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Nanocrystallization of aluminized surface of carbon steel for enhanced resistances to corrosion and corrosive wear
Electrochimica Acta 55 (2009) 118–124
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Nanocrystallization of aluminized surface of carbon steel for enhanced resistances to corrosion and corrosive wear C. Chen a,b , D.Y. Li b,∗ , C.J. Shang a a b
Dept. of Materials Physical and Chemical, University of Science and Technology Beijing, Beijing 100083, China Dept. of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2V4
a r t i c l e
i n f o
Article history: Received 4 June 2009 Received in revised form 12 August 2009 Accepted 13 August 2009 Available online 21 August 2009 Keywords: Nanocrystallization Aluminizing Corrosion Wear Corrosive wear Carbon steel
a b s t r a c t Aluminizing is often used to improve steel’s resistances to corrosion, oxidation and wear. This article reports our recent attempts to further improve aluminized carbon steel through surface nanocrystallization for higher resistances to corrosion and corrosive wear. The surface nanocrystallization was achieved using a process combining sandblasting and recovery heat treatment. The entire surface modification process includes dipping carbon steel specimens into a molten Al pool to form an Al coat, subsequent diffusion treatment at elevated temperature to form an aluminized layer, sandblasting to generate dislocation network or cells, and recovery treatment to turn the dislocation cells into nano-sized grains. The grain size of the nanocrystallized aluminized surface layer was in the range of 20–100 nm. Electrochemical properties, electron work function (EWF), and corrosive wear of the nanocrystalline alloyed surfaces were investigated. It was demonstrated that the nanocrystalline aluminized surface of carbon steel exhibited improved resistances to corrosion, wear and corrosive wear. The passive film developed on the nanocrystallized aluminized surface was also evaluated in terms of its mechanical properties and adherence to the substrate. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Aluminized steels have found wide applications, such as components in vehicles, bridges, pressure containers, and house appliances, which possess higher resistances to corrosion and oxidation at elevated temperatures [1–3] than carbon steel. However, compared to ferritic and austenite stainless steel, the aluminized steel is relatively softer [4,5] with lower resistance to corrosion [6]. In recent years, we used an inexpensive surface nanocrystallization process to improve surfaces of passive alloys for higher resistances to corrosion and wear. The surface nanocrystallization is achieved using a process combining sandblasting and recovery treatment. The former generates plastic deformation in the surface layer and the latter turns the resultant dislocation cells into nano-sized grains. This technique has been applied to aluminum, brass, and stainless steel [7–9]. Previously, we demonstrated positive effects of surface nanocrystallization on properties of an Al coat on carbon steel (by hot dipping), which, however, contained little Fe. The present study is focused on aluminized carbon steel
∗ Corresponding author. E-mail address: [email protected] (D.Y. Li). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.08.016
whose surface layer contained sufficient iron content achieved by diffusion treatment of a hot dipping Al coat on the steel at 850 ◦ C for 2 h. The purpose of the surface nanocrystallization treatment applied to aluminized carbon steel is to obtain increased resistances to corrosion and corrosive wear. In the present work, the entire surface modification process included surface aluminizing by hot dipping steel samples into a molten aluminum pool and subsequently annealing them in order to change the aluminum coat to an aluminized surface layer, which contained intermetallics of Fe and Al. The aluminized surface was then nanocrystallized by sandblasting followed by recovery treatment that turned generated dislocation cells or network into grains at nano-scale. The expected improvement in corrosion resistance would improve the performance of aluminized components made of carbon steel used in exhaust systems of vehicles as well in general corrosive environments. The enhanced resistances of aluminized carbon steel to corrosive wear would prolong the service life of the components during corrosion when mechanical attacks are involved simultaneously, e.g., corrosion–erosion and erosion by exhaust gases. The objective of this study is to determine the effectiveness of the surface nanocrystallization in enhancing the overall resistances of aluminized carbon steel to corrosion and corrosive wear. Relevant issues were also investigated, such as the effects of sur-
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Table 1 Chemical composition of synthetic exhaust gas condensate (ppm). Ion
Cl−
SO3 2−
SO4 2−
CO3 2−
NO3 −
NO2 −
CH4 COO−
HCHO
HCOO−
NH4 +
Activate carbon
Content
50
250
1250
2000
100
20
400
250
100
1934
50 g/L
face nanocrystallization on properties of passive film and surface electron stability. Mechanisms responsible for observed changes in surface properties of aluminized carbon steel modified using the current treatment are discussed.
2. Experimental details The substrate material for this study was commercial 1020 carbon steel, consisting of C: 0.18–0.23 wt.%, Si: 0.15–0.35 wt.%, Mn: 0.30–0.60 wt.%, P: 0.03 wt.%, S: 0.035, balanced by Fe. Specimens having a size of 10 mm × 10 mm × 10 mm were cut from a piece of the steel and polished using #600 grit papers. All specimens were cleaned with acetone and water, and then were dipped into a molten aluminum (purity >99.9%) pool at 850 ◦ C for 30 s. The specimens with an Al coat were then embedded in the high purity aluminum powder at 850 ◦ C for 120 min to generate a thicker aluminized layer. The aluminized specimens were polished and vertically blasted by a flow of silica particles (15–40 mesh) under a pressure of 240 K Pa for 15 min. The specimens were polished with alumina particles of 0.05 m to smoothen their sandblasted surfaces, followed by recovery treatment. Several recovery temperatures, 200 ◦ C, 300 ◦ C and 400 ◦ C, were respectively used to obtain different nano-grain sizes. The microstructure of the aluminized and nanocrystallized surfaces was examined using a Hitachi S-2700 scanning electronic microscope (SEM) and their composition was analyzed using a PGT PRISM IG (Intrinsic Germanium) detector for X-ray energydispersive spectrometer (EDS) analysis. The morphology and grain size of the treated surfaces were determined using an atomic force microscope (AFM, Digital Instruments, CA, USA). Vickers-hardness of these surfaces was measured using a micro-indentation probe (Fisher Technology Ltd., Winsor, CT, USA) with a diamond tip under a maximum load of 98 mN. The electrochemical stabilities of the aluminized nanocrystalline surfaces that experienced recovery treatments at different temperatures were evaluated using a scanning Kelvin probe having a gold tip of 1 mm in diameter (KP Technology Ltd., Wick, UK) in the ambient air, 0.5 M room-temperature H2 SO4 solution and a synthetic exhaust gas condensate [6,10]. The composition of this synthetic exhaust gas condensate is given in Table 1. The oscillation frequency of the SKP tip was 173.5 Hz.
Dynamic polarization experiments were performed using a commercial electrochemical system (Model PC4-750, Gamry Instruments Inc., Warminster, PA, USA). The scan rate was 5 mV s−1 and the experiments were performed in the 0.5 M H2 SO4 solution and the synthetic exhaust gas condensate (at 25 ◦ C and 80 ◦ C), respectively. A saturated calomel electrode (SCE) was used as the reference electrode, and a platinum plate (Pt) was used as the counter electrode. Wear and corrosive wear of all specimens’ surface were evaluated using a tribometer (Center for Tribology Inc., Campbell, CA, USA). During the wear test, an alloy steel ball with its diameter equal to 3.95 mm slid on a specimen along a circle of 5 mm in diameter at a velocity of 120 mm/s under an applied load of 2.5 N for 10 min. A special container was made for wear tests in a corrosive solution. The corrosive solution was similar to that used for the electrochemical tests. The corrosive wear tests were carried out under the same loading condition as that for the dry wear test. The failure resistances of passive films on various specimens were evaluated using a micro-scratcher with a tip made of tungsten carbide. During the scratch test, the normal load was linearly increased from 0 to a designated level and the tip of the scratcher moved at a velocity of 0.1 mm/s. During this process, changes in the contact electrical resistance (CER) with respect to the load were recorded. When a passive film failed under a critical force, the CER dropped steeply. The critical load corresponding to the drop in CER reflects the resistance of the passive film to scratch failure. Critical loads for passive films on various specimens were determined. Surface mechanical properties of specimens experienced recovery treatments at various temperatures were evaluated using a triboscope which is a combination of a nano-mechanical probe and an atomic force microscope. Nano-indentation tests were performed under a maximum force of 50 N.
