Chen Jun*ab
aSchool of Materials Science and Engineering, Henan University of Science and Technology, 263, Kaiyuan Road, Luoyang 471023, Henan 471003, PR China. E-mail: chenjun318822200@163.com; Tel: +86 15225508091
bCollaborative Innovation Center of Nonferrous Metals of Henan Province, Luoyang 471023, PR China
First published on 3rd May 2017
The corrosion wear behaviors of TC4, 316 stainless steel, and Monel K500 in artificial seawater were systematically investigated in this study. The contribution of corrosion and wear on corrosion-wear synergism and damage-recovery process of passive film for the three alloys were addressed. It can be concluded that wear loss for these three alloys at cathodic polarization was lower than that at anodic polarization and OCP. The passive film was damaged due to mechanical wear, resulting in low OCP and high corrosion rate. Electrochemical test showed that the passive film in the wear track area was only partially destroyed during the corrosion wear test. The ratio of the actual damage area in the wear track Se/Sm of 316 stainless steel and TC4 was much larger than that of Monel K500.
Generally, stainless steel, Ti alloy, Ni alloy, Al alloy, and CoCrMo alloy have been frequently used for corrosion wear research due to their excellent corrosion resistance and good mechanical properties.7,18–23 Excellent corrosion resistance of these alloys was provided by spontaneous formation of oxide film on the metallic surface.16,17,24 Marine tribology has been developed in the recent years due to the increasing exploitation of ocean resources. The metal material, high polymer material, and nonmetal material are widely used in marine engineering equipment. A large number of metal materials are used in ships, oil gathering, offshore wind power generation devices, deep-sea drilling, underwater robots, underwater equipment, and other marine equipment. Ceramic materials have been used in marine lubricating ceramic bearings and friction pairs as well. Because seawater is corrosive, the resistance of marine equipment, especially the friction pairs, to corrosion and corrosive wear should be considered. In the present study, the selected alloys were TC4, 316 stainless steel, and Monel K500. Corrosion wear behaviors of these three alloys including corrosion-wear synergism, abrasion and electrochemical control mechanisms, and corrosion wear maps have been studied by many researchers. Various electrochemical methods mainly including open circuit potential, potentiodynamic polarization, polarization, and impedance were normally performed to provide essential information on corrosion evaluation. However, the damage-recovery process and destruction proportion of the passive film have not been fully explored. In the present study, the electrochemical and corrosion wear behaviors of TC4, 316 stainless steel, and Monel K500 were investigated. Specifically, the contribution of corrosion and wear to synergism and the destruction degree of the passive film were analyzed in detail.
The metallic materials used in the present study are TC4, 316 stainless steel, and Monel K500. Their chemical compositions are listed in Table 1. The ring samples of metallic materials were machined into a size with an outer diameter of 54 mm and inner diameter of 38 mm. Before carrying out the corrosion wear test, each metallic specimen was mechanically abraded using a series of SiC grinding papers up to 1500 grit, followed by cleaning in running water and acetone. Before being immersed into the electrochemical cell, all the surfaces except the upper surface of the ring were covered with paint for insulation. The area in contact with artificial seawater was about 11.5 cm2. A rotating alumina pin with a diameter of 4.7 mm was matched with a stationary metallic specimen. The wear track was a ring with a mean diameter of 46 mm and a width of 4.7 mm (diameter of the Al2O3 pin). The area of the wear track was 8 cm2. The corrosive solution in this study was artificial seawater, which was prepared according to the ASTM D1141-98 standard. In this study, the temperature of artificial seawater during experiment was maintained at 25 °C. The corrosion wear tests were carried out at the rotation speed of 200 rpm, which means the linear velocity was 0.54 m s−1. A normal load of 100 N was applied, corresponding to a nominal contact stress of about 1.5 MPa. This normal load was selected to generate a contact stress much smaller than the yield strength of the metallic materials to avoid plastic deformation. Sliding tests were conducted for 60 min. After corrosion wear, ring specimens were ultrasonically cleaned with acetone. The gravimetric measurements were carried out using a TE214S-OCE analytical balance. Then, the volume loss of the metallic specimens could be obtained. The morphology of the worn surfaces was obtained using a JEM-5600LV scanning electron microscope (SEM). All the tests were repeated at least three times to check for reproducibility.
