Garry W. Warren

Water Adsorption and Surface Conductivity Measurements on alpha -Alumina Substrates

Water adsorption and surface conductivity on ceramic substrates are important parameters in the electrochemical corrosion of thin-film metallizations and the associated growth of dendrites in humid environments. The adsorption of water provides the necessary electrolyte through which ion migration and electrochemical corrosion is possible, and the surface conductivity of the adsorbed moisture layer is a direct measure of the ion migration capability. The water adsorption isotherm of or-alumina at 23°C was determined using thermo-gravimetric analysis. It is found to be of type III according to Brunauers classification. Similar to results for other oxides, the water adsorption on \alpha -alumina is much lower than that on metal surfaces. An equivalent of one monolayer of water is adsorbed when the relative humidity (RH) is about 35 percent. For RHs higher than 70 percent, water adsorption is more than five monolayers and increases dramatically with increasing water vapor pressure. The surface conductivity was measured using an interdigitated thin-film circuit of pure copper sputtered on an \alpha -alumina substrate. The results show that the conductivity of the water layer increases with the accumulation of water monolayers. A moisture film equivalent to less than five monolayers (RH less than about 70 percent) has a conductivity approximately two orders of magnitude lower than that of bulk water. At 95 percent RH, a water film of more than 20 monolayers has a conductivity which is still an order of magnitude lower than that of an equivalent film of bulk water. These results are attributed to a discontinuous or patchy nature of the water film on the surface of the substrate.

The role of electrochemical migration and moisture adsorption on the reliability of metallized ceramic substrates

Data presented here have resulted from an extensive investigation into fundamental mechanisms involved in electrochemical migration and dendrite growth on metallized ceramic substrates. Significant new results are presented, and pertinent data from previous studies are outlined to provide a comprehensive, coherent analysis of the complex process of electrochemical migration. A number of critical issues are addressed including formation of an absorbed moisture layer and the effect of humidity on the thickness and conductivity of this layer. Migration has been quantified by examining dendrite morphology and dendrite growth rates. Morphology and growth rates are nearly identical for immersion tests in both bulk and thin layer electrolytes, but is quite different in absorbed layers of moisture. Immersion tests are still useful in determining the effects of solution variables that would be difficult or impossible to quantify in adsorbed moisture layers. In addition the maximum velocity theory for predicting dendrite growth rates can be used for predicting general trends, but requires further refinement for more precise predictions. Clearly the most important parameters affecting reliability are the adsorption of moisture combined with surface contamination.

Corrosion Behavior of Binary Titanium Aluminide Intermetallics

Abstract The corrosion behavior of arc-melted binary titanium aluminide intermetallics TiAl, Ti3Al, and TiAl3 in aqueous sodium sulfate (Na2SO4) and sodium chloride (NaCl) solutions was measured an...

Corrosion of NdFeB permanent magnet materials

NdFeB is an important class of new magnetic materials, however corrosion resistance is a serious concern and literature on the electrochemical behavior of NdFeB is scarce. This paper reports the results of an electrochemical investigation of the corrosion behavior of sintered NdFeB magnets obtained from three manufacturers. Linear polarization (cyclic voltammetry) experiments were conducted in aqueous solutions ranging in pH from 0.7 to 13.5. A limited degree of passivation was observed in all solutions which is believed to be due to the formation of a complex Fe‐Nd oxide and/or hydroxide film. The presence of a small amount of chloride ion, 10 to 100 ppm, shows only a slight effect, but higher concentrations (1000 ppm) cause a total breakdown in passivity and a dramatic increase in anodic current. The cathodic potential sweep shows an abrupt and unusual oxidation process, giving rise to an oxidation peak not commonly seen. This peak may result from dissolution of the film or preferential attack of intergranular phases.

Electrochemical Examination of Dendritic Growth on Electronic Devices in HCl Electrolytes

Abstract As part of an investigation of electrochemical failure mechanisms occurring in electronic devices, factors affecting the formation and growth rate of dendrites on a model microelectronic d...

