Low Corrosion Resistance Research Articles

Optimizing parameter for electrophoretic deposition of hydroxyapatite coating with superior corrosion resistance on pure titanium

Bioactive hydroxyapatite (HA) coating is applied on a commercially pure Ti by electrophoretic deposition (EPD). Optimizing the coating structure is necessary to obtain a stable layer and the best corrosion protection. The EPD was conducted at a constant voltage of 20, 30, and 40 V for 30 min in a HA/DMF (dimethylformamide) suspension. Uniform HA layers with a Ca/P ratio of 1.82 were successfully deposited on the Ti surface. The layers, which consisted of HA grains with the size of 1–5 μm, exhibited a gradual increase in thickness of 32, 50, and 60 μm with formation voltage. For the biomedical application, the suitable coating thickness was at least 50 μm. The high compaction of HA grains deposited at 30 V led to an order magnitude higher polarization resistance and ten times lower corrosion current density relative to the other specimens. The porous HA layer formed at 20 V, and the presence of cracks in the 40 V-coating led to a lower corrosion resistance relative to the 30-V coating. The 20 V- and 30 V-coatings remained intact and triggered the deposition of HA during immersion in simulated body fluid for 28 days, while the 40 V-coating dissolved into the solution. The optimum EPD voltage for depositing a stable HA coating with reasonable coating thickness and the best corrosion resistance was 30 V.

The Electrochemical and Microstructure Effects of TiB2 and SiC Addition to AA5052/Al2O3 Surface Composite Coatings in 0.5 M H2SO4 Solution

In this work, Al2O3, SiC, and TiB2 ceramic coatings with different formulations were produced on the surface of AA5052 matrix by gas tungsten arc welding (GTAW). The corrosion behavior of the composites and the base metal (BM) was evaluated in a solution of 0.5 M H2SO4 with electrochemical techniques such as open circuit potential (OCP) monitoring, potentiodynamic polarization (PDP), and electrochemical impedance spectroscopy (EIS). The morphology of the cladded surfaces was monitored by scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS). Based on the PDP and EIS results, BM had the lowest corrosion rate in the 0.5 M H2SO4 solution. All composite coatings shifted to higher corrosion rate values than that of BM, indicating that AA5052 composites had lower corrosion resistance compared to the BM. SEM images of the corroded surface of BM exhibit minimum pitting corrosion in sulfuric acid media. According to electrochemical studies, AA5052/Al2O3 + SiC + TiB2 composite had the lowest corrosion resistance due to the formation of micro-galvanic sites on the aluminum matrix surface. However, the AA5052/Al2O3 + TiB2 composite showed the best corrosion performance compared to other composites. In general, by adding SiC and TiB2 ceramics to Al2O3 and fabricating binary composites, the corrosion resistance of AA5052/Al2O3 composite is improved. Although the corrosion resistance of AA5052 composites is lower than the BM, their higher surface hardness makes their use justified in applications where high mechanical properties are required if the environment is not too aggressive.Graphical Abstract

Electrodeposition of Ni-W Alloy from Citric Acid Free Aqueous Electrolyte As a Substitute for Hard Chrome Coating and the Effect of Tungsten Content on Coating Hardness

