Alumina Coatings Research Articles

Hot Corrosion Performance of Plasma-Sprayed Multiwalled Carbon Nanotube–Al2O3 Composite Coatings in a Coal-Fired Boiler at 900 °C

The present study investigates the high-temperature corrosion behavior of plasma-coated boiler steel for 1000 h in a 110-MW water tube boiler, which had encountered severe corrosion and the tube failure. The coating was carried out by alumina and MWCNTs (multiwalled carbon nanotubes)-reinforced alumina composite powders. The uncoated and coated steel substrates were hung at a platen superheater at 900 °C. After the exposure, thermogravimetric analysis was carried out to evaluate the kinetics of corrosion. XRD, SEM, TEM and EDS analysis was carried out to evaluate the corrosion products at the end of the cyclic high-temperature corrosion. The present research work concludes that CNT-reinforced alumina coatings (4.0 wt.% CNT + Al2O3 coating) have decreased the corrosion rate to 30.72 mpy, which was 113.53 mpy for conventional alumina coatings and 376.96 mpy for bare T11 boiler steel in the boiler environment. This manuscript submits the failure of tube at the platen superheater, exposure conditions and the analysis of the coated and uncoated samples of the steel tubes.

A simple repair method of fatigue cracks using stop-holes reinforced with wedge members: applicability to reinitiated cracks and effects of an anti-fatigue smart paste

ABSTRACT A simple method of repairing fatigue cracks using stop-holes reinforced with wedge members, that was previously proposed by the author, has been examined for the case of reinitiated cracks. By using this method, the stress intensity factor range around a reinitiated crack tip is expected to be reduced by the wedge load effect of the wedge members. The chief advantages of this method are that the repair work can be easily performed from only one side of a cracked structure, and that the wedge member can be set so adaptive as to maintain the wedge load automatically and effectively as the reinitiated crack grows. Specifically, slope-type wedge members have been adopted, and an adaptive mechanism of the wedge member has been devised using a pulley and a wire-type displacement metre. Fatigue tests were performed on a steel plate specimen with a drill hole and a notch, and validity of the above repair method was experimentally examined using both of simple and adaptive wedge members. In addition, an anti-fatigue smart paste, which consists of fine alumina particles and silicone grease, was applied to the periphery of the drill hole, and its effects on automatic restraint and visual detection of crack growth were investigated. FE analyses using contact elements were also carried out for a comparative study. As a result, it was found that the strain range on the specimen side is reduced to 44.0 % and the fatigue life is prolonged by 12.4 times by application of the adaptive wedge member as compared with the case of the conventional stop-hole. As for the anti-fatigue smart paste, it was found that the fatigue life is further prolonged by about three times as compared with the case of the adaptive wedge member and the paste has an evident function to facilitate visual detection of cracks.

Preparation of multi-purpose TiO2 pigment with improved properties for coating applications

A rutile-grade multipurpose TiO2 pigment with surface coatings of zirconia and alumina is prepared under controlled conditions. HRTEM, FTIR, ξ-Potential etc. were used to characterize the morphology, structure and electrokinetics. A continuous and compact film of zirconia was anchored on the base TiO2 particles surface as the innermost coating. A composite coating of alumina was precipitated under controlled conditions which facilitated the precipitation of both dense, compact and porous, boehmite alumina on the surface of zirconia-coated TiO2 particle surface. Comparative evaluation of the pigmentary properties of the coated pigment revealed superiority in durability and optical properties of the modified pigment over conventional enamel-grade and general-purpose grade TiO2 pigments respectively.

