comparative study of wear and corrosion properties of electric...

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Comparative Study of Wear and Corrosion Properties of Electric Arc Thermally Sprayed Coatings C. R. C. Lima, R. Barrera and F. Camargo, São Paulo/ BR Artigo apresentado no ITSC 2005 – International Thermal Spray Conference – ASM International - TSS The use of thermally sprayed coatings to improve wear and corrosion resistance of industrial components has been found to be an effective and viable choice. Coatings based on several materials can be used for such applications, including situations when they are combined, using distinct deposition methods. The analysis for choice of the ideal coating and application process should generally take into account both coating performance and cost. In this work a comparative study of three commercial coating materials is performed. The used coating materials are High Chromium (HCr), High Chromium + 420 Stainless Steel and 420 Stainless Steel + 1080 Carbon Steel wires applied by Electric Wire Arc Spaying. The obtained coatings are tested for wear (ASTM-G-65-91 rubber wheel test) and corrosion (ANSI/ASTM-B117 and Electrochemical Measurements). Coatings microstructure, microhardness and adhesion (ASTM-C633-85) are also evaluated. 1 Introduction The use of coatings to improve wear and corrosion resistance of mechanical components has been a common practice. Various application techniques have been commonly used, including welding, electroplating, physical (PVD) and chemical (CVD) vapor deposition. Thermal spraying has been found to be an effective and economically viable alternative for those processes in such applications. Coatings of several types can be used for wear and corrosion protection, including situations when they are combined. The analysis for choice of the ideal coating and of the application process should take into account the environment and related limitations and specificities of each coating material and application process. Despite of the availability of new high-energy techniques in thermal spraying like Plasma, Detonation Gun, HVOF and Cold Spray, Electric Wire Arc Spray is a very interesting thermal spray process that combines efficiency, high spray rate, easy operation and relatively low cost, being therefore a widely used process mainly in applications on large areas [1,2]. This technique utilizes the heat from an electric arc to melt two consumable wire electrodes fed automatically to the arc zone. An inherent advantage of the electric arc spray process is its simplicity and the low cost of both equipment and wire type materials [2,3], as well as the simple possibility of feeding two distinct materials to produce an engineered coating for an specific purpose. Coatings of various types can be used effectively to combat abrasive, adhesive or erosive wear, including applications that combine wear and corrosion, either at ambient or elevated temperatures [4,5]. Martensitic stainless steels and carbide-based coatings have been widely used in industrial applications and have proven to be a good choice for several wear and corrosion applications. In the case of electric arc sprayed coatings, the use of cored or composite wires allows the spraying of metals and metal alloys as well as composites like carbides in a metallic matrix [1]. A recent use of high chromium and high carbon alloys based coatings has been documented leading to good results in several industrial applications including hard chrome replacement [6]. This work presents a study of three different commercial materials applied by Electric Wire Arc Spraying on metallic substrates for wear and corrosion applications. The materials were combined in systems for comparison. Microhardness, adhesion and specific wear and corrosion tests evaluation are carried out as well as metallographic characterization. The results are presented in a comparative way for the three obtained coatings. 2 Experimental Procedure The arc sprayed coatings were produced by using an electric wire arc spray equipment model 8830 (Praxair-Tafa, Inc., Concord, NH, USA). The applied materials were 1,6 mm size wires of High Chromiun, here referred as HCr (EuTronic Arc 532, Eutectic, Sao Paulo, Brazil), 420 Stainless Steel (AWS ER 420, DiMartino, Sao Paulo, BR) and 1080 Carbon Steel (ATC MN21084, Met. Nhozinho, Sao Paulo, BR). The coatings were applied in three distinct systems: HCr only (two wires of the same material fed into the arc region); HCr-420 (one wire of HCr and one of 420 stainless steel fed simultaneously), and 1080-420 system (one wire of 1080 steel and one wire of 420 stainless steel fed simultaneously). The spraying conditions were settled at 200 A, 30 V for all the three systems. A spray pressure of 60 psi and wire pressure of 65 psi were used for the HCr-420 and 1080-420 systems. For the HCr system both the spray and wire pressure were 80 psi. The High Chromiun composite wire is referred sometimes as CrC, since upon fusion the constituints form hard-phase particles like carbides and borides. The used substrate was a low carbon steel (SAE 1020). All samples were degreased and grit blasted with aluminum oxide (#60) before spraying. The average thickness of the coatings was 0,55 mm (550 µm) with a variation of ± 50 µm. The samples with dimensions of 20 x 80 x 4,54 mm and 25,4 x 25,4 mm long were mounted in a carrousel rotating at 80 rpm with the spray torch at a fixed distance from the substrate, with vertical displacement of 0,5 m/min. An

