Surface Repassivation Research Articles

Effect of Single Particle High-Speed Impingement on the Electrochemical Step Characteristics of a Stainless-Steel Surface.

In the process of particle erosion and electrochemical corrosion interaction, the electrolyte flow state change, product film destruction, and matrix structure change caused by particle impact affect the electrochemical corrosion process. Such transient, complex physical and electrochemical changes are difficult to capture because of the short duration of action and the small collision area. The peak, step time, and recovery time in this transient step cycle can indirectly reflect the smoothness and reaction rate of the electrochemical reaction system, and thus characterize the resistance to scouring corrosion coupling damage of metals in liquid-solid two-phase flow. In this study, in order to obtain the electrochemical response at the moment of particle impact, electrochemical monitoring experiments using a specially designed miniature three-electrode system were used to test step-critical values, including step potential, current, and resistance, among others. Meanwhile, an electrochemical step model under particle impact considering boundary layer perturbation was developed. The experimental results reflect the effect law of particle impact velocity and particle size on the peak step and recovery period. Meanwhile, the effect of particle impingement on the electrochemical step of stainless steel in different electrolyte solutions was obtained by comparing the step curves in distilled water and Cl-containing water. The connection between the parameters in the electrochemical step model and in the particle impact, as well as the effect of the variation of these parameters on the surface repassivation process are discussed in this paper. By fitting and modeling the test curves, a new mathematical model of electrochemical step-decay under single-particle impact was obtained, which can be used to characterize the change pattern of electrochemical parameters on the metal surface before and after the impingement.

Tribo-corrosion response of additively manufactured high-entropy alloy

High-entropy alloys (HEAs) with multiple principal elements represent a paradigm shift in structural alloy design and show excellent surface degradation resistance in corrosive environment. Here, the tribo-corrosion response of laser-engineered net-shaped CoCrFeMnNi HEA was evaluated in 3.5 wt% NaCl solution at room temperature. The additively manufactured (AM-ed) CoCrFeMnNi showed five times lower wear rate, regenerative passivation, and nobler corrosion potential during tribo-corrosion test compared to its arc-melted counterpart. A significant anisotropy was seen in the tribo-corrosion response with 45° to the build direction showing better performance compared to tests along the build direction and perpendicular to it. The open circuit potential curves were characterized by a sharp drop to more negative values as wear began, followed by continuous change for the active tribo-corrosion duration and finally a jump to nobler value at the end of the test indicating excellent surface re-passivation for the AM-ed alloy. The superior tribo-corrosion resistance of AM-ed CoCrFeMnNi was attributed to the refined microstructure and highly protective surface passivation layer promoted by the sub-grain cellular structure formed during additive manufacturing. These results highlight the potential of utilizing additive manufacturing of HEAs for use in extreme environments that require a combination of tribo-corrosion resistance, mechanical durability, extended service life, and net shaping with low dimensional tolerance.

Zirconia-toughened alumina coated Ti6Al4V via additive manufacturing

CoCr alloy-based femoral heads have failed prematurely due to galvanic-induced corrosion when coupled with a titanium hip stem. Coupling a titanium based-femoral head with the titanium hip stem is ideal in addressing this failure mode. Ti6Al4V (Ti64) alloy was reinforced with zirconia-toughened alumina (ZTA) by directed-energy deposition (DED)-based additive manufacturing (AM) to address that concern. Preliminary materials processing work resulted in failed samples due to cracking, porosity, and delamination. After careful parameter optimization, a Ti64 + 5wt.%ZTA (5ZTA) composition produced a metallurgically sound and coherent interface, minimal porosity, and bulk structures. Hardness was observed to increase by 27%, normalized wear rate reduced by 25%, and contact resistance increased during in vitro tribological testing along with faster surface re-passivation.

Smart Coating Embedded with pH-Responsive Nanocapsules Containing a Corrosion Inhibiting Agent.

