An Investigation of Surface Corrosion Behavior of Inconel 718 After Robotic Belt Grinding

materials

Article An Investigation of Surface Corrosion Behavior of Inconel 718 after Robotic Belt Grinding

Junwei Wang 1, Jijin Xu 1,* , Xiaoqiang Zhang 1, Xukai Ren 1, Xuefeng Song 1 and Xiaoqi Chen 1,2,*

1 Shanghai Key Laboratory of Materials Laser Processing and Modiﬁcation, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China; [email protected] (J.W.); [email protected] (X.Z.); [email protected] (X.R.); [email protected] (X.S.) 2 Department of Mechanical Engineering, University of Canterbury, Christchurch 8140, New Zealand * Correspondence: [email protected] (J.X.); [email protected] (X.C.); Tel.: +86-21-3420-2640 (J.X.); +64-33692190 (X.C.) Received: 22 October 2018; Accepted: 30 November 2018; Published: 2 December 2018

Abstract: Surface corrosion resistance of nickel-based superalloys after grinding is an important consideration to ensure the service performance. In this work, robotic belt grinding is adopted because it offers controllable material processing by dynamically controlling process parameters and tool-workpiece contact state. Surface corrosion behavior of Inconel 718 after robotic belt grinding was investigated by electrochemical testing in 3.5 wt % NaCl solution at room temperature. Specimens were characterized by morphology, surface roughness and residual stress systematically. The potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) analysis indicate the corrosion resistance of the specimen surface improves remarkably with the decrease of abrasive particle size. It can be attributed to the change of surface roughness and residual stress. The energy dispersive X-ray spectroscopy (EDS) indicates that niobium (Nb) is preferentially attacked in the corrosion process. A plausible electrochemical dissolution behavior for Inconel 718 processed by robotic belt grinding is proposed. This study is of signiﬁcance for achieving desired corrosion property of work surface by optimizing grinding process parameters.

Keywords: robotic belt grinding technique; corrosion behavior; nickel-based superalloy; roughness; residual stress

1. Introduction Inconel 718 superalloy has been widely utilized in gas turbine engineering, submarine, nuclear reactors, oil and gas production parts because of its good service performance [1–3]. For ship gas turbine and submarine which mainly work in the marine environment, the corrosion behavior of Inconel 718 must be fully understand to adopt effective countermeasures ensuring the safety of the alloy system. In addition, a good corrosion resistance is beneﬁcial to the service life of the parts because most cracks commence at the surface of the material. Corrosion performance is mainly affected by material compositions [4] and surface states such as surface roughness and surface stress [5]. The high nickel content of Inconel 718 makes the alloy have a relatively strong resistance to chloride stress cracking corrosion. At the same time, due to the presence of chromium, corrosion resistance is even better than pure nickel in the oxidizing environment. Wang et al. [6] investigated the corrosion behavior of Inconel 718 in electrochemical machining, suggesting that the generated niobium carbide and niobium oxide have an important inﬂuence on the corrosion process of nickel-based superalloy. Jebaraj et al. [7] reported hydrogen permeation of Inconel 718 in different states and found that the hydrogen trapping in cold rolled and precipitation hardened Inconel 718 is irreversible. These studies objectively analyze

