Path Corrosion Research Articles

Characterisation of coelectrodeposited Ni–Al composite coating

Ni–Al composite coating was prepared by coelectrodeposition technique in Watt’s bath containing dispersed Al particles to be codeposited. Pure Ni coating was also prepared as a comparison. Morphology and composition studies showed that Al particles were uniformly distributed in the Ni matrix. Microhardness measurements and microstructure studies revealed that the incorporation of Al particles lowers the microhardness of Ni coating due to the high ductility of Al and weak bonds between Al and Ni. The results of potentiodynamic polarisation tests demonstrated that the corrosion resistance of Ni–Al composite coating is lower than that of pure Ni. The microstructure and corrosion path studies revealed that the lower corrosion resistance of Ni–Al composite coating is resulted from weak bonds between Al particles and Ni matrix and formation of active–passive cells on the surface of the coating.

Corrosion behavior of spray formed 7055 aluminum alloy joint welded by underwater friction stir welding

Abstract Friction stir welding with water cooling method had modified the microstructure of spray formed 7055 aluminum alloy but the corrosion behavior of alloy had been affected as well. Despite the tensile property having increased 15%, the corrosion behavior of underwater joint was more complex than base metal and traditional joint. Underwater joint characterized with an atypical pattern of intergranular corrosion (IGC) that its corrosion paths were not along the grain boundary though it still suffered a rank 4 IGC. At the same time, underwater joint was slightly sensitive to exfoliation corrosion (EXCO) and crossed 3-level-leap of corrosion rank within 24 h. Tafer curves illustrated more negative corrosion potential and lower corrosion current density for underwater joint than base metal and traditional joint. High potential phase MgCu 2 in traditional joint blocked corrosion process to some extend but non-uniform distribution of MgCu 2 led to weak zone for corrosion. In comparison with traditional joint, underwater joint avoided the formation of MgCu 2 and kept a large quantity of efficient strengthening particles (MgZn 2 ) in the matrix. Though MgZn 2 with more negative potential than that of MgCu 2 would lead to a higher sensitivity to corrosion, underwater joint still showed better corrosion resistance compared with base metal. In the process of underwater friction stir welding, MgZn 2 had been stirred into grains leaving grain boundary a high potential zone. Since corrosion paths were discontinuous and not along with the grain boundaries, corrosion process was blocked so it performed better resistance to corrosion and corrosion rate of underwater joint had decreased a lot compared with traditional joint and base metal.

Modeling of Combined Anodic Dissolution/Hydrogen-Assisted Stress Corrosion Cracking of Low-Alloyed Power Plant Steels in High-Temperature Water Environments

High-temperature slow strain rate testing (SSRT) – ductility data from cathodic hydrogen charging of 10CrMo910 and 7CrMoVTiB1010 are implemented into a mathematical model of stress corrosion cracking (SCC) at various operational conditions including temperatures, pH, and dissolved hydrogen/oxygen contents as well as global stresses and material properties. The results show that, as a consequence of experimentally verified local acidification at initial anodic path corrosion, subsequent local hydrogen-assisted cracking can be a controlling factor for SCC in high-temperature water. As a particular effect at global stresses close to the yield point, operational temperatures around 270°C exhibit peak crack growth rates depending on dissolved hydrogen, oxygen, pH, and global stress, which has been found to be consistent for both experimental studies and mathematical modeling.

A Model for Predicting the Stress Concentration of Intergranular Corrosion around a Fastener Hole

This paper presents a model that predicts the stress field around intergranular corrosion. The stress analysis is conducted in ABAQUS via a Python input script, which is written in Igor Pro. The intergranular corrosion path is described using a Monte-Carlo Markov Chain based on the materials grain size distribution and probability that the corrosion will turn at a grain boundary junction. The model allows a complete analysis of the stresses resulting from intergranular corrosion around a fastener hole of any size. As fatigue initiation is most likely to occur at the highest stress concentration, this model gives an understanding of which of the features of intergranular corrosion are most critical and can allow for the development of beta solutions for crack growth. This model has been applied to 7075-T651 extruded aluminum alloy from a legacy era aircraft but can be readily applied to any material where the microstructure is known and can be described using a statistical distribution.

