Mechanics of Materials
Material Removal Characteristics of Martensitic Stainless Steel in Acetic Acid Aqueous Solutions and in Acetic Acid Ethanolic Solutions
2022 Volume 63 Issue 7 Pages 1028-1036
Details
2022 Volume 63 Issue 7 Pages 1028-1036
To evaluate the effect of acetic acid on material removal from stainless steel, sliding tests were performed. Martensitic stainless steel pins were unidirectionally slid against smooth and rough alumina (Al2O3) disks in water, in ethanol, in acetic acid aqueous solutions and in acetic acid ethanolic solutions. Morphological and chemical analyses of the worn pin and disk surfaces were then performed. The addition of acetic acid to water and ethanol resulted in reduction of adhesion of stainless steel to Al2O3, inhibition of the formation of metal oxides, and increased wear of the stainless steel. The surface roughness of the mating Al2O3 disk affected the wear mechanism of the stainless steel pin. The solvents and total concentration of acetic acid in the solution affected electrochemical actions such as anodic dissolution and galvanic corrosion.
Fig. 3 Wear volume of 440C stainless steel pins slid against (a) smooth Al2O3 disks and (b) rough Al2O3 disks.
Stainless steels have been used for various applications because they have excellent corrosion resistance, high strength, and high toughness.1) The purpose of this study was to improve the efficiency of material removal processing of stainless steel with the aid of environmentally friendly working fluids. We previously investigated the wear properties of martensitic stainless steel SUS440C in ethanol (C2H5OH) with and without acetic acid (CH3COOH).2) The wear volume of stainless steel in ethanol containing acetic acid was larger than that in ethanol alone.2) The anodic dissolution caused by acetic acid promoted the wear of the stainless steel pin.2) On the sliding surface of stainless steel in ethanol, metal oxides were formed.2) Anodic dissolution inhibited the formation of metal oxides in ethanol containing acetic acid.2) Metal oxides that form on machined surfaces damage the machining tools.3) Alumina (Al2O3) cutting tools and abrasive grains have been used for the removal machining of stainless steels because Al2O3 hardly reacts with iron.3) On the other hand, iron oxides and chromium oxides react with Al2O3, which promotes the wear of Al2O3 cutting tools and abrasive grains of grinding wheels.3) Therefore, the anodic dissolution caused by acetic acid can be expected to assist in material removal from stainless steel by mechanical processing.
Many researchers have investigated the anodic dissolution of ferrous materials in solutions containing acetic acid.4–8) Acetic acid can dissociate into acetate anions (CH3COO−) and protons (H+) in polar protic solvents such as water and ethanol [Scheme (1)].9)
| \begin{equation*} \text{CH$_{3}$COOH}\leftrightarrows \text{CH$_{3}$COO$^{-}$} + \text{H$^{+}$}\qquad \text{Scheme (1)} \end{equation*} |
| \begin{equation*} \text{Fe}\to \text{Fe$^{2+}$} + \text{2e$^{-}$}\qquad \text{Scheme (2)} \end{equation*} |
| \begin{equation*} \text{Cr}\to \text{Cr$^{2+}$} + \text{2e$^{-}$}\qquad \text{Scheme (3)} \end{equation*} |
| \begin{equation*} \text{Fe$^{2+}$} + \text{2CH$_{3}$COO$^{-}$}\to \text{Fe(CH$_{3}$COO)$_{2}$}\qquad \text{Scheme (4)} \end{equation*} |
| \begin{equation*} \text{Cr$^{2+}$} + \text{2CH$_{3}$COO$^{-}$}\to \text{Cr(CH$_{3}$COO)$_{2}$}\qquad \text{Scheme (5)} \end{equation*} |
| \begin{equation*} \text{2H$^{+}$} + \text{2e$^{-}$}\to \text{H$_{2}$}\qquad \text{Scheme (6)} \end{equation*} |
Water does not emit CO2. Ethanol and acetic acid derived from plant biomass are carbon neutral and biodegradable fluids.14) Therefore, acetic acid aqueous solution and acetic acid ethanolic solution are expected to be environmentally friendly working fluids for stainless steel.
