Local Electrochemical Corrosion Research Articles

Local Electrochemical Corrosion of 6061 Aluminum Alloy with Nano-SiO2/MAO Composite Coating.

To improve the corrosion resistance of 6061 Al in electric vehicle battery packs, a composite coating of nano-SiO2/Micro-Arc oxidation (MAO) ceramic structure was prepared on its surface. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves (PDP) were used to evaluate the corrosion resistance of the specimens after 7 days immersion in a 3.5% NaCl solution. The corrosion resistance of the prefabricated coatings was measured via local electrochemical impedance spectroscopy (LEIS). Confocal microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used to characterize the microstructure and phase composition of the specimens. An energy dispersive spectrometer (EDS) was used to detect the elemental composition of the surface of the specimen. The results showed that the specimen with nano-SiO2/MAO composite coating had the least amount of micropores and superior corrosion resistance. The global electrochemical impedance of nano-SiO2/MAO composite coating was 1.1 times higher than that of the MAO coating and 8.4 times higher than that of the 6061 Al. When the coating was defective, the local electrochemical impedance of the nano-SiO2/MAO composite coating was still two times higher than that of the MAO coating. In the presence of scratches, the nano-SiO2/MAO composite coating still showed high corrosion resistance. The collapse corrosion mechanism of the nano-SiO2/MAO composite coating was proposed.

Role of second phases in the corrosion resistance and cerium conversion coating treatment of as-extruded Mg–8Al–4Ca magnesium alloy

The presence of a second phase in the Mg–8Al–4Ca (at. %) alloy plays a significant role on both its corrosion behavior and the chemical conversion coating processes. Using scanning Kelvin probe force microscopy (SKPFM), a lower Volta-potential of the second phase present on the surface has been measured. The β-Al-Ca phase has a higher electrochemical activity than the α-Mg matrix and may act as the micro-galvanic anode in a local electrochemical corrosion process. Transmission electron microscopy (TEM) examinations reveal that the β-Al-Ca phase is more susceptible to corrosion than the α-Mg matrix in an aqueous solution, and its higher activity and higher corrosion rate accelerate the hydrogen evolution rate on the α-Mg matrix in the cerium (Ce) conversion coating process. It's also been discovered that by immersing the bare Mg–Al–Ca alloy in deionized (DI) water, the β-Al-Ca phase, exposed on the surface, can be dissolved and converted in situ into aluminum hydroxide (Al(OH)3), and the Ce conversion coating can be deposited via replacement reactions in the subsequent conversion coating process. A thicker Ce coating with smaller blisters has then been produced on the DI-treated Ce-coated Mg specimen; it indeed improves the corrosion resistance.

Local electrochemical corrosion performance of nano-SiC/MAO composite coating on 6061-Al alloy

PurposeThe purpose of this paper is to improve the corrosion resistance of the 6061-Al alloy as the battery pack material for electric vehicles, and the nano-SiC/MAO composite coating was prepared.Design/methodology/approachThe corrosion resistance of coatings was evaluated by the global electrochemical test, and the local electrochemical impedance spectroscopy (LEIS) was used to study the local corrosion mechanism. The laser confocal microscope, scanning electron microscope and X-ray diffractometer (XRD) were used to characterise coatings.FindingsResults showed that the impedance of nano-SiC/MAO coating was 1–2 times higher than MAO coating, and the main corrosion product was Al(OH)3. LEIS results showed that the impedance of the nano-SiC/MAO coating was two times higher than the MAO coating. The defective SiC/Micro-arc oxidation coating still had high corrosion resistance compared to the MAO coating.Originality/valueThe physical model of the local corrosion mechanism for SiC/MAO composite coating in “cavity-fracture collapse” mode was proposed.

Local Electrochemical Corrosion Properties of a Nano-SiO2/MAO Composite Coating on an AM60B-Mg Alloy.