Modelling the effects of intergranular corrosion around a fastener hole in 7075-T651 aluminium alloy

Intergranular corrosion (IGC) is potentially a major concern for the structural integrity of aerospace aluminium alloys; the crack paths and networks formed make it particularly difficult to predict its impact on structural integrity, even when the likely location of the corrosion is known. IGC is a particular problem for the Royal Australian Air Force AP-3C Orion aircraft, where it is causing a significant increase to maintenance loads due to the unknown impact on structural integrity.In this paper a brick wall-style computational analysis is combined with a python script to create a finite element (FE) model of a plate-with-hole containing a random IGC path. The brick wall style analysis was chosen over measuring actual corrosion paths as analysing a large number of paths takes significant amounts of material, which is at a premium as it needs to be taken from the aircraft. The FE model produced realistic stress concentrations at the bore of the hole and showed that an IGC path that deviates vertically along its length will produce a small but noticeable change in the local stress concentration.

Effect of particle size and co-deposition technique on hardness and corrosion properties of Ni–Co/SiC composite coatings

Ni–Co/SiC alloy matrix composite coatings were electrodeposited in a modified Watt's bath containing micro and nano sized SiC particles by using conventional electro-co-deposition (CECD) and sediment co-deposition (SCD) techniques. The deposits were characterized using SEM, EDX and XRD analyses, and microhardness and potentiodynamic polarization measurements. The maximum incorporation of the SiC micro- and nano-particles was obtained using the SCD technique at deposition current densities of 2 and 3A/dm2, respectively. It was found that in the composite coatings, incorporation of SiC particles improves the microhardness of unalloyed Ni and Ni–Co alloy matrices. The nanocomposite coatings exhibit higher microhardness values than microcomposite ones. The potentiodynamic polarization measurements in 3.5% NaCl solution revealed that the corrosion resistance of the Ni–Co/SiC nanocomposite coatings is much higher than the Ni–Co alloy and Ni–Co/SiC microcomposite coatings. Moreover, corrosion resistance of Ni–Co/SiC nanocomposite coatings deposited by SCD technique is higher than the ones deposited by CECD technique. Corrosion resistance of the studied Ni–Co/SiC composite coatings was considerably affected by Co content, SiC particle size and content. Hardness enhancement was related to the structural features, and corrosion behavior was discussed based on the formation of corrosion micro cells, diminishing the effective metallic area, and increasing and hindering the corrosion paths.

Investigation of the corrosion behaviors of continuously hot-dip galvanizing Zn–Mg coating

Corrosion behaviors of continuously hot-dip galvanizing Zn–Mg (ZM) coating with 0.2wt.% Al containing 1–3wt.% Mg and Zn (GI) coating with 0.2wt.% Al in an aerated and quiescent 5wt.% NaCl solution were investigated. The linear polarization method was used to study corrosion path of GI and ZM coatings. The experimental results show that the grain corrosion in the ZM coatings can be significantly inhibited by Mg after 24hour immersion especially for the Zn coating containing 2wt.% Mg. Localized corrosion occurred within Zn grains or Mg-rich regions when the Mg content is not 2wt.%. The results of electrochemical impedance spectroscopy (EIS) also proved the results mentioned above. It was also found that Mg can significantly improve the adherence of the corrosion product layer and reduce surface cracks especially for a content of 2wt.% Mg in the coating after 11days of linear polarization treatment. The corrosion products of GI coating are as same as ZM coatings after the linear polarization treatment. The enhanced corrosion resistance of ZM coatings is mainly due to grain corrosion inhibition and strong adherence of corrosion products.

