Mohammadreza Daroonparvar

Effect of fluoride treatment on corrosion behavior of MG-CA binary alloy for implant application

Abstract The influence of hydrofluoric acid (HF) treatment on the corrosion behavior of the Mg-0.5Ca alloys was investigated by immersion specimen in sodium hydroxide and HF solutions with various concentrations and durations at room temperature. Microstructural evolutions of the specimens were characterized by atomic force microscopy, X-ray diffraction, field-emission scanning electron microscopy. The corrosion resistance was examined through potentiodynamic polarization and immersion test in Kokubo solution. The results revealed that the fluoride treated Mg-0.5Ca alloys produced by immersion in 40% HF provided more uniform, dense and thicker coating layer (12.6 μm) compared with the 35% HF treated specimen. The electrochemical test showed that the corrosion resistance of fluoride treated specimen was 35 times higher compared with the untreated Mg-0.5Ca alloy specimen in Kokubo solution. In vitro degradation rate of the fluoride treated specimens was much lower than untreated Mg-0.5Ca alloy in Kokubo solution. After immersion test the surface of 40% HF treated sample showed a few corrosion dots, while untreated specimens were fully covered by corrosion products and delamination. Fluoride treated Mg-0.5Ca alloy with 40% HF is a promising candidate as biodegradable implants due to its low degradation kinetics and good biocompatibility.

In-vitro degradation behavior of Mg alloy coated by fluorine doped hydroxyapatite and calcium deficient hydroxyapatite

Fluorine-doped hydroxyapatite (FHA) and calcium deficient hydroxyapatite (CDHA) were coated on the surface of biodegradable magnesium alloy using electrochemical deposition (ED) technique. Coating characterization was investigated by X-ray diffraction (XRD), Fourier-transformed infrared spectroscopy (FTIR), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The result shows that nano-FHA coated sample presents nano needle-like structure, which is oriented perpendicular to the surface of the substrate with denser and more uniform layers compared to the nano-CDHA coated sample. The nano-FHA coating shows smaller crystallite size (65 nm) compared to the nano-CDHA coating (95 nm); however, CDHA presents thicker layer (19 μm in thickness) compared to the nano-FHA (15 μm in thickness). The corrosion behaviour determined by polarization, immersion and hydrogen evolution tests indicates that the nano-FHA and nano-CDHA coatings significantly decrease corrosion rate and induce passivation. The nano-FHA and nano-CDHA coatings can accelerate the formation of bone-like apatite layer and significantly decrease the dissolution rate as compared to the uncoated Mg alloy. The nano-FHA coating provides effective protection to Mg alloy and presents the highest corrosion resistance. Therefore, the nano-FHA coating on Mg alloy is suggested as a great candidate for orthopaedic applications.

Investigation of three steps of hot corrosion process in Y2O3 stabilized ZrO2 coatings including nano zones

Abstract Phase transformation of tetragonal ZrO2 to monoclinic phase and also increment of bond coat oxidation kinetic (TGO thickening) can substantially restrict the life time of thermal barrier coating systems (TBCs). So, nanostructured and conventional Y2O3 stabilized ZrO2 coatings were evaluated in fused V2O5-Na2SO4 salts during thermal exposure in air. Microstructural characterization showed lower hot corrosion products (monoclinic zirconia, YVO4 crystals) formation and reduction of TGO thickness in thermal barrier coating system consisting of nanostructured Y2O3 stabilized ZrO2 (YSZ) top coat. It was found that inhomogeneities, pores and micro-cracks played a principal role in the molten salts infiltration into the YSZ coating during three steps of hot corrosion process. In the nanostructured YSZ coating with tri-model structure, nano zones which surrounded by fully molten parts could fill the aforementioned defects and could act as barrier for the oxygen and corrosive molten salts penetration into the TBC.

Improvement of thermally grown oxide layer in thermal barrier coating systems with nano alumina as third layer

Abstract A thermally grown oxide (TGO) layer is formed at the interface of bond coat/top coat. The TGO growth during thermal exposure in air plays an important role in the spallation of the ceramic layer from the bond coat. High temperature oxidation resistance of four types of atmospheric plasma sprayed TBCs was investigated. These coatings were oxidized at 1000 °C for 24, 48 and 120 h in a normal electric furnace under air atmosphere. Microstructural characterization showed that the growth of the TGO layer in nano NiCrAlY/YSZ/nano Al2O3 coating is much lower than in other coatings. Moreover, EDS and XRD analyses revealed the formation of Ni(Cr,Al)2O4 mixed oxides (as spinel) and NiO onto the Al2O3 (TGO) layer. The formation of detrimental mixed oxides (spinels) on the Al2O3(TGO) layer of nano NiCrAlY/YSZ/nano Al2O3 coating is much lower compared to that of other coatings after 120 h of high temperature oxidation at 1000 °C.

