Localized Corrosion Characteristics Research Articles

Localized corrosion characteristics of Al-Mg-Si aluminum alloy with aging time and pre-delayed aging condition

The effects of single aging, delayed aging and pre-delayed aging treatment on the corrosion resistance of Al-Mg-Si alloy were studied by scanning Kelvin probe force microscopy (SKPFM), transmission electron microscopy (TEM), scanning electron microscopy (SEM), three-dimensional X-ray tomography (3D-XRT), confocal laser scanning microscopy (CLSM) and electrochemical measurements. The surface potentials of Al(MnFe)Si, Mg2Si, and Si-rich remnant phases are determined, identifying Al(MnFe)Si phase as the crucial cathodic site for corrosion initiation, and the increase in surface potential implies an accelerated corrosion rate after immersion. The intergranular corrosion susceptibility of the alloy is predominantly influenced by the microstructure caused by aging conditions. The factors affecting corrosion resistance are summarized as (i) adjustment of electrochemical reaction activity by controlling the area fraction of Al(MnFe)Si phase, (ii) the spacing of grain boundary precipitates (GBPs), and (iii) the width of the precipitation-free zone (PFZ). The delayed aging (DA) sample exhibits inferior corrosion resistance due to the continuous GBPs and extended sub-grain boundaries corrosion propagation paths. The pre-delayed aging treatment successfully mitigates the corrosion degree of DA by widening the GBPs spacing and narrowing the PFZ width. This suggests that pre-aging treatment is a potent strategy for enhancing corrosion resistance without compromising the alloy's strength.

Corrosion Behavior of Nickel–Titanium Continuous-Casted Alloys

Variations in the corrosion behavior of biomedical NiTi alloys in Cl− containing and acidic environments present a problem with their biological implantation. The objective of this research was to evaluate the synergy of the microstructure, the corrosion behavior, and the biocompatibility of novel continuous-cast NiTi alloys and to compare them with commercial NiTi alloys. The two alloys have a practically identical nominal chemical composition, but they differ in production technology. The continuous casting technology involved vacuum induction melting of the basic components and vertical continuous casting, while the commercial NiTi alloy was produced through a process of casting, hot rolling, and forming into square shapes. The microstructure was revealed to determine the surface area and size of grains. The corrosion of a commercial nickel–titanium alloy and one prepared by a novel continuous casting method in acidic and chloride-containing solutions was studied via analytical and electrochemical tests. Localized corrosion characteristics related to oxide properties, when exposed to 9 g L−1 NaCl solution, were examined with focused ion beam analysis and subsequent microchemical analysis of the formed corrosive products. Corrosion potential over time and the oxide film resistance were analyzed using linear polarization measurements. To obtain a preliminary estimate of biocompatibility, human fibroblast cells were used in indirect contact, i.e., alloy conditioning medium. The continuous casting method resulted in a reduction in the average grain size in comparison to the commercial sample and better corrosion stability of the sample in an acidic environment. Also, in a solution of 9 g L−1 NaCl the commercial sample showed high values for the corrosion current density (jcorr = 6 μA cm−2), which indicated low corrosion resistance, while the continuous casting sample possessed much better corrosion stability and lower values for the corrosion current density (jcorr = 0.2 μA cm−2). In line with that, elemental analysis of the corroded surfaces showed higher Cl− ion deposition over the surface layer of the commercial sample, suggesting local oxide breakdown. Moreover, NiTicc reached a value three times higher for polarization resistance (Rp = 270 kΩ cm2) over time in comparison to the commercial sample (Rp~100 kΩ cm2). Biocompatibility evaluation showed that human fibroblast cells exhibited altered metabolic activity. An MTT assay showed that cells’ mitochondrial activity dropped below that of control cells in the presence of both materials’ supernatants.

