2019 Volume 60 Issue 8 Pages 1629-1637
In this paper, Nickel-Aluminum Bronze (NAB) alloy was subjected to the shot-peening (SP) treatment, and the corrosion resistance properties of SPed NAB alloy was systematically studied by scanning electron microscope, transmission electron microscopy, electrochemical workstation and immersion tests. The results show that SP treatment can improve the corrosion resistance properties of NAB alloy by controlling shot peening intensity. The SP treatment can result in rough surfaces, high-density dislocations and grain refinement of α and β′ martensitic phases in NAB alloy surface. In the corrosion medium, the corrosion resistance properties of SPed NAB alloys are related to not only the surface microstructures but also the surface roughness. The refined and homogenized microstructures favors the rapid formation of the protective passive film and promotes the occurrence of uniform corrosion on shot-peened NAB alloy surface, thus significantly improving their corrosion resistance properties. However, as the shot-peening intensity exceeds a critical value, the higher roughness values due to the large cracks, chips and flaking appearing on the shot-peened sample surface can deteriorate corrosion resistance properties.
As a typical and high frequently used Cu–Al alloy, Nickel-Aluminum Bronze (NAB) alloy has attracted extensive attentions in marine materials due to its better combination of high mechanical strength and corrosion resistance.1,2) The as-cast NAB alloy has complex microstructures, including α phase, β′ martensitic phase and four intermetallic precipitates (κI, κII, κIII, and κIV).3,4) The various phases in NAB alloy have different morphologies, crystal structure, and chemical composition, and thus the as-cast NAB alloy is easily subjected to electrochemistry and selective phase corrosion.1) Also, as-cast NAB alloy has coarse microstructures, which are detrimental to both its mechanical and corrosion resistance properties. Thus, considering practical service condition, it is highly desirable to obtain the combination of higher mechanical and better corrosion resistance properties of NAB alloy.5)
To improve the corrosion resistance properties of NAB alloy, many methods have been performed such as friction stir processing (FSP),6,7) ion implantation,8) electroplate9) and laser cladding10) and so on. Qin et al.8) reported that NAB alloy had better corrosion resistance properties after nickel ion implantation because of the compact Cu2O film with the incorporation of nickel ions. Luo et al.9) prepared gradient Ni–Cu layer on NAB alloy surface via thermal diffusion processing and found that the gradient layer could significantly improve the corrosion resistance properties of NAB alloy. Among the aforementioned methods, obtaining refined grains and uniform microstructures is a well-accepted method to enhance the corrosion resistance properties of NAB alloy. Ni et al.5) reported that the NAB sample subjected to friction stir processing exhibited better corrosion resistance properties than as-cast sample due to the refinement of grain and alleviation of the intermetallic precipitates. Song et al.11,12) considered that the formation of better protectiveness film on the as-FSP NAB alloy during corrosion was the main reason for its better corrosion resistance properties.
As one of the severe plastic deformation methods, shot-peening treatment is also an effective way to refine and uniform the surface microstructures of alloys.13) Meanwhile, compressive residual stress is also formed on the shot peened alloy surface, which will be of great benefit to the mechanical properties of alloys. However, during shot peening treatment, high-density dislocations, nanograins, and microstrain etc. are also induced on the surface of alloy because of severe mechanical effect.14) There is a great deal of disagreement as to the corrosion resistance properties of the residual stress and finer microstructure.15–17)
This is because the Gibbs free energy increases and the electric potential decreases according to the Nernst equation for a sample containing high stress. Some investigations also reported that low-carbon steel with nanograin exhibited a higher corrosion rate in the acidic environment and decreasing grain size can increase the corrosion rate.18,19) They considered that surface nanocrystallization of alloys can increase the number of the active sites. Many other investigations also reported that nanograins formed on the alloy surface can contribute to developing better protectiveness film, which can significantly improve its corrosion resistance properties.20–22) Besides, shot peening intensity is an essential parameter for the SPed sample. Increasing shot-peening intensity can result in finer microstructures and higher compressive residual stress, but also induce defects on the surface of SPed samples such as the cracks, chips and flaking and so on. There is no doubt that the aforementioned features formed during SP have significant influence on the corrosion resistance properties of NAB alloy. However, the effect of shot peening intensity on the corrosion resistance properties of NAB alloy has rarely been mentioned in previous investigations. Therefore, in this paper, NAB alloy was treated by shot-peening and relation between shot peening intensity and corrosion resistance properties of SPed NAB alloy were systematically studied by scanning electron microscope (SEM), transmission electron microscopy (TEM), electrochemical workstation and immersion tests.
