Wear 376-377 (2017) 1298–1306
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Micro abrasion-corrosion of ferritic stainless steels W.S. Labiapari a,b, M.A.N. Ardila a, H.L. Costa a, J.D.B. de Mello a,n a b
Universidade Federal de Uberlândia, Uberlândia, Brazil Aperam South America, Brazil
art ic l e i nf o
a b s t r a c t
Article history: Received 21 December 2016 Received in revised form 19 January 2017 Accepted 20 January 2017
The synergistic effect between abrasion and corrosion has been widely investigated and debated, where according to some works this synergy accentuates wear, and for others attenuates it. Despite the considerable effort to understand the synergy between abrasion and corrosion, little progress has been made to understand this phenomenon for ferritic stainless steels. This paper analyses the micro abrasioncorrosion performance of ferritic stainless steel with different chemical compositions (11%wt Cr with and without Ti stabilization; 16%wt Cr with and without Nb stabilization) and, for comparative purposes, austenitic stainless steel (18%wt Cr 8%wt Ni) and carbon steel (0.2%wt C). The specimens were tested for corrosion (turbulent and aerated environment), micro abrasion and micro abrasion-corrosion. For the corrosion tests, a 1 N H2SO4 solution was used, a 10%wt mixture of SiO2 in distilled water for the micro abrasion tests, and an abrasive-corrosive environment of 10%wt SiO2 in 1 N H2SO4 for the micro abrasioncorrosion tests. In the micro abrasive wear tests there was not a clear trend between the different materials tested, despite differences in their chemical composition, mechanical properties and microstructure. On the other hand, their performance was ruled by their chemical composition, in particular the Chromium content, under abrasion-corrosion conditions. This indicated the predominant role played by corrosion in abrasive-corrosive environments in this particular tribo-system. For all materials tested, micro abrasion wear coefficients were higher (4 ) than those measured under abrasion-corrosion conditions. Friction coefficients could also be measured by a 3D load cell strategically positioned in the specially developed micro abrasion-corrosion device, showing a strong reduction (2 ) in friction coefficient under abrasion-corrosion conditions when compared with solely abrasion conditions. This was attributed to the formation of a corrosion product, mainly constituted of iron sulphate (identified via FTIR), which has lubricant properties. & 2017 Elsevier B.V. All rights reserved.
Keywords: Micro abrasion-corrosion Synergy ferritic stainless steels Friction coefficient Ti and Nb stabilization
1. Introduction In various engineering applications, mechanical components are simultaneously subjected to a combination of mechanical wear and corrosion [1,2]. Due to their high corrosion resistance, stainless steels could be interesting candidates, despite their relatively low mechanical resistance when compared to other hard wearresistant materials. For example, in conditions of moderate corrosion in combination with mechanical wear, such as in the initial stages of sugar cane plants for ethanol production, a previous work proved an exceptional performance of inexpensive ferritic stainless steels at low cost-benefit ratios [3]. In systems where corrosion and mechanical wear occur simultaneously, phenomena such as plastic deformation influence mechanical failure by wear, but also lead to the removal of n
Corresponding author. Tel.: þ 55 34 3239 4409; fax: þ55 34 3239 4273. E-mail address:
[email protected] (J.D.B. de Mello).
http://dx.doi.org/10.1016/j.wear.2017.01.083 0043-1648/& 2017 Elsevier B.V. All rights reserved.
passivating layers present in the metal surface. Exposed metal surfaces may be highly reactive to the environment, which can accelerate corrosion [4]. Moreover, the presence of stresses changes the electrochemical response of mechanical components and structures [5]. Typically, corrosion and abrasion are investigated separately. However, attempts to include the influence of a corrosive environment on wear via a mathematical approach have incurred in great dispersion and considerable errors [6]. According to Wood et al. [7], these errors may be reduced for tests that impose the combined effect of corrosion and wear “in situ”. In this case, the mechano-electrochemical results dispense the mathematical approach for the synergistic effects. The combined effect between abrasion and corrosion, the so-called synergistic effect, has been widely debated in the literature. According to some authors, abrasion accentuates corrosion [2,6–11], whereas for others abrasion attenuates it [9,10]. However, even if considerable effort has been done to understand the synergy between abrasion and
W.S. Labiapari et al. / Wear 376-377 (2017) 1298–1306
corrosion, little progress has been made to quantify this phenomenon for stainless steels [6]. The corrosion resistance of stainless steels is attributed to the formation of a protective passivated layer and they are generally regarded as materials that are easily repassivated. The dynamics involving removal of the passive layer by mechanical action and repassivation plays an important role on the abrasion-corrosion resistance of stainless steels. The complex tribochemical mechanisms of stainless steels depend on the microstructure and chemical composition of the material surface, the solution pH, the abrasive (size, type and concentration), and the imposed electrochemical conditions [7]. Bello et al. [6] performed abrasion-corrosion tests for AISI 304, AISI 316 and duplex stainless steel using SiC (diameter ϕ ¼ 4.5 mm) abrasive in a 3.5%wt sodium chloride solution in distilled water. The abrasion corrosion tests showed higher passivation