3. Results and discussion 3.1. Microstructure Fig. 1 presents a typical cross-section backscattered electron SEM image and element distribution profiles of the aluminized surface layer on carbon steel, determined with the energy-dispersive X-ray spectroscopy (EDS). The line scanning EDS analysis indicates
Fig. 1. A cross-section backscattered electron SEM image of aluminized carbon steel and X-ray energy-dispersive spectrometer (EDS) analysis: Al and Fe distributions across the aluminized layer and the steel substrate. Dark and light dark regions in the aluminized layer are Al-rich and Fe-rich, respectively.
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Fig. 4. Average hardness values of different surfaces: (1) aluminized surface of C-steel; (2) aluminizing, sandblasting and recovery at 200 ◦ C; (3) aluminizing, sandblasting and recovery at 300 ◦ C; and (4) aluminizing, sandblasting and recovery at 400 ◦ C. Fig. 2. A representative XRD diffraction pattern of aluminized carbon steel.
that there is an aluminized layer of 900 m in thickness on the specimen and the distributions of Al and Fe are not very homogeneous. Point EDS analysis indicated that the dark and light dark regions in the aluminized layer (see Fig. 1) are Al-rich and Fe-rich, respectively. Fig. 2 illustrates a X-ray diffraction pattern of the aluminized carbon steel, which shows several co-existing phases, including Al, Al86 Fe14 , and a small amount of AlFe3 and Al5 Fe2 . These intermetallic phases are hard [11–13], which render the aluminized surface strong and resistant to wear in addition to desired resistance to corrosion. These Fe–Al intermetallic phases cannot form if only dipping samples in the molten aluminum pool, which generates an aluminum coat but not an aluminized layer. The annealing treatment is necessary to activate the reactions in order to form the intermetallic phases. The objective of this study is to further enhance the resistances of aluminized carbon steel to corrosion, wear and corrosive wear through surface nanocrystallization treatment using a process combining sandblasting and recovery heat treatment. Fig. 3(a) presents a typical optical micrograph of the untreated aluminized layer. As shown, the size of grains was approximately 20–80 m. Microstructures of aluminized layers experienced sandblasting and recovery treatments at different temperatures were examined under an atomic force microscope (after slight polishing and etching). Representative AFM images are illustrated in Fig. 3(b)–(d). As shown, the grain size was around 10–100 nm when the sandblasted specimen was annealed at 200 ◦ C for 60 min. The grain size became slightly larger when the recovery temperature was increased to 300 ◦ C. There is no obvious difference in grain size between surfaces recovered respectively at 300 ◦ C and 400 ◦ C. The recovery treatment helped dislocation cells or network generated by sandblasting to move or be rearranged, which were turned into nano-sized grains with sharp boundaries. At lower recovery temperatures, the thermal driving force was smaller
and the rearrangement of dislocations proceeded more locally, thus leading to smaller grains. However, if temperature is too low, nanocrystallization cannot be achieved or may be partially achieved. As the recovery temperature was increased, larger grains could form but the situation was saturated as the recovery temperature reached a certainly level. These have been illustrated in Fig. 3(b)–(d). If the temperature is higher than the recrystallization temperature, recrystallization will occur. 3.2. Surface hardness Surface micro-hardness of the specimens was measured under a maximum load of 98 mN. Fig. 4 illustrates average Vickershardness values of differently treated surfaces. As demonstrated, the hardness of aluminized nanocrystalline surfaces is about 60% higher than the untreated aluminized surface. The increase in hardness of the nanocrystallized surfaces is largely attributed to the nanocrystalline structure with high-density grain boundaries that effectively block the movement of dislocations. As a result, the aluminized nanocrystalline surface is harder than the aluminized microcrystalline one. Possible influences of other factors, such as densification of material and adding residual stresses caused by sandblasting, on the increase in hardness could be excluded to a large content after the sandblasted surfaces experienced recovery treatment at elevated temperatures. It needs to be indicated that the situation was different if the surface was sandblasted only. In this case, nano-sized “sub-grains” or more precisely the dislocation cells were generated by heavy deformation in the sandblasted surface layer with diffuse boundaries. Based on extensive studies on dislocation and well-established dislocation theory [14,31], the walls between adjacent dislocation cells are diffused in nature. The diffuse grain boundaries may not block dislocations effectively. Although high-density dislocations causes strain-hardening, the material is actually “damaged” with lower ductility/toughness or reduced capability for energy absorption before failure. The recovery treatment diminished or rearranged dislocations and sharpened the boundaries with increased misori-
Fig. 3. (a) An optical microscopic (OM) image of a untreated aluminized surface and AFM images (top view) of sandblasted aluminized surfaces experienced recovery treatments at different temperatures: (b) 200 ◦ C, (c) 300 ◦ C, and (d) 400 ◦ C.
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Fig. 5. Polarization curves of various specimens in a 0.5 M H2 SO4 solution. Fig. 6. Polarization curves of various specimens in the synthetic condensate solution at 25 ◦ C.
entation between adjacent grains or sub-grains [15]. As result, the nano-sized perfect crystal grains with improved grain boundaries certainly improve the mechanical properties of the treated surfaces. The authors would like to point out that the hardness values of different surfaces in Fig. 4 are presented only to show a general trend that reflects the effect of surface nanocrystallization on surface hardness without intention to provide accurate quantitative hardness values. Since the surface layers containing various intermetallic phases were rather inhomogeneous, the fluctuation in the range of 20–30% was not unexpected during the hardness measurement. 3.3. The electrochemical behavior Nanocrystallization has strong influence on the electrochemical properties of materials. Polarization behaviors of specimens experienced aluminizing and surface nanocrystallization in a 0.5 M H2 SO4 solution at room temperature and in a synthetic condensate at room temperature and 80 ◦ C, respectively, were evaluated. Potentiodynamic polarization curves of different specimens in different solutions were determined at a scanning rate of 5 mV/min. Fig. 5 illustrates polarization curves of various specimens in the 0.5 M H2 SO4 solution. Significant difference in polarization behavior among the specimens was observed. The potentiodynamic polarization curves demonstrate that the nanocrystallize surfaces of aluminized carbon steel showed not only higher corrosion potential (Ecorr ) but also reduced corrosion current (Icorr ). The measured values are provided in Table 2. Compared to the aluminized carbon steel, the nanocrystalline aluminized specimens experienced recovery treatments at 200 ◦ C, 300 ◦ C and 400 ◦ C exhibit enhanced corrosion resistance in the 0.5 M H2 SO4 solution. Polarization curves of various specimens determined in the synthetic condensate respectively at room temperature and 80 ◦ C are illustrated in Figs. 6 and 7 , respectively. As illustrated, aluminized
Fig. 7. Polarization curves of various specimens in the synthetic condensate solution at 80 ◦ C.
