Multi-physics analysis of the galvanic corrosion of Mg-steel couple under the influence of time-dependent anisotropic deposition film

The anisotropic deposit film formed during the galvanic corrosion can impede the mass transfer of the involved species,thereby affecting the electro-chemical behavior and the evolution of galvanic corrosion.The limitations of experimental studies in the spatial-temporal scales restrict a deeper understanding of the corrosion mechanism,which can be complemented by numerical simulation.A multi-physics coupled model is proposed in this work to systematically investigate the temporal and spatial evolution of galvanic corrosion of the Mg-steel couple with the growing anisotropic deposition layer.By utilizing the multi-physics field coupled technique,various coupled physical-chemical processes underlying the corrosion behavior are built into the model,including chemical reactions,ionic mass transfer in the bulk solution and the deposition layer,interfacial reaction,deposition of corrosion products as well as the morphological transitions caused by metal dissolution and deposition.In particular,the anisotropic deposit film is considered to be a porous layer with a porosity varying in time and space as the corrosion evolves.The predicted corrosion morphology by this model is better than the previous models.The coupled relationship between the electrochemical behavior(e.g.,electrode reaction kinetics,current density,surface potential)and the physical processes(e.g.,ionic transport,geometric evolution of metal surface and film interface)is revealed.The results indicate that a porous deposition layer with a denser inner layer and a loose outer layer is generated,leading to more significant inhibition of mass transfer in the inner layer than the outer layer.The anisotropism of the deposition layer results in a non-uniform conductivity distribution and a discontinuous current density distribution in the electrolyte.The current density on the electrode surface is inhibited by the deposition layer and the variation in the cathode/anode area ratio during the corrosion process.The competition between the transport process and the electrochemical reaction determines the spatial-temporal evolution of the ion concentration.

Wear Resistances of CO_2 Corrosion Product Films in the Presence of Sand Particles

Wear resistances of CO2 corrosion product films formed on P110 carbon steel at different CO2 partial pressures were investigated in water sand two-phase flow by weight loss method, and the microstructures and compositions of corrosion product films were analyzed by scanning electron microscope(SEM) and X-ray diffraction(XRD), respectively. The results showed that the wear rate of CO2 corrosion product films increased until a maximum and then decreased with the increasing of the film-forming pressure, and the maximum occurred at 2 MPa. However, the maximal corrosion rate and the loose and porous CO2 corrosion product films were obtained at 4 MPa. And the wear rate decreased and then went to be flat with increasing test time. Furthermore, the microstructures and compositions of corrosion product films and the impact and wear of sand particles played an important role on wear resistances. In addition, the wear rate and corrosion rate were fitted by cubic polynomial, respectively, which were well in accordance with the measured results.

Protective Characteristics and Formation Mechanism of the Corrosion Product Films of B30 Tubes Exposed to Seawater

The corrosion product films of two kinds of B30 tubes(similar to CDA 715)exposed to seawat- er for various periods of time were investigated by SEM,AES and XPS.The results show that the pro- tective corrosion product film is thin,uniform and adherent.FeOOH is found to be present in the film surface,which confirms the hypothesis that iron hydroxide segregates at the surface of the film.The FeOOH promotes Ni enrichment in the corrosion layer by preventing Ni from running off.The corro- sion product film with no protectiveness is of lay- ered structure,loose and bad adherence.The for- mer film is formed through direct oxidation and the latter by precipitation and redeposition from dis- solved species.

Research on the Corrosion of J55 Steel Due to Oxygen-Reducing Air Flooding in Low-Permeability Reservoirs

Oxygen-reducing air flooding is a low-permeability reservoir recovery technology with safety and low-cost advantages.However,in the process of air injection and drive,carbon in the air is oxidized through the crude oil reservoir to generate CO_(2),and this can cause serious corrosion in the recovery well.In this study,experiments on the corrosion of J55 tubular steel in a fluid environment with coexisting O_(2)and CO_(2)in an autoclave are presented.In particular,a weight loss method and a 3D morphometer were used to determine the average and the local corrosion rate.The corrosion surface morphology and composition were also measured by means of scanning electron microscopy(SEM)and an X-ray diffractometer(XRD).The corrosion pattern and morphological characteristics of J55 steel were analyzed in the O2/CO_(2)environment for different degrees of oxygen-reduction.As made evident by the experimental results,the corrosion products were mainly ferrous carbonate and iron oxide.In general,air injection increases the degree of oxygen reduction,from oxygen corrosion characteristics to CO_(2)corrosion-based characteristics.As a result,the corrosion product film becomes denser,and the corrosion rate is lower.

