ELECTROCHEMICAL STUDIES ON ANODIC DISSOLUTION OF SYNTHETIC NICKEL SULFIDES

The anodic behavior of synthetic nickel sulfides in sulfuric acid solutions was studied.When the anodes of Ni_3S_2 ,Ni_7S_6 and NiS are actively dissolved respectively,a solid layer forms on the electrode surface with its nature and composition depending on the potentials applied. Based on the electrochemical phenomena,phase and elemental compositions of the solid layer formed on the electrode surface were determined.The apparent number of electrons transfer- red during dissolution of Ni^(2+)was also measured.It can be considered that there are three parallel reactions ocurred on the electrode:the reaction with formation of HSO_4^- or SO_4^(2-), the reaction with formation of sulfides as intermediates and the reaction of Jorming elemental sulfur.With NiS_2 as an anode,formation of SO_4^(2-)or HSO_4^- can be considered as the only re- action occurred on the electrode.

Energy Principle of Corrosion Environment Accelerating Crack Propagation During Anodic Dissolution Corrosion Fatigue

A general method to predict the crack propagation of anodic dissolution corrosion fatigue is developed in this paper. Crack propagation of corrosion fatigue is presented as the result of the synergistic interactions of mechanical and environmental factors, and corrosive environment accelerates crack propagation mainly in term of anodic dissolution. By studying the variation of mechanical energy and electrochemical energy of anodic dissolution during the crack propagation process, an explicit expression of crack propagation rate is derived by the conservation of energy. The comparisons with experimental data demonstrate the validity of the proposed model. Moreover, the applicable upper-limit crack length for steady crack propagation is determined and the crack propagation life is evaluated.

Effect of Cl^-ion on Anodic Iron Dissolution in H_2S-Containing Sulfuric Acid Solutions

The effect of Cl- ion on the anodic iron dissolution in H2SO4 solutions containing H2S has been studied by using electrochemical polarization curve measurements. The competitive adsorption for Cl- and HS- ions at an anodic potential has been calculated using the CNDO/2method. The results show that a ceftain concentration of the Cl- ion can be adsorbed steadily and inhibit the anodic reaction of iron catalyzed by HS-. However, when the Cl- ion reaches the saturation adsorption, it begins to promote the anodic reaction of iron due tO the increased negative charge of iron atoms.

A Mg alloy with no hydrogen evolution during dissolution

Hydrogen evolution is normally associated with the corrosion or dissolution of Mg alloys in aqueous solutions.This work studied the corrosion behavior of sputtered pure Mg,Mg82Zn18(at.%),Mg64Zn36(at.%),and pure Zn in 3.5%Na Cl solution.Mg64Zn36had(ⅰ)an amorphous microstructure with some nano-scale grains,(ⅱ)a corrosion rate substantially lower than that of pure Mg,and(ⅲ)no hydrogen evolution during corrosion or anodic dissolution,because the positive corrosion potential retarded the cathodic hydrogen evolution.This is a new route to prevent hydrogen evolution during Mg corrosion,which has never previously been realized.

Anodic dissolution mechanism of TA15 titanium alloy during counter-rotating electrochemical machining

Counter-rotating electrochemical machining(CRECM)is a novel shaping method with advantages in processing revolving parts,especially engine casings.However,few researchers have studied the anodic behaviour of the counter-rotating state.This paper aims to analyse the anode dissolution behaviour of TA15 and obtain desired surface qualities in CRECM.The anodic characteristics were measured by polarization and cyclic voltammetry curves,and the passive-trans passive behaviour of TA15 was revealed.The electrode surface structures at different stages were analysed using electrochemical impedance spectroscopy(EIS),and a quantitative dissolution model was established to illustrate the electrochemical dissolution and structural evolution of the revolving surfaces.A series of CRECM experiments were conducted,and three stages(pitting corrosion,pitting expansion,and smoothing)were detected according to the current signals.In the first stage,an oxide film with small pores was formed initially on the metal surface when exposed to air.This air-formed oxide film was broken down locally during the electrochemical reaction process,resulting in the occurrence of pitting.With the increase of electricity,the oxide layer became thinner,and the pitting areas expanded and began to connect with each other.In this stage,the surface quality was poor owing to the uneven material dissolution.When the amount of electricity approximately reached a constant,the oxide layer was completely removed,and a very thin salt film was generated at the metal-electrolyte interface.In this stage,the material was dissolved uniformly,and the surface was smooth without pitting corrosion.Based on the obtained results,anode workpieces with grid-like convex structures were fabricated using CRECM.Both the revolving surface and the sidewalls of the convex structures can be controlled from pitting to smoothness.