Development of trimetallic cobalt-nickel-vanadium oxide anticorrosion coatings for the protection of AA2024 Al alloy specimens against sodium chloride solutions

Trimetallic cobalt-nickel-vanadium oxide (CoNiVOx) thin films were fabricated on aerospace grade AA2024 Al alloy specimens to investigate their behavior as anticorrosion coatings in a sodium chloride (NaCl) medium. Compact and homogenous nanostructured CoNiVOx coatings were developed using aerosol-assisted chemical vapor deposition by varying the deposition duration from 45 to 150 min at a constant temperature (475 °C). The structure and composition of the trimetallic CoNiVOx thin films were comprehensively investigated using X-ray diffraction, transmission electron microscopy, scanning electron microscopy/energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy, which demonstrated the successful synthesis of CoO/NiO/VO composite materials. The electrochemical corrosion properties of the trimetallic CoNiVOx films in a 3.5 % NaCl solution were determined using Tafel extrapolation and impedance spectroscopic tests, whereas the electrochemical stability and localized corrosion characteristics were determined using scanning electrochemical microscopic analyses. This study demonstrates that the trimetallic CoNiVOx oxide layer fabricated for 150 min exhibited an effective barrier protection performance on Al surfaces against corrosive attack in NaCl media.

Simultaneous enhancement of mechanical properties and corrosion resistance of as-cast Mg-5Zn via microstructural modification by friction stir processing

Magnesium alloys are ideal lightweight materials; however, their applications are extremely limited due to their low strength, poor ductility, and weak corrosion resistance. In the present study, a friction stir processing (FSP) treatment was employed to optimize the mechanical properties and corrosion resistance of an as-cast Mg-5Zn alloy. The average grain size of the Mg-5Zn alloy was refined from 133.8 µm to 1.3 µm as a result of FSP. Along different directions, FSP exhibited the enhancement effects on different mechanical properties. Furthermore, according to the potentiodynamic polarization results, the corrosion current density at the free-corrosion potential of the FSPed sample, was 4.1 × 10−6 A/cm2 in 3.5 wt.% NaCl aqueous solution, which was significantly lower than that of the as-cast sample. Electrochemical impedance spectroscopy revealed that the polarization impedance, Rp, of the FSPed sample was 1534 Ω/cm2 in 3.5 wt.% NaCl aqueous solution, which was 71.4% greater than that of the as-cast sample. The corrosion morphology of the FSPed sample in 3.5 wt.% NaCl aqueous solution exhibited largely uniform corrosion, rather than severe localized corrosion characteristics, which further reduced the corrosion depth on the basis of reducing the corrosion current density. The results presented herein indicate that FSP is a viable technique for simultaneously improving the mechanical properties and corrosion resistance of the as-cast Mg-5Zn alloy.

STUDY ON CORROSION OF TI-6AL-4V ALLOY IN 3.5% NACL SOLUTION BY WIRE BEAM ELECTRODE

This work aims to study the variation mechanism of corrosion-resistance performance of the oxidization film spontaneously generated on Ti-6Al-4V (TC4) titanium alloy surface by using the open circuit potential (OCP), Alternating Current (AC) impedance, and polarization curve. Combined with the method of wire beam electrode, the corrosion behaviors of the microregion of TC4 titanium alloy immersed in 3.5% NaCl solution during the formation of film were investigated. Results show that the corrosion-resistance performance of TC4 titanium alloy increased with the formation of oxidization film, with the electrochemical testing results showing that the OCP and AC impedance values were increased, while the corrosion current density reduced. When the time of film formed on the surface of the alloy was 24[Formula: see text]h, the impedance value of the TC4 alloy was an order of magnitude greater than that before oxidation, then the impedance value was no longer increased and the corrosion-resistance performance was best. The anode current and localized corrosion characteristics of the TC4 titanium alloy submerged in 3.5% NaCl solution during the earlier formation stage of film were great. With the formation of oxidized film, the peak value of anode current and its region gradually reduced, and both standard deviations of the circuit potential and circuit were narrowed, as well as the reduction in the difference of localized corrosion.

