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JTTEE5 21:304–313 DOI: 10.1007/s11666-011-9729-7 1059-9630/$19.00  ASM International

Corrosion Protection of Light Alloys Using Low Pressure Cold Spray D. Dzhurinskiy, E. Maeva, Ev. Leshchinsky, and R.Gr. Maev (Submitted July 19, 2011; in revised form December 1, 2011) Corrosion attack of aluminum- and magnesium-based alloys is a major issue worldwide. This study provides a report on the electrochemical behavior of several types of protective metal coatings obtained by low pressure cold spray (LPCS) and describes the performance of the latters corrosion resistance properties. In this manner several metal feedstock compositions were cold sprayed on AA2024-T3 Alclad substrate. Electrochemical methods, such as open circuit potential and potentiodynamic polarization, were used in combination with materials characterization techniques to assess the performance of LPCS protective coating layers. All sprayed samples were tested in the accelerated corrosion salt spray chamber for a time period of up to 500 h to obtain corrosion kinetics data, and with specific attention being focused on the characterization of the coatings microstructural and mechanical properties. The overall conclusion of this study is that the LPCS process could be utilized to deposit corrosion protection coatings of light alloys as well as to repair aluminum and aluminum cladding structures during overhaul maintenance schedule in industry.

Keywords

AA2024, aluminum, coating, corrosion, corrosion protection, low pressure cold spray

1. Introduction High-strength aluminum alloys such as AA2024 and AA7075 are widely used in aircraft manufacture as well as in many other industries as structural components due to their high-strength/weight ratio (Ref 1, 2). Nevertheless, those alloys can be affected by different forms of corrosion such as pitting and galvanic corrosion, intergranular corrosion, stress corrosion cracking or exfoliation corrosion (Ref 1–6). It is well known that in chloride ion containing solution Al-Zn-Mg-Cu alloys are susceptible to severe localized corrosion, such as exfoliation corrosion, intergranular or crevice corrosion, and most severely, pitting (Ref 7). Localized corrosion usually sets in at heterogeneities in the microstructure of the alloy, such as coarse intermetallic phases, constituent particles, inclusions, dispersoids, or even precipitates or segregations at grain boundaries. Among these, coarse intermetallic phases always attracted attention as they are the most prominent initiation sites in alloys due to galvanic coupling effects. It is well known that Cu, Fe and Ti containing intermetallic phases are cathodic with respect to the matrix and thus promote the dissolution of the surrounding matrix (Ref 8). The Mg, Zn, or Si rich intermetallics are usually anodic D. Dzhurinskiy and R.Gr. Maev, Institute for Diagnostic Imaging Research, University of Windsor, Windsor, ON, Canada; and E. Maeva and Ev. Leshchinsky, Department of Physics, University of Windsor, Windsor, ON, Canada. Contact e-mail: [email protected].

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with respect to the matrix and hence dissolve first. The localized corrosion can lead to sudden and unpredicted failures and possible catastrophic disasters. In order to protect light alloys from localized and other forms of corrosion, protective coatings must be used (Ref 9). The main function of protective coatings is to isolate structural reactive elements from environmental corrosives to provide long-term life. Common protection methods for high-strength aluminum alloys include anodizing, cladding, and priming. Most metallic coatings with high corrosion protection performance are deposited by thermal spraying processes (Ref 10). Many studies have been conducted in the past to evaluate the corrosion behavior of metal coatings produced by thermal spray methods (e.g. flame, plasma, HVOF, HVAF, etc.). Those studies reveal that corrosion behavior of metal protective coatings is highly sensitive to its microstructure which is difficult to control (Ref 11–15). The main reasons for this are the high internal stresses and multiphase structure of the deposited layers. Thus, the development of Cold Spray methods designed to avoid the above mentioned drawbacks is of great importance. In the present study, an emerging low pressure cold spray (LPCS) technique was selected as a coating method to produce composite coatings with near free density (porosity is about of 3%, Ref 16). LPCS is a low-temperature spraying process in which the feedstock materials are not melted during spraying but rather kinetically deposited at relatively low temperatures in the solid state on the substrate (Ref 17–19). Consequently, there are no thermal effects such as oxidation, distortion, residual stresses and/or undesirable metallurgical transformations that usually occur during the spray process (Ref 20). The potential and unique features of LPCS have attracted the attention of researchers worldwide to study various aspects of this technology (Ref 21–23). The creation of

Journal of Thermal Spray Technology

2. Experimental Methods and Materials 2.1 Materials Spraying of corrosion protection coatings was performed on commercially available aluminum alloy AA2024-T3 Alclad (composition in wt.%, 3.8-4.9 Cu, 1.21.8 Mg, 0.3-0.9 Mn). Aluminum, zinc atomized powders, and alumina fine particles were provided by Atlantic Equipment Engineers, a powder supply company, with a particle size of 325 mesh. Figure 1 represents the typical morphology of the powders used in this study. The powder blends were mechanically mixed for 10 h and the compositions are listed in Table 1. AA2024-T3 Alclad has been chosen as a reference material for corrosion tests as well.