In the industry, chrome coating is an important player in the functional layer application. Due to its high corrosion and wear resistance as well as high hardness [1], hard chrome plating is a popular choice in the automotive and aerospace industry. However, since September 2017, the application of chrome (VI) in surface treatment is banned by the European Chemical Agency (ECHA) due to chrome (VI) being toxic and environmentally hazardous [2]. Chrome (III) compound is a popular substitute for chrome (VI) due to it being less toxic compared to chrome (VI). However, chrome (III) coating has a lower corrosion resistance, and the electroplating process is more sensitive to metallic impurities.Many new innovations have since been developed such as the plasma electrolytic oxidation (PEO), extreme highspeed laser deposition (EHLA). Even established technologies such as physical vapour deposition (PVD), thermal spraying and case hardening process have tried to replace hard chrome coating with a degree of success. Admittedly, the above-mentioned technologies have their fair share of disadvantages. PEO, a green procedure [3], causes a metal substrate to develop a highly resistant oxide which is extremely strong and corrosion as well as wear resistant. The process however needs a high temperature and pressure, continuous replacement of electrolytes and consumes a great amount of power which made the process not feasible for a wide industrial application [4-6]. EHLA and thermal spraying on the other hand are restricted to rotational symmetric components. PVD is restricted to substrates that can handle high processing temperature like nitrable ferrous base materials. Lastly, case hardening processes requires long heat treatments which contribute to a higher process cost. [7]In our presentation, another alternative for chrome coating based on electrodeposition will be introduced. The element tungsten is considered in our experiments because tungsten not only has an excellent corrosion properties and high tensile strength at high temperature, but it also has a microhardness of 650 Vickers. However, it is not possible to deposit tungsten from an aqueous solution because tungstate cannot be directly reduced to metallic tungsten due to a very low overvoltage for hydrogen evolution on tungsten. Thus, an induced co-deposition mechanism is needed, and it is known that tungsten can be easily codeposited with iron group metal. [8-9]In this study a modified Watt’s bath is used as electrolyte to deposit NiW alloy. Our preliminary tests have shown that tungsten can also be deposited with nickel without the help of citric acid as complexing agent. The usage of citric acid does indeed inhibit the deposition of nickel but show no influence in the deposition of tungsten. Hence, in our study, experiments with the rotating disc electrode (RDE) will be done to see if tungsten content in the coating could be increased when the hydrodynamic and coating parameters are manipulated. The chemical composition of the layer is measured using SEM/EDX and the surface morphology is pictured using SEM. Metallographic cross-sections are made to see if there are any fissures in the layer and the hardness of the layer is measured using the microhardness test. The dependency of hardness on the tungsten content in the layer and the annealing parameter will also be shown in the presentation. XRD measurement is conducted for phase analysis and grain size calculation. Figure 1

Gold-Reduced Graphene Oxide Composite Coating on Stainless Steel 316L As Bipolar Plate for Proton Exchange Membrane Fuel Cell

Proton exchange membrane fuel cell (PEMFC) are regarded as one of the most promising fuel cell types for a new generation of power source in portable application due to no harmful byproducts emission, rapid startup and low operating temperature. Although PEMFC has a low output with a single cell, it is overcome by the series of unit stack by using bipolar plate (BPP) to improve the total efficiency. BPP, which conveys electrical current, separates reactant gases and connects single stack unit in series, is one of the most important part of PEMFC, along with catalytic electrode and proton exchange membrane. In recent years, stainless steel 316L (SUS316L) has been touted to as a promising candidate material for BPP with the numerous advantages such as low cost, high machinability and ductility. However, it has critical issues such as low corrosion resistance and high electrical resistance. For this reason, a coating process must be carried out on SUS316L with non-corrosive and conducting materials such as Au, Rh, Ag, Ti etc. Because these materials are high cost or rare-earth metals, anti-corrosive coating process inevitably causes the increase of BPP manufacturing cost.In this study, Au-reduced graphene composite coating is introduced on SUS316L to improve anti-corrosion resistance and electrical conductivity with in cost-effective manner by minimizing the consumption of Au deposit and the using electrochemical fabricated graphene with low cost. Corrosion properties and electrical properties are investigated with the as-coated samples and in simulated operating condition. By addition of graphene coating process, the consumption of Au is dramatically reduced with the outstanding performance compared to with only Au coating.

(Digital Presentation) Electrochemical Corrosion Evaluation of Metallic Materials in Hahm Pyrolysis Bio-Oil

Biomass-derived pyrolysis oils contain organic acids, water and oxygenates that can degrade metallic materials with low corrosion resistance. To construct storage tanks for such bio-oils, the selection of metallic materials with sufficient corrosion compatibility is of high importance. This work evaluated corrosion compatibility of Cr-alloyed steels and stainless steels in a pyrolysis bio-oil using electrochemical impedance spectroscopy (EIS). High-Ash High-Moisture (HAHM) bio-oil, produced from a forest residue feedstock by National Renewable Energy Laboratory in Golden, CO, was used as the test liquid for EIS measurements. The corrosion reaction resistance, determined from the impedance data, indicated that stainless steels with the critical value of Cr or Cr + Mo content exhibit corrosion resistance in the bio-oil. In post-exposure optical inspection, no change was found for the specimens with high corrosion reaction resistance while visually changed surfaces were observed from the specimens with low corrosion reaction resistance.

