Correlation between current frequency and electrochemical properties of Mg alloy coated by micro arc oxidation

The present work investigated the electrochemical behavior of Mg alloy subjected to micro arc oxidation coating for 120 s with respect to current frequencies from 60 Hz to 2000 Hz. The microstructure and chemical compound of thin coating layers with a thickness of ~ 3 μm were analyzed using scanning electron microscopy and X-ray photoelectron spectroscopy. The microstructural observations on the surface and cross sections revealed that both the size of pores and the number of discharge channels decreased significantly as the current frequency increased, resulting in a compact coating layer. This was primarily attributed to the transition time of the alternating electrical wave, which was determined by the current frequencies tested. Based on potentiodynamic polarization tests, the sample coated at a frequency of 2000 Hz demonstrated the highest polarization resistance of 6.37 × 10⁵ Ω cm², implying that the corrosion resistance was superior to that obtained under other conditions due to its condensed structure. This electrochemical response was also interpreted in relation to the equivalent circuit model.     

Effect of Si addition to CrN coatings on the corrosion resistance of CrN/stainless steel coating/substrate system in a deaerated 3.5 wt.% NaCl solution

CrSiN coatings with different Si concentration (Si/(Cr + Si) ratio: 0, 3.7, 11.7, 20%) were deposited on stainless steel substrates using a closed field unbalanced magnetron sputtering (CFUBM) system. The variation in the microstructure of the films with the Si concentration was measured by XRD. The corrosion behavior of the CrSiN coatings in a deaerated 3.5% NaCl solution was investigated by potentiodynamic tests, electrochemical impedance spectroscopy (EIS) and surface analyses. The microstructure of the CrSiN film was found to depend on the Si concentration. The results of the potentiodynamic polarization tests showed that the corrosion current density and porosity decreased with increasing Si/(Cr + Si) ratio. The EIS measurements showed that the corrosion resistance of the Si-bearing CrN was improved by the phase transformation of the film, which led to an increase in the pore resistance and charge transfer resistance. The Si-bearing CrN possesses the best corrosion resistance at a Si/(Cr + Si) ratio of 20%, measured by the maximization of the pore resistance and charge transfer resistance.     

Fretting corrosion characteristics of electrodeposited and hot dipped tin coating contacts

Electrodeposited (ED) tin was coated on the copper substrate for electrical contact instead of conventional hot dipped (HD) tin and subjected to fretting tests. The fretting wear behavior was investigated with various fretting cycles at ± 25 μm displacement amplitude, 0.5 N normal load, 3 Hz frequency, 45-50% relative humidity, and 25 ± 1 °C temperature. The contact resistance variation was recorded with fretting cycles. The fretting corrosion performance of ED tin enhanced with that of HD tin. The grain structure of ED tin was not removed even at 9000 fretting cycles, whereas HD tin removed at 1000 fretting cycles. The interdependence of extent of wear and oxidation on the fretted zone increases the complexity of the fretting corrosion behavior. The extent of wear and the elemental distribution at the center and edges of the fretting zone was characterized using scanning electron microscope (SEM) and energy dispersive analyzer of X-ray (EDAX). The surface profile of the fretted surface was examined using laser-scanning microscope (LSM). The fretting behavior difference between ED tin and HD tin was correlated to the grain structure as well as the absence of the abrasive debris in ED tin.     

Corrosion and mechanical properties of epoxy-polyurethane/bronze composite coatings with low infrared emissivity

Low infrared emissivity coatings were prepared by using epoxy-polyurethane (EPU) and bronze as adhesives and fillers, respectively. Due to the increasing electrical conductivity with increasing bronze content, the infrared emissivity is decreasing obviously. Mechanical properties of EPU/bronze composite coatings were studied by using adhesion test and impact strength test. We found that EPU/bronze composite coating had good adherence and impact strength at bronze content below 50 wt.%, and then the mechanical properties decreased in the bronze content range from 50 wt.% to 60 wt.%. Moreover, the composite coatings were tested for the corrosion behavior with potentiodynamic polarization. Corrosion test results showed that the low emissivity EPU/bronze composite coatings exhibited favorable corrosion resistance. By comparing EPU/bronze, PU/Cu, (ball-milled Ag-Cu)/PU and PU/Al coatings, the EPU/bronze coatings with 40 wt.% bronze have the best adherence, low infrared emissivity and good corrosion resistance.     