Influence of ammonium sulfate on the corrosion behavior of AZ31 magnesium alloy in chloride environment

Electrochemical corrosion of AZ31 magnesium alloy in the NH_(4)^(+)-SO_(4)2−-Cl−environment is studied.Effect of NH_(4)^(+)overshadows that of Cl−as the(NH_(4))_(2)SO_(4) concentration is 0.005 M or higher,yielding an evolution from localized corrosion to uniform corrosion.Acceleration effect of NH_(4)^(+)can be attributed to that(i)NH_(4)^(+)dissolves the inner MgO and hinders the precipitation of Mg(OH)_(2) and(ii)the buffering ability of NH_(4)^(+)provides H+,enhances the hydrogen evolution,and expedites the corrosion process.The latter is demonstrated as the dominant factor with the results in unbuffered and buffered environments.The severe corrosion and hydrogen process in NH_(4)^(+)-containing solution results in a high Hads coverage and yields an inductive loop within the low frequency.Meanwhile,SO_(4)^(2−)is helpful in generating cracked but partially protective corrosion products,while Cl−could broaden the corrosion area beneath the corrosion product.

SCC MECHANISM OF STAINLESS STEEL AISI 321 IN ACIDIC CHLORIDE SOLUTION

It is verified that stainless steel AISI 321 is in the active anodic dissolution state in 0.5 mol/L HCll+0.5 mol/L NaCl solution at 55℃.The SCC of the steel in the solution can not be reasonably explained by passive film rupture-nepasivation theory and by hydrogen embrittlement theory.There are evidences that the fracture stress at the tip of the cracks is reduced by anodic dissolution due to its role in relieving strain hardening layer at crack tip.

Corrosion Resistance of API5L X52 Carbon Steel in Sulfide Polluted Environments

In the present work, the corrosion behavior of base and welded API5L X-52 carbon steel coupons in synthetic ground water solution NS-4 was studied. Subsequently, the morphology and composition of the corrosion product deposits were determined using scanning electron microscopy (SEM). Based on weight loss as well as Tafel’s measurements, increasing the sulfide ions content above 0.4 mM, had an inhibition effect on the corrosion behavior of carbon steel. According to EDAX analyses, increasing of sulfide concentration from 0.4 to 4 mM Na2S, the Fe/S ratio decreased from 12.02 at 0.4 mM to 2.6 at 4 mM Na2S.

Corrosion Behavior of Al-Zn-Sn-Ga Alloy in NaCl Solution

The corrosion behavior of Al-Zn-Sn-Ga alloy in 3.5% NaCl solution was investigated by corrosion morphology observation and electrochemical testing. The experimental results show that there exist three stages during the course of corrosion. At the initial stage, pitting occurs surrounding the precipitates, which is driven by the galvanic couple effect. At the middle stage, pitting rapidly extends toward the horizontal direction, forming the shallow circular structure caused by the deposition of Ga ions around Sn. At the final stage, the new active sites led to the continuous corrosion of the alloy.

BEHAVIOUR AND MECHANISM OF STRESS CORROSION CRACKING FOR Al-Li-Cu-Mg ALLOY

Studies were made of the influence of aging conditions and applied potentials on the stress corrosion carcking(SCC)susceptibility for an Al-Li-Cu-Mg alloy by slow strain rate technique.The relationship between the relative hydrogen content on specimen surface,the applied potentials and elapsed time has also been examined.The SCC susceptibility was found to be dependent on aging conditions in which the peak aged condition gave the worst SCC resistance and the natural aged condition had the best one. The SCC susceptibility and surface hydrogen content relates to the applied potentials. The anodic potentials increase SCC susceptibility,while the cathodic ones below the critical potential accelerate SCC.It is considered that both the anodic dissolution and

Crack Propagation in Pipelines Under Extreme Conditions of Near-Neutral PH SCC

Stress Corrosion Cracking(SCC)process through which cracks occur in a variety of susceptible materials is a result of a combination of residual or applied stresses and corrosion.In oil and gas field,buried pipeline steels are made of low-alloy steels with a ferritic-pearlitic structure,such as X70.In dilute solutions,these materials are prone to SCC failure.The Near-neutral simulated soil solution(NS4)solution is established to imitate SCC conditions and subsequently became the industry requirement for crack growth experiments in the majority of laboratories.The strainassisted active crack pathways are considered while modelling SCC growth as an oxide film rupture and anodic dissolution process.It’s been hypothesized that increasing the strain concentration can help with dissolution at the filmfree crack tip.This research focuses on estimating the SCC crack growth rate under various environmental conditions in oil and gas pipelines using finite element modelling.The simulation is carried out using the J-integral theory in the COMSOL Multiphysics program.Simulations are performed to model the crack growth rate(CGR)using slip anodic dissolution(film rupture)mechanism.The plastic strain gradient is required to compute the SCC CGR(da/dt).Because the plastic strain located at crack tip increases proportionally to the crack length as it propagates,the CGR increases as the stress intensity factor(SIF)increases.The crack growth rates increase when constant loads are applied and as the temperature rises,and elevating the cathodic potential has a minimal influence on the propagation rate of cracks but raises the material yield strength and imparts brittle behavior to it.

