Corrosion behaviors of coatings fabricated using bulk metallic glass powders with the composition of Fe68.5C7.1Si3.3B5.5P8.7Cr2.3Mo2.5Al2.0

Two different types of coatings were prepared, by a high velocity oxy-fuel spraying method and a laser spraying method, respectively, using bulk metallic glass powders with the nominal composition of Fe_68.5C_7.1Si_3.3B_5.5P_8.7Cr_2.3Mo_2.5Al_2.0. The corrosion behaviors of the two coatings in 1M HCl, H₂SO₄, NaCl and NaOH solutions were investigated based upon the microstructural differences originating from the different coating methods. The amorphous coating layer formed by the high velocity oxy-fuel spraying method exhibited higher, excellent corrosion resistance in the 1M HCl solution. The coating layer formed by the laser spraying method exhibited a high pitting tendency attributed to the dendritic microstructure with various borides and carbides. Due to a great number of pores, the HVOF coating exhibits slightly lower corrosion resistance than the LS coating in alkaline solution.     

Study on laser-surface melting to enhance intergranular corrosion resistance of SUS 304 weld

A laser-surface melting method was studied with the aim of increasing resistance to intergranular corrosion of welded SUS 304 stainless steel. An Nd:YAG laser beam at a laser beam size of about 1 mm and power of 170 Wm was employed and the laser power density was varied to optimize the depth of the laser melted layers. The microstructures of TIG welded and laser-surface melted regions were measured via optical, scanning electron, and transmission electron microscopy. In addition, a comparative evaluation of the intergranular corrosion properties of as-TIG welded and as-LSM surfaces of the SUS 304 weld was carried out using a doubleloop electrochemical potentiodynamic reactivation(DL-EPR) polarization method in a 1 L aqueous solution of 0.5 M H₂SO₄ and 0.01 M KSCN. According to the test results, the maximum melted depth was obtained at a beam scan rate of 600 mm/min and laser power density of 20 J/mm². The laser-surface melted (LSM) region was observed to have a very fine, homogenous, and cellular microstructure compared to that of the TIG welded region. Grain growth in the laser-surface melted region from the substrate occurred epitaxially. The absence of Cr depletion along the grain boundary in the LSM region, which would result in increased resistance to intergranular corrosion of the welded SUS 304, was confirmed using an energy dispersive X-ray spectroscope attached to the electron microscope.     

The initial stage of pitting corrosion on coated steels investigated by photon rupture in chloride containing solutions

A photon rupture method, film removal by a focused pulse of pulsed Nd-YAG laser beam irradiation, has been developed to enable oxide film stripping at extremely high rates without contamination from the film removal tools. In the present study, Zn-55mass%Al alloy and Al-9mass%Si alloy-coated steel specimens covered with protective nitrocellulose film were irradiated with a focused pulse of a pulsed Nd-YAG laser beam at a constant potential in 0.5 kmol m⁻³ H₃BO₃-0.05 kmol m⁻³ Na₂B₄O₇ (pH = 7.4) with 0.01 kmol m⁻³ of chloride ions to investigate the initial stage of localized corrosion. At low potentials, oxide films on both coated alloys were reformed after the nitrocellulose films were removed by this method. The oxide film formation kinetics follows an inverse logarithmic law, in agreement with Cabrera-Mott theory. However, at high potentials, localized corrosion producing corrosion products occurs at the area where nitrocellulose film was removed. Nevertheless, when the applied potential is less noble, the dissolution current of the Zn-55mass%Al-coated steel samples is higher than that of Al-9mass%Si-coated samples.     

Corrosion behavior of Zr-based metallic glass coating on type 304L stainless steel by pulsed laser deposition method

The surface morphology and corrosion behavior of Zr-based amorphous metallic glass (MG) of Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈ alloy and MG coated type 304L stainless steel in different nitric acid media of 1 M, 6 M and 11.5 M HNO₃ is reported. Zirconium based MG of Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈ alloy was successfully deposited on type 304L stainless steel using pulsed laser deposition technique. The SEM morphology revealed a scattered particles of “Donut” shaped features distributed in the amorphous matrix. The atomic force microscope measurement indicated the formation of dense metallic deposited layer of agglomerate of granular clusters with negligible pores or micro-crack in metallic glass coated sample. The results of the potentiodynamic polarization shows that the amorphous MG coated type 304L stainless steel exhibited marginally lower corrosion resistance than MG alloy which is attributed to the presence of corrosion-induced defects in the coated layer. This work reports suitability of using pulsed laser deposition for the preparation of thin film amorphous metallic coating to achieve improved corrosion resistance in nitric acid medium.     