In order to improve the corrosion resistance of the automotive AM60B-Mg alloy, a nano-SiO2/MAO composite coating was prepared on the surface of the alloy. The electrochemical properties were studied in an 80 °C corrosion environment using potentiodynamic polarization tests. Local Electrochemical Impedance Spectroscopy (LEIS) was used to study the corrosion mechanisms of coating defect zone. The microstructure and phase of the samples were observed by confocal laser microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD). Macroscopic electrochemical test results showed that the impedance of the nano-SiO2/MAO coating was much higher than that of the MAO coating, by about 433 times. Local electrochemical test results showed that the minimum impedance of the nano-SiO2/MAO coating was 1–2 orders of magnitude higher than the maximum impedance of the MAO coating. The defective SiO2/MAO coating still had high corrosion resistance compared to the defective MAO coating. A physical model of local corrosion mechanisms was proposed.

Stability of Octadecyltrimethoxysilane-Based Coatings on Aluminum Alloy Surface.

Long-term stability in contact with water of organosilane layers formed by octadecyltrimethoxysilane (ODTMS) on polished aluminum alloy (AA2024) through dip-coating was studied by combining SEM, water contact angle measurements, and X-ray photoelectron spectroscopy. Similar organosilane layers were formed on AA2024 coated with permanganate conversion coating, 1,2-bis(triethoxysilyl)ethane (BTSE) and hydrated SiOx as under-layers, after which their long-term durability was also tested. During immersion in water for about one month, all the samples exhibited a decrease in hydrophobicity, implying the prepared organosilane layer was not stable over time, gradually hydrolyzing and letting water interact with the underlying layer. In parallel, SEM images of one-layer samples taken after immersion showed clear signs of local electrochemical corrosion, while XPS analysis confirmed a loss of silicon from the surface layer. The highest stability over time was demonstrated by a one-layer sample prepared in an ethanol/water bath for 5 min and by a similar ODTMS layer prepared on hydrated MnOx as an under-layer.

Structure and characteristics of Nb2O5 nanocoating thin film for biomedical applications

Nanocoating thin films are a synthesize of coatings layers to improve different required properties especially corrosion resistance for various biomedical applications. In this study, Nb2O5 thin film has been synthesized on Ni-Cr-Mo alloy surface by hydrothermal technique at 150 °C for 160 hr immersing time with surface annealing at 600 °C. Scanning electron microscope (SEM)/ Energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), atomic force microscopy (AFM) and Ellipsometry spectroscopy were used to characterize the microstructure and morphology. The electrochemical test was studied in simulated body fluid (SBF) at 37 °C. The results indicated that there is no contamination, defects, or micro-cracks with spherical nanoparticles morphology for Nb2O5 thin-film with 200–500 nm diameter range. The created thin film presented Nb as a major element with crystal structure peaks related to ultra-thin-film of Nb2O5, with 1.87 nm roughness average (Ra) and 2.75 nm root mean square (Rq). The localized electrochemical corrosion resistance of nanocoated sample was improved compared to the uncoated substrate by decreasing the icorr. from 1.9 (µA.cm−2) to 1.2*10-2 (µA.cm−2) and increasing the Ecorr from 172 mV to 293 mV. The corrosion rate by miles per year (mpy) was also decreased from 0.6 to 0.004 with 99.3 % corrosion resistance improvement.

Method of investigation of local corrosion processes on samples from clad steel

The article describes a method developed to evaluate the mechanism of local corrosion processes on the surface of a clad layer of steel specimens, which makes it possible to identify the difference in electrode potentials in artificially created holes. As a result of the study it was found that in a corrosive medium containing Cl- and S2O32- ions, the diffusion of oxidants into a hole of smaller diameter is difficult, which causes a large potential difference between the wall of the hole and its bottom. This process contributes to the development of local electrochemical corrosion.