Achieving ultra-high corrosion-resistant Mg-Zn-Sc alloys by forming Sc-assisted protective corrosion product film

The weak corrosion resistance of magnesium and its alloys greatly limited the industrial application.Though functional self-healing coatings have been proposed as countermeasures,repeated damages on coatings under practical installation and complex external environments could require self-adaptive cor-rosion protection against multiple abrasions.In this study,an ultra-high corrosion-resistant Mg-1Zn-1Sc(wt.%)alloy with a corrosion rate of 0.087 mm/y has been designed and prepared,which has fine grains and uniform structure of a nano-scale ScZn phase with low potential.A unique and dense corrosion prod-uct film with a three-layered structure was found and studied on Mg-1Zn-1Sc alloy,providing excellent corrosion protection.In addition,the formation and protection mechanisms of the three-layered corrosion product film on Mg-1Zn-1Sc alloy have been discussed and proposed.The growth behavior of protective corrosion product film could be driven by the synergy of Sc and Zn elements.Furthermore,with the in-crease of Sc content,the strength,plasticity,and corrosion resistance of Mg-1Zn-x Sc(x=0,0.2,0.6,1.0,in wt.%)alloys increased simultaneously.The high corrosion resistance and moderate mechanical perfor-mance qualify Mg-1Zn-1Sc alloy as a promising candidate for diverse industrial applications.

INFLUENCE OF THE MICROSTRUCTURE OF Cu-Ni ALLOY ON ITS CORROSION RESISTANCE

The microstructures of two kinds of Cu-Ni alloys were observed by TEM.The results show that one of the alloys is a homogeneous solid solution.The other contains discontinuous precipitates at some grain boundaries,and the precipitate is a phase rich in Fe-Ni.By monitoring the corrosion potential(E_(con))in artifical seawater and exposure to natural seawater for a long time,it is found that the E_(con)of the former alloy steadily decreases,while the E_(con)of the latter decreases a little and fluctuates,and the corrosion rate of the former is clearly lower than that of the latter.Aanalyses of SEM and EDX show that the corrosion product film of the former is thin,uniform,compact and rich in nickel,and the film of the latter is thick,loose and covered with numerous deposits.Additionally serious intergranular corrosion occurs in the underlying substrate of the latter.The author proposes that the intergraular corrosion results from preferential dissolution of discontinuous precipitates at grain boundaries.In addition,the protective characteristics of corrosion product films are related not only to the enrichment of nickel but also to their compactness.

CO_(2)corrosion prediction on 20#steel under the influence of corrosion product film

Based on corrosion thermodynamics and kinetics,considering the multi-field coupling effects of fluid flow,electrochemical reaction and mass transfer process,a new corrosion prediction mechanistic model was proposed by introducing the influence factor of corrosion product film on diffusion coefficient of ion mass transfer,which is based on the CO_(2) corrosion prediction model proposed by Nesic et al.The influence of temperature,flow rate and pH value on CO_(2) corrosion behavior on 20#steel was studied by orthogonal tests.Scanning electron microscopy(SEM)and energy spectrum analysis(EDS)was used to analyze the surface and cross section morphology of the corrosion product film,and the thickness of the corrosion product film was measured.The results show that the introduced influence factor can simplify the ion mass transfer calculation in the presence of corrosion product film,and the relative error between the predicted value of the modified model and the experimental results is satisfactorily controlled less than 10%.Compared with the prediction model without considering the influence of corrosion product film,the influence factor can effectively correct the high prediction value of the mechanistic model under the influence of corrosion product film,improve the accuracy and applicability of corrosion prediction,and provide important theoretical guidance for the design,manufacturing,operation and maintenance of oil and gas production pipelines and related facilities.