Laser nanocrystallisation and corrosion behaviour of electroless Ni–W–P coating with high phosphorus content

This paper reports an investigation of microstructural characteristics in electroless Ni–3·9W–13·4P (wt.%) coating by laser nanocrystallisation using a quantitative X–ray diffraction (XRD) method and scanning electron microscopy (SEM) with energy dispersive X–ray spectroscopy (EDX). The corrosion resistance of the coatings before and after laser treatment has been evaluated in 0·5 M H₂SO₄ solution by potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) techniques. The results show that the laser treatment improves the corrosion resistance of the coatings. The corrosion mechanism has been studied and correlated to the microstructural characteristics including volume fraction of I_Ni3P/I_total, microstrain, and the crystallite size of the Ni and Ni₃P phases.     

Effect of laser repairing on corrosion behaviour of metal–inert gas welding joint of 7N01 aluminium alloy

The heat affected zone (HAZ) on the metal–inert gas (MIG) welding joint of 7N01 aluminium alloy was repaired by multipass narrow gap laser welding. The Y–X direction precracked three‐point bending sample was used in the alternate immersion test. The morphology of specimen surfaces demonstrated that the exfoliation corrosion in the HAZ after laser repair (HAZ_a) was more serious than that before laser repair (HAZ_b). The electrochemical impedance spectroscopy after different immersion corrosion time indicated that the HAZ_a and HAZ_b had similar corrosion potentials. However, the pitting corrosion resistance of HAZ_a was lower than that of HAZ_b at the beginning of exfoliation corrosion. The stress corrosion crack (SCC) of 7N01P‐T4 aluminium alloy displayed a multicrack source and an intergranular crack propagated along the rolling grain boundary under the test condition. An unusual method was taken to measure the length of SCC. The results showed that laser repairing did not weaken the stress corrosion resistance of the original joint.     

Microstructure and corrosion behavior of laser surface-melted high-speed steels

Three high-speed steels (HSSs) M2, ASP23, ASP30 were surface-melted by a CW 2.5-kW Nd:YAG laser. The microstructure of the laser surface-melted HSSs was investigated by optical microscopy, scanning electron microscopy and X-ray diffractometry, and the hardness profiles of the laser surface-melted layers were determined by a Vickers hardness tester. The corrosion behavior in 0.6 M NaCl and 0.5 M NaHCO₃ solutions at 25 °C was studied by potentiodynamic polarization technique. Metallographical as well as electrochemical corrosion studies illustrated the beneficial effects of laser surface melting (LSM) in refining the microstructure and in enhancing the corrosion resistance of the HSSs. The large carbide particles of annealed HSSs were dissolved after LSM and ultrafine dendrites of austenite and martensite with submicroscopic carbide precipitation were formed in the melt zones of the laser surface-melted HSSs. LSM of M2, ASP23 and ASP30 produced surface layers of hardness 615, 580 and 665 Hv, respectively. The hardness of the laser surface-melted ASP23 and ASP30 reached about 0.75 to 0.80 that of the conventionally hardened ones, while the hardness of laser-melted M2 was comparable to that of conventionally hardened M2. The corrosion resistance of all laser surface-melted HSSs in both solutions was significantly improved, as evidenced by a noble shift of the corrosion potential and a reduction in the corrosion current density. Among the HSSs, laser surface-melted ASP23 possessed the highest corrosion resistance in both solutions. The presence of cobalt in ASP30 has no beneficial effect on enhancing its corrosion resistance. The enhancement in the corrosion resistance of the laser surface-melted HSSs is attributable to the combined effects of dissociation and refinement of large carbides and the increase of the passivating alloying elements such as Cr, Mo and W in solid solution.     

