Pitting of single crystals of A1 and homogenized A1-Cu alloys has been studied on low index surfaces as a function of surface orientation and alloy content in 0.5M NaC1. The potentials at which pitting phenomena occurred were determined potentiodynamically using single-cycle pitting tests at a scan rate of 0.5 mV/s. Galvanostatic tests were used for pit density studies. The potential at which pits initiated (Epit) was determined, together with a pit transition potential (Ept~,) that appeared as an abrupt potential discontinuity on the decreasing potential scan and was shown to be related to pit repassivation events. The observations indicated that pitting behavior was anisotropic. Pitting densities were dependent on crystallographic orientation. Consistent with this, the pitting potential showed a small dependence on surface orientation in the order (Epit)~0el~ > (E,it){~H} > (Epit){111}, whereas Eptp was found to be independent of orientation. Walls of pits on A1 were composed of {001} facets with <001> step edges:The presence of alloyed Cu raised pitting potentials, reduced the dependence of Epit on surface orientation, and modified the pit morphologies. The dependence of pitting behavior on surface orientation and alloy concentration is discussed in terms of a kinetic model of pitting in which pits are Considered to initiate at the base of flaws in the surface oxide film.

Polarization measurement and an accelerated life test of the oxide-coated titanium anodes in relatively dilute solutions of NaCl and in HClO4 solution were conducted. The OCTA failed at potentials higher than 1.3V vs. SCE, probably due to anodic oxidation of RuO2 and passivation of Ti substrate. An accelerated life test based on those results has been investigated to estimate the OCTA. The polarizing current decreases quickly when the OCTA becomes inactive, and the service life of material is evaluated by the time-to-failure from the start-up.A n u m b e r of the oxide-coated titanium anodes (OCTA) consisting of RuO2 and TiO~ are being used because of durability and low chlorine overvoltage in chlor-alkali cells. But eventually the OCTA becomes inactive and a part of the oxide drops out (arrow in Fig. 1). Low oxygen overvoltage is another problem for this oxide anode in chlorine cells.The OCTA is stable and its chlorine overvoltage is p r e f e r a b l y low in concentrated NaC1 solutions even a t high current densities, whereas it is attacked g r a d u a l l y and the anode potential becomes high in dilute N a C 1 solutions.Since the OCTA has come into the m a r k e t in early 1970's, m a n y articles on its preparation, modification, physico-chemical properties, electrochemical characteristics, and applications have been published. Degradation and deactivation of its unique material have also been discussed. With those articles and patents, degradation of the OCTA in chlor-alkali cells can be classified into three types: (i) coating dissolution, (ii) substrate a t t a c k , and (iii) substrate oxidation leading to electrical insulation, as stated by W a r r e n et al., who have studied the P t -I r alloy coated anodes for chlorate cells (1, 2).Extensive studies on the OCTA as well as the solid Ru metal anodes in sulfuric acid solution and solid p o l y m e r electrolyte have been conducted as a part of new developments in high-performance water electrolysis. Low oxygen overvoltage of those materials is a reason. However, dissolution a n d / o r degradation of the anode material at high potential ranges, and hence at high current densities, is a problem (3).Experiment of the OCTA under the operating conditions of chlorine cells are time-consuming, about one y e a r or more for only one run. Consequently, this paper deals mainly~with an accelerated life test and its procedure for the OCTA. The procedure is simple and requires minimum labor even for a prolonged period. The test specimen of the OCTA is electrolyzed in a mixed solution of HC104 and NaCI or in HC104 single solution under a constant terminal voltage, and the current is recorded until the test specimen breaks down. The concept is based on gradual degradation of the active material due to oxygen evolution. ExperimentsTest specimen.--Titanium sheet of 5 m m wide, about 150 m m long, and 0.5 m m thick was pickled with 10% oxalic acid at about 80~ for about 3 hr, rinsed Fig. I. SEM photograph ot degraded surface of OCTA 1439 ) unless CC License in place (s...

in the laboratory, Thlibi and Petit [1994] found that the reduction of HNO3 to NO occurred on carbon. Rogaski et al. [1997] found that the major products of the heterogeneous reaction of HNO3 with BCA were H20, NO2, and NO and reported an uptake coefficient of 3.8 x 10 •. They measured two NOx molecules produced for every three HNO3 molecules adsorbed. More recently, however, Choi and Leu [1998] on carbon between 10 -3 and 10 -5 depending on carbon sample surface history, with CO, CO2, and 02 as products. The presence of these products suggests that the ozone reaction on carbon is, at lest in part, noncatalytic.This paper discusses the measurement of BCA by the Ames Wire Impactors and assesses the role of BCA in stratospheric photochemistry. A new method of analyzing impactor samples is described that accounts for particle bounce and models the BCA as fractal aggregates to modify the aerodynamic collection efficiency and determine particle surface area. Results from the present analysis are compared with previous techniques and with BCA surface area values used in previous model simulations. The photochemical trajectory model used in the present simulations is described. An assessment of the importance of heterogeneous BCA reactions is made by comparing modeled and measured NOJNOy ratios and effects on ozone loss. Finally, the implications of mass-balancing these reactions on the lifetime of BCA particles in the stratosphere will be discussed. The Ames Wire ImpactorThe Ames Wire Impactor (AWI) has been used extensively in the past on such missions as AAOE, AASE, AASE II, ASHOE/MAESA, and POLARIS to sample stratospheric aerosol. It has proven to be an accurate and reliable means for obtaining atmospheric aerosol size distributions, and sulfate and BCA burdens. The sampler consists of palladium and/or gold wires of 75-and 500-p,m diameter that are strung across support rings. The wires are carbon coated and modules are assembled in a class 100 clean room. Up to six modules are mounted on the wingtip of the ER-2. Each module has the capability of holding several rings with several wires on each ring. The modules are exposed to the free stream by preselected triggers of time, altitude, or aircraft position. Aerosol particles suspended in the ambient air impact on the wires. A new automated sample actuation system was designed, fabricated, and successfully used to retrieve data during the POLARIS campaign. Samples were exposed typically for 3 min. Particles stick to the wire upon impact by virtue of van der Waals forces. After exposure, the wires are retracted into sealed modules containing concentrated ammonia vapor. The ammonia combines with the sulfuric acid aerosol to form ammonium sulfate which is very stable under laboratory conditions and allows for the sample to be stored indefinitely. Particles are manually identified, counted, and sized using a Hitachi S-4000 field emission scanning electron microscope. Under the conditions used for imaging, the lateral resolution of the microscope is better than 10 nm....

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Formation of the mixed oxide consisting of RuO2 and TiO2 was studied by the DTA/TGA technique and x‐ray diffraction. The coating agent containing RuCl3 and tetra‐n‐butyl titanate converted into the mixed oxide of RuO2 and TiO2 at about 400°C on the Ti substrate, and the reaction was exothermic. The electric resistance through the oxide film depends much on the fire temperature, and an optimum condition arises between 450° and 500°C. The solution composition and the time of preparation are also important factors. The anodic polarization behavior in the normalNaCl solution was also affected by the material or the condition of preparation of the oxide‐coated Ti electrode.