Materials 2018, 11, 2440; doi:10.3390/ma11122440 www.mdpi.com/journal/materials Materials 2018, 11, 2440 2 of 14 the chemical elements influencing the corrosion performance of the alloy. Additional effective measures need to be devised to improve corrosion performance. In order to enhance the corrosion resistance of nickel-based superalloy, various processing technologies were globally employed. Karthik et al. [8] reported the enhanced corrosion performance in Inconel 600 through laser shock peening without coating. They found the larger and deeper compressive residual stresses and smaller surface roughness are the main reasons for the increased corrosion performance. However this study was conducted on Inconel 600, and was not supported by an actual corrosion experiment, such as immersion tests. Huang et al. [9] investigated the electrochemical corrosion behavior of Inconel 718 sheets treated by electron beam welding in 3.5 wt % NaCl solution. To reduce the adverse impact on corrosion performance, they adopted a method of subsequent heat treatment. Khan et al. [10] studied the corrosion performance of Inconel 718 in the simulated body fluid. The results show that laser surface-modified process can improve the corrosion resistance of materials effectively. Akyol et al. [11] deposited Ni-P-W-CNF (Carbon Nanofibers) composites on the works by electroless method, and obtained a good wear and corrosion resistance. However, this improvement approach causes pollution. Narayanan et al. [12] analyzed the change of impingement angle to compare the corrosion resistance of nickel-based superalloy which was laser surface treated. The corrosion performance was increased by about 1.5 times due to the minimized energy transfer of laser. It is a feasible process to improve erosion resistance under the premise of ensuring efficiency. Arrizubieta et al. [13] combined laser material deposition and laser surface processes for the complete manufacturing of Inconel 718 components. The surface quality was improved and the roughness was reduced. Nevertheless, the process is relatively complicated. Karthik et al. [5] reviewed laser peening without protective coating technology as a mechanical surface modification method. The method can significantly improve the corrosion resistance of metallic materials due to the factors including surface roughness and compressive residual stress. These studies analyze the corrosion properties and influencing factors of nickel-based alloys under different conditions. However, methods to improving corrosion resistance require additional processing, such as material deposition, laser peening, etc. These measures not only consume time and resources, but also inevitably increase production costs. It is desired that a robotic belt grinding process could improve the corrosion performance by optimizing its process parameters and without additional processing. Robotic belt grinding offers controllable material processing by dynamically controlling process parameters and tool-workpiece contact state, which is otherwise difficult to achieve by manual grinding [14]. While conventional CNC grinding machine offers position control, a robotic grinding system can readily incorporate both position and force control. As a result, desired profile finishing accuracy and surface properties can be obtained by online tool condition monitoring and optimal process control [15,16]. As such, we have adopted robotic belt grinding in this study. Pradhan et al. [17] analyzed the influence of surface roughness on corrosion behavior of Inconel 718 in a simulated marine environment at high temperature for a relatively long time. The results show that the higher roughness increases the surface area for the corrosion behavior and reduces the corrosion resistance of the works. However, roughness is only one of the contributing factors affecting corrosion resistance. Based on our previous study [18], we found that the finished surface obtains a considerable residual stress, which has a significant impact on surface corrosion performance. This phenomenon, which is neither fully understood nor discussed by existing works, is investigated in detail in this paper. By controlling appropriate parameters, the robotic belt grinding process can achieve the ideal compressive residual stress in the sub-surface and lower surface roughness, which improves surface corrosion performance. To the best of our knowledge, the mechanism of residual stress affecting corrosion Ni-based superalloys has not been reported elsewhere. Tressia et al. [19] investigated the influence of abrasive particle size and different aqueous solution on the abrasive wear mechanisms in the grinding process. Turnbull et al. [20] reported the sensitivity of stress corrosion cracking of stainless steel to surface machining and grinding procedure. The above Materials 2018, 11, 2440 3 of 14 Materials 2018, 11, x FOR PEER REVIEW 3 of 13 inconsistency.research works Robotic mostly basedbelt grinding on manual is grindinga relatively suffer new from precision their uncontrollability machining technique, and inconsistency. and has attractedRobotic beltgreat grinding attention is for a relatively its advantages new precision over conventional machining manual technique, grinding and hasprocedure attracted [21,22 great]. Althoughattention forgreat its progress advantages on overthe corrosion conventional behavior manual of nickel grinding-based procedure superalloy [21, 22has]. been Although made, great the progressrelated research on the corrosionon surface behavior corrosion of nickel-basedbehavior and superalloyinfluencing has factors been of made, nickel the-based related superalloy research afonter surface robotic corrosion belt grinding behavior has been and barely influencing reported. factors of nickel-based superalloy after robotic belt grindingIn this has work, been electrochemical barely reported. corrosion of Inconel 718 was investigated to evaluate the corrosion behaviorIn this of work,the alloy electrochemical under robotic corrosion belt grinding. of Inconel The 718experiment was investigated was carried to evaluateout in 3.5 the wt corrosion % NaCl solutiobehaviorn, which of the is alloy a commonly under robotic used beltcorrosive grinding. solution The in experiment electrochemical was carried corrosion out experiments in 3.5 wt % NaCl [23]. Thesolution, potentiodynamic which is a commonly polarization used corrosivecurves and solution electrochemical in electrochemical impedance corrosion spectra experiments (EIS) were [23]. Themeasured. potentiodynamic Furthermore, polarization the corroded curves specimens and electrochemical were examined impedance by scanning spectra e (EIS)lectron were micrograph measured. (SEM)Furthermore, and energy the corroded dispersive specimens X-ray spectroscopy were examined (EDS). by The scanning influencing electron factors micrograph including (SEM) surface and roughnessenergy dispersive and residual X-ray stress spectroscopy were discussed (EDS). The in detail. influencing factors including surface roughness and residual stress were discussed in detail. 2. Materials and Methods 2. Materials and Methods 2.1. Materials 2.1. Materials The nickel-based superalloy Inconel 718 used in this study was provided by Baoshan Iron & The nickel-based superalloy Inconel 718 used in this study was provided by Baoshan Iron & Steel Steel Co., Ltd. (Shanghai, China). The Inconel 718 was hot forging state, and annealed at 980 °C for Co., Ltd. (Shanghai, China). The Inconel 718 was hot forging state, and annealed at 980 ◦C for 10 min 10 min followed by air cooling. Then, the materials were acid pickled. The acid used is a mixed followed by air cooling. Then, the materials were acid pickled. The acid used is a mixed solution, solution, whose concentration is 10 vol % HNO3 + 7 vol % HF. The purpose of acid pickling is to whose concentration is 10 vol % HNO + 7 vol % HF. The purpose of acid pickling is to remove the remove the oxide formed on the surface3 of the material during heat treatment. Its chemical oxide formed on the surface of the material during heat treatment. Its chemical compositions were compositions were investigated by inductively coupled plasma-optical emission spectroscopy (ICP- investigated by inductively coupled plasma-optical emission spectroscopy (ICP-OES) and the results OES) and the results are listed in Table 1. are listed in Table1. The as-received material was cut into square bars with dimensions of 15 mm × 15 mm × 500 mm The as-received material was cut into square bars with dimensions of 15 mm × 15 mm × 500 mm using electric discharge machine wire cutting. The annealing process can improve the ductility and using electric discharge machine wire cutting. The annealing process can improve the ductility and formability, which is meaningful for the subsequent processing. formability, which is meaningful for the subsequent processing.

Table 1. Chemical compositions of Inconel 718 (wt %). Table 1. Chemical compositions of Inconel 718 (wt %). C Si Mn Al Co Ti Mo Nb Cr Ni Fe C Si Mn Al Co Ti Mo Nb Cr Ni Fe 0.05 0.23 0.25 0.43 0.65 0.9 2.86 5.25 18.2 52.8 Balance 0.05 0.23 0.25 0.43 0.65 0.9 2.86 5.25 18.2 52.8 Balance 2.2. Experiment Set-Up 2.2. Experiment Set-Up The robotic belt grinding system used in this study mainly consists of four components which are controlThe robotic cabinet, belt industrial grinding system PC, FANUC used in (Fuji this studyAutomatic mainly Numerical consists of Control) four components robot with which a force are control sensor cabinet, and industrial a belt machine, PC, FANUC as shown (Fuji Automaticin Figure 1. Numerical The abrasive Control) belt was robot supplied with a by force 3 M control China sensor and a belt machine, as shown in Figure1. The abrasive belt was supplied by 3 M China Limited, Limited, and its abrasive particles is special Al2O3 stocked on an elastic paper strip reinforced with fibers.and its abrasive particles is special Al2O3 stocked on an elastic paper strip reinforced with ﬁbers.

Figure 1. Schematic diagram of experimental setup.

Materials 2018, 11, 2440 4 of 14

The prepared specimens were ground by the optimum processing parameters of grinding force 178 kPa and belt speed 21 m/s. Three kinds of belt with different particle size were selected, which are 36, 80 and 120 M, corresponding to grain sizes of about 500, 178, and 125 µm, respectively. The mesh number is inversely proportional to the particle size. The larger the mesh is, the smaller the abrasive particle. Therefore, there are three sets of parameters in this experiment: 36 M with 178 kPa and 21 m/s, 80 M with 178 kPa and 21 m/s, 120 M with 178 kPa and 21 m/s.