The effect of processing gas on corrosion performance of electroless Ni-W-P coatings treated by laser

An investigation of the microstructural and corrosion characteristics of electroless Ni-5.5 W-6.5P coatings on steel substrates after laser treatment in argon and air is presented. The microstructural characteristics of the coatings, in terms of crystallisation, grain size, microstrain, porosity as well as surface chemistry, were examined using quantitative X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Electrochemical tests, using potentiodynamic polarisation in 0.5 M H₂SO₄ solution and electrochemical impedances spectroscopy (EIS) in 3.5% NaCl solution, were undertaken to evaluate the corrosion behaviour of the coatings. The results indicated that the laser-treated coatings consisted of nanocrystalline Ni and Ni₃P phases, along with retained amorphous phase; further, the dimensions of the Ni crystallites were larger than those of Ni₃P. The laser-treated coating in argon revealed the presence of submicron scale porosity, while no porosity was evident in the coating surface treated by laser in air. The uniform corrosion revealed in 0.5 M H₂SO₄ solution is mainly determined by the microstructural characteristics of the coating. Pitting corrosion in 3.5% NaCl solution depended on the amount of porosity on the surface. The laser-treated coating in air exhibited better corrosion resistance in both acidic and chloride environments than that laser-treated in argon.     

Microstructure,microhardness and corrosion resistance of laser cladding Ni–WC coating on AlSi5Cu1Mg alloy

The microstructure, microhardness, and corrosion resistance of laser cladding Ni–WC coating on the surface of AlSi5Cu1Mg alloy were investigated by scanning electron microscopy, X-ray diffraction, microhardness testing, immersion corrosion testing, and electrochemical measurement. The results show that a smooth coating containing NiAl, Ni₃Al, M₇C₃, M₂₃C₆ phases (M=Ni, Al, Cr, W, Fe) and WC particles is prepared by laser cladding. Under a laser scanning speed of 120 mm/min, the microhardness of the cladding coating is 9–11 times that of AlSi5Cu1Mg, due to the synergistic effect of excellent metallurgical bond and newly formed carbides. The Ni–WC coating shows higher corrosion potential (−318.09 mV) and lower corrosion current density (12.33 μA/cm²) compared with the matrix. The crack-free, dense cladding coating obviously inhibits the penetration of Cl⁻ and H⁺, leading to the remarkedly improved corrosion resistance of cladding coating.     

Intergranular corrosion resistance of nickel-based alloy 690 weldments

This study focused on the temperature distributions and thermal histories of alloy 690 weldments, as well as the residual stress, grain boundary character distribution, degree of sensitization, and carbide precipitation, to evaluate the combined effects of these factors on the intergranular corrosion resistance of the weldments. Both laser beam welding and gas tungsten arc welding were performed for comparison. The results show that laser beam welding, with its narrower temperature distribution and rapid heating/cooling, resulted in more low energy Σ (1 ≦ Σ ≦ 9) boundaries, lower residual stress, and considerable suppression of Cr₂₃C₆ carbide formation. Therefore, laser weldment had a low degree of sensitization, and thus the intergranular corrosion resistance was significantly improved.     

Corrosion behaviour of laser gas nitrided Ti–6Al–4V in HCl solution

Laser surface nitriding of Ti–6Al–4V alloy was carried out with a Nd:YAG pulsed laser. The microstructure and corrosion behaviour of the nitrided samples were examined, using SEM, XRD, XPS, and anodic polarization tests in 2 M HCl solution. Laser nitriding produced a thin continuous TiN layer followed by TiN dendrites and TiN_0.3 needles. The laser nitrided specimen exhibited less corrosion current density, passivated more readily and also, maintained a lower current density over the duration of the experiment. This was correlated with the formation of very thin, continuous TiN_xO_y film, which could retard chloride ions ingress into the substrate.     