Local electrochemical characteristics of pure iron under a saline droplet II: Local corrosion kinetics

The local electrochemical corrosion kinetics of pure iron under a 3.5% NaCl droplet was investigated by newly developed concentric three-electrode array (CTEA) measurements. Electrochemical impedance spectra and polarization curves at different locations under a droplet were measured on the CTEA. A mathematical model for the impedance responses was developed to analyse the electrochemical corrosion processes for different areas under saline droplets with different volumes. The local corrosion kinetics parameters, including anodic faradaic impedance, cathodic faradaic impedance, diffusion resistance, effective capacitance, and local interfacial oxygen concentration, were obtained by the regression and subsequent calculation on EIS data for selected electrode units. There are significant differences in the control mechanisms of local corrosion kinetics between different places under the droplets, resulting from the uneven reaction distribution and the divergent evolution of the corrosion process caused by droplet geometry. Under a droplet with a volume of 25 μL, both the anodic and the cathodic electrode units were determined by oxygen diffusion. Under a droplet with a volume of 110 μL, the central anode units was mainly controlled by the mass transfer of oxygen, and the peripheral cathode unit was determined by both anodic faradaic impedance and oxygen diffusion. The local corrosion process was mass-transfer determined under the central part of 340 μL droplet; whereas, the peripheral part was controlled by both anodic charge transfer and the mass transfer of oxygen.

Retrospective analysis of corrosion and ion release from retrieved cast stainless steel tibia plateau leveling osteotomy plates in dogs with and without peri-implant osteosarcoma.

OBJECTIVE To evaluate and compare surface and cross-sectional structure as well as localized electrochemical corrosion and ion release for cast stainless steel (SS) tibia plateau leveling osteotomy (TPLO) plates retrieved from dogs with and without osteosarcoma (OSA) and to compare these findings with similar variables for forged SS TPLO plates retrieved from dogs. SAMPLE 47 TPLO plates explanted from 45 client-owned dogs (22 cast plates from dogs with OSA, 22 cast plates from dogs without OSA, and 3 forged plates from dogs without OSA). PROCEDURES Histologic evaluations of tissue samples collected from implant sites at the time of plate retrieval were performed to confirm implant site tumor status of each dog. Surfaces and metallographic cross sections of retrieved plates were examined, and the microcell technique was used to obtain local electrochemical corrosion and ion release measurements. RESULTS Findings indicated that all cast SS plates demonstrated high spatial variability of their electrochemical surface properties and inhomogeneous superficial and cross-sectional composition, compared with forged plates. Greater metal ion release was observed in cast plates than in forged plates and in cast plates from dogs with OSA than in cast or forged from dogs without OSA. CONCLUSIONS AND CLINICAL RELEVANCE Results suggested that accumulation of metal ions from implants could be a trigger for neoplastic transformation in neighboring cells. Metal ion release caused by corrosion of implants that do not comply with recommended standards of the American Society for Testing and Materials International or the International Organization for Standardization could potentially place patients at increased risk of tumor development.

Comparative corrosion resistance properties between (Cu, Ce)-DLC and Ti co-doped (Cu, Ce)/Ti-DLC films prepared via magnetron sputtering method

Element co-doping is an effective fashion to improve the properties of carbon-based film and amplify its application area. Here, (Cu, Ce)-DLC and (Cu, Ce)/Ti-DLC films were successfully prepared via magnetron sputtering. Evolutions of surface/interface morphology, microstructure and wettability of as-prepared films were investigated systematically to in-depth analysis the relationship between their microstructure and anti-corrosion capability. (Cu, Ce)/Ti-DLC film possessed a thicker thickness compared to (Cu, Ce)-DLC film. Raman spectra of as-prepared films exposed quintessential features of carbon peak. Scanning vibrating electrode technique revealed the local electrochemical corrosion process of films. Particularly, (Cu, Ce)/Ti-DLC film could demonstrate superior anti-corrosion ability.