2.2 LPCS Parameters For all deposition experiments, the inlet air temperature and pressure were fixed at 500 C and 0.62 MPa, respectively. The LPCS spray gun produced by Centerline Ltd. was held by a 3 axes robot at a constant stand-off distance of 15 mm and moved across the substrate surface at a transverse speed of 2 mm/s. The powder feeding rate was about 8-12 g/min using an external powder feeder system. Prior to the deposition process, the substrates were grit blasted using alumina powders with a particle size 100 mesh. The LPCS system was equipped with a custom made nozzle extension prepared by electrical discharge machining (EDM) (Ref 16). This nozzle was also equipped with a one of a kind cooling system developed for the purpose of sufficiently reducing the nozzle clogging effect during the spraying of the aluminum coating.

2.3 Characterization Methods An accelerated salt spray corrosion test was performed on coated coupons according to the ASTM B117 test procedure. The samples were exposed for 100, 200, 300, and 500 h in a salt fog environment using the ATLAS CXX advanced cyclic corrosion exposure system. Corrosion kinetics was evaluated using electrochemical methods. The kinetics data were depicted as average values

Table 1 Specimen designations and the component contents Composition, vol.% Specimen designation

Al

a-Al2O3

Zn

CP1 CP2 CP3 CP4 CP5

100 75 50 30 35

0 25 50 50 40

0 0 0 20 25

Fig. 1 SEM images showing the shape and morphology of the feedstock powders: (a) 75%Al-25%Al2O3 (CP2) and (b) 30%Al-20%Zn50%Al2O3 (CP4)

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composite metallic coatings for corrosion protection is one of the many potential applications of LPCS. There are several studies on the corrosion properties of cold sprayed coatings available to date (Ref 24–26). Regrettably, most of them only discuss the corrosion protection properties of the coatings made by high pressure cold spray systems. Also, there is a lack of information on the corrosion protection behavior over time in a corrosion environment. Thus, the study of corrosion properties of the multiphase composite coatings made by LPCS seems to be of primary importance. A goal of this work is to define and validate how the protective composite coatings made by LPCS will behave over the time in a corrosion environment. The specific objectives are as follows: (i) to select the optimal composition providing the highest corrosion resistance properties of LPCS coatings; (ii) to investigate the influence of a-Al2O3 particle content on the mechanical and corrosion properties of composite coatings; (iii) to analyze the specific features of corrosion protection mechanisms of LPCS coatings.

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from a set of three experiments per plotted point. The measurement of DC polarization potentials was made in accordance with ASTM G69 using the Biologic SP-150 system. According to this procedure, all electrochemical potentials were measured at room temperature in naturally aerated 1 M sodium chloride solution with reference to a saturated calomel electrode (SCE). Data of polarization potential measurements were collected using a typical three electrodes system, consisting of (1) the sample investigated as a working electrode (1 cm2 exposed surface), (2) a graphite rod as the counter electrode and (3) the SCE as the reference electrode. The tests consisted of potentiokinetic polarization of the samples from the cathodic potential of 1500 mV/SCE to the highest potential of 0 mV/SCE. The potential sweep rate dE/dt was 5 mV/s for the purpose of minimizing the timedependent probability of developing crevices during the test (Ref 27, 28). The Tafel extrapolation of the anodic and cathodic slopes was performed using Tafel Fit, where the corrosion current (Icorr) was yielded upon the extrapolation matching the corrosion potential (Ecorr). Open circuit potential (OCP) measurement was performed by immersion test where the coated samples suspended in the 1 M NaCl solution at 20 C where the potential recorded each 0.5 s over 30 min before DC polarization measurements take place. Corrosion rate (CR) was calculated in accordance with ASTM G102 standard: CR ¼

examination of the LPCS coating corrosion performance. In addition, specific attention was focused on the characterization of coating mechanical properties.

3.1 Coating Electrochemistry Characterization 3.1.1 Open-Circuit Potential Measurements. OCP measurement is one of the electrochemical methods used to evaluate and estimate the corrosion performance of coating layers. The OCP is a parameter which indicates the thermodynamically tendency of a material to electrochemical oxidation/passivation in a corrosive medium, and the goal of its measurement is to analyze the potential of the specimen without affecting, in any way, the electrochemical reactions on the specimen surface. After a period of immersion OCP usually stabilizes around a stationary value. Nevertheless, the potential may vary with time and oscillate because of changes in the state of the electrode surface (oxidation or formation of the passive layer) (Ref 30). In the present study, the OCP was used as a criterion to estimate the corrosion behavior of the investigated samples. Figure 2(a) shows an evolution of the OCP for the samples immersed in chloride solution.