Coating of Ca(OH)2 / γ-Al2O3 pellets with mesoporous Al2O3 and its application in thermochemical heat storage for CSP plants

Thermochemical heat storage using the Ca(OH)2 dehydration/hydration reaction for CSP plants requires the development of advanced materials that avoids the breakage shown by conventional CaO pellets. In this work, spherical cores made up of pelletized 60 wt % Ca(OH)2 and 40% γ-Al2O3 powders were coated with different mesoporous alumina coverings by means of a dip-coating route, in order to enhance its structural integrity for dehydration/hydration cycles. Three strategies of preparation were devised using as coatings alumina gels containing cetyltrimethylammonium bromide as surfactant in neutral (A2 CS) and acid pH (A4 CS) and without surfactant in neutral medium (A3 CS). In both A2 CS and A4 CS materials, denser alumina coatings were attained originating higher crushing strengths, pointing out the positive role played by the surfactant in the gel makeup. SEM images indicated that A2 presented small alumina grains (0.3–3 μm) highly dispersed over their surface, while over A4 CS, long sheets were appreciated. All the materials exhibited high hydration capacities after 10 cycles (>80%, >800 kJ/kg) at 250 °C, but only A2 CS hold its structural integrity that was preserved even after 20 cycles without loss of its hydration capacity.

Enhancement in the corrosion resistance of nanocrystalline aluminium coatings by incorporation of graphene oxide

Non-aqueous electrolyte containing aluminium chloride (AlCl3) and 1-ethyl-3-methylimidazolium chloride ([EMIm]Cl) topped with n-decane was used to electrodeposit aluminium (Al) coating via chronoamperometric method at ambient atmosphere. Aluminium-graphene oxide (Al-GO) composite coating was further obtained from the Al electrolytic bath by dispersing GO in it. GO was prepared by oxidation of high purity graphite. The Al deposition potential was noted by cyclic voltammetry (CV) method. Influence of GO on the morphology and microstructure of Al metal matrix was analysed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction spectroscopy (XRD). Inclusion of GO into the Al matrix enhanced the coating compactness thus nullifying the surface defects which were observed for the pristine Al deposit. The texture co-efficient calculation showed that the addition of GO altered the growth texture from growth predominately along (002) plane for pure Al coating to growth predominantly along (111) and (022) plane for Al-GO composite coating. The morphological and microstructural changes occurred in the Al coating from GO incorporation influenced the anti-corrosion behaviour of the coatings. The electrochemical corrosion analysis by Tafel polarization and Impedance Spectroscopy confirmed that the GO reinforced Al deposit possessed two-fold enhanced anticorrosive property than pure Al coating.

Effect of titanium addition on structure, corrosion resistance and mechanical properties of aluminum coatings on NdFeB by ion-beam-assisted magnetron sputtering

Titanium has been added in the Al coatings on sintered NdFeB magnets by ion-beam-assisted magnetron sputtering to improve the mechanical properties of coatings without corrosion resistance deterioration. The effect of Ti addition on structure, mechanical and electrochemical properties of Al coatings has been investigated, complemented by x-ray diffraction and transmission electron microscopy. The results show that the ion-beam-assisted magnetron co-sputtering of Al and Ti obtains a single-phase polycrystalline structure of Al–Ti solid solution. With the increasing Ti content, the coating structure becomes denser and the grain size is reduced. Both the mechanical and wear properties of the coatings are improved with the addition of Ti where the hardness of the Al–Ti coating increases with the increasing amount of Ti. Polarization and EIS testing results show that the titanium-doped coatings have an enhanced corrosion resistance by comparison with pure Al coatings when Ti content is lower than 4.3 at.%.

CFD Modeling of Ventilation and Dust Flow Behavior in Polishing and the Design of an Innovative Wet Dust Removal System.

Fine aluminum dust pollution in the polishing process was detected during a field survey. To obtain a fundamental understanding of the airflow patterns and the fine dust dispersion characteristics during a polishing process, computational fluid dynamics simulations were first performed to analyze the data collected in field measurements. The inappropriate ventilation arrangement and lack of effective dust control measures were identified as the main reasons for the high dust exposure levels (in excess of 1000 μg/m3). Simulation results showed that inhalable dust particles (PM10) could be significantly diluted at the operator’s breathing level by adding a supply air inlet above the operating area. Moreover, dry dust collection systems create a risk of aluminum dust explosion accidents. An innovative design of wet dust removal system which could mitigate the occurrence of dust explosions was proposed and then implemented on site. An independent field dust assessment showed that a reduction of fine dust particles up to 95% in the worker’s breathing area and the fine dust in the vents was reduced to 80%. Therefore, the proposed strategies are implemented immediately to address the combustible dust in the polishing working environment and can provide guidance for operators.