Machining Performance for Ultrasonic-Assisted Magnetic Abrasive Finishing of a Titanium Alloy: A Comparison with Magnetic Abrasive Finishing

Titanium alloys are widely used in aerospace, the military industry, electronics, automotive fields, etc., due to their excellent properties such as low density, high strength, high-temperature resistance, and corrosion resistance. Many components need to be finished precisely after being cut in these applications. In order to achieve high-quality and high-efficiency finishing of titanium alloys, ultrasonic-assisted magnetic abrasive finishing (UAMAF) was introduced in this research. The machining performance for UAMAF of a titanium alloy was studied by experimentally comparing UAMAF and magnetic abrasive finishing (MAF). The results show that the cutting force of UAMAF can reach 2 to 4 times that of MAF, and it decreases rapidly with the increase in the machining gap due to the energy loss of ultrasonic impact in the transmission between magnetic abrasives. The surface roughness of UAMAF can reach about Ra 0.075 μm, which is reduced by about 59% compared with MAF. The main wear type of the magnetic abrasive is that the diamond grits fell off the magnetic abrasive in both UAMAF and MAF. The uniform wear of the magnetic abrasive is realized, and the utilization ratio of the magnetic abrasive is obviously improved in UAMAF.

Mechanically robust and superhydrophobic concrete based on sacrificial template approach

Superhydrophobic (SH) concrete with low adhesion, self-cleaning, and corrosion resistance has the potential to significantly meet the challenges of injury and fatality under various external conditions such as irradiation, chemical corrosion, and biological contamination. However, the traditional process to realize the superhydrophobicity normally brings the concern of compressive strength loss. Herein, we introduced a facile approach to fabricate the superhydrophobic concrete with mechanical robustness based on the sacrificial template strategy. The concrete exhibits prominent water-repellency and admirable compressive strength simultaneously. The superhydrophobicity was well maintained after cyclic robustness evaluations, including sandpaper abrasion, knife scratch, water impacting, corrosive solution immersion, and UV radiation. The superhydrophobic concrete also exhibits broad-spectrum self-cleaning capability, such as easy removal of common dust, bacteria detachment, and resistance of ice formations. We believe that the SH concrete can potentially address the mechanical robustness and long-term water repellency towards the real applications in the future.

Corrosion performance of carbide/nitride/oxide (C/N/O)-based reactor during thermochemical hydrogen production by Na redox reaction

The corrosion of carbide/nitride/oxide(C/N/O)-based reactor materials were investigated to solve the corrosion problems in sodium redox reactions for thermochemical hydrogen production in this work. Various types of carbides, nitrides, and oxides were used as the potential materials of reaction containers for the sodium redox cycle to expect strong corrosion resistance. Comparison of corrosion of materials was performed by heat treatment with Na2O at 500 °C and corrosive phases were identified by X-ray diffraction. All the carbides materials showed the low corrosion resistance, while AlN among all the nitrides and Al2O3 among all the oxides were proved to possess high corrosion resistance with a very less corrosive phase from XRD results and non-discoloration of the test plate. The order of corrosion resistance of various materials was determined from the width of the corrosive layer observed by SEM and from the evaluation of Gibbs energy corresponding to the corrosion reaction from the thermodynamic point of view. The order of corrosion resistance was found to be AlN > Al2O3> ZrO2> TiO2. As a result, AlN was the most corrosive-resistant material according to SEM, XRD and EDS methods.

