Spall Off Research Articles

Anti-coking performance of Al/Si/Cr/Ce ceramic coating during naphtha steam cracking applied on Cr25Ni35Nb alloy

To reduce the coke formation, a novel Al/Si/Cr/Ce composite ceramic coating was prepared on the surface of the Cr25Ni35Nb alloy substrate by pack cementation. The microstructure, phase composition, surface morphology and element distribution of substrate and coating were characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The results reveal that the outer layer of the coating forms a protective layer with a high proportion of AlN and Cr3Si and trace amounts of Ce. The inter-diffusion layer is mainly aluminide coating (AlNi and AlFe) and transition layer contains a small amount of AlN. The anti-coking performance of the coating was assessed using naphtha steam cracking. To study coking behavior, the morphology and microstructure of the coke layer were analyzed and characterized by SEM and Laser Micro-Raman Spectrometer. The coke layer on uncoated sample has a higher focal density and is more likely to spall and peel off when subjected to coking experiment. A large number of filamentous and granular cokes appear on the surface of the uncoated sample, whereas only less granular cokes appear on the surface of the coated sample, and the coke on the uncoated sample has a more disordered and amorphous carbon structure. The coating significantly inhibited the growth of filamentous coke and reduced coke accumulation.

Rolling contact fatigue resistance and damage evolution of carburized bearing steel treated by surface ultrasonic rolling process (SURP)

The impacts of the surface ultrasonic rolling process (SURP) on the surface morphology, surface microstructure, residual stress distribution, and rolling contact fatigue (RCF) properties of G20Cr2Ni4A carburized bearing steel were studied in this research. The results show that when the RCF rolling direction is perpendicular to the grinding direction, the fatigue peeling of the raceway surface is more serious. SURP not only reduces surface roughness but also enhances the residual compressive stress and hardness of the surface layer, allowing it to resist the initiation and propagation of surface cracks and enhancing RCF life. Fatigue cracks are easily formed between the ground layer and the matrix, according to microscopic analyses of fatigue crack initiation and propagation on the original sample surface. These cracks have the potential to spread down the ground layer, causing the shallow layer of approximately 1 μm to spall off. They also promote crack propagation into the matrix and shorten fatigue life. SURP can cause the ground layer to peel off, making fatigue cracks on the surface more difficult to form and enhancing contact fatigue life.

Role of Al and Cr on cyclic oxidation behavior of AlCoCrFeNi2 high entropy alloy

In this work, cyclic oxidation behavior of AlCoCrFeNi 2 , Al 1.5 CoCrFeNi 2 , AlCoCr 1.5 FeNi 2 , and Al 1.5 CoCr 1.5 FeNi 2 High Entropy Alloys (HEAs) has been studied at 1050 °C for 10 cycles to understand the effect of Al and Cr in AlCoCrFeNi 2 based HEAs. X-ray photoelectron spectroscopy, X-ray diffraction, electron backscattered diffraction, scanning electron microscopy with energy dispersive spectroscopy were used to study the base metals and oxidized samples. It was found that in all the samples, face centered cubic, and body centered cubic phases were present in different phase fractions. Al 1.5 CoCrFeNi 2 alloy shows lower oxidation rate with oxidation rate constant (K p ) in the order of 10 −13 g 2 cm −4 s −1 . The lowest oxidation rate of Al 1.5 CoCrFeNi 2 alloy is attributed to the presence of higher Al content as it forms protective continuous thin Al 2 O 3 oxide layer which suppress further oxidation by preventing ingress of oxygen into the substrate. Presence of higher Cr content results in forming protective Cr 2 O 3 oxide layer but eventually it spalls off leading to the exposure of substrate for further oxidation. • Cyclic oxidation behavior of AlCoCrFeNi 2 , Al 1.5 CoCrFeNi 2 , AlCoCr 1.5 FeNi 2 , and Al 1.5 CoCr 1.5 FeNi 2 alloys was studied at 1050 °C for 10 cycles. • Al 1.5 CoCrFeNi 2 alloy shows lower oxidation rate with oxidation rate constant (K p ) in the order of 10 −13 g 2 cm −4 s −1 . • Formation of continuous Al 2 O 3 oxide layer is observed in Al 1.5 CoCrFeNi 2 alloy. • Presence of higher Cr content results in forming protective Cr 2 O 3 oxide layer but eventually it spalls off.