The as-cast NAB alloy was prepared by nonvacuum melting. The chemical component and microstructures have been reported in our previous investigation,23,24) as shown in Table 1 and Fig. 1. Then the NAB alloy was subjected to SP treatment with shot-peening intensity of 0.15, 0.2 and 0.25 respectively, and the detailed preparation process has been reported in our previous study.23) To investigate the effect of roughness on corrosion resistance, no additional grinding or polishing was applied on the surface of samples after SP treatment. The surface morphologies of samples was characterized using a JEOL 7600F field emission gun scanning electron microscope (SEM). Residual stresses information was evaluated by X-ray stress analyzer (Proto LXRD) with Cu-Kα radiation and a Ni filter at the voltage of 30 kV and current of 25 mA. The shift of Cu (420) diffraction profile was tested according to the standards of ASTM-E915-2010, EN15305-2008 and GB7704-2008. Transmission electron microscope (TEM, JEM200 EX) was used to examine the microstructures in the subsurface of SPed NAB alloy (Within 50 µm from upper surface).
SEM image showing microstructures of as-cast NAB alloy.24)
The corrosion behaviors of SPed samples were examined by potential dynamic polarization and electrochemical impedance tests (EIS) using a traditional three-electrode system with the calomel reference electrode and the platinum counter electrode in a glass cell. The corrosive medium was 3.5% NaCl solution. Samples were connected to copper wires and then coated with paraffin wax leaving the peened side with the dimension of 10 mm × 10 mm as the working electrode. Before the electrochemistry test, Open-circuit potential (OCP) test was first applied on samples for 15 minutes to reach the equilibrium potential. The potential dynamic polarization tests were carried out at the scanning speed of 1.0 mV/s from −500 to 1000 mV. For EIS test, the frequency is set from 100 kHz to 0.01 HZ with 5 mV amplitude. In order to guarantee the accuracy of experiment data, the potential dynamic polarization and EIS tests of each group were performed at least three times. In order to understand the effect of shot-peening intensity on the static corrosion behavior of NAB alloy, SPed NAB samples were corroded in 3.5% NaCl salt solution for 20 days. The surface morphologies were observed using SEM, and elements mapping analysis was carried out using EDX attached to the SEM.
Figure 2 shows the top surface and cross-section morphologies of SP samples with different shot-peening intensity. From the top surface topographies (Fig. 2(a), (b) and (c)), it can be seen that the surfaces become quite uneven and rough with indentations after SP treatment. In addition, fragmentation, chipping, and micro-crack (indicated by yellow arrows) can be seen on the peened surfaces. There is the little variation on surface topographies between SP-0.15 and those SP-0.20 samples. In contrast, the roughness of SP-0.25 samples progressively increases and chipping has multiplied. The cross-sectional images of SP surfaces are also illustrated (see Fig. 2(d), (e) and (f)). It is evident that the edges of SP-0.15 and SP-0.20 samples are smooth, and severe flaking, cracks and visible plastic deformation layer can be observed on the surface of SP-0.25 samples, which indicates that the SP-0.25 samples are severely deformed. The values of roughness parameters are presented in Table 2. The Ra is surface arithmetic mean roughness, the Rq is surface root-mean-square roughness, and the Rz is the height between surface maximum peak and valleys. As apparent in Table 2, the roughness of the SP samples is significantly increased as compared to the as-cast samples, and SP-0.25 samples reach the highest roughness values of 0.348 µm while SP-0.15 and SP-0.20 samples show lower roughness values of 0.177 µm and 0.218 µm separately, which further indicates that surface roughness of SP samples is highly correlated with the values of shot-peening intensity. The increase in surface roughness after SP treatment was attributed to the presence of indentations and chipping formed by the ceramic balls.25) In the SP process, the ceramic balls hit the surface with high energy and the indentations were created with a pair of valley and peak. As the processing time increases, the peak and valley area around the indentations were continuously hit by repeated impacts, and then, due to excessive work hardening, the chips, microcracks, and fragment occurred. With the same other parameters, more chipping and higher roughness values were observed with increasing shot-peening intensity.26) Similar findings are also reported by Wang.27) The reason of these can be explained as follows: with the same other parameters, higher shot-peening intensity means higher collapse energy of ceric ball resulting in expanding plastic deformation and deep indention. After repeated impact, the deformation of larger valleys and peaks are easily subjected to excessive work hardening and then the cracks, chips, and flaking appear which can result in higher roughness values.
