Hot Corrosion Resistance of TB8 Titanium Alloy after ECAP and Heat Treatment

The metastableβtitanium alloy TB8(Ti-12.76Mo-2.13Nb-2.73A1-0.16Si)was used as the original material,and the secondary processing method combining equal channel angular pressing(ECAP)and heat treatment was adopted.With the help of optical microscope(OM),scanning electron microscope(SEM)and X-ray diffractometer(XRD),the corrosion behavior of TB8 titanium alloy after different secondary processing(800℃/850℃solid solution-520℃aging,ECAP-800℃/850℃solid solution-520℃aging,and800℃/850℃solid solution-ECAP-520℃aging)was studied.The experimental results show that the hot corrosion products of the six samples are similar,mainly Na_(2)Si_(2)O_(5),MoS_(2),TiCl_(2),Ti(SO_(4))_(2),and TiS.Due to the grains of the TB8 titanium alloy treated by 850℃solid solution-ECAP-520℃aging are obviously refined,the surface structure is the most smooth and dense,forming a continuous Al2O3protective film,and the surface defects are the least after corrosion.Its corrosion layer thickness is the lowest(102.3μm),only 36.5%-81.4%of that of other secondary processing titanium alloys.In addition,the corrosion kinetics curves of the six materials all follow parabolic laws,and the minimum corrosion weight gain of the samples after 850℃solutionECAP-520℃aging treatment is 0.7507 mg·mm^(-2),showing better hot corrosion resistance.

Effect of Sputtered TiAlCr Coatings on Oxidation and Hot Corrosion Resistance of Ti-24Al-14Nb-3V

The effect of sputtered Ti-50Al-10Cr and Ti-50A1-20Cr coatings on both isothermal and cyclic oxidation resistance at 800-900℃ and hot corrosion resistance at 850℃ of Ti-24Al-14Nb-3V was investigated. Results indicated that Ti-24Al-14Nb-3V alloys exhibited poor oxidation resistance due to the formation of Al2O3+TiO2+AlNbO4 mixed scales in air at 800-900℃ and poor hot corrosion resistance due to the spoliation of scales formed in Na2SO4+K2SO4 melts at 850℃. Both Ti-50Al-10Cr and Ti-50Al-20Cr coatings remarkably improved the oxidation and hot corrosion resistance of Ti-24Al-14Nb-3V alloy.

Microstructural analysis and hot corrosion behavior of HVOF-sprayed Ni–22Cr–10Al–1Y and Ni–22Cr–10Al–1Y–SiC(N) coatings on ASTM-SA213-T22 steel

The present paper deals with the investigation of microstructure and high-temperature hot corrosion behavior of high-velocity oxy fuel(HVOF)-produced coatings. Two powder coating compositions, namely, Ni22Cr10Al1Y alloy powder and Ni22Cr10Al1Y(80 wt%;microsized)–silicon carbide(SiC)(20 wt%;nano(N)) powder, were deposited on a T-22 boiler tube steel. The hot corrosion behavior of bare and coated steels was tested at 900°C for 50 cycles in Na2SO4–60 wt%V2O5 molten-salt environment. The kinetics of corrosion was established with weight change measurements after each cycle. The microporosity and microhardness of the as-coated samples have been reported. The X-ray diffraction,field emission-scanning electron microscopy/energy dispersive spectroscopy, and X-ray mapping characterization techniques have been utilized for structural analysis of the as-coated and hot-corroded samples. The results showed that both coatings were deposited with a porosity less than2%. Both coated samples revealed the development of harder surfaces than the substrate. During hot corrosion testing, the bare T22 steel showed an accelerated corrosion in comparison with its coated counterparts. The HVOF-sprayed coatings were befitted effectively by maintaining their adherence during testing. The Ni22Cr10Al1Y–20 wt%SiC(N) composite coating was more effective than the Ni–22Cr–10Al–1Y coating against corrosion in the high-temperature fluxing process.

Hot Corrosion Behavior of Hf-Doped NiAl Coating in the Mixed Salt of Na_(2)SO_(4)+K_(2)SO_(4)at 900℃

The hot corrosion behavior of the NiAl coating and the 5Hf-NiAl coating induced by mixed salt at 900℃was investigated.Comparing with the NiAl coating,the 5Hf-NiAl coating exhibited superior hot corrosion resistance because the addition of Hf promoted the formation of protective oxide scale and reduced the growth rate of oxide scale.Therefore,internal sulfides were not present in the 5Hf-NiAl coating after hot corrosion for 140 h.Cr exhibited different distribution in the two coatings since the addition of Hf changed the hot corrosion process of the coating.Hf and Ti in the 5Hf-NiAl coating trapped and captured sulfur,preventing the penetration of sulfur into the coating.The hot corrosion mechanism of the two coating and the effects of Hf on this process were discussed in this work.