current densities than pure corrosion tests and this was attributed to the removal of the passive layer. Another point regarding abrasion corrosion tests of passivating materials such as stainless steels is that they usually show more dispersion of the electrochemical current than pure corrosion tests, which is partly attributed to competition between the removal of the passive layer and repassivation [7]. Several studies have measured abrasion-corrosion for biomaterials and various coatings [7,8,10], including CoCrMo alloys and duplex stainless steels, materials that are easily repassivated. Those works showed higher current densities under abrasioncorrosion conditions when compared with pure corrosion conditions, due to removal or damage of the passive layer. For carbon steels, current density increases with the applied normal load, showing the abrasive effect on the passive film [2]. For WC-metal composites tested using basic slurries, the metallic matrices that formed passive layers showed higher resistance under abrasion-corrosion conditions when compared with pure abrasion conditions, i.e., a negative synergy between abrasion and corrosion. On the other hand, for the matrices where corrosion resulted in the loss of matrix support for the carbides, resistance decreased under abrasion-corrosion conditions, showing a positive synergy between abrasion and corrosion [9]. Most studies about abrasion-corrosion resistance of stainless steels are mainly focused on more traditional materials, such as AISI 304 and AISI 316 stainless steels, and, more recently, on AISI 2205 duplex stainless steels. The investigation of cost effective ferritic stainless steels for those applications is often neglected. Despite this, they have found an important application niche in the biofuel industry [3], which certainly involves abrasion and corrosion. Within ferritic stainless steels, it is relevant to investigate the effect of Cr content on tribocorrosion, as well as the effect of stabilization. The cheapest stainless steel is 11Cr (DIN 14003) and it has vast application in the sugar-cane biofuel industry. 11CrTi (ASTM S40910) is also a low-cost ferritic stainless steel stabilized with Ti, largely used in automobile exhausts systems. 16Cr (ASTM S43000) steels are slightly more expensive ferritic stainless steel, mainly used in the cutlery industry, but they are still cheaper than austentic steels. 16CrNb (ASTM S43000) is the same stainless steel stabilized with Nb, used in cutlery and stamping. On the other hand, austenitic stainless steels, such as
1299
18Cr8Ni (AISI 304), present higher cost, but are extremely versatile in their use, with high corrosion resistance, good formability and weldability. This paper analyses the micro abrasion-corrosion performance of ferritic stainless steel with different chemical compositions:
11%wt Cr with (11CrTi) and without Ti (11Cr) stabilization; 16%wt Cr with (16CrNb) and without Nb (16Cr) stabilization; For comparative purposes, one austenitic stainless steels with 18%wt Cr and 8%wt Ni (18Cr8Ni) and one carbon steel with 0.2% wt C (ASTM A36). The specimens were characterized in terms of corrosion, microabrasion and micro-abrasion-corrosion. For all materials tested, friction coefficients were measured by a 3D load cell.
2. Methodology 2.1. Materials Four ferritic stainless steels were selected for this work and were compared with one austenitic stainless steel for its great versatility in terms of use. The ferritic stainless steels had different Cr contents and were tested both in stabilized (with Ti or Nb) and non-stabilized conditions. All the stainless steel specimens were used after industrial hot rolling and annealing (see hot rolling temperatures and annealing conditions in Table 1). The stainless steels were also compared with a commercial low carbon steel, ASTM A36, in the hot rolled condition, due to its low cost and large applicability in engineering applications. Their chemical composition, which was evaluated by different techniques [infrared absorption, (Leco, CS444s), thermo conductivity (Leco, TC436s), X-ray fluorescence spectrometry (Thermo ARL, 9900) and optical emission spectrometry (Thermo ARL, 4460)] is presented in Table 2. The specimens were thoroughly characterized in terms of mechanical properties [tensile tests (Instron, 5583) and Vickers hardness (Instron Wolpert, Testor 930s)] and microstructure, details can be found in [12]. Table 3 summarizes the main mechanical properties of the materials tested. For each material, samples were cut into coupons with dimensions of 35x25 5 mm3 and then sanded (grit sizes of 220 and 600). After this step, they were subjected to ultrasonic cleaning in acetone for 15 min and dried. All materials were tested for pure corrosion, pure microabrasion and microabrasion-corrosion. 2.2. Corrosion tests A standard 2 cm2 of active area for the corrosion and abrasioncorrosion tests was obtained using nail varnish and wax. A copper wire bonded by a silver glue established the electrical contact. To ensure the solidification and fixation of the wax on the specimen and consequently the reproducibility of the results, the entire preparation process occurred at least 8 h before each test. The electrolyte was a 1 N H2SO4 (F. Maia Ind. e Com. Ltda Brazil) solution in distilled water. A platinum gauze was the
Table 1 Industrial hot rolling and annealing conditions. Specimen
11Cr DIN 14003
11CrTi ASTM S40910
16Cr 16CrNb ASTM S43000
18Cr8Ni AISI 304
Final hot rolling temperature (°C) Annealing soaking temperature (°C) Annealing time
10007 50 7607 50 8h
950 7 50 950 7 30 30 s
1000 750 780 720 8h
11007 50 10407 20 30 s
950 7 50 930 7 20 30 s
1300
W.S. Labiapari et al. / Wear 376-377 (2017) 1298–1306 0.7
Table 2 Chemical composition of the specimens.