Microstructural examination of interactions between chromia‐based refractory and nuclear glass in a melter

AbstractThis study shows the effects of nuclear waste glass production on Monofrax K‐3 refractory corrosion. A continuously fed research‐scale melter containing an Fe‐ and Ni‐rich simulated nuclear waste feed with borosilicate glass‐forming chemicals was cyclically melted at 1150°C and idled at 1050°C for a total of 11 weeks. Chemical maps using scanning electron microscopy show the interactions between the quenched melt and the refractory. Nanoscale X‐ray‐computed tomography was used for a three‐dimensional visualization of certain parts of the interface. Unreacted K‐3 consists of primarily corundum (Al,Cr)2O3 and spinel (Fe2+,Mg)(Al,Cr)2O4 interlocking crystalline phases. Some of the Cr from the refractory interacts with the Ni and Fe from the melt to form a reaction layer comprising (Ni,Fe2+)(Cr,Fe3+)2O4 spinel crystals. Simultaneously, melt components (Na,Si) infiltrate into the refractory. This interaction proceeds at the expense of the integrity of the refractory structure. Intact refractory grains (e.g., (Al,Cr)2O3) as well as the reaction layer itself can lose mechanical integrity and spall off into the melt, especially near the top of the melter. As the reaction layer can be a protective boundary for the refractory against further melt infiltration, a reduction in the reaction layer thickness allows an increase in refractory corrosion.

Durability degradation of tunnel-lining-shotcrete exposed to nitric acid: Neutralization and nitrate ion migration

The tunnel-lining-shotcrete, in the long highway tunnel, was usually neutralization jeopardized by NxOy attack caused by automobile exhaust. To investigate the neutralization process of lining shotcrete, the nitrate acid solution, simulated NxOy gas, with the pH value of 1, 2 and 3 were used as aggressive medium. Meanwhile, the pH value and nitrate ion content in shotcrete neutralization zone were measured using potentiometry method. After that, the composition and relative content of mineral phases and microstructure were charactered via X-ray diffraction (XRD), thermogravimetric-differential thermal scanning (TG-DSC) and scanning electron microscope (SEM). With increased of the hydrogen ion concentration on nitric acid solution, the pH value and nitrate ion content in neutralization zone were decreased and increased respectively. Meanwhile, those of shotcrete were higher and smaller than those of normal concrete. The loss rate of weight, relative compressive and splitting tensile strength of shotcrete was lower than that of normal concrete as the pH value of 1, while that of shotcrete was higher when the nitric acid pH value of 2 and 3. The neutralization process of nitric acid-exposed-shotcrete was divided into three stages, namely, neutralization reaction, hydration product decomposition, and concrete surface spalling off. The pH value of concrete declined, whereas nitrate ion content in the neutralization zone increased with prolonged immersion time. Moreover, the maximum depth of nitrate ion content was deeper. The yellow-white spongy corrosion product layer formed and covered the specimen surface. Additionally, crisscross macro-cracks appeared on the concrete specimen surface, which caused large amount of aggregates was peeled off.

Effect of pre-oxidation on hot corrosion resistance of HR3C stainless steel in sulfate salt with or without Fe2O3

HR3C steel exhibits high oxidation resistance in air at 700 °C owing to the formation of a dense chromia scale. In the presence of sulfate salt deposition (Na2SO4:K2SO4 = 1:1), hot corrosion occurs. It leads to the dissolution and re-precipitation of Cr2O3 platelets and severe internal corrosion. Pre-oxidation can notably enhance corrosion resistance with almost no internal corrosion zone. When the deposited sulfate salt contains Fe2O3 (Na2SO4:K2SO4:Fe2O3 = 1:1:1), HR3C steel suffers even severer hot corrosion. Its scale spalls off easily and internal corrosion develops deep into the steel. Meanwhile, pre-oxidation played almost no positive effect on corrosion in sulfate salt containing Fe2O3.