It has been reported that the corrosion rate of stainless steel in aqueous solutions of acetic acid increased with increasing concentration of acetic acid in the range from 1%–80%.8) Therefore, the characteristics of mechanical material removal assisted by the anodic dissolution will be affected by the concentration of acetic acid.
This paper describes the material removal characteristics of martensitic stainless steel slid against Al2O3 in acetic acid aqueous solutions and in acetic acid ethanolic solutions of various concentrations. Smooth and rough alumina disks were used for material removal by fixed abrasive grains. The material removal characteristics are discussed based on the friction coefficient, the wear volume, and observation and elemental analysis of the worn surfaces. For comparison, the material removal characteristics of the stainless steel in water and in ethanol were also investigated.
Martensitic stainless steel (ISO: X110Cr17, ASTM: A276 440C, JIS: SUS440C) hemispherical pins (radius: 2 mm) were slid against smooth or rough Al2O3 disks (purity: 99.5%; diameter: 20 mm; thickness: 5 mm) in fluids using a unidirectional pin-on-disk machine (FPR-2200, RHESCA), which has been described elsewhere.15) Hereafter, the martensitic stainless steel (ISO: X110Cr17, ASTM: A276 440C, JIS: SUS440C) is referred to as the 440C stainless steel. The surface roughness (Ra) of the smooth Al2O3 disk was 0.011 µm and that of the rough Al2O3 disk was 0.434 µm. The physical properties of the test materials are summarized in Table 1.16) The chemical composition of the 440C stainless steel pin is summarized in Table 2. Figure 1 shows scanning electron microscopy (SEM) image and energy-dispersive X-ray spectroscopy (EDS) maps of the unworn surface of the 440C stainless steel pin. The island-shaped Cr-rich areas were dispersed in the matrix. It has been reported that Cr-rich ferrite grains and/or chromium carbide were dispersed in the matrix of the martensitic phase in the structure of martensitic stainless steels such as SUS440C stainless steel, AISI 403 stainless steel, AWS ER420 stainless steel and AISI 420 stainless steel.6,17–19) The Cr-rich areas in Fig. 1 were presumed to be Cr-rich ferrite grains and/or chromium carbide in the martensite matrix, although the chemical compositions and heat treatment conditions of the 440C stainless steel pin used in our experiment and the materials used in the literatures were different.6,17–19) The fluids used in the sliding test were distilled water, dehydrated ethanol (purity: 99.5%; H2O < 50 ppm), aqueous solutions of acetic acid and ethanolic solutions of acetic acid. The total concentration of acetic acid, i.e., the sum of the concentrations of dissociated and undissociated acetic acid, in the solutions was 0.05, 0.5 and 5 vol%. The applied load was 0.49 N (initial mean Hertzian contact pressure: 0.52 GPa), the sliding speed was 40 mm/s, the sliding time was 60 min, and the sliding distance was 144 m. Each test was performed two or three times. After the sliding test, the disk and pin were rinsed with ethanol, then rinsed with a mixture of acetone and petroleum benzine, and then dried in air.
SEM image and EDS maps of unworn 440C stainless steel pin: (a) SEM image, (b) Cr Kα map of (a) and (c) Fe Kα map of (a). Brighter contrast in the EDS maps indicates a higher concentration of the element.
The material removal volume of the 440C stainless steel pin was approximated by the wear volume. The wear volume of the pin (V) was calculated from the wear diameter using:20)
| \begin{equation} V = \pi d^{4}/(64r) \end{equation} | (1) |
The surface roughness (Ra) of the as-received Al2O3 disks was measured using a stylus-type surface texture and contour measurement instrument (SURFCOM 1500, TOKYO SEIMITSU). Optical microscopic images of the wear track on the Al2O3 disks were obtained by a laser microscope (VK-9500, KEYENCE). The morphologies and chemical compositions of the surfaces of the 440C stainless steel pins were analyzed by tungsten filament scanning electron microscopy (W-SEM, JSM-6060, JEOL: accelerating voltage, 6 kV) and energy-dispersive X-ray spectroscopy (EDS, JED-2300, JEOL: accelerating voltage, 15 kV), respectively.