A Study into the Localized Corrosion of Magnesium Alloy Magnox Al-80

Magnesium (Mg) nonoxidizing alloy, known as Magnox, was historically used as a fuel cladding material for the first generation of carbon dioxide gas-cooled nuclear reactors in the United Kingdom. Waste Magnox is currently stored in cooling ponds, pending final disposal. The corrosion resistance of Mg and its alloys is relatively poor compared to modern cladding materials, such as zirconium alloys, so it is important to have a knowledge of the chloride concentration/pH dependence on breakdown and localized corrosion characteristics prior to waste retrievals taking place. These results show that Magnox exhibits passivity in high-pH solutions, with charge transfer resistance and passive film thicknesses showing an increase with immersion time. When chloride is added to the system, the higher pH maintains Magnox passivity, as shown through a combination of potentiodynamic and time-lapse/post-corrosion imaging experiments. Potentiodynamic polarization of Magnox reveals a −229 mV/decade linear dependence of breakdown potential with chloride ion concentration. The use of the scanning vibrating electrode technique enabled the localized corrosion characteristics to be followed. At high pH where Magnox is passive, at low chloride concentrations, the anodes that form predominantly couple to the visually intact surface in the vicinity of the anode. The high pH, however, means that visually intact Magnox in the vicinity of the anode is less prone to breakdown, restricting anode propagation such that the anodes remain largely static. In high-chloride concentrations, the higher conductivity means that the anode and cathode can couple over greater distances, and so propagation along the surface can occur at a much faster rate, with the visually intact surface acting as a distributed cathode. In addition, the chloride anion itself, when present at high concentration, will play a role in rapid passive film dissolution, enabling rapid anode propagation.

The Influence of Chloride Ion Concentration on the Localized Corrosion of E717 Magnesium Alloy

The localized corrosion behavior of E717 magnesium alloy immersed in chloride-containing electrolyte is investigated using an in situ scanning vibrating electrode technique (SVET) coupled with time-lapse imaging. It is shown that initiation of localized corrosion in chloride-containing electrolyte is characterized by the appearance of discrete local anodes, corresponding with the leading edges of dark, filiform-like features, which combine with time to produce a mobile anodic front. The size and growth rate of these features are highly dependent on the chloride ion concentration of the electrolyte. SVET-derived current density maps reveal that the corroded surface left behind the anodic front is cathodically activated, where cathodic current density values progressively decline with increasing distance away from the anodic leading edge. The intensity of localized anodes is highly dependent on the chloride ion concentration, where progressively higher local anodic current density values are observed with increasing chloride ion concentration along with progressively higher rates of volumetrically determined hydrogen evolution. Breakdown potential, measured using time-dependent free corrosion potential transients and potentiodynamic polarization at neutral and elevated pH, respectively, is shown to vary with the logarithm of chloride ion concentration, and the time for localized corrosion initiation is progressively increased with decreasing chloride concentration. From the combination of results that are presented herein, the underlying reasons for the influence of chloride ion concentration on the localized corrosion characteristics of E717 alloy will be discussed.

The study of under deposit corrosion of carbon steel in the flowback water during shale gas production

Serious under deposit corrosion could occur in the shale gas flowback water environment. In this work, under deposit corrosion of Q235 carbon steel in a simulated CO2/O2-containing shale gas flowback water was studied using wire beam electrode, potentiodynamic polarization curves, electrochemical impedance spectroscopy, scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The results show that there is a galvanic corrosion effect between the deposit-covered electrode and bare electrode. The polarity of the deposit-covered electrode and bare electrode is reversed at 3 h. Before 3 h immersion, deposit-covered electrode acts as cathode and bare electrode acts as anode. However, after 3 h immersion, deposit-covered electrode turns into anode while bare electrode converts into cathode. The obvious localized corrosion characteristics can be observed under deposit and the corrosion products mainly consist of Fe2O3, Fe3O4 and FeCO3. WBE can be used to in situ monitor the occurrence and development of local corrosion under deposit. The current maps of WBE show that severe local corrosion has occurred on the electrodes covered by deposit.

Effect of Manganese on Microstructure and Corrosion Behavior of the Mg-3Al Alloys