Modeling of SCC initiation and propagation mechanisms in BWR environments

During operation of mainly BWRs’ (Boiling Water Reactors) excursions from recommended water chemistries may provide favorite conditions for stress corrosion cracking (SCC). Maximum levels for chloride and sulfate ion contents for avoiding local corrosion are therefore given in respective water specifications. In a previously published deterministic 288 °C – corrosion model for Nickel as a main alloying element of BWR components it was demonstrated that, as a theoretically worst case, bulk water chloride levels as low as 30 ppb provide local chloride ion accumulation, dissolution of passivating nickel oxide and precipitation of nickel chlorides followed by subsequent local acidification. In an extension of the above model to SCC the following work shows that, in a first step, local anodic path corrosion with subsequent oxide breakdown, chloride salt formation and acidification at 288 °C would establish local cathodic reduction of accumulated hydrogen ions inside the crack tip fluid. In a second step, local hydrogen reduction charges and increasing local crack tip strains from increasing crack lengths at given global stresses are time stepwise calculated and related to experimentally determined crack critical cathodic hydrogen charges and fracture strains taken from small scale SSRT tensile tests pieces. As a result, at local hydrogen equilibrium potentials higher than those of nickel in the crack tip solution, hydrogen ion reduction initiates hydrogen crack propagation that is enhanced with increasing global stresses. In accordance with respective experimental literature data it is shown that decreasing chloride and increasing pH levels of the primary bulk water at 288 °C reduce the total crack propagation rates including anodic path corrosion as well as hydrogen cracking. It is also demonstrated that crack propagation rates can be significantly suppressed by hydrogen water chemistry (HWC) that leads to reduction of bulk surface corrosion potentials. As a conclusion the extended SSC-model for nickel supplies quantitative insight into the frequently controversially discussed high temperature SCC mechanisms of a basic alloying element of BWR components.

Modeling the Effects of Local Anodic Acidification on Stress Corrosion Cracking of Nickel

Abstract In the author's recent work, effects of pH, chlorides, and sulfides on local anodic acidification has been modeled by coupling electrochemical polarization and phase precipitation inside occluded cells of nickel and iron. In the present work, the model is experimentally verified and extended to demonstrate the capability of accumulated and reduced hydrogen ions to provide critical conditions for local hydrogen-assisted crack propagation. Based on experimentally determined crack critical hydrogen reduction charge densities and crack initiation strains from small scale slow strain rate testing (SSRT), the model explains quantitatively the effects of chlorides, hydrogen sulfide (H2S), pH, and global mechanical stress on cracking by hydrogen that is locally produced by ion reduction at the tip of an advancing anodic corrosion path. In particular, the known effects of pH, chlorides, and H2S on cracking of metallic materials applied in the oil and gas industry are reflected by the results of the calcul...

Correlation Between Sn Grain Orientation and Corrosion in Sn-Ag-Cu Solder Interconnects

The impact of a marine environment on Sn-Ag-Cu interconnect reliability is examined using salt spray exposure followed by thermal cycling. Sn-Ag-Cu solder alloy wafer-level packages, with and without pretreatment with 5% NaCl salt spray, were thermally cycled to failure. The prior salt spray reduced the characteristic lifetime of the Sn-Ag-Cu solder joints by over 43%. Although Sn-based materials show strong resistance to corrosion, the nature of localized corroded areas at critical locations in the solder joint caused significant degradation in the Sn-Ag-Cu solder joints. An important link between the corrosion path and Sn grain orientation was observed using orientation imaging microscopy (OIM). A strong correlation between the corrosion path and grain orientation was identified, indicating that the corrosion attack preferentially followed the basal plane of the Sn lattice.

Hydrogen Embrittlement Type Stress Corrosion Cracking Behavior of Wrought Magnesium Alloy AZ31

Stress corrosion cracking (SCC) behavior of wrought magnesium alloy, AZ31, was investigated. SCC tests were performed using compact tension (CT) specimens in 3%NaCl solution under hydrogen charged conditions, where the cathodic potentials were -1.4, -2.5 and -3.0V. According to Pourbaix diagram, the cathodic potentials of -2.5 and -3.0V correspond to the immunity region. Under the immunity condition, hydrogen embrittlement (HE) type SCC occurred where the KIscc value was much higher than that of active path corrosion (APC) type SCC. The KIscc value decreased and the crack propagation rates, da/dt, increased with decreasing cathodic potential in the immunity region where HE type SCC occurred. At the cathodic potential of -3.0V, da/dt was insensitive to the stress intensity factor.

THE EFFECT OF HIGH TEMPERATURE CORROSION ON MECHANICAL BEHAVIOR OF A GAMMA-TiAl ALLOY

The mechanical properties of Ti -48 Al -2 Cr -2 Nb alloy were discussed after the high temperature corrosion tests carried out with salt mixture of 75wt. % Na 2 SO 4 and 25wt. % NaCl at 800°C. The microstructure of the alloy after corrosion was observed by SEM and the fracture behavior of the corroded and uncorroded alloys was investigated by means of the three-point bending tests. It has been shown that the corrosion path was mainly along the lamellar structure and rough surface with a large number of corrosion pits formed during the high temperature corrosion. The experimental results also indicated that the bearing capacity of bending fracture descended evidently due to the molten salt corrosion at high temperature, which only had remarkable effects on the surface state of the alloy. The microcracks inside the alloy always propagated along the phase interfaces and grain boundaries while the corrosion pits on salt-deposited surface became the main crack initiation location in corroded alloy. The stress concentration caused by corrosion was considered as the essential reason of the property reduction, which decreased the energy barrier of crack nucleation and shortened the incubation period.