Triethanolamine (TEA), an amine-based corrosion inhibitor, was encapsulated and then embedded into an epoxy coating to provide long-term corrosion protection of aluminum alloy 3003. TEA was encapsulated by means of free-radical polymerization, yielding an average particle size of 450 nm. An applied epoxy coating containing 10 wt % of the nanocapsules successfully protected an artificially defective area for a long period due to TEA adsorption, which resulted in the formation of an inhibiting layer. Electrochemical impedance spectroscopy showed an inhibition efficiency surpassing 85% when the metal experienced mild corrosion in its early stages, and the inhibiting layer started to form by local pH level change, which triggered the release of TEA. On the basis of the results of a scanning vibrating electrode technique, the current density of the metal surface in the presence of an encapsulated TEA coating was significantly lower than that of the control coating with no nanocapsules. Optical microscope and scanning electron microscope images revealed that the sample surface that had encapsulated TEA nearby was considerably less stained after 60 days of testing compared with that of the control sample, which indicated that the metal substrate was protected by an inhibiting layer. Elemental analysis performed by energy-dispersive X-ray spectroscopy further confirmed that the metal surface was well preserved, and a remarkably low oxygen content indicated suppressed metal oxidation. The nuclear magnetic resonance spectra also evidenced a successful release of TEA throughout the experimental period. These findings reflected a spontaneous surface repassivation with the help of encapsulated TEA coating.

In situ spectroelectrochemical ellipsometry using super continuum white laser: Study of the anodization of magnesium alloy

This work shows the interest to use a real time, white laser-based ellipsometer to characterize a complex electrolyte|electrode interface during an electrochemical process in an aqueous-based medium. This method is proposed to probe electrochemical interfaces that are usually not suitable to the full extent application of ellipsometry due to great disturbance of the reflected light flux provoked by gas evolution or roughness. In situ spectroelectrochemical ellipsometry combining such a visible super continuum fiber laser-band source was not previously reported to the best of the authors’ knowledge. The setup was employed to monitor an electrochemical process whose mechanism was previously incompletely described: the prespark anodization regime of the plasma electrolytic oxidation process of the Mg alloy AZ91D in the 3M KOH electrolyte. Above the anodization voltage of 4 V, the side water oxidation reaction induced light diffusion that reduces reflected light beam intensity. The process is monitored in an extended voltage range from 4 to 40 V and in an extended spectral range (495–800 nm). In the presented case, the use of a visible super continuum fiber laser-band source enhanced the signal-to-noise ratio giving access to a deeper picture of the triplex layer structure during surface repassivation by monitoring the evolution of the outer, inner, and interfacial layers.

Tribocorrosion and the surface repassivation behavior of Monel 400 alloy in artificial seawater

PurposeThis paper aims to study the tribocorrosion and the surface repassivation behaviors of Monel 400 alloy in artificial seawater.Design/methodology/approachIn this study, the tribocorrosion behavior of Monel 400 alloy was studied under different applied loads in artificial seawater by using a pin-on-disk tribometer equipped with an electrochemical workstation. The applied loads were selected ranging from 50 to 200 N. The surface repassivation behavior of Monel 400 alloy was studied by X-ray photoelectron spectroscopy.FindingsIt was demonstrated that mass loss was determined by the combined effect of mechanical wear and chemical corrosion. The surface repassivation mechanism of the alloy is that layer corrosion product film formed on the surface of Monel 400 alloy, which can protect metal matrix from future corrosion.Originality/valueThis research adds original content in revealing the tribocorrosion and surface repassivation behaviors of Monel 400 alloy under different loads, which offer a theoretical basis for the application under the corrosion and wear environment of Monel 400.

Influence of Ionizing Radiation and the Role of Thiol Ligands on the Reversible Photodarkening of CdTe/CdS Quantum Dots.