First-Principles Study on the Structural Stability and Segregation Behavior of γ-Fe/Cr2N Interface with Alloying Additives M (M = Mn, V, Ti, Mo, and Ni)

This study investigated the structural stability and electrochemical properties of alloying additives M (M = Mn, V, Ti, Mo, or Ni) at the γ-Fe(111)/Cr2N(0001) interface by the first-principles method. Results indicated that V and Ti were easily segregated at the γ-Fe(111)/Cr2N(0001) interface and enhanced interfacial adhesive strength. By contrast, Ni and Mo were difficult to segregate at the γ-Fe(111)/Cr2N(0001) interface. Moreover, the results of the work function demonstrated that alloying additives Mn reduced local electrochemical corrosion behavior of the γ-Fe(111)/Cr2N(0001) interface by cutting down Volta potential difference (VPD) between clean γ-Fe(111) and Cr2N(0001), while alloying additives V, Ti, Mo, and Ni at the γ-Fe(111)/Cr2N(0001) interface magnified VPD between clean γ-Fe(111) and Cr2N(0001), which were low-potential sites that usually serve as local attack initiation points.

Corrosion Currents in the Cathodic and Anodic Sections of a Circular Stress Concentrator

A quantitative relationship between the mechanical stress field and local distribution of the shifts of electrode potentials and corrosion current densities in the metal containing a circular stress concentrator filled with corrosive medium is analytically determined. The dependences of the sizes of cathodic and anodic sections on the applied forces and the contents of Na + and Cl − ions in the medium are analyzed. The boundaries of the cathodic and anodic sections on the surface of the circular hole are determined. In order to evaluate the serviceability and durability of structural elements in the course of long-term opera- tion under the action of static loads and working media, it is important to establish the relationship between the stress-strain state of the metal and its electrochemical interaction with the medium, which substantially affects the strength, load-carrying ability, and fracture processes in the products. The force loading and the correspond- ing inhomogeneous stress field induce the space redistribution of current carriers in metallic bodies. In the pro- cess of tension of this body, the zone of the highest concentration of stresses turns into the anode, whereas the remaining part of the surface plays the role of cathode. Even small-size anodic and cathodic regions cause the motion of charges and, hence, the local electrochemical corrosion of the surface, where the metal body contacts with the corrosive medium. However, the mutual influence of the stress-strain state and the electrochemical in- teraction of media with metals is studied insufficiently. Fr this reason, we estimate the shifts of the electrode potentials and current density on the boundary of a circular hole in the metal placed in a corrosive medium in the process of deformation.

The main mechanisms of stress corrosion cracking in natural gas trunk lines

Stress corrosion cracking in a pipe material presents a most critical hazard to natural gas trunk lines. The paper addresses the causes (mechanism) of crack formation and growth. Understanding of these issues will enable a proper implementation of methods for early identification, diagnostic, and prediction of this type of failure. A most comprehensive explanation of a joint influence of internal and external factors on the corrosion cracking can be provided by means of the probabilistic local electrochemical corrosion theory. It is based on the probabilistic nature of initiation of a local corrosion damage. A probabilistic approach is used to develop a concept of stress corrosion cracking. The factors responsible for the formation of local damage nuclei have been determined. The pipe manufacturing operations are shown to have a certain effect on the pipeline susceptibility to stress corrosion cracking. Modeling has been performed for the corrosion processes that may occur in a gas trunk line section subjected to a stress corrosion cracking hazard.

Corrosion of steel under the defected coating studied by localized electrochemical impedance spectroscopy

Corrosion of steel under the defected coating in near-neutral pH solution was investigated by localized electrochemical impedance spectroscopy (LEIS) measurements. The LEIS response is dependent on the size of the defect. For small defects, e.g., less than 200 μm in diameter, localized corrosion process and mechanism of steel, as indicated by the measured LEIS plots, change with time. The diffusion process dominates the interfacial corrosion reaction, which is due to the block effect of the deposited corrosion product combined with the geometrical factor of a large coating thickness/defect width ratio. In the presence of a big defect, e.g., up to 1000 μm, the LEIS responses measured at the defect are always featured by a coating impedance in the high-frequency range and an interfacial corrosion reaction in the low-frequency range. The block effect of corrosion product does not apply due to the relatively open geometry. Conventional EIS measurements on a macroscopic-coated electrode reflect the “averaged” impedance results from both coating and defect. The information of the localized electrochemical corrosion processes and mechanisms at the small defect is lost, and the coating impedance information is “averaged” out when a big defect is contained. LEIS measurement provides an essential technique to characterize microscopically the local electrochemical corrosion reaction of steel under the defected coating.