High-Temperature Oxidation in Dry and Humid Atmospheres of the Equiatomic CrMnFeCoNi and CrCoNi High- and Medium-Entropy Alloys

Surface degradation phenomena of two model equiatomic alloys from the CrMnFeCoNi alloy system were investigated in 2% O2 and 10% H2O (pO2 = 0.02 and 10−7 atm, respectively) at 800 °C for times up to 96 h. The crystallographic structures, morphologies, and chemical compositions of the corrosion layers developing on CrMnFeCoNi and CrCoNi were comparatively analyzed by mass gain analysis, X-ray diffraction, and scanning electron microscopy combined with energy-dispersive X-ray spectroscopy and electron backscatter diffraction. The oxidation resistance of CrMnFeCoNi is relatively poor due to the fast growth of porous Mn-oxide(s). CrCoNi forms an external chromia layer that is dense and continuous in a dry 2% O2 atmosphere. This layer buckles and spalls off after exposure to 10% H2O atmosphere. Beneath the chromia layer, a Cr-depleted zone forms in the CrCoNi alloy in both environments. As the oxide scale spalls off in the H2O-containing atmosphere, a secondary chromia layer was observed and correspondingly enlarges the Cr-depleted zone. In contrast, as the chromia layer remains without significant spallation when CrCoNi is exposed to a dry oxidizing atmosphere, the region depleted in Cr is narrower.Graphic

Studies on corrosion behavior of a single-crystal superalloy and its sputtered nanocrystalline coatings with solid NaCl deposit in O2 + 38 vol.% H2O environment at 700 °C

Corrosion behavior of the N5 superalloy (Ni-7.0 Cr-6.2 Al-7.5 Co-5.0 W-1.5 Mo-6.5 Ta-3.0 Re) and two nanocrystalline coatings prepared by magnetron sputtering is investigated in chlorine and water vapor environment at 700 °C. The alloy shows low corrosion resistance. Its oxide scale spalls off readily and includes porous NiO outer layer and NiCr(Al)2O4 inner one. The nanocrystalline coating performs better. Its oxide scale consists of Al2O3, NiO and Ta2O5. Chlorination and/or oxidation of Ta ruined compactness of scale. The yttrium modified nanocrystalline coating provides the highest corrosion resistance. Its scale is exclusively alumina. Yttrium inhibits the outward diffusion of Ta.

A new spallation mechanism of thermal barrier coatings and a generalized mechanical model

Multilayer thermal barrier coating (TBC) systems typically consist of three layers of materials: A thermal barrier top coat (TC), a thermally-grown oxide (TGO), and a bond coat (BC) in addition to the substrate. Local strain energy concentrations, called ‘pockets of energy concentration (PECs)’ in this work, often occur around the interface between the TGO and the BC. They have various causes, including local phase changes, and non-uniform creep and plastic relaxation. It is discovered that both PECs and buckling drive the spallation of a TBC in a new spallation mechanism. A PEC-based mechanical model is developed that describes, explains and predicts how blisters nucleate in a TBC under constant biaxial compressive residual stress, steadily and then unsteadily grow, and finally spall off. Two conditions are established for the occurrence of TBC spallation, which depend on the compressive residual strain energy density in the TC and the TGO, and the interface fracture toughness. Experimental validation of the model was performed using aircraft jet engine turbine blades with electron beam physical vapor deposition (EBPVD) TBCs. The predictions from the developed PEC-based mechanical model for the radii of spallation in the TBC are in a good agreement with experiment results.