2.3. Test Methods After the grinding process, a series of tests were carried out. The surface roughness of the ground specimens was tested with a roughness tester (SJ-410, Mitutoyo, Kawasaki, Japan) and the measuring direction was perpendicular to the grinding direction. The residual stress states on the ground surface were measured by an X-ray Stress Analyzer (LXRD, Proto, Sacramento, CA, USA) 2 with Mn-Kα radiation and Cr ﬁlter, using standard sin ψ method under 18 kV (voltage) and 4 mA (current). The (311) plane with a 2θ of 165.32 was chosen as the shifts of diffraction peaks. To evaluate the corrosion resistance of the alloy, electrochemical testing was employed at room temperature in 3.5 wt % NaCl solution and free air. The samples were held in corrosive solution for 65 min, including 20 min of immersion time and 45 min of corrosion time. It was conducted using a CHI 660E electrochemical system with Saturated Calomel Electrode (SCE), Pt wire and the ground specimen as reference, auxiliary and working electrode respectively. The dimension of the test specimen was 10 mm × 10 mm with an area of 100 mm2. Potentiodynamic polarization testing was performed over the applied potential ranging from −1.2 V to 1.5 V at a sweep rate of 1 mV/s. Each potentiodynamic polarization measurement was repeated three times to obtain a reliable result. Tafel extrapolation method was used to calculate the corrosion potential (Ecorr) and corrosion current (Icorr). EIS testing was conducted with the same device under the same condition. The perturbation voltage is 10 mV. The frequency ranges from 100 kHz to 10 mHz. The EIS test was carried out at open circuit potential (Ecorr). EIS test was also repeated three times. Immersion tests were also conducted after the grinding process. The ground specimens of 15 mm × 15 mm were rinsed carefully by acetone, ethanol and distilled water in turn. The immersion tests were carried out in 3.5 wt % NaCl solution at room temperature. Corrosion products were removed by chemical cleaning with 10 vol % HNO3 + 7 vol % HF. The mass loss of the specimens were measured by an analytical balance (FA124, Sunny Hengping, Shanghai, China) with a precision of 10 µg. Each mass loss was measured for three times to obtain a reliable result. Morphology characterization was accomplished using a JSM-7600F ﬁeld-emission scanning electron microscope (SEM, JEOL, Akishima, Japan) at the accelerating voltage of 5 kV and probe current of 2 × 10−10 A. The SEM machine was equipped with an energy dispersive X-ray spectroscopy (EDS) to identify the compositions of specimen surface. The accelerating voltage is 20 kV and the working distance is 15 mm.

3. Results and Discussion

3.1. Electrochemical Analysis Figure2 shows potentiodynamic polarization curves of the ground surface with different abrasive particle sizes. The potentiodynamic polarization curves exhibit similar shape for the three specimens and there are obvious passivation regions in accord with the current response [24]. The current density has an increasing trend as the applied anodic voltage increases. In working electrodes, there exists a cathodic reduction process, which is a hydrogen evaluation reaction [25]. In the anodic regions, there is a typical active-passive-trans-passive behavior, displaying a limiting current density with the increase of the corrosion potential. This is owing to the existence of the compact oxidation ﬁlm on the ground surface [26]. The passivation region begins near the corrosion potentials and there is a rapid increase of current density when the applied potential increases to a Materials 2018, 11, 2440 5 of 14

Materials 2018, 11, x FOR PEER REVIEW 5 of 13 certain value around 1.25 V due to the dissolution of the oxidation ﬁlm. In addition, the pit corrosion pitoccurs corrosion according occurs to the according steep increase to the of steep the current. increase Simultaneously, of the current. the Simultaneously, oxidation reaction the ofoxidation Inconel reaction718 proceeds, of Inconel leading 718 to proceeds, a passivation leading oxidation to a passivation ﬁlm on the oxidation specimen film surface. on the specimen surface.

Figure 2. Potentiodynamic polarization curves of the ground surface with different abrasive particle sizes. Saturated Calomel Calomel Electrode Electrode (SCE). (SCE).

To comparecompare the the corrosion corrosion performance performance of theof groundthe ground specimens specimens clearly, clearly, corrosion corrosion current current density (Idensitycorr) and (Icorr corrosion) and corrosion potential potential (Ecorr) are (E calculatedcorr) are calculated from the potentiodynamicfrom the potentiodynam polarizationic polarization curves [27], curvesas presented [27], as in Tablepresented2. The in corresponding Table 2. The errors corresponding are also listed errors and are have also a goodlisted repeatability. and have a Itgood can repeatability.be found that It corrosion can be found potential that corrosion (Ecorr) increases potential forwards (Ecorr) increases the positive forwards direction the andpositive the corrosiondirection andcurrent the densitycorrosion (Icorr current) decreases density with (Icor ther) decreases decrease of with the particlethe decrease size. Itof is the well particle known size. that It the is morewell positiveknown that the corrosionthe more potentialpositive the is, thecorrosion higher potential corrosion is, resistance the higher [28 ].corrosion In addition, resistance the corrosion [28]. In propertyaddition, isthe also corrosion inﬂuenced property by corrosion is also rate,influenced which by can corrosion be represented rate, which by the can corrosion be represente currentd density by the (Icorrosioncorr) according current to density Faraday’s (Icorr) lawaccording [29]. The to Faraday’s specimen law ground [29]. The by particlespecimen size ground of 120 by M particle shows size the ofhighest 120 M corrosion shows the potential highest andcorrosion the lowest potential corrosion and the current lowest density, corrosion indicating current density, the best indicating corrosion property.the best corrosion From the property. analysis From above, the it analy can besis concluded above, it can that be the concluded corrosion that property the corrosion of the specimenproperty ofsurface the specimen increases surface remarkably increases with remarkably the decrease with of particle the decrease size. of particle size.

Table 2. Tafel polarization parameters of the ground specimens with different abrasive particle sizes.

2 2 ParticleParticle Size Size (M) (M) E Ecorrcorr (mV(mV vs. SCE) IcorrI(corrµA/cm (μA/cm) ) 36 36 −−800800 ((±± 13)13) 35.7435.74 (± 2.5) (± 2.5) 80 80 −−680 ((±± 9) 10.6410.64 (± 1.2) (± 1.2) 120 120 −−627627 ((±± 11)11) 5.285.28 (± 0.6) (± 0.6) In order to further understand the electrochemical corrosion behavior of Inconel 718 treated by roboticIn orderbelt grinding, to further EIS understand testing was the carried electrochemical out. Typical corrosion Nyquist behaviorplots of the of Inconelspecimens 718 are treated shown by inrobotic Figure belt 3. There grinding, is an EIS obvious testing capacitance was carried loop, out. because Typical Nyquistthe corrosion plots process of the specimens is dominated are shownby the chargein Figure transfer3. There step. is anThe obvious capacitance capacitance loop is loop, not a because standard the semici corrosionrcle due process to the is dominatedheterogeneous by corrosionthe charge surface transfer [30] step.. The The diameter capacitance of capacitance loop is not a loop standard increases semicircle with duethe todecrease the heterogeneous of abrasive particlecorrosion size. surface The value [30]. of The the diameter diameter ofrepresents capacitance the loopcorrosion increases resistance with of the the decrease test specimen of abrasive [31]. particleSamples size.ground The valueusing oflarg theer diameter particle representssizes have the better corrosion corrosion resistance performance of the test [32] specimen. It can [ 31be]. concludedSamples ground that a usingsmaller larger abrasive particle particle sizes size have is better beneficial corrosion to the performance corrosion resistance [32]. It can of the be concludedspecimen surface.that a smaller The results abrasive of EIS particle testing size are is consistent beneﬁcial with to the the corrosion potentiodynamic resistance polari of thezation specimen tests well. surface. The results of EIS testing are consistent with the potentiodynamic polarization tests well.