Hot corrosion behaviors of 921A alloy and HVAF-sprayed Fe-based amorphous coating covered with KCl-ZnCl_(2) salts

Hot corrosion behaviors of the 921A alloy and Fe-based amorphous coating induced by KCl-10% ZnCl_(2) and KCl-55% ZnCl_(2) salts at 450℃ in air for 40 h were investigated.Results show that the 921A alloy suffers more serious corrosion damage than the coating and KCl-55% ZnCl_(2) salts are more corrosive than KCl-10% ZnCl_(2) salts.In the two salts,an Fe_(2)O_(3) layer is formed on the 921A alloy surface,while an outer Fe-rich oxide layer and an inner Cr-rich oxide layer are formed on the surface of the coating.Moreover,a certain amount of metal chloride can be found at the oxide/alloy(coating)interface,which can be explained by "active oxidation".However,the corrosion resistance of the Fe-based amorphous coating did not achieve the desired results,probably because the intersplats in the coating serve as corrosion diffusing channels,which facilitate the corrosion damage rate.Nevertheless,the coating is still in amorphous state after hot corrosion exposure.

A review on molten sulfate salts induced hot corrosion

Hot corrosion has been extensively observed in various high temperature applications,which might cause large economic losses.To deepen the insight and understanding of hot corrosion,herein,we provide a detailed discussion on hot corrosion induced by molten sulfate salt,in which Na_(2)SO_(4)is the main chemical reactant.The hot corrosion is triggered and sustained by the negative solubility gradient of protective oxide from the oxide/salt interface to salt/gas interface.In this work,we reviewed several key factors,including temperature,gas composition,molten salt composition,alloy element and external stress,influencing the hot corrosion of alloy beneath the molten salt.The application of anti-corrosion coating seems to be the most effective and commercial mothed to mitigate hot corrosion.Therefore,the progress of the development of effective coatings,e.g.,Ni-Cr coatings,aluminide coatings,silicide coatings and MCrAlY(X)coatings,has also been summarized.

Corrosion Behaviors of Nitride Coatings on Titanium Alloy in NaCl-Induced Hot Corrosion

TiN and TiAlN coatings were deposited by arc ion plating on titanium alloys to study their hot corrosion resistance when they were exposed to NaCl at 600℃.The microstructure and corrosion behaviors of nitride coatings were studied using scanning electron microscope,X-ray diffraction,electro-probe microanalyzer and X-ray photoelectron spectroscopy.The results showed that nitride coatings with the different compositions and the ones with the same composition but different thicknesses presented different hot corrosion resistance.TiN and thin TiAlN coatings showed poor corrosion resistance.Serious internal oxidation attacked the alloy substrate.Their corrosion products were mainly consisted of non-protective TiO2 and sodium salt.By contrast,the thick TiAlN coating presented outstanding corrosion resistance.Besides sodium salt,the corrosion products were composed of protective Al_(2)O_(3).The increasing thickness of TiAlN significantly enhanced the hot corrosion resistance.The corrosion mechanisms of alloy,TiN and TiAlN coatings were discussed in detail.

Hot corrosion of surface-modified Sm_(2)Co_(17) high-temperature magnet with Ni and Ni/Cr bilayer coatings in 75 wt% Na_(2)SO_(4)–NaCl mixture

Hot corrosion behavior of Sm_(2)(Co, Fe, Cu,Zr)17-type high-temperature magnetic alloy without and with a protective coating of Ni and Ni/Cr bilayer at 500 ℃ in a corrosive mixture of 75 wt% Na_(2)SO_(4)–NaCl for 100 h was reported in this paper. The obtained results of timedependent weight change established the parabolic growth of oxidized surface of bare magnets(8.87 mg·cm^(-2)),which caused a rapid loss of magnetic properties measured both at room and high temperatures(500 ℃). X-ray diffraction(XRD) analysis revealed the growth of oxides for bare magnet and Cr–oxides(Cr_(2) O_(3)), top layer of Ni/Cr bilayer coating, but in the case of single coating, Ni–sulfides formation indicated penetration of sulfur, which is further verified by electron probe microanalysis with energy dispersive spectroscopy(EPMA/EDS) study.Results showed that bilayer-coated samples(Ni/Cr) performed better than Ni-coated and bare samples.

Hot corrosion mechanism of yttria-stabilized zirconia powder in the presence of molten Na_(2)SO_(4)+V_(2)O_(5) salts

The hot corrosion behavior of yttria-stabilized zirconia(YSZ)powder specimens exposed to Na_(2)SO_(4)/V_(2)O_(5) salts mixtures at high temperature was evaluated.Initial tests were carried out at 1000℃ for 10 h,the salt concentrations varied from 0.1 wt%to 1.00 wt%,and the Na_(2)SO_(4)/V_(2)O_(5) mass ratios were between 0.20 and 0.44 following a factorial design 22.X-ray diffraction(XRD)analyses of the tested samples showed with a confidence of95%that the mixture composed of 32 wt%Na_(2)SO_(4)+68 wt%V_(2)O_(5),and 1.00 wt% salt concentration led to high destabilization of the t’-YSZ phase and formation of YVO_(4) products.A second set of experiments were conducted to assess the influence of temperature on the hot corrosion response of the YSZ in the range between 490 and 1100℃.Thermogravimetric analysis(TGA)experiments and Rietveld adjustments of XRD patterns showed that the mass loss of the samples varied with testing temperature and also that the major destabilization of tetragonal phase occurred at 900℃.