Effect of Y2O3 stabilized ZrO2 coating with tri-model structure on bi-layered thermally grown oxide evolution in nano thermal barrier coating systems at elevated temperatures

Abstract Bi-layered thermally grown oxide (TGO) layer plays a major role in the spallation of Y2O3 stabilized ZrO2 (YSZ) layer form the bond coat in the thermal barrier coating (TBC) systems during oxidation. On the other hand, bi-layered TGO formation and growth in the TBC systems with nanostructured YSZ have not been deeply investigated during cyclic oxidation. Hence, Inconel 738/NiCrAlY/normal YSZ and Inconel 738/NiCrAlY/nano YSZ systems were pre-oxidized at 1000 °C and then subjected to cyclic oxidation at 1150 °C. According to microstructural observations, nanostructured YSZ layer over the bond coat should have less micro-cracks and pinholes, due to the compactness of the nanostructure and the presence of nano zones that resulted in lower O infiltration into the nanothermal barrier coating system, formation of thinner and nearly continuous mono-layered thermally grown oxide on the bond coat during pre-oxidation, lower spinels formation at the Al2O3/YSZ interface and finally, reduction of bi-layered thermally grown oxide thickness during cyclic oxidation. It was found that pre-heat treatment and particularly coating microstructure could influence microstructural evolution (bi-layered TGO thickness) and durability of thermal barrier coating systems during cyclic oxidation.

The Role of Nanostructured Al2O3 Layer in Reduction of Hot Corrosion Products in Normal YSZ Layer

YVO4 crystals and monoclinic ZrO2 are known as hot corrosion products that can considerably reduce the lifetime of thermal barrier coatings during service. The hot corrosion resistance of two types of air plasma sprayed thermal barrier coating systems was investigated: an Inconel 738/NiCrAlY/YSZ (yttria-stabilized zirconia) and an Inconel 738/NiCrAlY/YSZ/nano-Al2O3 as an outer layer. Hot corrosion test was accomplished on the outer surface of coatings in molten salts (45% Na2SO4 + 55% V2O5) at 1000°C for 52 hour. It was found that nanostructured alumina as outer layer of YSZ/nano-Al2O3 coating had significantly reduced the infiltration of molten salts into the YSZ layer and resulted in lower reaction of fused corrosive salts with YSZ, as the hot corrosion products had been substantially decreased in YSZ/nano-Al2O3 coating in comparison with normal YSZ coating after hot corrosion process.

Effects of bond coat and top coat (including nano zones) structures on morphology and type of formed transient stage oxides at pre-heat treated nano NiCrAlY/nano ZrO2-8%Y2O3 interface during oxidation

Abstract Bond coat geometry is able to significantly influence thermally grown oxide (TGO) layer formation and growth in thermal barrier coating systems at the onset of oxidation. Moreover, nanostructured yttria stabilized zirconia coating with three model structure (including nano zones) could reduce oxygen partial pressure to the formation pressure of continues alumina oxide scale which was able to protect the substrate from extra oxidation and corrosion. Thus, nano NiCrAlY/nano YSZ and normal NiCrAlY/nano YSZ coatings were prepared by air plasma spray method and then evaluated by high temperature oxidation test at 1000 °C for 160 h. As-sprayed samples were also pre-oxidized at 1000 °C for 48 h and then examined by electrochemical polarization test. The corrosion rate of the pre-oxidized nano NiCrAlY/nano YSZ coating was estimated to be the lowest compared to that of the other samples. It might be related to the formation of continues alumina layer on the nano NiCrAlY at the onset of oxidation. This continuous layer could reduce the penetration of the aggressive solution into the NiCrAlY coating and acted as a protective layer (with the lowest anodic current density). This layer also lessened TGO growth rate at the stages II and III of the growth.