Figure 2 shows the average friction coefficient of the 440C stainless steel/Al2O3 disk pairs. The friction in water was high. The addition of acetic acid to water resulted in a decrease in friction [Figs. 2(a) and (b)]. The friction coefficient of the 440C stainless steel pin/smooth Al2O3 disk pair in the 5 vol% acetic acid aqueous solution decreased with an increase of sliding distance. The result indicated that the substances which had lubricating ability were formed on the sliding surface in acetic acid aqueous solution.21) The friction coefficient of the 440C stainless steel pin/smooth Al2O3 disk pair in ethanol with and without acetic acid were ca. 0.2 [Fig. 2(c)]. The friction coefficient of the 440C stainless steel pin/rough Al2O3 disk pair in 5 vol% acetic acid ethanolic solution was higher than that in ethanol [Fig. 2(d)]. The friction in 0.05 vol% and 0.5 vol% acetic acid solutions showed almost the same behavior as in 5 vol% acetic acid solutions.
Friction coefficient of the tested materials as a function of sliding distance: (a) 440C stainless steel pin/smooth Al2O3 disk pair in aqueous environment, (b) 440C stainless steel pin/rough Al2O3 disk pair in aqueous environment, (c) 440C stainless steel pin/smooth Al2O3 disk pair in ethanolic environment, and (d) 440C stainless steel pin/rough Al2O3 disk pair in ethanolic environment.
Figure 3 shows the average wear volume of the 440C stainless steel pins, where the error bars represent the standard deviation. The range of the horizontal axis for the 440C stainless steel pins slid against rough Al2O3 disks [Fig. 3(b)] is ten times larger than that for the pins slid against smooth Al2O3 disks [Fig. 3(a)]. The addition of acetic acid to water and ethanol resulted in an increase of the wear volume of the 440C stainless steel pins. In each fluid, wear volume of the 440C stainless steel pins slid against rough Al2O3 disks was larger than that for the case of smooth Al2O3 disks. The wear volume increased with increasing total acetic acid concentration in both aqueous and ethanolic solutions. For solutions of the same total acetic acid concentration, the wear volume of the pins slid against smooth Al2O3 disks in aqueous solutions was larger than that in ethanolic solutions [Fig. 3(a)]. In contrast, the wear volume of the pins slid against rough Al2O3 disks in ethanolic solutions was larger than that in aqueous solutions [Fig. 3(b)]. The total acetic acid concentration dependence of the wear volume of the pins for the case of a smooth Al2O3 disk was higher than that for a rough Al2O3 disk.
Wear volume of 440C stainless steel pins slid against (a) smooth Al2O3 disks and (b) rough Al2O3 disks.
Figure 4 shows optical microscopic images of the worn Al2O3 disk surfaces. The wear tracks on the smooth Al2O3 disk surfaces worn in aqueous solutions of acetic acid are not shown because they are indistinguishable from the unworn areas. More adhesive substances were observed on the Al2O3 disk surfaces worn in solvent alone compared to the Al2O3 disk surfaces worn in solvent containing acetic acid. The adhesive substances appeared to be wear debris of the mating 440C stainless steel pin. The adhesion between abrasive wheel and workpiece decreased the machining efficiency.3,22) Adhesive substances cause loading of an abrasive wheel.22) The adhesion caused adhesive wear of the abrasive grains.3) In our experiment, the addition of acetic acid to water and ethanol resulted in reduction of the adhesion of the stainless steel to Al2O3.