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Page 1: Comparative Study of Wear and Corrosion Properties of Electric …ogramac.com.br/artigos/ITSC2005Ogramac.pdf · 2008. 11. 27. · electroplating, physical (PVD) and chemical (CVD)

Comparative Study of Wear and Corrosion Properties of Electric Arc Thermally Sprayed Coatings C. R. C. Lima, R. Barrera and F. Camargo, São Paulo/ BR Artigo apresentado no ITSC 2005 – International Thermal Spray Conference – ASM International - TSS The use of thermally sprayed coatings to improve wear and corrosion resistance of industrial components has been found to be an effective and viable choice. Coatings based on several materials can be used for such applications, including situations when they are combined, using distinct deposition methods. The analysis for choice of the ideal coating and application process should generally take into account both coating performance and cost. In this work a comparative study of three commercial coating materials is performed. The used coating materials are High Chromium (HCr), High Chromium + 420 Stainless Steel and 420 Stainless Steel + 1080 Carbon Steel wires applied by Electric Wire Arc Spaying. The obtained coatings are tested for wear (ASTM-G-65-91 rubber wheel test) and corrosion (ANSI/ASTM-B117 and Electrochemical Measurements). Coatings microstructure, microhardness and adhesion (ASTM-C633-85) are also evaluated. 1 Introduction The use of coatings to improve wear and corrosion resistance of mechanical components has been a common practice. Various application techniques have been commonly used, including welding, electroplating, physical (PVD) and chemical (CVD) vapor deposition. Thermal spraying has been found to be an effective and economically viable alternative for those processes in such applications. Coatings of several types can be used for wear and corrosion protection, including situations when they are combined. The analysis for choice of the ideal coating and of the application process should take into account the environment and related limitations and specificities of each coating material and application process. Despite of the availability of new high-energy techniques in thermal spraying like Plasma, Detonation Gun, HVOF and Cold Spray, Electric Wire Arc Spray is a very interesting thermal spray process that combines efficiency, high spray rate, easy operation and relatively low cost, being therefore a widely used process mainly in applications on large areas [1,2]. This technique utilizes the heat from an electric arc to melt two consumable wire electrodes fed automatically to the arc zone. An inherent advantage of the electric arc spray process is its simplicity and the low cost of both equipment and wire type materials [2,3], as well as the simple possibility of feeding two distinct materials to produce an engineered coating for an specific purpose. Coatings of various types can be used effectively to combat abrasive, adhesive or erosive wear, including applications that combine wear and corrosion, either at ambient or elevated temperatures [4,5]. Martensitic stainless steels and carbide-based coatings have been widely used in industrial applications and have proven to be a good choice for several wear and corrosion applications. In the case of electric arc sprayed coatings, the use of cored or composite wires allows the spraying of metals and metal alloys as well as composites like carbides in a metallic matrix [1]. A recent use of high chromium and high carbon alloys based coatings has been