Corrosion properties of micro- and nanocomposite copper matrix coatings produced from a copper pyrophosphate bath under pulse current

The aim of this work was the deposition of copper matrix composites under direct and pulse current at different frequencies and the evaluation of their protective properties, in the frame of the research domain of the production of metal matrix composite coatings.The deposits were produced using copper pyrophosphate bath in which 20 g/l of either micro- (mean diameter 2 μm) or nano- (mean diameter 45 nm) SiC particles have been added. A squared current waveform was used with the frequencies 0.01, 0.1, 1, and 10 Hz for the pulse current deposition.The microstructure of all deposits, both on the top surface and in cross section, was studied by Scanning Electron Microscopy. The SiC content was evaluated by EDXs in the case of the micro-composite deposits and by Glow Discharge Optical Emission Spectroscopy for the nanocomposite ones. The protective properties were examined by potentiodynamic measurements in different corrosive environments and by salt spray exposure combined with Electrochemical Impedance Spectroscopy measurements.Both the nanoparticles incorporation and the use of pulse current lead to a noticeable grain refinement and thus to a corrosion resistance increase. The nanocomposite deposits present the highest resistance to both uniform and localized corrosion, strongly correlated to their compact structure. The micro-composite deposits produced under direct current present gaps between the particles and the copper matrix, which lower the corrosion resistance. The use of the pulse current managed to partially close these gaps and increase the corrosion resistance to values similar to those of the pure copper deposits.     

Development of high Mn-N duplex stainless steel for automobile structural components

A new high Mn-Ni free (duplex stainless steel) DSS containing 18Cr-6Mn-1Mo-0.2N has been developed by examining the effects of manganese on the corrosion and mechanical properties of high Mn SSs containing 18Cr-4 ∼ 11Mn-0 ∼ 2Ni-0 ∼ 1Mo-0.2N. The alloy with 45% ferrite is found to be an optimum alloy with much higher mechanical strength and similar corrosion resistance compared with those of standard SS304. In addition, the alloy was free of precipitation of sigma phase and Cr-nitride when exposed to high temperatures due primarily to relatively low contents of Cr, N and Mo. With an increase in Mn content, the resistance to pitting and metastable pitting corrosion of high Mn DSS decreased since the number of (Mn, Cr) oxides, acting as preferential sites of pitting, increased with the Mn content.     

The beneficial role of Y-implantation on the aqueous corrosion of stainless steel

The corrosion behaviour of Y-implanted austenitic stainless steel AISI 321 samples was investigated in 0.5 M H₂SO₄ at ambient temperature using potentiodynamic polarization and cyclic voltammetry. The implantation of 1 × 10¹⁶ Y-ions/cm² of 40 keV energy did not lead to an improvement of the corrosion resistance of the material because of sputtering effects. On the other hand, a significant improvement of the corrosion resistance was observed by increasing of the dose (2 × 10¹⁷ Y-ions/cm² implanted in the presence of oxygen) and the implantation energy (55 and 80 keV). The elemental composition of the near-surface layers of the implanted steel samples prior and after the corrosion attack was determined by Rutherford backscattering spectrometry (RBS) and Nuclear Reaction Analysis (NRA) using alpha particles, protons and deuterons as projectiles. The surface morphology and microstructure of the non-corroded and corroded samples were examined by Scanning Electron Microscopy (SEM). The corrosion resistance of the implanted materials was found to be related with the thickness and the composition of the implanted layer.     