specimens after sandblasting and annealing at 300 ◦ C and 400 ◦ C showed higher corrosion potentials and lower current densities in the synthetic condensate at both room temperature and 80 ◦ C than aluminized carbon steel without nanocrystallization. This demonstrates that surface nanocrystallization provides an effective approach to improve the corrosion resistance of aluminized carbon steel. However, sandblasting and annealing at 200 ◦ C did not show marked positive effect on the corrosion behavior of aluminized carbon steel. This could be attributed to the incomplete nanocrystallization process at the relatively lower temperature that might not able to provide sufficiently large thermal driving force to fully turn dislocation cells into nano-sized grains. Taking a close look at Fig. 3(b), one may see that some grains contain smaller sub-grains or cells, which may imply that the surface nanocrystallization treatment was only partially completed. Residual dislocations can retain the strain-hardening effect to a certain degree but negatively affect the electrochemical behavior of the material with higher activity
Table 2 Ecorr and Icorr of various specimens in different solutions. Ecorr (mV, SCE)
Icorr (A/cm2 )
Solution
Specimen
0.5 M H2 SO4 , 25 ◦ C
Aluminized carbon steel Sandblasted-annealed at 200 ◦ C Sandblasted-annealed at 300 ◦ C Sandblasted-annealed at 400 ◦ C
−674 −594 −638 −580
2.66 × 10−3 2.47 × 10−4 1.57 × 10−4 1.40 × 10−4
Condensate, 25 ◦ C
Aluminized carbon steel Sandblasted-annealed at 200 ◦ C Sandblasted-annealed at 300 ◦ C Sandblasted-annealed at 400 ◦ C
−1180 −1080 −1070 −1110
4.68 × 10−6 1.14 × 10−5 1.76 × 10−6 2.34 × 10−6
Condensate, 80 ◦ C
Aluminized carbon steel Sandblasted-annealed at 200 ◦ C Sandblasted-annealed at 300 ◦ C Sandblasted-annealed at 400 ◦ C
−1220 −1270 −1202 −1211
5.58 × 10−5 9.52 × 10−5 8.83 × 10−6 3.81 × 10−5
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Fig. 9. Volume losses of aluminized specimens with different additional treatments caused by wear and corrosive wear.
Fig. 8. (a) Schematic illustration of the electron work function; (b) surface electron work functions of various specimens after exposed to different environments.
(i.e. more anodic), bearing in mind that dislocations increase the activity of electrons and thus promote electrochemical reactions [16]. The polarization behavior or corrosion resistance of the specimens is related to their surface chemical stability, which may be evaluated by their surface electron work function that is the minimum energy required to move electrons from the inside a metal (at Fermi energy level) to surface with zero kinetic energy [17] (see Fig. 8(a)). EWF, as a sensitive indicator of the surface condition [18–20], is often used for corrosion studies [19,21,22]. The activity or inertness of a metal is characterized by its absolute electrode potential (Em ), which is dependent on the electron work function (EWF: ϕm ) of the metal and the contact potential difference (m ) s at the metal (M)–solution (S) interface [28]: E m = m + m s Therefore, the surface reactivity or corrosion behavior of a metal is intrinsically determined by the electron work function and the surrounding medium or environment influences the surface electrochemical reactions as well. In a specific environment, if a passive film or a thin oxide scale forms on the metal, escape of electrons from the metal will be affected, which equivalently changes the apparent or measured EWF. In this study, electron work functions of nanocrystalline aluminized surfaces layers and untreated aluminized surface after being immersed in different solutions, respectively, for 60 min were measured using a scanning Kelvin probe. During the measurement, an area of 280 m × 280 m was scanned for each specimen. Fig. 8(b) presents surface electron work functions of various specimens after exposure to four environments: air (i.e. the natural state), a H2 SO4 solution, and condensates at room temperature and 80 ◦ C, respectively. As demonstrated, the nanocrystallized surfaces show increased EWF, compared to the untreated aluminized surface. In particular, the nanocrystalline aluminized surfaces experienced annealing respectively at 300 ◦ C and 400 ◦ C for 60 min exhibit considerably higher EWF. The results of the EWF measurement suggest that the surface nanocrystallization helped to develop a more protective passive film associated with enhanced corrosion resistance. Consistent with the electrochemical properties (polarization, corrosion potential and current), the surface sandblasted and annealed at 200 ◦ C showed lower EWF than those annealed at 300 ◦ C and 400 ◦ C. As explained earlier, at lower annealing temperatures, the recovery process may only cause incomplete nanocrystallization during which dislocation cells are not fully turned into nano-sized grains. The residual dislocations can lower the EWF and make the surface more anodic than those with fewer dislocations [19,29,30]. It may also need to indicate that in the
hot condensate all surfaces show similar EWFs, which should be attributed to the formation of thicker chemical reaction product (aluminum salt) scales. Although their actual thickness was not determined, the product scales were apparently thicker with obvious changes in surface color after exposure to the hot condensate. This scale reduced the interaction between the surface electrons and the surrounding medium. 3.4. Resistances to wear in corrosive environments The surface nanocrystallization is not designed to resist severe wear, since the nanocrystallized surface layers produced by sandblasting and recovery is not very thick (in the range of 30–60 m) [7,32]. However, when mechanical attacks are simultaneously involved during corrosion, if not very severe, the improved mechanical strength of the nanocrystallized surface will certainly help to suppress the synergy of corrosion and wear, thus prolonging the service life of the surfaces. The resistances of the nanocrystalline aluminized surfaces of carbon steel to both mechanical attack (wear) and that in corrosive environments (corrosive wear) were evaluated under a load of 2.5 N using a pin-on-disc tribometer. Fig. 9 illustrates volume losses of various specimens during wear tests in various environments, which were the same as those used for the polarization tests. It was demonstrated that the nanocrystalline specimens showed considerably increased resistances to wear and corrosive wear. The volume losses of the specimens caused by corrosive wear in the 0.5 M H2 SO4 solution and room-temperature condensate were smaller than those caused by dry wear. This happened probably because the H2 SO4 solution or room-temperature condensate, to which aluminized surface are electrochemically resistant, could act partially as lubricants and thus decrease the wearing force. However, the volume losses of the specimens caused by corrosive wear in the condensate at 80 ◦ C were the highest, since at the elevated temperature the condensate became more aggressive and the resultant higher corrosivity enhanced the synergistic attack of corrosion and wear to the surfaces. After sandblasting and recovery treatment, the resistances of aluminized surfaces to wear and corrosive wear were enhanced, resulting from the increases in surface hardness and corrosion resistance due to the surface nanocrystallization treatment. The increases in both of the wear and corrosion resistances reduced the synergy of wear and corrosion in addition to suppressing individual failure processes. 3.5. Passivated surface The corrosion resistance of a surface with a passive film is strongly influenced by mechanical properties of the passive film and its adherence to the substrate. The resistances of passive films
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Fig. 10. Sample changes in the CER as the normal load was increased with time. The curves were obtained from the scratch test of an aluminized steel sample in air.
Fig. 11. Critical loads of aluminized specimens with different additional treatments in various environments during micro-scratching.