Optical microscopic images of the worn Al2O3 disk surfaces: smooth Al2O3 disks worn in (a) water, (b) ethanol and (c) 0.05 vol% acetic acid ethanolic solution; rough Al2O3 disks worn in (d) water, (e) 0.05 vol% acetic acid aqueous solution, (f) ethanol and (g) 0.05 vol% acetic acid ethanolic solution.
Figure 5 shows SEM images of the surfaces of the 440C stainless steel pins slid against the smooth Al2O3 disks in water with and without acetic acid. On the surface of the pin worn in water, many grooves along the sliding direction and adhered or embedded substances were observed [Figs. 5(a) and (e)]. The surfaces of the pins worn in aqueous solutions of acetic acid were relatively flat and had many hollows [Figs. 5(b)–(d) and (f)–(h)]. On the surface of the pin worn in 0.05 vol% acetic acid aqueous solution, shallow grooves were observed [Figs. 5(b) and (f)].
SEM images of 440C stainless steel pins slid against smooth Al2O3 disks in water with and without acetic acid: (a), (b), (c) and (d) SEM images of the worn pin surfaces; (e), (f), (g) and (h) magnified views of (a), (b), (c) and (d), respectively.
Figure 6 shows SEM images of the surfaces of the 440C stainless steel pins slid against the smooth Al2O3 disks in ethanol with and without acetic acid. On the surface of the pin worn in ethanol, many grooves along the sliding direction, pits and embedded substances were observed [Figs. 6(a) and (e)]. The pits appeared to be formed by the adhesion of stainless steel to the mating smooth Al2O3 disk. The surfaces of the pins worn in ethanolic solutions of acetic acid were relatively flat [Figs. 6(b)–(d) and (f)–(h)]. The flat wear scars appeared to be formed by corrosive wear caused by anodic dissolution.2) On the surface of the pin worn in 5 vol% acetic acid ethanolic solution, several hollows were observed [Fig. 6(d) and (h)].
SEM images of 440C stainless steel pins slid against smooth Al2O3 disks in ethanol with and without acetic acid: (a), (b), (c) and (d) SEM images of the worn pin surfaces; (e), (f), (g) and (h) magnified views of (a), (b), (c) and (d), respectively.
To evaluate the difference in chemical composition between worn surface and unworn surface, an EDS analysis was performed. Figure 7 shows EDS maps of the circumferential area of the boundary between wear scar and unworn surface of the 440C stainless steel pins slid against the smooth Al2O3 disks. The chemical composition of the unworn surface of the pin shown in Fig. 1 was 1 mass% C, 1 mass% O, trace Al, 17 mass% Cr and 81 mass% Fe. The EDS analysis revealed that the O concentration of the adhered or embedded substances [point A in Fig. 7(a), composition (mass%): C, 2; O, 8; Al, trace; Cr, 19; Fe, 71] on the pin surface worn in water was higher than that of the unworn surface. In part of the adhered and embedded substances [point B in Fig. 7(a), composition (mass%): C, 2; O, 16; Al, 1; Cr, 12; Fe, 69], a small amount of Al was detected. The O and Al concentrations of the wear scar of the pin in 5 vol% acetic acid aqueous solution were almost the same as those of the unworn surface [Figs. 7(b), (f) and (j)]. The O concentration on the wear scar formed in ethanol was higher than that of the unworn surface [Figs. 7(c) and (g)]. The chemical composition of the embedded substances [point C in Fig. 7(c)] was 2 mass% C, 4 mass% O, trace Al, 14 mass% Cr, and 80 mass% Fe. In part of the embedded substances [point D in Fig. 7(c), composition (mass%): C, 1; O, 5; Al, 2; Cr, 13; Fe, 79], small amount of Al was detected. The O and Al concentrations of the wear scar of the pin in 5 vol% acetic acid ethanolic solution were almost the same as those of the unworn surface [Figs. 7(d), (h) and (l)]. For both aqueous and ethanolic solutions, there were also almost no difference in the O and Al concentrations between the inside and outside of the wear scar of the pins slid against the smooth Al2O3 disks in 0.05 vol% and 0.5 vol% acetic acid solutions (data not shown).