Materials 2018, 11, 2440 6 of 14 Materials 2018, 11, x FOR PEER REVIEW 6 of 13 Materials 2018, 11, x FOR PEER REVIEW 6 of 13

Figure 3. Nyquist plots of the ground surface with different abrasive particle sizes. Constant phase FigureFigure 3. NyquistNyquist plots plots of of the the ground ground surface surface with with different different abrasive abrasive particle particle sizes. sizes. C Constantonstant phase phase elements (CPE). elementselements (CPE).(CPE). To further explain the the corrosion corrosion behavior, behavior, an an equivalent equivalent circuit circuit diagram diagram was was used, used, as asshown shown in To further explain the corrosion behavior, an equivalent circuit diagram was used, as shown in inthe the insert insert of ofFigure Figure 3.3. T Thehe R RS Srepresentsrepresents the the solution solution resistance, RCT representsrepresents charge transfertransfer the insert of Figure 3. The RS represents the solution resistance, RCT represents charge transfer resistance at thethe interface.interface. Moreover, purepure capacitorscapacitors werewere substitutedsubstituted byby constantconstant phasephase elementselements resistance at the interface. Moreover, pure capacitors were substituted by constant−1 phase elements (CPE)(CPE) andand thethe impedanceimpedance ofof CPECPE waswas defined defined by by equation: equation:Z == [Q((jω))n] , ,where where Q is thethe (CPE) and the impedance of CPE was defined by equation: CPE = [ ( ) ] , where is the 𝑛𝑛 −1 frequencyfrequency independentindependent constant; constant; n n is is the the exponential exponential coefficient, coefficient, the the𝐶𝐶𝐶𝐶𝐶𝐶 value value of of which which𝑛𝑛 −1 is is between between 0 and0 and 1; frequency independent constant; n is the exponential coefficient, 𝑍𝑍the𝐶𝐶𝐶𝐶𝐶𝐶 value𝑄𝑄 of𝑗𝑗𝑗𝑗 which is between𝑄𝑄 0 and j1; is j anis an imaginary imaginary unit unit and andω ωis is an an angular angular frequency frequency [33 [33]]. The. The fitted fitted𝑍𝑍 resistance resistance𝑄𝑄 𝑗𝑗𝑗𝑗 values values R RCTCT were𝑄𝑄 listed 1; j is an imaginary unit and ω is an angular frequency [33]. The fitted resistance values RCT were listed inin FigureFigure4 4.. It It could could be be found found that that the the R RCTCT increasesincreases as as the the abrasive abrasive particle size decreases, indicating in Figure 4. It could be found that the RCT increases as the abrasive particle size decreases, indicating thatthat thethe corrosioncorrosion resistanceresistance increasesincreases withwith thethe decreasedecrease ofof abrasiveabrasive particleparticle sizesize inin NaClNaCl solution.solution. that the corrosion resistance increases with the decrease of abrasive particle size in NaCl solution.

Figure 4. RCT of the equivalent circuit evaluated by fitting the impedance spectra of the ground surface FigureFigure 4. 4. RRCTCT ofof the the equivalent equivalent circuit circuit evaluated evaluated by by fitting ﬁtting the the impedance impedance spectra spectra of of the the ground ground surface surface with different abrasive particle sizes. with different abrasive particle sizes. 3.2. Corrosion Morphology Analysis 3.2.3.2. Corrosion Corrosion Morphology Morphology Analysis Analysis Ideally, electrochemical corrosion of metals should be uniform and homogeneous. Nevertheless, Ideally,Ideally, electrochemical corrosion of metals should should be be uniform uniform and and homogeneous. homogeneous. Nevertheless, Nevertheless, Inconel 718 contains many other elements to improve its mechanical properties, resulting in non- InconelInconel 718 718 contains contains many many other other elements elements to improve to improve its mechanical its mechanical properties, properties, resulting resulting in non in- uniformity and inhomogeneity. To observe the corrosion morphology, the corroded surfaces of the uniformitynon-uniformity and inhomogeneity. and inhomogeneity. To observe To observe the corrosion the corrosion morphology, morphology, the corroded the corroded surfaces surfaces of the ground specimens are investigated by SEM equipped with EDS to determine the change of alloying groundof the ground specimens specimens are investigated are investigated by SEM by equipped SEM equipped with EDS with to determine EDS to determine the change the of change alloying of elements after the measurement of potentiodynamic polarization curves. Figure 5 shows SEM images elementsalloying elementsafter the measurement after the measurement of potentiodynamic of potentiodynamic polarization polarization curves. Figure curves. 5 Figureshows5 SEM shows images SEM of the corroded surface under different abrasive particle sizes. There are many corrosion pits and ofimages the corroded of the corroded surface surfaceunder diff undererent different abrasive abrasive particle particle sizes. sizes.There Thereare many are many corrosion corrosion pits and pits corrosion products with a size of about 50 μm, which indicates serious corrosion in the surface of corrosionand corrosion products products with witha size a sizeof about of about 50 μm, 50 µ whichm, which indicates indicates serious serious corrosion corrosion in the in thesurface surface of specimens. To clearly compare the degree of corrosion behavior, the average number of corrosion specimens. To clearly compare the degree of corrosion behavior, the average number of corrosion pits per unit area is counted. Twenty different regions are chosen to calculate the average number of pits per unit area is counted. Twenty different regions are chosen to calculate the average number of corrosion pits. The results are presented in Figure 6. It can be seen that the number of corrosion pits corrosion pits. The results are presented in Figure 6. It can be seen that the number of corrosion pits

Materials 2018, 11, 2440 7 of 14 of specimens. To clearly compare the degree of corrosion behavior, the average number of corrosion pits per unit area is counted. Twenty different regions are chosen to calculate the average number of Materials 2018, 11, x FOR PEER REVIEW 7 of 13 corrosion pits. The results are presented in Figure6. It can be seen that the number of corrosion pits has an obvious decreasing trend as the abrasiveabrasive particle size decreases. It can be concluded that the size reduction of abrasive particle facilitatesfacilitates thethe improvementimprovement ofof corrosioncorrosion performance.performance.