documented leading to good results in several industrial applications including hard chrome replacement [6]. This work presents a study of three different commercial materials applied by Electric Wire Arc Spraying on metallic substrates for wear and corrosion applications. The materials were combined in systems for comparison. Microhardness, adhesion and specific wear and corrosion tests evaluation are carried out as well as metallographic characterization. The results are presented in a comparative way for the three obtained coatings. 2 Experimental Procedure The arc sprayed coatings were produced by using an electric wire arc spray equipment model 8830 (Praxair-Tafa, Inc., Concord, NH, USA). The applied materials were 1,6 mm size wires of High Chromiun, here referred as HCr (EuTronic Arc 532, Eutectic, Sao Paulo, Brazil), 420 Stainless Steel (AWS ER 420, DiMartino, Sao Paulo, BR) and 1080 Carbon Steel (ATC MN21084, Met. Nhozinho, Sao Paulo, BR). The coatings were applied in three distinct systems: HCr only (two wires of the same material fed into the arc region); HCr-420 (one wire of HCr and one of 420 stainless steel fed simultaneously), and 1080-420 system (one wire of 1080 steel and one wire of 420 stainless steel fed simultaneously). The spraying conditions were settled at 200 A, 30 V for all the three systems. A spray pressure of 60 psi and wire pressure of 65 psi were used for the HCr-420 and 1080-420 systems. For the HCr system both the spray and wire pressure were 80 psi. The High Chromiun composite wire is referred sometimes as CrC, since upon fusion the constituints form hard-phase particles like carbides and borides. The used substrate was a low carbon steel (SAE 1020). All samples were degreased and grit blasted with aluminum oxide (#60) before spraying. The average thickness of the coatings was 0,55 mm (550 µm) with a variation of ± 50 µm. The samples with dimensions of 20 x 80 x 4,54 mm and ∅ 25,4 x 25,4 mm long were mounted in a carrousel rotating at 80 rpm with the spray torch at a fixed distance from the substrate, with vertical displacement of 0,5 m/min. An

Page 2: Comparative Study of Wear and Corrosion Properties of Electric …ogramac.com.br/artigos/ITSC2005Ogramac.pdf · 2008. 11. 27. · electroplating, physical (PVD) and chemical (CVD)

80 µm tick NiAl bond coat was applied for all samples. Vickers microhardness measurements were accomplished computing the average of 10 readings for each sample using a Shimadzu modelo–HMV durometer. Wear resistance measurements were done in a Rubber Wheel Test machine in agreement with ASTM-G-65-91 standard [7]. The abrasive used in the test was quartz round sand. The applied load was 50 N with 4200 total test revolutions at 140 rpm for the rubber wheel. The weight lost was used to compare the abrasion resistance and to calculate the abrasive wear rate of the different samples. The corrosion resistance of the samples was evaluated by means of Salt Fog Test according to ANSI/ASTM-B117 [8] and electrochemical measurements (Open circuit potential x time) in 80 mL of aerated and unstirred 3.4% NaCl solution. An Ag/AgCl KCl, saturated electrode connected to the solution through a Luggin capillary was used as reference electrode. The auxiliary electrode was a Pt-network. A working electrode of each coated sample was fixed at the bottom of the electrochemical cell, exposing an area of 1 cm2 to the solution. Tensile Adhesion Tests (TAT) were performed according ASTM-C-633-85 standard [9]. Metallographic specimens of the obtained coatings as well as of abrasion wear and adhesion tested surfaces were observed in a metallographic test stand by Carl Zeiss, Neophot-32. Scanning Electronic Microscopy (SEM) was carried out using a JEOL electronic microscopy Model 5310 coupled to an Energy Dispersive Spectrometer Analyzer (EDS). 3 Results and Discussion 3.1 Microhardness

Microhardness measurements results are showed in Table 1 for both materials. It can be observed from Table 1 that a higher hardness is obtained for the HCr containing coatings, been maximun for the HCr system samples. This can be observed for the mean value as well as for the maximum and minimum values of Table 1. The high dispersion of values is due to the anisotropic microstructure of the alloyed coatings, even in the HCr one that is an alloy itself . Table 1. Microhardness measurements (Kg/mm2)

HCr HCr-420 1080-420 HV0,3 HV0,3 HV0,3

Min. 312 281 258Max. 484 432 369

Mean (x) 363 330 298 3.2 Adhesion Adhesion test results are presented in Table 2. It can be seen that the HCr samples have the highest adhesion strength. Since all the tree systems samples

have a bond coat, it should be observed that HCr samples were sprayed in their standard spraying conditions, leading to a more defect free coating. Individually, the repeatibility of adhesion results was better for HCr samples that have a good adhesion and basicaly have had a cohesive rupture. Table 2. Adhesion Test Results ASTM-C633 (MPa)

Sample HCr HCr-420 1080-420 1 30,61 30,10 16,65 2 36,61 27,48 22,91 3 36,02 15,22 16,32 4 30,44 25,19 32,38