TC4 | Monel K500 | 316 stainless steel | |||
---|---|---|---|---|---|
Al | 6.25 | Cu | 30 | Cr | 17.2 |
V | 4.21 | Al | 3 | Ni | 10.2 |
H | 0.0073 | Ti | 0.6 | Mo | 2.1 |
O | 0.19 | Fe | 1 | C | 0.04 |
Fe | 0.22 | C | 0.1 | S | ≤0.03 |
Ti | Balance | Ni | Balance | Si | ≤1 |
Mn | ≤2 | ||||
Fe | Balance |
Several electrochemical measurements were conducted to investigate the effect of wear on corrosion. (1) Open circuit potential (OCP) variation curves were monitored before, during, and after corrosion wear. (2) Potentiodynamic polarization measurements, which involve measurement of the polarization curves of the metallic specimens under corrosion-only and corrosion wear conditions, were performed after attaining a stable OCP. The potential was swept from −1 V to +1 V at the sweep rate of 1.67 mV s−1. CHI software was used to analyze the polarization data. (3) During potentiostatic polarization measurement, a constant potential E was imposed on the metallic specimen. The selected potential was maintained at a fixed value, and the current was measured as a function of time to follow the evolution of corrosion kinetics. (4) For simulating the damage and restoration process of the passive film and evaluating the destruction proportion of the passive film, potential pulse method (PPM) was applied. A constant cathodic potential E was imposed on the metallic specimen to eliminate the passive film. Then, the potential was increased to a given value within 1 ms. With the change of potential, the current rapidly increased and then exponentially decreased. This electrochemical variation was considered to simulate the exposure and recovery of the passive film. An example of the change in potential and current is shown in Fig. 2.
Fig. 2 An example of the change in potential and current of PPM method: (a) variation of potential and (b) variation of current. |
The evolutions of OCP for TC4, 316 stainless steel, and Monel K500 are shown in Fig. 3. It can be observed that at the start of the corrosion wear, OCP sharply dropped to a more negative value. The similar cathodic shift phenomenon during the corrosion wear has been observed in other passive metallic materials at different corrosion wear systems.10,26,27 Passive oxide film on the metal surface is mechanically destroyed or partially removed by sliding.25,28 Active surface can increase the anodic reaction, and the released electrons cathodically polarize the surrounding surface. Therefore, OCP shifts towards the cathodic direction. A relative steady state is reached in potential owing to the establishment of an equilibrium level between mechanical depassivation and electrochemical repassivation rates. When friction stops, OCP starts to abruptly increase (anodic shift). This indicates the re-establishment of the passive state in the worn area.25
The polarization curves of TC4, 316 stainless steel, and Monel K500 under both corrosion-only and corrosion wear conditions are shown Fig. 4. The aim of corrosion-only test was to investigate the chemical stability of the passive film without any mechanical damage. Typical passivation phenomenon can be clearly observed for the three alloys. Polarization curves during corrosion wear can analyze corrosion and repassivating ability when the passive film is mechanically destroyed. However, the weak point of this method is the unsteady state of the electrochemical test system, which results in significant oscillations of the current data. Moreover, the contact positions of the pin-ring system continuously change, which contributes to the oscillations and instabilities of the polarization curves.29 The polarization curves of the three alloys during corrosion wear shown in Fig. 4 exhibit clear current oscillations. As the contact area of the friction couple is constant, the calculation of the corrosion current density is based on the area of the wear track. It can be seen that friction obviously affects the shape and position of the potentiodynamic polarization curves with respect to the corrosion-only reference curves. Table 2 presents the values of the corrosion potential E, corrosion current density it, ic, and the ratio of it/ic. The corrosion current during corrosion wear is much higher than that under the corrosion-only condition. This indicates that a rapid dissolution occurs in the worn surface. The it/ic value was calculated to explain the effect of wear on corrosion. The it/ic values indicated that the effect of wear on corrosion for TC4 was more significant than that for 316 stainless steel and Monel K500. In addition, Monel K500 exhibited a much lower corrosion current compared to that of 316 stainless steel and TC4. Low wear-accelerated-corrosion effect occurred for Monel K500, which may be the reason for its low OCP value, as shown in Fig. 3.26
Fig. 4 The polarization curves under corrosion-only and corrosion wear conditions: (a) TC4; (b) 316 stainless steel; and (c) Monel K500. |
Materials | Corrosion-only | Tribocorrosion | it/ic | ||
---|---|---|---|---|---|
ic (μA cm−2) | E (V) | it (μA cm−2) | E (V) | ||
TC4 | 1.86 | −0.497 | 317 | −0.627 | 170.4 |
316 stainless steel | 10.89 | −0.625 | 192 | −0.712 | 17.6 |
Monel K500 | 2.44 | −0.396 | 38.4 | −0.562 | 15.7 |
In the potentiostatic polarization test, the applied cathodic potentials were −0.9 V, −0.8 V, and −0.6 V for TC4, 316 stainless steel, and Monel K500, respectively, and the applied anodic potentials were 0.2 V, −0.2 V, and 0.1 V for TC4, 316 stainless steel, and Monel K500, respectively. The anodic potential was selected to maintain the alloys in a passive state.2,26,30
Fig. 5 exhibits the evolution of the current density under cathodic and anodic polarization conditions during corrosion wear. It can be observed that the measured currents were cathodic (negative) before, during, and after sliding under cathodic polarization conditions for the three alloys, confirming that no corrosion occurred. Clearly, under this cathodic condition, material loss was only caused by pure mechanical wear.10,26,27 At anodic potential, the current sharply increased at the beginning of sliding. The current curves exhibit significant oscillations, demonstrating the unsteady state of the electrochemical test system. TC4 and 316 stainless steel exhibit large corrosion currents as compared to Monel K500 under anodic conditions. Mechanical wear leads to thinning or local removal of the passive film, which subsequently repairs itself by oxidation of the metals. As a consequence, corrosion wear leads to a sharp increase of anodic current. The cyclic abrasion maintains the current at relatively high values. After corrosion wear stops, the current decreases again to a lower value. It proves that the current change is indeed caused by mechanical wear.