Flat Absorber Black PEO coatings on Ti6Al4V for spacecraft thermal control application

Black appearance and flat absorber, i.e., high solar absorptance (αs) and high IR emittance (εir) surfaces, are essential for optical benches and optical mounts in astronomical and scientific payloads/spacecraft. Generally, these optical benches and optical mounts are fabricated by Ti6Al4V alloy due to its superior mechanical and thermal properties. However, bare Ti6Al4V is restricted for spacecraft thermal control applications due to its low emittance, intermediate absorptance, and moderate corrosion resistance behaviors. Hence, plasma electrolytic oxidation (PEO) technique was utilized to develop black and flat absorber coatings on bare Ti6Al4V alloy. Sodium silicate (SS), sodium hypophosphate (SHP), and potassium fluoride (KF) were used for the preparation of the electrolytic bath. Process parameters such as current density, duty cycle, frequency, the concentration of electrolytes, and duration were judiciously altered to get uniform PEO coatings with the flat absorbing property. Six numbers of PEO coatings were finally chosen for further in-depth characterizations as they showed adequate flat absorbing property. The phase composition, microstructure, roughness, and wettability of those PEO coatings were thoroughly studied by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), profilometry, and water contact angle measuring techniques, respectively. Thermo-optical properties viz., εir and αs of the coatings were recorded by portable IR emissometer and solar reflectometer, respectively, and electrical sheet resistance was measured by two probe method. The Nanomechanical properties of the coatings, such as modulus (E) and nanohardness (H), were evaluated by the nanoindentation technique. The PEO coatings were also investigated with respect to potentiodynamic polarization to investigate corrosion resistance of the PEO coatings. The highest εir value of PEO coating was achieved as 0.87 for a 20% duty cycle with a combination of SS + SHP electrolytes, while the maximum αs value was achieved as 0.83 for the same 20% duty cycle with a combination of SS + SHP + KF electrolytes.

Erosion Resistant Hydrophobic Coatings for Passive Ice Protection of Aircraft

Novel polymeric coatings, namely slippery polyurethane (SPU) coatings, with high surface hydrophobicity and superior erosion resistance against high speed solid particles and water droplets were successfully developed to protect the leading edge of fast moving aerodynamic structures, such as aircraft wings and rotor blades, against ice accretion. The coatings comprise newly synthesized surface-modifying polymers (SMPs) bearing fluorinated and polydimethylsiloxane branches at a loading level of 1–5 wt.%, based on the total resin solid, which showed good compatibility with the erosion-resistant polyurethane matrix (PU-R) and rendered effective surface hydrophobicity and slipperiness to the coatings, as evidenced by the high water contact angles of 100–115°. The coatings can be easily be sprayed or solution cast and cured at ambient temperature to provide highly durable thin coating films. X-ray photoelectron spectroscopy (XPS) investigation showed concentration of fluorine on the surface. The presence of 1–5 wt.% of SMPs in the polyurethane matrix slightly reduced the tensile modulus but had no significant impact on the tensile strength. All coating films exhibited good thermal stability with no material softening or degradation after heating at 121 °C for 24 h. DSC heating scans revealed no thermal transitions in the temperature range of −80 °C to 200 °C. Ice adhesion strength (IAS) tests using a static push rig in a cold room of −14 °C showed IAS as low as 220 kPa for the SPU coatings, which is much lower than that of PU-R (i.e., about 620 kPa). Sand erosion tests using 50 μm angular alumina particles at an impinging speed of 150 m/s and an impinging angle of 30° revealed very low erosion rates of ca. 100 μg/g sand for the coatings. Water droplet erosion tests at 175 m/s using 463 μm droplets with 42,000 impingements every minute showed no significant coating removal after 20 min of testing. The combination of the high surface hydrophobicity, low ice adhesion strength and superior erosion resistance makes the SPU coatings attractive for ice protection of aircraft structures, where the coatings’ erosion durability is of paramount importance.

A review on surface coating techniques on Mg based bio-degradable implants

Cobalt-chromium alloys, Stainless steel and alloys of titanium are the traditional implant materials used in orthopaedic applications. However, these implants induce stress shielding effect and release harmful ions in the human body. Also, on the completion of bone healing, these must be removed by performing additional surgery. Magnesium (Mg), being biocompatible and biodegradable, can be a potential replacement for these traditional implants. But, Mg is having low corrosion resistance in the physiological environment. Surface coating proves to be an elixir to ameliorate the corrosion resistance of Mg. Biocompatibility and biodegradability are the paramount requirements of the coating material. Therefore, Hydroxyapatite (HA) is preferential material for coating. This review is intended to study the microstructure, characteristics and degradation behaviour of HA-coats deposited on Mg-based implants by electrochemical methods. The beneficiaries of this study will have a guideline for selecting particular compatible process and parameters to maximise the performance of implants.