Microstructure and corrosion properties of plasma-sprayed NiCr-Cr3C2 coatings comparison with different post treatment

The effects of laser and plasma arc remelting on the microstructure and properties of plasma-sprayed NiCr-Cr₃C₂ coatings on steel substrates have been investigated. The microstructure of the coatings has been analyzed by X-ray diffraction (XRD) and scanning electron microscope (SEM). It is found that the Cr₃C₂, δ-(Cr,Ni), Cr₇C₃ and Cr₂₃C₆ phases were obtained for both coatings, before and after remelting treatment. The laser remelting was operated in a continuous way with 800 W power in different scan speed, while the plasma arc remelting was operated with a plasma cladding machine under different scan currents. However, the denser microstructure of both remelted coatings can be obtained, especially for the plasma arc remelted coating. The Vickers microhardness measurement showed certain enhancement values for both remelted coatings. The corrosion behavior was evaluated through salt spray corrosion (SSC) method. Energy-dispersive spectroscopy (EDS) showed that the chloride was produced during SSC process. The higher corrosion resistance for plasma arc remelted coating may be due to the more compact microstructure, less porosity rate and tensile residual stress. Compared with laser remelting method, plasma arc remelting is a cheap, convenient and effective remelting method.     

Corrosion behaviour of AISI type 304L stainless steel in nitric acid media containing oxidizing species

The corrosion behaviour of AISI type 304L stainless steel (SS) in different concentration of 0.01 M, 1 M and 5 M HNO₃ in presence of oxidizing ions at different temperatures has been evaluated. The main objective of this study is to assess the corrosion resistance of type 304L SS in non-radioactive conditions encountered during storage of liquid nuclear waste. Electrochemical impedance spectroscopy (EIS) and laser Raman spectroscopy (LRS) has clearly brought out the deleterious effect of oxidizing species on the passive film leading to increased corrosion along with increase in HNO₃ concentration and higher temperature.     

The corrosion of potlining refractories: A unified approach

The study of corrosion chemistry of aluminosilicate refractories in potlinings has historically focused on a single corrosive agent: either molten sodium fluoride or metallic sodium. Each approach can explain only a fraction of in-field observations. This article presents a unified approach, taking into account the effect of both corrosion agents on aluminosilicates. Along with the model, mathematical tools have been developed to help interpret experimental results as well as to make predictions of in-service behaviors. Those predictions are based on the use of corrosion maps, which vary according to the sodium ratio [R_Na=N_Na(M)/(N_NaF+N_Na(M))], as well as other parameters. Predictions made with the model match almost perfectly with the results obtained in a wide range of corrosion conditions. Depending on the quantity of fluorides that a lining lets percolate and the amount of metallic sodium diffusing, cells can be divided into two distinct groups: wet or dry. This article discusses the wet/dry distinction, including industrial aspects. For more information, contact C. Allaire, CIREP-école Polytechnique, 8475 Christophe-Colomb, Montréal, Québec, Canada, H2M 2N9; e-mail claude.allaire@courriel.polymtl.ca.     

Inhibiting effects of butyl triphenyl phosphonium bromide on corrosion of mild steel in 0.5 M sulphuric acid solution and its adsorption characteristics

Butyl triphenyl phosphonium bromide (BuTPPB) has been evaluated as a corrosion inhibitor for mild steel in 0.5 M H₂SO₄ solutions using galvanostatic polarisation and potentiostatic polarisation measurements. The study was also complemented by infra red (IR) spectroscopy, scanning electron microscopy (SEM) and quantum chemical calculations. Galvanostatic polarisation measurements showed that the presence of BuTPPB in aerated 0.5 M H₂SO₄ solutions decreases corrosion currents to a great extent and the corrosion rate decreases with increasing inhibitor concentration at a constant temperature. At 298K, inhibition efficiency was found to be 94.5% for 10⁻⁷ M BuTPPB which increased to about 99% for the BuTPPB concentration of 10⁻² M. The effect of temperature on the corrosion behaviour of mild steel was studied at five different temperatures ranging from 298 to 338K. The polarisation curves clearly indicate that BuTPPB acts as a mixed type inhibitor. Adsorption of BuTPPB on the mild steel surface follows the Langmuir isotherm.Potentiostatic polarisation measurements showed that passivation was observed only for lower BuTPPB concentrations (10⁻⁵ and 10⁻⁷ mol l⁻¹) for the mild steel in 0.5 M H₂SO₄. IR and SEM investigations also confirmed the adsorption of BuTPPB on the mild steel surface in 0.5 M H₂SO₄ solutions. The molecular parameters obtained using PM3 semi-empirical method, were correlated with the experimentally measured inhibitor efficiencies.     

