The use of microelectrodes in the study of localized corrosion of aluminum 6111-like alloys (original) (raw)
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Electrochimica Acta, 2010
A method has been developed to map electrochemical data from constituent particles comprising intermetallic compounds found in aluminum alloys onto real or virtual alloy microstructures with micrometer resolution. This method, referred to as "electrochemical microscopy" facilitates presentation of phasespecific microelectrochemical data by presenting it visually within the context of alloy microstructure. In this paper, several examples of the approach are given using electrochemical reaction rates of the various phases in Al alloy 7075 at the prevailing alloy corrosion potential (−0.8V sce ). In near-neutral dilute chloride solutions, the resulting maps show expected high net anodic reaction rates at Mg 2 Si particles and high net cathodic reaction rates at Al 7 Cu 2 Fe and Al 3 Fe particles. When the pH is varied from 2.5 to 12 and chloride concentration is varied from 0.01 to 0.6 M, the maps capture the changes in relative reaction rates. A comparison to corrosion morphologies developed during free corrosion experiments with the alloy show that electrochemical microscopy captures forms and intensity of localized corrosion as well as trends associated with systematic changes in environment chemistry.
Corrosion Science, 2006
The eect of microconstituents on the corrosion and electrochemical behaviour of AZ91D alloy prepared by die-casting and ingot casting route has been investigated in 3.5% NaCl solution at pH 7.25. The experimental techniques used include constant immersion technique, in-situ corrosion monitoring, and potentiodynamic polarisation experiments. Surface examination and analytical studies were carried out using optical and scanning electron microscopy, EDX and XRD. The corrosion behaviour of microconstituents namely primary a, eutectic a and b phases was signi®cantly dierent. Coring of aluminum showed in¯uence on corrosion behaviour more signi®cantly in ingot material. Areas with aluminium concentration less than about 8% were found to be prone to corrosion attack compared with either those with higher amount of aluminium or b phase. Die-cast material with smaller grain size and ®ne b phase oered marginally lower corrosion rate and better passivation compared with the ingot. In die-cast and ingot, hydrogen evolution took place preferentially on b phase. XRD pattern of non-corroded and corroded surface revealed the removal of b phase from alloy surface during corrosion. The corrosion products for ingot consisted of Mg(OH) 2 with small amounts b phase, magnesium-aluminium oxide and MgH 2 while for die-cast, the product showed a highly amorphous structure. 7
Materials Science Forum, 2006
The influence of surface roughness on the pitting potential using the electrochemical microcell technique is reported. Two Al-Mg-Si model alloys of varying manganese content were investigated. With potentiodynamic anodic polarization in 0.1 M NaCl solutions and with different sizes of exposed area it is shown, that a critical defect density is present which controls the pitting potential at small polarization potentials. This defect density is related to the crevice distance. With smoother polished surfaces a larger crevice distance is measured. The pitting potential measured on an exposed area smaller than the crevice distance reveals that small effects of the alloying element Mn become visible. For the fine polished surface it is shown that the addition of 0.5 wt% Mn to the model alloy lowers the pitting potential.
Journal of The Electrochemical Society, 2008
The corrosion behavior of different tempers of two aluminum alloys, AA7050 and an experimental Al-Mg-Cu-Si alloy, was studied in NaCl solution by anodic polarization and scanning electron microscopy and was correlated with differences in the microstructure. Potentiodynamic polarization experiments were performed on samples from the exact sheets used by others to study the microstructure evolution during the early stages of the precipitation sequence by high-resolution characterization tools ͓i.e., high-resolution transmission electron microscopy and atom probe tomography ͑APT͔͒. The usefulness of information from these state-of-the-art tools to lead to a better understanding about the effects of nanoscale segregation on localized corrosion of aluminum alloys is discussed. APT was able to provide information about the composition of the solid solution matrix region between the fine-scale hardening particles, which is not possible by any other technique. Some of the changes in corrosion behavior, e.g., the breakdown potentials, with temper could be rationalized based on changes in the matrix composition. The formation of corrosion-susceptible surface layers on as-polished AA7050 depended on the predominant type of hardening particle. The lack of detailed knowledge of the grain boundary region limited the applicability of the microstructural information generated by previous studies for understanding intergranular corrosion.
