Environmentally Friendly Solvent‐ and Water‐Based Coatings for Mitigation of Crystallization Fouling

The surface properties of solvent‐based (SB) and water‐based (WB) coatings and their impact on fouling during convective heat transfer of CaSO₄ solutions were investigated. Experiments demonstrated that the SB coatings had generally better non‐adhesive characteristics, especially at higher values of the electron donor component since the deposits could easily be washed away. For the SB coatings, a longer induction period compared to those of untreated surfaces was observed and a significant reduction of the fouling rate could be achieved. Further analysis of surfaces revealed that SB coatings enhanced the acid‐base repulsive force and thus reduced the deposit/solid adhesion energy. For the WB coatings, the Liftshitz‐van der Waals attractive force plays a decisive role in the adhesion process due to the higher apolar component of the surface energy.     

板式换热器Ni-P-TiO2复合纳米镀层微生物污垢特性

换热器微生物污垢问题普遍存在于能源化工领域,污垢的聚集会导致设备的流动阻力、燃料消耗和维护成本支出大幅度增加。本文采用复合纳米镀层来抑制和减轻换热表面的微生物污垢的附着和沉积。首先采用化学镀的方式,在板式换热器的不锈钢316板上镀覆 Ni-P-TiO₂复合纳米镀层和对照性的Ni-P 镀层。基于板式换热器的微生物污垢在线监测实验系统,研究了镀覆Ni-P-TiO₂复合纳米镀层的板式换热器微生物污垢特性。结果表明,清洁状态下,镀覆两种镀层的板式换热器其摩擦系数（f）和Nusselt数（Nu）相较未镀覆板式换热器均略有增加;微生物污垢实验后,相比较未镀覆的板式换热器,镀覆Ni-P镀层的板式换热器污垢热阻减少了8.36%~23.07%,而镀覆Ni-P-TiO₂复合纳米镀层的板式换热器污垢热阻减少了16.6%~30.96%;在相同微生物污垢实验工况下,镀覆Ni-P-TiO₂复合纳米镀层的板式换热器的摩擦系数（f）相比Ni-P镀层的低2.54%~11.82%,但Nu却明显高于Ni-P镀层达8.47%~9.45%,并且污垢热阻明显小于Ni-P镀层达10.66%~18.18%,镀覆Ni-P-TiO₂复合纳米镀层的板式换热器展现了优异的强化传热性能和抑垢性能。     

Calcium phosphate fouling on TiN-coated stainless steel surfaces: Role of ions and particles

Modifying the surface properties of the stainless steel used in heat exchangers to decrease its tendency for the building-up of milk deposits seems to be a promising strategy to reduce fouling during heat treatment in the dairy industry. In this work, several modified stainless steel surfaces, obtained by reactive magnetic sputtering, were used as fouling supports for a milk-simulating mineral solution under constant conditions of temperature, pH and calcium concentration. The aim of the work was to quantify deposition and removal parameters, based on the influence of surface energy properties on the type of deposit. The fouling process was characterized by the surface reaction coefficient of the ions present in solution, as well as the adhesion coefficients of the particles formed in the bulk. An insight on the mechanisms of deposition and removal processes was achieved, as well as by the dependence between those two processes and the electron-donor component (γ^-) of the surface energy of the stainless steel-based materials. The surfaces with lower γ^- were found to have lower amounts of deposit, and this deposit could be more easily cleaned than for the other surfaces (including the non-modified stainless steel).     

Coal fouling characteristic to deposit probe with different temperatures under the gasification condition

Coal gasification was carried out to verify the coal fouling characteristic in a drop tube furnace (DTF). Four pulverized coal samples, in the range of bituminous and sub-bituminous, were used. To analyze the fouling characteristic by different temperature of deposit probe, a two-stage deposit probe was used in the experiment. Ash deposition rate was at upper deposit probe higher than at lower one. The X-ray fluorescence (XRF) results indicated that coal fouling included acid minerals such as SiO₂ and Al₂O₃ at upper deposit probe more than that at lower deposit probe. The results of X-ray diffraction (XRD) and scanning electron microscopy (SEM) indicated that the fouling particles at high deposit temperature were agglomerated more than those at low deposit temperature. And the convective heat transfer efficiency was reduced by ash deposition on probe. Especially, the convective heat transfer coefficient substantially declined with small particle size of fouling and Fe₂O₃, CaO, and MgO.     

