Improving Corrosion Resistance Properties of Nickel-Aluminum Bronze (NAB)Alloys via Shot Peening Treatment

Yuting Lv1, Bingjie Zhao2, Hongbin Zhang1, Chunjian Su1, Bin Nie1, Rui Wang1,Lianmin Cao1,+ and Fuyan Lyu1

1College of Mechanical and Electronic Engineering, Shandong University of Science and Technology, Qingdao, Shan dong 266590, China2State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China

In this paper, Nickel-Aluminum Bronze (NAB) alloy was subjected to the shot-peening (SP) treatment, and the corrosion resistanceproperties of SPed NAB alloy was systematically studied by scanning electron microscope, transmission electron microscopy, electrochemicalworkstation and immersion tests. The results show that SP treatment can improve the corrosion resistance properties of NAB alloy by controllingshot peening intensity. The SP treatment can result in rough surfaces, high-density dislocations and grain refinement of ¡ and ¢ A martensiticphases in NAB alloy surface. In the corrosion medium, the corrosion resistance properties of SPed NAB alloys are related to not only the surfacemicrostructures but also the surface roughness. The refined and homogenized microstructures favors the rapid formation of the protective passivefilm and promotes the occurrence of uniform corrosion on shot-peened NAB alloy surface, thus significantly improving their corrosion resistanceproperties. However, as the shot-peening intensity exceeds a critical value, the higher roughness values due to the large cracks, chips and flakingappearing on the shot-peened sample surface can deteriorate corrosion resistance properties. [doi:10.2320/matertrans.M2019031]

(Received January 29, 2019; Accepted May 7, 2019; Published June 7, 2019)

Keywords: nickel-aluminum bronze (NAB), shot-peening treatment, corrosion resistance properties, microstructures

1. Introduction

As a typical and high frequently used Cu­Al alloy, Nickel-Aluminum Bronze (NAB) alloy has attracted extensiveattentions in marine materials due to its better combinationof high mechanical strength and corrosion resistance.1,2) Theas-cast NAB alloy has complex microstructures, including ¡

phase, ¢A martensitic phase and four intermetallic precipitates(¬I, ¬II, ¬III, and ¬IV).3,4) The various phases in NAB alloyhave different morphologies, crystal structure, and chemicalcomposition, and thus the as-cast NAB alloy is easilysubjected to electrochemistry and selective phase corrosion.1)

Also, as-cast NAB alloy has coarse microstructures, whichare detrimental to both its mechanical and corrosionresistance properties. Thus, considering practical servicecondition, it is highly desirable to obtain the combination ofhigher mechanical and better corrosion resistance propertiesof NAB alloy.5)

To improve the corrosion resistance properties of NABalloy, many methods have been performed such as frictionstir processing (FSP),6,7) ion implantation,8) electroplate9) andlaser cladding10) and so on. Qin et al.8) reported that NABalloy had better corrosion resistance properties after nickelion implantation because of the compact Cu2O film with theincorporation of nickel ions. Luo et al.9) prepared gradientNi­Cu layer on NAB alloy surface via thermal diffusionprocessing and found that the gradient layer couldsignificantly improve the corrosion resistance properties ofNAB alloy. Among the aforementioned methods, obtainingrefined grains and uniform microstructures is a well-acceptedmethod to enhance the corrosion resistance properties ofNAB alloy. Ni et al.5) reported that the NAB samplesubjected to friction stir processing exhibited better corrosionresistance properties than as-cast sample due to the refine-ment of grain and alleviation of the intermetallic precipitates.

Song et al.11,12) considered that the formation of betterprotectiveness film on the as-FSP NAB alloy duringcorrosion was the main reason for its better corrosionresistance properties.