Breakaway oxidation of a low-Al content nanocrystalline coating at 1000 °C

Nanocrystalline coating though with low Al reservoir shows high oxidation and spallation resistance, owing to its high density of defects and grain boundaries. It now attracts growing attention. However, with oxidation going on, its stability is always a significant concern since such a high-temperature protective coating should be designed with a long life-expectancy when the nano-sized grains have grown and the reserved Al has been consumed a lot. Yet, this issue has never been clarified. To identify the exact mode of breakaway oxidation, a nanocrystalline coating was prepared with very low Al content that could decrease fast to a critical value in a very short oxidation time. Its oxidation behaviour related with the stability was studied in detail at 1000 °C. Results indicate that an intact external alumina scale still forms at the surface of the nanocrystalline coating initially. However, after 100 h exposure, grains growing and the continuous consuming of Al of the nanocrystalline coating lead to oxidation of Ti and doping of titanium oxide in the alumina scale, which was accompanied with the formation of abundant micropores and cracks. Thereafter, scale spalls off and abundant less-protective oxides, e.g. TiO2, NiCr2O4, NiTiO3, form, and breakaway oxidation occurs.

Algorithmic clustering of LiDAR point cloud data for textural damage identifications of structural elements

This study explored the potential of combining point cloud data (PCD) and data clustering algorithms for textural damage detection of commonly seen structural elements in Taiwan. Intensity and RGB (red, green, and blue color model) information acquired by ground LiDAR (light detection and ranging) were used for clustering analysis. Four data clustering algorithms, k-means (KM), fuzzy c-means (FCM), subtractive clustering (SC), and density-based spatial clustering of applications with noise (DBSCAN) were employed to detect the textural damages and to compare the corresponding efficiency and accuracy. The structural elements being studied were rusted rolling doors representing general metal materials with corrosion, walls with tile spall off representing structural elements with erosions and physical damages, and washing finish walls with water staining representing the aging and lichen covering of structural elements. Our study results suggested that both KM and FCM gave preferable clustering performance than SC and DBSCAN. They exhibited desired accuracy for the damage/anomaly identification as well as computational efficiency, suggesting that KM and FCM were more appropriate for these types of application when PCD was used. It was also concluded that intensity rather than RGB data was more appropriate for reflecting the damaged areas. Intensity data was less interfered by environmental effects such as sunlight and rainwater when used to detect the textural changes. The clustering results were also shown to be associated with the clustering number and with the nature of the textural damage type.

Hot corrosion of arc ion plating NiCrAlY and sputtered nanocrystalline coatings on a nickel-based single-crystal superalloy

Hot corrosion in sulfate salt at 850°C of three metallic coatings is investigated comparatively. The NiCrAlY coating loses its protectiveness after 200h corrosion. Its oxide scale spalls off partly and becomes porous as a consequence of basic fluxing. The nanocrystalline coating (SN) performs better than the NiCrAlY one, but its scale is porous as well. Oxidation and/or sulfidation of Ta account for the formation of pores. The yttrium modified nanocrystalline coating (SNY) provides the highest corrosion resistance. Yttrium completely inhibits oxidation and sulfidation of Ta. Its scale is intact and adherent, and exclusively composted of alumina.

Rolling contact fatigue: Spalling versus transverse fracture of rails

Rolling contact fatigue (RCF) defects in the running band of the rail may develop, as a function of born tonnage, either superficially and spall off, or penetrate into the subsurface. In practice, the first type is found to occur notably (but not exclusively) on heat-treated pearlitic rails. Both possibilities involve an essentially different operational risk with respect to transverse rail fracture and require therefore different inspection and maintenance regimes. This study presents a validated hypothesis that explains both similarities and differences of spalling defects and classical squat defects that develop also in depth. It is shown that their microstructural/-mechanical initiation mechanism is different and not necessarily governed by the local tangential stress history in the case of spalling. A model is set up and validated for subsurface crack propagation directivity, distinguishing a spalling and a transverse fracture domain for development of running band defects for both straight track and high and low legs of curves. This model allows for understanding and recognition of the nature of running band defects and for adjustment of control actions.