Pool boiling fouling and corrosion properties on liquid‐phase‐deposition TiO2 coatings with copper substrate

Fouling on the heat transfer surfaces of industrial heat exchangers is an intractable problem, and several techniques have been suggested to inhibit fouling. Surface coatings are of such techniques by which the adhesion force between fouling and heat transfer surface can be reduced with low surface free energy thin films. In this article, liquid phase deposition was applied to coat titanium dioxide thin films on the red copper substrates with film thickness in micro‐ or nano‐meter scale. Coating thickness, contact angle, roughness, surface topography, and components were measured with X‐ray diffraction, contact angle analyzer, stylus roughmeter, scanning electron microscopy, and energy dispersive X‐ray spectroscopy, respectively. Surface free energy of coating layers was calculated based on the contact angle. Heat transfer and fouling characteristics in pool boiling of distilled water and calcium carbonate solution on coated surfaces were investigated. Heat transfer enhancement was observed on coated surfaces compared with untreated or polished surfaces due to the micro‐ and nano‐structured surfaces which may increase the number of nucleation sites. The nonfouling time on the coated surfaces is extended than that on the untreated or polished surfaces due to the reducing of the surface free energy of coated surfaces. Corrosion behavior of coated surfaces soaked in the corrosive media of hydrochloric acid, sodium hydroxide alkali, and sodium chloride salt solutions with high concentration at room temperature a few hours was also explored qualitatively. Anticorrosion results of the coated surfaces were obtained. The coatings resisted alkali corrosion within 7.2 × 10⁵ s, acidic corrosion within 3.6 × 10⁵ s and salt corrosion within 2.16 × 10⁶ s. The present work may open a new coating route to avoid fouling deposition and corrosion on the heat transfer surfaces of industry evaporators, which is very important for energy saving in the related industries. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Protective properties of Al2O3 + TiO2 coating produced by the electrostatic spray deposition method

Mechanical resistance of Al₂O₃ + TiO₂ nanocomposite ceramic coating deposited by electrostatic spray deposition method onto X10CrAlSi18 steel to thermal and slurry tests was investigated. The coating was produced from colloidal suspension of TiO₂ nanoparticles dispersed in 3 wt% solution of Al₂(NO₃)₃, as Al₂O₃ precursor, in ethanol. TiO₂ nanoparticles of two sizes, 15 nm and 32 nm, were used in the experiments. After deposition, coatings were annealed at various temperatures, 300, 1000 and 1200 °C, and next exposed to cyclic thermal and slurry tests. Regardless of annealing temperature and the size of TiO₂ nanoparticles, the outer layer of all coatings was porous. The first five thermal cycles caused a rapid increase of aluminum content of the surface layer to 30–37 wt%, but further increase in the number of thermal cycles did not affect the aluminum content. The oxidation rate of coating-substrate system was lower during the thermal tests than during annealing. The oxidation rate was also lower for smaller TiO₂ particles (15 nm) forming the coating than for the larger ones (32 nm). The protective properties of Al₂O₃ + TiO₂ coating against intense oxidation of substrate were lost at 1200 °C. Slurry tests showed that coatings annealed at 1000 °C had the best slurry resistance, but thermal tests had weakened this slurry resistance, mainly due to decreasing adhesion of the coating.     

Effect of substrate temperature on microstructure and nanomechanical properties of Gd2Zr2O7 coatings prepared by EB-PVD technique

In the present work, gadolinium zirconate (Gd₂Zr₂O₇) coatings have been developed on Inconel-718 substrates by electron beam physical vapor deposition (EB-PVD) technique. The structural, morphological and mechanical properties as a function of substrate temperature have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), nanoindentation and scratch tests. XRD analysis revealed that the coatings showed cubic defect fluorite phase, and no secondary phase formation was observed in the coatings during deposition. The decrease in the lattice constant of the fluorite phase with increasing deposition temperature was explained on the basis of strain relaxation and vacancy concentration. Increased surface roughness of the coatings has been found with increasing substrate temperature as a result of increased crystallite size. An improved coating adhesion achieved for the coating deposited at higher substrate temperature of 973?K was confirmed by scratch test. Nanoindentation measurements indicated higher hardness (7.7?GPa) and resistance to plastic deformation and better capability to accommodate deformation energy for the coatings prepared at higher deposition temperature.     