Corrosion behavior of nickel alloyed and austempered ductile irons in 3.5% sodium chloride

In this study, the nickel alloying and austempering effects on corrosion behavior of ductile irons were investigated. The microstructure of austempered ductile iron (ADI) was analyzed by XRD, and the polarization corrosion tests were conducted using 3.5 wt.% NaCl solution. The results showed Ni-alloyed as-cast has less nodule counts than the unalloyed one; therefore, the former is more corrosion resistant than the latter. For the ADI, the nickel addition increases the retained austenite content, resulting in having better corrosion inhibition than the unalloyed ADI. Comparatively, the order of corrosion resistance in 3.5 wt.% NaCl solution is as follows: 4%Ni-ADI > ADI > 4%Ni-DI > DI.     

Corrosion and passivation of low-temperature nitrided AISI 304L and 316L stainless steels in acidified sodium sulphate solution

304L and 316L steels were nitrided at 425 °C for 30 h and examined at various depths in 0.1 M Na₂SO₄ acidified to pH 3.0. In the near-surface region with about 7-14 wt% N, at potentials of active state anodic currents were much higher than those for untreated steels, whereas in deeper regions with <7 wt% N the currents were only slightly increased in comparison with untreated steels or they were even lower in passive and transpassive states. Surface films were composed of oxygen-containing species on top and of Cr-N species in deeper layers. It is suggested that strong corrosion of near-surface regions is associated with nitride precipitates. Beneficial effect of low nitrogen concentrations can be due to initially accelerated corrosion which leads to larger amounts of passivating species and to the accumulation of corrosion resistant chromium nitrides.     

Corrosion behavior of CrN, Cr2N and π phase surfaces on nitrided Ni-50Cr for proton exchange membrane fuel cell bipolar plates

The relationships between nitridation, microstructure and corrosion were studied for a thermally nitrided model Ni-50Cr (wt.%) alloy to gain insight into the protection of metallic bipolar plates in proton exchange membrane fuel cells (PEMFCs). Surface layers containing CrN, Cr₂N or ternary Cr-Ni-N π phase nitride were formed, dependent on the nitridation conditions. Studies in aerated pH 3 sulfuric acid at 80 °C indicated that the best corrosion resistance was exhibited by CrN + Cr₂N surface layers, which were more corrosion resistant than Ni-50Cr metal, Cr metal, and nitrided Cr. X-ray photoelectron spectroscopy (XPS) analyses indicated oxygen enrichment of the polarized surfaces in the form of Cr-oxide and Cr-oxynitride species. Such oxygen enrichment does not appear to result in detrimental increases in interfacial contact resistance/electrical properties of the surface.     

The application of electrochemical noise resistance to evaluate the corrosion resistance of AISI type 304 SS in nitric acid

The paper presents the application of noise resistance to evaluate the corrosion behaviour of sensitized AISI type 304 SS in nitric acid of varying concentration (4 N, 12 N, 16 N) and temperature (298 K, 323 K, 348 K). Electrochemical noise data was acquired from a three identical electrode configuration in the required conditions at open circuit potential. The noise resistance was evaluated as the ratio of the standard deviation of the potential to that of the current noise after removing the DC component. The inverse relationship between noise resistance and corrosion rate was exploited to qualitatively assess the corrosion behaviour of AISI type 304 SS in nitric acid. Noise resistance decreased with increase in concentration implying an increase in corrosion rate with increase in nitric acid concentration. An increase in temperature from 298 K to 323 K and 348 K decreased the noise resistance in 4 N and 12 N nitric acid implying higher corrosion rates at higher temperatures. The corrosion rates were similar at 323 K and 348 K in these concentrations. The simultaneous measurement of current and potential noise facilitated the evaluation of the frequency dependence of the noise data to determine the spectral noise resistance (R_sn) and the DC limit of the spectral noise resistance . The results from R_sn and also indicated higher corrosion rates at higher concentration and temperature. Also R_n and correlated well in 4 N and 12 N nitric acid at 323 K and 348 K while disparity was observed at room temperature in 4 N and 12 N nitric acid.     