We investigate the influence of high energy photons and thiol ligands on the photophysical properties of sub-monolayer CdTe/CdS quantum dots (QDs) immobilized in porous silica (PSiO2) scaffolds. The highly disperse, uniform distributions of QDs in a three-dimensional PSiO2 framework ensure uniform interaction of not only radiation but also subsequent surface repassivation solutions to all immobilized QDs. The high optical densities of QDs achieved using PSiO2 enable straightforward monitoring of the QD photoluminescence intensities and carrier lifetimes. Irradiation of QDs in PSiO2 by high energy photons, X-rays, and γ-rays leads to dose-dependent QD photodarkening, which is accompanied by accelerated photooxidative effects in ambient environments that give rise to blue-shifts in the peak QD emission wavelength. Irradiation in an oxygen-free environment also leads to QD photodarkening but with no accompanying blue-shift of the QD emission. Significant reversal of QD photodarkening is demonstrated following QD surface repassivation with a solution containing free-thiols, suggesting reformation of a CdS shell, etching of surface oxidized species, and possible reduction of photoionized dark QDs to a neutral, bright state. Permanent lattice displacement damage effects may contribute toward some irreversible γ radiation damage. This work contributes to an improved understanding of the influence of surface ligands on the optical properties of QDs and opens up the possibilities of engineering large area, low-cost, reuseable, and flexible QD-based optical radiation sensors.

Surface-Dependent Localized Surface Plasmon Resonances in CuS Nanodisks

We demonstrated revertible shifts of surface-dependent localized surface plasmon resonances (LSPRs) in CuS nanodisks. Oleylamine (OYA) served as a solvent and surface ligand covering on CuS nanodisks during the thermolysis of single-source precursor copper ethylxanthate (Cu(ex)2). When OYA ligand was unloaded and reloaded on the surface of CuS nanodisks, the wavelength of LSPRs blue-shifted due to more oxygen exposure and then reverted through surface repassivation. The surface-dependent shift of LSPRs was dominated by the concentration of free holes in CuS nanodisks, which was modulated by the coverage and exchange of surface ligands, and the oxygen exposure dose and time. The semiconductor nanocrystals with tunable LSPRs have great potential in advanced plasmonics.

Carbon steel passivity examined in solutions with a low degree of carbonation: The effect of chloride and nitrite ions

The effect of nitrite ions as inhibitors against steel corrosion in carbonated and chloride-contaminated concrete pore simulating solutions (pH 9) is investigated. Various inhibitor dosages and two chloride contents are tested. The inhibition efficiency is evaluated measuring the corrosion potential, cyclic voltammograms, anodic polarization curves, pitting and repassivation potentials, polarization resistance and performing electrochemical impedance spectra, together with weight-loss measurements. Independently of the chloride content, when [NO 2 −] = 0.2 mol l −1, the inhibition efficiency reaches 99% and if pitting occurs, surface repassivation is possible. At high chloride contents, if inadequate dosages of inhibitor are used, the risk of pitting attack is high because E pit is close to the OCP. The [NO 2 −]/[Cl −] ratio is not an appropriated parameter to evaluate the inhibitor efficiency for varying chloride concentrations in a carbonated concrete. An optimal nitrite ions concentration ([NO 2 −] = 0.2 mol l −1) can be established as the most suitable parameter to guarantee the inhibitor efficiency in the different electrolytes studied.

Electrochemical Noise Analysis of Tribocorrosion Processes under Steady-State Friction Regime

Abstract Tribocorrosion processes of austenitic stainless steel in a medium close to that used in pressurized water reactor nuclear plants have been investigated using electrochemical impedance and noise measurements. The experiments were carried out under a steady-state friction regime yielded by the continuous circular displacement of a fretting probe against a large disk electrode. Finite-element-method simulations of the current distribution in a conventional three-electrode cell showed that a significant part of the anodic current generated by the film removal and surface repassivation at the wear track does not flow to the counter electrode and, therefore, is not measured by the potentiostat. A specific arrangement with two working electrodes, a small disk electrode inserted at the center of the large one and electrically isolated from it, was devised to monitor the anodic current induced by the tribocorrosion process. Results have shown that only a small part of the current lines flows to the centr...