Microstructure and corrosion behavior of the Mg-3Al-xMn (x = 0, 0.12, 0.21, 0.36, 0.45) (hereafter in wt.%) alloys were experimentally investigated by electron probe microanalysis (EPMA), scanning electron microscope equipped with energy dispersive X-ray spectroscopy (SEM/EDX), X-ray diffraction (XRD), electrochemical, and hydrogen evolution tests. A new self-constructed Mg-Al-Mn-Fe thermodynamic database was used to predict the solidification paths of the alloys. The addition of Mn showed no grain refinement in the cast Mg-3Al alloys. According to the microstructure observation, Al-Fe phases were observed in the non-Mn-added alloy, while Al8Mn5(LT) (Al8Mn5 in low temperature) became the main intermetallic phase in the Mn-added alloys, and the amount increased gradually with the Mn addition. The τ–Al0.89Mn1.11 phase with lower Al/(Fe + Mn) ratio was observed in the alloys with 0.36 and 0.45 wt.% Mn content. According to the electrochemical tests, all five alloys showed localized corrosion characteristics in 3.5 wt.% NaCl solution. Compared with the Mg-3Al alloy, the corrosion resistance of Mn-added alloys were significantly improved and increased gradually with the Mn addition, which was due to the variation of Al-containing intermetallic compounds. The present experimental investigations and thermodynamic calculations confirmed the mechanism that the increasing amount of Al8Mn5(LT) with Mn addition could encapsulate the B2-Al(Mn,Fe) phase with higher Fe. Therefore, it could prevent this detrimental phase from contacting magnesium matrix, thus suppressing micro-galvanic corrosion and improving corrosion resistance gradually.

Effect of Aging on the Corrosion Resistance of 2209 Duplex Stainless Steel Weldments

In this study, we investigated the correlation between the microstructure and the localized corrosion characteristics for a weld metal made up of 22Cr-9Ni duplex stainless steel (DSS) aged in the range of 400–1000 °C for 30 min. The results showed that changes in the microstructure of the DSS weldments, owing to the formation of precipitates during the aging process, affected its pitting corrosion resistance. The microstructural evolution was found to be extremely sensitive to the secondary phase precipitations such as chi (χ), sigma (σ), chromium nitride (Cr2N), and secondary austenite (γ2) in the aging range of 700–900 °C. The potentiodynamic polarization tests confirmed that the pitting potential representing the corrosion resistance decreased gradually from 500 °C, attained the lowest at 850 °C, and increased again at 900 °C. In addition, the Cr or Mo deficient area around σ phase was transformed into the secondary austenite (γ2) phase during aging where the pitting was found to occur.

Localized Corrosion Characteristics of Nickel Alloys: A Review

There are a great variety of commercial nickel alloys mainly because nickel is able to dissolve a large amount of alloying elements while maintaining a single ductile austenitic phase. Nickel alloys are generally designed for and used in highly aggressive environments, for example, those where stainless steels may experience pitting corrosion or environmentally assisted cracking. While nickel alloys are generally resistant to pitting corrosion in chloride-containing environments, they may be prone to crevice corrosion attack. Addition of chromium, molybdenum and tungsten increases the localized corrosion resistance of nickel alloys. This review on the resistance to localized corrosion of nickel alloys includes specific environments such as those present in oil and gas upstream operations, in the chemical process industry and in seawater service.

アルカリ水溶液中におけるアルミニウム合金クラッド材の局部腐食

Aluminum alloy three-layered brazing sheets, clad with sacrificial anode alloy on one side of them, have widely been used for the tube of radiators for automotive in which cooling water circulates. Inner pitting corrosion resistance of the tube has been excellent both in acidified and in neutral corrosive aqueous solutions by the effect of sacrificial anode. On the other hand, it has been reported that the sacrificial anode does not work effectively in alkaline solution, and perforation by pitting corrosion occurs in earlier testing period of time. In this report, localized corrosion characteristics in alkaline solution were investigated by immersion corrosion tests and electrochemical measurements. Based on the experimental results, new corrosion mechanism was proposed in terms of solution environment formed on cathode surface. Pitting corrosion of the core alloy grew not with the attack by the alkaline bulk solution, but with accelerated dissolution by strong alkaline solution derived from the electrochemical cell formation between sacrificial anode and cathodic core alloy. Corrosion products formed in the pit were effective for maintaining strong alkaline environment on the core alloy surface.

海水環境における窒素添加ステンレス鋼の局部腐食特性に及ぼす合金元素の影響とすき間腐食の発生評価

海水環境における窒素添加ステンレス鋼の局部腐食特性に及ぼす合金元素の影響とすき間腐食の発生評価

Al–Si合金ろう材の電気化学的性質に及ぼすCuとZn添加の影響

Electrochemical measurements as well as immersion corrosion tests were carried out in order to investigate the effect of Cu and Zn content in the filler on localized corrosion characteristics and corrosion rate of the filler, and on pitting corrosion resistance of Al–Mn brazing sheet. Both Cu and Zn segregated at eutectic α–phase of filler after brazing. Selective dissolution of eutectic α-phase occurred not only in Al–Si alloy filler but also in Al–Si–Zn and Al–Si–Cu alloys after brazing. In Al–9%Si–2%Zn alloy fillers containing more than 0.1%Cu, both inner primary α and eutectic α–phase of filler dissolved and the outer primary α undissolved. Dissolution characteristics of filler were closely related to the pitting potential difference between the phases depending on solid solubility of Si, Cu and Zn. Brazing sheet clad with the Zn containing filler can be pitting corrosion resistant, because corrosion rate of filler was not so high and the filler was effective as sacrificial anode for the core.

