Wear 260 (2006) 116–122
Wear and corrosion wear of medium carbon steel and 304 stainless steel M. Reza Bateni a,∗ , J.A. Szpunar a,1 , X. Wang b,2 , D.Y. Li b,2 a
Department of Mining, Metals and Materials Engineering, McGill University, M.H. Wong Building, 3610 University, Montreal, Que., Canada H3A 2B2 b Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alta., Canada T6G 2G6 Received 12 January 2004; received in revised form 27 August 2004; accepted 20 December 2004 Available online 5 March 2005
Abstract Wear and corrosive wear involve mechanical and chemical mechanisms and the combination of these mechanisms often results in significant mutual effects. In this paper, tribological behavior, X-ray peak broadening, and microstructure changes of carbon steel AISI 1045 and stainless steel AISI 304 samples under simultaneous wear and corrosion were investigated and the results were compared with those obtained from dry wear tests. 3.5 wt.% NaCl solution was used as the corrosion agent and a pin-on-disk tribometer was employed to perform wear and corrosive wear tests. X-ray diffraction measurements have shown that by increasing the applied load, the worn surfaces of carbon steel samples reached a constant strain at which fracture and wear occurred. Whereas in 304 stainless steel samples, by increasing the applied load, broadening of X-ray diffraction peaks was decreased. Wear tests of carbon steel and stainless steel samples have shown smaller weight losses and lower friction coefficient in the presence of corrosive environment. Study of worn surfaces suggested that depending on wear environment and applied load, different features of wear mechanisms were involved. © 2005 Elsevier B.V. All rights reserved. Keywords: Corrosive wear; Wear; Tribology; friction; X-ray peak broadening; Strain
1. Introduction Wear and corrosion damage to materials used in mining, metals and materials processing, directly or indirectly, impact the nation financially in terms of material loss, associated equipment downtime for repairing and finally the replacement of worn and corroded components [1,2]. Wear and corrosion wear experiments on AISI 304 stainless steel showed that corrosion played a minor role on corrosive wear of the stainless steel in synthetic Ni–Cu mine water and synergetic effects were very small. However, the synergetic effects were quite pronounced on alloy steels . The interactions ∗
Corresponding author. Tel.: +1 514 398 4755; fax: +1 514 398 4492. E-mail addresses: reza firstname.lastname@example.org (M.R. Bateni), email@example.com (J.A. Szpunar), firstname.lastname@example.org (X. Wang), email@example.com (D.Y. Li). 1 Tel.: +1 514 398 4755; fax: +1 514 398 4492. 2 Tel.: +1 780 492 6750; fax: +1 780 492 2881. 0043-1648/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2004.12.037
among wear and corrosion could significantly increase total weight losses and reduction of either wear or corrosion could considerably decrease the total weight loss . In order to decide whether to choose materials according to their mechanical properties or corrosive characteristics, for any particular applications, sufficient information will be necessary . The chemical degradation of materials, corrosion, destroys the materials by chemical reactions with aggressive environments, mostly liquid or gases. In contrast, the mechanical failure is the end-point of elastic and/or plastic deformation processes . Corrosive wear takes place when an active environment results in material dissolution or produces a reaction product on one or both of the rubbing surfaces. The reaction products are usually poorly bonded to the surface and further rubbing causes their removal. Corrosion wear requires both corrosion and rubbing. The growth rate of corrosion product, oxide film, decreases with time, and therefore unless the oxide film is
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removed by rubbing, the corrosion of metal rapidly becomes small . Sliding contact between the surfaces of ductile materials is often accomplished by severe plastic deformation of materials, adjacent to the contact surface. The process of debris formation, especially in un-lubricated systems, is closely related to magnitude and distribution of local strain and strain gradients as well as the variation of stress within the deformed subsurface zones . On the other hand, the ridges on both sides of an isolated wear track are plastically deformed regions containing a high density of dislocations, which present a high internal energy and in the presence of corrosive environment, corrosion attack occurs in the deformed layer very quickly . X-ray diffraction can be used to determine both macroscopic and microscopic residual stresses. Macro-stresses are determined from the shift in the position of diffraction peaks. While micro-stresses, could be quantified from the broadening of the diffraction peaks. As a metal is cold worked, the dislocation density increases, thereby perfectly crystalline areas are decreased and the average micro-strain in the crystal lattice is increased. The reduced crystallite size and increased micro-strain both produce broadening of diffraction peaks . For many significant applications, the actual understanding of the synergism between wear and corrosion processes in sliding contacts is rather limited . In this paper, wear performances, frictional behavior, X-ray peak broadening and microstructure of medium carbon steel and austenitic stainless steel samples under wear and corrosion wear conditions have been investigated.