Microstructure and corrosion behaviour of Al2O3–13 wt-% TiO2 laser alloyed magnesium alloys

Nanostructured Al₂O₃–13?wt-% TiO₂ was prepared on AZ91D magnesium alloy surface by laser surface alloying to improve its corrosion resistance. The microstructure of the laser surface alloyed specimens before and after corrosion tests was characterised by scanning electron microscope and optical microscope (OM). The phase and element composition were investigated by X-ray diffractometer and energy-dispersive spectrometry. An electrochemical workstation was used to evaluate the corrosion behaviour of the specimens. Results showed that the laser surface alloyed layer was primarily composed of Mg and Mg₁₇Al₁₂. Al₂O₃ and TiO₂ existed in the form of agglomerated particles. The corrosion resistance was improved after laser surface alloying.     

The protective effect of polypyrrole coating on the corrosion of steel electrode in acidic media

In this study, the corrosion parameters of stainless steel containing 12% Cr, have been determined by Tafel extrapolation method in 1 M HCl, H₂SO₄ and H₃PO₄ media. Later, steel was coated with polypyrrole in 0.1 M Pyrrole + 0.3 M Oxalic acid solution by cyclic voltametric method. The corrosion parameters and percentage inhibition efficiencies of coated electrodes were investigated according to immersion times in the same media. In all acidic media studied, increases in immersion time, produced increased corrosion densities and a decrease in percentage inhibition efficiencies were determined.     

Open circuit potential transients and electrochemical quartz crystal microgravimetry measurements of dissolution of copper in acidic sulfate solutions

It is demonstrated that open circuit potential transients measurements are able to give the realistic and fast information on the rate of metal corrosion. The results of the studies of the rate of copper corrosion in naturally aerated unstirred acidic sulfate solutions obtained by this method are close to the results of electrochemical quartz crystal microgravimetry measurements. It was found that within the concentration range CuSO₄: 0.001-0.01 M, H₂SO₄: 0.005-0.5 M, independently of whether 0.1 M K₂SO₄ is present in the solution or not, the rate of corrosion of thin (about several tens monolayers) and fresh deposits of copper does not depend on the solution composition and is limited by the transport of the dissolved oxygen. The rate of corrosion of such deposits sometimes exceeds the rate of corrosion of compact copper samples.     

Fe-Based Amorphous Coatings on AISI 4130 Structural Steel for Corrosion Resistance

The current study focuses on synthesizing a novel functional coating for corrosion resistance applications, via laser surface alloying. The iron-based (Fe₄₈Cr₁₅Mo₁₄Y₂C₁₅B) amorphous precursor powder is used for laser surface alloying on AISI 4130 steel substrate, with a continuous wave ytterbium Nd-YAG fiber laser. The corrosion resistance of the coatings is evaluated for different processing conditions. The microstructural evolution and the response of the microstructure to the corrosive environment is studied using x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Microstructural studies indicate the presence of face-centered cubic Fe-based dendrites intermixed within an amorphous matrix along with fine crystalline precipitates. The corrosion resistance of the coatings decrease with an increase in laser energy density, which is attributed to the precipitation and growth of chromium carbide. The enhanced corrosion resistance of the coatings processed with low energy density is attributed to the self-healing mechanism of this amorphous system.     

FIB-SEM investigation on corrosion propagation of aluminium–lithium alloy in sodium chloride solution

Abstract

Microstructural characterisation of 2A97-T4 aluminium–lithium alloy was carried out using electron probe microanalysis and transmission electron microscopy (TEM). Scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy facilities has been employed to examine localised corrosion sites after immersion in sodium chloride solution. A dual beam microscope, which integrates a focused ion beam and an electron beam in one powerful instrument, has also been employed to investigate the development of intergranular corrosion from both surface and cross-section. It was found that localised corrosion is generally initiated at θ phase particles, which represents only 8.4% of the intermetallic (IM) particles in 2A97-T4 aluminium–lithium alloy. θ phase particles exhibit preferential dissolution of aluminium during corrosion testing, with trench formed at their periphery as well. Initiation of intergranular corrosion is relatively late with respect to the attack of IM particles. Owing to the presence of θ phase particles at intergranular corrosion sites and non-uniform distribution of T1 (Al₂CuLi) grain boundary precipitates, it is supposed that dealloyed θ phase particles and grain boundary precipitates cooperate to provide the driving force for grain boundary attack.