Effect of halide concentration on tribocorrosion behaviour of 304SS in artificial seawater

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Effect of halide concentration on tribocorrosion behaviour of 304SS in artificial seawater Yue Zhang a,b , Xiangyu Yin a , Fengyuan Yan a,∗ a b

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China

a r t i c l e

i n f o

Article history: Received 23 April 2015 Received in revised form 1 July 2015 Accepted 28 July 2015 Available online xxx Keywords: A. Stainless steel C. Alkaline corrosion C. Interfaces C. Pitting corrosion

a b s t r a c t Recently, increasing efforts have been made to study the tribocorrosion mechanism and influencing factors, and yet little relevant to the influence of electrolyte characteristics. In this work, the dependence of tribocorrosion on halide ions concentration in seawater is taken into consideration, and it follows that halide can improve solution lubricity and reduce friction coefficient and wear rate remarkably. However, high halide ions concentration may increase the corrosion susceptibility of 304SS, especially pitting susceptibility. Although over the entire range of halide ions concentration corrosion and wear interact positively, high halide ions concentration is beneficial for decreasing total material loss. © 2015 Published by Elsevier Ltd.

1. Introduction Austenitic stainless steels are used in a wide variety of industries for their exceptional general corrosion resistance, good weldability and fabricability. However, the steels are vulnerable to a number of localized corrosion. For example, when these steels are operating in halide ions containing solutions, they will suffer from pitting corrosion. In the past decades, a great deal of attention has been paid to investigating the mechanism of pitting corrosion and the conditions affecting pitting nucleation and growth [1–9]. This is due not only to their greater aggressiveness, but also to the wider distribution of halide ions in nature, being constituent of seawater, brackish water, de-icing salts, and airborne salts, that is, to greater chance of their presence in aggressive media, including many technically employed situations. It is well reported that under static condition, the susceptibility of austenitic stainless steels to pitting corrosion depends radically on the environmental parameters [10–20], as well as the chemical composition and metallurgical characteristics of the steels [21–23]. One of the most critical environmental parameters is the aggressive ions concentration, because it accounts for more of the pitting corrosion resistance of steels. Till now, great progress has been achieved in studying the pitting tendency of stainless steels by employing two popular methods: one is carried out under droplets with various aggressive

∗ Corresponding author. E-mail address: [email protected] (F. Yan).

ions concentrations accomplished through controlling the relative humidity in the atmosphere to determine the critical halide ions concentration, especially chloride ions concentration (cCl− ). It had been found that the critical cCl− for the initiation of pitting corrosion for 304 stainless steel is around 6 mol L−1 at room temperature [18,24–27]. The other method is performed in excessive solution with a certain aggressive ions concentration and uses electrochemical techniques to confirm the pitting potential (Epit ) under this aggressive condition. It is often reported that the Epit value has a direct relationship with cCl− [11,28–32]. In general, the higher the chloride ions concentration, the lower the Epit . And, different logarithmic formulas are typically for establishing the relations between Epit and cCl− as follows [11,33,34]: Epit = A − BlogcCl−

(1)

where A and B are constant, and both values vary from metal to metal. Recently, special attention has been put on the subject that the role of cCl− plays in tribocorrosion systems, because tribocorrosion phenomena are also encountered in many halide containing environments, such as body fluid, cooling water and seawater, where they often cause damage to installations, machines and devices, thus resulting in greater economic loss [35–42]. Consequently, this subject is of great interest in a wide range of practical situations, and the ability to understand and predict the nature and intensity of such a process is highly expected and important. In the case of austenitic stainless steels, as observed by Abd El-Kader and ElRaghy, the dissolution rate of 304 stainless steel at constant solution