Icorr  K  EW dA

where Icorr is the corrosion current (in A); K the constant that defines the units of the CR (3272 mm/(A-cm-year), for CR in mmpy); EW the equivalent weight (in g/equivalent); d the density (in g/cm3); and A is the sample area (in cm2). Microstructure characterization of coatings was performed by field emission scanning electron microscopy according to ASTM E1588 using FEI Quanta 200F SEM with EDAX attachment. The fully automated tensile testing system Zwick/Roell Z150 was employed to evaluate coating adhesion strength and five samples per numerical value were tested in accordance with the ASTM C633 standard. All samples were prepared by spraying about 200-300 lm thick coatings on the top flat surface of cylinder coupons 25.4 cm in diameter. F-1000 Adhesive Film (Cytec Industries Inc.) was used to glue the counter block to get threatened coupling. Microhardness was measured randomly at the cross-section of the coated specimens using DUH-W201S in accordance with ASTM E384 where the maximum loading force was at 1961 mN.

3. Experimental Results As stated by Ref 29 the cold spray coatings could be considered powder consolidated compounds or metal matrix composites. Assuming that the corrosion resistance of powder compounds is worse than that of the corresponding wrought product there is a need for the

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Fig. 2 Evolution of the OCP (EOC) for the investigated samples: (a) EOC as a function of immersion time in corrosive media and (b) EOC as a function (linear regression) of accelerated salt fog corrosion test

Journal of Thermal Spray Technology

Journal of Thermal Spray Technology

of alumina content in the CP1, CP2, CP3 coatings. The corrosion potential is poised at 720 mV/SCE for the AA2024-T3 Alcald, whereas for the CP1 coating it is 738 mV/SCE which is in agreement with data reported by Ref 3 and 34. For the CP2 and CP3 coating layers in assprayed condition it becomes about 1080 mV/SCE. The polarization curves of the examined composite coatings CP2 and CP3, depicted in Fig. 3(a), also exhibit two breakdown potentials: the more active Eb1 is close to 1080 mV/SCE and the more noble Eb2 is close to 760 mV/SCE, where the region between Eb1 and Eb2 is a passivity area at which the current density is constant. Each of these breakdown potentials presents a threshold of anodic current density sharp growth. As explained by Maitra and English (Ref 35) the breakdown potential Eb2 corresponds to the dissolution potential of an Al matrix whereas Eb1 is associated with the dissolution of the aluminum along the boundaries between aluminum and alumina particles. Thereby, the corrosion protection

Fig. 3 Evolution of the corrosion potential (Ecorr) for the investigated samples: (a) potentiodynamic DC polarization curves before corrosion test and (b) Ecorr as a function (linear regression) of accelerated salt fog corrosion test

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Around the 15 min mark during the immersion period, an abrupt decrease in OCP of Alclad sample toward negative potentials was observed indicating that the thermodynamic tendency of corrosion is increased. This initial fall seems to be related to the destruction of a virgin aluminum oxide film formed during the cold mill rolling process. This effect disappears after further holding the samples in the aerated 1 M NaCl solution during the test. The OCP of the Alclad is stabilized at 730 mV/SCE after 15 min of immersion test. The corrosion behavior of the examined cold sprayed coatings depends on the powder feedstock composition. The OCP of the cold sprayed coating made of pure aluminum (CP1) does not depend on immersion time in sodium chloride media. That reveals the presence of a relatively stable protective oxide film on the coating surface. The as-sprayed CP2 and CP3 coatings exhibit the slight change of the OCP toward to the negative value during immersion time, and the potential stabilizes at 762 mV/SCE for CP2, and 801 mV/SCE for CP3 respectively whereas the Al-Zn-Al2O3 coatings exhibit the decrease of the potential up to 1050 mV/SCE. The higher amount of Zn within the cold sprayed coating layer causes more negative OCP potentials due to the sacrificial behavior of the Zn. This is in agreement with Ref 31 and 32. Another observation made from the OCP monitoring is data noise caused by the potential oscillation behavior. This oscillation feature may be attributed to the formation of new interfaces such as pitting, and to a reaction at the interparticle and grain boundaries. The parameters of the oscillation depend on the surface roughness, reaction type and kinetics that result in a changing surface oxide layer (Ref 33). While an amplitude of the OCP oscillation of approximately 8 mV is obtained for the Alclad (Ra = 0.6 lm), that of the Al coating (Ra = 12.5 lm) is about 2 mV. The possible reason of such behavior is the more uniform dissolution of the aluminum coating oxide film as compared with Alclad. A more detailed investigation of this effect will be made in further work. The kinetics of the corrosion process may be evaluated in terms of the dependence of the OCP on the corrosion time. The experimental data of the OCP variation with time Ecorr = f(t) may be approximated by linear functions as shown in Fig. 2(b). It can be seen that the OCP of pure Al coating remains stable during the entire test time whilst the Al-Al2O3 coatings become more noble due to permanent oxidation in a corrosion environment which results in a growth of the dense oxide film. The behavior of Al-Zn-Al2O3 coatings differs from those of Al based ones due to the sacrificial effect of Zn, which results in a higher CR providing sacrificial corrosion protection to the AA2024-T3 Alclad material. 3.1.2 DC Polarization Measurements. The potentiodynamic DC polarization testing was conducted for further detailed understanding of the coating corrosion properties. The potentiodynamic polarization curves of uncoated AA2024-T3 Alclad substrate and as-sprayed CP1, CP2, and CP3 coating layers are summarized in Fig. 3(a). A comparison of results shows the effect of the decrease of the corrosion potential Ecorr with an increase

