Erosion-corrosion corrosion studies of nanostructured Ni-Cr Ni Cr coatings on SAE 213-(T22) 213 and SA 516 (Gr -70) steels

Abstract

Fireside degradation of boiler tubes in coal-fired water tube boilers is a combined effect of erosion as well as corrosion. The corrosion products (oxides and sulfides) are formed on boiler tubes due to chemical reactions with the combustion gases, simultaneously the flue gases present in the boiler environment erode these corrosionproducts deposited on the tubes. Fireside erosion-corrosion of boiler tubes leads to continuous thinning of the boiler tubes in the working environment, which can further result into instantaneous rupture of the tubes. Erosion-corrosion is recognized as the main cause of downtime (50-75% of their total arrest time) at power-generating plants, which leads to enormous economic losses (Koch et al., 2002; Hidalgo et al., 1997). Keeping in view of these crucial facts, erosion-corrosion control of boiler tubes has invited considerable attention and among the various alternatives the thermal sprayed coatings have the potential to provide suitable protection to boiler steels from the aggressive fireside corrosion (McCune et al., 1995). Among various thermal spray coating techniques, high-velocity oxy-fuel (HVOF) spraying is a frequently considered process to provide high temperature E-C protection to boiler tube. Further, a new entrant in the category of thermal spray processes, namely cold-spray process has great potential to be used for such applications. Cold-spray process works well below the melting point of feedstock powders (Papyrin et al., 1996). The major role played in deposition of coating is by the momentum transfer from the supersonic gas jet to powder particles, results in high particle jet. These powder particles on striking with substrate surface plastically deformed, to form interlinked splats resulting in a surface coating. No oxidation of coating powders takes place during in-flight and hence no phase transformation occurs during deposition process. These features give cold-spraying process a capacity to develop nano-structured coatings. In the current investigation, hot corrosion behaviour of uncoated and coated boiler tube materials namely SA213-T22 and SA 516-Grade 70 was studied. The nanoparticles of Ni and Cr were synthesized using bottom-up and top-down approaches. These nano-particles were further characterized using several characterization techniques such as XRD, SEM/EDS, TEM, DLS and CHNS-O analyses. The average particle size of oxide-free Ni nano-particles synthesized by chemical route was found to be 29 nm, with a spherical morphology. Average particle size of Ni and Cr nano-particles synthesized by mechanical milling (top-down approach) was found to be 67 nm and 59 nm respectively, with irregular flake-type morphology. Four novel variants of nanocrystalline Ni-20Cr powders were synthesized by blending nano-sized and micron-sized powders in a planetary ball mill, in such a way that overall wt.% Ni:Cr was maintained as 80:20. Average particle size of four different variants of Ni-20Cr nanocrystalline powders synthesized by mechanical route (designated as Powders A, B, C and D), was found to be 14 µm, 6 µm, 13 µm and 11 µm respectively. XRD and TEM analyses have been carried out to confirm the nanocrystallinity of the synthesized powders and the crystallite size was found to be 8 nm, 12 nm, 13 nm and 10 nm respectively. Then, these nanocrystalline powders were deposited on the selected substrate steels using HVOF and cold-spray techniques. The developed HVOF-sprayed coatings have been designated as AHT, BHT, CHT, DHT, AHS, BHS, CHS, DHS, whereas cold-sprayed as ACT, CCT, DCT, ACS, CCS, DCS; here A, B, C, D refer to the coating variants, H to HVOF-spray, C to coldspray, T to T22 steel substrate and S to SA 516 steel substrate. It has been found that the deposited coatings could maintain the nanocrystalline structure similar to that of feedstock powders, although slight grain growth occurred during deposition process. The developed coatings were subsequently characterized with regard to their metallurgical and mechanical properties. The HVOF-sprayed coatings in general, were found to have the presence of Ni and NiO phases. The presence of NiO phase may be due to in flight oxidation of Ni particles during deposition process. On the other hand γ-Ni was observed as the main phase in all the cold-spray coatings without the formation of NiO or any other oxide. In this way, cold-spray coatings were found to be oxide-free, which is useful attribute from the point of view of high temperature corrosion resistance. Surface and cross-sectional optical and SEM micrographs of the investigated HVOF-spray nanostructured coatings indicated a typical splat-like morphology, which led to the formation of a lamellar crosssectional microstructure. Most of the splats/particles exhibited a flattened appearance, which is an indication of severe plastic deformation during deposition process. The coatings in general were found to have a continuous contact with the steel substrate. The investigated cold-sprayed coatings had a dense morphology, in general, formed as a consequence of proper coalescence of feed stock powder particles. It was further observed from the X-ray mappings that the inter-diffusion of various elements between the substrate and the HVOF/cold sprayed coatings was found to be marginal. The XRD results were well supported by the surface as well as cross-sectional SEM/EDS analysis. TEM analysis revealed the consistent distribution of grain size indicating that the nanocharacteristics of both the Ni-rich phases and Cr-rich phases were well preserved in the cold-spray coating. Various mechanical properties such as microhardness, scratch resistance, elastic modulus, and fracture toughness of as-sprayed coatings were studied. The average microhardness of HVOF and cold-sprayed coatings on T22 and SA 516 steel was found to be higher than the conventional Ni-20Cr coatings (micron-sized) on the same steels. Elastic modulus of the coating is the indicator of the stiffness of the coatings. HVOF-sprayed coatings on T22 steels had shown stiffer behavior in comparison to coldsprayed coatings on T22 steels. But in case of SA 516 steels, cold-spray coatings had offered more stiffness in comparison to HVOF-spray coatings. The fracture toughness of the HVOF-spray coatings was found to be quite high in comparison to cold-spray coatings. The cyclic air oxidation studies of uncoated and coated steels were conducted at 900ºC for 50 cycles. The analyses regarding the oxide scale thickness, XRD, SEM/EDS and X-ray mapping analyses of the oxidized samples have been carried out. T22 steel showed a relatively higher air oxidation resistance in comparison with SA 516 steel, which may be attributed to the presence of Cr (2wt.