0.56 0.59
0.6
C (%wt) Cr (%wt) Ni (%wt) Nb (%wt) Ti (%wt) N (%wt)
11Cr
11CrTi
0.011 11.23 0.31 0.006 0.003 0.0145
16Cr
0.009 11.29 0.12 0.002 0.144 0.0087
16CrNb
0.049 16.10 0.27 0.014 0.003 0.0528
0.025 16.19 0.19 0.416 0.004 0.0202
18Cr8Ni 0.055 18.28 8.01 0.005 0.001 0.0421
0.60 0.51
0.53
A36
Friction Coefficient
Specimen
0.138 0.01 0.01 0.001 0.001 0.0026
0.51
0.5 0.4 0.3 0.2
0.1
Table 3 Mechanical properties of the materials tested.
0.0 18Cr8Ni
Specimen
Vickers hardness (MPa)
Yield stress (MPa)
Ultimate stress (MPa)
Elongation to fracture (%)
11Cr 11CrTi 16Cr 16CrNb 18Cr8Ni ASTM A36
14917 39 14717 29 1638 7 10 1510 710 19127 29 1402 7 10
323 7 6 3167 4 343 7 4 3367 3 3477 9 305 7 4
4127 2 4007 1 488 7 3 4477 2 7067 7 4457 3
38 71 40 72 317 2 36 72 62 72 34 71
A36
16CrNb
11CrTi
16Cr
11Cr
Materials Fig. 3. Average friction coefficients during the pure microabrasion tests.
7 6
100
Fraction Accumulated
90
Fractoin [%]
70 60
4
50 3
40 30
2
20
1
Accumulated [%]
80 5
10
2.3. Micro-abrasion tests
0
0.00 0.49 0.58 0.67 0.78 0.91 1.06 1.24 1.44 1.68 1.95 2.28 2.65 3.09 3.60 4.19 4.88 5.69 6.63 7.72 9.00 10.48 12.21
0
area, creating a saline bridge. The anodic polarization curve was obtained using an increasing potential rate of 1 mV/s, which gives a test duration of 42 min. ASTM G5-94 (2004) [13] recommends a rate of 0.167 mV/s, which gives a test duration of 4 h. The reduction of the test time and consequent increase in the potential variation rate was necessary because the subsequent micro-abrasion corrosion tests required a test time of around 40 min. Therefore, the use of this higher rate ensures the imposition of equivalent electrochemical conditions in both the corrosion and abrasion-corrosion tests. Moreover, the rate of 1 mV/s has already been used in other works [9,14–16] or even higher values (2 mV/s) as reported by Perret et al. [17]. For each condition tested, at least 3 repetitions were performed to ensure the reproducibility of the results and the same applies to the subsequent pure microabrasion and microabrasion-corrosion tests.
Granulometry [mm] Fig. 1. SiO2 particle size distribution analyzed by laser granulometry and abrasive morphology by SEM.
counter electrode and a saturated calomel electrode (SCE) was the reference electrode. The corrosion tests were carried out using a potentiostat/galvanostat model Biologic SP150. During the tests, the specimens were partially submerged in the solution. The same solution was then pumped at a rate of 1.7 ml/min over the exposed
The so-called pure abrasion tests were carried out in a specially designed hybrid equipment, which allows both pure microabrasion and micro-abrasion-corrosion tests to be carried out. The test rig consists of a fixed-ball micro-abrasion tester, where the specimen is supported by a three-axis load cell, enabling the measurement of forces and moments acting on the contact. This test rig is well described in a previous paper [18]. Abrasive slurries composed of silica (SiO2) particles (SigmaAldrich) suspended in distilled water, at a concentration of 10 g cm 3, were used. Distilled water can be considered as a neutral medium, leading to negligible corrosion of stainless steels,
2.0
1.0
Ip [mA/cm2].10-3
Potential [V] vs. SCE
1.5
10000
11Cr 11CrTi 16Cr 16CrNb 18Cr8Ni A36
0.5
0.0
3,030.0
3,158.7
11Cr
11CrTi
1000
100
35.7 19.8
11.5 10
-0.5 -1.0 1.E-05
1.E-03
1.E-01
1.E+01
Current Density [mA/cm2]
-a-
1.E+03
1 18Cr8Ni
16Cr
16CrNb
Materials
-b-
Fig. 2. Abrasion behaviour. a- Typical potentiodynamic polarization curves. b- Average passivation current density.
