Hugh S. Isaacs

Studies of the Formation of Cerium‐Rich Protective Films Using X‐Ray Absorption Near‐Edge Spectroscopy and Rotating Disk Electrode Methods

The deposition of cerium-rich films on copper under cathodic polarization was studied as a model system for understanding the mechanism of corrosion inhibition of copper-containing aluminum alloys. Deposition was also studied on gold and iron for comparison with copper. Inhibition of corrosion of the aluminum alloys is achieved by deposition of a cerium-rich film on the copper-containing intermetallics that blocks the cathodic reduction of oxygen at these sites. X-ray absorption near-edge structure measurements show that cerium-rich films precipitated from aerated solutions are in the tetravalent state. Thermodynamically, the Pourbaix diagram predicts that under these conditions cerium should be in the trivalent state. This indicates that cerium chemistry is determined by processes in the solution rather than the potential of the electrode. Cerium-rich film formation is dependent on reduction of oxygen which influences the oxidation of Ce(III) to Ce(IV) in solution and precipitation of the film by changing the local pH at the electrode. The generation of hydrogen peroxide by oxygen reduction is considered to enhance cerium-rich film formation by oxidizing Ce(III) to Ce(IV) in solution. This was confirmed by addition of hydrogen peroxide to the solution.

The mechanism of corrosion inhibition by chromate conversion coatings from X-ray absorption near edge spectroscopy (xanes)

Abstract X-Ray absorption spectra of chromate conversion coatings on Al 2024-T3 and 3003-H14 have been obtained as a function of processing time and environmental exposure. The spectra reveal the presence of a significant ratio of unreacted hexavalent chromium in the chromate conversion coatings which decays with the time of exposure in air-equilibrated 0.5 M NaCl. These results suggest that the unique corrosion-protective property of chromated Al lies in the presence of a source of the hexavalent chromium species as an active inhibiting species.

A Novel Method for Generating Quantitative Local Electrochemical Impedance Spectroscopy

A local electrochemical impedance spectroscopy (LEIS) technique for mapping the ac impedance distribution, as a function of frequency, of an electrode has been developed. In LEIS, as in traditional ac impedance methods, a sinusoidal voltage perturbation between the working and reference electrode is maintained by driving an ac current between the working electrode and a distant counterelectrode with a potentiostat. Local ac impedances are then derived from the ratio of the applied ac voltage and the local ac solution current density. The local ac current density is obtained from potential difference measurements near the electrode surface using a probe consisting of two micro‐electrodes. By measuring the ac potential difference between the micro‐electrodes, and knowing their separation distance and the solution conductivity, the local ac solution current density is derived. The accuracy of the local ac impedance data generated with this technique was established by investigating two model systems. The first provided a homogeneous electrode which allowed LEIS measurements to be compared to traditional EIS, while the second system provided a heterogeneity of known size and location whose components were easily characterized with traditional techniques. It is shown that area‐normalized scanning ac impedance measurements of the homogeneous electrode agreed well with traditional results. In addition, because LEIS maps the impedance properties of an electrode, the defect in the heterogeneous electrode was easily detected, while traditional ac impedance of this electrode gave little indication of its presence.

The localized breakdown and repair of passive surfaces during pitting

Abstract The behavior of localized corrosion sites has been studied using scanning techniques, potential monitoring and simulated pits. The early stages of pitting were found to depend markedly on the properties of the passive oxide film present prior to pitting. These measurements demonstrated that the stability of pitting depended on the role of the passive oxide in confining the aggressive anolyte, keeping it in contact with the actively dissolving surface and preventing its concentration from falling below a level where repassivation can take place. The concentrations of chloride required to maintain active surfaces were determined using pencil-type artificial pits giving a one-dimensional geometry and coupling diffusional and electrochemical effects. The dissolution kinetics were found to depend on the concentration of chloride in contact wih the dissolving surface. Stainless steel and nickel alloys repassivated at high chloride concentrations whereas iron and nickel in general remained active as the chloride concentration decreased. Potential transients and currents from both the passive surfaces and localized corrosion sites were monitored during the early stages of localized corrosion. The measurements have emphasized the difficulties in determining the early stages of localized corrosion.

XANES investigation of the role of cerium compounds as corrosion inhibitors for aluminum

Abstract Cerium-containing films on aluminum and Alloy 5052 have been investigated using XANES (X-ray Absorption Near Edge Structure). The Ce L III and Ce L I absorption edges allow a clear distinction to be made between 3- and 4-valent cerium. Measurements were made of the fluorescent X-ray signal with X-rays incident at glancing angles to give surface sensitivity. Cerium in the 3-valent state was deposited by galvanostatic reduction from solutions of sufficiently high pH for oxygen to oxidize the cerium to a 4-valent state. From XANES measurements, it was determined that these films contained cerium mainly in the 3-valent state. Exposure of these films to a NaCl solution converted the cerium to the 4-valent state. Open circuit exposure of aluminum to low concentrations of cerium ions gave films with either 3- or 4-valent cerium depending on the exposure time.