2. Materials and experimental techniques Carbon steel AISI 1045 and stainless steel AISI 304 specimens were used as experimental specimens. Both stainless steel and carbon steel samples were annealed at 875 ± 25 ◦ C for an hour. The aim of annealing treatment was decreasing and relieving internal stresses. The origins of the internal stresses were previous mechanical working and forming processes. The harness of stainless steel and carbon steel samples, after annealing treatment, were 179 and 163 HB, respectively and carbide precipitations were not observed in stainless steel sample after annealing treatment. A pin-on-disk tribometer was employed to perform wear tests. AISI 3Cr12 steel Pins (220 HB) were employed as the counter face. The dimension of pin specimens for wear test was 6 mm × 12 mm × 40 mm. Test surface of the samples was polished on a lathe and the final roughness was about 1.0 m. Wear tests were conducted at room temperature and a velocity of 0.53 m/s. The contact area in all experiments was 72 mm × 72 mm. Corrosion wear behavior of carbon steel and stainless steel samples were studied in 3.5 wt.% NaCl solution. Wear tests were carried out under different loads of 9.6, 32, 54 and 71N
and for all experiments the sliding distance of 160 m was used. X-ray diffraction (XRD) and scanning electron microscope (SEM) were used for determination of X-ray peak broadening, microstructure of worn surfaces, and wear mechanisms. XRD patterns were recorded using Cu K␣ radiation at a step scan mode of 0.02◦ .
3. Results and discussion 3.1. X-ray peak broadening of the worn surfaces The K␣ radiation generally used for residual stress measurement produces overlapped double diffraction peaks. The K␣ doublet (consisting of peaks produced by K␣1 and K␣2 radiations) could be separated to determine the width of the stronger peak generated by K␣1 . Most of the difficulties encountered in determining both the diffraction peak position and width can be overcome if the measured diffraction peaks can be accurately described by a suitable function fitted by regression analysis. Pearson VII functions, which are bellshaped curves ranging from Cauchy to Gaussian distributions, have been shown to describe accurately the profiles of diffraction peaks in the back-reflection region used for residual stress measurement . The K␣1 diffraction peak position, width and intensity can be determined from the fitted function. The diffraction peak width is usually taken by the width at half value of maximum intensity (FWHM). Fig. 1 shows the variation of X-ray peak broadening against applied load for carbon steel samples under corrosive and dry conditions. By increasing the applied load, broadening reaches a constant value at which fracture occurred. It seems that further increasing of applied load does not have any effect on X-ray peak broadening. Corrosive media more easily removed the stressed surface layers and X-ray peak broadening of these samples is less than dry tested samples (Fig. 1). On the other hand, lower friction led to less wear when worn in the NaCl solution. By increasing the applied load, X-ray peak broadening starts
Fig. 1. X-ray peak broadening vs. applied load for carbon steel samples.
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Fig. 2. X-ray peak broadening vs. applied load for stainless steel samples.
to increase. As the load increased, the contribution due to corrosion changed. When wear continuously occurred under high loads, the percentage of corrosion effects became less important, whereas, under lower loads, the part of corrosion effects were high . Fig. 2 shows the effect of applied load on X-ray peak broadening of 304 stainless steels in both corrosive and non-corrosive environments. Stainless steel possesses excellent corrosion resistance in the absence of wear. However, it readily wears and corrodes when abrasive particles remove its corrosion resistance passive film . Corrosion wear was preceded by successive stages of buildup and removal of oxides on the rubbing surfaces. Under dynamic conditions or rubbing, reaction Fe/Fe(OH)2 is thermodynamically favored in the early stages of film formation . This reaction led to re-passivation of the abraded surface. The passive state only existed for a short time due to continual abrading. Accordingly, corrosion proceeded as long as rubbing was taking place. Under this condition, layerrupturing action, which is constant at fixed conditions of abrasion, became rate controlling for corrosion-wear process . On the other hand, during the wear test, temperature increased at the contact area between substrate and counter face. Austenitic stainless steels have shown the lowest thermal conductivity among carbon steels and other stainless steels. Therefore, by increasing the load, the contact areas were increased and more heat was generated at contact areas. Increasing the temperature at the contact area and low thermal conductivity of austenitic stainless steels, caused heat accumulation at the contact area. The accumulation of heat and subsequent stress relief process could be the main reasons for decreasing X-ray diffraction peak broadening by increasing the applied load. Such process of heat accumulation and subsequent stress relief could however, be suppressed when worn in the NaCl solution due to the reduced friction and faster heat conduction through the solution. As a result, the decrease in the XRD peak broadening was smaller than that occurred in the case of dry wear test (Fig. 2).
