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The role of the crevice-opening dimension, a, on the stability of crevice corrosion was investigated in the Ni/i N H2S04 system. The electrode potential and current distributions inside the crevice were measured and calculated, respectively. Other variables, in particular the composition of the electrolyte, were constrained from changing during the experiment. The passive/active boundary, x ass' moved further into the crevice for increasing a, and was in close agreement with the calculated value using the IR = A6I relation. A sharp upper limiting value, aim, for a given crevice depth, 1, above which crevice corrosion does not occur, was predicted and experimentally observed. The potential Epa,s at Xpass on the crevice wall corresponded approximately to the potential of the passive/active transition of the bulk solution polarization curve.

Alloys that depend on oxide films or passive layers for corrosion resistance are susceptible to crevice corrosion. This is for instance the case for iron in a sodium acetate-acetic acid ͑NaAc-HAc͒ buffer solution. This material-environment combination shows an active-passive transition in the polarization behavior around Ϫ150 mV ͑SCE͒ and, hence, is susceptible to crevice corrosion. Indeed, a potential drop into the crevice might bring part of the crevice into the active region, resulting in crevice corrosion. The potential drop into a crevice in a Fe/NaAc-HAc buffer system has been computed and compared to the experimentally measured profile. Mathematically, the potential drop into the crevice is a Poisson-type field problem with nonlinear boundary conditions and it has been described in a one-dimensional finite difference framework. The subsequent critical depth calculations, determining the onset of crevice corrosions, were solely based on the geometry of the crevice, the conductivity and the polarization behavior of iron in a NaAc-HAc buffer solution. In order to achieve this, a Weibull transition function has been used to describe the active-to-passive transition in the polarization behavior. Due to the strongly nonlinear and inherently nonmonotonic character of the boundary conditions ͑the active-passive transition in the polarization behavior͒, the resulting equations are solved using a generally applicable homotopy method.

The roles of applied potential, solution resistance, pH, and temperature on passive and active metal dissolution and hydrogen evolution inside crevices were evaluated using in situ measurements of potential distribution, pH, and other data, for systems with active/passive transitions in their polarization curves. Their roles can be explained in terms of a requisite JR voltage drop, z4", for active metal dissolution to occur inside the crevice: JR > IX". Accordingly, crevice corrosion was suppressed in the systems studied by: (i) increased pH (or deacidification) of the crevice solution (the increased A4* and decreased J dominated over the increased R), (ii) increased solution conductivity (decreased R), (iii) decreased solution temperature (the increased AV and decreased J dominated over the increased R), and (iv) a more noble applied or corrosion potential in the passive region (increased A4*). A decrease in oxidant availability during open-circuit corrosion reduces the susceptibility by decreasing J. InfroductionCrevice corrosion is a form of localized corrosion which occurs on areas of the metal surface where current flow, mass transport, and fluid flow are limited by the system geometry. Metal is dissolved within the crevice while the outer metal surface suffers little or no attack. These local corrosion cells can be formed by a depleted supply of oxygen or other oxidant inside the crevice. The cathodic reaction then takes place mainly on the outer surface. In this situation the anodic and cathodic reactions are physically well separated, i.e., the current path between the anode (inside the crevice) and cathode (outer surface) is typically orders of magnitude longer than in the case of uniform corrosion where the path length is conceptually viewed on the subnanometer scale. Thus, sizable JR voltages can develop in the crevice solution and cause steep potential and current distributions inside the crevice. These distributions can be strongly modified by changes in the composition of the alloy and/or of the electrolyte within the cavity. The JR voltage shifts the local electrode potential, E(x), in the negative direction with increasing distance into the crevice. Hence, beyond some distance into the crevice, Xp,,,, the passive film is not stable (active region of the polarization curve), allowing high rates of metal dissolution to occur. The other boundary of the actively dissolving region on the crevice wall (besides .rpa,,) occurs at a larger value of x (deeper into the crevice), Xhm, where Eiim(X), in the case of base metals, has the value of the mixed potential established by the metal dissolution reaction and the hydrogen evolution reaction (HER).' The rest of the crevice wall at x> x,,,,, is at the E,,, value and, therefore, dissolves at a uniform and relatively low rate to produce an etched surface. These features of the JR criterion for corrosion inside cavities are presented more completely in the literature."2 Experimental results for iron,3' nickel,7 and steel' in sulfate and other electrolytes spann...

. The collected PM 2.5 -samples were analyzed by ICP-MS for determination of lead. The average atmospheric PM 2.5 concentration was 50.8 mg/m 3 . Atmospheric PM 2.5 -concentrations were higher than the 24-h U.S. National Ambient Air Quality Standards (NAAQS) in 14 sample events. The average lead concentration for all samples was 0.07326 mg/m 3 . Atmospheric lead concentration was dependent on the sampling location. Concentrations at the two southern locations were higher than at the two northern locations. Southern locations had higher lead concentrations due to very high traffic density, in addition to their proximity to industrial zone. In general, the results of this study show a considerable decrease in atmospheric lead concentration 7 years after phasing out leaded gasoline. The study recommends further studies to accurately determine the current sources of atmospheric lead.