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efficiency was different with an increase of ceramic particles content in the coatings. The kinetics of the corrosion potential evolution during an accelerated corrosion test is shown in Fig. 3(b) as a function of holding time. Results indicate the two types of functions Ecorr = f(t): (i) Ecorr increases with time, and (ii) Ecorr falls with time. This reveals two coating corrosion mechanisms: noble for CP1, CP2, and CP3 (associated with the growth of oxide film), and sacrificial for CP4 and CP5 (associated with the dissolution of the coating layer). The behavior of corrosion current (Icorr) is shown in Fig. 4. The Icorr is poised at 0.2 lA/cm2 for the AA2024T3 Alcald and at 0.8 lA/cm2 for the CP1 coating. Contrary, the Icorr dependence of the CP2 and CP3 coatings falls from 1.3 to 1.05 lA/cm2 and from 0.9 to 0.3 lA/cm2 respectively. The experimental results (Fig. 3 and 4) reveal that a LPCS coating layer made of pure aluminum powder has the best corrosion protection properties as compared to those of other investigated composite coatings. Also, the results of DC polarization measurements are in agreement with the OCP measurements described above (Fig. 2) where the potential depends on the content of Al2O3 within the coating. Al-Zn-Al2O3 coatings exhibit significantly higher Icorr values than those of AA2024-T3 Alclad substrate (46 lA/ cm2 for CP4 and 50 lA/cm2 for CP5). The Ecorr of CP4 and CP5 coatings is significantly lower than that of Alclad material, which reveals an active dissolution of the Al-ZnAl2O3 composite. This behavior confirms that the CP4 and CP5 coatings are capable of providing a sacrificial corrosion protection mechanism. A comparison of the corrosion behavior of LPCS Al-Al2O3 and Al-Zn-Al2O3 composite coatings clearly shows that the presence of corrosion products, i.e. the formation of the new interfaces may significantly change the corrosion mechanism from a noble to sacrificial corrosion protection. 3.1.3 Corrosion Rate. The CR calculation results depicted in Fig. 5 reveal that all Al-Al2O3 based LPCS coatings have relatively the same CR. This data may be explained by the fact that galvanic corrosion between Al2O3 and Al particles is unlikely because the resistivity of

alumina is greater than 1014 XÆcm (Ref 36). The higher CRs of the Al-Zn-Al2O3 sprayed coatings (CP4, CP5) may be explained by the action of both the Cl ions and the galvanic coupling effect between the noble Al and the much more active Zn particles resulting in the formation of Zn5(OH)8Cl2ÆH2O and ZnO corrosion products (Ref 37). One has to note, an accurate overall CR could not be expected from the potentiodynamic polarization technique. It is important to note that the presented CR values should be considered as CR estimations only.

3.2 Coating Mechanical Properties 3.2.1 Tensile Strength. Table 2 lists the average results of the adhesion tensile strength measurements performed on coated specimens. An increase in the content of Al2O3 within the aluminum matrix of the coating leads to an increase of the adhesion strength from 37 to 45 MPa for CP1, CP2 and CP3 coatings. It should be noted from test results analysis that in most cases the failure type was cohesive; where the fracture occurred through the coating layers was consistent, indicating cohesion strength (Fig. 6a) and that the true adhesion strength of these coatings must be greater. It is believed that the presence of ceramic particles within the coating reduces the total surface area of the bonded particle interfaces and results in cohesive failure rather than adhesive; this has recently been confirmed (Ref 38). However, the tensile strength of CP4 and CP5 composites was of 40 MPa and irrespective of the reinforcement phase. It is believed that the mutual

Fig. 5 Evolution of CR as a function (linear regression) of accelerated salt fog corrosion test environment