60
60
50
50
k [m3/N.m].10-15
k [m3/N.m].10-15
W.S. Labiapari et al. / Wear 376-377 (2017) 1298–1306
40 30 20
Test 1 Test 2 Test 3
10 0
0
3
6
9
12
15
18
21
24
27
30
1301
40
30
20
Test 1 Test 2 Test 3
10
33
0
0
3
6
9
12
15
18
21
24
27
30
33
Time [min]
Time [min]
-a-
-b-
Fig. 4. Examples of the evolution of the micro-abrasive wear coefficient k with time: (a) 11Cr, (b) 11CrTi.
20 18
17.3 13.6
k [m3/N.m].10-15
16 14
12.7 11.3
12
10.2 9.0
10 8 6 4 2 0 A36
11CrTi
11Cr
16CrNb
16Cr
18Cr8Ni
Materials Fig. 5. Average wear coefficients k.
so that the microabrasion tests carried out using this abrasive slurry were considered pure microabrasion tests [18]. The granulometric distribution of the abrasive particles, determined by laser granulometry (Malvern Instruments - Mastersizers), is shown in Fig. 1, where the abrasives have semi angular shape and an asymmetric normal size distribution, mean particle size of 3.4 70.2 mm, 85% of the particles with diameter between 1 and 10 mm. The counter body used was a zirconia ball (Só Esferas- Brazil) with diameter ϕ ¼ 25.4 mm. As reported by some authors [20] and quantified by Costa et al. [21], the ball roughness strongly influence the wear mechanisms and wear rates. In order to maintain this influence negligible, the arithmetic surface roughness (Ra) was controlled and kept in the range 0.32 mm o Ra o0.38 mm. The rotary speed was set to 150 rpm, and the normal load was set to 1.42 N. For each test, wear coefficients (k) were calculated as k¼ (πb4 /64R)/(s.N), where b is the diameter of the wear scar, R is the sphere radius, s is the sliding distance and N is the normal load, as proposed in [19]. The tests were interrupted at every 3 min to evaluate the evolution of k with time. After the tests, the spherical shape of the wear craters was verified by 3D laser interferometry, since this equation for k only applies to spherical craters. 2.4. Micro-abrasion-corrosion tests The microabrasion tests were carried out using the electrochemical cell present in the same hybrid equipment used for the pure abrasion tests. The same imposed parameters used for the separate abrasion and corrosion tests were applied simultaneously
in the abrasion–corrosion tests. For that, an electrolyte of 1 N H2SO4 instead of distilled water was used in the abrasive slurry at the same abrasive concentration (10%wt. SiO2 ϕ ¼ 4.5 mm). To minimize the effect of transients, the abrasion–corrosion tests were performed only after the abrasion process reached a steady state condition. For that, the tests started without applying any external potential difference during 15 min. The use of the corrosive slurry assured a pickling process that was able to remove the passive layer. This initial time of 15 min was established as the time necessary to reach a steady-state abrasion condition under pure microabrasion conditions, as it will be shown in the Results section. After that, a potentiodynamic corrosion test was triggered, so that a micro-abrasion-corrosion test could be carried out. The conditions for the potentiodynamic scan were identical to those used in the pure corrosion tests. In order to verify whether corrosion occurs mainly in the wear crater, after the abrasion-corrosion test, one sample (11Cr) was immersed in a solution containing potassium ferricyanide, which helps to evidence the corrosion products. In fact, the corrosion products flow from the wear crater, evidencing that corrosion and abrasion indeed occur in the same location [12]. After the tests, the surfaces were investigated by optical microscopy and scanning electron microscopy (SEM- Philips, XL-30s) to identify wear mechanisms.
3. Results and discussion 3.1. Corrosion tests Fig. 2 synthesizes the corrosion behaviour of the materials investigated in this work. In Fig. 2-a, typical potentiodynamic polarization curves for all studied steels are illustrated. Different repetitions for each material tested showed very stable and reproducible results, so that for clarity only one curve is presented for each material. Fig. 2-b presents the average value of the passivation current density for the stainless steels. The arrow in this figure indicates that materials with lower passivation current densities exhibit higher corrosion resistance. There is a strong influence of the Chromium content on the passivation current density and the shape of the passivation curves. The higher Cr content (18Cr8Ni steel) induced the lower passivation current whereas the higher passivation currents corresponded to the lower Cr content regardless of the stabilization by Ti (11CrTi and 11Cr). The effect of stabilization by Nb on current density was also negligible for the intermediate Cr content, 16%wt [22]. The low
1302
W.S. Labiapari et al. / Wear 376-377 (2017) 1298–1306
Fig. 6. SEM of wear craters after the pure abrasion tests: (a) A36 (b) 18Cr8Ni (c) 16Cr.
B
Removed Material
µm 1.2
0.8 0.6
83.5 µm
1.3 µm
0.4 0.2
µm 0.2
Height
1.0
Profile BB
0 -0.2
Removed Material
-0.4
-0.6 -0.8 0
0.0
10
20
30
40
50
60
70
80 µm
Length [µm]
B 35.2 µm
-a-
-b-
Fig. 7. Topographic analysis of the wear craters evidencing large portions of material removal, 18Cr8Ni: (a) 3D map and (b) 2D profile.
carbon steel (A36) did not exhibit passivation in this medium, as normally reported in the literature [23,24]. 3.2. Micro-abrasion tests Due to the use of the 3D load cell, friction coefficients could be obtained during the tests. Average friction coefficients were computed for each material (Fig. 3). No significant difference was detected between the friction coefficients presented by the different materials. Fig. 4 exemplifies the evolution of the microabrasion wear coefficient k with time for two of the materials tested. It shows that after a transient period of around 15 min, a steady-state regime is achieved. This initial transient period during microabrasion tests has been widely reported in many other works [21,25,26]. Similar behaviour was observed for all the materials tested.
