm/y, illustrating that mechanical wear obviously promotes electrochemical corrosion. Figure 4b,c shows the influence of DO content on the wear rate (W_c) and total material loss rate (T) of Monel 400 alloy. The results show that the DO content has no effect on the mechanical wear rate after cathodic protection tests. However, the total material loss rate after tribo-corrosion tests decreases first then increases with the increase in DO content. The lowest value of total material loss rate is reached at 5 ppm and 8 ppm, and the highest value is reached at 11 ppm. In addition, it is clearly noted that the inhibiting influence of corrosion on mechanical wear is strongest at 8 ppm.

Figure 5 shows the damaged morphologies of Monel 400 alloy after tribo-corrosion and cathodic protection tests under various DO contents. First of all, the phenomena of furrow and peeling exist on the damaged surface of Monel 400 alloy under any condition, which is a representative feature of delamination wear. Second, under the same DO content, the surface of Monel 400 alloy after the cathodic protection tests is damaged more severely, and the damaged surface is rougher, which is consistent with the above results. This shows that electrochemical corrosion has an inhibiting effect on mechanical wear. After tribo-corrosion tests, when the DO content is 2 ppm and 11 ppm, there is a large spalling pit on the damaged surface, suggesting that the total material loss rate is larger. When the DO content is 5 ppm and 8 ppm, the damage extent of Monel 400 alloy is relatively slight. After cathodic protection tests, there is no significant difference in the damaged degree of Monel 400 alloy under various DO contents. This indicates that the DO content has little effect on the wear rate of Monel 400 alloy after cathodic protection tests.

Next, the authors observe the cross-section morphologies of Monel 400 alloy after tribo-corrosion and cathodic protection tests under various DO contents (Figure 6). The results show that there are cracks in the subsurface of Monel 400 alloy after friction, indicating that the wear mechanism of Monel 400 alloy is delamination wear. At the same DO content, the deformation layer on the subsurface of Monel 400 alloy is thicker, and cracks are wider after cathodic protection tests than that after tribo-corrosion tests. This further indicates that electrochemical corrosion inhibits mechanical wear. Similarly, after tribo-corrosion tests, cracks formed on the subsurface of Monel 400 alloy are fewer and the deformation layer is thinner when the DO content is 5 ppm and 8 ppm, while the phenomenon of crack propagation is more obvious when the DO content is 2 ppm and 11 ppm.

4. Discussion

Based on the above results, the DO content has a significant effect on the corrosion and wear behavior of Monel 400 alloy. First, the DO content greatly affects electrochemical corrosion of Monel 400 alloy. The DO content usually affects the corrosion kinetics of alloys by controlling the cathodic and anodic processes in corrosive environments [18,31]. In general, the cathodic reaction is the oxygen consumption reaction, and the anodic reaction is the dissolution reaction of the metal. From Figure 1b, with the increase in DO content, the corrosion current of Monel 400 alloy in seawater increases significantly in the cathodic process, while the corrosion current of Monel 400 alloy in the anodic process decreases. This is because at a higher DO content, corrosion products are quickly formed on the surface of Monel 400 alloy, which can prevent the corrosive medium from further eroding the surface. In order to reveal the chemical composition of corrosion products, the XPS spectra of Monel 400 alloy under static corrosion condition are analyzed (Figure 7). First, the corrosion product of Monel 400 alloy in the air is characterized as a control group. In Ni 2p_3/2 spectra, three characteristic peaks are found. The peak located at 852.3 eV belongs to metal Ni, while the binding energy of Ni(OH)₂ and its satellite peak are 856.0 eV and 861.0 eV, respectively [32,33,34,35]. In Cu 2p_3/2 spectra, the only characteristic peak located at 932.2 eV is assigned to Cu/Cu₂O. Generally, the binding energy of metal Cu and Cu₂O is similar, and it is difficult to distinguish them [34,36]. In O 1s spectra, the peaks of O²⁻, OH⁻, and H₂O are observed. The binding energy is located at 531.2 eV, 531.6 eV, and 532.4 eV, respectively [20]. This indicates that Cu₂O exists in corrosion products on the surface of Monel 400 alloy in the air. When Monel 400 alloy is immersed in seawater, the species of Ni has no change, while the peak of CuO and Cu(OH)₂ appears and its binding energy is located at 933.0 eV and 935.0 eV [37,38]. Therefore, the corrosion products of Monel 400 alloy under static corrosion condition are composed of Ni(OH)₂, Cu₂O, CuO, and Cu(OH)₂. The cathodic and anodic reaction can be presented as follows:

The cathodic reaction is an oxygen consumption reaction:

$${2\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2+{4\mathrm{e}}^{-}\to {4\mathrm{O}\mathrm{H}}^{-}.$$

(6)

The anodic reaction is the dissolution reaction of metals:

$$\mathrm{N}\mathrm{i}+2{\mathrm{O}\mathrm{H}}^{-}-2{\mathrm{e}}^{-}\to {\mathrm{N}\mathrm{i}\left(\mathrm{O}\mathrm{H}\right)}_2,$$

(7)

$${\mathrm{C}\mathrm{u}}_2\mathrm{O}+{2\mathrm{O}\mathrm{H}}^{-}-{2\mathrm{e}}^{-}\to 2\mathrm{C}\mathrm{u}\mathrm{O}+{\mathrm{H}}_2\mathrm{O},$$