Table 2 Adhesion strength measurement results Coating layer

Fig. 4 Evolution of the corrosion current (Icorr) as a function (linear regression) of accelerated salt fog corrosion test

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CP1 CP2 CP3 CP4 CP5

Average adhesion tensile strength, MPa 37 45 43 40 40

± ± ± ± ±

3 2 2 2 2

Failure type Adhesion Cohesion Cohesion Cohesion Cohesion

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Fig. 6 SEM images showing cohesive failure of the CP2 coating layer

penetration/interlocking of the sprayed Al-Zn materials plays a critical role and specifies enhanced bonding strength of the coating. The bonding between Zn and Al particles is mainly attributed to the plastic deformed mechanical embedding effect (Ref 39). According to Hawthorne and Xie (Ref 40) the bonding strength of sprayed material is closely linked to its resistance of contact deformation and its ability to deform without fracture. Furthermore, SEM examination of the surface topography (Fig. 6b) examined after cohesion failure reveals the presence of fine submicron Al2O3 particles embedded into the coating matrix (Fig. 6c and d). This is similar to the results shown by Coddet and co-workers (Ref 41) where the jet milling process of ceramic particles takes place during the kinetic impacting process of agglomerated particles upon the substrate. 3.2.2 Coating Microhardness. As mentioned earlier, an increase of ceramic content in the coating allows the improvement of the adhesion strength. Figure 7 shows the average microhardness of examined coating layers. The microhardness of AA2024-T3 Alclad was found to be of 135 MPa, similar to the results of Ref 42. It can be seen that the microhardness of Al-Al2O3 coating depends on the content of the reinforcement ceramic phase within the coating. The mean microhardness of the coatings increases from 55 to 83 MPa for CP1 and CP3 respectively. The

Journal of Thermal Spray Technology

Fig. 7 Distribution of microhardness along cross section of the coating layers

possible reason is believed to be an effective hardening of the coating layers due to reinforcement by Al2O3 particles, which is in agreement with Ref 25. Aluminum coating has a lower microhardness because of difficulties of interparticle bonds formation due to the fracture of native aluminum oxide film on the particle surface. Furthermore, the microhardness of CP4 and CP5 coatings exhibits 86 MPa

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Peer-Reviewed Fig. 8 SEM images of the coating layers topography: (a) Al feedstock powder (CP1); (b) 75%Al-25%Al2O3 feedstock powder (CP2); and (c) 30%Al-20%Zn-50%Al2O3 feedstock powder (CP4)

Fig. 9 SEM images showing the cold spray surface topographies after given 500 h of accelerated corrosion test environment (ASTM B117): (a) Al feedstock powder (CP1); (b) 75%Al-25%Al2O3 feedstock powder (CP2); and (c) 30%Al-20%Zn-50%Al2O3 feedstock powder (CP4)

due to the presence of Zn particles which are being severely deformed (Fig. 11c). It is believed that the content of zinc within the coating intensifies the local interparticle bonding effect and enhances coating microhardness.

4. Discussion The specific features of the passivation behavior define a corrosion protection mechanism of the LPCS coatings. It is well known (Ref 43–46) that the passivation of wrought aluminum material depends on the formation of a strong oxide layer which is essentially inert and prevents corrosion. Foley and Nguyen (Ref 43) observed that in chloride solution aluminum ionizes rapidly to the Al3+ ion, which also hydrolyses rapidly. These two particles react further to form a reasonably stable basic aluminum chloride (Al3+ + 3Cl fi AlCl3; and AlCl3 + 6H2O fi 2Al(OH)3 + 3HCl) that is transformed slowly to Al(OH)3 and finally to Al2O3Æ2H2O which plays an important role in the passivation of aluminum (Ref 44–46). The detailed analysis of the passivation behavior was based on an evaluation of the coating morphology. With this

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aim in mind, the coating surface was examined using scanning electron microscopy (SEM) and the chemical composition of the corrosion product formed at the coating layers was defined by energy dispersive x-ray spectroscopy (EDX). The images depicted in Fig. 8 show that the coating morphology is quite different for the as-sprayed coatings. An increase of the reinforcing ceramic phase in the coating leads to an increase of the coating roughness thereby increasing the number of sites where corrosion attack can be initiated. Hong and Li reported (Ref 47, 48) that the coating morphology plays a critical role in corrosion propagation over the sprayed surface where the higher coating roughness corresponds to lower pitting potential (the potential at which pits start to form on the surface). Analyzing the results of the coatings oscillation behavior demonstrates that the amplitude of the OCP oscillation is decreased with an increase in the coating roughness. Figure 9 shows micrographs of the coating surface after 500 h in the ASTM B117 environment. The homogeneous and dense scale formed looks similar for the CP1, CP2, and CP3 coating layers. However, image analysis revealed that the area fraction occupied by white rust increases with an