on various specimens to scratching failure were evaluated using a micro-scratching technique. During the micro-scratching test, the normal load was gradually increased and the contact electronic resistance (CER) was measured simultaneously. When the normal load reached a critical value, the passive film failed, accompanied with a drop in the CER. As an example, Fig. 10 illustrates changes in CER of an aluminized surface during scratching as the normal load was increased. The critical load, corresponding to the drop in CER, was a measure of the passive film’s resistance to scratching. Critical loads for passive films formed on various specimens in different environments are given in Fig. 11. The drop in CER of the specimen, which was aluminized, sandblasted and annealed at 400 ◦ C for 60 min, occurred when the normal load reached 115.2 g in air, while that of aluminized carbon steel occurred when the load reached 62.1 g. Clearly, the natural passive film or the oxide
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film on the aluminized nanocrystalline surface formed in air had a considerably higher resistance to scratching failure. The observed increase in corrosion resistance of the surface nanocrystallized specimens is largely attributed to the improvement of their passivation capability with faster formation of a stronger passive film (Al2 O3 ). The high-density grain boundaries promoted atomic diffusion and helped the oxide film to peg into the grain boundaries, which made passivation faster or enhanced the self-repair capability of the passive film and strengthened the interface between the passive film and the substrate. However, the critical load of the sandblasted sample annealed at the lower temperature of 200 ◦ C is comparable to that of the aluminized one, possibly due to relatively lower adherence of the passive film to substrate when it formed on an incomplete nanocrystalline surface with residual dislocation cells. The situation is similar for passive films formed in the H2 SO4 solution and the room-temperature condensate. Critical loads of all specimens were measured after they were exposed in a 0.5 M H2 SO4 solution and room-temperature condensate for 60 min, respectively (presented in Fig. 11). However, when specimens were immersed in the condensate at 80 ◦ C for 60 min, the specimens’ surface was covered by a thicker chemical reaction product (aluminum salt) film as mentioned earlier. So the critical loads of all specimens in the condensate at 80 ◦ C cannot reflect the true passive film’s resistance to scratching in this specific environment. Nano-indentation is an effective technique to evaluate the mechanical behavior of passive films [23,24]. In this study, nanoindentation tests were performed on surfaces of all specimens. A sample force-depth curve obtained under a maximum test load of 50 N is illustrated in Fig. 12. The maximum depths and values of various specimens are presented in Table 3. The value is defined as the ratio of the elastic deformation energy (the area under the unloading curve) to the total deformation energy (the area under the loading curve) [25], which reflects the elastic behavior of the surface under test. In Table 3, one may see that the surface of nanocrystalline aluminized surface recovered at 300 ◦ C for 60 min had the smallest indentation depth and the largest value for almost all environments, which reflects superior mechanical properties (strength and elasticity) of the passivated surface of the nanocrystallized specimen (in natural state and after exposure to the corrosive environments). A passivated surface is covered by an oxide film i.e. the passive film that prevents the surface from continuous electrochemical attacks. It needs to indicate that the passive film on aluminum alloys is thin with its thickness in the range of a few nanome-
Table 3 The maximum indentation depth and ratio of different specimens. Specimens
Condition
(%)
Maximum depth (nm) Test load (50 N)
Estimated load (20 N)
Aluminized carbon steel
Natural passive film Exposed to 0.5 M H2 SO4 for 60 min Exposed to 25 ◦ C—condensate for 60 min Exposed to 80 ◦ C—condensate for 60 min
8.3 5.1 9.6 19.1
3.3 2.0 3.8 7.6
41.6 83.7 35.7 20.1
Sandblasted and annealed at 200 ◦ C
Natural passive film Exposed to 0.5 M H2 SO4 for 60 min Exposed to 25 ◦ C—condensate for 60 min Exposed to 80 ◦ C—condensate for 60 min
9.4 5.5 10.2 18.4
3.8 2.2 4.1 7.4
48.5 79.0 39.3 25.4
Sandblasted and annealed at 300 ◦ C
Natural passive film Exposed in 0.5 M H2 SO4 for 60 min Exposed to 25 ◦ C—condensate for 60 min Exposed to 80 ◦ C—condensate for 60 min
6.2 3.8 7.1 10.5
2.5 1.5 2.8 4.2
84.2 96.5 83.5 27.4
Sandblasted and annealed at 400 ◦ C
Natural passive film Exposed to 0.5 M H2 SO4 for 60 min Exposed to 25 ◦ C—condensate for 60 min Exposed to 80 ◦ C—condensate for 60 min
6.4 4.3 6.8 11.2
2.6 1.7 2.7 4.5
79.3 94.8 81.0 22.1
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work function of the nanocrystalline aluminized surface. Performances of the specimens during wear and corrosive wear were also evaluated. It is demonstrated that the resistances of aluminized carbon steel to corrosion, wear and corrosive wear were considerably improved by the surface nanocrystallization treatment. These improvements are attributed to the high-density grain boundaries, which accelerate atomic diffusion and thus passivation, block dislocation movement more effectively, and enhance the passive films’ adherence to the substrate. The passive film on the nanocrystalline surface also showed increased hardness and higher resistance to scratching.
Fig. 12. A typical load-depth curve of nano-indentation test.
ters, which however varies with the surface treatment [26] and the environment in which the passive film develops. The thickness of passive film can be as large as ∼101 nm or even larger in certain environments. In the present study, without precisely measuring the thickness of passive films we cannot exclude the influence of the aluminized steel substrate on the force-depth curves obtained for various surfaces. However, if estimating the indentation depth under a smaller load, e.g., 20 N, one may see that the indentation depths on the microcrystalline specimen (i.e. aluminized only) and nanocrystallized specimens experienced recovery treatment at 300 ◦ C and 400 ◦ C are about 3.3 nm, 2.5 nm and 2.6 nm, respectively (in natural state). The indentation depths at this scale more or less reflect higher hardness of the natural passive film on the nanocrystallized specimens, compared to the hardness of that on the microcrystalline specimen. Based on the slope of indentation curves in the loading period, one may estimate the indentation depths under even lower loads, which more represent the hardness of passive films. The accuracy of the estimation could be improved using the regression treatment. With such estimation we can draw the same conclusions: the passive films developed on the nanocrystalline aluminized surface in different environments were harder than those on microcrystalline specimens. Since we only deal with a portion of the loading curves to determine the indentation depth under light loads, the value cannot be estimated in this case. When specimens were immersed in an 80 ◦ C condensate for 60 min, thicker chemical reaction films were produced. In this case, the difference in the response to indentation among all specimens is smaller with lower as given in Table 3. The nano-indentation test result indicated that the chemical product film was less elastic with lower hardness, related to its chemical composition and structure [27]. However, the aluminized specimens, which experienced sandblasting and annealing at 300 ◦ C and 400 ◦ C, still showed harder product films than that on the untreated aluminized specimen. The faster diffusion and enhanced interfacial bonding, e.g., oxide pegging into the high-density grain boundaries, could be responsible for the improvement. However, detailed characterization of the product film developed in the synthetic condensate is beyond the scope of this study. Further studies are needed in order to obtain solid information for clarification of this issue. 4. Conclusions Surface nanocrystallization of aluminized carbon steel produced by a process combining aluminizing, sandblasting and recovery treatment was investigated, including microstructure, hardness, electrochemical behavior in different environments, and electron
