Barbara A. Shaw

Assessing the Corrosion Resistance of Nonequilibrium Magnesium-Yttrium Alloys

Abstract Magnesium-yttrium (Mg-Y) alloys containing 7 at% to 26 at% solute were fabricated using magnetron cosputter deposition. X-ray diffraction (XRD) revealed that no second phases were present in any of the alloys and that all but two of the alloys (Mg-7% Y and Mg-14% Y) were nanocrystalline and/or amorphous. Anodic potentiodynamic polarization performed in 0.1 M sodium chloride (NaCl) at both near-neutral and high pH values showed improved anodic polarization behavior for most of the alloys when compared to either pure Mg or one of the most corrosion-resistant commercial Mg alloys (alloy WE43 [UNS M18430]). In the high-pH solution, it was not uncommon for breakdown potentials (Eb) of the Mg-Y alloys to be 2,000 mV higher than values for alloy WE43. Long-term galvanic and potentiostatic testing was performed on selected alloys to verify anodic polarization behavior. Potentiostatic testing also was performed in a pH 12 buffer solution. When defects did not affect corrosion behavior, low current density...

Measurement of pH Gradients in the Crevice Corrosion of Iron Using a Palladium Hydride Microelectrode

The study of localized corrosion is hindered by the inability to characterize electrolyte compositions inside crevices and pits during the corrosion process. This paper describes the development of a microelectrode systemfor in situ monitoring of the pH in artificial crevices and small recesses. This system permits determination of the pH gradient inside a crevice, even in the presence of a pronounced potential drop. The pH sensor, consisting of a palladium hydride electrode, can be positioned at any desired depth into the crevice while observing the position of the passive to active transition. With increasing time of crevice corrosion, one finds a pronounced pH change in the crevice electrolyte. This work lends further insight toward understanding of crevice corrosion and other localized corrosion phenomena.

Corrosion Protection Assessment of Barrier Properties of Several Zinc-Containing Coating Systems on Steel in Artificial Seawater

Abstract The present investigation assessed the corrosion protection performance of different Zn sacrificial coating system configurations during constant immersion in artificial seawater. Visual o...

The Role of Grain Boundary Regions in the Localized Corrosion of a Copper-Free 6111-like Aluminum Alloy

Abstract The localized corrosion behavior of a Cu-free 6111-like alloy was studied in solutions with different chloride concentrations. It was found that the alloy was susceptible to pitting and intergranular corrosion (IGC). The IGC attack on the alloy was less conspicuous unless the pits were cross sectioned. The pitting attack propagates via crystallographic tunneling regardless of the applied potential or heat treatment conditions. Scanning transmission electron microscopy (STEM)/x-ray microanalysis detected preferential segregation of Mg and Si at some high-angle grain boundaries (GB) and approximately 30-nm-wide Mg- and Si-depleted regions across GB of the alloy. Such findings have experimentally confirmed the assumptions made by others regarding the IGC susceptibility of 6XXX Al alloys, which was based on the noble GB precipitates and/or the preferential segregation of noble elements and adjacent solute-depleted regions. As a result, a link between the GB elemental profiles and the alloys IGC susc...

Technical Note: Improving Corrosion Resistance of Magnesium by Nonequilibrium Alloying with Yttrium

Abstract Significant improvements were achieved in the anodic polarization behavior of magnesium (Mg) by nonequilibrium alloying with 9 at% to 22 at% yttrium (Y). Alloys were fabricated using the nonequilibrium technique of magnetron cosputter deposition. Anodic potentiodynamic polarization experiments were conducted at various scan rates and pH values to assess the corrosion resistance of several MgY alloys. These alloys exhibited significantly enhanced corrosion behavior compared to pure Mg. Differences included higher breakdown potentials and lower passive current densities. X-ray diffraction of each alloy revealed the Y was retained in solid solution.

Microprobe Study of pH during the Induction Period Preceding Crevice Corrosion

Potential, pH, and their gradients are important parameters in determining the electrochemical reactivity of metals in crevices. Although an IR voltage alone can be responsible for the activation of crevice corrosion thatoccurs immediately, changes in pH may be necessary for crevice corrosion to occur in the delayed form of crevice corrosion. In cases where acidification forms an active peak in the polarization curve, pH. corrosion products, and electrode potential, E, inside the crevice play synergistic roles in determining the onset of crevice corrosion. This synergistic relationship is explored by measuring gradients in acidity, pH(x), and electrode potential, E(x), within crevices during the induction period preceding crevice corrosion of iron in 0.2 M Na 2 SO 4 + 0.025 M K 2 CrO 4 of pH 9.1. Crevice corrosion was found to initiate when the IR voltage caused the E(x) to have a value within the active peak of the polarization curve of the crevice electrolyte. The active peak formed due to acidification of the crevice electrolyte.

Comparison of Testing Techniques Used to Analyze the Corrosion Resistance of Sacrificial Coating Systems

Abstract The present investigation assessed the corrosion performance of 14 different Zn and Al sacrificial coating system configurations based on several different accelerated corrosion testing te...

Nonequilibrium Alloying of Graphite-Reinforced Aluminum Metal Matrix Composites

Abstract With corrosion behavior superior to pure aluminum (Al), nonequilibrium Al alloys produced by cosputter deposition have great potential for use as matrix metals in graphite (Gr)- reinforced...

Technical Note: Observations Regarding the Effects of Nitrogen Addition to the Aluminum- Tungsten System by Reactive Sputter Deposition

Abstract Reactive direct-current magnetron sputter deposition was applied to produce aluminum-tungsten-nitrogen (Al-W-N) ternary alloys. By this process, nitrogen-containing solid solution alloys were formed through careful regulation of the nitrogen partial pressure. Nitrogen content was controlled such that an intermetallic subnitride material formed, rather than a stoichiometric nitride. Nitrogen addition to the Al-W binary system imparted exceptional resistance to localized corrosion in 0.1 M sodium chloride (NaCl). Surface analysis by x-ray photoelectron spectroscopy (XPS) indicated nitrogen promoted oxidation of tungsten relative to the binary Al-W, as evident from comparison of the native air-formed oxides. The enhanced oxidation of tungsten was believed to result from a reduction in the activity of the aluminum metallic species due to substoichiometric Al-N-type bonding. Nitrogen alloying in the Al-W-N system also was associated with distinct changes in peak shape in the direct Bremsstrahlung exci...

A comparison of the corrosion response of zinc-rich coatings with and without presence of carbon nanotubes under erosion and corrosion conditions

Electrochemical impedance spectroscopy tests were conducted on carbon nanotubes (CNTs) enriched zinc-rich epoxy coating and a commercial zinc-rich coating. Coating performances were examined after ...