Average wear coefficients were calculated for each material. To compute the average values, all points in the steady-state regime (after 15 min of test) for each repetition (3) were used, which gives 18 values for each material. Fig. 5 summarizes the average wear coefficients k obtained. The ferritic steels showed higher abrasion resistance than the low carbon steel A36. The best performance was presented by the austenitic stainless steel. Attempts were carried out to correlate wear coefficients with hardness and some microstructural characteristics, such as grain size, but no clear pattern was identified. Fig. 6 shows SEM images of some selected wear craters produced after the microabrasion tests. In all tests the prevailing mechanism was the sliding of the abrasive particles (grooving) [27]. It is noteworthy the detachment of large portions of material for the carbon steel (A36, Fig. 6-a) and for the austenitic stainless steel (18Cr8Ni, Fig. 6-b), indicated by arrows. Similar localized removal did not occur for the ferritic stainless steels (Fig. 6-c),
W.S. Labiapari et al. / Wear 376-377 (2017) 1298–1306 2.0
2.0
18Cr8Ni: Aerated - Turbulent 18Cr8Ni: Abrasion - Corrosion
1.0 0.5 0.0
16CrNb: Aerated - Turbulent 16CrNb: Abrasion - Corrosion
1.5
Potential [V] vs. SCE
1.5
Potential [V] vs. SCE
1303
1.0 0.5
0.0
-0.5
-0.5
-1.0 1.E-06
1.E-04
1.E-02
1.E+00
-1.0 1.E-06
1.E+02
1.E-04
Current Density [mA/cm2]
1.E-02
1.E+00
1.E+02
Current density [mA/cm2]
-a-
-b-
1.0
Abrasion Abrasion-Corrosion
Friction Coefficient
0.9
0.8 0.7 0.6 0.5 0.4 0.3 0.2
0.1 0.0
0
30
60
90
120
150
180
Time [s]
-c-
Fig. 8. Typical results for the microabrasion tests. (a) potentiodynamic curve, 18Cr8Ni, (b) potentiodynamic curve, 16CrNb. (c) Evolution of friction coefficient with test time, 18Cr8Ni.
0.8
90
0.7
63.5
Friction Coefficient
k [m3/N.m].10-15
80 70
81.3
Abrasion Abrasion-Corrosion
60
50.6 50
42.0 40
36.7
33.9
30 20
17.3
13.6
12.7
11.3
10
10.2
9.0
0.6
Abrasion Abrasion-Corrosion 0.59
0.53
0.52
0.60 0.55
0.51
0.5 0.33 0.4
0.28
0.31
0.26
0.3
0.34 0.27
0.2 0.1 0.0
0 A36
11CrTi
11Cr
16CrNb
16Cr
18Cr8Ni
A36
11CrTi
11Cr
16CrNb
Materials
Materials
-a-
-b-
16Cr
18Cr8Ni
Fig. 9. Evaluation of (a) wear rate coefficient k, and (b) friction coefficient in pure micro-abrasive and micro-abrasive-corrosive environment.
which showed lower wear coefficients than the carbon steel and the austenitic stainless steel. A 3D topographic assessment of the worn surfaces by laser interferometry (Fig. 7) confirms that the regions indicated by arrows correspond indeed to areas with intense material removal. 3.3. Micro-abrasion-corrosion test Fig. 8-a and Fig. 8-b shows some selected potentiodynamic curves obtained during the microabrasion-corrosion tests. It should be again pointed out that the time necessary to scan the
whole potential range investigated (42 min) was longer than the transient period in the pure microabrasion tests (15 min), so that the final portion of the test should occur after a steady-state wear regime was achieved. First, it was observed for all material an increase in the passivation current density when compared with the pure corrosion tests. The curves obtained for pure corrosion are again presented in this figure for comparison. In the literature [2,7,28], this increased passivation current density Ip observed in abrasion corrosion tests is attributed to a competition between of the removal of the passive layer and repassivation. In our tests, the current density showed a slight increase in the “passivation
1304
W.S. Labiapari et al. / Wear 376-377 (2017) 1298–1306
Fig. 10. FTIR analysis of 11Cr steel before and after immersion in 1 N H2SO4 solution.