Figure 5.5. ScanningScanning electron electron microscope microscope (SEM) (SEM) images images of the corrodedof the corroded surface ofsurface samples of preparedsamples preparedusing under using different under particledifferent sizes: particle (a) 36sizes: M, ((ba)) 8036 M,M, ((cb)) 120 80 M, M. (c) 120 M.

Figure 6. The number of corrosion pits per unit area at the corroded surface.

To furtherfurther investigate investigate the the corrosion corrosion behavior behavior of the of Inconelthe Inconel 718 under 718 under robotic robotic belt grinding belt grinding system, system,the change the ofchange alloying of elementsalloying elements on the corroded on the corroded surface are surface determined are determined by EDS. Figure by EDS.7 shows Figure the 7 showsEDS results the EDS for differentresults for regions different (uncorroded regions (uncorroded region, corrosion region, products, corrosion and products, corrosion and pits) corrosion on the pits)corroded on the surface. corroded Compared surface. with Compared the uncorroded with the regionuncorroded and the region corrosion and pits,the corrosion the content pits, of Nbthe contentand O is of the Nb highest and O inis the corrosion highest products, in corrosion which products, is similar which to the is similar previous to resultsthe previous [6]. There results might [6]. There might exist Nb-rich regions in the corrosion products. The boundary of the Nb-rich region was firstly eroded. The content of Nb and O in the corrosion pits decreases with the shedding of corrosion products.

Materials 2018, 11, 2440 8 of 14 exist Nb-rich regions in the corrosion products. The boundary of the Nb-rich region was ﬁrstly eroded. The content of Nb and O in the corrosion pits decreases with the shedding of corrosion products. Materials 2018, 11, x FOR PEER REVIEW 8 of 13

Figure 7.7. SEMSEM and and energy energy dispersive dispersive X-ray X spectroscopy-ray spectroscopy (EDS) results (EDS) for results the corroded for the specimens: corroded (specimens:a) SEM image; (a) SEM (b) Uncorroded image; (b) Uncorroded surface; (c) Corrosionsurface; (c products;) Corrosion (d products;) Corrosion (d pits.) Corrosion pits.

Materials 2018, 11, 2440 9 of 14

3.3. Mass Loss Analysis Figure8 shows the average mass loss of the specimens ground by different abrasive particles after different immersion times. The average mass loss of the specimens increases with the extension of immersionMaterialsMaterials 2018 time.2018, 11, 11, x It, FORxis FOR also PEER PEER found REVIEW REVIEW that the average mass loss decreases with the decrease of the9 of abrasive9 13of 13 particle size. The results indicate that the corrosion resistance of the ground surface is improved with abrasive particle size. The results indicate that the corrosion resistance of the ground surface is theabrasive decrease particle of abrasive size. particleThe results size. indicate that the corrosion resistance of the ground surface is improvedimproved with with the the decrease decrease of of abrasive abrasive particleparticle size.

FigureFigure 8. Average 8. Average mass mass loss loss of of the the specimens specimens after immersion immersion for for 1, 1,5, 10, 5, 10, 15, 15,25, 25,35, and 35, and50 days. 50 days. Figure 8. Average mass loss of the specimens after immersion for 1, 5, 10, 15, 25, 35, and 50 days. 3.4. Surface3.4. Surface Roughness Roughness Influence Influence 3.4.In Surface orderIn order Roughness to furtherto further Influence elucidate elucidate the the influencing influencing factors factors of of corrosion corrosion resistance resistance on nickel on nickel-based-based superalloyssuperalloyIn order unders underto further robotic robotic elucidate belt belt grinding, grinding, the influencing thethe surfacesurface factors roughness roughness of corrosion w wasas tested. tested. resistance The The average average on roughnessnickel roughness-based (Ra) is measured with 2D stylus profiler over a length of 6 mm. The surface roughness of treated (Ra)superalloy is measureds under with robotic 2D stylus belt grinding, profiler overthe surface a length roughness of 6 mm. was The tested. surface The roughnessaverage roughness of treated specimens as a function of different abrasive particle size is presented in Figure 9. There is an specimens(Ra) is measured as a function with of 2D different stylus profiler abrasive over particle a length size isof presented 6 mm. The in surface Figure9 roughness. There is an of obviouslytreated obviously monotonous decrease with the decrease of abrasive particle size. This is due to different monotonousspecimens decreaseas a function with theof decreasedifferent ofabrasive abrasive particle particle size size. is Thispresented is due toin differentFigure 9. surface There ablationis an surface ablation of materials during the grinding process. The larger the abrasive particle size is, the obviously monotonous decrease with the decrease of abrasive particle size. This is due to different of materialsgreater surface during ablation the grinding and the process. higher surface The larger roughness. the abrasive The corrosion particle behavior size is, is theaccelerated greater by surface ablationsurfacthe epresence and ablation the higherof of large materialssurface peaks during and roughness. valleys the grinding which The corrosionyields process. a la The behaviorrger larger ground isthe acceleratedsurface abrasive area particle by[34] the. A size presencelarger is, the of largegreatersurface peaks surface area and derived valleys ablation from which and higher the yields higher roughness a larger surface facilitates ground roughness. surface the diffusion The area corrosion [of34 the]. A electrolytebehavior larger surface isand accelerated promotes area derived by fromthethe higherpresence corrosion roughness of ratelarge [17] facilitatespeaks, which and is the consistentvalleys diffusion which with of yields the improvement electrolyte a larger andground of promotescorrosion surface performance the area corrosion [34]. Aas ratelargerthe [17 ], whichsurfaceabrasive is consistentarea particle derived withsize from decreases. the higher improvement roughnessMoreover, offacilitatesthe corrosion rough thesurface performancediffusion is harmful of the as toelectrolyte the the abrasive formation andparticle promotes of the size the corrosion rate [17], which is consistent with the improvement of corrosion performance as the decreases.passive Moreover, film [35], which the rough is a protective surface isfilm harmful of the parts. to the formation of the passive film [35], which is a abrasive particle size decreases. Moreover, the rough surface is harmful to the formation of the protective film of the parts. passive film [35], which is a protective film of the parts.

Figure 9. Surface roughness of treated specimens as a function of different abrasive particle sizes.