Surface topographies and cross-section morphologies of SPed samples with different shot-peening intensity. (a, b) SP-0.15 samples (c, d) SP-0.20 samples (e, f) SP-0.25 samples.
Figure 3 shows TEM images of SP samples with different shot-peening intensity. At the shot-peening intensity of 0.15, a significant number of dislocation cells are observed and finer α grains were formed via the dislocation segmentation (Fig. 3(a) and (b)). Lots of twins in α phase were also found, as shown in Fig. 3(c). It is indicated that high-density dislocation areas were formed in the surface of SP-sample due to severe plastic deformation during SP, while the α grains were refined via dislocation activities and mechanical twins.23) As increasing the shot-peening intensity, the morphologies of β′ martensitic transforms into twins (Fig. 3(d)). Many investigations have reported the formation reason of the martensitic twins in NAB alloy. In our previous investigation,28) we found that during hot rolling treatment, martensitic nanotwins were formed via emission of partial dislocation on adjacent planes. With further increasing the shot-peening intensity to 0.25, more high-density dislocations and α nanograins were formed (Fig. 3(e) and (f)). Therefore, it can be concluded that due to the hitting effect of the ceramic balls on NAB alloy, severe plastic deformation occurs on the NAB surface. In this case, high-density dislocations and dislocation cells were formed and α and β′ martensitic phases were refined into nanoscale.
TEM images of SP samples with different shot-peening intensity. (a, b) SP-0.15 samples (c, d) SP-0.20 samples (e, f) SP-0.25 samples.
The residual stress in the surface layer of all samples was investigated by X-ray analyzer using the sin2 ψ method. The results are shown in Fig. 4. It can be observed that SP treatment can produce high compressive residual stresses into surfaces. As-cast samples show a quite small compressive residual stresses value of 9.6 MPa, which may be caused by pretreatment of grinding. In contrast, the values of compressive residual stresses for SPed samples increase from 376.5 MPa for SP-0.15 samples to 449.1 MPa for SP-0.25 samples indicating higher compressive residual stresses due to purely mechanical effect of indentation can be obtained at higher shot-peening intensity. Trdan et al.29) reported the AA6082-T651 aluminum alloy subjected to laser shock peening had increased compressive residual stresses with the increase of pulse intensities. Moreover, Wang27) also confirmed that the larger value and depth of compressive residual stresses can be induced into the surface layer by enhancing peening intensity, which is in agreement with ur results.
The variation of compressive residual stresses in the surface region of NAB with different shot-peening intensity.