Mean 33,42 24,49 22,06 On the other hand, the HCr-420 samples have presented lower adhesion values probably due to the higher oxides content. The 1080-420 samples have presented the lowest adhesion values, 52% lower than HCr ones, that could be related to a higher oxide and pores content compared to the other coated materials. Optical microscopy has shoun that only the sample 4 of 1080-420 coating has failed inside the coating itself (coesively). All the other samples for this system have failed basically in the bond coat/ top coat interface. 3.3 Abrasive Wear The results of abrasive wear tests accomplished in conformity with ASTM-G-65-91 are represented in Figure 1. Observing the graphic, it can be noted that HCr samples present the worst wear behaviour since the wear speed is the most acenptuated, refering to a fast degradation of coating integrity. The better behaviour is that of HCr-420 samples, followed by 1080-420 ones. It should be observed that all the tests were conducted in the as sprayed condition,

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Fig. 1. Rubber wheel wear test results (ASTM G-65)

in higly rough surfaces. Normally such materials are machined before application and therefore one should consider the final part of the graphic of Fig. 1, when after stabilization the samples have almost a similar behaviour with wear speed values of 1,86 x 10-4

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mm3/N.m for HCr, 1,71 x 10-4 mm3/N.m for 1080-420 and 1,41 x 10-4 mm3/N.m for HCr-420 samples. 3.4 Corrosion The corrosion results of the Salt Fog Tests have shown a better behaviour for the 1080-420 samples that have started corroding after 100 hs of test. Both HCr and HCr-420 samples have started corroding after 70 hs. After corrosion starts, the 1080-420 samples tend to rapidly spread to the overall surface probably due to the higher 1080 afinity to air corrosion. Electrochemical measurements results are presented in Figure 2 that shows the open-circuit potential curves for tested samples. A potential decay was observed due to the dissolution of surface oxides and the electrolyte penetration in the coating. After 18 hours of testing all the three systems samples have stabilized at the same potential and very close to the substrate potential itself (0,690 V). However, corrosion products can be observed at the surfaces corresponding to iron oxides due to substrate corrosion. Different behaviors among the three systems were observed in the first test hours. Sample of HCr-420 and sample of HCr show both a less abrupt potential decay than the 1080-420 one indicating a slower penetration of the electrolyte until the first 6 hours of testing. It could be stated that samples HCr-420 and HCr have presented better corrosion protection to the substrate compared to the sample 1080-420. A deeper electrochemical study would be necessary to exactly define corrosion resistance of each individual coating system but the obtained results can be used as an indicative of corrosion behavior.

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Fig. 3. Open Potential versus Time results 3.5 Microstructure Figures 4 to 6 show some representative SEM cross sectional micrographs (backscattered) of the obtained coatings. It can be observed an homogeneous coating for both systems with a few pores. The microstructure and morphology of the evaluated coatings, as well as the obtained microhardness values suggest a quite

similar behavior for the three systems in abrasive wear test evidenced by the results. Eventhought HCr has the highest hardness the dispersion of hardness

Figure 4. Cross sectional SEM image of 1080-420

Figure 5. Cross sectional SEM image of HCr-420

Figure 6. Cross sectional SEM image of HCr results agrees with wear behaviour. Optical and scanning electron microstructural analysis has evidenced the morphology and microstructural features of the three applied coating systems. It can be observed a quite uniform distribution of phases in the coatings according to specific systems. In the 1080-420 samples (Fig. 4), some chromium and iron

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oxides (gray phases) in an iron matrix evidenced by EDS analiys. Samples of HCr-420 (Fig. 5) show a more balanced distribution of chromium and iron oxides (gray phases) as well as some evidence of fine and dispersed chromium carbides. Samples of HCr (Fig. 6) have presented a more complex three phases distribution including iron and chromium oxides (dark gray), chromium carbides and other complex phases (light gray) in an iron matrix as verified in spectroscopy analysis. X-rays diffraction of the HCr sample has indicate the presence of some Fe3O4 as well as Cr23C6. Figure 7 shows details of HCr deposited microsctructure in a higher magnification. Some cold sprayed particles can be observed.

Figure 7. Cross sectional micrograph of HCr deposit Since oxidation during the “in-flight” stage of spraying in the general thermal spray processes is a common ocurrence, and this ocurrence is even more accemptuated in conventional electric arc spraying (that uses compressed air as spraying gas), several oxidation products can occur in the deposited coating of high alloyed materials, for instance FeCr2O4 or other several phases from the martensitic stainless steel deposits [10]. This ocurrence suggests that oxide formation is a preferential process instead of carbide formation in the high chromium alloys. Or, as stated by some authors, the time of wire fusion and particle in-flight residence are too short to allow the whole reactions of alloying and hard phase formation [2] .