(8)

$$\mathrm{C}\mathrm{u}+{2\mathrm{O}\mathrm{H}}^{-}-2{\mathrm{e}}^{-}\to {\mathrm{C}\mathrm{u}\left(\mathrm{O}\mathrm{H}\right)}_2.$$

(9)

When Monel 400 alloy slides against the Al₂O₃ pin in seawater, the alloy will suffer serious tribo-corrosion, resulting in material loss. Generally, electrochemical corrosion and mechanical wear interact in a synergistic way; that is, corrosion will promote wear, and wear, in turn, will promote corrosion [39,40,41]. However, in this paper, the corrosion of the alloy significantly inhibits its wear, that is, ΔW_c < 0, resulting in significantly greater material loss and frictional coefficient after mechanical wear tests than that after tribo-corrosion tests (Figure 3 and Figure 4). Moreover, compared with tribo-corrosion, the spalling on the damaged surface is more obvious after cathodic protection tests, and the crack propagation on the subsurface surface is more significant (Figure 5 and Figure 6). In order to explore the mechanism of electrochemical corrosion inhibiting mechanical wear, the damaged surface of Monel 400 alloy after the tribo-corrosion test is observed under high-power electron microscopy, as shown in Figure 8. The results show that granular corrosion products are formed on the damaged surface under different DO contents, which is consistent with our previous study. The effects of load, rotational speed, applied potential, and temperature on the tribo-corrosion behavior of Monel 400 alloy in seawater were studied [42,43,44]. Corrosion products with good lubricating properties were found on the damaged surface. Analogously, the XPS spectra are used to analyze the chemical composition of the corrosion product after the tribo-corrosion test (Figure 9). At 2 ppm and 5 ppm, the corrosion product consists of Ni(OH)₂, CuO, and Cu(OH)₂. When the DO content increases to 8 ppm and 11 ppm, the chemical composition of corrosion product is Ni(OH)₂, Cu₂O, CuO, and Cu(OH)₂. After exploring Monel 400 alloy as a nickel–copper alloy, Ni-selective corrosion exists in seawater, which causes copper-rich particles to be raised on the damaged surface of the alloy. Under cyclic shear stress, these copper-rich particles, which have good viscoelasticity, finally gather together to form a layer of corrosion products with good lubrication properties. The shear stress during tribo-corrosion tests is greatly weakened, leading to the total material loss rate (T) and frictional coefficient (COF) being reduced. In order to observe the thickness of the corrosion products, the profile of damaged surface is observed using transmission electron microscopy (Figure 10). It is obvious that with the DO content increasing from 2 ppm to 11 ppm, the thickness of the corrosion product layer on the damaged surface increases from 12 nm to 36 nm. Compared with the DO content of 2 ppm, the thickness of corrosion product layer increases three times at 11 ppm. However, from Figure 3, when the DO content is 11 ppm, the total materials loss rate of Monel 400 alloy after tribo-corrosion tests is the highest, and the frictional coefficient is not the lowest. The temperature set in this test is 25 °C, and the saturation solubility of oxygen in seawater is 8 ppm at this temperature. When the DO content is 11 ppm, the entire corrosive environment is in a state of oxygen supersaturation. A large number of oxygen bubbles are generated in the solution, and the corrosion products on the damaged surface gradually break under the attack of the oxygen bubbles and finally peel off from the surface, as shown in Figure 8d and Figure 11. Therefore, when the DO content is 11 ppm, the material loss rate and frictional coefficient are higher. However, the corrosion of Monel 400 alloy never stops, which increases the thickness of the corrosion products on the damaged surface at 11 ppm. When the DO content is 8 ppm, the surface of the alloy is not only in an oxygen-rich state; also, a little oxygen bubble is formed, which causes the lower total material loss rate and frictional coefficient. Under cathodic protection conditions, the electrochemical corrosion of Monel 400 alloy is inhibited, so the DO content has little effect on the wear rate. This suggests that the DO content indirectly affects the wear of the alloy by affecting its corrosion.

5. Conclusions

In this work, the influence of different DO contents on the tribo-corrosion characteristics of Monel 400 alloy is researched, and the variation trend of various components of on tribo-corrosion process under different DO contents is revealed in detail through in situ electrochemical characterization and quantitative calculation. The specific results are as follows:

(1)

Under the same DO content, the material loss rates (T) after tribo-corrosion tests are significantly lower than those after mechanical wear tests, which is attributed to the corrosion products with excellent lubrication properties generated on the damaged surface of the alloy under tribo-corrosion conditions.

(2)

Under tribo-corrosion conditions, with the increase in DO content, although the shrouded area and thickness of the corrosion products on the surface of Monel 400 alloy increases, the wear rate of the alloy decreases first and then increases because the oxygen bubbles generated under DO content of 11 ppm (supersaturated DO) attack the corrosion product layer on the damaged surface of the alloy, causing the corrosion product layer to break and eventually fall from the damaged surface, resulting in material loss.

(3)

Because DO controls the corrosion kinetics of the alloy, it indirectly affects the wear of the alloy under tribo-corrosion conditions. If the corrosion is inhibited (cathodic protection test), the DO content has little effect on the wear of the alloy.