As one of the severe plastic deformation methods, shot-peening treatment is also an effective way to refine anduniform the surface microstructures of alloys.13) Meanwhile,compressive residual stress is also formed on the shot peenedalloy surface, which will be of great benefit to the mechanicalproperties of alloys. However, during shot peening treatment,high-density dislocations, nanograins, and microstrain etc.are also induced on the surface of alloy because of severemechanical effect.14) There is a great deal of disagreementas to the corrosion resistance properties of the residual stressand finer microstructure.15­17)

This is because the Gibbs free energy increases and theelectric potential decreases according to the Nernst equationfor a sample containing high stress. Some investigations alsoreported that low-carbon steel with nanograin exhibited ahigher corrosion rate in the acidic environment anddecreasing grain size can increase the corrosion rate.18,19)

They considered that surface nanocrystallization of alloyscan increase the number of the active sites. Many otherinvestigations also reported that nanograins formed on thealloy surface can contribute to developing better protective-ness film, which can significantly improve its corrosionresistance properties.20­22) Besides, shot peening intensity isan essential parameter for the SPed sample. Increasing shot-peening intensity can result in finer microstructures andhigher compressive residual stress, but also induce defects onthe surface of SPed samples such as the cracks, chips andflaking and so on. There is no doubt that the aforementionedfeatures formed during SP have significant influence on thecorrosion resistance properties of NAB alloy. However, theeffect of shot peening intensity on the corrosion resistanceproperties of NAB alloy has rarely been mentioned inprevious investigations. Therefore, in this paper, NAB alloy+Corresponding author, E-mail: 13793293776@163.com

Materials Transactions, Vol. 60, No. 8 (2019) pp. 1629 to 1637©2019 The Japan Institute of Metals and Materials

was treated by shot-peening and relation between shotpeening intensity and corrosion resistance properties ofSPed NAB alloy were systematically studied by scanningelectron microscope (SEM), transmission electron microsco-py (TEM), electrochemical workstation and immersion tests.

2. Experimental Details

The as-cast NAB alloy was prepared by nonvacuummelting. The chemical component and microstructures havebeen reported in our previous investigation,23,24) as shown inTable 1 and Fig. 1. Then the NAB alloy was subjected to SPtreatment with shot-peening intensity of 0.15, 0.2 and 0.25respectively, and the detailed preparation process has beenreported in our previous study.23) To investigate the effect ofroughness on corrosion resistance, no additional grindingor polishing was applied on the surface of samples after SPtreatment. The surface morphologies of samples wascharacterized using a JEOL 7600F field emission gunscanning electron microscope (SEM). Residual stressesinformation was evaluated by X-ray stress analyzer (ProtoLXRD) with Cu-K¡ radiation and a Ni filter at the voltage of30 kV and current of 25mA. The shift of Cu (420) diffractionprofile was tested according to the standards of ASTM-E915-2010, EN15305-2008 and GB7704-2008. Transmissionelectron microscope (TEM, JEM200 EX) was used toexamine the microstructures in the subsurface of SPedNAB alloy (Within 50 µm from upper surface).

The corrosion behaviors of SPed samples were examinedby potential dynamic polarization and electrochemicalimpedance tests (EIS) using a traditional three-electrodesystem with the calomel reference electrode and the platinum

counter electrode in a glass cell. The corrosive medium was3.5% NaCl solution. Samples were connected to copperwires and then coated with paraffin wax leaving the peenedside with the dimension of 10mm © 10mm as the workingelectrode. Before the electrochemistry test, Open-circuitpotential (OCP) test was first applied on samples for 15minutes to reach the equilibrium potential. The potentialdynamic polarization tests were carried out at the scanningspeed of 1.0mV/s from ¹500 to 1000mV. For EIS test, thefrequency is set from 100 kHz to 0.01HZ with 5mVamplitude. In order to guarantee the accuracy of experimentdata, the potential dynamic polarization and EIS tests of eachgroup were performed at least three times. In order tounderstand the effect of shot-peening intensity on the staticcorrosion behavior of NAB alloy, SPed NAB samples werecorroded in 3.5% NaCl salt solution for 20 days. The surfacemorphologies were observed using SEM, and elementsmapping analysis was carried out using EDX attached tothe SEM.