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pH was independent of the chloride concentration ranging from 1% to 10% [35]. However, according to the data obtained by Yahagi and Mizutani, although the corrosion rate decreased gradually as the NaCl concentration increases from 0.1 wt.% to around 25 wt.%, the corrosive wear rate of 316 stainless steel reached a maximum in about 0.1 wt.% NaCl solution [43]. Later, Hong and Pyun claimed that the corrosive wear rate for 304 stainless steel increased linearly with increasing chloride ion concentration from 0 to 23.4 g L–l below the critical cCl− . They attributed this increase of corrosive wear rate to the decreased current efficiency for the formation of a passivating oxide film caused by the presence of chloride ions [37]. Unfortunately, the effects of cCl− on tribocorrosion behavior have been studied for many years, yet crucial phenomena remain unclear, because it is of system dependence. In this study, we investigate the tribocorrosion behavior of 304SS in seawater with different halide ions concentrations (cA− ). The importance of seawater as an inherently chemically aggressive environment has been increasing for the last few decades due to the greatly developed offshore and deep-sea engineering. Under static corrosion, mass loss of austenitic stainless steels as structural materials fully immersed in seawater is generally quite low, but these alloys are vulnerable to pitting attack for the presence of halide ions (Cl– , Br– , F– ) in such an environment. It should not be ignored that when the structural materials experience sand erosion, impingement attack, scratching, etc., these mechanical actions will interact with corrosion and have a major impact on material loss and pitting corrosion tendency of passive metals and alloys [44,45]. Although the actual application of 304SS in seawater is constrained due to its poor localized corrosion resistance, as a typical and widely-used representative of passive metals, the tribocorrosion behavior of 304SS was studied by a number of scientists in simulated seawater [46–49], because a deep understanding on tribocorrosion behavior and mechanism of 304SS must have guiding significance for the optimization and for the protection of other passive metals used in seawater. So, in this study we also select 304SS as the research subject to reveal the effect of halide ions concentration on the tribocorrosion behavior of 304SS in seawater. We hope this work will be helpful to understand and predict the nature and intensity of material degradation during sliding wear in seawater, as well as to select the optimal protection for structural materials and to promote the development of sea economy in the future. 2. Experiment 2.1. Material The samples in ring form (thickness: 80 mm, outer diameter: 54 mm, inner diameter: 38 mm) studied in this work were made of 304 austenitic stainless steel (304SS) with a composition in mass contents percent of 18.4Cr, 8.1 Ni, 0.81 Mn, 0.06C, 0.47 Si, 0.011 S, 0.02 P and balance Fe. The ring samples of 304SS were annealed at 1050 ◦ C for an hour before tribocorrosion experiments to relieve internal stresses. Prior to tribocorrosion tests, each sample was ground with SiC grinding papers (from grade 600 to 1500 grit), degreased and cleaned ultrasonically, and dried with N2 . Before immersed into tribocorrosion cell, all surfaces except the uppersurface were sealed with insulating glue. 2.2. Solution preparation Analytical reagents and distilled water (0.74 × 10−3 S m−1 ) were used to prepare the seawater with different halide ions concentrations (cA− ) ranging from 0 to 1.04 mol L−1 according to ASTM D1141-98. The main halide ions (A– ) contained in seawater were chloride ion (Cl– ), fluorine ion (F– ) and bromine ion (Br– ), while

other anion groups still existed, such as sulfate ion and bicarbonate ion. Besides, sulfate ion and bicarbonate ion existed in all test solutions by dissolving a fixed amount of Na2 SO4 and NaHCO3 in distilled water. Except Na2 SO4 and NaHCO3 , no halide existed in halide ions free solution (0 mol L−1 ). In halide–containing solutions (0.10, 0.49, 0.77, and 1.04 mol L−1 ), the concentrations of fluorine ion (0.071 × 10−3 mol L−1 ) and bromine ion (0.779 × 10−3 mol L−1 ) were constant, and the total halide ions concentration was adjusted by controlling the weight of added NaCl. It should be noted that pH value of all solutions was a constant (pH 8.2) adjusted by 0.1 mol L−1 NaOH prior use. The conductivity of each solution was measured using a conductivity meter (WTW ProfiLine Cond 197i, Germany). The conductivity and the concentrations of main compound in five corrosive solutions are listed in Table 1. 2.3. Tribocorrosion tests MMW-1 sliding pin-on-disc tribometer was used in this work, where the tribological contact between Al2 O3 pin and 304SS specimen was totally immersed in the test electrolyte. The specimen was connected to an electrochemical workstation, and its schematic view of components was the same as already described in the previous publication [42]. During each test, a rotating Al2 O3 pin ( 4 mm) slid against a stationary 304SS specimen with fixed tribological parameters of 50 N, 0.24 m s−1 (100 R min−1 ) for 1 h. The torque (T, N mm) was measured by an attached strain gauge and recorded by computer acquisition system. Then the friction coefficient (, dimensionless ratio) can be calculated by the following equation: =