Mapping Corrosion Kinetics Using the Wire Beam Electrode in Conjunction with Electrochemical Noise Resistance Measurements

A wire beam electrode (WBE) has been used, in conjunction with the electrochemical noise resistance method, to determine the kinetics of nonuniform corrosion of mild steel in a carbon dioxide saturated brine. Local electrochemical parameters including corrosion potential, galvanic current, and electrochemical noise resistance were measured from local areas of a WBE surface. These parameters were used to calculate the kinetics of local electrochemical corrosion processes. A map showing instantaneous corrosion rate distribution over a WBE surface has been produced. Corrosion depths were calculated, based on instantaneous corrosion rate data, at various stages of the corrosion exposure period, and these were summed up to produce an accumulated corrosion depth map. This accumulated corrosion depth map was found to be quantitatively comparable with a microscopically observed corrosion depth map. This work shows that electrochemical corrosion kinetics can be determined using kinetic equations previously derived based on the WBE concept. This work also shows that the WBE is a novel electrode design which is suitable for performing electrochemical noise resistance measurement and that the WBE is applicable to practical corrosion conditions. © 2000 The Electrochemical Society. All rights reserved.

On interpretation of the mechanism of stress-corrosion crack growth and relevant terminology

Currently, there is an established concept that there exist two alternative mechanisms of growth of stresscorrosion cracks, namely, local anodic dissolution and hydrogen embrittlement of material in the vicinity of a crack tip [1-3]. Although there is no denying the correctness of this approach in general, it should be noted that the terminology used (local anodic dissolution and hydrogen embrittlement) is inadequate for processes that take place in a crack and, thus, it is not completely valid. Local anodic dissolution refers to the case where a crack grows due to the localization of corrosion immediately at its tip, which is possible when the corrosion rate is significantly higher in a small vicinity of the crack tip than on its sides. In this case, active corrosion occurs, for all practical purposes, right at the crack tip, and corrosion of the walls is small and usually neglected. Theoretical electrochemistry regards dissolution of metals as the following two processes: dissolution caused by an external current (anodic dissolution) and dissolution caused by interaction with the components of the medium, i.e., the corrosion itself [4]. Thus, anodic dissolution of a metal is understood as its oxidation under anodic polarization by an external current. Hence, corrosion inside a crack bears no relation to anodic dissolution. There is one more significant difference between anodic dissolution and corrosion that should be mentioned, namely, anodic dissolution is a nonspontaneous process sustained by external energy (electric current), whereas corrosion is a spontaneous process, during which energy is released in the form of corrosion current. Consequently, it is totally incorrect to apply the term "local anodic dissolution" to local corrosion in a crack. We think that the pure corrosion mechanism of crack growth should be treated as a mechanical--electrochemical mechanism because it is the mechanical effect that initiates galvanic cells in a crack, which give rise to local electrochemical corrosion of the crack tip. It is also incorrect to call the alternative mechanism according to which a crack goes deeper into metal weakened by hydrogenation, "hydrogen embrittlement of material in a vicinity of a crack". This term gives a simplistic idea of abrupt crack growth. We consider the so-called hydrogen embrittlement mechanism in greater detail. During this process, a crack propagates abruptly [1, 3] and a highly corrosion-active freshly formed surface emerges in the jump area. The corrosion of the freshly formed surface acting together with the crack walls as a galvanic cell is exactly the main cause of local purely corrosion deepening of a crack as well as hydrogen regeneration and charging of the metal with it, i.e., embrittling hydrogenation. Hydrogen charging of metal gives rise to cells of differential hydrogenation in a crack [5], which accelerates corrosion of the tip. Purely corrosion deepening of a crack can be quite significant and commensurable with purely mechanical growth per jump. Thus, at this stage, a crack exhibits not only mechanical but also corrosion growth, and the mechanism of its growth is of a mechanical-hydrogen-electrochemical nature. Thus, in view of the argument above, it is more appropriate to call the mechanism of local anodic dissolution and the mechanism of hydrogen embrittlement "the mechanical-electrochemical mechanism" and "the mechanicalhydrogen-electrochemical mechanism."