EDS maps of the surfaces of 440C stainless steel pins slid against smooth Al2O3 disks: (a), (b), (c) and (d) secondary electron images; (e), (f), (g) and (h) O Kα maps of (a), (b), (c) and (d), respectively; (i), (j), (k) and (l) Al Kα maps of (a), (b), (c) and (d), respectively. Brighter contrast in the EDS maps indicates a higher concentration of the element.
It has been reported that iron oxides and chromium oxides were formed on the surfaces of stainless steels worn in water and ethanol by tribo-oxidation.2,23,24) The adhered or embedded substances on the pin surface worn in water and in ethanol appeared to consist mainly of iron oxides and chromium oxides formed by tribo-oxidation. The iron oxides and chromium oxides seemed to abrade the mating Al2O3 disk. The detected Al was likely derived from the wear debris of the Al2O3 disk. The grooves observed on the surfaces of the pin worn in water and ethanol appeared to be formed by three-body abrasive wear, i.e., abrasive action of wear debris which consisted of metal oxides such as iron oxides, chromium oxides and aluminum oxide. Adhered or embedded metal oxides and corrosion products were hardly observed on the surfaces of the pins worn in aqueous solutions of acetic acid and in ethanolic solutions of acetic acid. The main corrosion product, that is, the main product of the reaction of the 440C stainless steel with acetic acid, appeared to be iron acetate.4) Iron acetate is soluble in water and in ethanol.4) Iron acetate formed on the sliding surface seemed to dissolve in the acetic acid solutions and ethanol used as a rinsing liquid after the sliding test. The results of SEM and EDS indicated that acetic acid inhibited the formation of metal oxides and that the main wear mechanism of the 440C stainless steel pin in acetic acid solution was corrosive wear caused by anodic dissolution.2)
On the surfaces of the pins worn in acetic acid aqueous solutions and in 5 vol% acetic acid ethanolic solution, hollows were observed. Figure 8 shows EDS maps of the areas around the hollows in the wear scars. The EDS analysis revealed that the hollows were formed in the Cr-rich areas. The Cr-rich areas seemed to be ferrite grains and/or chromium carbide in the matrix of the martensitic phase.6,17–19) The nobility of the ferrite grains was lower than that of martensitic matrix.6) The chromium carbide seemed to act as a cathode and cause the anodic dissolution of ferrite matrix and/or martensite matrix.25,26) The hollows in the wear scar of the pins appeared to have resulted from galvanic corrosion of the ferrite grains and/or martensite matrix.
EDS maps of the areas around the hollows on the wear scar of 440C stainless steel pins slid against smooth Al2O3 disks in 5 vol% acetic acid aqueous solution and in 5 vol% acetic acid ethanolic solution: (a) and (b) secondary electron images; (c) and (d) Cr Kα maps of (a) and (b), respectively; (e) and (f) Fe Kα maps of (a) and (b), respectively. Brighter contrast in the EDS maps indicates a higher concentration of the element.
Figure 9 shows SEM images of the surfaces of the 440C stainless steel pins slid against the rough Al2O3 disks in water with and without acetic acid. On the surface of the pin worn in water, many grooves along the sliding direction were observed [Figs. 9(a) and (e)]. On the edge of the wear scar, adhered substances were observed [Fig. 9(a)]. On the surfaces of the pin worn in aqueous solutions of acetic acid, many grooves along the sliding direction and serpentine cracks were observed [Figs. 9(b)–(d) and (f)–(h)]. The grooves seemed to be formed by the abrasive action of the protrusions of the rough Al2O3 disks, i.e., two-body abrasion. It has been reported that pits, cracks, and holes were formed by a combination of corrosive action and abrasive action on the surface of the stainless steel abraded under corrosive environment.27,28) The cracks appeared to be formed by a combination of corrosive action and abrasive action.27,28) The results suggested that the 440C stainless steel pins slid against the rough Al2O3 disks in acetic acid aqueous solutions were subjected not only to mechanical wear by two-body abrasion but also corrosive damage.