Journal of Thermal Spray Technology

Peer-Reviewed Fig. 10 SEM images showing: (a) Al2O3 nucleation around the aluminum particle; (b) dense passivation oxide layer on the coating surface after 500 h of salt fog environment; and (c) EDX analysis of corrosion protection layer

Fig. 11 Cross-section microphotographs of the coating layers cold sprayed on AA2024-T3 Alclad: (a) Al feedstock powder (CP1); (b) 75%Al-25%Al2O3 feedstock powder (CP2); and (c) 30%Al-20%Zn-50%Al2O3 feedstock powder (CP4)

increase of alumina content, to approximately 55% for CP1 and 65% for CP2 respectively. The granular oxide scale around the aluminum particle is seen at higher magnification. Figure 10(b) shows the region of CP1 coating where aluminum phase islands (dark gray) are found. EDX analysis confirmed that dark gray areas on the image associate with aluminum whereas white gray areas correspond to tetrahydroxoaluminate oxide layers. The significant increase of the oxygen content is believed to be associated with the formation of Al2O3Æ2H2O (Ref 46). Figure 10(a) shows the needle crystal nucleation of the aluminum oxide. Fine-grain lamellas have a leaf-like structure with leaves the size of about tens of nanometers. Such needle structure of Al2O3Æ2H2O is known to be typical during aluminum oxide nucleation and its crystal growing (Ref 49, 50). The morphology of Al-Zn-Al2O3 coating after 500 h of the salt fog test is depicted in Fig. 9(c). It is likely that the corrosion of CP4 and CP5 coatings begins from a galvanic action between aluminum and zinc phases leading to the dissolution of the coating as reported by Ref 51 and 52. The main corrosion products formed are zinc hydroxychloride (Zn5(OH)8Cl2ÆH2O) and zinc oxide (ZnO) as confirmed by EDX. The kinetics of Zn5(OH)8Cl2ÆH2O formation greatly depends on the content of zinc in the coating and also became dominant at high chloride ion concentration as reported by Long and Yin (Ref 53, 54).

Journal of Thermal Spray Technology

The SEM analysis of the coatings as depicted in Fig. 11 represents typical cold spray microstructures (Ref 25, 55) where the metal particles are severely deformed. The structure of the coating interfaces formed a relatively rough surface (Ra = 10-15 lm) caused by the grid blasting process prior to deposition. In addition, there has been a substantial amount of particle deformation upon impact with the substrate leading to relatively low porosity of the sprayed coatings (Fig. 11). Figure 11(a) reveals the presence of closed micropores at the coating interface (less than 1.0 vol.% as measured by image analysis) whereas the rest of the coating was fully dense. The low porosity level and the absence of cracks at the coating/substrate interface are the major coating characteristics produced by LPCS system (Ref 16, 26). Furthermore, the uniform dispersion of both randomly distributed Al and Al2O3 particles was observed for Al-Al2O3 composites (Fig. 11b). The coating consists of deformed aluminum particles forming a matrix reinforced with ceramic particles (Al2O3—darker gray spots).

5. Conclusions Based on the results obtained in this study, the following conclusions can be drawn:

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LPCS offers unique opportunities to spray uniformly dense composite corrosion-resisting coatings. The use of a portable LPCS machine makes the process an attractive method for on-site and in-field restoration by reducing or eliminating further corrosion damage. The results of the accelerated corrosion test demonstrate that corrosion of the sprayed composites intensifies as the concentration of alumina particles increases within the coating. The coatings made of pure aluminum feedstock may be recommended as corrosion protection coating to protect aluminumbased alloys through the noble corrosion protection mechanism. In Al-Zn-Al2O3 coating composites, the process of degradation is accelerated by the Al-Zn galvanic couple. Those coatings could be considered sacrificial anode materials. The increase of Al2O3 in the coatings results in enhanced adhesion strength and microhardness of the sprayed composites.

Acknowledgments The authors would like to acknowledge the financial support of the Auto 21 Program and NSERC CRD PG320679 Grant.