3. Results and Discussion

3.1 Surface morphologies and microstructures of SPedsamples

Figure 2 shows the top surface and cross-sectionmorphologies of SP samples with different shot-peeningintensity. From the top surface topographies (Fig. 2(a), (b)and (c)), it can be seen that the surfaces become quite unevenand rough with indentations after SP treatment. In addition,fragmentation, chipping, and micro-crack (indicated byyellow arrows) can be seen on the peened surfaces. Thereis the little variation on surface topographies between SP-0.15 and those SP-0.20 samples. In contrast, the roughness ofSP-0.25 samples progressively increases and chipping hasmultiplied. The cross-sectional images of SP surfaces are alsoillustrated (see Fig. 2(d), (e) and (f )). It is evident that theedges of SP-0.15 and SP-0.20 samples are smooth, andsevere flaking, cracks and visible plastic deformation layercan be observed on the surface of SP-0.25 samples, whichindicates 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 issurface root-mean-square roughness, and the Rz is the heightbetween surface maximum peak and valleys. As apparentin Table 2, the roughness of the SP samples is significantlyincreased as compared to the as-cast samples, and SP-0.25samples reach the highest roughness values of 0.348 µmwhile SP-0.15 and SP-0.20 samples show lower roughnessvalues of 0.177 µm and 0.218 µm separately, which furtherindicates that surface roughness of SP samples is highlycorrelated with the values of shot-peening intensity. Theincrease in surface roughness after SP treatment wasattributed to the presence of indentations and chippingformed by the ceramic balls.25) In the SP process, the ceramicballs hit the surface with high energy and the indentationswere created with a pair of valley and peak. As the processingtime increases, the peak and valley area around theindentations were continuously hit by repeated impacts, andthen, due to excessive work hardening, the chips, micro-cracks, and fragment occurred. With the same other

Table 1 The elemental composition of as-cast NAB alloy.23)

Fig. 1 SEM image showing microstructures of as-cast NAB alloy.24)

Y. Lv et al.1630

parameters, more chipping and higher roughness values wereobserved with increasing shot-peening intensity.26) Similarfindings are also reported by Wang.27) The reason of thesecan be explained as follows: with the same other parameters,higher shot-peening intensity means higher collapse energyof ceric ball resulting in expanding plastic deformation anddeep indention. After repeated impact, the deformation oflarger valleys and peaks are easily subjected to excessivework hardening and then the cracks, chips, and flaking appearwhich can result in higher roughness values.

Figure 3 shows TEM images of SP samples with differentshot-peening intensity. At the shot-peening intensity of 0.15,a significant number of dislocation cells are observed andfiner ¡ grains were formed via the dislocation segmentation(Fig. 3(a) and (b)). Lots of twins in ¡ phase were also found,as shown in Fig. 3(c). It is indicated that high-densitydislocation areas were formed in the surface of SP-sampledue to severe plastic deformation during SP, while the ¡

grains were refined via dislocation activities and mechanicaltwins.23) As increasing the shot-peening intensity, themorphologies of ¢ A martensitic transforms into twins(Fig. 3(d)). Many investigations have reported the formationreason of the martensitic twins in NAB alloy. In our previousinvestigation,28) we found that during hot rolling treatment,martensitic nanotwins were formed via emission of partialdislocation on adjacent planes. With further increasing the

shot-peening intensity to 0.25, more high-density dislocationsand ¡ nanograins were formed (Fig. 3(e) and (f )). Therefore,it can be concluded that due to the hitting effect of theceramic balls on NAB alloy, severe plastic deformationoccurs on the NAB surface. In this case, high-densitydislocations and dislocation cells were formed and ¡ and ¢ Amartensitic phases were refined into nanoscale.