Fig. 3. Weight losses of the samples under dry wear and corrosive wear: (a) 1045 carbon steel; (b) 304 stainless steel.
3.2. Tribological behavior Wear performances of carbon steel and stainless steel samples during both dry sliding and corrosion wear in NaCl solution were evaluated. Fig. 3 illustrates weight losses of carbon steel and stainless steel samples under different conditions. As the load is increased, the weight loss is increased. It is observed that under corrosive wear condition, weight loss is smaller than dry wear. This could be attributed to the following reasons: (1) The dilute NaCl solution was not very corrosive and therefore did not result in severe synergistic attack of wear and corrosion. On the other hand, the NaCl solution reduced the friction between the substrate and the counter-face, which consequently decreased the wearing force, thus leading to less damage to the substrate. (2) Heat generation, due to friction, generally resulted in softening of the substrate and thus increased the weight loss. The NaCl solution decreased the temperature rise, which could further reduced the weight loss. Variation of friction coefficient versus time in carbon steel and stainless steel samples at a load of 9.6N are shown in Figs. 4 and 5. It is observed that the coefficient of friction of carbon steel sample shows a decrease under corrosive environment (Fig. 4). The corrosive solution reduced the friction between the substrate and the counter-face, which consequently decreased the coefficient of friction. On the other hand, in the presence of NaCl solution, the formation of surface oxide layer was accelerated. Such an oxide film acted as intermediate layer on the surface, which reduced the
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Fig. 4. Variation of friction coefficient vs. time in carbon steel sample: (a) dry condition; (b) corrosive condition.
Fig. 5. Variation of friction coefficient vs. time in stainless steel sample: (a) dry condition; (b) corrosive condition.
coefficient of friction [12,13]. The ease of formation of the surface oxide layer and decreasing metallic contact between substrate and counter-face are main reasons for reducing the coefficient of friction in the presence of corrosive environment. On the other hand, in the presence of corrosive solution, less fluctuations in the friction coefficient is observed. The NaCl solution reduced the adhesion and friction between the substrate and the counter-face, which consequently decreased the wearing force, thus leading to less fluctuation in friction coefficient. The variation of friction coefficient versus sliding distance of stainless steel samples shows in Fig. 5. Higher coefficient of friction value in stainless steel samples is due to stronger adhesion bonds. A remarkable stick-slip behavior in friction coefficient of stainless steel samples under dry wear and corrosive wear conditions is observed (Fig. 5). When two surfaces contacted each other, the adhesion took place at the contact area and caused stick to take place. The friction force and, consequently, the friction coefficient rose sharply. At some points the tangential forces were sufficient to overcome the adhesive bonds at the interface, fracture accrued, and the friction force dropped sharply. As long as the wear test continued, adhesion, force build up, interfacial ad-
hesion bonds fracture and slip process repeated, respectively . The variation of specific wear rate versus applied load is shown in Fig. 6. By increasing the applied loads, specific wear rates stared to decrease and approach to constant values, steady-state wear. On the other hand, wear mechanism
Fig. 6. Variation of specific wear rate vs. applied load.
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Fig. 7. Wear surface of carbon steel sample after dry wear test, under 9.6N load.
Fig. 8. Cross-section of carbon steel sample under dry wear test, 71N load.
changes from sever regime to mild one. At higher loads, a hard surface layer is formed, most likely martensite, on surfaces because of high flash temperature, followed by rapid quenching as the heat was conducted into the underlying bulk material. The higher flash temperature also caused the local oxidation rate to increase . On the other hand, increasing the applied load caused work hardening of subsurface layers and the surface oxide layer supported by the hardened sublayers. The higher oxidation rate formed thicker oxide layer on the surface. The formed oxide layer prevented further direct metallic contact and reduced the specific wear rates.
area is the real contact area and wear and friction forces are directly related to that area. That area varies with the number of contacting points, the roughness, and the elasto-plastic deformation of each contact point. During wear process, some parts of the surface area loss their surface film due to a mechanical loading . In the presence of corrosive environment, the formation of galvanic cell might be responsible for the formation of crack networks on the worn surface. The galvanic cells were formed due to the abraded area on the surface as anode and undamaged area as cathode. In the presence of corrosive environment, the formation of cracks and voids is observed in the cross-section (Fig. 10). Corrosive media reduced the friction forces between two surfaces and the depth of the deformed layer decreased. However, regardless of the formation of cracks and voids due to the synergism of wear and corrosion, the total weight loss is smaller than that under dry wear condition. It seems that the reduction of friction forces by the NaCl solution might be responsible for less damage on the specimen surface, when worn in the NaCl solution than in the specimen treated without it.