Progress of laser surface treatment on magnesium alloy.

Magnesium (Mg) metals have been widely used in various fields as one of the most promising lightweight structural materials. However, the low corrosion resistance and poor mechanical properties restrict its applications. Surface treatments are common approach to enhance the mechanical strength and corrosion resistance of Mg metals. Among them, laser surface treatment generates novel tissues and structures in situ on the sample surface, thereby improving properties of mechanical strength and corrosion resistance. We briefly describe the changes in surface organization that arise after laser treatment of Mg surfaces, as well as the creation of structures such as streaks, particles, holes, craters, etc., and provide an overview of the reasons for the alterations. The effect of laser processing on wettability, hardness, friction wear, degradation, biocompatibility and mechanical properties were reviewed. At last, the limitations and development trend of laser treatment on Mg metals research were further pointed out.

Latest research and developments of ceramic reinforced magnesium matrix composites—A comprehensive review

High-performance lightweight materials give rise to a demand for magnesium-based composites. Magnesium metal matrix composites have found extensive applications in the aerospace and automotive industries due to their remarkable mechanical properties. Limitations in dispersion, strength, and interface strength of magnesium-based composites are seen through the addition of some hybrid reinforcements and different fabrication techniques. In this regard, previous studies have demonstrated the ability of various reinforcements such as graphene, carbon nano tubes, silicon carbide, and titanium carbide to a magnesium matrix for the enhancement of its metallurgical properties. There are still several challenges in the development of magnesium metal matrix composites, such as non-homogeneous distribution, poor creep resistance at elevated temperatures, limited cold work ability, and low corrosion resistance. The challenges that need to be overcome and suggestion for improving the wear resistance and frictional properties of magnesium metal matrix composites were studied and discussed. This review evaluates the importance of different reinforcement percentages as well as the development of magnesium metal matrix composites. The different types of fabrication techniques that are well suited to overcome the challenges of poor dispersing, non-homogeneous distribution, interfacial problems, and poor wettability are discussed. Microstructure analysis, the agglomerating effect, and matrix bonding strength are also discussed. The challenges and future scope of research are discussed for the demonstration of the importance of more scientific studies in magnesium metal matrix composites.

Additive manufacturing of corrosion-resistant maraging steel M789 by directed energy deposition

Conventional maraging steels feature a combination of high strength and toughness, but they often suffer from low corrosion resistance. Hence, maraging steel M789 was developed to alleviate this issue while maintaining its strength. Most studies of maraging steels processed by additive manufacturing (AM) focus on utilizing laser powder bed fusion (LPBF). However, the research in laser-directed energy deposition (L-DED) fabrication of corrosion-resistant maraging steels is limited. The different cooling rates experienced by materials during L-DED and LPBF processing give rise to differing microstructures and mechanical properties. In this study, the L-DED process was adopted to manufacture nearly-fully dense M789 parts, which were subsequently subjected to direct aging and solutionizing + aging heat treatments. Electron backscatter diffraction (EBSD) analysis reveals a martensitic structure in both as-fabricated and heat-treated samples with the presence of austenite in the as-fabricated and directly aged samples. Scanning transmission electron microscopy (STEM) and transmission Kikuchi diffraction (TKD) reveal the presence of Ti- and Al-rich precipitates within the martensites after the solution and aging treatment, suggesting that Orowan looping around precipitates, grain boundary strengthening, and solid solution strengthening are responsible for the high yield strength of L-DED M789. Besides, the as-fabricated alloy shows higher pitting potential than solutioned and aged sample. This work serves as a guidance for the fabrication of corrosion-resistant maraging steels by L-DED and accelerate the implementation of maraging steels for marine and offshore applications.