region”, i.e. a less effective repassivation occurred. Fig. 8-c exemplifies, for the stainless steel, a comparison between friction coefficients obtained during pure abrasion and abrasion-corrosion tests. Under abrasion-corrosion conditions the friction coefficients obtained were substantially lower. Similar behaviour was observed for the other materials tested. Average wear coefficients were calculated for the microabrasion-corrosion tests using the same methodology described for the pure microabrasion tests. Fig. 9-a presents the values obtained, which for comparison also presents the values obtained under pure microabrasion conditions. The first strong fact evidenced by this figure is the substantially lower wear coefficient (3–8 lower) observed under microabrasion-corrosion conditions when compared with pure microabrasion conditions. A similar tendency was also observed using NaCl solutions by Bateni et al. [29] for AISI 304 stainless steels and AISI 1045 carbon steel and by Sinnett-Jones et al. [14] for AISI 304 stainless steels. This behaviour was associated to the change in the repassivation kinetics due to the presence of the corrosive media and to the external application of a potential difference. The comparative behaviour between the different materials was very different from that observed under pure abrasion conditions, where the austenitic stainless steels had presented the worst behaviour. Under abrasion-corrosion conditions, the increase in Chromium content resulted in a steady reduction of the wear coefficient. Higher Cr contents increase the stability of the passive film in most corrosive environments [22]. Under abrasioncorrosion conditions, the passive film stability seems to govern the behaviour of stainless steels [6,14]. The increase in Cr content could improve the film stability by either hindering depassivation
or accelerating repassivation. The exact mechanism still needs to be elucidated and should be a niche of further research. Fig. 9-b compares the average friction coefficients of each material measured during the pure abrasion and the micro-abrasion tests. No significant difference was observed between the different materials, which was confirmed by Analysis of Variance (ANOVA). This figure confirms the lower friction coefficients under abrasion-corrosion conditions when compared with pure microabrasion. When Wu et al. [30] measured the friction coefficient of AISI 304 stainless steel during sliding corrosion tests (fretting conditions) using H2SO4 solution, they found friction coefficients between 0.25 and 0.3, which is the same range found here under microabrasion-corrosion conditions. According to Kolesnichenko et al. [31], the formation of FeSO4 in tribocorrosive systems containing sulfuric acid is the main factor responsible for the reduction of the friction coefficient. Reciprocating sliding-corrosion tests of iron also showed a reduction of the friction coefficient when sliding in H2SO4 solutions when compared with pure sliding tests using water [32]. Rowson and Azouz [33] associated friction reduction to the formation of FeSO4 as an extreme pressure agent to the contact. We had tried, using FTIR, to detect the presence of FeSO4 in the wear scars. It was impossible to detect any variation by comparing the spectrum obtained outside and within the wear mark. In fact, the instrument indicated that the bands were very weak and therefore unreliable. The thickness of FeSO4 films formed on iron surfaces in the presence of H2SO2 was measured by Auger microscopy in the literature as around 3 nm [34]. Therefore, it is difficult to identify this film, but it induces significant smoothening of the surface. In presence of a H2SO4 solution as electrolyte, iron dissolution leads to the formation of FeSO4 on the metallic surface, as indicated by the reaction shown in Eq. (1).
Fe(s) + H2 SO4 (aq) → FeSO4 (aq) + H2 (g)
(1)
To demonstrate the formation of this film, a 11Cr steel sample was immersed in a 1 N H2SO4 solution at room temperature for 1 h. Then the sample was subjected to FTIR analysis, Fig. 10. The FTIR spectrum identified humidity bands in the region between 3570 and 2940 cm 1 and 1650 cm 1, and more importantly, it confirmed the formation of FeSO4 in the region between 1168 and 1068 cm 1. In order to verify the kinetics response of the effects of corrosion on friction coefficient, two further test sequences were carried out. In the first, the abrasion-corrosion test started using a slurry of abrasive and water, which was then changed to a slurry of
Fig. 11. Friction coefficients measured during microabrasion-corrosion tests of 18Cr8Ni:(a) started with water slurry and replaced by 1 N H2SO4 slurry; (b) started with 1 N H2SO4 slurry and replaced by water slurry.
W.S. Labiapari et al. / Wear 376-377 (2017) 1298–1306
1305
Fig. 12. Analysis by SEM of the surfaces of steel samples after testing 18Cr8Ni: (a) micro-abrasion-corrosion and (b) micro-abrasion test.
Microhardeness HV0.05 [MPa]
4000
Before test Abrasion Abrasion - Corrosion
3500 3000 2500
2145
2301
3255 2972
2810
2972
2527 2400 2363 2363 2143 2186 2186 2143
1971 1971 2000 1842 1842
1500
1000 500 0 A36
11CrTi
11Cr
16CrNb
16Cr
18Cr8Ni
Materials Fig. 13. Micro hardness Vickers HV0,05 at the samples of the material measured within the wear scar before and after micro-abrasion and micro-abrasion-corrosion test.