The corrosion performance of anodised magnesium alloys

The corrosion performance of anodised magnesium and its alloys, such as commercial purity magnesium (CP-Mg) and high-purity magnesium (HP-Mg) ingots, magnesium alloy ingots of MEZ, ZE41, AM60 and AZ91D and diecast AM60 (AM60-DC) and AZ91D (AZ91D-DC) plates, was evaluated by salt spray and salt immersion testing. The corrosion resistance was in the sequential order: AZ91D ≈ AM60 ≈ MEZ ? AZ91D-DC ? AM60-DC > HP-Mg > ZE41 > CP-Mg. It was concluded the corrosion resistance of an anodised magnesium alloy was determined by the corrosion performance of the substrate alloy due to the porous coating formed on the substrate alloy acting as a simple corrosion barrier.     

Liquid metal corrosion of T91 and A316L materials in Pb-Bi eutectic at temperatures 400-600 °C

The corrosion resistance of T91 and A316L materials was tested in stagnant liquid lead-bismuth eutectic (LBE). The materials were exposed for 175, 500, 1250, 2300 and 3000 h at temperatures from 425 to 600 °C under 5%H₂ + Ar cover gas atmosphere. Severe corrosion occurred at temperatures above 500 °C where three corrosion modes were distinguished: stable oxide film mode, transition mode, and final dissolution mode featuring Cr and Ni leaching and material loss. The principle corrosion mechanisms were uniform penetration and dissolution of the penetrated volumes. At these temperatures (>500 °C) T91 had a better corrosion resistance (corrosion rate ∼ ?137 μm/year) compared to A316L (?250 μm/year). The transition corrosion mode continued 2-3 times longer for T91 material due to residual oxides found even after 3000 h of exposure. At low temperatures (<450 °C) both materials showed good corrosion resistance but A316L performed better than T91 with corrosion rates 2-5 times lower.     

Effects of non-metallic inclusions on the initiation of pitting corrosion in 11% Cr ferritic stainless steel examined by micro-droplet cell

This paper explores the effects of non-metallic inclusions (NMIs) on the initiation of pitting corrosion in type 409L stainless steels refined by the argon oxygen decarburization (AOD) and vacuum oxygen decarburization (VOD) processes. The dominant NMIs in the AOD and VOD samples were (Ti, Ca)-oxides and Ti-nitrides, respectively. In-situ electrochemical noise (EN) and micro-electrochemical analyses were conducted to investigate quantitatively the inherent effects of the NMIs on the pitting corrosion of the alloys. Pitting corrosion was initiated mostly around the (Ti, Ca)-oxides in the AOD samples, while little such corrosion occurred around the Ti-nitrides in the VOD samples. In addition, the pitting resistance of the AOD samples increased with increasing Ti content and decreasing Ca content in the (Ti, Ca)-oxides.     

Metallurgical assessment of critical defects in continuous hot dip galvanized steel sheets

Development of hot dip zinc coated sheets in new applications increased demand for production of high quality galvanized coatings, but the presence of surface defects reduces the quality of these products. In order to alleviate the problem, one needs to know the extent to which the properties of a galvanized sheet are influenced by the presence of a given defect. In this paper, specimens including any of the four major defects of continuously galvanized steel sheets produced in an industrial continuous process have been studied. The defects, including scratches, bare spots, pimples and wrinkle bands, were microstructurally characterized and their influence on corrosion behavior of the coated sheet was evaluated. The defects, originating from insufficient cleaning procedure, improper quality of steel substrate or adhered metallic particles to the substrate surface, exert their main effects on corrosion resistance and surface quality. Corrosion behavior was examined via standard salt spray test and polarization test. Based on the experimental results, it was concluded that the corrosion resistance was influenced by severity of defects; bare spots reduced the overall corrosion resistance of galvanized sheet by 39% ± 1% and pimples by 10% ± 1% as compared to defect free specimens.     