T rN

(2)

where N is the load, Newtons. r is the sliding radius, mm. The mass loss of 304SS was obtained by weighing the sample before and after each test. In addition, cathodic protection technology was performed by applying cathodic potential of –0.9 V versus Ag/AgCl reference electrode to eliminate electrochemical corrosion during rubbing, and therefore it is possible to assess separately the role of corrosion and wear in the total degradation of material, and to evaluate the synergy between them. Specially, mass loss due to both corrosion and wear was normalized to wear track area (that is, eroding area, 5.78 cm2 ). In order to investigate the effect of cA− on the corrosion behavior of 304SS in the absence and presence of rubbing process (also known as static corrosion and tribocorrosion process), a typically three-electrode component electrochemical cell was adopted: an Ag/AgCl (3.5 M KCl filled) electrode served as reference, cylindrical platinum gauge as auxiliary electrode, and 304SS ring as working electrode. Firstly, the evolution of open circuit potential (OCP) was monitored before (20 min), during (60 min) and after (20 min) rubbing process to ascertain the effect of rubbing on corrosion resistance of 304SS in different seawater. Besides, potentiodynamic polarization tests were carried out to study the corrosion behavior of 304SS during rubbing in different seawater at a scanning rate of 2 mV s−1 from –250 mV to +1000 mV with respect to OCP. The electrochemical parameters, such as corrosion potential (Ecorr ), pitting potential (Epit ), corrosion current density (icorr ), anodic Tafel slopes (ˇa ), and cathodic Tafel slopes (ˇc ) were obtained by the Tafel extrapolation method and are listed in Table 2. Unless stated, all potentials in this paper are referred to the Ag/AgCl reference electrode. The corrosion rate under static corrosion (C0 , mm y−1 ) and tribocorrosion (C, mm y−1 ) conditions were calculated according to ASTM G 102-89. All the tests in this work were conducted at room temperature (21 ± 1 ◦ C) and open to the air. In addition, all tests were performed a minimum of three times under the identical conditions, and the average results were reported in this paper.

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Table 1 Conductivity and main chemical composition of seawater. cA− (mol L−1 )

Conductivity(S m−1 )

0 0.10 0.49 0.77 1.04

0.57 1.81 5.15 7.23 8.91

Compound concentration (mol L−1 ) NaCl

MgCl2

CaCl2

KCl

NaF

KBr

Na2 SO4

NaHCO3

0 0.028 0.419 0.692 0.965

0 0.055 0.055 0.055 0.055

0 0.010 0.010 0.010 0.010

0 0.009 0.009 0.009 0.009

0 0.071 × 10−3 0.071 × 10−3 0.071 × 10−3 0.071 × 10−3

0 0.779 × 10−3 0.779 × 10−3 0.779 × 10−3 0.779 × 10−3

0.029 0.029 0.029 0.029 0.029

0.002 0.002 0.002 0.002 0.002

Table 2 Electrochemical parameters for 304SS in the electrolyte solutions containing different halide ions concentrations ranging from 0 to 1.04 mol L−1 . cA – (mol L−1 )

Ecorr /VAg/AgCl Epit /VAg/AgCl icorr (␮A cm−2) ˇc (mV dec−1 ) ˇa (mV dec−1 )

Static corrosion

Tribocorrosion

0

0.10

0.49

0.77

1.04

0

0.10

0.49

0.77

1.04

−0.042 – 0.77 −38.96 47.29

−0.149 +0.501 1.01 −36.59 41.79

−0.099 +0.434 0.85 −44.13 40.78

+0.031 +0.402 0.71 −40.68 41.04

+0.097 +0.375 0.67 −35.90 44.84

−0.586 – 150.62 −56.85 53.50

−0.644 – 179.43 −58.04 56.72

−0.661 −0.029 226.21 −61.26 53.67

−0.693 −0.066 293.67 −59.37 55.22

−0.705 −0.110 531.02 −61.32 53.56

2.4. Characterization

The corrosion augmentation factor is:

After tribocorrosion tests, the worn surface, subsurface, and wear debris morphologies of 304SS samples were examined by scanning electron microscope (SEM, JEOL 5600, Japan). Of note is that before SEM examination, no pretreatment is necessary for worn surface and wear debris. For subsurface morphologies examination, however, etching is requisite with duration of 15 s in etchant consisting of 12 mL H2 O, 3 mL HCl and 1 g FeCl3 . 2.5. Calculation The mass loss rate and the synergistic effect between corrosion and wear in different electrolytes were calculated according to ASTM G119-09. Based on the corrosion rate with friction (C, mm y−1 ) and total mass loss (T, mm y−1 ), the wear rate with corrosion (W, mm y−1 ) can be calculated by the following equation: W =T −C

(3) mm y−1 )

Thereupon, the wear increment (Wc , and the corrosion increment (Cw , mm y−1 ) can be deduced as follows: WC = W − W0

(4)

CW = C − C0

(5)

(mm y−1 )

where W0 is the pure mechanical wear rate obtained by applying of cathodic protection technique, and C0 (mm y−1 ) is pure corrosion rate obtained under static corrosion. Thence, the synergistic effect between mechanical and electrochemical material loss can be described by Eqs. (6) and (7): S = T − W0 − C0

(6)

S = WC + CW

(7)

Besides, three dimensionless factors, namely, the total synergism factor, corrosion augmentation factor, and wear augmentation factor, are used to depict the synergism degree of corrosion and wear. The total synergism factor is: T T −S

(8)

The wear augmentation factor is: W0 + WC W0

(9)

C0 + CW C0

(10)

3. Results and discussion 3.1. Effect of cA− on friction and wear Friction coefficient is a very important characteristic for understanding the nature of rubbing process between 304SS and Al2 O3 in solutions with different halide ions concentrations (0–1.04 mol L−1 ), because the value of this friction coefficient depends both on the nature and surface particularities of the sample and its antagonist, and on the lubricating property of solutions. It can be seen from Fig. 1(a) that the occurrence of corrosion will increase friction coefficient between 304SS/Al2 O3 tribocouples under each testing condition. This increase is because of its especial susceptibility to changes in the surface state of 304SS caused by corrosion in different solutions as we described in the earlier article [50]. Besides, high cA− solution exhibits good lubricity under both pure mechanical wear and tribocorrosion conditions, evidenced by the decreased friction coefficient with increasing cA− in seawater. Additionally, total mass loss is another essential measure of friction and wear. Fig. 1(b) shows a similar changing trend: the occurrence of corrosion significantly increases the material loss of 304SS, and the lower the cA− in seawater, the larger the increment. Besides, the total mass loss of 304SS, whether caused by pure mechanical wear or tribocorrosion, decreases gradually with increasing cA− , which should also be attributed to the good lubricity of high cA− seawater. 3.2. Effect of cA− on electrochemical corrosion In general, conventional electrochemical techniques were adopted to study the depassivation and repassivation behavior of metals and alloys caused by mechanical abrasion or scratching [51–56] in various solutions. In this work, open circuit potential (OCP) and potentiodynamic polarization tests were carried out to extract relevant electrochemical parameters, and then to determine the damage and regeneration of protective passive film on 304SS surface caused by Al2 O3 rubbing in halide–containing seawater. Fig. 2 shows that under static corrosion when the 304SS specimens are immersed in different seawater, regardless of in

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Fig. 1. Friction coefficient (a) and total mass loss (b) curves for 304SS in seawater with different halide ions concentrations ranging from 0 to 1.04 mol L−1 . Mechanical conditions: 50 N, 0.24 m s−1 . Test duration: 1 h.