3.3. Microstructure of worn samples Worn surface of carbon steel sample in the presence of corrosive environment indicates the formation of plate-like debris on the surface (Fig. 7). The presence of plate like debris has confirmed that the delamination took place in those samples [16,17]. The delamination wear process consists of plastic deformation of a surface layer of finite thickness, void nucleation and crack propagation below the surface . Increasing the applied load did not have any effects on wear mechanisms. Scanning electron microscopy (SEM) image of the subsurface layers of carbon steel sample under dry wear test shows the presence of subsurface cracks and deformed layers under the surface (Fig. 8). The cracks propagate at a depth of 5–30 m under the surface. By increasing the applied load, the depth of the deformed layer increased. It is reasoned that voids around inclusions and any imperfections like dislocations can be nucleated only at a certain depth from the surface. Indeed, cracks nucleated at a finite distance below the surface and the location of possible crack nucleation sites was affected by the magnitude of the normal forces at the asperity-surface contact . Under corrosive condition, the presence of surface crack networks is shown on the worn surface (Fig. 9). When two surfaces came into contact with each other, only part of the apparent contact area carried the applied load. This load bearing
Fig. 9. Wear surface of carbon steel sample after corrosive wear test, under 71N load.
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Fig. 10. Cross-section of carbon steel sample under corrosive wear test, 71N load.
Studying of worn surfaces of stainless steel under dry wear tests have shown that at lower loads, delamination is the predominant wear mechanism. Whereas, by increasing the applied load, the adhesive wear was observed more often. The cross-section of worn stainless steel sample under dry condition is presented in Fig. 11. The presence of voids under the surface is illustrated. Under repeated load, the voids were enlarged and linked. By increasing the applied load, the number of voids considerably increased. On the other hand, highly deformed areas were not observed under the surface. The higher hardness and lower ductility of stainless steel and adhesion wear mechanism are the main reasons for lack of highly deformed areas below the specimen surface. SEM micrograph of worn surface of stainless steel sample under corrosive environment shows in Fig. 12. Under lower loads, the presence of plate like particles was observed. Whereas, abrasive wear and adhesive wear became predominant wear mechanisms by increasing the applied load to 71N. The presence of protective chromium oxide layer on the surface of stainless steel is the main reason for changing wear mechanism. At lower loads, the oxide layer lowered adhe-
Fig. 11. Cross-section of stainless steel sample under dry wear test, 71N load.
Fig. 12. Wear surface of stainless steel sample under corrosive wear test, 71N load.
sion between two surfaces. However, as the load increased, the oxide layer broke down and metallic contact was created between two surfaces. The break down of the oxide layer and metallic contact are main reasons for the existence of adhesion wear. On the other hand, the relative motion of two surfaces induced severe plastic deformation and rupture of deformed junctions produced hard wear debris. The ploughing actions of hard wear particles could caused abrasive wear mechanism .
4. Conclusions 1. By increasing the applied load, X-ray peak broadening of worn carbon steel surfaces, under both dry and corrosive conditions, reached to a constant value. 2. Increasing of temperature at the contact area and low thermal conductivity of 304 stainless steels are responsible for heat accumulation and consequently stress relief and lower X-ray peak broadening in the deformed surface layers. 3. The weight loss and XRD peak broadening were lower when the substrate was worn in the NaCl solution. Decreasing of the friction coefficient should be responsible for these changes. 4. Studying of worn surfaces has shown that delamination is main wear mechanism in carbon steel sample treated without corrosive solution, whereas under corrosive conditions, the surface damage was caused by synergy of wear and corrosion attacks. 5. In stainless steel samples tested under both dry wear and corrosion wear condition, delamination of surface layers was predominant wear mechanism during testing at lower loads. By increasing the load, under dry wear tests, the adhesion wear was observed and under corrosion wear conditions, the combined adhesive and abrasive wear were identified. 6. High wear rates of carbon steel and stainless steel samples, of order 10−3 to 10−4 mm3 /N m, could be due to the
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Received12January2004;receivedinrevisedform27August2004;accepted20December2004 Availableonline5March2005 ∗ Correspondingauthor.Tel.:+1514398...