Microstructure, mechanical and tribological properties of WC/a-C:H coatings deposited by cathodic arc ion-plating

Nanocomposite WC/a-C:H coatings were deposited in a cathodic arc ion-plating system, in which a WC target in Ar + C2H2 atmosphere and a variable acetylene flow of 0–200 sccm were used. The synthesized coatings contain 6–29 at.% of tungsten with a thickness of 1–2 μm. X-ray diffraction revealed the presence of only cubic WC1-x phase with a grain size of 2.5 nm. Rutherford backscattering technique was utilized to study elemental composition and structure of the coatings. X-ray photoelectron spectroscopy and Raman scattering were used to study the amorphous carbon component. The nanohardness is greatly influenced by tungsten concentration and is decreased from 26 GPa to 15 GPa by decreasing tungsten concentration. The critical load Lc2 demonstrates more complex behavior and the coatings spall off at 35 N. The coatings with tungsten concentration of 6–10 at.% show good tribological properties with coefficient of friction of 0.12–0.13 against Si3N4 ball and wear rate of 6.7 × 10−7 mm3/N·m.

Preparation and Corrosion Performance of Lanthanum Nitrate Conversion Coating on Hot-dip Galfan Steel

Lanthanum nitrate conversion coating was prepared on a hot-dip Galfan steel by dipping in a passivation solution of La(NO3)3. The surface morphology, chemical composition and structure of the coating were characterized by scanning electron microscopy (SEM) with X-ray energy dispersive spectrometer (EDS), X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM). The corrosion resistance of the conversion film coated steel was assessed by neutral salt spray tests (NSS), electrochemical polarization curve and electrochemical impedance spectroscopy (EIS). The results showed that the conversion coating was not uniform, which had grown preferentially on grainand phase-boundaries as well as other active sites. The thickness of the coating increased, while cracks in the coating expanded gradually with the increasing dippingtime. The coating spall off after dipping for more than 30 min, and therewith the protective performance of the coating degraded. In comparison with the blank Galfan steel, the corrosion rate of the conversion film coated Galfan steel lowered down significantly.

Effects of surface finish of single crystal superalloy substrate on cyclic thermal oxidation of its nanocrystalline coating

Cycling oxidation behavior of the nanocrystalline coating on the N5 superalloy substrate of different surface finishing is investigated at 1050°C. Results indicate that the surface finishing of the alloy substrate affects oxidation behavior of the sputtered nanocrystalline coating. After cyclic oxidation, oxide scale on the nanocrystalline coating rumples heavily and spalls off following a way of layer-by-layer in case that the alloy substrate is pretreated by sand blasting. However, it just roughens rather than rumples when the alloy substrate is pre-polished or ground by 2000# SiC paper and its spallation follows a way of bulk fracture.

Seismic retrofit of circular RC columns through using tensioned GFRP wires winding

The aim of this paper is to suggest a jacketing method using glass fiber reinforced polymer (GFRP) wires for reinforced concrete (RC) columns with lap splice of longitudinal reinforcement. For this study, four RC columns were prepared of 400 mm in diameter and 1400 mm in height with an aspect ratio of 3.5; two were lap spliced and the other two had continuous longitudinal reinforcement. One specimen of each of the two types was jacketed by GFRP wires that had a diameter of 1.0 mm. The GFRP wires were tensioned with small force during winding each column. The jacket comprised stepped layers along the variation of bending moment of the column. Cyclic lateral force was applied at the top of the columns, and the top displacement of the columns as well as corresponding force was measured during the bending tests.This study considered the failure of the four tested columns and analyzed their later force–displacement behavior. Additionally, the effective stiffnesses of the force–displacement curves were evaluated. The GFRP wire winding jacket prevented splitting of the lap-spliced reinforcement in the lap-spliced column and delayed buckling of the longitudinal reinforcement. The jacket protected the continuous reinforcement column against steel buckling and concrete spalling off and, thus, induced shear failure in the column. The GFRP wire winding jacket increased the failure drifts of both jacketed columns compared with those of the references.