4 Conclusions Coatings of High Chromiun Alloy (HCr), High Cromium plus 420 Stainless Steel and 1080 Carbon Steel plus 420 Stainless Steel were applied onto low carbon steel substrates. Such materials mixtures were produced by simultaneously feeding distinct wire materials in the arc spray system. All the three coating systems were tested for abrasive wear as well as corrosion resistance. Coatings adhesion, microhardness and microstructure were also evaluated.

The higher hardness is obtained for the HCr system samples and the lower value is that of the 1080-420 system. A high dispersion of values is evidencied by the results, basicaly due to the anisotropic microstructure of the alloyed coatings. Adhesion of the coatings is higher for the HCr coating system. HCr-420 and 1080-420 samples have presented similar adhesion values, 52% lower than the HCr system. The results of abrasive wear show that HCr samples present the worst wear behaviour, i.e., the the most acenptuated wear speed. The better behaviour is that of HCr-420 samples, followed by 1080-420 ones. It should be observed that all the tests were conducted in the as sprayed condition. The corrosion resistance of the samples in the Salt Fog Test have shown a better behaviour for the 1080-420 samples that have started corroding after 100 hs of test. Both HCr and HCr-420 samples have started corroding after 70 hs. In electrochemical measurements HCr-420 and HCr systems have presented better corrosion protection to the substrate compared to the sample 1080-420. The former system have presented a more abrupt potential decay than the 1080-420 one. The obtained results can be used as an indicative of corrosion behavior. 5 Acknowledgments

The authors would like to thank N. Espallargas and CPT - Thermal Spray Center of Barcelona University for helping the corrosion and wear tests. Carlos Lima would like to thanks the support of CNPq-Brazil. 6 Literature

[1] Steffens, H.D., Babiak, Z. and M. Wewel: Recent developments in arc spraying. IEEE Transactions on Plasma Science 18 (1990), Issue 6, pp. 974/79. [2] Verstak, A. and V. Baranovski: HVAF arc spraying. Proceedings of ITSC 2004 – International Thermal Spray Conference and Exposition, Osaka, Japan, ASM-International / DVS (2004), pp. 1/6 [3] Wang, R., Lin, X., Tianjian, Z. and H. Xiaoou: The properties of the high productive high velocity arc sprayed coatings and its applications. Proceedings of ITSC 2004 – International Thermal Spray Conference and Exposition, Osaka, Japan, ASM-International / DVS (2004), pp. 1/5 [4] Lima,C.R.C. and F. Camargo: Study and characterization of high velocity oxy-fuel thermally sprayed wear coatings In. Proceedings of ITSC 2002- International Thermal Spray Conference and Exposition, Essen, Germany, ASM-International/ DVS, 2002, v. 01, n.1 , pp. 654-657

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[5] Tucker, R. C.: An overview of alternative coatings for wear and corrosion resistance, Thermal Spray: Meeting the Challenges of the 21st Century, Proceedings of the 15th International Thermal Spray Conference, vol. 1 (1998), May 25-29, Nice, France, pp. 103/107. [6] Hishinuma, A., Takaki, S. and K. Abiko: Recent progress and future R&D for high –chromium iron-base and chromium-base alloys. Phys. Stat. Sol. 189 (2002), Issue 1, pp. 69/78. [7] ASTM- American Society for Testing and Materials. (1991), Designation G-65-Standard Practices for Conducting Dry Sand/Rubber Wheel Abrasion Tests, Philadelphia, PA, USA.

[8] ASTM. Designation: B117: Metals Test and Analytical Procedures; Erosion and Wear, Metal Corrosion. 1986; Volume 03.02. 43p. [9] ASTM- American Society for Testing and Materials. (1985), Designation C-633- Standard Method of Test for Adhesion or Cohesive Strength of Flame-Sprayed Coatings, Philadelphia, PA, USA. [10] Voleník, K., Hanousek, F., Chráska, P., Ilavský, J. and K. Neufuss: In-flight oxidation of high-alloy steels during plasma spraying. Materials Science and Engineering A272 (1999), pp. 199/206.