Effect of the microstructure on the properties of Ni–P deposits on heat transfer surface

In the present work Ni–P deposits with various microstructures were prepared on the low carbon steel substrate by adjusting the electroless plating processing parameters. The results of anti-fouling experiments show that Ni–P deposits can greatly inhibit the adhesion of crystallization fouling in comparison with uncoated copper, stainless steel and carbon steel surfaces. The further experiments indicated that the amount of nanocrystalline phase in the Ni–P deposit affects the surface free energy, as well as changing the anti-fouling and corrosion-resistant properties of the deposit. When the phase structure of the Ni–P deposit is almost amorphous phase, the Ni–P deposit exhibits the best anti-fouling and corrosion-resistant properties. The amorphous Ni–P deposit exhibits high corrosion resistance which is attributed to its extreme structure homogeneity without preferential corrosion paths like grain boundaries or other structural defects, and the higher corrosion resistance could ensure this deposit is not easy to form “transitional interface” connecting fouling and matrix.

In situ monitoring of corrosion processes within the bulk of AlMgSi alloys using X-ray microtomography

Susceptibility to localized corrosion of AlMgSi (AA6016 and AA6111) alloys in certain aggressive environments is high. In this study, synchrotron X-ray microtomography was used to monitor non-destructively corrosion processes within bulk materials. In the selected aggressive solutions, surface–deformed layers showed high localized corrosion susceptibility, but are more stable than the bulk of the alloy during corrosion propagation. In addition, exfoliation-like attack was observed as a transition from intergranular attack. This directed corrosion is not related to grain or crystallographic structure. Finally, intermetallic particles dissolution inside the materials after contact with aggressive solution of the intergranular corrosion path was evidenced.

The long-term behaviors of passivation and hydride layer of commercial grade pure titanium in TRU waste disposal environments

Preservation of the passivity under reducing environmental conditions for extended periods of time and the behavior of hydrogen evolution as the results of the preservation of the passivity of several candidate commercial grade pure titanium related to the small amount of palladium addition, such as Ti–Gr.17 for metallic containers to be buried under deep ground for disposing of transuranic (TRU) waste is investigated. The present investigation has revealed the following corrosion paths for the titanium alloys investigated. The passivity of the alloys is preserved as the result of repeated destruction and recovery of the surface films on the alloys. The long-term corrosion rate under the preserved passivity is of the order of 10 −6–10 −8 my −1 with evolution of hydrogen. The substrate alloys absorb parts of the hydrogen generated to form lath-type hydride phase before forming hydride layers at the final stage.

ＳＳＲＴ条件下におけるＭＩＭ法により作製されたＳＵＳ３０４ステンレス鋼の腐食特性

In this study, the corrosion behavior of MIM (Metal Injection Molding) specimen has been investigated by SSRT (Slow Strain Rate Tensile) test and EIS (Electrochemical Impedance Spectroscopy) method in comparison with I/M (Ingot Metallurgy) specimen. In the SSRT test, the aqueous solution composed of 2.5 kmol·m−3 H2SO4 and 0.20 kmol·m−3 NaCl (hereafter shortened as SCC solution) in which SCC occurs was used. In addition to this solution, the aqueous solution of 2.5 kmol·m−3 H2SO4 and 3.0 kmol·m−3 NaCl (hereafter shortened as general corrosion solution) in which general corrosion occurs was used in the EIS test.Cdl (Electric Double Layer Capacitance) of I/M specimen has increased abruptly at 7.2 ks during holding in the SCC solution under applied stress, while it has increased gradually without applied stress. On the other hand, the Cdl of MIM specimen has increased gradually increased irrespective of with or without applied stress for MIM specimen. The remarkable Rct difference was observed in two cases with or without applied stress in the SCC solution for both I/M and MIM specimen while the slight Rct difference was observed in the general corrosion solution. The fracture surface appearance by SEM showed that the typical transgranular SCC occurred on I/M specimen while the intergranular SCC along grain boundaries occurred on MIM specimen. It was also confirmed that the more significant HE-SCC (Hydrogen Embrittlement) and the weaker APC-SCC (Active Path Corrosion) were observed on the MIM specimen compared with the I/M specimen.