3.5. Residual Stress Analysis FigureFigure 9. 9.Surface Surface roughness roughness of of treatedtreated specimensspecimens as a function function of of different different abrasive abrasive particle particle sizes. sizes. Residual stress is also a significant factor affecting the corrosion property in the grinding process [36]. During processing, metals often form part deformation and non-uniform stress. In grinding 3.5. Residual Stress Analysis process, the residual stress forms on the specimen surface mainly due to the plastic deformation and Residual stress is also a significant factor affecting the corrosion property in the grinding process

[36]. During processing, metals often form part deformation and non-uniform stress. In grinding process, the residual stress forms on the specimen surface mainly due to the plastic deformation and

Materials 2018, 11, 2440 10 of 14

3.5. Residual Stress Analysis Residual stress is also a significant factor affecting the corrosion property in the grinding process [36]. During processing, metals often form part deformation and non-uniform stress. In grinding process, the residual stress forms on the specimen surface mainly due to the plastic deformation and temperature change during grinding process [37]. Normally, the corrosion occurs easily in high tensile stress and high energy state [38]. A considerable residual stress occurs in ground metallic structure [39]. Based on the importance of stress, the surface residual stress were tested in two directions (the grinding direction is defined as X and material flowing direction is defined as Y) under different abrasive particle sizes, as depicted in Figure 10. The residual stresses in both directions exhibit a similar change trend. As the abrasive particle size decreases, the residual stress in the X direction changes from the tensile stress to the compressive stress. The residual stress in the Y direction is shown as the compressive stress, and the compressive stress gradually increases with the decrease of abrasive particle size. The effect of residual stress on corrosion performance is similar to that of stress corrosion. In stress corrosion, the metal exerts brittle cracking below yield strength in the specific corrosive medium and the tensile stress (including the applied stress and residual stress) [40]. The open circuit potential is always lower in the tensile stress concentration zone which acts as the anode of a corrosive cell. However, the compressive stress is beneficial to the formation of the passive film and enhances the corrosion resistance of the specimen surface [41,42]. The specimen surface ground by abrasive particle size of 120 M with the largest comprehensive residual stress in both directions has the best corrosion performance. This further consolidates our research result.

Figure 10. The relationship between surface residual stress and abrasive particle size: (a) X direction and (b) Y direction.

3.6. Electrochemical Dissolution Behavior Based on the results and discussion above, a plausible electrochemical dissolution behavior for Inconel 718 ground by the robotic belt grinding system can be schematically illustrated, as shown in Figure 11. The oxidation film has a porous structure which provides many natural active sites for corroding [43], and the dissolution process predominantly begins from the defective pores of the oxide film, as shown in Figure 11a. It is reasonable that the open circuit electrochemical potential of Nb is more negative, compared with other elements of Inconel 718. The Nb-rich regions occur at the defective pores of the oxide film during the corrosion process. Some corrosion products are generated in this regions. As the corrosion process continues, the dissolution occurs on the boundary between the Nb-rich region and nickel matrix according to the steep increase of current in Figure2. The grooves are generated at the boundary which is in a high energy state and easily corroded. This is similar to the traditional pitting process [44]. With the further extension of corrosion time, the groove deepens

Materials 2018, 11, x; doi: FOR PEER REVIEW www.mdpi.com/journal/materials Materials 2018, 11, 2440 11 of 14 due to the potential difference and high stress concentration, as presented in Figure 11b. With the corrosion process evolving, more products accumulate on the specimen surface and then shed into the electrolyte, leaving a large number of corrosion pits. During the shedding process, undissolved components is peeled off with the removal of the corrosion products [6]. Finally, the corrosion products shed to the electrolyte leaving the corrosion pits, as displayed in Figure 11c. In this experiment, the specimen surface ground by abrasive particle size of 120 M obtains the smallest roughness and the largest compressive residual stress. The smallest roughness generates a Materials 2018, 11, x FOR PEER REVIEW 11 of 13 small corrosion surface area and the largest compressive residual stress protects corrosion products from shedding shedding to to electrolyte. electrolyte. Moreover, Moreover, a small a small roughness roughness can can reduce reduce the stress the stress concentration. concentration. The Thecombination combination of these of these factors factors ultima ultimatelytely leads leads to tothe the best best corrosion corrosion performance. performance. The The results results further validate the electrochemicalelectrochemical dissolutiondissolution behaviorbehavior ofof InconelInconel 718.718.

Figure 11. Schematic of the electrochemical dissolution behavior of Inconel 718 ground by robotic belt grinding system:system: (a) Uncorroded surface,surface, ((b)) CorrosionCorrosion products,products, ((bb)) CorrosionCorrosion pits.pits.

4. Conclusions This paperpaper hashas focusedfocused on on the the electrochemical electrochemical corrosion corrosion behavior behavior of Inconelof Inconel 718 718 after after robotic robotic belt grinding.belt grinding. From From the analysisthe analysis above, above, the followingthe following conclusions conclusions can becan made: be made: 1. TheThe corrosion corrosion resistance of the specimen surface improves remarkably with the decrease of surfacesurface roughness roughness and and residual stress, which result from the abrasive particle size. 2. CorrosionCorrosion of of Inconel Inconel 718 718 ground by the robotic belt grinding systemsystem proceeds from oxide ﬁlmfilm defectdefect occurrence and Nb-richNb-rich region formation,formation, to corrosioncorrosion productproduct generation.generation. Then the corrosioncorrosion products products shed shed into into the the electrolyte electrolyte due to the dissolution of the boundary, leaving a largelarge number number of of corrosion corrosion pits. 3. TheThe small small roughness reduces the corrosion surface area and oxide ﬁlmfilm defect, increasing the resistanceresistance to to corrosion product formation. In In addition, addition, the the compressive residual stress cancan impedeimpede the the exfoliation of corrosioncorrosion products.products. This This reasonably reasonably explains that small abrasiveabrasive particleparticle sizesize improves improves the corrosionthe corrosion performance performance by producing by producing small roughness small androughness compressive and compressiveresidual stress. residual stress.

This work provides signiﬁcantsignificant insights in understanding of corrosion behavior of Inconel 718 treated byby robotic belt system, and analyzing the influencing inﬂuencing factors including surface roughness and residual stress. Further research in precision robotic grinding of nickel nickel-based-based superalloyssuperalloys will revolve around modelling and optimaloptimal controlcontrol ofof thethe processprocess parametersparameters toto achieveachieve desireddesired properties.properties. Author Contributions: J.W. and X.C. proposed the concept of the research. J.W. and X.Z. designed and performed the experiments. X.R. and X.S. contributed the analysis tools. J.W. and J.X. wrote the paper.