References 1. ASM Handbook, Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, 10th ed., ASM International Committee Handbook, vol. 2, 1992 2. R.G. Buchheit, R.P. Grant, P.F. Hlava, B. Mckenzie, and G.L. Zender, Local Dissolution Phenomena Associated with S-Phase (Al2CuMg) Particles in Aluminum Alloy 2024-T3, J. Electrochem. Soc., 1997, 144, p 2621-2628 3. V. Guillaumin and G. Mankowski, Localized Corrosion of 2024 T351 Aluminium Alloy in Chloride Media, Corros. Sci., 1999, 41, p 421-438 4. C. Blanc, S. Gastaud, and G. Mankowski, Mechanistic Studies of the Corrosion of 2024 Aluminum Alloy in Nitrate Solutions, J. Electrochem. Soc., 2003, 150, p 396-404 5. G.O. Ilevbare, O. Schneider, R.G. Kelly, and J.R. Scully, In Situ Confocal Laser Scanning Microscopy of AA 2024-T3 Corrosion Metrology I: Localized Corrosion of Particles, J. Electrochem. Soc., 2004, 151, p 453-464 6. R.G. Buchheit, L.P. Montes, M.A. Martinez, and P.F. Hlava, The Electrochemical Characteristics of Bulk-Synthesized Al2CuMg, J. Electrochem. Soc., 1999, 146, p 4424-4428 7. F. Andreatta, H. Terryn, and J.H.W. de Wit, Corrosion Behaviour of Different Tempers of AA7075 Aluminium Alloy, Electrochim. Acta, 2004, 49, p 2851 8. N. Birbilis and R.G. Buchheit, Electrochemical Characteristics of Intermetallic Phases in Aluminum Alloys: An Experimental Survey and Discussion, J. Electrochem. Soc., 2005, 152, p 140-151 9. ASM Handbook, Corrosion: Fundamentals, Testing, and Protection, vol. 13A, 2003, p 1779-1793 10. T. Burakowski, Surface Engineering of Metals, CRC Press, Boca Raton, 1999 11. G. Bolelli, L. Lusvarghi, and M. Barletta, Heat Treatment Effects on the Corrosion Resistance of Some HVOF-Sprayed Metal Alloy Coatings, Surf. Coat. Technol., 2008, 202, p 4839-4847

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12. G. Ballerini, U. Bardi, R. Bignucolo, and G. Ceraolo, About Some Corrosion Mechanisms of Magnesium Alloy, Corros. Sci., 2005, 47, p 2173-2184 13. J. Kawakita, S. Kuroda, T. Fukushima, and T. Kodama, Corrosion Resistance of HVOF Sprayed HastelloyC Nickel Base Alloy in Seawater, Corros. Sci., 2003, 45, p 2819-2835 14. J. Kawakita, S. Kuroda, T. Fukushima, and T. Kodama, Development of Dense Corrosion Resistant Coatings by an Improved HVOF Spraying Process, Sci. Technol. Adv. Mater., 2003, 4, p 281-289 15. M.B. Kannan, W. Dietzel, R. Zeng, R. Zettler, and J.F. Santos, A Study on the SCC Susceptibility of Friction Stir Welded AZ31 Mg Sheet, Mater. Sci. Eng. A, 2007, 460/461, p 243-250 16. D. Dzhurinskiy, R. Maev, and V. Leshchynsky, Particle Consolidation by Low Pressure Cold Spray: Modeling, Experiments and Applications, 2nd Canadian Cold spray Conference (Montreal, Canada), 2010 17. R.G. Maev and V. Leshchinsky, Low Pressure Gas Dynamic Spray: Shear Localization During Particle Shock Consolidation, Building on 100 Years of Success: Proceedings of the 2006 International Thermal Spray Conference, 15-18 May 2006 (Seattle, WA, USA), ASM International 18. R.Gr. Maev and V. Leshchynsky, Introduction to Low Pressure Gas Dynamic Spray, John Willey and Son-VCH, New York, 2007, p 181-214 19. Department of Defense Manufacturing Process Standard, Materials Deposition: Cold Spray, MIL-STD-3021, 2008 20. H. Assadi, F. Gartner, T. Stoltenhoff, and H. Kreye, Bonding Mechanism in Cold Gas Spraying, Acta Mater., 2003, 51, p 43794394 21. T.H. Van Steenkiste, J.R. Smith, R.E. Teets, J.J. Moleski, D.W. Gorkiewicz, R.P. Tison, D.R. Marantz, K.A. Kowalsky, W.L. Riggs, P.H. Zajchowski, B. Pilsner, R.C. McCune, and K.J. Barnett, Kinetic Spray Coatings, Surf. Coat. Technol., 1999, 111, p 62-71 22. H.J. Choi and M. Lee, Application of a Cold Spray Technique to the Fabrication of a Copper Canister for the Geological Disposal of CANDU Spent Fuels, Nucl. Eng. Des., 2010, 240, p 2714-2720 23. N. Bala, H. Singh, and S. Prakash, Characterization and HighTemperature Oxidation Behavior of Cold-Sprayed Ni-20Cr and Ni-50Cr Coatings on Boiler Steels, Metall. Mater. Trans. A, 2011, 42, p 3399-3416 24. H. Lee, S.H. Jung, S.Y. Lee, Y.H. You, and K.H. Ko, Correlation Between Al2O3 Particles and Interface of Al-Al2O3 Coatings by Cold Spray, Appl. Surf. Sci., 2005, 252, p 1891-1898 25. K. Spencer, D.M. Fabijanic, and M.-X. Zhang, The Use of Al-Al2O3 Cold Spray Coatings to Improve the Surface Properties of Magnesium Alloys, Surf. Coat. Technol., 2009, 204, p 336-344 26. Y. Tao, T. Xiong, C. Sun, H. Jin, H. Du, and T. Li, Effect of AlfaAl2O3 on the Properties of Cold Sprayed Al/A-Al2O3 Composite Coatings on AZ91D Magnesium Alloy, Appl. Surf. Sci., 2009, 256, p 261-266 27. P.E. Morris and R.C. Scarerry, Predicting Corrosion Rates with the Potentiostat, Corrosion, 1972, 28, p 444 28. S. Frangini and N. De Cristofaro, Analysis of the Galvanostatic Polarization Method for Determining Reliable Pitting Potentials on Stainless Steel in Crevice-Free Condition, Corros. Sci., 2003, 45, p 2769-2786 29. V. Champane, The Cold Spray Materials Deposition Process, Woodhead Publishing Ltd, Cambridge, 2007 30. I.J. Hwang, D.Y. Hwang, Y.M. Kim, B. Yoo, and D.H. Shin, Formation of Uniform Passive Oxide Layers on High Si Content Al Alloy by Plasma Electrolytic Oxidation, J. Alloys Compd., 2010, 504, p 527-530 31. M.A. Baker, W. Gissler, S. Klose, M. Trampert, and F. Weber, Morphologies and Corrosion Properties of PVD Zn-Al Coatings, Surf. Coat. Technol., 2000, 125, p 207 32. L. Guzman, M. Adami, W. Gissler, S. Klose, and S. De Rossi, Vapour Deposited Zn-Cr Alloy Coatings for Enhanced Manufacturing and Corrosion Resistance of Steel Sheets, Surf. Coat. Technol., 2000, 125, p 218 33. Y. Tao, T. Xiong, C. Sun, L. Kong, X. Cui, and T. Li, Microstructure and Corrosion Performance of a Cold Sprayed Aluminium Coating on AZ91D Magnesium Alloy, Corros. Sci., 2010, 52, p 3191-3197