AYVALITOHMA ÇAYI HAVZASI AŞAĞI ÇIĞIRININ JEOMORFOLOJİSİ

Ayvalıtohma Çayı Havzası, Fırat Nehri’nin batıdaki kollarından olan Tohma Çayı’nın alt havzası durumundadır. Darende- Hekimhan (Malatya) Gürün, Kangal (Sivas) arasında kalan çalışma alanının jeomorfolojik özellikleri saha çalışmaları ve sentetik yüzeyler üzerinde yapılan jeomorfolojik analizler çerçevesinde belirlenmiştir. Çalışma alanı genel olarak Uzunyayla Platosu ile Malatya Havzası arasında kalan ve akarsularla derin bir şekilde yarılmış platolardan oluşmaktadır. Havzanın batısındaki Köroğlu, doğusundaki Akbabaçalı, güneyindeki Cuma Dağı gibi dağ sıraları ise bu platolar üzerinde yumuşak sırtlar halinde yükselmektedir. Ayvalıtohma Çayı, açtığı dar ve derin boğaz ve kanyon vadilerle, Uzunyayla, Ayvalı-Kuluncak, Darende-Balaban depresyonlarını birbirine bağlamaktadır. Gürün, Darende, Kuluncak dolaylarında tipik olarak görülen ve karstik temele gömülen bu boğaz ve kanyonlar, Ayvalıtohma Çayı boyunca gelişen taşkın ovalarının oluşumunda önemli bir yerel kaide seviyeyi rolü üstlenirler. Çalışma alanının merkezini oluşturan Ayvalı-Kuluncak Depresyonu ise Oligosen’de gelişmiş dağ arası havza özelliği taşımaktadır. Oligosen’de depolanan ve aşınım direnci düşük olan çakıltaşı, kumtaşı, çamurtaşı gibi detritikler havzada en fazla erozyona uğrayan alanları ortaya çıkarmışlardır. Bu dirençsiz kayalar erozyona neden olurken, dağlık ve plato sahalarındaki kalın kireçtaşı istifleri özellikle 1700-2100 m aralığında karstik şekillerin gelişmesine sebep olmuştur.

The effect of powder particle size on the corrosion behavior of atmospheric plasma spray-Y2O3 coating: Unraveling the corrosion mechanism by fluorine-based plasma

The atmospheric plasma spray (APS) -Y2O3 coatings are widely used to minimize damage to internal parts of the chamber caused by the strong corrosive effect of fluorine-based plasma used in dry etching; however, the poor density of APS-Y2O3 coatings results in low plasma corrosion resistance. This is a major cause of the generation of contamination particles during CF4/O2 plasma etching. Hence, defects inside the coating must be eliminated to improve the plasma corrosion resistance of APS-Y2O3 coatings.The correlation between the decrease in the number of defects inside the APS-Y2O3 coatings according to the powder particle size and the surface corrosion process by CF4/O2 plasma etching was confirmed. We verified that the internal defects in APS-Y2O3 coatings can be controlled with small powder particles because the powder particles in the APS layer melt efficiently under the APS coating conditions; in addition, the fluorine content rate inside the coating is improved. After the focused ion beam (FIB) process, by examining YOxFy layer with a field emission scanning electron microscope (FE-SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS); we observed the formation of a thick YOxFy layer with decreasing number of defects inside the APS-Y2O3 coatings; thereby, the bonding strength among the Y2O3 powder particles was increased, and corrosion of YOxFy layer due to physical and chemical reactions decreased; as a result, the etch amount was significantly reduced.Therefore, it can be seen that the generation of contamination particles in the CF4/O2 plasma etching can be reduced by controlling the occurrence of defects inside the APS-Y2O3 coatings. The results of this study demonstrate the possibility of minimizing the effect of contamination particles generated in the dry etching process to remove YOxFy layer using chamber self-cleaning such as waferless step. This paper presents a new direction for controlling contamination particles in the inner coating parts of the plasma etching chamber.