Funding: This research received no external funding.

Acknowledgments: This work is supported by Shanghai Jiao Tong University Start-up Grant of China 1000Plan. The authors acknowledge Minnesota Mining and Manufacturing (3M) Company (Shanghai, China) and Baoshan Iron & Steel Co., Ltd (Shanghai, China) for its support to the experimental work.

Conflicts of Interest: The authors declare no conflict of interest.

References

Materials 2018, 11, 2440 12 of 14

Author Contributions: J.W. and X.C. proposed the concept of the research. J.W. and X.Z. designed and performed the experiments. X.R. and X.S. contributed the analysis tools. J.W. and J.X. wrote the paper. Funding: This research received no external funding. Acknowledgments: This work is supported by Shanghai Jiao Tong University Start-up Grant of China 1000Plan. The authors acknowledge Minnesota Mining and Manufacturing (3M) Company (Shanghai, China) and Baoshan Iron & Steel Co., Ltd. (Shanghai, China) for its support to the experimental work. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. González, H.; Pereira, O.; Asier Fernández-Valdivielso, A.; López de Lacalle, L.; Calleja, A. Comparison of Flank Super Abrasive Machining vs. Flank Milling on Inconel® 718 Surfaces. Materials 2018, 11, 1638. [CrossRef][PubMed] 2. Zhao, J.; Liu, Z.; Shen, Q.; Wang, B.; Wang, Q. Investigation of Cutting Temperature during Turning Inconel 718 with (Ti, Al) N PVD Coated Cemented Carbide Tools. Materials 2018, 11, 1281. [CrossRef][PubMed] 3. Wang, B.; Liu, Z.; Hou, X.; Zhao, J. Inﬂuences of Cutting Speed and Material Mechanical Properties on Chip Deformation and Fracture during High-Speed Cutting of Inconel 718. Materials 2018, 11, 461. [CrossRef] [PubMed] 4. Khan, M.A.; Sundarrajan, S.; Natarajan, S.; Parameswaran, P.; Mohandas, E. Oxidation and hot corrosion behavior of nickel-based superalloy for gas turbine applications. Mater. Manuf. Processes 2014, 29, 832–839. [CrossRef] 5. Karthik, D.; Swaroop, S. Laser peening without coating—An advanced surface treatment: A review. Mater. Manuf. Process. 2017, 32, 1565–1572. [CrossRef] 6. Wang, D.; Zhu, Z.; Wang, N.; Zhu, D.; Wang, H. Investigation of the electrochemical dissolution behavior

of Inconel 718 and 304 stainless steel at low current density in NaNO3 solution. Electrochim. Acta 2015, 156, 301–307. [CrossRef] 7. Jebaraj, J.J.; Morrison, D.J.; Suni, I.I. Hydrogen diffusion coefficients through Inconel 718 in different metallurgical conditions. Corros. Sci. 2014, 80, 517–522. [CrossRef] 8. Karthik, D.; Swaroop, S. Laser shock peening enhanced corrosion properties in a nickel based Inconel 600 superalloy. J. Alloys Compd. 2017, 694, 1309–1319. [CrossRef] 9. Huang, C.; Wang, T.; Han, W.; Lee, C. A study of the galvanic corrosion behavior of Inconel 718 after electron beam welding. Mater. Phys. 2007, 104, 293–300. [CrossRef] 10. Khan, M.A.; Prasad, N.R.; Krishnan, S.N.; Raja, S.K.; Jappes, J.W.; Duraiselvam, M. Laser-treated austenitic steel and nickel alloy for human implants. Mater. Manuf. Process. 2017, 32, 1635–1641. [CrossRef] 11. Akyol, A.; Algul, H.; Uysal, M.; Akbulut, H.; Alp, A. A Novel Approach for Wear and Corrosion Resistance in the Electroless Ni-P-W alloy with CNFs Co-Depositions. Appl. Surf. Sci. 2018, 453, 482–492. [CrossRef] 12. Narayanan, B.K.; Duraiselvam, M.; Natarajan, S.; Anaz Khan, M. Laser material processing of nickel superalloy for improved erosion resistance. Mater. Manuf. Process. 2017, 32, 1596–1605. [CrossRef] 13. Arrizubieta, J.; Cortina, M.; Ruiz, J.; Lamikiz, A. Combination of laser material deposition and laser surface processes for the holistic manufacture of inconel 718 components. Materials 2018, 11, 1247. [CrossRef] [PubMed] 14. Chen, X.; Devanathan, R.; Fong, A.M. Advanced Automation Techniques in Adaptive Material Processing; World Scientific: Singapore, 2002. 15. Zhang, X.; Chen, H.; Xu, J.; Song, X.; Wang, J.; Chen, X. A novel sound-based belt condition monitoring method for robotic grinding using optimally pruned extreme learning machine. J. Mater. Process. Technol. 2018, 260, 9–19. [CrossRef] 16. Chen, J.; Chen, H.; Xu, J.; Wang, J.; Zhang, X.; Chen, X. Acoustic signal-based tool condition monitoring in belt grinding of nickel-based superalloys using RF classifier and MLR algorithm. Int. J. Adv. Manuf. Technol. 2018, 98, 859–872. [CrossRef] 17. Pradhan, D.; Mahobia, G.S.; Chattopadhyay, K.; Singh, V. Effect of surface roughness on corrosion behavior of the superalloy IN718 in simulated marine environment. J. Alloys Compd. 2018, 740, 250–263. [CrossRef] 18. Wang, J.; Xu, J.; Wang, X.; Zhang, X.; Song, X.; Chen, X. A comprehensive study on surface integrity of nickel-based superalloy Inconel 718 under robotic belt grinding. Mater. Manuf. Process. 2018, 1–9. [CrossRef] Materials 2018, 11, 2440 13 of 14