Many investigations have reported the electrochemical corrosion behavior of NAB alloy in 3.5% NaCl salt solution.5,11,12) It is well reported that the main anodic reaction process of NAB alloy is the Cu element dissolution and cathodic reaction is oxidation of oxygen. The chemical equation is: Cu + 2Cl− − e− → CuCl−2; O2 + 2H2O + 2e− → 4OH−; 2CuCl−2 + OH− → Cu2O + H2O + 4Cl−. The Al element is beneficial to improve the corrosion resistance of NAB alloy because of the formation of Al(OH)3. The chemical equation is: Al + 4Cl− → AlCl−4 + 3e−; AlCl−4 + 3H2O → Al(OH)3 + 3H+ + 4Cl−. SP treatment only modify the surface microstructures of NAB alloy without the variation of chemical component, thus we consider that the anodic and cathodic reactions of SPed NAB alloy in the polarization curves is same with the NAB alloy in previous investigations. Figure 5(a) depicts the polarization curves of all samples in initial corrosion period (15 minutes), and the values of corrosion potential (Ecorr) and corrosion current density (jcorr) fitted by the Tofel method are shown in Table 3. The polarization curves of both SP and as-cast samples are similar except that SP treatment reduces the corrosion current density and shifts the corrosion potential into cathodic value. The SP-0.25 samples achieve the smallest corrosion current density value of 7.759 mA/mm2 with the most negative corrosion potential value of −3.40 V. The corrosion potential and corrosion current density are two parameters that describe the corrosion behaviors. The corrosion potential represents the thermodynamics factor of corrosion, and corrosion current density directly reflects dynamic corrosion behaviors and corrosion rate. The lower corrosion potential may be due to the increase of dislocations, residual stress and Gibbs free energy.5,30) As shown in our results, SP samples have more negative corrosion potential, but it doesn’t means that SP samples are susceptible to corrosion in the initial stage of corrosion. In Fig. 5, it can be seen that anodic polarization curve of SPed sample firstly become passive like state. It is indicated that SPed sample can form rapidly corrosion protective film compared with as-cast NAB alloy. Corrosion current density results shows that all SPed samples have lower corrosion rate than that of as-samples. Thus we consider that SP treatment can lead to rapid formation of corrosion protective film and improve the electrochemical corrosion resistance of NAB alloy.
The polarization curves of as-cast samples and SP samples at different shot-peening intensity in 3.5% NaCl solution, after (a) 15-minute immersion, (b) 20-day immersion.
The potentiodynamic anodic polarization of as-cast and SP samples after immersion in 3.5% NaCl medium for 20 days were also investigated (see Fig. 5(b)). The corrosion potentials of SP samples are more positive than that of as-cast sample which indicates that the passive films of SP samples become more thermodynamically stable than that of as-cast samples after 20-day immersion. Among three different shot-peening intensity, SP-0.20 samples have the highest corrosion potential following by SP-0.25 and SP-0.15 samples.
EIS tests are employed to exam the growth and electrochemical properties of the corrosion protective films formed on all samples after different immersion times (i.e. 15 min, 2, 5, and 10 days) in 3.5% NaCl solution. As shown in Fig. 6, the Nyquist plots of all samples exhibit a Warburg line at low frequency and a capacitive semicircle at high frequency. It is well known that the magnitude of the semicircle diameter represents the impendence and corrosion resistance of the protective film. The diameters of capacitive semicircles for both as-cast and SPed samples are increased with the increase of immersion time. The rise in diameters of capacitive semicircles indicates that corrosion protective films gradually grow on the surfaces of SPed samples. In the initial period of corrosion (15 min), the diameters of capacitive semicircles for SPed samples are smaller than that for as-cast samples (Fig. 6(a)). As the immersion time increases, the diameters of capacitive semicircles for SPed samples grow more rapidly. After 2-day immersion (Fig. 6(b)), SPed samples begin to have larger semicircles than as-cast sample. After 10-day immersion (Fig. 6(d)), the diameter of the capacitive circle for SP-0.20 samples is as much twice as that for as-cast samples. From the results of Nyquist curves, it is apparent that the protective film formed on SPed samples is more protective.
The Nyquist plots of as-cast NAB and SPed samples with different shot-peening intensity in 3.5% NaCl solution after different immersion time. (a) 15 min (b) 2-day (c) 5-day (d) 10-day.