SEM images of 440C stainless steel pins slid against rough Al2O3 disks in water with and without acetic acid: (a), (b), (c) and (d) SEM images of the worn pin surfaces; (e), (f), (g) and (h) magnified views of (a), (b), (c) and (d), respectively.
Figure 10 shows SEM images of the surfaces of the 440C stainless steel pins slid against the rough Al2O3 disks in ethanol with and without acetic acid. On the surface of the pin worn in ethanol, many grooves along the sliding direction, pits and embedded particles were observed [Figs. 10(a) and (e)]. On the surfaces of the pins worn in ethanolic solutions of acetic acid, many grooves along the sliding direction and pits were observed [Figs. 10(b)–(d) and (f)–(h)]. The grooves appeared to be formed by the abrasive action of the protrusions of the rough Al2O3 disks, while the pits appeared to be formed by adhesion and shear. Little damage due to corrosion was observed on the surfaces of the pins worn in acetic acid ethanolic solutions, unlike the case for acetic acid aqueous solutions.
SEM images of 440C stainless steel pins slid against rough Al2O3 disks in ethanol with and without acetic acid: (a), (b), (c) and (d) SEM images of the worn pin surfaces; (e), (f), (g) and (h) magnified views of (a), (b), (c) and (d), respectively.
Figure 11 shows EDS maps of the circumferential areas of the boundary between wear scar and unworn surface of the 440C stainless steel pins slid against the rough Al2O3 disks. An EDS analysis revealed that the adhesive substances [point E in Fig. 11(a), composition (mass%): C, 1; O, 18; Al, 1; Cr, 13; Fe, 67] on the edge of the wear scar of the pin worn in water contained a high concentration of O and small amounts of Al. This result indicated that the adhesive substances consisted of iron oxides, chromium oxides and a small amount of aluminum oxide. The O and Al concentrations of the wear scar of the pin in 5 vol% acetic acid aqueous solution were almost the same as those of the unworn surface [Figs. 11(b), (f) and (j)]. The black particles [point F in Fig. 11(c), composition (mass%): C, 2; O, 16; Al, 20; Cr, 7; Fe, 55] embedded in the wear scar of the pin worn in ethanol contained high concentrations of O and Al. This result indicated that the black particles were Al2O3 grains that had fallen out of the mating Al2O3 disk due to adhesion. The O and Al concentrations of the wear scar of the pin in 5 vol% acetic acid ethanolic solution were almost the same as those of the unworn surface [Figs. 11(d), (h) and (l)]. For both aqueous and ethanolic solutions, there were also almost no difference in the O and Al concentrations between the inside and outside of the wear scar of the pins slid against the rough Al2O3 disks in 0.05 vol% and 0.5 vol% acetic acid solutions (data not shown). In the case of the pins slid against the rough Al2O3 disks in acetic acid solutions, the formation of metal oxides appeared to be inhibited.
EDS maps of the surfaces of 440C stainless steel pins slid against rough Al2O3 disks: (a), (b), (c) and (d) secondary electron images; (e), (f), (g) and (h) O Kα maps of (a), (b), (c) and (d), respectively; (i), (j), (k) and (l) Al Kα maps of (a), (b), (c) and (d), respectively. Brighter contrast in the EDS maps indicates a higher concentration of the element.
The main actions that caused material removal from the 440C stainless steel pins in the acetic acid solutions are summarized in Table 3. The wear behavior of the 440C stainless steel pin was influenced by the surface roughness of the mating Al2O3 disk, the total concentration of acetic acid in the solution, and the solvent.
The surface roughness of the mating Al2O3 disk affected the wear mechanism of the 440C stainless steel pins in acetic acid solutions. The 440C stainless steel pins slid against the smooth Al2O3 disks were worn mainly by corrosive wear, which is a chemo-mechanical action. On the other hand, the 440C stainless steel pins slid against the rough Al2O3 disks were worn mainly by two-body abrasion, which is a mechanical action.
