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Lower temperature aluminizing and its effect on improving corrosion resistance of iron treated by surface mechanical attrition treatment.

[C] ACA and OCCA 2010

Abstract An ultrafine-grained surface layer with the average grain size of about 28 nm in the surface layer was fabricated on a pure Fe plate by the surface mechanical attrition treatment (SMAT). Lower temperature aluminizing treatments of the SMAT samples were investigated by scanning electron microscope and X-ray energy dispersive spectroscope. The electrochemical corrosion behavior of the aluminized SMAT sample was studied in 0.05 mol/L [Na.sub.2][SO.sub.4] + 0.05 mol/L [H.sub.2][SO.sub.4] solution, in comparison with the original SMAT and the coarse-grained sample. The results showed that SMAT had a negative effect on the corrosion resistance of Fe. An aluminized surface layer was formed on SMAT sample by aluminizing treatment at 400[degrees]C, which was much lower than that of the conventional aluminizing treatment. A successive lower temperature aluminizing process made the aluminized layer thicker and continuous. The SMAT sample treated by a successive lower temperature aluminizing had much higher corrosion resistance and exhibited passive behavior, which was due to the formation of a protective passive film.

Keywords Surface mechanical attrition treatment, Nanocrystalline materials, Lower temperature aluminizing, Corrosion resistance

Introduction

Surface mechanical attrition treatment (SMAT) has been recently developed to fabricate a nanocrystal-line surface layer of various materials to effectively improve the overall mechanical properties of bulk materials. (1-3) The basic principle of SMAT is the generation of plastic deformation in the surface layer of a bulk material by means of the repeated multidirectional impact of flying balls (GCr15 steel in most cases) on the sample surface. The plastic deformation in the surface layer under the high strain rate results in a progressive refinement of coarse grains into a nanometer regime. Up to date, most of the studies on SMAT have focused on optimization of nanostructured surface layer, (4) characterization of nanocrystalline surface layer, (5), (6) investigation of grain refinement mechanism, (7), (8) and properties of nanocrystalline surface layer. (9-13) Furthermore, since SMAT can significantly improve the mechanical and tribological properties of materials, recent studies have paid much attention to mechanical properties, such as hardness, tensile strength, friction, and wear properties. (11-13)

However, there is still insufficient information on corrosion properties of the materials treated by SMAT, which is also an important issue for its practical application. Reports on the effect of SMAT on the corrosion resistance are somewhat conflicting. (14-16) For example, Miyamoto et al. (14) reported that ultrafine-grained copper exhibited remarkably lower corrosion current in comparison with that in its coarse-grained counterpart. However, Li et al. (15) found that the corrosion rate of the surface nanocrystallized low carbon steel prepared by ultrasonic shot peening (USSP) was higher than that of the bulk steel.

Aluminizing is an effective surface coating technology to economically improve the corrosion resistance of materials. (17), (18) Nevertheless, conventional pack aluminizing processes are carried out at very high temperatures (900-1100[degrees]C) with a duration of several to dozens hours, as limited by the diffusion and reaction kinetics. Holding at such high temperatures might lead to serious distortion of workpieces and grain coarsening of the materials, hence, degradation of mechanical properties. Apparently, lowering the aluminizing temperatures is of great importance for minimizing the negative effects during the treatment and widening the application of aluminizing techniques. (19) Previous work from our group (20) showed that the diffusion of Al in the ultrafine-grained Fe layer prepared by SMAT is much enhanced compared with that in the coarse-grained counterpart in the lower temperature range, which is attributed to a large volume fraction of non-equilibrium grain boundaries (GBs). Therefore, it is expected that SMAT may provide a practical means to lower the aluminizing temperature, and to improve the corrosion resistance of the materials treated by SMAT.

In view of these considerations, in this work, a pure Fe plate was employed to study the effect of SMAT on the corrosion behavior and the lower temperature aluminizing treatment. The aluminized layer and the Al-diffusion depth in the treated surface layer were examined, and the temperature effect on the micro-structure of the aluminized surface layer of SMAT sample was discussed. The corrosion resistance of the Fe sample with and without the lower temperature aluminizing treatment was investigated by electrochemical measurements, in comparison with that of the SMAT sample and the coarse-grained counterpart.