Evolution of the Corrosion Product Film and Its Effect on the Erosion–Corrosion Behavior of Two Commercial 90Cu–10Ni Tubes in Seawater

The composition and structural evolution of the corrosion product film of two commercial 90Cu-10Ni tubes, namely TubeA and Tube B, after being immersed in natural seawater for 1, 3, and 6 months were characterized by scanning electronmicroscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy, and its effecton the erosion--corrosion behavior of the tubes was determined through a rotating cylinder electrode system using variouselectrochemical techniques. For the freshly polished samples used as contrast samples, the flow velocity mainly enhancedthe cathodic reaction at low flow velocities while both the anodic and the cathodic reactions were remarkably accelerated athigher flow velocities. The corrosion product films formed on the two commercial 90Cu-10Ni tubes after being immersedin seawater for up to 6 months are of a complex three-layer or multilayer structure. The structural evolution of the films isout of sync for the two tubes. A continuous residual substrate layer depleted of Ni was observed in the inner layer of thefilms on Tube B after 30, 90, and 180 days＇ immersion, while it was observed in the film on Tube A only after 180 days＇immersion. The nature of the inner layer plays a crucial role in the erosion-corrosion resistance of the 90Cu-10Ni tubes athigher flow velocity. The film with a compact and continuous inner layer of Cu20 doped with Ni2＋ and Ni3＋ which bondsfirmly with the substrate could survive and even get repaired with the increased flow velocity. The film on Tube Bpossessing a hollow and discontinuous inner layer composed of the residual substrate was degraded rapidly with increasingrotation speed in spite of its quite good resistance at the stagnant or lower speed conditions.

Effect of Zn on mechanical and corrosion properties of Mg-Sc-Zn alloys

The effect of different Zn concentrations(0 wt.%,1 wt.%,3 wt.%,and 6 wt.%)on the microstructure,cor-rosion property,and mechanical property of Mg-0.3Sc-x Zn(x=0 wt.%,1 wt.%,3 wt.%,and 6 wt.%)alloys was investigated.Here,MSZ1 alloy exhibits the highest corrosion resistance(0.194 mm/y)and appropriate mechanical properties with an ultimate tensile strength of 228 MPa and elongation of 19%.The superior corrosion resistance of Mg-0.3Sc-1Zn alloys is attributed to the homogeneous volta-potential distribution and the dense corrosion product film.With the increase in zinc content,the strength and plasticity of Mg-0.3Sc-x Zn alloys(x=0 wt.%,1 wt.%,3 wt.%,6 wt.%)improved to some extent.The precipitated ScZn phase plays the role of the second phase strengthening,which enables MSZ6 to obtain the maximum tensile strength.However,the ScZn phase with low volta potential intensifies the galvanic corrosion,re-sulting in the decline of the corrosion performance.

Corrosion Evolution of Low Alloy Steel in Deaerated Bicarbonate Solutions

Corrosion evolution during immersion tests （up to 43 days） of NiCu steel in deaerated 0.1 mol/L bicarbonate solutions was investigated by electrochemical measurements, scanning electron microscopy （SEM） and X-ray diffraction （XRD）. Results show that NiCu steel transformed from the anodic dissolution in the early stage of immersion to a metastable passive state in the final stage as the open-circuit potential value shifted positively, which was aroused by the precipitation of corrosion products. This process was mainly promoted by the trace amount of oxygen. Simultaneously, dominant cathodic reaction transformed from the hydrogen evolution in early stage to reduction processes of corrosion products in later stages. Possible corrosion processes were discussed with the assistance of a corresponding Pourbaix diagram.

Corrosion Performance of Carbon Steel in CO_(2) Aqueous Environment Containing Silty Sand with Different Sizes

Corrosion performance of carbon steel in CO2 aqueous environment containing silty sand with different sizes was investigated by immersion tests and electrochemical measurements. Silty sand could form an adsorption layer on steel surface in initial period, and the sand adsorption layer was turned into a mixture film of silty sand with corrosion product in last period. The adsorption layer in 325 mesh condition （large size） had the fewest pores for H2CO3 transport, exhibiting the highest cathodic current inhibition. In spite of little corrosion product, the sand adsorption film formed in 325 mesh condition induced the lowest corrosion rate. For 1000 and 5000 mesh silty sand, the sand adsorption layer had some pores for H2CO3 transport, leading to low cathodic current inhibition and much matrix dissolution. But the adsorption layer for 5000 mesh silty sand （small size） had the largest special surface area to accelerate heterogeneous precipitation of corrosion product FeCO3. Therefore, the mixture film in 5000 mesh condition was more compact, exhibiting stronger anodic inhibition and lower corrosion rate than those in 1000 mesh condition.