Tribocorrosion testing of stainless steel (SS) and PVD coated SS using a modified scanning reference electrode technique

A conventional scanning reference electrode technique (SRET) has been modified to provide a novel method for characterising the real-time localised tribocorrosion behaviour of uncoated and physical vapour deposition (PVD) coated samples. The modification was carried out based upon the principle of a ball-on-cylinder contact. Here a sliding probe, in this case a changeable alumina ball, is positioned diametrically opposite a pseudo (Pt) reference electrode probe which is positioned 100 μm away from the surface of a cylindrical specimen which rotates at a set speed. The contact probe, specimen and reference probe are immersed within a chosen electrolyte, which for this study, is aqueous 0.01% NaCl. The dynamic corrosion activity is monitored via the Pt probe by recording variations in ionic flux emanating at the surface of the corroding sample. The SRET can operate in two modes, namely line scan, in which corrosion activity along the circumference of the specimen is measured, or map scan in which a 2D corrosion activity area map of the surface of the specimen is obtained. This paper presents the design, calibration procedures, and real-time tribocorrosion behaviour of uncoated and PVD (C/Cr) coated 316 stainless steel as a function of the contact load, and test conditions. Evidence of the effects of wear damage due to contact by the alumina ball was obtained by monitoring the SRET activity during loading (contact) and unloading conditions. The real-time local tribocorrosion activity can be detected immediately a load was applied. Such activity was found to stop, due to surface repassivation, upon removal of the load (alumina ball). The results show that the applied loads affect the free corrosion potential, local current density, active anodic areas and the sample recovery time of the tested samples. The wear tracks were investigated using optical and scanning electron microscopy and laser profilometer. Correlation of tribocorrosion damage and contact load has been investigated.

The role of chloride ions in pitting of carbon steel studied by the statistical analysis of electrochemical noise

The role of chloride ions in metastable pitting of A516-70 carbon steel was studied based on the statistical analysis of recorded current and potential noise. It is shown that the amplitude and the occurrence frequency of current and potential transients, indicating the initiation of metastable pits, increase with the concentration of chloride ions. The time dependence of pit initiation rate ( λ) is well described by equations of λ( t)= λ(0)exp( at), 0≤ t< τ and λ( t)= λ( τ)exp(− bt), t≥ τ. The interaction between chloride ions and passive film has the reaction order of 3. The average peak potential depends upon the chloride concentration by a logarithm law. An analysis of the probability distribution of pit repassivation time vs. chloride concentration shows that the main role of chloride ions in pitting is to increase the chance of the breakdown of passive film, rather than to inhibit the surface repassivation.

Influence of loading frequency on transient current behavior of Fe26Cr1Mo during low cycle corrosion fatigue in 1M H2SO4 and 0.6M NaCl solutions

To understand the corrosion fatigue (CF) mechanism, in recent years many studies have been done on the transient current behavior of cyclic straining electrode for mild steel although there have been some studies on the transient current evolution under cyclic deformation, detailed information concerning the effect of loading frequency in corrosive solutions with different passivation characteristics is lacking. Crack initiation mechanisms are generally analyzed from the effect of surface dissolution, which is related to competition between the kinetics of depassivation by film rupture and repassivation by refilming of new metal surface. Two important factors should affect this surface reaction process: (1) surface dissolution behavior of metals which is dependent on the strain rate in the case of Fe-26Cr-1Mo stainless steel and gives a distribution of passive film damage; (2) repassivation kinetics, which are determined by the refilming ability of the corrosive medium. In the condition of CF, frequency is therefore an important factor which simultaneously determines both the material plastic deformation rate and the new surface repassivation time. On the other hand, the surface refilming process will be controlled by the characteristics of the selective medium. The main purpose of this paper is to study the effect of loading frequencymore » on the transient current behavior for Fe-26Cr-1Mo stainless steel under a low cycle CF condition, and to compare the difference of current response in 1M H[sub 2]SO[sub 4] solution.« less