abrasive in 1 N H2SO4 solution. In the second, the abrasion-corrosion test started using a slurry of abrasive in 1N H2SO4 solution, which was then changed to a slurry of abrasive and water. The results are shown in Fig. 11. Both curves show a short interval between the two situations that corresponds to the time necessary to remove the test load and change the slurry. When H2SO4 is added to the slurry (Fig. 11-a), the film seems to form very promptly, reducing friction coefficient. In Fig. 11-b, abrasion removes the film, which is not restored in the absence of H2SO4, causing friction to increase. We believe that the FTIR technique does not present sufficient sensitivity and is therefore not suitable for the detection of FeSO4 in the wear marks produced by abrasion-corrosion. More sensitive techniques such as Auger Microscopy, XPS and TOF-SIMS are necessary and will be used in further investigations. Fig. 12 compares for the austenitic stainless steel the wear crater produced after abrasion-corrosion (Fig. 12-a) with that produced after pure abrasion (Fig. 12-b). This figure shows that abrasion-corrosion leads to the formation of a significantly smoother surface, which gives lower friction. On this way, it was observed the smoother surface, Fig. 12-a compared to Fig. 12-b, as a consequence of low friction coefficient. The reduction of friction coefficient under microabrasion corrosion conditions decreases the amount of mechanical energy that is dissipated in the active interface in the form of friction. Vickers microhardness measurements were carried out inside the wear
scars for all the materials tested under pure microabrasion and microabrasion-corrosion conditions. The results are shown in Fig. 13. First, all the samples showed an increase in hardness when compared with microhardness of the samples before the tests. Strain hardening is a common phenomenon during abrasion of metals and has been vastly reported in the literature [35]. However, strain hardening was less intense under abrasion-corrosion conditions than under pure abrasion conditions. Comparing pure microabrasion, which is mechanically dominated, with abrasioncorrosion, less energy is used in mechanical phenomena such as strain hardening. This agrees with the lower friction values measured under abrasion-corrosion than under pure abrasion, since abrasion-corrosion only needs frictional energy for the mechanical removal of passive film, but not for the tribocorrosion of the active areas.
4. Conclusions Abrasion-corrosion of stainless steels was assessed using fixedball microabrasion tests with 1 N H2SO4 solution slurry and simultaneous imposition of potentiodynamic curves. The results showed that: 1. The mechanical effects of turbulence and abrasion accelerate the corrosion process, mainly evidenced by increase in the passivation current density. 2. Although the austentic stainless steel stainless steel (18Cr8Ni), with the highest Cr content within the materials tested, showed the worst performance under pure abrasion conditions, it presented the best performance in the microabrasion-corrosion tests. Under abrasion-corrosion conditions, the tribological performance improved with the increase in Cr content. 3. The test rig allowed the measurement of friction coefficients during microabrasion tests. This allowed to detect a reduction in friction coefficient during abrasion-corrosion conditions when compared with pure abrasion tests. Friction reduction was attributed to the presence of FeSO4 as a corrosion product, which was confirmed by FTIR analysis. 4. Microhardness Vickers measurements inside the wear scars showed a less intense strain hardening under abrasion corrosion conditions than under pure abrasion conditions. This is probably due to the reduction in the energy dissipated as friction in the contact and therefore available for plastic deformation.
1306
W.S. Labiapari et al. / Wear 376-377 (2017) 1298–1306
Acknowledgements The authors would like to thank Capes/Proex, CNPq (Gant 477286/2011-9), CBMM S.A. (Grant FEMEC01-2011) and Fapemig for financial support.
References [1] M.M. Stack, K. Chi, Mapping sliding wear of steels in aqueous conditions, Wear 255 (2003) 456–465. [2] M.M. Stack, H. Jawan, M.T. Mathew, On the construction of micro-abrasion maps for a steel/polymer couple in corrosive environments, Tribol. Int. 38 (2005) 848–856. [3] W.S. Labiapari, C.M. Alcântara, H.L. Costa, J.D.B. de Mello, Stainless steel as an antiwear material for the bio-fuel industry, Wear 302 (2013) 1536–1545. [4] D. Landolt, S. Mischler, M. Stemp, S. Barril, Third body effects and material fluxes in tribocorrosion systems involving a sliding contact, Wear 256 (2004) 517–524. [5] V. Vignal, N. Mary, P. Ponthiaux, F. Wenger, Influence of friction on the local mechanical and electrochemical behaviour of duplex stainless steels, Wear 261 (2006) 