Patrick J. Moran

Effect of Reinforcement on the Pitting Behavior of Aluminum‐Base Metal Matrix Composites

This paper examines the effect of graphite and silicon carbide reinforcements on the pitting behavior of graphite/aluminum (Gr/Al) and silicon carbide/aluminum (SiC/Al) metal matrix composites. Electrochemical corrosion tests were performed on both Gr/Al and SiC/Al composite specimens. Identical tests were completed on powder metallurgy processed aluminum and wrought aluminum of the same composition. The electrochemical behavio of the SiC/Al composites was essentially identical to that of the powder processed and wrought aluminum alloys; however, the pitting attack on the SiC/Al composites was distributed more uniformly across the surface, and the pits penetrated to significantly less depths. The presence of graphite in the Gr/Al composites did not cause an electropositive shift in corrosion potential as anticipated, but caused a substantial decrease in resistance to passive film breakdown. This effect is the predominant reason for the poor performance of Gr/Al composites in marine environments.

The Passivity of 304 Stainless Steel in Propylene Carbonate Solutions

This paper reports that the passivation behavior of 304 stainless steel in anhydrous propylene carbonate (PC) containing 0.5M LiAsF{sub 6} or 0.5M LiClO{sub 4} was studied. The air-formed film on 304SS is stable up to the oxidation potential of PC (PC{sub ox}). Scratch tests show that the bared 304SS surface repassivates in the anhydrous PC solutions of either electrolyte by chemisorption of PC molecules below PC{sub ox}. In PC/0.5M LiAsF{sub 6} solutions, the 304SS is not passivated at potentials above PC{sub ox}. This is attributed to the formation of a thin metastable perchlorate salt film or an adsorbed layer of perchlorate anions. When the perchlorate anions oxidize, the passivation becomes unstable and pitting occurs. Small (3-8 volume percent) additions of PC/0.5M LiClO{sub 4} to PC/0.5M LiAsF{sub 6} solutions raised the passive range to the perchlorate oxidation potential. Small quantities of water, propylene glycol, and propylene oxide added to PC lightly improve the passive range of the 304 stainless steel.

An electrochemical monitor for the detection of coating degradation in atmosphere

Electrochemical impedance spectroscopy (EIS) is ideally suited for detection and monitoring of atmospheric corrosion of painted metals based on its sensitivity, essentially non-destructive nature, and its sensitivity, essentially non-destructive nature, and its applicability for continuous monitoring. EIS has been used for several applications in recent years to assess coating degradation in immersion environments. The authors report an atmospheric electrochemical monitor (ATMEIS) capable of detection of paint degradation in the atmosphere via EIS. The ATMEIS consists of a painted steel coupon (total coating thickness, 100-150 microns) upon which a sputter coated electrode (covering less than 10% of the front surface of the sample coupon) has been deposited to serve as a reference/counter electrode combination. The monitor can be used, therefore, in the absence of a remote reference or counter electrode. This monitor can be used to generate EIS data in air (with the aqueous acid electrolyte removed).

Electrochemical Monitoring of Organic Coating Degradation During Atmospheric or Vapor Phase Exposure

Abstract This manuscript reports an atmospheric electrochemical monitor (ATMEIS) designed to assess degradation of painted metal substrates during atmospheric exposure using electrochemical impedan...

The Influence of Incubation Time on the Passive Film Breakdown of Aluminum Alloys in Sea Water

Abstract : Potential-controlled electrochemical methods were used to characterize the pitting behavior of 6061 alloy aluminum in synthetic ocean water. Irreproducible breakdown potentials (E sub bd) and reproducible repassivation potentials (E sub rp) were determined from cyclic anodic polarization (CAP). Reproducible breakdown and repassivation potentials were found from the quasi-stationary anodic polarization method; however, the breakdown potential values were shifted in the electronegative direction relative to the CAP method due to the allowance of a longer incubation time. These results support the theory that with sufficient incubation time, pit initiation and propagation will occur at potentials at or slightly above the repassivation potential, and that the repassivation potential value is the only characteristic potential for aluminum. The quasi-stationary anodic polarization method used for E sub rp determination provides a reproducible electrochemical method for obtaining E sub rp after minimal pit growth or surface damage. Cyclic anodic polarization can also be used for determining a representative potential if surface damage subsequent to pit initiation is minimized.