CaCO3 fouling on microscale–nanoscale hydrophobic titania–fluoroalkylsilane films in pool boiling

Micrometer–nanometer hydrophobic titania–fluoroalkylsilane composite coatings were prepared on substrates based on liquid‐phase deposition. Coatings and crystallization forms were characterized with instruments of surface analyses. Experimental facilities of pool boiling were established to evaluate heat and mass transfer on coated surfaces in deionized water and saturated calcium carbonate solution. Obvious pool boiling enhancement was observed on thinner microscale–nanoscale hydrophobic titania–fluoroalkylsilane composite films at higher heat fluxes compared to that on thicker titania–fluoroalkylsilane coatings or on titania coatings and stainless steel surfaces. Lower fouling resistance was obtained on titania–fluoroalkylsilane coatings in pool boiling of saturated calcium carbonate solution and crystal form was aragonite, which was different from calcite on titania coatings. Results of inhibition of fouling and enhancement of heat transfer on titania–fluoroalkylsilane coatings were contributed to special surface microscale–nanoscale structure and material wettability. Asymptotic model was used to fit experimental data of fouling resistance, and reasonable agreement was obtained. © 2013 American Institute of Chemical Engineers AIChE J, 59: 2662–2678, 2013     

Cathode plasma electrolytic deposition of Al2O3 coatings doped with SiC particles

Semiconductor particles doped Al₂O₃ coatings were prepared by cathode plasma electrolytic deposition in Al(NO₃)₃ electrolyte dispersed with SiC micro- and nano-particles (average particle sizes of 0.5–1.7?µm and 40?nm respectively). The effects of the concentrations and particle sizes of the SiC on the microstructures and tribological performances of the composite coatings were studied. In comparison with the case of dispersing with SiC microparticles, the dispersion of SiC nanoparticles in the coatings was more uniform. When the concentration of SiC nanoparticles was 5?g/L, the surface roughness of the composite coating was reduced by 63%, compared with that of the unmodified coating. Friction results demonstrated that the addition of 5?g/L SiC nanoparticles reduced the friction coefficient from 0.60 to 0.38 and decreased the wear volume under dry friction. The current density and bath voltage were measured to analyze the effects of SiC particles on the deposition process. The results showed that the SiC particles could alter the electrical behavior of the coatings during the deposition process, weaken the bombardment of the plasma, and improve the structures of the coatings.     

Properties of TiN–Al2O3–TiCN–TiN,TiAlN, and DLC-coated Ti(C,N)-based cermets and their wear behaviors during dry cutting of 7075 aluminum alloys

In the work, TiAlN for physical vapor deposition (PVD), multilayer TiN-Al₂O₃-TiCN-TiN for chemical vapor deposition (CVD), and diamond-like carbon (DLC) for plasma-enhanced chemical vapor deposition (PECVD) were deposited on the cermet inserts. Characteristics and wear behaviors of the three coated cermets during dry cutting of 7075 aluminum alloys were observed. The results show that TiN-Al₂O₃-TiCN-TiN coatings have highest adhesion strength and hardness. At the cutting speed of 1100 r/min, the depth of 0.2 mm, and the feed rate of 0.1 mm/r, the three coated inserts show the best wear-resistant properties. In this case, TiN/Al₂O₃/TiCN/TiN shows the worst wear-resistant properties (value of the flank wear [VBB] = 0.062 mm), while DLC coatings show the most excellent wear-resistant properties (VBB = 0.046 mm). During the cutting of aluminum alloys, which have high plasticity and low melting point, adhesive wear dominate on the flank of the inserts. The thickest coating of TiN/Al₂O₃/TiCN/TiN results in the bluntest cutting edge, which form the most serious adhesive worn zone. For the TiAlN and DLC coatings, due to a smaller cutting force, the two coatings have much better wear resistance. Further, the self-lubricating properties of DLC show excellent effect on protecting the inserts. Thus, the DLC-coated cermets have the best wear-resistant properties. Further, the TiAlN-coated cermets have the widest wear-affected zone while the DLC coating has the narrowest.     

Effect of heat treatment on microstructure and properties of powder immersion reaction assisted chromium carbide coatings prepared on Q235 steel

In this work, powder immersion reaction assisted (PIRAC) chromium carbide coatings prepared on Q235 steel at 800 and 900°C were heat-treated in vacuum at 900 and 1000°C, respectively. The effects of heat treatments on the phase compositions, microhardness, and oxidation behavior in the air of the as-prepared PIRAC chromium carbide coatings were investigated. Under further heat treatments, Cr₂₃C₆ in the chromium carbide coatings was carburized due to the diffusion of carbon and transformed into Cr₃C₂ and Cr₇C₃ and subsequently, the microhardness of the coatings increased. The higher the heat treatment temperature, the higher the amount of Cr₃C₂ and Cr₇C₃ and the greater microhardness of the coatings. The as-prepared and heat-treated chromium carbide coatings experienced similar parabolic oxidation weight gains with time. Further heat treatments led to the oxidation-induced weight gain and the parabolic oxidation rate constant decreased.     