Journal of Thermal Spray Technology

Journal of Thermal Spray Technology

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Peer-Reviewed

34. B. DeForce, T. Eden, J. Potter, V. Champagne, P. Leyman, and D. Helfritch, Application of Aluminum Coatings for the Corrosion Protection of Magnesium by Cold Spray, Tri-Service Corrosion Conference, Denver, USA, 2007 35. S. Maitra and G.C. English, Mechanism of Localized Corrosion of 7075 Alloy Plate, Metall. Trans. A, 1981, 12A, p 535 36. R.E. Bolz and G.L. Tuve, CRC Handbook of Tables for Applied Engineering Science, CRC Press, Boca Raton, 1973, p 262 37. H. Li, X. Li, M. Sun, H. Wang, and G. Huang, Corrosion Resistance of Cold-Sprayed Zn-50Al Coatings in Seawater, J. Chin. Soc. Corros. Prot., 2010, 30, p 62-66 38. M. Yandouzi, P. Richer, and B. Jodoin, Particulate Reinforced Al-12Si Alloy Composite Coatings Produced by the Pulsed Gas Dynamic Spray Process: Microstructure and Properties, Surf. Coat. Technol., 2009, 203, p 3260 39. L. Hai-xiang, S. Ming-xian, L. Xiang-bo, and W. Hong-ren, Residual Stress Measurement of Metal Surface, Chin. J. Nonferr. Met., 2010, 20, p 1353-1359 40. H.M. Hawthorne and Y. Xie, An Attempt to Evaluate Cohesion in WC/Co/Cr Coatings by Controlled Scratching, Meccanica, 2001, 36, p 675-682 41. W. Li, C. Zhang, H. Liao, J. Li, and C. Coddet, Characterizations of Cold-Sprayed Nickel-Alumina Composite Coating with Relatively Large Nickel-Coated Alumina Powder, Surf. Coat. Technol., 2008, 202, p 4855-4860 42. W. Zhang and G.S. Frankel, Transitions Between Pitting and Intergranular Corrosion in AA2024, Electrochem. Acta, 2003, 48, p 1193-1210 43. R.T. Foley and T.H. Nguyen, Chemical Nature of Aluminum Corrosion-2. The Initial Dissolution Step, J. Electrochem. Soc., 1982, 192, p 129 44. C.B. Breslin, G. Treacy, and W.M. Cornell, Studies on the Passivation of Aluminium in Chromate and Molybdate Solutions, Corros. Sci., 1994, 36, p 1143-1154