Measurement of electrolytic conductivity in highly conducting solutions

A new technique, suitable specifically for the determination of specific conductivity in highly conducting solution is described. The technique combines the advantages of four-electrode measurement with those of fast current interruption to yield a clear signal, proportional to the resistivity of the solution and independent of any polarization effects.The conductivity of concentrated aqueous solutions of KOH is measured over a range of temperatures from 25 to 70°C and compared to literature data on the one hand and to measurements with a commercial two-electrode a.c. conductometer on the other. Agreement with literature data is good. Two-electrode a.c. measurements yield consistently lower values because polarization is not eliminated. The technique is simple and easily adaptable for on-line operation in the electrolytic industry where continuous monitoring of the conductivity may be important for process control.

The Anodic Behavior of Iron in Anhydrous Dimethoxyethane and Passivation by Solvent Electropolymerization

The corrosion and passivity of high purity iron in anhydrous dimethoxyethane with 0.5M LiAsF 6 have been studied by various electrochemical and surface analytical techniques. The motivations for this study were twofold; (i) to develop an understanding of the passivity of metals and alloys in anhydrous organic solutions at a fundamental level, and (ii) to apply what is learned to improve the performance and useful life of high energy density lithium batteries which employ organic solvents. The data show that iron displays a stable passive region with low current densities over a large range of anodic potentials. Several different passivation mechanisms have been identified in this stable region. Among these mechanisms is passivation by solvent chemisorption

Crystallization, embrittlement and fracture morphology of annealed Fe81B13.5Si3.5C2

Crystallization of Fe81B13.5Si3.5C2 (Metglas 2605SC) was investigated using angular dispersive X-ray diffractometry and energy dispersive X-ray diffractometry. Transition temperatures were determined by annealing an array of specimens from 0 to 97 h at temperatures ranging from 140 to 490° C. Samples of Metglas 2605SC began to transform in about 2.2 h at 365° C. Crystallization temperatures and products were compared to those previously reported in the literature. In addition, the embrittlement behaviour of the alloy was investigated and the fracture topographies of the as-received and annealed samples were examined and categorized using scanning electron microscopy. The Metglas tended to embrittle easily within short periods of annealing at temperatures as low as 240° C.

Electron channeling contrast imaging of crystalline deformation due to high power ultrasound

Abstract The ability of ultrasound to reduce markedly the static load needed for material deformation has important implications for the future of material processing; however, the underlying mechanism is still controversial. Wide area electron channeling patterns (ECPs) and channeling contrast were used to examine a chemically polished single-crystal aluminum rod following high power insonification at 20.5 kHz. ECPs taken from the rods local deformation region showed a marked reduction in channeling bandwidths, by as much as 45%. Spatial information from channeling contrast micrographs revealed details of progressive crystalline misorientation within the deformation region.

Structural homogeneity, crystallization, and embrittlement behavior of amorphous Fe81B13.5Si3.5C2

Abstract Structural homogeneity and crystallization of 50 mm wide Metglas 2605SC (Fe81B13.5Si3.5C2) were investigated using angular dispersive diffractometry and energy dispersive diffractometry. Inspection of numerous areas of a 1.2 m length of the Metglas to determine the degree of crystallinity indicated a lack of homogeneity. Transition temperatures and embrittlement behavior were determined by annealing an array of specimens for 0 to 97 h at temperatures ranging from 140°C to 490°C. Crystallization temperatures and products witnessed were compared with those previously reported.