Effect of Mo and Co additions on the microstructure and properties of WC-TiC-Ni cemented carbides

In this work, the effects of 1.0 wt.% additions of Mo and Co on the microstructure and properties of WC-TiC-Ni cemented carbides were investigated using scanning electron microscope, mechanical properties tests, corrosion resistance and abrasion resistance tests. The results show that 1.0 wt.% Mo addition can refine the WC grains and increase the hardness. Moreover, with the addition of minor Mo, the corrosion resistance and abrasion resistance of alloys improved significantly. The addition of 1.0 wt.% Co can inhibit the growth of WC grains, improve the density and hardness slightly, and enhance the abrasion resistance of cemented carbides. However, the minor Co has negative effect for the corrosion resistance.     

Investigation of Plasma Electrolytic Oxidation (PEO) coatings on a Zr-2.5Nb alloy using high temperature/pressure autoclave and tribological tests

A Plasma Electrolytic Oxidation (PEO) process was used to produce thin oxide coatings on a Zr-2.5 wt% Nb alloy. Effects of current density on surface morphologies and wear properties of PEO coatings were investigated and compared to the uncoated substrate and a commercially used black oxide coating. Corrosion properties at ambient and high temperature/pressure conditions were studied using potentiodynamic polarization tests and autoclave tests, respectively. Up to 30-day autoclave experiments were carried out in an aqueous condition of 300 °C and 10 MPa in 0.05 M LiOH solutions. It was found that most of the micro-pores which were produced during the PEO treatment were closed after the autoclave experiments. PEO coatings had larger weight gains in the first 10 exposure days than the black oxide coating. However, after 10 days, the corrosion rate of black oxide coating accelerated and exhibited a similar weight gain to PEO coatings after 30 days. PEO coatings prepared at low current densities had lower weight gains. Although the black oxide coating exhibited a good corrosion resistance, it had a much lower wear resistance than the PEO coatings. Compared with the uncoated substrate, all PEO coatings had a higher corrosion resistance, lower weight gain during autoclave tests and better wear resistance.     

Enhanced corrosion resistance of WC-Co with an ion beam mixed silicon carbide coating

Strong adhesion of a silicon carbide (SiC) coating to a WC-Co substrate was achieved through an ion beam mixing technique and the corrosion resistance of the SiC coated WC-Co was investigated by means of a potentiodynamic electrochemical test. In a 1 M NaOH solution, the corrosion current density of SiC-coated WC-Co after heat treatment at 500 °C was about 50 times lower than that for the as-received WC-Co. In addition, the corrosion resistance systematically increases with increasing the SiC coating thickness. On the other hand, for a 0.5 M H₂SO₄ solution, the corrosion current density for SiC-coated WC-Co was about 3 times lower than that for the as-received WC-Co. We discuss the physical reasons for the changes in the corrosion current density with the different electrolytes.     

Corrosion behaviour and characteristics of reforming chromized coatings on SS 420 steel in the simulated environment of proton exchange membrane fuel cells

Low-temperature pack chromization is utilized in combination with various activating pretreatment methods, including rolling and electrical discharge machining, to improve the performance of SS 420 stainless steels in the simulated environments of proton exchange membrane fuel cells (PEMFCs). The corrosion resistance and hydrophobicity of chromized steels are enhanced with the pretreatments. The rolled and chromized steel possessing a continuous, uniform, and hydrophobic coating exhibits the best corrosion resistance and stability among all tested specimens. Furthermore, the specimen exhibits the smallest interfacial contact resistance at a compaction pressure of 140 N cm⁻², among all tested specimens.     

Influence of the β phase on the corrosion performance of anodised coatings on magnesium-aluminium alloys

The effect of the β phase in Mg-Al alloys on the corrosion performance of an anodised coating was studied. It was found that the corrosion resistance of the anodised coating was closely associated with the corrosion performance of the substrate alloy. In particular, Mg alloys with a dual phase microstructure of α + β with intermediate aluminium contents (namely 5%, 10% and 22% Al) after anodisation had the highest corrosion rate and the worst corrosion resistance provide by the anodised coating. The poor performance of an anodised coating was attributed partly to lower corrosion resistance of the substrate alloy and partly to the higher porosity of the anodised coating.