halide-free seawater, OCP shifts to more and more positive direction as the cA− increases. When halide-free seawater is taken into account, however, the corrosion susceptibility of 304SS is in the middle among all solutions, that is, in low cA− (0.10 and 0.49 mol L−1 ) seawater the corrosion tendency of 304SS increases, compared with that in halide–free seawater, however high cA− (0.77 and 1.04 mol L−1 ) reverses this tendency. A sharp decrease in OCP is detected once the periodic movement of rubbing starts. At this point, the corrosion susceptibility, which relates closely to the integrity of protective passive film on 304SS surface [57,58], increases gradually with increasing cA− in seawater, demonstrating that rubbing by Al2 O3 makes the protective passive film rupture or even removal, and leaves the fresh metal surface with higher electrochemical activity exposure to the electrolyte. Consequently, it is more vulnerable to corrosion for 304SS under rubbing condition due to the establishment of galvanic cell between mechanically depassivated surface (anode) and the surrounding passive surface (cathode) as described previously by Lucas [59]. Besides, the higher the cA− in seawater, the greater the corrosion susceptibility for 304SS. It is worth noting that when wear and corrosion are involved simultaneously, more pronounced potential fluctuations are displayed. We attribute the fluctuations to the periodic removal–recovery of passive film on 304SS during the continual sliding–over processes. And for every potential fluctuation, the value of the highest and lowest points is determined by the maximum level of passive film removal and recovery, respectively. Once the rubbing stops, OCP moves to positive direction immediately, proclaiming a “recovery” of the damaged passive film on 304SS. Clearly, longer period of recovery is requisite after severe damage, evidenced by lower OCP than those for relatively minor damage surface in high cA− seawater after the same recovery time. In order to further determine the corrosion behavior of 304SS in different electrolytes, potentiodynamic polarization curves were obtained in the presence and absence of tribological contact, and an essential different role was played by halide between the course of the polarization of 304SS under static conditions and under tribocorrosion as shown in Fig. 3. The polarization curves of 304SS obtained under static corrosion are shown in Fig. 3(a). A consistent result is got on the corrosion susceptibility for 304SS to different cA− in alkaline seawater. In addition to affecting the corrosion thermodynamics, the halide concentration in seawater also has a significant effect on the dissolution kinetics of 304SS as shown in Table 2. It is obvious that with increasing cA− , the corrosion current density (icorr ) has the same changing tendency with the corrosion potential. In the previous work [60–63], the dissolution kinetics of iron in alkaline chloride solution was interpreted in accordance with the Bockris mechanism with the additional consideration of the formation of an iron chloride intermediate complex. In the case of stainless steel, the present results can also be interpreted on the same basis, and the anodic reaction in terms of Bockris mechanism is composed of the dissolution of the alloying elements (M Fe, Cr, Ni · · ·) to produce Fe2+ , Cr3+ and Ni2+ ions as follows [11,34]: M + OH− → M(OH) + e−

(11)

M(OH) → M(OH)n−1 + (n − 1)e−

(12)

M(OH)

Fig. 2. Variations in OCP for 304SS before (20 min), during (60 min) and after (20 min) rubbing by Al2 O3 with fixed tribological parameters of 50 N, 0.24 m s−1 in the electrolyte solutions containing different halide ions concentrations ranging from 0 to 1.04 mol L−1 .

n−1

n+

→M

−

+ OH

(13)

Simultaneously, the aggressive anions, especially Cl– ions, are attached and migrate to active surface, and some even react with metal cations to form Fe2+ , Cr3+ and Ni2+ chlorides at the anode areas or pits, destroying passivity and preventing its recurrence as follows: Mn+ + Cl− + OH− → MOCln−3 + H+

(14)

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0.10 to 1.04 mol L−1 . By linear regression, the following relation is established between Epit and logcA− : Epit = 0.3865 − 0.1183logcA−

Fig. 3. Potentiodynamic polarization curves for 304SS obtained (a) under static corrosion and (b) under tribocorrosion in the electrolyte solutions containing different halide ions concentrations ranging from 0 to 1.04 mol L−1 .