Hot Corrosion Behavior of Ti45Al8.5Nb Alloy: Effect of Anodization and Pre-oxidation

The hot corrosion behavior of Ti45 A18.5 Nb alloy was studied in the salt of Na_(2)SO_(4)and/or NaCl at 700℃.To improve the hot corrosion resistance,Ti45 A18.5 Nb alloy was anodized in fluorine-containing solution and pre-oxidized in air.Results showed that the combination of anodization and pre-oxidation can efficiently enhance the hot corrosion resistance of Ti45 A18.5 Nb alloy contaminated with Na_(2)SO_(4)or Na_(2)SO_(4)+NaCl deposits.This is because anodization and pre-oxidation result in the formation of compact AlOlayer which can act as a diffusion barrier to prevent sulfur,chlorine,and oxygen from attacking the alloy,therefore providing good resistance against hot corrosion.When exposed to NaCl deposit,however,no obvious improvement was achieved on the hot corrosion behavior no matter the alloy was anodized or further pre-oxidized in air.

Mechanism of adding rhenium to improve hot corrosion resistance of nickel-based single-crystal superalloys

Hot corrosion behavior in sulfate salt at 950℃ of Rene N5 single-crystal superalloys with 3 wt%rhenium(NSR)was investigated compared with that of nickel-based single-crystal superalloys without rhenium(NS).After 30-h corrosion,the surface of the NS superalloy is seriously corroded.Many holes and exfoliation appear on the surface.The NSR superalloys exhibit better hot corrosion resistance than the NS superalloys.After 30-h corrosion,a continuous and compact Al_(2)O_(3) film is observed on its surface.The Al_(2)O_(3) film with dense structure formed on the surface provides protection for the matrix.The characterization results show that A1 is aggregated in theγ’phase,while Re is aggregated in the y phase during the formation of oxide scale.Considering that Re can inhibit the diffusion of A1 in the nickel matrix,it is inferred that Re can inhibit the outward diffusion of A1 and prevent the decrease of Al concentration in theγ’phase.High concentration of Al hinders the decomposition of Al_(2)O_(3) due to the reaction of acid and basic dissolution.Al_(2)O_(3) keeps its structure intact and provides protection for the matrix.

Preparation and characterization of LPPS NiCoCrAlYTa coatings for gas turbine engine

NiCoCrAlYTa coatings have been deposited onto an aircraft gas turbine engine blade using a LPPS unit equipped with a computerized robot. Optimal processing conditions, including spray parameters, the trajectory of the robot, and the synchronized movements between the torch and the blade, have been developed for superior coating properties. Transferred arc treatment, providing a preheating and a cleaning of the substrate surface, enhances the adherence of the coatings to the substrate. The resulting LPPS coatings show dense and uniform characteristics with ideal hardness, and good corrosion resistance to cycle oxidation.

Corrosion mechanism investigation of TiN/Ti coating and TC4 alloy for aircraft compressor application

It is imperative to develop multifunctional erosion and corrosion resistant coatings for compressor blades of aircraft engines in harsh environment.PVD(Physical Vapor Deposition)technology has the advances in processing erosion-resistant coatings;however,the performance of PVD coatings to combat corrosion depends on various coating defects.Determining and comparing the corrosion performances of PVD TiN/Ti coating and uncoated TC4 alloy was the main objective of present work.The 960 h salt spray corrosion and 116 h hot corrosion tests were conducted to simulate the grounding and working environments of the aircraft compressors.The corrosion mechanisms due to the coating defects such as pinhole,columnar boundary and large grain were analyzed based on the OM,Confocal microscope,electrochemical measurements,SEM,XRD and EDS results.Owing to the disordered state associated with the columnar boundary and the coating defect,nitrogen could be easily replaced by oxygen in the hot corrosion process,these structures were channels for fast diffusion of oxygen.Moreover,the Gibbs energy changes of Ti oxidation and TiN oxidation were thermodynamically calculated according to the working condition of aircraft compressors,and considerable research effort was focused on mapping out the phase diagram of Ti,TiN and high pressure gases.The findings of this research can provide insights into developing multifunctional coatings for future aircraft engines.

Corrosion behavior of Ti_(3)AlC_(2) in molten KOH at 700℃

This paper investigated the corrosion behaviors of Ti_(3)AlC_(2) at 700℃in molten KOH with various mass ratios.If the mass ratio of KOH:Ti_(3)AlC_(2)≤2,Ti_(3)AlC_(2) can resist KOH hot corrosion in 2 h.Ti_(3)AlC_(2) suffered serious corrosion attack if the mass ratio≥3.The main compositions of corroded samples were amorphous graphite and potassium titanates(K_(2)O·nTiO_(2)).If the samples were washed by acid and dried,potassium titanates could decompose to K_(2)O and amorphous rutile.Based on the experimental results,a corrosion mechanism of Ti_(3)AlC_(2) in molten KOH was proposed.