Synergistic effect of surface textures and DLC coatings for enhancing friction and wear performances of Si3N4/TiC ceramic

To enhance the tribological properties of Si₃N₄ based ceramics, surface textures of dimples combined with DLC coatings are fabricated on Si₃N₄/TiC ceramic surface by nanosecond laser and plasma enhanced chemical vapor deposition (PECVD). The dry friction and wear performances are evaluated by unidirectional sliding friction tests using a rotary ball-on-disk tribometer. Results reveal that the friction and wear properties of Si₃N₄/TiC ceramics are significantly enhanced by DLC coatings or dimpled textures, and the DLC coatings combined with dimpled textures show the best efficiency in reducing friction, adhesion and wear. This improvement can be explained by the synergistic effect of DLC coatings and surface textures, and the synergistic mechanisms are attributed to the formation of lubrication film and secondary lubrication, debris capture of dimpled textures, increased surface hardness and mechanical interlocking effect, and reduced contact area.     

Effects of surface pretreatments on the deposition of adherent diamond coatings on cemented tungsten carbide substrates

Well-adhered microcrystalline diamond (MCD) coatings have been deposited on WC–Co substrates by the microwave plasma enhanced chemical vapor deposition (MPECVD) method. A multi-interlayer system Cr/CrN/Cr was deposited on the cemented carbide substrate before diamond deposition to act as a diffusion barrier. The interlayer-coated substrate was shortly peened by friable diamond powders with an average size of 150 μm to roughen the surface. Diamond coatings deposited on short peened substrates show higher nucleation density and stronger adhesion properties. The X-ray diffraction (XRD) pattern showed that an additional carbide compound layer (Cr₃C₂ and Cr₇C₃) was formed during the CVD diamond deposition to work as an intermediate bonding layer for better adhesion. Rockwell indentation tests with a load of 1470 N were conducted to investigate the coating's adhesion. No delamination outside of the indentation zone was observed for the diamond coating deposited on the roughened sample. Electron probe microanalysis (EPMA) results showed that the delamination in the indentation zone occurred mainly at the diamond/Cr interface and very little Co (less than 1 wt.%) was detected on the Cr failure surface. This suggests that during the CVD process Co/C inter-diffusion was successfully prevented by the Cr/CrN/Cr buffer layers.     

Optimization of the mechanical properties of magnetron sputtered diamond-like carbon coatings

Diamond-like carbon coatings containing hydrogen, a-C:H, were deposited by use of reactive DC magnetron sputtering with an industrial deposition system. The reactive gas C₂H₂ was used in combination with carbon targets. Using Raman spectroscopy, nanoindentation and Rockwell C indentation, the mechanical properties of the coatings were optimized. Excessively high compressive stresses, which were measured with Raman spectroscopy, were found in the coatings with high hardness, resulting in poor adhesion to the substrates. By thermal annealing, these compressive stresses were reduced without altering the hardness, resulting in diamond-like carbon coatings with good adhesion.     

The effectiveness of an antifouling compound coating based on a silicone elastomer and colored phosphor powder against Navicula species diatom

Silicone elastomers coatings can “release” fouling organisms and, thus, they are referred to as fouling-release coatings. In this study, compound coatings based on silicone elastomer and various colored glow-in-the-dark phosphor powders were prepared to investigate the initial settlement density and adhesion strength of diatom Navicula sp. on the coatings. It was found that settlement densities on samples with phosphor layers (diatom coverage area on Day 7 was ~30%) were significantly lower than those on samples without phosphor layers (coverage area was ~60%), while both coatings showed similar fouling-release characteristics (~90% of removal at wall shear stress value of 90 Pa). In addition, the phosphor color played an important role in the diatom fouling activities. Those experimental phenomena were partly explained according to surface characteristics and our previous study on the relationship between diatom settlement and light conditions. To sum up, this type of preliminary compound coating could both reduce diatom initial settlement and keep the advantage of fouling release.     