19. Tressia, G.; Penagos, J.; Sinatora, A. Effect of abrasive particle size on slurry abrasion resistance of austenitic and martensitic steels. Wear 2017, 376, 63–69. [CrossRef] 20. Turnbull, A.; Mingard, K.; Lord, J.; Roebuck, B.; Tice, D.; Mottershead, K.; Fairweather, N.; Bradbury, A. Sensitivity of stress corrosion cracking of stainless steel to surface machining and grinding procedure. Corros. Sci. 2011, 53, 3398–3415. [CrossRef] 21. Klocke, F.; Soo, S.L.; Karpuschewski, B.; Webster, J.A.; Novovic, D.; Elfizy, A.; Axinte, D.A.; Tönissen, S. Abrasive machining of advanced aerospace alloys and composites. CIRP Ann. Manuf. Technol. 2015, 64, 581–604. [CrossRef] 22. Saha, P.K. Aerospace Manufacturing Process.; CRC Press: Boca Raton, FL, USA, 2016. 23. Liu, F.; Wang, X.; Zhou, B.; Huang, C.; Lyu, F. Corrosion Resistance of 2060 Aluminum–Lithium Alloy LBW Welds Filled with Al-5.6 Cu Wire. Materials 2018, 11, 1988. [CrossRef][PubMed] 24. Tonpe, S.; Kamachi Mudali, U. Effect of thermomechanical process on microstructural evolution, mechanical and corrosion properties of zircaloy-4 tubes of mock-up dissolver vessel. Mater. Manuf. Process. 2017, 32, 27–33. [CrossRef] 25. Liu, X.; Yin, M.; Zhang, S.; Wei, H.; Liu, B.; Du, H.; Hou, L.; Wei, Y. Corrosion Behavior of the As-Cast and As-Solid Solution Mg-Al-Ge Alloy. Materials 2018, 11, 1812. [CrossRef][PubMed] 26. De Vito, E.; Marcus, P. XPS study of passive films formed on molybdenum-implanted austenitic stainless steels. Surf. Interface Anal. 1992, 19, 403–408. [CrossRef] 27. Chai, Z.; Jiang, C.; Zhu, K.; Zhao, Y.; Wang, C.; Cai, F.; Chen, M.; Wang, L. Pretreatment Behaviors and Improved Corrosion Resistance for Cu/Co-Ni-Cu Coating Electrodeposition on Magnesium Alloy. J. Electrochem. Soc. 2016, 163, D493–D499. [CrossRef] 28. Ma, C.; Han, E.-H.; Peng, Q.; Ke, W. Effect of polishing process on corrosion behavior of 308L stainless steel in high temperature water. Appl. Surf. Sci. 2018, 442, 423–436. [CrossRef] 29. Shi, Y.; Pan, Q.; Li, M.; Huang, X.; Li, B. Effect of Sc and Zr additions on corrosion behaviour of Al–Zn–Mg–Cu alloys. J. Alloys Compd. 2014, 612, 42–50. [CrossRef] 30. Dominguez-Crespo, M.A.; Torres-Huerta, A.M.; Rodil, S.; Ramírez-Meneses, E.; Suárez-Velázquez, G.; Hernández-Pérez, M. Effective corrosion protection of AA6061 aluminum alloy by sputtered Al–Ce coatings. Electrochim. Acta 2009, 55, 498–503. [CrossRef] 31. Jiang, B.; Jiang, S.; Ma, A.; Zheng, Y. Effect of heat treatment on erosion-corrosion behavior of electroless Ni-P coatings in saline water. Mater. Manuf. Process. 2014, 29, 74–82. [CrossRef] 32. Kim, Y.-S.; Park, J.; An, B.-S.; Lee, Y.; Yang, C.-W.; Kim, J.-G. Investigation of Zirconium Effect on the Corrosion Resistance of Aluminum Alloy Using Electrochemical Methods and Numerical Simulation in an Acidified Synthetic Sea Salt Solution. Materials 2018, 11, 1982. [CrossRef] 33. Pillis, M.F.; Geribola, G.A.; Scheidt, G.; de Araújo, E.G.; de Oliveira, M.C.L.; Antunes, R.A. Corrosion of thin, magnetron sputtered Nb2O5 films. Corros. Sci. 2016, 102, 317–325. [CrossRef] 34. Sivakumar, B.; Pathak, L.C.; Singh, R. Role of surface roughness on corrosion and fretting corrosion behaviour of commercially pure titanium in Ringer’s solution for bio-implant application. Appl. Surf. Sci. 2017, 401, 385–398. [CrossRef] 35. Balusamy, T.; Narayanan, T.S.; Ravichandran, K.; Park, I.S.; Lee, M.H. Influence of surface mechanical attrition treatment (SMAT) on the corrosion behaviour of AISI 304 stainless steel. Corros. Sci. 2013, 74, 332–344. [CrossRef] 36. Fredj, N.B.; Sidhom, H.; Braham, C. Ground surface improvement of the austenitic stainless steel AISI 304 using cryogenic cooling. Surf. Coat. Technol. 2006, 200, 4846–4860. [CrossRef] 37. Xin, H.; Shi, Y.; Ning, L.; Zhao, T. Residual stress and affected layer in disc milling of titanium alloy. Mater. Manuf. Process. 2016, 31, 1645–1653. [CrossRef] 38. Prabhakaran, S.; Kulkarni, A.; Vasanth, G.; Kalainathan, S.; Shukla, P.; Vasudevan, V.K. Laser shock peening without coating induced residual stress distribution, wettability characteristics and enhanced pitting corrosion resistance of austenitic stainless steel. Appl. Surf. Sci. 2018, 428, 17–30. [CrossRef] 39. Ding, W.; Zhang, L.; Li, Z.; Zhu, Y.; Su, H.; Xu, J. Review on grinding-induced residual stresses in metallic materials. Int. J. Adv. Manuf. Technol. 2017, 88, 2939–2968. [CrossRef] 40. Neves, F.O.; Braga, D.U.; Silva, A.S.C.D. Study of residual stresses on cold-forming metals using stress corrosion. Mater. Manuf. Process. 2015, 30, 1278–1282. [CrossRef] Materials 2018, 11, 2440 14 of 14

41. Takakuwa, O.; Soyama, H. Effect of residual stress on the corrosion behavior of austenitic stainless steel. Adv. Chem. Eng. Sci. 2015, 5, 62–71. [CrossRef] 42. Ralston, K.; Birbilis, N. Effect of grain size on corrosion: A review. Corrosion 2010, 66, 13. [CrossRef] 43. Ghosh, S.; Dey, G.; Dusane, R.; Grover, A. Improved pitting corrosion behaviour of electrodeposited nanocrystalline Ni–Cu alloys in 3.0 wt. % NaCl solution. J. Alloys Compd. 2006, 426, 235–243. [CrossRef] 44. Du, J.; Ding, D.; Zhang, W.; Xu, Z.; Gao, Y.; Chen, G.; Chen, W.; You, X.; Chen, R.; Huang, Y. CeLa enhanced corrosion resistance of Al-Cu-Mn-Mg-Fe alloy for lithium battery shell. Appl. Surf. Sci. 2017, 422, 221–227. [CrossRef]

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).