The residual stress in the surface layer of all samples wasinvestigated by X-ray analyzer using the sin2¼ method. Theresults are shown in Fig. 4. It can be observed that SPtreatment can produce high compressive residual stresses intosurfaces. As-cast samples show a quite small compressiveresidual stresses value of 9.6MPa, which may be causedby pretreatment of grinding. In contrast, the values ofcompressive residual stresses for SPed samples increase from376.5MPa for SP-0.15 samples to 449.1MPa for SP-0.25samples indicating higher compressive residual stresses dueto purely mechanical effect of indentation can be obtainedat higher shot-peening intensity. Trdan et al.29) reported theAA6082-T651 aluminum alloy subjected to laser shockpeening had increased compressive residual stresses with theincrease of pulse intensities. Moreover, Wang27) alsoconfirmed that the larger value and depth of compressiveresidual stresses can be induced into the surface layer byenhancing peening intensity, which is in agreement withur results.

Table 2 The roughness parameters values of the as-cast NAB and SP-samples.

Fig. 2 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.

Improving Corrosion Resistance Properties of Nickel-Aluminum Bronze (NAB) Alloys via Shot Peening Treatment 1631

3.2 The electrochemical corrosion behaviors of SPsamples

Many investigations have reported the electrochemicalcorrosion behavior of NAB alloy in 3.5% NaCl saltsolution.5,11,12) It is well reported that the main anodicreaction process of NAB alloy is the Cu element dissolutionand cathodic reaction is oxidation of oxygen. The chemical

equation is: Cu + 2Cl¹ ¹ e¹ ¼ CuCl¹2; O2 + 2H2O +2e¹ ¼ 4OH¹; 2CuCl¹2 + OH¹ ¼ Cu2O + H2O + 4Cl¹.The Al element is beneficial to improve the corrosionresistance of NAB alloy because of the formation ofAl(OH)3. The chemical equation is: Al + 4Cl¹ ¼AlCl¹4 + 3e¹; AlCl¹4 + 3H2O¼ Al(OH)3 + 3H+ + 4Cl¹.SP treatment only modify the surface microstructures ofNAB alloy without the variation of chemical component, thuswe consider that the anodic and cathodic reactions of SPedNAB alloy in the polarization curves is same with the NABalloy in previous investigations. Figure 5(a) depicts thepolarization curves of all samples in initial corrosion period(15 minutes), and the values of corrosion potential (Ecorr) andcorrosion current density ( jcorr) fitted by the Tofel methodare shown in Table 3. The polarization curves of both SPand as-cast samples are similar except that SP treatmentreduces the corrosion current density and shifts the corrosionpotential into cathodic value. The SP-0.25 samples achievethe smallest corrosion current density value of 7.759mA/mm2 with the most negative corrosion potential valueof ¹3.40V. The corrosion potential and corrosion currentdensity are two parameters that describe the corrosionbehaviors. The corrosion potential represents the thermody-namics factor of corrosion, and corrosion current densitydirectly reflects dynamic corrosion behaviors and corrosion

Fig. 3 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-0.25samples.

Fig. 4 The variation of compressive residual stresses in the surface regionof NAB with different shot-peening intensity.

Y. Lv et al.1632

rate. The lower corrosion potential may be due to the increaseof dislocations, residual stress and Gibbs free energy.5,30) Asshown in our results, SP samples have more negativecorrosion potential, but it doesn’t means that SP samplesare susceptible to corrosion in the initial stage of corrosion.In Fig. 5, it can be seen that anodic polarization curve ofSPed sample firstly become passive like state. It is indicatedthat SPed sample can form rapidly corrosion protective filmcompared with as-cast NAB alloy. Corrosion current densityresults shows that all SPed samples have lower corrosion ratethan that of as-samples. Thus we consider that SP treatmentcan lead to rapid formation of corrosion protective film andimprove the electrochemical corrosion resistance of NABalloy.