Development of titanium bipolar plates fabricated by additive manufacturing for PEM fuel cells in electric vehicles

Bipolar plates (BPs) are one of the main parts of proton exchange membrane (PEM) fuel cell stacks, which constitute a significant percentage of a PEM fuel cell system in terms of cost, weight, and structural strength. Although frequently used graphite BPs have low density, high conductivity, and high corrosion resistance, machining the desired flow channels on these plates is challenging. On the other hand, BPs made of various materials rather than graphite can be also fabricated by additive manufacturing methods. These methods can be considered as a reasonable alternative to conventional machining for the fabrication of graphite BPs in PEM fuel cells regarding material cost, fabrication of flow channels, and some post-processes in which the large-scale manufacturing of graphite BPs is more complex. This study offers a comparison of formed stainless-steel, additive manufactured titanium and machined composite graphite plates having the same flow-field geometry as a bipolar plate. In addition, titanium BPs are coated with gold and their performances are compared. Among the cells tested, the highest peak power of 639 mWcm−2 is measured from the cell with 450 nm gold coated titanium BP, whereas those of the cell with conventional graphite and stainless-steel BP are only around 322 mWcm−2 and 173 mWcm−2, respectively. Moreover, a new titanium bipolar plate design providing high specific power density is also presented.

Corrosion Resistance of Aluminum Alloy AA2024 with Hard Anodizing in Sulfuric Acid-Free Solution.

In the aeronautical industry, Al-Cu alloys are used as a structural material in the manufacturing of commercial aircraft due to their high mechanical properties and low density. One of the main issues with these Al-Cu alloy systems is their low corrosion resistance in aggressive substances; as a result, Al-Cu alloys are electrochemically treated by anodizing processes to increase their corrosion resistance. Hard anodizing realized on AA2024 was performed in citric and sulfuric acid solutions for 60 min with constant stirring using current densities 3 and 4.5 A/dm2. After anodizing, a 60 min sealing procedure in water at 95 °C was performed. Scanning electron microscopy (SEM) and Vickers microhardness (HV) measurements were used to characterize the microstructure and mechanical properties of the hard anodizing material. Electrochemical corrosion was carried out using cyclic potentiodynamic polarization curves (CPP) and electrochemical impedance spectroscopy (EIS) in a 3.5 wt. % NaCl solution. The results indicate that the corrosion resistance of Al-Cu alloys in citric acid solutions with a current density 4.5 A/dm2 was the best, with corrosion current densities of 2 × 10−8 and 2 × 10−9 A/cm2. Citric acid-anodized samples had a higher corrosion resistance than un-anodized materials, making citric acid a viable alternative for fabricating hard-anodized Al-Cu alloys.

Improving wear and corrosion protection of AISI 304 stainless steel by Al2O3-TiO2 hybrid coating via sol-gel process

ABSTRACT Al2O3, TiO2, and Al2O3-TiO2 coatings were deposited on AISI 304 stainless steel plates by the sol–gel dipping technique, and the effects of the coating composition on microstructure, wear, and corrosion characteristics were examined. Tribological features were determined by ball-on-disc sliding tests against an Al2O3 ball under a 10 N load, while corrosion properties were evaluated by potentiodynamic polarisation tests performed in 3.5% wt. NaCl solution. γ-Al2O3, α-Al2O3, and rutile phases were detected in the hybrid coatings. The highest coating thickness was obtained in single Al2O3 film and decreased with an increasing amount of TiO2 nanoparticles. The single Al2O3 layer had both the highest wear resistance and the lowest corrosion resistance. Although TiO2 addition caused an increase in the wear rate, it improved the corrosion resistance of the coating by filling the voids in the Al2O3 film. The Al2O3-TiO2 hybrid film with a relative ratio of 6:1 as a candidate for the wear- and corrosion-resistant coating decreased wear and corrosion losses of the 304 substrates by 83% and 97%, respectively.

Shot Peening Induced Corrosion Resistance of Magnesium Alloy WE43

Mechanical surface treatments like shot peening provide a pathway to stabilize degradation of magnesium alloys without compositional change. The resultant subsurface grain refinement, twin dislocations, and nanoprecipitation improves fatigue life but lowers corrosion resistance of magnesium. The lower corrosion resistance is justified by an increase in surface roughness after shot peening. However, a skewed estimate of surface area is expected to justify the inconsistent corrosion behavior of shot peening magnesium compared to other mechanical surface treatments. The objective of this research was to understand the influence of surface topography due to shot peening on the corrosion kinetics of a magnesium alloy WE43. Results showed that the corrosion resistance increased after shot peening WE43 when using an adjusted surface area that better accounts for the topographical features under exposure to corrosion attack.