Bond Coat Engineering Influence on the Evolution of the Microstructure, Bond Strength, and Failure of TBCs Subjected to Thermal Cycling

Different types of thermal spray systems, including HVOF (JP5000 and DJ2600-hybrid), APS (F4-MB and Axial III), and LPPS (Oerlikon Metco system) were employed to spray CoNiCrAlY bond coats (BCs) onto Inconel 625 substrates. The chemical composition of the BC powder was the same in all cases; however, the particle size distribution of the powder employed with each torch was that specifically recommended for the torch. For optimization purposes, these BCs were screened based on initial evaluations of roughness, porosity, residual stress, relative oxidation, and isothermal TGO growth. A single type of standard YSZ top coat was deposited via APS (F4MB) on all the optimized BCs. The TBCs were thermally cycled by employing a furnace cycle test (FCT) (1080 °C-1 h—followed by forced air cooling). Samples were submitted to 10, 100, 400, and 1400 cycles as well as being cycled to failure. The behavior of the microstructures, bond strength values (ASTM 633), and the TGO evolution of these TBCs, were investigated for the as-sprayed and thermally cycled samples. During FCT, the TBCs found to be both the best and poorest performing and had their BCs deposited via HVOF. The results showed that engineering low-oxidized BCs does not necessarily lead to an optimal TBC performance. Moreover, the bond strength values decrease significantly only when the TBC is about to fail (top coat spall off) and the as-sprayed bond strength values cannot be used as an indicator of TBC performance.

Structural performance assessment of deteriorated reinforced concrete bridge piers

The aim of this study is to assess the structural performance of deteriorated reinforced concrete bridge piers, and to provide method for developing improved evaluation method. For a deteriorated bridge piers, once the cover spalls off and bond between the reinforcement and concrete has been lost, compressed reinforcements are likely to buckle. By using a sophisticated nonlinear finite element analysis program, the accuracy and objectivity of the assessment process can be enhanced. A computer program, RCAHEST (Reinforced Concrete Analysis in Higher Evaluation System Technology), is used to analyze reinforced concrete structures. Material nonlinearity is taken into account by comprising tensile, compressive and shear models of cracked concrete and a model of reinforcing steel. Advanced deteriorated material models are developed to predict behaviors of deteriorated reinforced concrete. The proposed numerical method for the structural performance assessment of deteriorated reinforced concrete bridge piers is verified by comparing it with reliable experimental results. Additionally, the studies and discussions presented in this investigation provide an insight into the key behavioral aspects of deteriorated reinforced concrete bridge piers.

Cathodic Protection of X65 Carbon Steel in a Simulated Oilfield Produced Water

The effectiveness of cathodic protection for X65 carbon steel in a simulated oilfield pro- duced water was examined by means of measurements of polarization curve, constant potential ca- thodic polarization and weight loss. Then the morphology, composition and constituent of the corro- sion products were characterized by SEM, EDS and XRD. The result indicates: in the simulated oil- field produced water, X65 carbon steel suffered from serious pitting corrosion with a great weight loss rate; the cathodic protection potential in a range -800 mV to -1000 mV exhibits obvious inhibi- tion effect on corrosion of X65 in the environment; a sound scale of calcareous deposits can't formed on the surface of carbon steel by the potential of -800 mV, a dense and good adhesive scale of deposits may formed by -900 mV on the steel surface, resulting effectively in reduction of the ca- thodic protection current density. The deposits blister and easily spall off due to hydrogen evolution by potential -1000 mV. In comparison with the circumstance in seawater, hydrogen evolution po- tential of X65 carbon steel is much positive in the simulated oilfield produced water and the depos- its do not contain magnesium hydroxide.