The potentiodynamic anodic polarization of as-cast and SPsamples after immersion in 3.5% NaCl medium for 20 dayswere also investigated (see Fig. 5(b)). The corrosionpotentials of SP samples are more positive than that ofas-cast sample which indicates that the passive films of SPsamples become more thermodynamically stable than thatof as-cast samples after 20-day immersion. Among threedifferent shot-peening intensity, SP-0.20 samples have thehighest corrosion potential following by SP-0.25 and SP-0.15samples.

EIS tests are employed to exam the growth andelectrochemical properties of the corrosion protective filmsformed on all samples after different immersion times (i.e.15min, 2, 5, and 10 days) in 3.5% NaCl solution. As shownin Fig. 6, the Nyquist plots of all samples exhibit a Warburgline at low frequency and a capacitive semicircle at highfrequency. It is well known that the magnitude of thesemicircle diameter represents the impendence and corrosion

resistance of the protective film. The diameters of capacitivesemicircles for both as-cast and SPed samples are increasedwith the increase of immersion time. The rise in diameters ofcapacitive semicircles indicates that corrosion protectivefilms gradually grow on the surfaces of SPed samples. Inthe initial period of corrosion (15min), the diameters ofcapacitive semicircles for SPed samples are smaller than thatfor as-cast samples (Fig. 6(a)). As the immersion timeincreases, the diameters of capacitive semicircles for SPedsamples grow more rapidly. After 2-day immersion(Fig. 6(b)), SPed samples begin to have larger semicirclesthan as-cast sample. After 10-day immersion (Fig. 6(d)), thediameter of the capacitive circle for SP-0.20 samples is asmuch twice as that for as-cast samples. From the results ofNyquist curves, it is apparent that the protective film formedon SPed samples is more protective.

The rapid formation of corrosion protective film in SPsamples is mainly attributed to the nanocrystallization andnano-twins induced by SP treatment. Pan et al.31) inves-tigated the passive film growth mechanisms of differentcrystalline by electrochemical measurements and in situAFM, and revealed that the nanostructure crystalline of the304 stainless steel had the highest passive film growth ratesand also changed nucleation mechanism of passive film fromprogressive to instantaneous. The passive films formed on thenanocrystallization surface of 304 stainless steel were alsoinvestigated using nano/micro/-indention, micro-scratch,SKP (Scanning Kelvin probe) by Pandey et al.,32) and theydemonstrated that the passive film on the nanocrystallizationhad greater mechanical properties, more rapid capacity ofpassivation and higher chemistry stability. The similar resultswere found on commercial brass (70-30) and Ni­Ti shapememory alloy.33,34) As seen in Fig. 3, nanocrystallization aswell as nano-twins have been formed in the surface region.Therefore, the possible explanation for the rapid formation ofcorrosion protective films in the SP samples may be that themassive amounts of boundaries and sub-boundaries createdby nano-structure can promote more diffusion paths to formprotective films and enhance the densities of films at the sametime. This is supported by Balusamy et al.35) and Ye et al.,36)

who reported that high densities of grain boundaries couldpromote the diffusion of Cr to the surface and form thehomogeneous oxide layer. Wang et al.37) also demonstratedthe electrochemical corrosion behavior of nanocrystallization

Fig. 5 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.

Table 3 The electrochemical parameters (Ecorr and jcorr) of as-cast NAB andSP samples in 3.5wt.% NaCl solution after 15-minute immersion.

Improving Corrosion Resistance Properties of Nickel-Aluminum Bronze (NAB) Alloys via Shot Peening Treatment 1633

Co coatings by higher grain boundary densities. In theirregard, since the crystalline lattice defects was easier to bepassivated and there are lots of grain boundaries anddislocation in the nanocrystallization Co coating, and henceit was suggested that high density of grain boundaries couldprovide a large number of active sites to rapidly form auniform and protective passive film.