Acknowledgments The authors are grateful for financial support from Natural Science and Engineering Research Council of Canada, University of Science and Technology Beijing (USTB) and China Scholar Council (CSC). References [1] S. Higuchi, K. Asakawa, Tetsu to Hagane: Journal of The Iron and Steel Institute of Japan 78 (10) (1992) 1561. [2] W. Deqing, S. Ziyuan, Z. Longjiang, Applied Surface Science 214 (2003) 304. [3] C.J. Wang, S.M. Chen, Surface and Coatings Technology 200 (2006) 6601. [4] C.W. Su, J.W. Lee, C.S. Wang, C.G. Chao, T.F. Liu, Surface & Coatings Technology 202 (2008) 1847. [5] W. Deqing, Phase evolution of an aluminized steel by oxidation treatment, Applied Surface Science 254 (2008) 3026. [6] T. Ujiro, M. Kitazawa, S. Satoh, Journal of the Society of Material Science, Japan l45 (11) (1996) 1192. [7] X.S. Guan, Z.F. Dong, D.Y. Li, Surface nanocrystallization by sandblasting and annealing for improved mechanical and tribological properties, Nanotechnology 16 (2005) 2963. [8] L. Wang, D.Y. Li, Mechanical, electrochemical and tribological properties of nanocrystalline surface of brass produced by sandblasting and annealing, Surface & Coatings technology 167 (2003) 188. [9] X.Y. Wang, D.Y. Li, Wear 255 (2003) 836. [10] T. Ujiro, M. Kitazawa, F. Togashi, Materials Performance 33 (1994) 49. [11] E. Gopal, S. Baranidharan, J Sekhar, Materials Science and Engineering 99 (1988) 413. [12] W. Deqing, S. Ziyuan, Journal of Materials Science Letters 22 (2003) 1003. [13] L.N. Larikov, V.M. Falchenko, D.F. Polishebuk, V.R. Ryabov, A.V. Lonovskays, in: G.V. Samsonov (Ed.), Protective Coatings on Metals, vol. III, Consultant Bureau, New York, 1971, p. 56. [14] J.P. Hirth, J. Lothe, Theory of Dislocations, 2nd ed., Wiley/Interscience, 1982. [15] R.W. Cahn, P. Haasen, Physical Metallurgy, 4th ed., North Holland, The Netherlands, 1996. [16] T. Ohmori, Tetsu to Hagane: Journal of the Iron and Steel Institute of Japan 72 (5) (1986) 450. [17] W. Harrison, Electronic Structure and Properties of Solids: The Physics of the Chemical Bond, Freeman, San Francisco, 1980. [18] T. Kasai, X.Y. Fu, D.A. Rigney, A.L. Zharin, Wear 225 (1999) 1186. [19] S. Tao, D.Y. Li, Philosophical Magazine Letters 88 (2008) 137. [20] B. Bhushan, A.V. Goldade, Applied Surface Science 157 (4) (2000) 373. [21] A.K. Neufeld, I.S. Cole, A.M. Bond, S.A. Furman, Corrosion Science 44 (2002) 555. [22] W. Li, D.Y. Li, Acta Materialia 54 (2006) 445. [23] M. Seo, Y. Kurata, Electrochimica Acta 48 (2003) 3221. [24] X.Y. Wang, D.Y. Li, Surface & Coatings Technology 160 (1) (2002) 20. [25] R. Liu, D.Y. Li, Y.S. Xie, R. Liewellyn, H.M. Hawthorne, Scripta Materialia 41 (1999) 691. [26] S.V. Phadnis, M.K. Totlani, D. Bhattacharya, Transactions of the Institute of Metal Finishing 76 (1998) 235. [27] S. Higuchi, K. Asakawa, Tetsu to Hagane: Journal of the Iron and Steel Institute of Japan 77 (2) (1991) 258. [28] O’.M.B. John, U.M.K. Shahed, Surface Electrochemistry: A Molecular Level Approach, Springer, New York, 1993. [29] W. Li, D.Y. Li, Material Science & Technology 18 (2002) 1057. [30] D.Y. Li, Physica Status Solidi A 191 (2002) 427. [31] H.C. Thoma, Mechanical Behavior of Materials, 2nd ed., McGraw Hill, Boston, 2000. [32] X. Tang, D.Y. Li, Scripta Materialia 58 (2008) 1090.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Nanocrystallization of aluminized surface of carbon steel for enhanced resistances to corrosion and corrosive wear C. Chen a,b , D.Y. Li b,∗ , C.J. Shang a a b
Dept. of Materials Physical and Chemical, University of Science and Technology Beijing, Beijing 100083, China Dept. of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2V4
a r t i c l e
i n f o
Article history: Received 4 June 2009 Received in revised form 12 August 2009 Accepted 13 August 2009 Available online 21 August 2009 Keywords: Nanocrystallization Aluminizing Corrosion Wear Corrosive wear Carbon steel
a b s t r a c t Aluminizing is often used to improve steel’s resistances to corrosion, oxidation and wear. This article reports our recent attempts to further improve aluminized carbon steel through surface nanocrystallization for higher resistances to corrosion and corrosive wear. The surface nanocrystallization was achieved using a process combining sandblasting and recovery heat treatment. The entire surface modification process includes dipping carbon steel specimens into a molten Al pool to form an Al coat, subsequent diffusion treatment at elevated temperature to form an aluminized layer, sandblasting to generate dislocation network or cells, and recovery treatment to turn the dislocation cells into nano-sized grains. The grain size of the nanocrystallized aluminized surface layer was in the range of 20–100 nm. Electrochemical properties, electron work function (EWF), and corrosive wear of the nanocrystalline alloyed surfaces were investigated. It was demonstrated that the nanocrystalline aluminized surface of carbon steel exhibited improved resistances to corrosion, wear and corrosive wear. The passive film developed on the nanocrystallized aluminized surface was also evaluated in terms of its mechanical properties and adherence to the substrate. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Aluminized steels have found wide applications, such as components in vehicles, bridges, pressure containers, and house appliances, which possess higher resistances to corrosion and oxidation at elevated temperatures [1–3] than carbon steel. However, compared to ferritic and austenite stainless steel, the aluminized steel is relatively softer [4,5] with lower resistance to corrosion [6]. In recent years, we used an inexpensive surface nanocrystallization process to improve surfaces of passive alloys for higher resistances to corrosion and wear. The surface nanocrystallization is achieved using a process combining sandblasting and recovery treatment. The former generates plastic deformation in the surface layer and the latter turns the resultant dislocation cells into nano-sized grains. This technique has been applied to aluminum, brass, and stainless steel [7–9]. Previously, we demonstrated positive effects of surface nanocrystallization on properties of an Al coat on carbon steel (by hot dipping), which, however, contained little Fe. The present study is focused on aluminized carbon steel
∗ Corresponding author. E-mail address: [email protected] (D.Y. Li). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.08.016
whose surface layer contained sufficient iron content achieved by diffusion treatment of a hot dipping Al coat on the steel at 850 ◦ C for 2 h. The purpose of the surface nanocrystallization treatment applied to aluminized carbon steel is to obtain increased resistances to corrosion and corrosive wear. In the present work, the entire surface modification process included surface aluminizing by hot dipping steel samples into a molten aluminum pool and subsequently annealing them in order to change the aluminum coat to an aluminized surface layer, which contained intermetallics of Fe and Al. The aluminized surface was then nanocrystallized by sandblasting followed by recovery treatment that turned generated dislocation cells or network into grains at nano-scale. The expected improvement in corrosion resistance would improve the performance of aluminized components made of carbon steel used in exhaust systems of vehicles as well in general corrosive environments. The enhanced resistances of aluminized carbon steel to corrosive wear would prolong the service life of the components during corrosion when mechanical attacks are involved simultaneously, e.g., corrosion–erosion and erosion by exhaust gases. The objective of this study is to determine the effectiveness of the surface nanocrystallization in enhancing the overall resistances of aluminized carbon steel to corrosion and corrosive wear. Relevant issues were also investigated, such as the effects of sur-
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Table 1 Chemical composition of synthetic exhaust gas condensate (ppm). Ion
Cl−
SO3 2−
SO4 2−
CO3 2−
NO3 −
NO2 −
CH4 COO−
HCHO
HCOO−
NH4 +
Activate carbon
Content
50
250
1250
2000
100
20
400
250
100
1934
50 g/L
face nanocrystallization on properties of passive film and surface electron stability. Mechanisms responsible for observed changes in surface properties of aluminized carbon steel modified using the current treatment are discussed.
2. Experimental details The substrate material for this study was commercial 1020 carbon steel, consisting of C: 0.18–0.23 wt.%, Si: 0.15–0.35 wt.%, Mn: 0.30–0.60 wt.%, P: 0.03 wt.%, S: 0.035, balanced by Fe. Specimens having a size of 10 mm × 10 mm × 10 mm were cut from a piece of the steel and polished using #600 grit papers. All specimens were cleaned with acetone and water, and then were dipped into a molten aluminum (purity >99.9%) pool at 850 ◦ C for 30 s. The specimens with an Al coat were then embedded in the high purity aluminum powder at 850 ◦ C for 120 min to generate a thicker aluminized layer. The aluminized specimens were polished and vertically blasted by a flow of silica particles (15–40 mesh) under a pressure of 240 K Pa for 15 min. The specimens were polished with alumina particles of 0.05 m to smoothen their sandblasted surfaces, followed by recovery treatment. Several recovery temperatures, 200 ◦ C, 300 ◦ C and 400 ◦ C, were respectively used to obtain different nano-grain sizes. The microstructure of the aluminized and nanocrystallized surfaces was examined using a Hitachi S-2700 scanning electronic microscope (SEM) and their composition was analyzed using a PGT PRISM IG (Intrinsic Germanium) detector for X-ray energydispersive spectrometer (EDS) analysis. The morphology and grain size of the treated surfaces were determined using an atomic force microscope (AFM, Digital Instruments, CA, USA). Vickers-hardness of these surfaces was measured using a micro-indentation probe (Fisher Technology Ltd., Winsor, CT, USA) with a diamond tip under a maximum load of 98 mN. The electrochemical stabilities of the aluminized nanocrystalline surfaces that experienced recovery treatments at different temperatures were evaluated using a scanning Kelvin probe having a gold tip of 1 mm in diameter (KP Technology Ltd., Wick, UK) in the ambient air, 0.5 M room-temperature H2 SO4 solution and a synthetic exhaust gas condensate [6,10]. The composition of this synthetic exhaust gas condensate is given in Table 1. The oscillation frequency of the SKP tip was 173.5 Hz.