947–953. [6] J.O. Bello, R.J.K. Wood, J.A. Wharton, Synergistic effects of micro-abrasion– corrosion of UNS S30403, S31603 and S32760 stainless steels, Wear 263 (2007) 149–159. [7] R.J.K. Wood, D. Sun, M.R. Thakare, A. de Frutos, Rozas, J.A. Wharton, Interpretation of electrochemical measurements made during micro-scale abrasion-corrosion, Tribol. Int. 43 (2010) 1218–1227. [8] D. Sun, J.A. Wharton, R.J.K. Wood, Micro-abrasion mechanisms of cast CoCrMo in simulated body fluids, Wear 267 (2009) 1845–1855. [9] M.R. Thakare, J.A. Wharton, R.J.K. Wood, C. Menger, Exposure effects of alkaline drilling fluid on the microscale abrasion–corrosion of WC-based hardmetals, Wear 263 (2007) 125–136. [10] M.R. Thakare, J.A. Wharton, R.J.K. Wood, C. Menger, Investigation of microscale abrasion–corrosion of WC-based sintered hardmetal and sprayed coating using in situ electrochemical current-noise measurements, Wear 267 (2009) 1967–1977. [11] A.J. Gant, M.G. Gee, A.T. May, The evaluation of tribo-corrosion synergy for WC–Co hardmetals in low stress abrasion, Wear 256 (2004) 500–516. [12] W.S. Labiapari, Abrasion-Corrosion of Ferritic Stainless Steels (Doctoral Thesis), Universidade Federal de Uberlandia 2015, p. 186, p, Uberlandia, MG, Brazil (in Portuguese). [13] ASTM, Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements, G5-94, 2004. [14] P.E. Sinnett-Jones, J.A. Wharton, R.J.K. Wood, Micro-abrasion–corrosion of a CoCrMo alloy in simulated artificial hip joint environments, Wear 259 (2005) 898–909. [15] I. Garcı ́a, D. Drees, J.P. Celis, Corrosion-wear of passivating materials in sliding contacts based on a concept of active wear track area, Wear 249 (2001)
452–460. [16] H.B. Lee, D.S. Wuu, C.Y. Lee, C.S. Lin, Wear and immersion corrosion of Ni–P electrodeposit in NaCl solution, Tribol. Int. 43 (2010) 235–244. [17] J. Perret, E. Boehm-Courjault, M. Cantoni, S. Mischler, A. Beaudouin, W. Chitty, J.P. Vernot, EBSD, SEM and FIB characterisation of subsurface deformation during tribocorrosion of stainless steel in sulphuric acid, Wear 269 (2010) 383–393. [18] M.B. Santos, W.S. Labiapari, M.A.N. Ardila, W.M. da Silva Jr, J.D.B. de Mello, Abrasion–corrosion: new insights from force measurements, Wear 332–333 (2015) 1206–1214. [19] D.N. Allsopp, R.I. Trezona, I.M. Hutchings, The effects of ball surface condition in the micro-scale abrasive wear test, Tribol. Lett. 5 (1998) 259–264. [20] P.H. Shipway, The role of test conditions on the microabrasive wear behaviour of soda-lime glass, Wear 233 (1999) 191–199. [21] H.L. Costa, M.A.N. Ardila, W.S. Labiapari, W.M. Silva, J.D.B. de Mello, Effect of surface topography on the dynamics of the abrasive particles during microabrasion, Wear 324–325 (2015) 129–139. [22] A.J. Sedriks, Corrosion of Stainless Steels, Wiley-Interscience Publication, New York, 1996. [23] Y.F. Cheng, J. Bullerwell, F.R. Steward, Electrochemical investigation of the corrosion behavior of chromium-modified carbon steels in water, Electrochim. Acta 48 (2003) 1521–1530. [24] J.A.L. Dobbelaar, E.C.M. Herman, J.H.W. de Wit, The corrosion behaviour of iron-chromium alloys in 0.5 M sulphuric acid, Corros. Sci. 33 (1992) 765–778. [25] W.M. da Silva, R. Binder, J.D.B. de Mello, Abrasive wear of steam-treated sintered iron, Wear 258 (2005) 166–177. [26] J. D. B de Mello, A. A. Polycarpou, Abrasive wear mechanisms of multi-components ferrous alloys abraded by soft, fine abrasive particles, Wear, 269, 2010, 911-920. [27] R.I. Trezona, D.N. Allsopp, I.M. Hutchings, Transitions between two-body and three-body abrasive wear: influence of test conditions in the microscale abrasive wear test, Wear 225 (1999) 205–214. [28] F. Ferrer, H. Idrissi, H. Mazille, P. Fleischmann, P. Labeeuw, A study of abrasion– corrosion of AISI 304L austenitic stainless steel in saline solution using acoustic emission technique, NDT E Int. 33 (2000) 363–371. [29] M.R. Bateni, J.A. Szpunar, X. Wang, D.Y. Li, The effect of wear and corrosion on internal crystalline texture of carbon steel and stainless steel, Wear 259 (2005) 400–404. [30] P.-Q. Wu, J.P. Celis, Electrochemical noise measurements on stainless steel during corrosion–wear in sliding contacts, Wear 256 (2004) 480–490. [31] L.F. Kolesnichenko, P.V. Trushko, Formation of boundary layers during friction in the presence of sulfur, Sov. Powder Metall. Met. Ceram. 9 (1970) 993–997. [32] K. Miyoshi, Studies of mechanochemical interactions in the tribological behavior of materials, Surf. Coat. Technol. 43 (1990) 799–812. [33] D.M.Rowson, A.Azouz, An investigation of extreme pressure films formed during wear test, Vacuum 31 (1981) 593–596. [34] S. Mischler, E. Rosset, G.W. Stachowiak, D. Landolt, Effect of sulphuric acid concentration on the rate of tribocorrosion of iron, Wear 167 (1993) 101–108. [35] K.L. Zum Gahr, Microstructure and Wear of Materials, Tribology Series 10, Elsevier, Amsterdam, 560p, 1987.