The electrochemical hydrogenation of NdFeB sintered alloys

The absorption of hydrogen by NdFeB alloys in aqueous solution has been investigated using dc electrochemical methods and x‐ray diffraction. Immersion of the NdFeB alloy in 0.1 M H2SO4 results in the formation of the Nd2Fe14BH2.7 hydride phase as indicated by x‐ray diffraction analysis. The hydrogen entering the NdFeB lattice is believed to result from the preferential acid dissolution or etching of the Nd‐rich phase. The hydrogen absorption and desorption behavior of sintered NdFeB alloys, (NdDy)15Fe78.3Al0.7B6 and (NdDy)15Fe67Co5V4Al2B7, have been compared with that of a LaNi5 alloy utilizing an electrochemical technique. LaNi5 is capable of absorbing extremely high quantities of hydrogen as a metal hydride. The level of hydrogen absorbed by the three alloys was compared by ‘‘charging’’ electrodes of each alloy in 6 M KOH at a constant cathodic current, followed by a constant anodic ‘‘discharge’’ current. An arrest in the resulting potential versus time curve during discharge indicates the oxidation of ...

Hydrometallurgical treatment of municipal solid waste fly ash for simultaneous detoxification and metal recovery

Abstract Fly ash from municipal solid waste (MSW) incinerators is frequently classified as a characteristic hazardous waste, due to low levels of Pb and Cd. In some cases the fly ash also contains a significant amount of Zn, up to 15%. The objective of this investigation was to lay the foundation for a potential hydrometallurgical process for simultaneous detoxification and recovery of metal values, particularly Zn, Pb, and Cd. This study has provided a more complete characterization of the fly ash particularly with regard to those parameters which are important to a hydrometallurgical process, such as the forms in which Pb and Zn occur and leaching characteristics in several different Iixiviants including various concentrations of hydrochloric, acetic, and sulfuric acid. Results show that Pb is present in at least two forms, PbSO4 and PbCl2. Similarly, three forms of Zn are indicated: (1) a water soluble form, probably chloride or sulfate or both, (2) an acid soluble form, likely Zn oxide, and (3) a much...

Corrosion behavior of NdFeB with Co and V additions

Two NdFeB materials were selected for study. The first is referred to as a base or standard alloy with a composition of (NdDy)15FebalAl0.7B6, and the second is similar, Nd13.5Dy1.5FebalCo5V4AlB8, but alloyed with Co and V. The corrosion behavior of magnets made with these two materials is compared by two different techniques, autoclave testing at elevated temperature and pressure for 96 h, and dc electrochemical testing in an aqueous electrolyte. For the former, the base alloy exhibited poor corrosion resistance with a weight loss on the order of 270 mg/cm2. Significantly, the Co‐V‐treated samples yielded excellent corrosion resistance, showing no change in weight. Two types of electrochemical tests were performed on electrodes of each material, potentiodynamic polarization and constant potential in sodium sulfate solutions at pH from 1.5 to 4.4. Consistent with the autoclave tests, electrochemical experiments show lower currents for the Co‐V alloys.

Corrosion and hydrogen absorption in melt spun NdFeB-TiC bonded magnets

The corrosion behavior of bonded NdFeB permanent magnet materials produced by melt spinning with and without TiC additions has been investigated. Typical anodic polarization experiments in 0.9 M Na/sub 2/SO/sub 4/ showed that the addition of 6 a/o TiC resulted in lower corrosion currents and reduced iron dissolution at fixed potential. The role of hydrogen was also investigated by means of galvanostatic charging and discharging experiments (hydrogen absorption and desorption) in 6M KOH which showed that the addition of TiC resulted in a reduction of absorbed hydrogen.

Stabilization and hydrometallurgical treatment of flyash from a municipal incinerator

Flyash from municipal solid-waste (MSW) incinerators is frequently classified as a characteristic hazardous waste due to lead and cadmium content. Two alternate processing schemes for the treatment of flyash obtained from the MSW incinerator in Tuscaloosa, AL have been investigated; one involves stabilization of the flyash with portland cement, and the other involves the recovery and recycling of metal values by leaching and subsequent precipitation. Results indicate that flyash can be successfully stabilized in a 1:1 mixture of flyash and portland cement which meets environmental requirements. It has also been demonstrated that extraction of the metal content from flyash, specifically lead, cadmium and zinc, can be accomplished quickly and efficiently by leaching in hydrochloric acid solutions. Preliminary experiments on the subsequent recovery or precipitation of metallic lead and cadmium from these leaching solutions by cementation with zinc dust appear to be promising. Various means were used for characterization of the as-received material and several leaching residues including X-ray diffraction, particle size analysis, inductively coupled plasma spectroscopy, and atomic absorption spectroscopy. Results indicated that at least some of the lead in the flyash is present in the form of PbSO4, with smaller amounts of PbCl2.