Experimental procedure

Sample preparation

An iron plate (7 mm by 100 mm by 100 mm in size) with a purity of 99.95 wt% was subjected to the SMAT processing to achieve a nanostructured surface layer. The experimental setup and the details of SMAT processing were described in reference (4). Because of the high vibration frequency of the system, the sample surface was impacted repetitively by a large number of balls within a short period of time. As a consequence, the sample surface is plastically deformed with a high strain rate and grains in the surface layer are expected to be effectively refined.

The SMAT sample was cleaned and then aluminized by a pack aluminizing method at various lower temperatures (400-700[degrees]C). The aluminizing agents consisted of aluminum powder, iron-aluminum alloy powder, and activators ([NH.sub.4]Cl). For comparison, the coarse-grained sample was also aluminized under the same conditions. After aluminizing, the surface of the sample was cleaned to remove adhering powders.

Structure characterization

The evolution of the surface layer structure of the Fe sample was characterized by X-ray diffraction (XRD) analysis using a Rigaku D/max-[gamma]B X-ray diffractometer with Cu [K.sub.[alpha]] radiation and at a 20 scanning rate of 1.2[degrees]/min. The XRD peak broadening due to instrumental effects was calibrated by means of pure Si powders. The average grain size in the SMAT surface layer was estimated from the breadths at half maximum intensity of measured Bragg diffraction peaks using the Scherrer--Wilson method. (21)

The cross-sectional morphologies of the samples were observed using a Philips-XL30 field emission scanning electron microscope (SEM). Meanwhile, Al distribution in the aluminized surface layers was examined by using an X-ray energy dispersive spectroscope (EDS).

Corrosion test

The potentiodynamic polarization measurements were conducted in 0.05 mol/L [Na.sub.2][SO.sub.4] + 0.05 mol/L [H.sub.2][SO.sub.4] solution to investigate the corrosion behavior of aluminized SMAT samples. The tests were carried out on an electrochemical workstation model of CHI660B with a scan rate of 0.3 mV/s under the ambient temperature. A three-electrode cell was employed with a reference electrode of saturated calomel electrode (SCE), a counter electrode of platinum plate (Pt) and a working electrode of the tested sample with the treated surface exposed an area of 10 x 10 [mm.sup.2].

A simple but unique corrosion test was also introduced in our work in order to observe the effect of SMAT on corrosion behavior of the Fe sample by SEM in a more direct way. SMAT sample and the original sample were connected by epoxy with the surfaces of the samples opposite to each other, as shown in Fig. 1. The cross section of both samples was then polished for further corrosion test. This allows exactly the same corrosion condition for two different samples.

[FIGURE 1 OMITTED]

Results and discussion

Microstructure characterization of the SMAT Fe sample

Figure 2 shows typical cross-sectional SEM morphologies of the original Fe sample (sample A), SMAT Fe sample (sample B), and local magnification SEM image of the treated surface layer. It is seen that prior to the SMAT, the original sample is featured by its coarse grain, and the grain size is in a range of 100-200 [micro]m (Fig. 2a). The microstructure morphology of the treated surface layer (of about 30 [micro]m thick) obviously differs from that in the original sample, as shown in Figs. 2b and 2c. Grain boundaries in the SMAT surface layer cannot be identified as in the matrix, indicating that grains in the treated layer are significantly refined compared with that in the coarse-grained matrix.

[FIGURE 2 OMITTED]

Figure 3 shows the XRD patterns for the Fe sample before and after SMAT. It is seen that after SMAT there is an evident broadening of the Bragg reflections relative to the coarse-grained sample. This is due to the grain refinement and microstrain development. Given the [CuK.sub.[alpha]] wavelength and its extinction depth in Fe, XRD patterns reflect the structural information from the surface layer of about 10 [micro]m thick. The average grain size of the surface layer after the SMAT is estimated to be 28 nm by the Scherrer-Wilson method.