Preparation and Performance of Plasma/Polymer Composite Coatings on Magnesium Alloy

A triplex plasma (NiCoCrAlHfYSi/Al2O3·13%TiO2)/polycaprolactone composite coating was successfully deposited on a Mg-1.2Ca alloy by a combination of atmospheric plasma spraying and dip-coating techniques. The NiCoCrAlHfYSi (MCrAlHYS) coating, as the first layer, contained a large number of voids, globular porosities, and micro-cracks with a thickness of 40-50 μm, while the Al2O3·13%TiO2 coating, as the second layer, presented a unique bimodal microstructure with a thickness of 70-80 μm. The top layer was a hydrophobic polymer, which effectively sealed the porosities of plasma layers. The results of micro-hardness and bonding strength tests showed that the plasma coating presented excellent hardness (870 HV) and good bonding strength (14.8 MPa). However, the plasma/polymer coatings interface exhibited low bonding strength (8.6 MPa). The polymer coating formed thick layer (100-110 μm) that homogeneously covered the surface of the plasma layers. Contact angle measurement showed that polymer coating over plasma layers significantly decreased surface wettability. The corrosion current density (icorr) of an uncoated sample (262.7 µA/cm2) decreased to 76.9 µA/cm2 after plasma coatings were applied. However, it was found that the icorr decreased significantly to 0.002 µA/cm2 after polymer sealing of the porous plasma layers.

Cold deformation and heat treatment influence on the microstructures and corrosion behavior of AISI 304 stainless steel

Abstract In the present study AISI 304 stainless steel with different degrees of cold deformation and annealing parameters were investigated. Microstructural evolutions by optical micrography and scanning electron microscopy showed relatively fully austenitic ultrafine-grained structure obtained after annealing at 700°C for 80 min. The volume fraction of α′-martensite increased with increasing compressive deformation rate and maximum volume fraction of α′-martensite was attained in the samples subjected 0⋅65% strain. The potentiodynamic polarization results in 3% NaCl indicated that the corrosion current density increased with cold deformation, while after annealing, it reduced from 2⋅86 to 2⋅29 μA cm−2, showing an enhancement of corrosion resistance. The immersion test showed that the austenitic ultrafine-grained structure exhibits moderate and more uniform pitting corrosion attack compared to the coarser grain in NaCl solution. Dans cette étude, on a examiné un acier inoxydable AISI 304 ayant différents taux de déformation à froid et différents paramètres de recuit. L’analyse de l’évolution de la microstructure par micrographie optique et par microscopie électronique à balayage a montré une structure à grains ultrafins et presque complètement austénitique obtenue après un recuit à 700°C pendant 80 minutes. La fraction volumique de martensite α′ augmentait avec l’augmentation du taux de déformation en compression et la fraction volumique maximale de martensite α′ était atteinte par les échantillons soumis à une déformation de 0·65%. Les résultats de polarisation potentiocinétique dans 3% de NaCl indiquaient que la densité du courant de corrosion augmentait avec la déformation à froid, alors qu’après le recuit, elle était réduite de 2·86 μA/cm2 à 2·29 μA/cm2, montrant ainsi une augmentation de la résistance à la corrosion. L’épreuve d’immersion dans la solution de NaCl a montré que la structure austénitique à grains ultrafins exhibait une attaque par corrosion localisée modérée et plus uniforme comparée au grain plus grossier.

Introducing a composite coating containing CNTs with good corrosion properties: characterization and simulation

A duplex composite coating containing carbon nanotubes (CNTs) was successfully deposited on a Mg–Zn–Ca alloy by an atmospheric plasma spraying (APS) technique. The NiCoCrAlYTa coating, as the first layer included a large number of voids, globular porosities, and micro-cracks with a thickness of 90–100 μm, while the nanostructure TiO2 (nTiO2) coating, as the second layer, presented a unique bimodal microstructure with a thickness of 140–150 μm. The incorporation of CNTs into TiO2 led to the reduction of surface defects as a result of the filling of porosities and pinholes of the surface by the CNTs. The corrosion current density (icorr) of an uncoated sample was about 208.2 μA cm−2 which decreased to 67.2 μA cm−2 after applying the second layer. In the most optimal mode, it was found that the icorr decreased significantly to 17.3 μA cm−2 after addition of CNTs to nTiO2 coatings. At the end, by the use of gene expression programming (GEP) for the first time, the corrosion rate of the nTiO2 coating containing CNTs was simulated and formulated. The results presented by the GEP software were compared to the experimental ones and the GEP based formulation for predicting corrosion rate, was evaluated. The results showed a negligible error of 0.5 mm per year in predicting the corrosion rate, which confirms a high accuracy simulation of the corrosion behavior of the composite coatings containing CNTs.