The measurement of the galvanic corrosion of soldered copper using the scanning vibrating electrode technique

Abstract The corrosion behavior of antimony-tin and lead-tin solders have been compared after soldering to copper under similar conditions. The measurements were made in an air-exposed dilute solution containing chloride and sulfate at room temperature. The vibrating probe was used to determine the current densities above the sample under freely corroding conditions and with external applied polarizing currents. The tests showed that the antimony-solder was susceptible to localized corrosion whereas the lead-solder was passive. Polarization resistance measurements were derived from convertional polarization measurement and from the polarization of the anodic and cathodic areas.

A high-resolution probe for localized electrochemical impedance spectroscopy measurements

A high-resolution probe consisting of two platinum microelectrodes, 10 {micro}m in diameter, that permits localized electrochemical impedance spectroscopy measurements was developed and tested. The measurements were made using platinum disks as current sources with defined geometry in an insulating plane. The ac current distribution measured in solution using the probe was compared with analytical solutions and computer simulations. The spatial resolution of the probe was tested and defined by varying the height of the probe and the distance between the microelectrodes.

Effects of molybdenum on the pitting of ferritic- and austenitic-stainless steels in bromide and chloride solutions

Abstract The effect of Mo on the pitting potential, dissolution kinetics, and repassivation behavior of high purity ferritic stainless steels, Fe–18%Cr–x%Mo, and austenitic stainless steels, Fe–18%Cr–12–15%Ni–x%Mo, was studied in solutions of bromide and of chloride. Large increases in the pitting potential of Fe–18%Cr–x%Mo in chloride solution were found with increasing Mo content, compared with the distinctly smaller increases in bromide solution. There was an excellent correlation between the increase in pitting potentials of ferritic alloys between Fe–18%Cr–2% and 5%Mo in the chloride and bromide solutions with the increased dissolution overvoltages in the saturated solution of dissolved products within artificial pits. Austenitic stainless steels also showed larger increases in pitting potential in chloride solution than in bromide solution. Higher pitting potentials were recorded for the austenitic than the ferrite steels with the same Mo content. For austenitic steels, the pitting potentials in bromide and chloride solutions were not attributable solely to the difference in dissolution rates or repassivation characteristics in saturated solution of the dissolved products. It was concluded that initiation processes play an important role in the pitting of austenitic stainless steels.

Electrochemical Behavior of Cr2 O 3 / Fe2 O 3 Artificial Passive Films Studied by In Situ XANES

The electrochemical behavior of thin sputter-deposited mixed Cr 2 O 3 /Fe 2 O 3 oxide films with Cr 2 O 3 -contents of 10, 20, 50, and 90% was studied with in situ X-ray absorption near edge spectroscopy (XANES). These measurements gave information on the chemical states and dissolution rates during anodic and cathodic polarization in different electrolytes. At low Cr oxide concentrations, the films dissolve when cathodically polarized and are resistant to dissolution when polarized in the anodic direction. At high Cr 2 O 3 concentrations, dissolution occurs when the films are anodically polarized, but the films are stable against cathodic dissolution. In the intermediate Cr oxide concentration range, the oxides neither dissolve under anodic nor cathodic polarization. However, in all the cases, even when no dissolution takes place, the species show electroactivity in that the redox reactions Fe 3+ → Fe 2+ and Cr 3+ → Cr 6+ can take place under cathodic and anodic polarization, respectively. In the mixed oxides a solid-state conversion takes place in the iron oxide phase during reduction, whereas the oxidation of the chromium oxide phase converts only the outermost layer. An acidic environment accelerates both anodic and cathodic dissolution, associated with chemical dissolution of the iron oxide. The results further show that critical threshold values exist for the dissolution resistance of the oxide. These values are different for anodic and cathodic reactions and further strongly depend on the solution chemistry.

Surface Charge—Induced Ordering of the Au(111) Surface

Synchrotron surface x-ray scattering (SXS) studies have been carried out at the Au(lll)/electrolyte interface to determine the influence of surface charge on the microscopic arrangement of gold surface atoms. At the electrochemical interface, the surface charge density can be continuously varied by controlling the applied potential. The top layer of gold atoms undergoes a reversible phase transition between the (1 x 1) bulk termination and a (23 x √3) reconstructed phase on changing the electrode potential. In order to differentiate the respective roles of surface charge and adsorbates, studies were carried out in 0.1 M NaF, NaCl, and NaBr solutions. The phase transition occurs at an induced surface charge density of 0.07 � 0.02 electron per atom in all three solutions.