Simulating corrosion conditions in the microvolumes of cracklike defects

A study of the electrochemical processes in limited microvolumes at the apices of sharp ended defects makes it possible to obtain data as ~ result of which certain aspects of crevice and pitting corrosion or corrosion cracking of metals may be explained. Access to a crevice defect by oxygen and the solution from the general volume is difficult and hydrolysis of metal ions under such conditions changes the pH of the solution and there is a shift in the electrode potential (Est) of the apex of a cracklike defect in comparison with the integral steady potential of the general surface of the investigated object. In this process a definite role must be assigned to the thermodynamic stability of the water since depending upon the pH of the solution and the value of the electrode potential either decomposition of the water with the liberation of hydrogen or oxygen or the formation of water may occur [i]. Despite attempts at direct measurement of some electrochemical characteristics (electrode potential, solution pH) in the microvolume at the apex of cracklike defects, there is still a lack of clarity with regards to this question. It has been clearly established only that under these conditions at the apex of a sharp-ended defect (or crack) in tests with low alloy and 18-8 chrome--nickel austenitic stainless steels and titanium alloys the pH of a chloride solution drops to 3.5-1 with a solution pH in the macrovolume of 6.3 [2-5]. The sharp change in the pH of the solution in the microvolume of a cracklike defect occurs as a result of separation of the anodic and cathodic elements when the products of the electrode reactions are not mixed. Depending upon the nature of the hydrating ions, there is a change in the concentration of hydrogen ions and, consequently, in the pH of the solution and the intensity of this change predetermines the course of the further local electrochemical process. To enlighten the role of the individual hydrating elements of alloys during local electrochemical corrosion, a series of experiments was made with separation of the anolyte and catholyte in corrosion in a 3% NaCI solution (pH = 6.5) with the application of anodic and cathodic polarization of 12KhI8NIOT steel, AT3 titanium alloy, DI6 aluminum alloy, and L-59 brass, and also of metals included in their composition either as bases or as alloying elements (Table I). The anodic polarization made it possible to judge the degree of change in the solution pH in the anolyte in intense corrosion of an alloy or metal. The cathodic polarization almost completely suppressed general corrosion and a change in the characteristics of the solution occurred as a result of decomposition of water: H20 + e- + 1/2 H2 + OH-. The tests were made in an electrochemical cell with volumes of anolyte and catholyte separated by a glass filter (Fig. I). The volume of anolyte solution in which the pH was determined and the chemical analysis of it for the presence and quantity of metal ions was made was 16 cm s. The working electrode was polarized with a P-5827M electronic potentiostat, a silver chloride electrode was used as the comparison electrode, and the auxiliary electrode was a platinum plate. In order to decrease the influence of oxygen on the electrochemical processes in the investigated material (working electrode) cleaned and dried argon was bubbled through the solution, which partially imitated the character of the solution in the microvolume of a developing microdefect or a narrow slit, access of oxygen to which from the macrovolume is difficult and in which convective solution exchange is absent and diffusion processes are weak. The lO-cm 2 working electrode was made of plates of the investigated materials rolled to a thickness of 0.i mm.

Mechanics of stress corrosion fracture

It should be emphasized that the influence of the environment is sometimes very substantial even under normal atmospheric conditions, due to the presence of moisture. Sometimes there is a combination of hydrogen embrittlement and electrochemical corrosion mechanisms, leading to characteristic linearconstant diagrams (hydrogen embrittlement prevails in the linear sector, and electrochemical corrosion in the sector where KI = const). A theoretical description of crack growth in metals exposed to various active environmental constituents is given in [5]. Local hydrogen absorption and local electrochemical corrosion at the crack tip are the most important (practical) mechanisms of subcritical crack growth [5]; [39–44] are also devoted to theoretical study of the latter mechanisms.