Seizure and Friction Properties of the DLC Coated Journal and Aluminum Alloy Bearing

Seizure and Friction Properties of the DLC Coated Journal and Aluminum Alloy Bearing

Residual Stress Measurement of Suspension HVOF-Sprayed Alumina Coating via a Hole-Drilling Method

The nature and magnitude of residual stresses in thermal-sprayed coatings determine their lifetime and failure mechanisms. The residual stresses of suspension high-velocity oxy-fuel (SHVOF) thermal sprayed alumina (Al2O3) coating were measured with hole-drilling and x-ray diffraction. The coating is dense and consists of amorphous and two crystalline phases: alpha and gamma. The residual stresses measured by hole-drilling in the Al2O3 coating was − 162 MPa (compression) in the longitudinal direction and − 104 MPa (compression) in the transverse direction. This is due to the peening stress and the high substrate–coating CTE ratio of ~ 2.1. The nature of the residual stress through the coating is related to the microstructure build-up shown from the cross section and the fracture surfaces of the coating.

Rupture of alumina coatings on C35 steel: A numerical simulation

The knowledge of the rupture mechanism of multilayer systems is necessary for improving the bonding of materials, particularly for the ceramic-to-metal junctions, and especially in the case of the ceramic coatings on metallic substrates. For this purpose, coupling experimental measurements of macroscopic interfacial adhesions with numerical tools of fracture mechanics allow understanding how the stress distribution and the cracking mechanisms are influenced by the local environment. This brings an explanation to the cracks initiation and propagation and to the role of local characteristics of the interfacial zone. The present work illustrates this, in the case of alumina coatings strongly bonded to C35 steel substrates previously pre-oxidized in CO2. The experimental tool is based on the “silver print test”. It consists in covering the central part of the samples with a thin layer of silver paint before coating in order to create a defect zone. Concerning the numerical part, the finite element method is used as a powerful tool to describe the mechanical phenomena at the local scale influencing the crack propagation. The obtained numerical results show a good agreement with the experimental observations and they allow a local description of the phenomena influencing the cracking mechanism. Moreover, they lead to a concrete proposition for improving the coatings adherence.

Hot corrosion behavior of aluminized and Si-modified aluminized coated IN-738LC produced by a novel hot-dip process

Hot-dip aluminum and Si-modified aluminum diffusion coatings were applied on the substrate of nickel-base superalloy IN-738LC to enhance its hot corrosion behavior for high-temperature applications. Aluminizing salt bath consisted of KCl, NaCl, NaF, Na3AlF6, and Al powder, while the Si-modified salt bath contained NaCl, NaF, KCl, Na2SiF6, and Na3AlF6. A temperature of 750 °C and an operating time of 30 min were applied for coating. At this condition, the thicknesses of about 48 and 35 µm were achieved for aluminum and Si-modified aluminum coatings, respectively. All coated samples were immersed in the molten salt of Na2SO4–25 wt%NaCl composition for 60 and 140 h for the hot corrosion test. It was found that the Si-modified coated samples had better corrosion resistance. The high hot corrosion resistance was believed to be due to the formation of the protective and continuous scale of Al2O3 and SiO2 and no sign of sulfidation was detected at 750 °C.