[FIGURE 3 OMITTED]

Electrochemical corrosion behavior of the SMAT Fe sample

Figure 4 shows typical potentiodynamic polarization curves obtained from Fe sample with and without SMAT during the immersion in 0.05 mol/L [Na.sub.2][SO.sub.4] + 0.05 mol/L [H.sub.2][SO.sub.4] solution. Both samples exhibit active dissolution. The shapes of SMAT sample and original sample are similar in spite of their different structures, which indicate that the electrochemical reaction processes on the two samples remain the same. However, the current density for the SMAT sample is larger than that of the coarse-grained sample at the same polarized potential, indicating that the SMAT Fe sample has a higher corrosion rate compared with the coarse-grained sample. Besides, the value of the corrosion potential is also lowed after SMAT. For example, the corrosion potential is lowered from -498 to -546 mV, while the current density increases from 1.17 to 4.68 mA c[m.sup.-2] at a fixed polarized potential (e.g., -430 mV) after SMAT.

[FIGURE 4 OMITTED]

The corrosion resistance of materials is dependent upon many factors. In this case, structural change at the surface of the SMAT sample plays an important role in its corrosion behavior. An ultrafine-grained structure is observed in the surface layer (Figs. 2b and 2c), and the average grain size calculated from XRD measurements is about 28 nm for the SMAT sample. The SMAT surface layer with mean grain size in the nanometer scale provides a large number of GBs. Because the atoms at GBs have high energies, they take part in the reaction first. Therefore, the SMAT sample provides a much higher population of active sites for the corrosion reaction compared with the coarse-grained sample, which results in the increasing current density. In addition, the considerable volume of GBs also enhances diffusion of the corrodent within the surface. These factors could be responsible for its higher dissolution kinetics compared with the coarsegrained sample.

Figure 5 shows the variation of the anodic current density with time for Fe sample with and without SMAT in 0.05 mol/L [Na.sub.2][SO.sub.4] + 0.05 mol/L [H.sub.2][SO.sub.4] solution. It is seen that the corrosion rate of the SMAT Fe is much higher than that of the coarse-grained Fe, e.g., the current density is five times higher than that of the coarse-grained sample at the initial stage of the immersion. Besides, the corrosion rate for the original Fe sample remains constant during all the immersion time while the corrosion rate of the SMAT Fe can be characterized by three stages. The first stage, during the first hour, is characterized by a relatively high current density about 3.5 mA [cm.sup.-2]. It is followed by a decrease in the current density with the increasing immersion time during the next 7 h. In the third stage, the current density is gradually slowed down to a constant value. Such variation of corrosion rate is related to the change of the grain size with the distance from the treated surface. Previous studies reported that the grain size evidently increases with the distance from the treated surface for many materials, which is identified by both XRD and TEM observations. For example, Lu and coworkers (7) found that the grain size increases gradually from 10 nm at the top surface layer to several micrometers at about 60 [micro]m deep, after SMAT to a pure Fe bulk sample. Therefore, for the SMAT sample, the volume fraction of GBs decreases with grain growth along the depth. This results in a decreased number of active atoms. As a result, the anodic current density drops with the increasing immersion time.

[FIGURE 5 OMITTED]

Figure 6 shows the cross-sectional morphologies of the corroded original sample, and SMAT sample prepared in the way as illustrated in Fig. 1 after potentiodynamic polarization test. During this test, the SMAT sample and the original sample are corroded under exactly the same condition (e.g., same solution, same polarization process). It is seen that both the original and the SMAT sample are characterized by obvious general active corrosion from optical microscopy observations (Fig. 6a), which is in agreement with the polarization test (Fig. 5). Furthermore, it is interesting to note that there is a groove (see the arrow) at the surface layer of the SMAT sample while there is no such feature on the original sample. This clearly demonstrates that the surface layer of the SMAT sample, i.e., the ultrafine-grained surface layer, has a much higher corrosion rate compared with the matrix. Figure 6b shows the local magnification SEM observation of the cross section of the SMAT sample. Obviously, the corrosion morphology of the treated surface differs from that in the matrix. A distinct groove is observed on the surface layer and the depth of the groove decreases with the increasing distance from the surface, suggesting that the corrosion extent increases with the decreasing distance from the surface. Such evolution of the geometrical feature of the groove is in good agreement with that illustrated by current density vs time curves (Fig. 5). This again provides the evidence that SMAT has a negative effect for corrosion resistance, and the corrosion resistance deteriorates with the refinement of the grain size for Fe sample. This result agrees well with the previous study carried by Li et al., (15) which showed a similar grain size effect on the electrochemical corrosion behavior for a surface nanocrystallized low carbon steel produced by USSP.