The rapid formation of corrosion protective film in SP samples is mainly attributed to the nanocrystallization and nano-twins induced by SP treatment. Pan et al.31) investigated the passive film growth mechanisms of different crystalline by electrochemical measurements and in situ AFM, and revealed that the nanostructure crystalline of the 304 stainless steel had the highest passive film growth rates and also changed nucleation mechanism of passive film from progressive to instantaneous. The passive films formed on the nanocrystallization surface of 304 stainless steel were also investigated using nano/micro/-indention, micro-scratch, SKP (Scanning Kelvin probe) by Pandey et al.,32) and they demonstrated that the passive film on the nanocrystallization had greater mechanical properties, more rapid capacity of passivation and higher chemistry stability. The similar results were found on commercial brass (70-30) and Ni–Ti shape memory alloy.33,34) As seen in Fig. 3, nanocrystallization as well as nano-twins have been formed in the surface region. Therefore, the possible explanation for the rapid formation of corrosion protective films in the SP samples may be that the massive amounts of boundaries and sub-boundaries created by nano-structure can promote more diffusion paths to form protective films and enhance the densities of films at the same time. This is supported by Balusamy et al.35) and Ye et al.,36) who reported that high densities of grain boundaries could promote the diffusion of Cr to the surface and form the homogeneous oxide layer. Wang et al.37) also demonstrated the electrochemical corrosion behavior of nanocrystallization Co coatings by higher grain boundary densities. In their regard, since the crystalline lattice defects was easier to be passivated and there are lots of grain boundaries and dislocation in the nanocrystallization Co coating, and hence it was suggested that high density of grain boundaries could provide a large number of active sites to rapidly form a uniform and protective passive film.
3.3 The static immersion corrosion behavior of SP samplesSEM was employed to observe the morphologies of corrosive surfaces after 20-day immersion in 3.5% NaCl medium. As shown in Fig. 7, the oxide products have been seen on all samples. Apparently, as-cast samples suffer from severely selective corrosion with large amounts of deep pitting. The pitting is about 40∼60 µm in size and initiates at the κ-rich location especially at the boundary of α/κIII. Some partially dissolved inclusions can also be observed inside the pitting as yellow arrows shown (Fig. 7(a)). In contrast, the corrosive surfaces of the SPed samples are totally different from that of as-cast samples and reveal the absence of severe pitting. For the SP-0.15 samples, the corrosion pitting with smaller amounts and shallower depth is exhibited on the surface. With increasing shot-peening intensity to 0.20 mmA, the corrosion pitting disappears, which is replaced by uniform corrosion protective films. However, as further increasing shot-peening intensity to 0.25 mmA, the micro-cracks begin to appear on the film as indicated by the yellow arrow (see Fig. 7(d)). Figure 8 and Fig. 9 present the element mapping of the corrosive surface of as-cast and SP-0.20 samples after immersion in 3.5% NaCl solution for 20 days. The presence of Cu, Al, Fe, Ni, O and Cl confirms that the protective films for both as-cast and SPed samples contain oxides of aluminum and copper, copper salts, copper hydrochlorides and the oxides of nickel and iron, which is in line with the results of previous researches.1,38,39) For the as-cast samples (Fig. 8), the elements distribute on different sites are various from each other, especially for Al and Cu element. The Al element is mainly abundant in the corrosive pits and surrounds the uncorroded κ phases, while Cu element is distributed on the whole α matrix. The Al-abundant in the corrosion pits shown by element mapping confirms that the as-cast samples suffer selective corrosion and κ phases are protective. For the SP-0.20 samples (Fig. 9), the elements such as Cu, Al, and O are distributed equally in the whole corrosion surface. The homogeneous distribution of all elements shown by element mapping also indicates that the corrosion products in the surface of SP-0.20 samples are more uniform.
The corrosion morphologies images of (a) as-cast NAB (b) SP-0.15 samples (c) SP-0.20 samples (d) SP-0.25 samples after 20-day immersion in 3.5% NaCl solution.
The element mapping images of as-cast samples after 20-day immersion in 3.5% NaCl solution.