3.3 The static immersion corrosion behavior of SPsamples

SEM was employed to observe the morphologies ofcorrosive surfaces after 20-day immersion in 3.5% NaClmedium. As shown in Fig. 7, the oxide products have beenseen on all samples. Apparently, as-cast samples suffer fromseverely selective corrosion with large amounts of deeppitting. The pitting is about 40³60 µm in size and initiates atthe ¬-rich location especially at the boundary of ¡/¬III. Somepartially dissolved inclusions can also be observed inside thepitting as yellow arrows shown (Fig. 7(a)). In contrast, thecorrosive surfaces of the SPed samples are totally differentfrom that of as-cast samples and reveal the absence of severepitting. For the SP-0.15 samples, the corrosion pitting withsmaller amounts and shallower depth is exhibited on thesurface. With increasing shot-peening intensity to 0.20mmA,the corrosion pitting disappears, which is replaced byuniform corrosion protective films. However, as furtherincreasing shot-peening intensity to 0.25mmA, the micro-cracks begin to appear on the film as indicated by the yellowarrow (see Fig. 7(d)). Figure 8 and Fig. 9 present the elementmapping of the corrosive surface of as-cast and SP-0.20samples 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 containoxides of aluminum and copper, copper salts, copperhydrochlorides and the oxides of nickel and iron, which isin line with the results of previous researches.1,38,39) For theas-cast samples (Fig. 8), the elements distribute on differentsites are various from each other, especially for Al and Cuelement. The Al element is mainly abundant in the corrosivepits and surrounds the uncorroded ¬ phases, while Cuelement is distributed on the whole ¡ matrix. The Al-abundant in the corrosion pits shown by element mappingconfirms that the as-cast samples suffer selective corrosionand ¬ phases are protective. For the SP-0.20 samples (Fig. 9),the elements such as Cu, Al, and O are distributed equally inthe whole corrosion surface. The homogeneous distributionof all elements shown by element mapping also indicates thatthe corrosion products in the surface of SP-0.20 samples aremore uniform.

Combined with the previous results of EIS tests, theseinvestigations further confirm that the corrosion protectivefilms formed on SPed samples are more uniform andprotective conspicuously in the surface of SP-0.20 samples.It is also evident that the selective corrosion has beenprohibited after SP treatment. As is known to all, the selectivecorrosion of as-cast NAB is caused by complicated structureand galvanic couple effect. Nevertheless, during the SPtreatment, apart from the formation of nanocrystallizationand nano-twins, the precipitates, as well as eutectoid structuresuch as ¬III phase in the surface layers, are fragmentizedleading to the reduction of the potential gap between thematrix and inclusion phases. Hence, the galvanic coupleeffect and the susceptibility to selective corrosion isdecreased. Pandey et al.32) found the similar result that after

Fig. 6 The Nyquist plots of as-cast NAB and SPed samples with different shot-peening intensity in 3.5% NaCl solution after differentimmersion time. (a) 15min (b) 2-day (c) 5-day (d) 10-day.

Y. Lv et al.1634

Fig. 8 The element mapping images of as-cast samples after 20-day immersion in 3.5% NaCl solution.

Fig. 7 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-dayimmersion in 3.5% NaCl solution.

Fig. 9 The element mapping images of SP-0.20 samples after 20-day immersion in 3.5% NaCl solution.

Improving Corrosion Resistance Properties of Nickel-Aluminum Bronze (NAB) Alloys via Shot Peening Treatment 1635

SP treatment, the precipitates were significantly dissolved inthe matrix resulting in the decreased tendency for localizedand pitting corrosion.