Dynamic polarization experiments were performed using a commercial electrochemical system (Model PC4-750, Gamry Instruments Inc., Warminster, PA, USA). The scan rate was 5 mV s−1 and the experiments were performed in the 0.5 M H2 SO4 solution and the synthetic exhaust gas condensate (at 25 ◦ C and 80 ◦ C), respectively. A saturated calomel electrode (SCE) was used as the reference electrode, and a platinum plate (Pt) was used as the counter electrode. Wear and corrosive wear of all specimens’ surface were evaluated using a tribometer (Center for Tribology Inc., Campbell, CA, USA). During the wear test, an alloy steel ball with its diameter equal to 3.95 mm slid on a specimen along a circle of 5 mm in diameter at a velocity of 120 mm/s under an applied load of 2.5 N for 10 min. A special container was made for wear tests in a corrosive solution. The corrosive solution was similar to that used for the electrochemical tests. The corrosive wear tests were carried out under the same loading condition as that for the dry wear test. The failure resistances of passive films on various specimens were evaluated using a micro-scratcher with a tip made of tungsten carbide. During the scratch test, the normal load was linearly increased from 0 to a designated level and the tip of the scratcher moved at a velocity of 0.1 mm/s. During this process, changes in the contact electrical resistance (CER) with respect to the load were recorded. When a passive film failed under a critical force, the CER dropped steeply. The critical load corresponding to the drop in CER reflects the resistance of the passive film to scratch failure. Critical loads for passive films on various specimens were determined. Surface mechanical properties of specimens experienced recovery treatments at various temperatures were evaluated using a triboscope which is a combination of a nano-mechanical probe and an atomic force microscope. Nano-indentation tests were performed under a maximum force of 50 N.
3. Results and discussion 3.1. Microstructure Fig. 1 presents a typical cross-section backscattered electron SEM image and element distribution profiles of the aluminized surface layer on carbon steel, determined with the energy-dispersive X-ray spectroscopy (EDS). The line scanning EDS analysis indicates
Fig. 1. A cross-section backscattered electron SEM image of aluminized carbon steel and X-ray energy-dispersive spectrometer (EDS) analysis: Al and Fe distributions across the aluminized layer and the steel substrate. Dark and light dark regions in the aluminized layer are Al-rich and Fe-rich, respectively.
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Fig. 4. Average hardness values of different surfaces: (1) aluminized surface of C-steel; (2) aluminizing, sandblasting and recovery at 200 ◦ C; (3) aluminizing, sandblasting and recovery at 300 ◦ C; and (4) aluminizing, sandblasting and recovery at 400 ◦ C. Fig. 2. A representative XRD diffraction pattern of aluminized carbon steel.
that there is an aluminized layer of 900 m in thickness on the specimen and the distributions of Al and Fe are not very homogeneous. Point EDS analysis indicated that the dark and light dark regions in the aluminized layer (see Fig. 1) are Al-rich and Fe-rich, respectively. Fig. 2 illustrates a X-ray diffraction pattern of the aluminized carbon steel, which shows several co-existing phases, including Al, Al86 Fe14 , and a small amount of AlFe3 and Al5 Fe2 . These intermetallic phases are hard [11–13], which render the aluminized surface strong and resistant to wear in addition to desired resistance to corrosion. These Fe–Al intermetallic phases cannot form if only dipping samples in the molten aluminum pool, which generates an aluminum coat but not an aluminized layer. The annealing treatment is necessary to activate the reactions in order to form the intermetallic phases. The objective of this study is to further enhance the resistances of aluminized carbon steel to corrosion, wear and corrosive wear through surface nanocrystallization treatment using a process combining sandblasting and recovery heat treatment. Fig. 3(a) presents a typical optical micrograph of the untreated aluminized layer. As shown, the size of grains was approximately 20–80 m. Microstructures of aluminized layers experienced sandblasting and recovery treatments at different temperatures were examined under an atomic force microscope (after slight polishing and etching). Representative AFM images are illustrated in Fig. 3(b)–(d). As shown, the grain size was around 10–100 nm when the sandblasted specimen was annealed at 200 ◦ C for 60 min. The grain size became slightly larger when the recovery temperature was increased to 300 ◦ C. There is no obvious difference in grain size between surfaces recovered respectively at 300 ◦ C and 400 ◦ C. The recovery treatment helped dislocation cells or network generated by sandblasting to move or be rearranged, which were turned into nano-sized grains with sharp boundaries. At lower recovery temperatures, the thermal driving force was smaller
and the rearrangement of dislocations proceeded more locally, thus leading to smaller grains. However, if temperature is too low, nanocrystallization cannot be achieved or may be partially achieved. As the recovery temperature was increased, larger grains could form but the situation was saturated as the recovery temperature reached a certainly level. These have been illustrated in Fig. 3(b)–(d). If the temperature is higher than the recrystallization temperature, recrystallization will occur. 3.2. Surface hardness Surface micro-hardness of the specimens was measured under a maximum load of 98 mN. Fig. 4 illustrates average Vickershardness values of differently treated surfaces. As demonstrated, the hardness of aluminized nanocrystalline surfaces is about 60% higher than the untreated aluminized surface. The increase in hardness of the nanocrystallized surfaces is largely attributed to the nanocrystalline structure with high-density grain boundaries that effectively block the movement of dislocations. As a result, the aluminized nanocrystalline surface is harder than the aluminized microcrystalline one. Possible influences of other factors, such as densification of material and adding residual stresses caused by sandblasting, on the increase in hardness could be excluded to a large content after the sandblasted surfaces experienced recovery treatment at elevated temperatures. It needs to be indicated that the situation was different if the surface was sandblasted only. In this case, nano-sized “sub-grains” or more precisely the dislocation cells were generated by heavy deformation in the sandblasted surface layer with diffuse boundaries. Based on extensive studies on dislocation and well-established dislocation theory [14,31], the walls between adjacent dislocation cells are diffused in nature. The diffuse grain boundaries may not block dislocations effectively. Although high-density dislocations causes strain-hardening, the material is actually “damaged” with lower ductility/toughness or reduced capability for energy absorption before failure. The recovery treatment diminished or rearranged dislocations and sharpened the boundaries with increased misori-
Fig. 3. (a) An optical microscopic (OM) image of a untreated aluminized surface and AFM images (top view) of sandblasted aluminized surfaces experienced recovery treatments at different temperatures: (b) 200 ◦ C, (c) 300 ◦ C, and (d) 400 ◦ C.