3.4.2 Total concentration of acetic acid in the solution, and solventIn our experiments, the higher the total concentration of acetic acid in the solution, the larger the wear of 440C stainless steel. This tendency was stronger in corrosive wear than in mechanical wear. The wear of 440C stainless steel in acetic acid aqueous solutions was affected by electrochemical actions such as anodic dissolution and galvanic corrosion more strongly than in acetic acid ethanolic solutions.
The anodic dissolution rate seemed to increase with increasing concentration of acetate anions [CH3COO−] and protons [H+] because acetate anions and protons consume metal ions (Fe2+ and Cr2+) and electrons, respectively [Schemes (4)–(6)].4,12,13) The concentrations of acetate anions and protons in each solution were estimated as follows. Acetic acid is classified as a weak acid in water. The concentrations of protons generated by dissociation of the solvents (water and ethanol) were negligibly lower as compared with that generated by dissociation of acetic acid.29) The concentrations of acetate anions and protons in the acetic acid solutions were calculated using:30)
| \begin{equation} [\text{CH$_{3}$COO$^{-}$}] = [\text{H$^{+}$}] = (\text{K}_{\text{a}}c)^{1/2} \end{equation} | (2) |
| \begin{equation} \mathrm{pK}_{\text{a}} = {-}{\log \text{K}_{\text{a}}} \end{equation} | (3) |
| \begin{equation} \mathrm{pK}_{\text{a}}(\mathrm{EtOH}) = \mathrm{pK}_{\text{a}}(\text{H$_{2}$O}) + 5.8 \end{equation} | (4) |
The main wear mechanism of the 440C stainless steel pins slid against the smooth Al2O3 disks in acetic acid solutions was corrosive wear caused by anodic dissolution. In general, the corrosive wear rate is determined by the corrosion rate.33) In our experiments, the corrosive wear rate of the 440C stainless steel pins seemed to be determined by the anodic dissolution rate. Therefore, the wear of the 440C stainless steel pins slid against the smooth Al2O3 disks increased with increasing concentrations of acetate anions and protons in the solution. In high-concentration acetate anion and proton solutions, not only corrosive wear but also galvanic corrosion occurred on the sliding surfaces of the 440C stainless steel pins. To avoid corrosive damage on the surface of 440C stainless steel in the chemo-mechanical removal process, processing fluids of low acetic acid ethanolic solutions were preferable.
The main wear mechanism of the 440C stainless steel pins slid against the rough Al2O3 disks in acetic acid solutions was two-body abrasive wear, which is mechanical wear. The addition of acetic acid to solvent resulted in increase of two-body abrasive wear because the corrosion products protected the rough Al2O3 disk from adhesion which caused loading and removal of Al2O3 abrasive grains. On the other hand, it has been reported that the abrasive wear amount of the stainless steel was reduced by the lubricating corrosion products that formed on the surface.34,35) The friction data suggested that the corrosion products on the sliding surface of the 440C stainless steel pin/Al2O3 disk had lubricating ability. The amount of the lubricating corrosion products in acetic acid aqueous solution was larger than that in acetic acid ethanolic solution. Therefore, the wear volume of the 440C stainless steel pins slid against rough Al2O3 disks in acetic acid ethanolic solutions was larger than that in acetic acid aqueous solutions. On the sliding surfaces of the 440C stainless steel pins slid against the rough Al2O3 disks in acetic acid aqueous solutions, the cracks were formed by a combination of corrosive action and abrasive action.27,28) The 440C stainless steel was effectively removed by two-body abrasion without corrosive damage in 0.05∼5 vol% acetic acid ethanolic solutions.
In our experiments, the material removal characteristics of 440C stainless steel pins slid against Al2O3 disks in acetic acid aqueous solutions and in acetic acid ethanolic solutions were evaluated. Our experimental results can be summarized as follows.