[FIGURE 6 OMITTED]

Microstructure characterization of the aluminized surface layer

Figures 7a and 7b show the cross-sectional SEM morphologies of SMAT samples after a single aluminizing treatment at 400[degrees]C for 180 min (sample C) with different scales. Figure 7c shows that of the original Fe sample treated under the same condition. Since the onset temperature for the dissociation of the activator [NH.sub.4]Cl is 380[degrees]C, the lowest aluminizing temperature is selected as 400[degrees]C. As can be seen in Fig. 7a, an aluminized surface layer of about 10 [micro]m at the maximum depth is formed in sample C. On the contrary, there is no detectable aluminized layer on the original coarse-grained Fe, as shown in Fig. 7c. However, surface morphology on a large scale shows that the aluminized surface layer is not continuous on the entire sample. As can be seen in Fig 7b, aluminized layer is not observed at some places for the SMAT sample.

[FIGURE 7 OMITTED]

Figures 8a and 8b show the corresponding EDS results of the surface layer of the SMAT sample (point A labeled in Fig. 7a) and the original Fe sample after a single aluminizing treatment. Figure 8c shows Al distribution in the aluminized surface layer of sample C. It is seen that there is a strong Al peak for the SMAT sample (Fig. 8a) while there is no delectable Al on the coarse-grained sample (Fig. 8b), which agrees well with the SEM observations. As shown in Fig. 8c, the atomic concentration of Al is about 46% at the top surface layer of sample C and its value gradually decreases with the increasing diffusion depth. The above experimental results clearly illustrate that an aluminized surface can be formed on the SMAT Fe sample during aluminizing at a much lower temperature than that for the conventional aluminizing treatment. This phenomenon has also been observed by several other investigations, such as nit riding of SMAT samples. (22), (23) The depressed aluminizing temperature is due to the much enhanced Al diffusion in the ultrafine-grained surface layer compared with the coarse-grained structure, which has been confirmed by secondary ion mass spectrometry studies in our previous work. (20) The thickness of the aluminized layer of the sample during the aluminizing is believed to be controlled by diffusion process, (24) which is described by

[FIGURE 8 OMITTED]

[d.sup.2]/t = [D.sub.[infinity]] exp (-E/KT). (1)

where d is the aluminized layer thickness, t is the aluminizing duration, [D.sub.[infinity]] is the effective diffusion coefficient at infinite temperature, E is the activation energy, k is the Boltzmann constant, and T is the temperature. The aluminized layer thickness is expected to increase with an increase of the diffusion coefficient and a decrease of the activation energy. In conventional aluminizing of coarse-grained Fe, Al diffusion in the Fe lattice dominates. However, in the SMAT Fe sample, Al diffusion is controlled by GB diffusion. The diffusivity of element along GBs is several orders of magnitude larger than the lattice diffusion. (25), (26) Thus, it is reasonable to assume that the ultrafine-grained surface layer of Fe sample provides numerous GBs that significantly promote the diffusion of Al, leading to the formation of an aluminized layer at a much lower temperature.

Although SMAT can significantly reduce the aluminizing temperature, the aluminized layer formed by a single aluminizing treatment at a low temperature is still not uniform enough from place to place by a large-scale observation (Fig. 7b). This may be attributed to the inhomogeneous distribution of active Al atmosphere since the activity of the [NH.sub.4]Cl activator is still low at this temperature. Though one may expect that increasing temperature would enhance the Al diffusion kinetics and contribute to the uniform active Al atmosphere, it should be noted that the treatment temperature influences the aluminizing of the SMAT sample in two different ways. On one hand, Al diffusion is enhanced with the increasing temperature. On the other hand, the superior ultrafine-grained surface layer will be changed during high temperature treatment, (27) which has a negative effect on the Al diffusion. Below certain temperature, the SMAT sample can maintain its ultrafine-grained surface layer since the temperature is still not high enough for severe grain growth. Thus, the Al diffusion is dominated by a fast GB diffusion controlled mechanism. Nevertheless, when the aluminizing treatment reaches to a high value, significant increment of the grain size is expected to occur, leading to the loss of large number of GBs. Therefore, the diffusion kinetic changes from the GB diffusion dominated mechanism at low temperature to conventional lattice diffusion dominated mechanism, which is far much slower and requires a much higher temperature.