%) in the former steel. Amongst all the developed coatings (HVOF and cold-sprayed) on T22 steel, the cold-sprayed coatings in general, showed comparatively high oxidation resistance. The ACT coating was found to have shown highest resistance to oxidation, which may be attributed to its finest grain (18 nm) size among all the coatings on T22 steel. Furthermore, this coating was even found to have a better oxidation resistance (two-fold) in comparison with the reported cold-sprayed Ni-20Cr microstructured coating on the same steel (Bala, 2010). Further it has been found from the cross-sectional EDS and x-ray mapping analyses of all the coldsprayed T22 steels that no internal oxidation of substrate has taken place and the basic elements have been preserved, with an exception of DCT coating. Overall it was observed that with the increase in grain size of the coatings, the capability to offer oxidation resistance decreased both for the HVOF and cold-spray coatings. The surface XRD analysis confirmed the presence of mainly NiO and Cr2O3 phases in the scale of all the investigated coatings, with the exception of DHT coating. These oxides (NiO and Cr2O3) may partially inhibit oxidation of substrate steels by blocking the diffusion of reacting species towards the substrate steel as has been suggested by Nicoll and Wahl (1983). However in case of DCT coating, O was penetrated to substrate region. In case of BHT, CHT and DHT coatings small superficial cracks were found near edges during oxidation studies. The cracking and some minor spallation may be due to thermal shocks due to difference in heat expansion coefficients of oxides, coatings and the substrate (Rapp et al., 1981 and Liu et al., 2001). But overall no depletion of basic elements was observed from the substrate steel. The cold-sprayed coatings outperformed the HVOF-spray coatings and showed high erosion-corrosion (E-C) resistance to base steels, amongst the investigated HVOF and cold-spray coatings on T22 steel. This may be attributed to the relatively lower oxide content and low porosity present in the cold-spray coatings. Moreover, the relatively higher hardness of the cold-sprayed coatings might have also imparted higher erosion resistance to coatings in comparison with HVOF-sprayed coatings. In terms of thickness loss, CCT and DCT coatings offered the highest E-C resistance. Further, the performance these coatings were found to be better than conventional Ni-20Cr coatings (Bala, 2010). Similarly AHT, ACT and DHT coatings were also successful to reduce the E-C of T22 steel. However for the other two variants, the scale-substrate interface was found to be severely damaged and some cracks were seen in the coating area. Therefore, these coatings could not provide necessary protection to the steel. Amongst all the developed HVOF and cold-sprayed coatings on SA 516 steel, the cold-sprayed coatings in general have shown high temperature oxidation resistance in comparison to HVOF-sprayed coatings, except AHS coating, which offered highest oxidation resistance among the investigated coatings on SA 516. AHS coating showed only a marginal weight change (2.96 mg/cm2 ) till the end of the experimentation and was able to reduce the weight gain of bare steel by 98.4%. This may be due to the strong presence of NiO and Cr2O3 phases. CCS coating was found to be the second best performing coating, which has the additional presence of a spinel phase NiCr2O4 in its oxide scale, alongwith NiO and Cr2O3. It has been reported that the spinel phase (NiCr2O4) has smaller diffusion coefficients of the cations and anions than those in their parent oxides (Chatterjee et al., 2001). Furthermore, it was observed that the CCS coating outperformed its counterpart cold-sprayed microstructured Ni-20Cr coating under similar conditions of oxidation testing (Bala, 2010). Both DCS and ACS coatings were also found to be better performing in comparison to the micron-sized Ni-20Cr coating. This reduction in oxidation rate in comparison to microstructured coatings may be attributed to rapid promotion of selective oxidation of Cr to form denser Cr2O3 scale due to enhanced grain boundary area (Chen et al., 1999; Peng et al., 2005). This was followed by DHS, CHS and BHS coatings in terms of oxidation resistance. In case of BHS and CHS coating the scale-substrate interface was damaged, so were not able to stop internal oxidation. Amongst all the investigated HVOF and cold-sprayed coatings on SA 516 steel, the cold-sprayed coatings were found to be relatively more effective to reduce the E-C rate, similar to T22 steel cases. In terms of thickness loss the CCS and DCS coatings provided the highest E-C resistance, followed by AHS, CHS, DHS and ACS coatings. The E-C resistance trend of HVOF-spray coatings could be co-related with grain size of the coating, with the exception of BHS coating. For all other cases, it was observed that E-C resistance increased with decrease in grain size of the coating. However, BHS coating showed the highest corrosion rate (83 mpy) irrespective of its lower grain size, which may be attributed to its lowest thickness. It is worth to mention that only 50 µm coating thickness could be achieved in this case. Due to low average particle size of feedstock powder B (6 µm), the flowability of the powder became the major issue and hence the lowest thickness of the coating was achieved in BHS case. Based upon the overall results of the current study, it has been recommended that CCS coating may be the best choice for the given boiler application. It is pertinent to mention that the cold-sprayed coatings in the current work were developed by a costeffective approach in which air was used as working medium (powder carrier gas), whereas during development of conventional cold-sprayed coatings (micron-sized) a high purity and costly helium gas was used (Bala, 2010). In this way the current research offers cost-effective solution to prospective boiler industry.