What activates the Mg surface——A comparison of Mg dissolution mechanisms

Several mechanisms have been proposed to interpret the widely reported phenomenon of Mg corrosion that the hydrogen evolution rate increases with increasing anodic potential or anodic current density. This paper critically analyzed the two main mechanisms,(1) "the incomplete film univalent Mg+ion mechanism" and(2) "the enhanced catalytic activity mechanism", aiming to clarify the current understanding of the Mg corrosion mechanism and to provide a profound insight into the Mg characteristic electrochemical behavior, anodic polarization accelerating both hydrogen evolution and Mg dissolution. It is expected that the deepened fundamental understanding from this comprehensive mechanistic review will provide a basis of practical applications for Mg alloys and open up a new way to the control of corrosion of Mg alloys in practice.

Effect of residual dissolved oxygen on the corrosion behavior of low carbon steel in 0.1 M NaHCO3 solution

The effect of residual dissolved oxygen （DO） on the corrosion behavior of carbon steel in 0.1 M NaHCO3 solution was investigated by electrochemical measurements, corrosion mass loss test, scanning electron microscopy （SEM） and X-ray diffraction （XRD）. In the initial immersion stage, the increase of the dissolved oxygen concentration led to the change of from a reductive state of active dissolution to an oxidizing state of pseudo passivation in low carbon steel. While in the final stage, all the steels transformed into the steady state of pseudo passivation. In the anaerobic solution, the formation of c^-FeOOH was attributed to the chemical oxidization of the ferrous corrosion products and the final cathodic process only included the reduction of c^-FeOOH, while in the aerobic solution, it included the reduction of oxygen and （x-FeOOH simultaneously. As the main corrosion products, the content of （x-FeOOH was increased while that of Fe6（OH）12CO3 was decreased with increasing concentration of dissolved oxygen. The total corrosion mass loss of the steel was promoted with the increase of dissolved oxygen concentration.

Influence of the lithium content on the negative difference effect of Mg-Li alloys

The so-called 'negative difference effect'(NDE) was often defined by the increasing rate of hydrogen evolution from magnesium(Mg) surface under anodic polarization.In this work,a series of electrochemical tests and microstructure observations were performed to provide an evidence that the NDE of Mg-Li alloys can be retarded by increasing lithium content.Potentiostatic,galvanostatic and potentiodynamic polarization experiments using Mg-xLi(x=4,7.5 and 14 wt%) alloys electrodes indicated that Mg-4 Li alloy maintained the enhancing NDE prior to anodic dissolution as that of conventional Mg alloys.However,the emergence of β-Li phase weakened the NDE of duplex Mg-7.5 Li alloy at a low anodic current density,but it was still enhanced apparently after a high applied anodic value(more than 2 mA/cm^2).The surface observations,including the plane and cross-sectional morphologies,confirmed that the cracked surface film derived from the anodic dissolution resulted in the catalytic activity of NDE for Mg-4 Li and Mg-7.5 Li alloys.Furthermore,the NDE of Mg-14 Li alloy was suppressed obviously after a prior applied anodic polarization,which was attributed to the persistent and integrated surface film which endured a higher level of applied anodic potential and current.

Experimental investigation into fabrication of microfeatures on titanium by electrochemical micromachining

Titanium machining is one of the challenging tasks to modem machining processes. Especially fabricat- ing microfeatures on titanium appear as a potential research interest. Electrochemical micromachining （EMM） is an effective process to generate microfeatures by anodic dis- solution. Machining of titanium by anodic dissolution is different than other metals because of its tendency to form passive oxide layer. The phenomenon of progression of microfeature by conversion of passive oxide layer into transpassive has been investigated with the help of mask- less EMM technique. Suitable range of machining voltage has been established to attain the controlled anodic disso- lution of titanium by converting passive oxide film of titanium into transpassive with nonaqueous electrolyte. The experimental outcomes revealed that the micromachining of titanium with controlled anodic dissolution could be possible even at lower machining voltage in the range of 6-8 V. This work successfully explored the possibility of generation of microfeatures on commercially pure titanium by anodic dissolution process in microscopic domain by demonstrating successful fabrication of various microfea- tures, such as microholes and microcantilevers.

Investigation of stress corrosion cracking behavior and mechanism analysis of a 1900 MPa-grade ultra-high-strength stainless steel

The stress corrosion cracking(SCC)behavior of a 1900 MPa-grade ultra-high-strength stainless steel in 3.5 wt.% NaCl solution was investigated by X-ray diffractometer,scanning electron microscopy,electron back-scattered diffraction,X-ray photoelectron spectroscopy,and potentiodynamic polarization curves.The results showed that USS122G stel has good SCC resistance,and the critical stress intensiy factor(K_(iscc))of USS122G steel was about 68.906 MPa m^(1/2) and Kiscc/K_(ic)=0.76(K_(ic) is plane strain fracture toughness).The existence of film-like austenite along the lath martensite boundary and the protective effect of thecc passivation flm were the main factors for its high Kiscc.Among them,the main components of the passivation film on the surface of USS122G steel were Cr_(2)O_(3),Cr(OH)_(3),FeOOH,and Ni(OH)_(2).The fracture morphology of SCC zone was intergranular and transgranular.Through the slow and fast scanning rate polarization curve test results,it can be concluded that SCC mechanism of USS122G steel in 3.5 wt.%NaCl solution at the open-circuit potential was a mixed mechanism involving hydrogen embritlement and anodic dissolution.