Pitting behavior of SiCp 2024 Al metal matrix composites

The effects of the volume fraction of SiC particulate reinforcements and the concentration of chloride ions in solution on the localized corrosion characteristics of SiCp/2024 Al metal matrix composites (MMC) were investigated. A scanning micro reference electrode (SMRE) technique was employed to study the dynamic process of pitting initiation and development on the surface of the composites at open-circuit potential. Potentiodynamic polarizations were performed to characterize the electrochemical behavior of the MMCs. The morphology of the localized attack on the MMC sample after corrosion tests were examined by scanning electron microscopy (SEM). The results of electrochemical measurement showed that the composites were less resistant to pit initiation than the corresponding unreinforced metrix alloy. Increase in the volume fraction of SiCp reinforcement in the SiCp/2024 Al composites resulted in a significant decrease of pitting potential. In situ potential mapping of active centers on the surfaces of the composites revealed that local breakdown of passivity and initiation of micro pitting corrosion could take place even at an open-circuit potential which was more negative than the pitting potential, and the number of active centers on the surfaces of the composites increased as the volume fraction of SiC particulates in MMCs increased. Micro-structural analysis indicated that pitting attack on the composites mainly occurred at SiCp-Al interfaces or inclusions-Al interfaces.

Effect of Ga Content on Localized Corrosion of Al-9 mass%Mg Alloys in H<SUB>2</SUB>SO<SUB>4</SUB>–NaCl Solution

The effects of Ga content and aging treatment on localized corrosion characteristics of Al-9 mass%Mg alloys which contain a small amount of Ga were investigated by the measurements of polarization curves, potentiostatic corrosion tests and immersion corrosion tests. The scanning electron micrographs of precipitates of the secondary phase with aging treatment and the surface of the specimens corroded in NaCI containing 1 x 10 -2 kmol/m 3 H 2 SO 4 solution at 303 K were also observed. The principal conclusions are as follows: (1) The Al 3 Mg 2 compounds of the secondary phase of Al-9 mass%Mg alloys aged at 453 K for 6.0 ks precipitate along the grain boundary irrespective of the specimens used. However, rodlet precipitates are observed even within the grain, and precipitate-free zones are formed along the grain boundary of the specimens aged at 453 K for 307.2 ks. (2) The pitting potential E pit of Al-9 mass%Mg alloys is shifted to a more basic potential with increasing NaCI concentration, Ga content in the alloys and aging time intervals, and thus the degradation of localized corrosion resistance were observed. (3) The apparent anodic dissolution rates under the potentiostatic condition are enhanced with increasing Ga content in the alloys and aging time. The rates are increased in the order of super-saturated solid solution<the specimens aged at 453 K for 6.0 ks<the specimens aged at 453 K for 307.2 ks. (4) The corrosion of super-saturated Al-9 mass%Mg alloys in H 2 SO 4 -NaCl solution is mainly pitting attack. However, the localized types of corrosion of the aged specimens are observed as intergranular and pitting attacks.

Ａｌ‐Ｓｉ合金ろう材の電気化学的性質

To investigate localized corrosion characteristics of Al–Si alloy brazing filler, electrochemical measurements as well as immersion corrosion tests were carried out. Selective dissolution of eutectic phase occurred in the filler after brazing. On the other hand, no selective dissolution occurred in the filler with solution heat treatment after brazing. Pitting potential increased with increasing Si content in solid solution. Though cathodic reaction rate increased with an increase in Si content of the filler, the effect of Si itself was rather small. It was concluded that the localized corrosion of filler is caused by smaller Si content in solid solution at eutectic than that at primary α phase, and that matrix/Si particle interface acts as a more active cathode than matrix and Si particle.