Journal of Electroanalytical Chemistry 732 (2014) 74–79. , 2014
Localized electrochemical activation of aluminium alloy AA6060 surfaces during immersion in chloride containing aqueous solution has been characterised using the scanning vibrating electrode technique (SVET). Ionic current flows in the electrolyte phase adjacent to corroding microcells are imaged in situ, allowing their evolution with time to be monitored with spatial resolution. A heterogeneous response due to the presence of intermetallic inclusions in the material is found on AA6060 surfaces. The highly localized anodic activity was thus detected, which has been interpreted as a result of the galvanic coupling between the matrix and the intermetallic particles. Metallic inclusions behave cathodically whereas the aluminium matrix undergoes metal dissolution due to the activation of anodic sites in the proximity of those inclusions. Hydrolysis of aluminium ions leads to local acidification of the electrolyte adjacent to the anodic sites, and it is accompanied by hydrogen evolution.
Localized Corrosion Behavior of Alloy 2090—The Role of Microstructural Heterogeneity
CORROSION, 1990
The pitting and intergranular corrosion behavior of alloy AAI') 2090 (AI-2Li-3Cu, UNSIZ A92090) in a 3.5 wt% NaCI solution was investigated. Techniques used included potentiodynamic polarization, galvanic coup/es, and pH measurements in simulated crevices. Polarization scans were performed on under-aged and peak-aged material to obtain the standard polarization parameters. Corroded specimens were examined with optical microscopy, scanning electron microscopy (SEM), and energy-dispersive x-ray spectroscopy (EDAX) to distinguish various local corrosion morphologies. Ingots were cast to approximate the subgrain boundary, T, (AI2CuLi) phase, and AI-Cu-Fe constituent phases inherent in the alloy. These were then galvanically coupled to solution heat treated (SHT) 2090 to identify their role in local corrosion processes. Simulated crevices were produced by inserting pH micro-electrodes into crevices machined in 2090 blocks to measure pH versus time response in occluded environments. Based on the experiments listed above, two different types of pitting mechanisms were identified. The first type was directly attributed to the dissolution of the subgrain boundary phase T,. A direct correlation between increased subgrain boundary precipitation and increased subgrain boundary pitting was observed. The second type of pitting involved enhanced local galvanic attack of the matrix material surrounding AI-Cu-Fe constituent particles found in this plate. With this type of attack, large pits formed around constituents which occurred randomly throughout the plate. Associated with constituent particle pitting was a form of localized attack that was designated as continuous subgrain boundary dissolution. This form of corrosion was observed on all specimens used in corrosion experiments. It was only seen, however, in regions adjacent to the largest pits on the specimen, suggesting the influence of an occluded (crevice) environment. Simulated crevice experiments revealed that an acidic crevice solution developed with time. This environment is suspected to contribute to the continuous subgram boundary dissolution in the vicinity of the constituent particles.
Corrosion Science, 2000
The localized corrosion behavior of Al 6061 and cast aluminum was investigated by using¯uorescein in combination with¯uorescence microscopy and atomic force microscopy in 0.1 M KCl solution. The formation of¯uorescent rings around the inclusions can be observed within a short time after open circuit initiation of localized corrosion at open circuit. These¯uorescent rings are associated with topographic features formed by precipitated corrosion products which are related to anodic dissolution sites. Hence, even a small number of anodic sites can easily be identi®ed among a large number of inclusions.
Sensors, 2021
Amperometric and potentiometric probes were employed for the detection and characterization of reactive sites on the 2098-T351 Al-alloy (AA2098-T351) using scanning electrochemical microscopy (SECM). Firstly, the probe of concept was performed on a model Mg-Al galvanic pair system using SECM in the amperometric and potentiometric operation modes, in order to address the responsiveness of the probes for the characterization of this galvanic pair system. Next, these sensing probes were employed to characterize the 2098-T351 alloy surface immersed in a saline aqueous solution at ambient temperature. The distribution of reactive sites and the local pH changes associated with severe localized corrosion (SLC) on the alloy surface were imaged and subsequently studied. Higher hydrogen evolution, lower oxygen depletion and acidification occurred at the SLC sites developed on the 2098-T351 Al-alloy.
Journal of Electroanalytical Chemistry 728 (2014) 148–157. , 2014
Corrosion processes occurring on stainless steel 304 surfaces under anodic polarization were characterized using scanning electrochemical microscopy (SECM) and the scanning vibrating electrode technique (SVET), complemented with conventional potentiodynamic polarization curves. Stable pit formation on the samples was monitored by SVET as result of surface modification under electrochemical control. The operation procedure may involve the previous electrochemical reduction of the passive oxide layer if the media is not aggressive enough to induce pitting at small overpotentials. Additionally, the sample generation – tip collection operation mode of the SECM enabled to detect local release of iron (II) ions, as well as their conversion to iron (III), both processes being greatly affected by the potential applied to the substrate.