Thus, the over-all anode reaction is: M + Cl− + OH− → MOCln−3 + H+ + ne−

(15)

This reaction is balanced by the cathodic reduction of adsorbed oxygen (Eq. (16)). O2 + 2H2 O + 4e− → 4OH−

(16)

Hence, the small number of halide ions (0.10 and 0.49 mol L−1 ) in electrolyte will promote the anode reaction, and therefore increase the corrosion rate of 304SS in such solutions. However, oxygen solubility in water decreases continuously with sodium chloride concentration, and excessive halide ions (0.77 and 1.04 mol L−1 ) should make themselves more prevalent in the competing adsorption between halide ions and adsorbed oxygen on 304SS [64]. As a result of this competing adsorption, the cathodic reduction of adsorbed oxygen is impeded, which in turn decreases the anodic dissolution of alloying elements constituting 304SS. Judging from the data obtained, it is obvious that the level of halide ions concentrations remarkably influences the characteristics of 304SS dissolution. Besides, it also plays an important role in stimulating pitting corrosion of 304SS. In pH 8.2 seawater, the Epit is found to be a function of halide ions concentration ranging from

(17)

where Epit is the pitting potential, V versus Ag/AgCl reference electrode, and cA− is the halide ions concentration, mol L−1 . It has been noted in a number of works that the initiation of pitting is a process, where Cl– ions constantly adsorb and accumulate on the passive surface of alloys by displacing adsorbed oxygen, until a sufficient concentration corresponding to the critical potential, Cl– ions succeed at favored sites in stimulating pitting corrosion of 304SS by nucleation of more metastable pits [39,65,66]. Besides, during the formation of metal chloride complex, pH of the electrolyte very near the active surface drops due to the formation of H+ . This causes further dissolution of 304SS, and facilitates the formation of stable pits. Consequently, as shown in Table 2, with increasing halide ions concentration, the pitting potential shifts to more active (or cathodic) direction, demonstrating a decreasing resistance to pitting corrosion in high halide-containing electrolyte. As is the case under static corrosion, the tribo-electrochemical corrosion of 304SS also appears to be halide ions concentration dependent. However, there is an essential difference between the course of the polarization of 304SS under static corrosion and under tribocorrosion. It is clear in Fig. 3(b) that with increasing halide ions concentration, both Ecorr and Epit shift to more cathodic direction, while icorr increases gradually, which is consistent with the literature reported [67]. As mentioned above, during tribocorrosion the continual rubbing-over processes must rupture or even remove the protective passive film and make the fresh surface exposed to aggressive electrolyte with higher electrochemical activity. As a result, the galvanic corrosion will occur because of the potential difference between the passive film (cathode) and the fresh surface (anode). Moreover, some experiments have also shown that the O2 reduction rate is higher on bare metal electrode than on oxide-covered surface [68,69]. The above two points account well for the sharp increase in icorr during tribocorrosion compared with those obtained under static corrosion conditions. Based on the values of Tafel slope listed in Table 2, when compared with those obtained under static corrosion conditions both the anodic Tafel slopes (ˇa ) and the absolute value of cathodic Tafel slopes (|ˇc |) increase obviously due to the periodic rubbing actions between 304SS and Al2 O3 pin, especially for |ˇc |. This illustrates that the friction-accelerated corrosion is mainly accomplished by promoting the cathodic reduction of oxygen, which in turn drives the anodic dissolution of alloying elements and consequently increases the corrosion rate of 304SS. It is noteworthy that, during tribocorrosion process pitting corrosion also occurs in halide-containing seawater once the polarization potential is higher than the pitting potential of 304SS under each halide ions concentration condition. Although it is difficult to distinguish the pitting potential for 304SS when polarized in 0.10 mol L−1 halide ions solution, pitting do happen as proven by the corrosion pits found within wear track (Fig. S1). The electrochemically measured Epit during tribocorrosion is found to be a linear function of the logarithm of halide ions concentration ranging from 0.49 to 1.04 mol L−1 . This linear dependence of Epit on log cA− can be represented as below. Epit = −0.1013 − 0.2431logcA−

(18)

Compared with Eq. (17), both the slope and intercept of Eq. (18) reduce evidently. That is, in the same electrolyte, friction facilitates the occurrence of local corrosion and makes the pitting sensitivity increase. More importantly, there is an essential difference between the course of pitting formation of 304SS under static corrosion and under tribocorrosion as shown in Fig. 3. It is clear that a larger scale of applied anodic potentials relative to Ecorr is needed to form stable pitting under tribocorrosion than under