Considering the negative effect caused by the high temperature treatment and the fact that the activator is still not active enough at 400[degrees]C, a successive aluminizing process at 500[degrees]C for 120 min and then at 700[degrees]C for 60 min is employed to aluminize the SMAT Fe sample. The low temperature treatment will allow effective diffusion of Al along GBs. In comparison with aluminizing at 400[degrees]C, aluminizing at 500[degrees]C provides higher activity of the activator, resulting in a more uniform distribution of active Al. Besides, this temperature is still lower than the onset temperature for severe grain growth, (27) so the Al diffusion is also enhanced with the increasing temperature. The following aluminizing treatment at the high temperature will further enhance the growth kinetics of the aluminized layer.

Figures 9a and 9b show the cross-sectional SEM morphologies of the SMAT sample after a successive aluminizing process (sample D). Figure 9c shows the Al distribution in the aluminized surface layer obtained by EDS measurements. As expected, compared with the aluminized layer achieved by a single low temperature aluminizing (Fig. 7a), the aluminized surface layer fabricated by a successive aluminizing process is much thicker (Fig. 9a). Furthermore, it is continuous on the entire Fe plate surface, as confirmed by SEM observation at a large scale (Fig. 9b). Besides, it is interesting to note that the aluminized surface layer seems to keep its original structure features such as the deformation plastic flows, and grain growth below the aluminized layer is observed compared with the original SMAT sample before aluminizing treatment (Fig. 2b). These observations indicate that the diffused Al during the first step of low temperature process provides extra stability of the aluminized layer. As shown in Fig. 9c, the measured Al profile shows that Al diffuses into a much deeper depth (about 20 [micro]m) compared to that obtained by a single aluminizing treatment (Fig. 8c).

[FIGURE 9 OMITTED]

Corrosion resistance of aluminized SMAT samples

Figure 10 shows the potentiodynamic polarization curves of the aluminized SMAT samples (samples C and D). The results for the original coarse-grained sample (sample A) and the SMAT Fe sample (sample B) are also given in the figure for comparison. Distinct differences in polarization behavior between the aluminized and untreated samples are observed. Relative to the corrosion potential of the samples without the aluminizing treatment, it is decreased after the aluminizing treatment, e.g., reaching about -0.66 V for sample C and -0.68 V for sample D. This is due to the lower corrosion potential of the iron aluminide compared with pure Fe. (28) It is generally accepted that there is no strong correlation between the corrosion potential and the corrosion rate for two different materials. However, the corrosion current density ([i.sub.corr]) is commonly utilized as an important parameter to evaluate the kinetics of corrosion reactions. By fitting the polarization curves, it is found that [i.sub.corr] of sample C is about one order of magnitude lower than that of the SMAT sample and [i.sub.corr] of sample D is about three orders of magnitude lower than that of the SMAT sample. Furthermore, it is worth mentioning that an apparent self-passivation is visible for sample D. Within the potential range of passivity, a very small anodic dissolution current which is slightly potential-dependent, is observed. This is attributed to the formation of a protective film containing aluminum oxide on the SMAT sample surface during polarization, which restricts the anodic dissolution. (29), (30) A strong potential-dependent dissolution current is observed beyond the passive region, indicating the onset of the pitting corrosion. It is clearly seen that the corrosion resistance is significantly improved after a successive aluminizing treatment on the SMAT sample due to the passivation by Al.