The element mapping images of SP-0.20 samples after 20-day immersion in 3.5% NaCl solution.
Combined with the previous results of EIS tests, these investigations further confirm that the corrosion protective films formed on SPed samples are more uniform and protective conspicuously in the surface of SP-0.20 samples. It is also evident that the selective corrosion has been prohibited after SP treatment. As is known to all, the selective corrosion of as-cast NAB is caused by complicated structure and galvanic couple effect. Nevertheless, during the SP treatment, apart from the formation of nanocrystallization and nano-twins, the precipitates, as well as eutectoid structure such as κIII phase in the surface layers, are fragmentized leading to the reduction of the potential gap between the matrix and inclusion phases. Hence, the galvanic couple effect and the susceptibility to selective corrosion is decreased. Pandey et al.32) found the similar result that after SP treatment, the precipitates were significantly dissolved in the matrix resulting in the decreased tendency for localized and pitting corrosion.
Taking together all results, it can be suggested that corrosion resistance of NAB can be enhanced by SP during long time immersion. And based on the results of EIS test and SEM images, it can be assumed that the higher corrosion resistance after SP is mainly attributed to the rapid formation of the protective film and uniform corrosion. Moreover, compressive residual stresses is also favorable to the corrosion resistance. Krawiec et al.40) investigated the effect of laser shocking processing on the micro-electrochemical behavior of AA2050-T8 aluminum alloy, and the results showed both charge transfer and oxide film resistance were significantly increased due to the presence of compressive residual stresses. Trdan et al. reported similar results that increase of pitting potential, as well as reduction of corrosion current density on AA6082-T651 aluminum alloy, were attributed to the higher compressive residual stresses caused by the laser shocking processing.41) Patrice et al. confirmed that the passive film formed on the high compressive residual stresses surface of 316L stainless steel was thinner but possibly more compact and resistant to breakdown when subjected to electrostrictive stress.42)
3.4 Effect of roughness on the corrosion resistance properties of SPed NAB alloyThe corrosion behaviors are not only related with the refined microstructure but also related with surface roughness. Roughness is an important factor that can retard the corrosion resistance.43,44) Lee et al.44) reported the ultrasonically peened samples showed equal or even better corrosion resistant than SP samples due to the increase of surface roughness. Azar et al.43) revealed that the roughness and heterogeneities could deteriorate the surface corrosion resistance considering the preferred locations for pitting corrosion. In our research, shown by the ESI results and SEM images, after long-time immersion in 3.5% NaCl medium, the SP-0.20 samples show the highest corrosion resistance and is followed by SP-0.25 samples implying that the corrosion resistance is not always proportional to the shot-peening intensity. The deterioration of corrosion resistance for SP-0.25 samples is mainly attributed to micro-cracks and defects which appear on corrosion protective film. Berzins et al.45) and Wood et al.46) both investigated the absorption nature for chloride on the corroded aluminum surface by using 36Cl as the radioactive tracer and other means and showed absorption was especially localized to the pit sites due to lower adsorption energy. Hence, defects and micro-cracks are susceptible to local adsorption of chloride ions which lead to further corrosion of the substrate beneath the passive film.47) As shown in Fig. 2(e), there are larger amounts of chips and flaking in the surface of SP-0.25 samples. After long time immersion, these defects can’t be healed, and still appear on protective film shown in Fig. 7(d). The micro-cracks and defects appearing on the films are the major susceptible sites which will allow the corrosive medium in and result in further corrosion of the subsurface. Thus, the formation of the stable protective film is prohibited and corrosion resistance for SP-0.25 samples is decreased.
Authors (Yuting Lv and Bingjie Zhao) contributed equally to this work. This work has been supported by the Fund for National Science Foundation under Grant No. 51801115, the Fund for Doctor of Shandong Province (No. ZR2018BEM005, ZR2018BEE014), and the scientific research foundation of Shandong University of Science and Technology for Recruited Talents under Grant No. 2017RCJJ025.