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Fig. 4. The morphologies of (a,b) worn surface, (c,d) wear debris, and (e,f) subsurface obtained after tribocorrosion in 0 and 1.04 mol L−1 cA− seawater, respectively. The sliding direction of Al2 O3 across 304SS surface is from left to right.

static corrosion. At the anodic potentials below Epit metastable pits are discerned by the fluctuations of current, pointing to the nucleation and subsequent death (or repassivation) events. Similar phenomenon was observed during erosion–corrosion of 304 L in 0.6 M NaCl solution, and prof. Burstein argued that this phenomenon arose both from exposure of fresh active sites during erosion as well as by altering their geometry [39,70]. In our case, the periodic rubbing process is responsible to the formation of metastable pits by destroying the geometry of the nucleation sites so that metal cations concentration at the nucleation sites is not dense enough to stimulate the further growth of metastable pits. Additionally, with increasing cA− the nucleation-death characteristic of metastable pits becomes more dramatic, and the average value of fluctuated current is also found to be proportional to the halide ions concentration which should be interpreted by the

negative influence of halide ions on instantaneous repassivation of the damaged areas of 304SS [71]. There is every indication that high cA− electrolyte increases the pitting tendency for 304SS by increasing the number of nucleation sites. 3.3. Effect of cA− on the morphologies of worn surface and wear debris To the end it will be useful to compare the SEM morphologies of worn surface, subsurface and wear debris after tribocorrosion tests in 0 mol L−1 (Fig. 4a,c,e) cA− seawater with the morphologies after tribocorrosion tests in 1.04 mol L−1 (Fig. 4b,d,f) cA− seawater. It is clear in Fig. 4(a) and (b) that the worn surface is coarser after tribocorrosion in halide-free solution due to the poor lubricity of this solution, and giving rise to the relatively large coefficient of

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Fig. 5. Synergetic contributions of mechanical wear and corrosion to each other and to total mass loss of 304SS in the electrolyte solutions containing different halide ions concentrations ranging from ranging from 0 to 1.04 mol L−1 .

friction between 304SS and alumina pin in such solution (Fig. 1a). Besides, many residual margins after laminar tearing (indicated by the yellow arrow) and grooves (indicated by the blue arrow) appear on the worn surfaces after tribocorrosion tests. And after rubbing in 1.04 mol L−1 cA− seawater for 1 h, the residual margins are more significant in sizes (except thickness) than those after testing in halide-free solution, which is in accordance with the sizes of flaky wear debris detached from worn surface as shown in Fig. 4(c) and (d), respectively. Besides, the length of wear debris in Fig. 4(c) is more mixed in contrast with that shown in Fig. 4(d). In order to shed light on the wear mechanism, the subsurface morphology below wear track of 304SS were also inspected. Clearly, the cyclicrubbing-induced plastic shear deformation and micro-cracks with different lengths can be found in Fig. 4(e) and (f), and some cracks even shear to the surface. Correlating the subsurface morphology with the feathers of worn surface and wear debris, it suggests that under these two kinds of lubrication conditions delamination is the main wear mechanism. However, it should be noted that under different lubrication conditions, varying degrees of delamination are displaying, since good lubricity of solution makes the contact stress between 304SS sample and harder Al2 O3 pin much reduced under the same mechanical conditions. In this case, the shear deformation below 304SS surface is slighter, and relatively small amounts of voids form and join neighboring ones around surface hard particles to nucleate cracks. After per cycle of rubbing by Al2 O3 pin, a shorter distance of crack propagation (wear rate determining step) is accomplished than that rubbing under bad lubricity condition [72,73]. Therefore, further rubbing and deformation often impel small flaky wear