[FIGURE 10 OMITTED]

Figure 11 shows the SEM images of corroded surfaces of the original SMAT sample (sample B), and the SMAT sample treated by a single lower temperature aluminizing (sample C) and by a successive aluminizing process (sample D), respectively. It is seen that the samples are featured with different corrosion morphology. As shown in Fig. 11a, the SMAT sample experiences severe corrosion featured by a uniform surface dissolution, which is in agreement with the polarization measurements (Fig. 4). For sample C, the surface morphology is dominated by a general corrosion. However, localized corrosion is also observed on some parts of the surface (marked as "B" in Fig. 11b). This is inconsistent with the cross-sectional SEM observations of the aluminized SMAT sample processed by a single lower temperature aluminizing. The cross-sectional morphology shows that, although aluminized layer is formed at some places (Fig. 7a), it is not continuous from a large-scale observation (Fig. 7b). As a result, the surface morphology of the corroded sample is featured by localized corrosion at the Al concentrated place due to the formation of the passive film, while it is characterized by uniform dissolution at some places where the surface of the sample fails to form a protective aluminized layer. The corrosion current density of sample C decreases compared with untreated sample A or B (Fig. 10), owing to its smaller area of the active dissolution. Note that the surface morphology of corroded sample D shows a remarkable difference compared with that of samples B and C (Fig. 11c). It is characterized by a distinct pitting corrosion with the pit dimension about 50 [micro]m instead of the severe general corrosion, which agrees with the above discussion of the polarization curves. This result clearly illustrates that a passive film is formed on the aluminized surface layer of the SMAT sample processed by a successive aluminizing treatment, which significantly improves the corrosion resistance and hence changes the corrosion mechanism of the sample. The results in the present study are consistent with studies on the corrosion behavior of Fe-Al alloys, (28-31) which showed that Al addition to Fe reduces the dissolution current density remarkably compared with pure Fe and improves the ability to form a protective passive film in [H.sub.2][SO.sub.4] solution. In this study, the enhanced corrosion resistance of sample D relative to that of sample C is attributed not only to the much thicker aluminized surface layer, but also to the continuous distribution of Al on the surface layer, as shown in Fig. 9. Previous XPS studies showed that the passive film consists of Al(III) oxide/hydroxide and additionally formed Fe(II)/Fe(III) oxide during passivation in [H.sub.2][SO.sub.4] solution. (32) Such a passive film is typically featured with localized corrosion in the chemically aggressive environments, (33) as is also the case in our study.

[FIGURE 11 OMITTED]

Conclusions

An ultrafine-grained surface layer with the average grain size about 28 nm in the surface layer on Fe sample is prepared using the SMAT technique. The SMAT sample exhibits a uniform corrosion with a larger corrosion rate compared with the coarse-grained sample, which is due to the high activity of surface atoms in the SMAT layer. The maximum Al-diffusion depth into the SMAT sample is about 10 [micro]m after a single aluminizing treatment at 400[degrees]C. This temperature is much lower than the conventional pack aluminizing treatment temperature. The greatly enhanced aluminizing kinetics for the SMAT sample can be attributed to its large volume fraction of GBs, which promote the Al diffusion. A much thicker and continuous aluminized surface layer is obtained on the SMAT sample after a successive aluminizing process. The aluminized surface layer in the SMAT sample greatly improves the corrosion resistance, owing to the formation of a protective passive film. The corrosion mechanism of the SMAT sample processed by the successive aluminizing is featured by a localized corrosion instead of uniform dissolution for the sample without aluminizing treatment.

It is anticipated that the process of the SMAT followed by a successive aluminizing treatment might be developed to significantly modify conventional aluminizing technologies to lower the treatment temperature and to greatly improve the corrosion resistance of some materials, whose corrosion resistance deteriorates after SMAT.

Acknowledgments The authors are grateful to Professor G. Hu for his great help with SEM analysis. This work was supported by National Nature Science Foundation of China (No. 50974088), Cultural Fund for Major Program of Technology Innovation Engineering in Higher Education Institutions, China (No. 707025), Shanghai Rising-Star Program (No. 10QA1403400).

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C. Zhong, W. Hu

State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China

Y. Jiang, B. Deng, J. Li ([??])

Department of Materials Science, Fudan University, 220 Handan Road, Shanghai 200433, China

e-mail: jinli@fudan.edu.cn; corrosion@fudan.edu.cn

DOI 10.1007/s11998-010-9280-3
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Author:Zhong, Cheng; Hu, Wenbin; Jiang, Yiming; Deng, Bo; Li, Jin
Publication:JCT Research
Date:Jan 1, 2011
Words:5490
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