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Fig. 5. Polarization curves of various specimens in a 0.5 M H2 SO4 solution. Fig. 6. Polarization curves of various specimens in the synthetic condensate solution at 25 ◦ C.
entation between adjacent grains or sub-grains [15]. As result, the nano-sized perfect crystal grains with improved grain boundaries certainly improve the mechanical properties of the treated surfaces. The authors would like to point out that the hardness values of different surfaces in Fig. 4 are presented only to show a general trend that reflects the effect of surface nanocrystallization on surface hardness without intention to provide accurate quantitative hardness values. Since the surface layers containing various intermetallic phases were rather inhomogeneous, the fluctuation in the range of 20–30% was not unexpected during the hardness measurement. 3.3. The electrochemical behavior Nanocrystallization has strong influence on the electrochemical properties of materials. Polarization behaviors of specimens experienced aluminizing and surface nanocrystallization in a 0.5 M H2 SO4 solution at room temperature and in a synthetic condensate at room temperature and 80 ◦ C, respectively, were evaluated. Potentiodynamic polarization curves of different specimens in different solutions were determined at a scanning rate of 5 mV/min. Fig. 5 illustrates polarization curves of various specimens in the 0.5 M H2 SO4 solution. Significant difference in polarization behavior among the specimens was observed. The potentiodynamic polarization curves demonstrate that the nanocrystallize surfaces of aluminized carbon steel showed not only higher corrosion potential (Ecorr ) but also reduced corrosion current (Icorr ). The measured values are provided in Table 2. Compared to the aluminized carbon steel, the nanocrystalline aluminized specimens experienced recovery treatments at 200 ◦ C, 300 ◦ C and 400 ◦ C exhibit enhanced corrosion resistance in the 0.5 M H2 SO4 solution. Polarization curves of various specimens determined in the synthetic condensate respectively at room temperature and 80 ◦ C are illustrated in Figs. 6 and 7 , respectively. As illustrated, aluminized
Fig. 7. Polarization curves of various specimens in the synthetic condensate solution at 80 ◦ C.
specimens after sandblasting and annealing at 300 ◦ C and 400 ◦ C showed higher corrosion potentials and lower current densities in the synthetic condensate at both room temperature and 80 ◦ C than aluminized carbon steel without nanocrystallization. This demonstrates that surface nanocrystallization provides an effective approach to improve the corrosion resistance of aluminized carbon steel. However, sandblasting and annealing at 200 ◦ C did not show marked positive effect on the corrosion behavior of aluminized carbon steel. This could be attributed to the incomplete nanocrystallization process at the relatively lower temperature that might not able to provide sufficiently large thermal driving force to fully turn dislocation cells into nano-sized grains. Taking a close look at Fig. 3(b), one may see that some grains contain smaller sub-grains or cells, which may imply that the surface nanocrystallization treatment was only partially completed. Residual dislocations can retain the strain-hardening effect to a certain degree but negatively affect the electrochemical behavior of the material with higher activity
Table 2 Ecorr and Icorr of various specimens in different solutions. Ecorr (mV, SCE)
Icorr (A/cm2 )
Solution
Specimen
0.5 M H2 SO4 , 25 ◦ C
Aluminized carbon steel Sandblasted-annealed at 200 ◦ C Sandblasted-annealed at 300 ◦ C Sandblasted-annealed at 400 ◦ C
−674 −594 −638 −580
2.66 × 10−3 2.47 × 10−4 1.57 × 10−4 1.40 × 10−4
Condensate, 25 ◦ C
Aluminized carbon steel Sandblasted-annealed at 200 ◦ C Sandblasted-annealed at 300 ◦ C Sandblasted-annealed at 400 ◦ C
−1180 −1080 −1070 −1110
4.68 × 10−6 1.14 × 10−5 1.76 × 10−6 2.34 × 10−6
Condensate, 80 ◦ C
Aluminized carbon steel Sandblasted-annealed at 200 ◦ C Sandblasted-annealed at 300 ◦ C Sandblasted-annealed at 400 ◦ C
−1220 −1270 −1202 −1211
5.58 × 10−5 9.52 × 10−5 8.83 × 10−6 3.81 × 10−5
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Fig. 9. Volume losses of aluminized specimens with different additional treatments caused by wear and corrosive wear.
Fig. 8. (a) Schematic illustration of the electron work function; (b) surface electron work functions of various specimens after exposed to different environments.
(i.e. more anodic), bearing in mind that dislocations increase the activity of electrons and thus promote electrochemical reactions [16]. The polarization behavior or corrosion resistance of the specimens is related to their surface chemical stability, which may be evaluated by their surface electron work function that is the minimum energy required to move electrons from the inside a metal (at Fermi energy level) to surface with zero kinetic energy [17] (see Fig. 8(a)). EWF, as a sensitive indicator of the surface condition [18–20], is often used for corrosion studies [19,21,22]. The activity or inertness of a metal is characterized by its absolute electrode potential (Em ), which is dependent on the electron work function (EWF: ϕm ) of the metal and the contact potential difference (m ) s at the metal (M)–solution (S) interface [28]: E m = m + m s Therefore, the surface reactivity or corrosion behavior of a metal is intrinsically determined by the electron work function and the surrounding medium or environment influences the surface electrochemical reactions as well. In a specific environment, if a passive film or a thin oxide scale forms on the metal, escape of electrons from the metal will be affected, which equivalently changes the apparent or measured EWF. In this study, electron work functions of nanocrystalline aluminized surfaces layers and untreated aluminized surface after being immersed in different solutions, respectively, for 60 min were measured using a scanning Kelvin probe. During the measurement, an area of 280 m × 280 m was scanned for each specimen. Fig. 8(b) presents surface electron work functions of various specimens after exposure to four environments: air (i.e. the natural state), a H2 SO4 solution, and condensates at room temperature and 80 ◦ C, respectively. As demonstrated, the nanocrystallized surfaces show increased EWF, compared to the untreated aluminized surface. In particular, the nanocrystalline aluminized surfaces experienced annealing respectively at 300 ◦ C and 400 ◦ C for 60 min exhibit considerably higher EWF. The results of the EWF measurement suggest that the surface nanocrystallization helped to develop a more protective passive film associated with enhanced corrosion resistance. Consistent with the electrochemical properties (polarization, corrosion potential and current), the surface sandblasted and annealed at 200 ◦ C showed lower EWF than those annealed at 300 ◦ C and 400 ◦ C. As explained earlier, at lower annealing temperatures, the recovery process may only cause incomplete nanocrystallization during which dislocation cells are not fully turned into nano-sized grains. The residual dislocations can lower the EWF and make the surface more anodic than those with fewer dislocations [19,29,30]. It may also need to indicate that in the
hot condensate all surfaces show similar EWFs, which should be attributed to the formation of thicker chemical reaction product (aluminum salt) scales. Although their actual thickness was not determined, the product scales were apparently thicker with obvious changes in surface color after exposure to the hot condensate. This scale reduced the interaction between the surface electrons and the surrounding medium. 3.4. Resistances to wear in corrosive environments The surface nanocrystallization is not designed to resist severe wear, since the nanocrystallized surface layers produced by sandblasting and recovery is not very thick (in the range of 30–60 m) [7,32]. However, when mechanical attacks are simultaneously involved during corrosion, if not very severe, the improved mechanical strength of the nanocrystallized surface will certainly help to suppress the synergy of corrosion and wear, thus prolonging the service life of the surfaces. The resistances of the nanocrystalline aluminized surfaces of carbon steel to both mechanical attack (wear) and that in corrosive environments (corrosive wear) were evaluated under a load of 2.5 N using a pin-on-disc tribometer. Fig. 9 illustrates volume losses of various specimens during wear tests in various environments, which were the same as those used for the polarization tests. It was demonstrated that the nanocrystalline specimens showed considerably increased resistances to wear and corrosive wear. The volume losses of the specimens caused by corrosive wear in the 0.5 M H2 SO4 solution and room-temperature condensate were smaller than those caused by dry wear. This happened probably because the H2 SO4 solution or room-temperature condensate, to which aluminized surface are electrochemically resistant, could act partially as lubricants and thus decrease the wearing force. However, the volume losses of the specimens caused by corrosive wear in the condensate at 80 ◦ C were the highest, since at the elevated temperature the condensate became more aggressive and the resultant higher corrosivity enhanced the synergistic attack of corrosion and wear to the surfaces. After sandblasting and recovery treatment, the resistances of aluminized surfaces to wear and corrosive wear were enhanced, resulting from the increases in surface hardness and corrosion resistance due to the surface nanocrystallization treatment. The increases in both of the wear and corrosion resistances reduced the synergy of wear and corrosion in addition to suppressing individual failure processes. 3.5. Passivated surface The corrosion resistance of a surface with a passive film is strongly influenced by mechanical properties of the passive film and its adherence to the substrate. The resistances of passive films
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Fig. 10. Sample changes in the CER as the normal load was increased with time. The curves were obtained from the scratch test of an aluminized steel sample in air.