Fig. 5 The evolution of the current density under cathodic and anodic polarization conditions: (a) TC4; (b) 316 stainless steel; and (c) Monel K500. |
The evolution of friction coefficient with time is shown in Fig. 7 for the three alloys. Friction coefficient rapidly reached a steady state, exhibiting significant peaks at fairly regular time intervals. These fluctuations are attributed to the formation and ejection of wear debris as well as the uneven wear surface.35 Friction coefficient was apparently large at cathodic polarization. When corrosion takes place, this leads to an increase in the surface roughness and, consequently, a reduction in the contact area.31 Moreover, 316 stainless steel exhibits a large friction coefficient compared with TC4 and Monel K500.
Fig. 7 The evolution of COF at different electrochemical conditions: (a) TC4; (b) 316 stainless steel; and (c) Monel K500. |
Kwc = Kc + Kw = Kwo + ΔKw + Kco + ΔKc | (1) |
The results of various contributions to total material loss are given in Table 3. Note that the contribution of Kco to total material loss for the three alloys is very small, indicating the negligible effect of pure corrosion. Overall, pure mechanical wear and synergistic effect between wear and corrosion are dominating factors. Fig. 8 shows the specific values of Kc/Kw, ΔKc/Kc, and ΔKw/Kw. It can be noticed that wear (Kw) is dominant for the three alloys, and the value of Kc/Kw is high under anodic conditions compared with that under OCP conditions. The proportion of Kc in wear loss for Monel K500 is higher than that for 316 stainless steel and TC4. Obviously, wear loss caused by corrosion is almost equivalent to the wear-induced corrosion. The values of ΔKw/Kw are in the range of 20–40% for the three alloys.
Potential | Materials | Kwc (mm3) | Kw (mm3) | Kc (mm3) | Kwo (mm3) | ΔKw (mm3) | Kco (mm3) | ΔKc (mm3) |
---|---|---|---|---|---|---|---|---|
OCP | 316 | 185 | 183.3 | 1.7 | 134.8 | 48.5 | 0.0049 | 1.7 |
TC4 | 68.7 | 66.5 | 2.23 | 43.8 | 22.7 | 0.031 | 2.2 | |
Monel K500 | 2.18 | 1.93 | 0.25 | 1.56 | 0.37 | 0.0025 | 0.25 | |
Anodic potential | 316 | 216 | 210.8 | 5.2 | 134.8 | 76 | 0.017 | 5.18 |
TC4 | 77.4 | 72.5 | 4.9 | 43.8 | 28.7 | 0.023 | 4.87 | |
Monel K500 | 3.42 | 2.76 | 0.66 | 1.56 | 1.2 | 0.0079 | 0.65 |
Wear-induced corrosion and corrosion-induced wear play a dominant role in material loss. Fig. 9 shows the fraction of ΔKc, ΔKw, and Kwo for the three alloys. Note that relative contribution of pure wear to total material loss is in the range of 45–72%. Moreover, the contribution of pure wear to total wear loss is high under OCP compared with that under anodic conditions, indicating that corrosion plays a great role in the material loss especially at high potential. Importantly, the contributions of wear-induced corrosion and corrosion-induced wear are very important, especially for Monel K500 at anodic potential. Note that although the electrochemical dissolution is significantly promoted and corrosion rate is highly increased due to sliding, the contribution of ΔKc to total material loss is not very large. Actually, it is in the range of 0.92–6.29% for 316 stainless steel and TC4. Moreover, it is not more than 20% for Monel K500. On the contrary, the ratio of corrosion-induced wear to total wear loss is very large, and it is in the range of 16.9–35.2%. The results obtained in this present study show that the ratio of wear-accelerated corrosion to the total wear loss is small, and pure mechanical and corrosion-induced wear are dominant factors during corrosion wear.