debris or only one tip of crack where stress is concentrated to shear to the surface first and cocks up from the surface. This cocking-up tip acting as asperity is removed to generate smaller wear debris by the following rubbing by Al2 O3 pin (Fig. 4d). By comparison, when the periodic rubbing proceeds in poor lubricity solution (halide ions free solution), the contact stress is much higher so that many cracks nucleate and propagate fast below 304SS surface, and eventually break through to the surface to form large wear sheets (Fig. 4c and e). Also of note is that under both lubrication conditions, wear debris is of different sizes, while grooves appear on the worn surface. This may perhaps be that after the wear debris are created during the above delamination wear or abrasion of asperities generated during deformation and delamination, some of them may be entrapped between the two rubbing surfaces and then be broken into small pieces or remain the same, leaving the grooves on the soft 304SS surface. 3.4. Synergy between corrosion and wear In order to ascertain the synergistic effect between corrosion and wear in different cA− solutions, calculation was performed, and the results are shown in Fig. 5. It is no doubt that all three synergism factors are bigger than 1 (Fig. 5a), proclaiming that corrosion and wear interact positively to bring about accelerated corrosion rate, wear rate, and total material degradation for 304SS. Besides, the acceleration of corrosion is more obvious than the acceleration of wear. In combination with the corrosion rate obtained under static and tribocorrosion conditions, it is shown that the corroding of 304SS accompanying with friction becomes

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more seriously in high halide ions concentration solution. For an electrode at steady-state potential, Icorr = Ia = −Ic

(19)

where Icorr is corrosion current, A. Ia is the anodic current of the metal, A, and Ic is the cathodic current, A. Considering the anodic ia and cathodic ic current densities (A cm−2 ), one equation can write for the case of tribocorrosion experiments: ia Aa = ic Ac ⇔ ia /ic = Ac Aa

(20)

where Aa and Ac correspond to the mechanically depassivated area and the passive area, respectively, cm2 . Hence, the absolute value of anodic-to-cathodic current density ratio (–ia /ic ) in the galvanic cell is determined by the cathode-to-anode area ratio (Ac /Aa ). Under good lubrication condition, the wear rate of 304SS is low, so the area of mechanically depassivated surface is small, and therefore a bigger Ac /Aa value is obtained. As a result, the absolute value of ia /ic and subsequent corrosion rate presents an upward trend as the cA− increases (Fig. 5b). Moreover, corrosion-accelerated wear rate, as well as pure wear rate, decreases greatly, even though the corrosion rate increases with increasing cA− (Fig. 5c). As discussed above, corrosion can affect the friction and wear behavior mainly by changing the contact surface characteristics between 304SS and Al2 O3 pin. However, good lubricity of solution can smooth the tribological contact and weaken the influence of corrosion on friction and wear. Hence, declined wear augmentation factor and wear rate are displayed as the cA− increases. In addition, seen from the augmentation factors, although the accelerated extent of wear to corrosion is much greater as depicted in Fig. 5(a), for the selection of severe tribological parameters, the changes of total synergism factor and the total material loss rate are remarkably consistent with those of wear, demonstrating that the problem of material loss is caused basically by pure mechanical wear and corrosion-accelerated wear under tribocorrosion in seawater. 4. Conclusions In this paper, the effect of halide ions concentration on tribocorrosion behavior for 304SS/Al2 O3 tribocouples in seawater was studied on a pin–on–disc tribometer. Within the entire studied ranges of halide ions concentration (0–1.04 mol L−1 ), corrosion and wear are claimed to accelerate each other. Moreover, high cA− solution increases susceptibility of 304SS to general and local corrosion, resulting in accelerated corrosion rate under both static corrosion and tribocorrosion conditions. And yet, high cA− solution also performs good lubricity, and under this lubrication condition, the rubbing between 304SS and Al2 O3 becomes smooth. As a result, friction-induced plastic shear deformation and delamination wear are mitigated. Besides, good lubricity of solution may weaken the influence of deteriorated contact surface properties caused by corrosion on friction and wear behavior. That is, this solution slows down the corrosion-accelerated wear. Hence, with increasing cA− in seawater, pure mechanical wear as well as corrosion-accelerated wear decreases distinctly. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 51405478), and National Basic Research Program of China (973 Program, Grant No. 2014CB643302). The authors also gratefully thank the reviewers for their detailed, rigorous and helpful comments.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.corsci.2015.07. 017

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