Mechanical and chemical robustness of the aluminum oxide-infiltrated block copolymer films and the resulting aluminum oxide coatings

Block copolymer (BCP) templates have been widely used for the design of functional organic-ceramic composites and ceramic structures. In this study, we explore the mechanical characteristics of such structures designed using sequential infiltration synthesis (SIS). We demonstrate that SIS significantly reinforces the BCP template and decreases their sensitivity to temperature and chemical environment. We also probe tribological characteristics of nanoporous alumina films obtained from the organic-ceramic composites after the BCP template removal. We determined with atomic force microscopy (AFM) that the lateral forces between the tip and the substrate are highly affected by the surface reactivity, surface structure, area of contact, environment, and porosity accessibility and interconnectivity. As confirmed by the quartz crystal microbalance (QCM) analysis, the high accessibility of the pores in the alumina film to the water penetration leads to a decrease in the friction for the ~4 μm in size colloidal tip sliding against the nanoporous alumina in contrast to the bulk alumina. The effect diminishes once the size of the AFM probe becomes comparable to the size of the pores, which is attributed to the limited water flow under the tip to support the applied during sliding stresses. Understanding the surface structure and its effect on the mechanical and frictional characteristics of materials is important for the predictive design of coatings that can sustain thermomechanical stresses and can be used to enhance the lubrication of fluids in various mechanical systems.

Hydrogen in Aluminium-Coated Steels Exposed to Synthetic Seawater

Thermally sprayed aluminium (TSA) coatings provide protection to offshore steel structures without the use of external cathodic protection (CP) systems. These coatings provide sacrificial protection in the same way as a galvanic anode, and thus hydrogen embrittlement (HE) becomes a major concern with the use of high strength steels. The effect of TSA on the HE of steel seems to remain largely unknown. Further, the location of hydrogen in TSA-coated steel has not been explored. To address the above knowledge gap, API 5L X80 and AISI 4137 steel coupons, with and without TSA, were prepared and the amount of hydrogen present in these steels when cathodically polarised to −1.1 V (Ag/AgCl) for 30 days in synthetic seawater was determined. One set of TSA-coated specimens was left at open circuit potential (OCP). The study indicates that the amount of hydrogen present in TSA-coated steel is ~100 times more than the amount found in uncoated steel, and that the hydrogen seems to be largely localised in the TSA layer.

Studies on the Low-Temperature Synthesis of Fine α-Al2O3 Powder by Precipitation Route

Fine alpha-alumina (α-Al2O3) powder was prepared by simple precipitation technique using aluminium salt and ammonia as a precipitating agent. In this method, an aqueous solution of aluminium nitrate and ammonia were mixed with continuous stirring both in the presence and in the absence of polyethylene glycol (PEG). A controlled pH range of 7.5–8.5 was maintained throughout the synthesis process. A comparative study between the alumina powder prepared in the presence and absence of PEG was drawn into attention. The washing of the precipitate was deliberately avoided in one case to study the effect of washing on phase formation temperature. The synthesized powders were calcined at different temperatures and characterized by X-ray diffraction study, thermal analysis, infrared analysis, microstructural study, and particle size analysis. The results showed less agglomerated fine alumina powder formation in its pure α-Al2O3 phase at 1050°C in the presence of PEG.

Effects of aluminum content and mechanical nano‐alloying time on the preparation of titanium‐aluminum composite coatings on magnesium‐lithium alloy

AbstractA titanium‐aluminum composite coating was successfully fabricated on magnesium‐lithium alloy substrate by mechanical nano‐alloying treatment (SMNAT). The effects of aluminum content and mechanical nano‐alloying time on the microstructure and properties of titanium‐aluminum composite coatings were investigated. The experimental results showed that the ratio of titanium‐aluminum powder has a significant effect on the preparation of the coating. Low aluminum content and high aluminum content in the mixture powder resulted in the formation of the coatings with inhomogeneous thickness. The titanium‐aluminum ratio of 3 : 1 was considered to be optimal for fabricating the coating with homogeneous thickness under the selected milling parameters. The hardness of top surface layer on titanium‐ aluminum coatings decreased from 292.0 HV 0.01 to 121.4 HV 0.01 with the increase of aluminum content. Additionally, the prolongation of treatment time facilitated to increase the thickness and density of the coatings, resulting to enhance their hardness and corrosion resistance.