Fig. 10 shows the typical images for the three alloys. It can be seen that these alloys exhibit different wear mechanisms. The worn surfaces of TC4 and Monel K500 are characterized by mainly flat regions interrupted by furrows parallel to the sliding direction. The worn surfaces of 316 stainless steel are dominated by sharp ridges and thin flakes.29,30 Moreover, the electrochemical state may affect the deterioration mechanism and surface morphology. At the cathodic potential, the wear track is generally very smooth and featureless, except for some isolated furrows for TC4 and Monel K500 and some ridges and thin flakes for 316 stainless steel. At OCP and anodic potential, the worn surfaces exhibit typical deformation features. These images also show some wear debris.
Fig. 10 The typical images of the wear surface: (a) TC4; (b) 316 stainless steel; and (c) Monel K500. |
Fig. 11 The evolution of current during PPM test: (a) TC4; (b) 316 stainless steel; and (c) Monel K500. |
The variation extent of the corrosion current is very large, as shown in Fig. 11, and this causes much difficulty in the quantitative analysis. Therefore, the concept of the electricity value Q and average current density Ipm was introduced to evaluate the passivation process. The electricity charge Q can be defined as follows:
(2) |
(3) |
For a special corrosion wear process, the passive film in a wear scan area may not be completely damaged. Therefore, the actual breakdown area of the passive film Se can be derived by the following formula.
(4) |
The results of various corrosion parameters using the PPM approach are given in Table 4. It can be observed that the average current density Ipm is obviously higher than the in situ measured corrosion current density I, indicating that passive film in the wear track is only partially destroyed in this experiment. Considering the actual damage area of the passive film Se, the three alloys exhibited different trends. The ratio of the actual damage area Se/Sm for 316 stainless steel and TC4 was very large. This was 66.7% and 71.9% at OCP and 88.1% and 87.1% at anodic polarization. However, the values of Se/Sm for Monel K500 were relatively small. It was only 13.8% at OCP and 21.9% at anodic polarization. It indicates that the passive film of Monel K500 exhibits high resistance to sliding damage compared with 316 stainless steel and TC4. More importantly, the electrochemical state had a significant influence on the destruction proportion of the passive film during corrosion wear, and the actual damage area of the passive film Se was high under the anodic conditions than that under the OCP conditions for the three alloys.
Alloy | Electrochemical condition | Ipm (mA cm−2) | I (mA cm−2) | Sm (cm2) | Se (cm2) | Se/Sm |
---|---|---|---|---|---|---|
TC4 | OCP | 4.75 | 3.17 | 8 | 5.34 | 66.7% |
Anodic | 6.99 | 6.15 | 8 | 7.04 | 88.1% | |
316 | OCP | 2.67 | 1.92 | 8 | 5.75 | 71.9% |
Anodic | 6.18 | 5.38 | 8 | 6.96 | 87.1% | |
Monel K500 | OCP | 1.88 | 0.26 | 8 | 1.11 | 13.8% |
Anodic | 2.69 | 0.59 | 8 | 1.75 | 21.9% |
(1) The cathodic shift of OCP was confirmed, and the current density was obviously increased during corrosion wear for the three alloys.
(2) The wear loss for the three alloys at cathodic potential was lower than that at OCP and anodic potential. The wear loss of 316 stainless steel was much larger than that measured for TC4 and Monel K500. Friction coefficients were apparently large in cathodic polarization compared with those in anodic polarization.
(3) The contribution of pure corrosion to total material loss for three alloys was very small. Considering the synergy between wear and corrosion, the ratio of wear-accelerated corrosion to the total wear loss was small. Pure wear and corrosion-induced wear were dominant during corrosion wear.
(4) The passive film in the wear track was only partially destroyed in this experiment. The ratios of the actual damage area Se/Sm for 316 stainless steel and TC4 were much larger than that for Monel K500. The actual damage area of the passive film Se was high under anodic conditions compared with that under OCP conditions for the three alloys.
This journal is © The Royal Society of Chemistry 2017 |