Taking together all results, it can be suggested thatcorrosion resistance of NAB can be enhanced by SP duringlong time immersion. And based on the results of EIS testand SEM images, it can be assumed that the higher corrosionresistance after SP is mainly attributed to the rapid formationof the protective film and uniform corrosion. Moreover,compressive residual stresses is also favorable to thecorrosion resistance. Krawiec et al.40) investigated the effectof laser shocking processing on the micro-electrochemicalbehavior of AA2050-T8 aluminum alloy, and the resultsshowed both charge transfer and oxide film resistance weresignificantly increased due to the presence of compressiveresidual stresses. Trdan et al. reported similar results thatincrease of pitting potential, as well as reduction of corrosioncurrent density on AA6082-T651 aluminum alloy, wereattributed to the higher compressive residual stresses causedby the laser shocking processing.41) Patrice et al. confirmedthat the passive film formed on the high compressive residualstresses surface of 316L stainless steel was thinner butpossibly more compact and resistant to breakdown whensubjected to electrostrictive stress.42)

3.4 Effect of roughness on the corrosion resistanceproperties of SPed NAB alloy

The corrosion behaviors are not only related with therefined microstructure but also related with surface rough-ness. Roughness is an important factor that can retard thecorrosion resistance.43,44) Lee et al.44) reported the ultrasoni-cally peened samples showed equal or even better corrosionresistant than SP samples due to the increase of surfaceroughness. Azar et al.43) revealed that the roughness andheterogeneities could deteriorate the surface corrosionresistance considering the preferred locations for pittingcorrosion. In our research, shown by the ESI results and SEMimages, after long-time immersion in 3.5% NaCl medium,the SP-0.20 samples show the highest corrosion resistanceand is followed by SP-0.25 samples implying that thecorrosion resistance is not always proportional to the shot-peening intensity. The deterioration of corrosion resistancefor SP-0.25 samples is mainly attributed to micro-cracks anddefects which appear on corrosion protective film. Berzinset al.45) and Wood et al.46) both investigated the absorptionnature for chloride on the corroded aluminum surface byusing 36Cl as the radioactive tracer and other means andshowed absorption was especially localized to the pit sitesdue to lower adsorption energy. Hence, defects and micro-cracks are susceptible to local adsorption of chloride ionswhich lead to further corrosion of the substrate beneath thepassive film.47) As shown in Fig. 2(e), there are largeramounts of chips and flaking in the surface of SP-0.25samples. After long time immersion, these defects can’t behealed, and still appear on protective film shown in Fig. 7(d).The micro-cracks and defects appearing on the films arethe major susceptible sites which will allow the corrosivemedium in and result in further corrosion of the subsurface.Thus, the formation of the stable protective film is prohibitedand corrosion resistance for SP-0.25 samples is decreased.

4. Conclusion

(1) After shot-peening treatment, the microstructures ofNAB alloy were refined and uniformed, and high-density dislocations, nano-twins and compressiveresidual stress were also induced in the surface ofNAB alloy.

(2) Increasing shot-peening intensity can result in theformation of more high-density dislocations and grainrefinement of ¡ and ¢A martensitic phases. As the shot-peening intensity exceeds a critical value, the cracks,chips and flaking appear on the SPed sample surface.

(3) The surface microstructures and roughness are twomain factors for the corrosion resistance properties ofSPed NAB alloy. In the corrosion medium, the refinedand homogenized microstructures formed on SPedNAB alloy surface contributes to rapid formation ofcorrosion protective film and occurrence of uniformcorrosion, thus significantly improving their corrosionresistance properties.

(4) As the shot-peening intensity exceeds a critical value,the defects caused by shot-peening can result in higherroughness values, leading to worse corrosion resistanceproperties of shot-peened NAB alloy.

Acknowledgments

Authors (Yuting Lv and Bingjie Zhao) contributedequally to this work. This work has been supported bythe Fund for National Science Foundation under GrantNo. 51801115, the Fund for Doctor of Shandong Province(No. ZR2018BEM005, ZR2018BEE014), and the scientificresearch foundation of Shandong University of Scienceand Technology for Recruited Talents under Grant No.2017RCJJ025.

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