Molten-salt fuel cells—Technical and economic challenges

This paper presents a personal view of the status and research needs of the MCFC and other molten-salt fuel cells. After an overview of current MCFC performance, compared with performance and cost of other fuel cells, improvements in power density and lifetime as well as cost reduction are identified as key priorities to accelerate the commercialization of the MCFC. In spite of its unfavorable public image (compared to, in particular, PEMFC and planar SOFC) MCFC technology has progressed steadily and cost reduction has been significant. Large-scale commercialization, especially in the distributed generation and cogeneration market, remains a possibility but its chances are highly dependent on a forceful and consistent energy policy, for example taking into account the externalities associated with various modes of electric power production from fossil fuels. In spite of steady improvements in performance, important defects in fundamental knowledge remain about wetting properties, oxygen reduction kinetics, corrosion paths and control mechanisms. These must be addressed to stimulate further simplification of design and find solutions to lifetime issues. Recently, alternative concepts of molten-salt fuel cells have been capturing attention. The direct carbon fuel cell (DCFC), reviving an old concept, has caught the attention of energy system analysts and some important advances have been made in this technology. Direct CO and CH 4 oxidation have also been a focus of study. Finally, the potential of nanotechnology for high-temperature fuel cells should not be a priori excluded.

Statistical Modeling of Minimum Intergranular Corrosion Path Length in High-Strength Aluminum Alloy

In this article a brick wall model is developed to describe the relationship between the minimum intergranular corrosion path length and the aspect ratio of grains of high-strength wrought aluminum alloy AA2024-T3. We study the distribution of the horizontal distance that a corrosion path will travel in the metal and fit the model to an actual corrosion dataset using the method of moments. The distribution of the horizontal distance of a corrosion path along a given grain is assumed to be uniformly distributed given the length of the grain, which is itself modeled by a gamma distribution. A modified brick wall model is proposed that also imposes a distribution on the vertical distance traveled by the corrosion path. Computer simulation is used to evaluate the fit of these models.

Delayed fracture of Ni-Ti superelastic alloys in acidic and neutral fluoride solutions.

Hydrogen-related degradation of the mechanical properties of a Ni-Ti superelastic alloy has been examined by means of delayed fracture tests in acidic and neutral fluoride solutions and hydrogen thermal desorption analysis. Delayed fracture took place in both solutions; the time to fracture was shorter in the acidic solutions than in the neutral solutions with the same fluoride concentration. The time to fracture was reduced in both solutions when applied stress exceeded the critical stress for martensite transformation. In the acidic solutions, Ni-Ti superelastic alloy underwent general corrosion and absorbed substantial amounts of hydrogen. Fractographic features suggested that the delayed fracture in the acidic solutions was attributable to hydrogen embrittlement, whereas in the neutral solutions, a different fracture mode appeared associated with localized corrosion only in the vicinity of the fracture initiation area. In the neutral solutions, the amount of absorbed hydrogen was much less than that in the acidic solutions, and the delayed fracture was likely to be induced by active path corrosion accompanying hydrogen absorption. The results of the present study imply that the hydrogen-related degradation of performance of Ni-Ti superelastic alloys occurs in the presence of fluoride.

Statistical modeling and computer simulation of intergranular corrosion growth in AA2024-T3 aluminum alloy

An extension of a brick wall model developed in a previous paper (Technometrics, to appear) was used to describe intergranular corrosion in AA2024-T3 aluminum alloys. The extended model simulates the behavior of corrosion paths at intersections of grain boundaries within the metal sample. Situations considered include the cases where a corrosion path might assume an upward turn, skip an intersection (not turn) or split into branches. The splitting of a corrosion path results in a smaller median of the minimum order statistic while the other factors increase the median of the minimum order statistic. Moreover, a larger number of grain layers increases the minimum path length for a sample with given thickness. With a proper combination of these factors, the extended model is able to provide a good fit to the experimental data developed by the foil penetration technique.