Thermal Sprayed Aluminium Coatings: A Review

Thermal Sprayed Aluminium Coatings: A Review

Thermal spray deposition of aluminum and zinc coatings on thermoplastics

Coatings of aluminum and zinc were applied on two thermoplastic materials, ultra-high molecular weight polyethylene (PE) and polytetrafluoroethylene (PTFE), using twin wire-arc spray spraying. Samples were either smooth (Ra = 0.2 μm) or roughened by grit blasting (Ra = 1.6 μm). Before the start of spraying samples were kept at either room temperature or preheated to 55 °C for PE and 95 °C for PTFE, so that the peak temperature during spraying reached the glass transition temperature of the polymer. Surface temperature was monitored during spraying using thermocouples. Coating adhesion strength was measured using pull tests. Single splats of metal on samples were examined using SEM images. Zinc coatings, about 260 ± 20 μm thick, formed on both polymer samples irrespective of their surface roughness. Adhesion strength was significantly higher on rough surfaces than smooth surfaces. Increasing the initial sample temperature also enhanced adhesion strength. Aluminum adhered only to the PTFE but not the PE. Aluminum droplets have a higher melting point than zinc and melted the sample upon impact, inhibiting mechanical bonding. These results suggest that the metal adhesion to thermoplastics can be significantly improved by pre-heating polymer samples prior to thermal spraying.

Alumina layer using low-cost direct liquid injection metal organic chemical vapor deposition (DLI-MOCVD) on AISI 1018 steel

Amorphous alumina layers were deposited on low carbon steel (AISI 1018) substrates by the direct liquid injection metal organic chemical vapor deposition (DLI-MOCVD) technique. For the DLI- MOCVD technique, two temperatures were used: a vaporization chamber temperature of 180 °C and a reaction chamber temperature of 370 °C. Liquid precursor aluminum tri-sec-butoxide was introduced in form of atomized liquid, each pulse of 1 Hz frequency and 3 ms opening time, the solution was pressurized at 40 psi under inert Argon atmosphere. 200 sccm of Argon were used as continuous carrier gas during all the deposition process. Microscopy and XRD techniques were used to characterize the deposited material. The thickness of the alumina coatings was of 100 μm, approximately.

Influence of seed layers on optical properties of aluminum in the UV range

The potential of titanium and copper seed layers to enhance the optical properties of aluminum films for ultra-violet (UV) applications is analyzed. The seed layers significantly influence the initial layer growth of aluminum films. For the titanium-seeded aluminum, a surface roughness of 0.34 nm was observed. UV spectral reflectance measurements showed an average higher reflectivity of 4.8% for wavelengths from 120 nm to 200 nm for the aluminum film grown on the titanium seed layer. Furthermore, the titanium-seeded aluminum coatings were stable at an elevated temperature of 225°C and showed no increase in surface roughness or pinholes.

A Study on the Microstructural Characterization and Phase Compositions of Thermally Sprayed Al2O3-TiO2 Coatings Obtained from Powders and Water-Based Suspensions.

In this work, the alumina (Al2O3) and alumina-titania coatings with different contents of TiO2, i.e., Al2O3 + 13 wt.% TiO2 and Al2O3 + 40 wt.% TiO2, were studied. The coatings were produced by means of powder and liquid feedstock thermal spray processes, namely atmospheric plasma spraying (APS), suspension plasma spraying (SPS) and suspension high-velocity oxygen fuel spraying (S-HVOF). The aim of the study was to investigate the influence of spray feedstocks characteristics and spray processes on the coating morphology, microstructure and phase composition. The results revealed that the microstructural features were clearly related both to the spray processes and chemical composition of feedstocks. In terms of phase composition, in Al2O3 (AT0) and Al2O3 + 13 wt.% TiO2 (AT13) coatings, the decrease in α-Al2O3, which partially transformed into γ-Al2O3, was the dominant change. The increased content of TiO2 to 40 wt.% (AT40) involved also an increase in phases related to the binary system Al2O3-TiO2 (Al2TiO5 and Al2−xTi1+xO5). The obtained results confirmed that desired α-Al2O3 or α-Al2O3, together with rutile-TiO2 phases, may be preserved more easily in alumina-titania coatings sprayed by liquid feedstocks.