Fig. 11. Critical loads of aluminized specimens with different additional treatments in various environments during micro-scratching.
on various specimens to scratching failure were evaluated using a micro-scratching technique. During the micro-scratching test, the normal load was gradually increased and the contact electronic resistance (CER) was measured simultaneously. When the normal load reached a critical value, the passive film failed, accompanied with a drop in the CER. As an example, Fig. 10 illustrates changes in CER of an aluminized surface during scratching as the normal load was increased. The critical load, corresponding to the drop in CER, was a measure of the passive film’s resistance to scratching. Critical loads for passive films formed on various specimens in different environments are given in Fig. 11. The drop in CER of the specimen, which was aluminized, sandblasted and annealed at 400 ◦ C for 60 min, occurred when the normal load reached 115.2 g in air, while that of aluminized carbon steel occurred when the load reached 62.1 g. Clearly, the natural passive film or the oxide
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film on the aluminized nanocrystalline surface formed in air had a considerably higher resistance to scratching failure. The observed increase in corrosion resistance of the surface nanocrystallized specimens is largely attributed to the improvement of their passivation capability with faster formation of a stronger passive film (Al2 O3 ). The high-density grain boundaries promoted atomic diffusion and helped the oxide film to peg into the grain boundaries, which made passivation faster or enhanced the self-repair capability of the passive film and strengthened the interface between the passive film and the substrate. However, the critical load of the sandblasted sample annealed at the lower temperature of 200 ◦ C is comparable to that of the aluminized one, possibly due to relatively lower adherence of the passive film to substrate when it formed on an incomplete nanocrystalline surface with residual dislocation cells. The situation is similar for passive films formed in the H2 SO4 solution and the room-temperature condensate. Critical loads of all specimens were measured after they were exposed in a 0.5 M H2 SO4 solution and room-temperature condensate for 60 min, respectively (presented in Fig. 11). However, when specimens were immersed in the condensate at 80 ◦ C for 60 min, the specimens’ surface was covered by a thicker chemical reaction product (aluminum salt) film as mentioned earlier. So the critical loads of all specimens in the condensate at 80 ◦ C cannot reflect the true passive film’s resistance to scratching in this specific environment. Nano-indentation is an effective technique to evaluate the mechanical behavior of passive films [23,24]. In this study, nanoindentation tests were performed on surfaces of all specimens. A sample force-depth curve obtained under a maximum test load of 50 N is illustrated in Fig. 12. The maximum depths and values of various specimens are presented in Table 3. The value is defined as the ratio of the elastic deformation energy (the area under the unloading curve) to the total deformation energy (the area under the loading curve) [25], which reflects the elastic behavior of the surface under test. In Table 3, one may see that the surface of nanocrystalline aluminized surface recovered at 300 ◦ C for 60 min had the smallest indentation depth and the largest value for almost all environments, which reflects superior mechanical properties (strength and elasticity) of the passivated surface of the nanocrystallized specimen (in natural state and after exposure to the corrosive environments). A passivated surface is covered by an oxide film i.e. the passive film that prevents the surface from continuous electrochemical attacks. It needs to indicate that the passive film on aluminum alloys is thin with its thickness in the range of a few nanome-
Table 3 The maximum indentation depth and ratio of different specimens. Specimens
Condition
(%)
Maximum depth (nm) Test load (50 N)
Estimated load (20 N)
Aluminized carbon steel
Natural passive film Exposed to 0.5 M H2 SO4 for 60 min Exposed to 25 ◦ C—condensate for 60 min Exposed to 80 ◦ C—condensate for 60 min
8.3 5.1 9.6 19.1
3.3 2.0 3.8 7.6
41.6 83.7 35.7 20.1
Sandblasted and annealed at 200 ◦ C
Natural passive film Exposed to 0.5 M H2 SO4 for 60 min Exposed to 25 ◦ C—condensate for 60 min Exposed to 80 ◦ C—condensate for 60 min
9.4 5.5 10.2 18.4
3.8 2.2 4.1 7.4
48.5 79.0 39.3 25.4
Sandblasted and annealed at 300 ◦ C
Natural passive film Exposed in 0.5 M H2 SO4 for 60 min Exposed to 25 ◦ C—condensate for 60 min Exposed to 80 ◦ C—condensate for 60 min
6.2 3.8 7.1 10.5
2.5 1.5 2.8 4.2
84.2 96.5 83.5 27.4
Sandblasted and annealed at 400 ◦ C
Natural passive film Exposed to 0.5 M H2 SO4 for 60 min Exposed to 25 ◦ C—condensate for 60 min Exposed to 80 ◦ C—condensate for 60 min
6.4 4.3 6.8 11.2
2.6 1.7 2.7 4.5
79.3 94.8 81.0 22.1
124
C. Chen et al. / Electrochimica Acta 55 (2009) 118–124
work function of the nanocrystalline aluminized surface. Performances of the specimens during wear and corrosive wear were also evaluated. It is demonstrated that the resistances of aluminized carbon steel to corrosion, wear and corrosive wear were considerably improved by the surface nanocrystallization treatment. These improvements are attributed to the high-density grain boundaries, which accelerate atomic diffusion and thus passivation, block dislocation movement more effectively, and enhance the passive films’ adherence to the substrate. The passive film on the nanocrystalline surface also showed increased hardness and higher resistance to scratching.
Fig. 12. A typical load-depth curve of nano-indentation test.
ters, which however varies with the surface treatment [26] and the environment in which the passive film develops. The thickness of passive film can be as large as ∼101 nm or even larger in certain environments. In the present study, without precisely measuring the thickness of passive films we cannot exclude the influence of the aluminized steel substrate on the force-depth curves obtained for various surfaces. However, if estimating the indentation depth under a smaller load, e.g., 20 N, one may see that the indentation depths on the microcrystalline specimen (i.e. aluminized only) and nanocrystallized specimens experienced recovery treatment at 300 ◦ C and 400 ◦ C are about 3.3 nm, 2.5 nm and 2.6 nm, respectively (in natural state). The indentation depths at this scale more or less reflect higher hardness of the natural passive film on the nanocrystallized specimens, compared to the hardness of that on the microcrystalline specimen. Based on the slope of indentation curves in the loading period, one may estimate the indentation depths under even lower loads, which more represent the hardness of passive films. The accuracy of the estimation could be improved using the regression treatment. With such estimation we can draw the same conclusions: the passive films developed on the nanocrystalline aluminized surface in different environments were harder than those on microcrystalline specimens. Since we only deal with a portion of the loading curves to determine the indentation depth under light loads, the value cannot be estimated in this case. When specimens were immersed in an 80 ◦ C condensate for 60 min, thicker chemical reaction films were produced. In this case, the difference in the response to indentation among all specimens is smaller with lower as given in Table 3. The nano-indentation test result indicated that the chemical product film was less elastic with lower hardness, related to its chemical composition and structure [27]. However, the aluminized specimens, which experienced sandblasting and annealing at 300 ◦ C and 400 ◦ C, still showed harder product films than that on the untreated aluminized specimen. The faster diffusion and enhanced interfacial bonding, e.g., oxide pegging into the high-density grain boundaries, could be responsible for the improvement. However, detailed characterization of the product film developed in the synthetic condensate is beyond the scope of this study. Further studies are needed in order to obtain solid information for clarification of this issue. 4. Conclusions Surface nanocrystallization of aluminized carbon steel produced by a process combining aluminizing, sandblasting and recovery treatment was investigated, including microstructure, hardness, electrochemical behavior in different environments, and electron
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