Prevention of crystallization fouling during eutectic freeze crystallization in fluidized bed heat exchangers

Eutectic freeze crystallization is a promising separation technique to produce salt and ice crystals with very high purities and requires less energy than competitive evaporative crystallization techniques. A drawback of this technique is crystallization fouling, which seriously reduces heat transfer rates. Solid–liquid fluidized bed heat exchangers may be attractive crystallizers for this purpose, since they have demonstrated to prevent severe crystallization fouling, for example of ice crystals. This paper presents crystallization experiments with a single-tube fluidized bed heat exchanger. A first experimental series showed that fluidized beds are also able to prevent salt fouling during cooling crystallization from KNO₃ or MgSO₄ solutions. A second series revealed that fouling during eutectic freeze crystallization is more severe than during separate salt or ice crystallization and is therefore difficult to prevent by the fluidized bed. The explanation for this phenomenon is that the salt crystallization process strongly reduces the solute mass transfer limitation for ice crystals growing on the wall resulting in an increased growth rate and more severe crystallization fouling. The addition of a non-crystallizing component strongly reduces fouling and enables to perform eutectic freeze crystallization in fluidized bed heat exchangers for industrial applications.     

Structural changes of hydroxyapatite coating electrophoretically deposited on NiTi shape memory alloy

The surface of the NiTi shape memory alloy was functionalized through the deposition of hydroxyapatite (HAp) coatings using the electrophoretic method (EPD). The electrophoresis carried out at the voltage of 40?V during the time of 120?s did not affect the crystalline structure of the initial HAp powder and, at the same time, ensured obtaining a homogeneous layer without visible cracks or discontinuities. Next, the coatings were subjected to heat treatment at 800?°C for 2?h in vacuum, wherein the applied conditions did not affect the decomposition of the deposited hydroxyapatite. The heat treatment resulted in the formation of carbonate apatite (C-HAp) in the HAp layer and in ceramic particles’ coalescence. Changes in the morphology and roughness of the layer as well as partial decomposition of the NiTi substrate parent phase into Ti₂Ni and Ni₄Ti₃ phases were also observed.     

Fouling von Wärmeübertragungsflächen

Fouling of heat transfer surfaces . Fouling of heat transfer surfaces in industry causes considerable costs. Overcoming fouling is therefore essential for technical and economic reasons. Solution of the problems requires a better understanding of the physical, chemical and biological processes causing fouling. It is shown, that the prediction of the fouling behaviour of heat transfer equipment based on existing mathematical models is not jet satisfactory. This article presents a new physical model describing particulate and precipitation fouling based on the assumption of a deposition and a removal process. Experiments to prove the results of the theoretical considerations were carried out using an aqueous CaSO₄ solution. The test unit which is also suitable for in-situ measurements is described in detail. Measured and predicted asymptotic fouling factors agree with acceptable accuracy.     

Structural investigation of Al2O3 coatings by PECVD with a high deposition rate

The deposition of alumina on thermal barrier coatings can effectively avoid hot corrosion and increase durability. Al₂O₃ coatings were prepared on an yttria-stabilized zirconia (YSZ) substrate by plasma chemical vapor deposition (CVD). The effects of microwave power (P_M) and total pressure (P_tot) on the crystalline phase and microstructure of Al₂O₃ coatings were investigated, and the effect of the mechanism on the deposition rate was also analyzed. The α-Al₂O₃ coatings with a needle-like microstructure were formed at a higher P_M and P_tot, whereas the γ-phase coatings exhibited a cauliflower-like microstructure at a lower P_M and P_tot. A maximum deposition rate (R_dep) of 58 μmh⁻¹ was obtained, which is significantly higher than those of conventional CVD methods.     

Fouling Mitigation in Tubular Exchangers Using Various Projectiles: Deposit Removal Mechanisms

Fouling of different types can severely compromise the thermal efficiency of heat exchangers. The removal of CaSO₄ deposits was experimentally investigated when projectiles of various types were propelled inside a heated tube. Projectiles were injected at a constant injection rate from the beginning of the fouling process until an asymptotic fouling resistance was approached. The removal rate was found to be inversely proportional to the fouling layer thickness. The removal of deposits due to exerted shear forces by projectiles decreased as the fouling layer developed due to hardening and intensification of the deposit layer. The results indicate that the asymptotic behavior is mainly due to the decrease in the deposition rate as the fouling layer hardens such that it becomes equal to the removal rate and not the other way round.