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Steel bars in reinforced concrete are protected from corrosion by the high pH environment of the surrounding concrete. This alkaline environment is destroyed by the reaction of atmospheric CO, with the Ca(OH), of the concrete mass. When this process, called carbonation of concrete, reaches the reinforcing bars, corrosion of the latter may commence. In this paper, the physiochemical processes in this phenomenon are presented and modeled mathematically. The mathematical model is fairly complex, but certain simplifying assumptions are possible, which lead to the formation of a "carbonation front" and to a simple analytical expression for the evolution in time of this front, in terms of the composition parameters of cement and concrete and of the environmental conditions. This simple expression is in very good agreement with experimental results obtained in this and in previous studies. The effect of some parameters on the carbonation front propagation is also discussed. Porosity at t,, -0 Porosity decrease due to hydration AeH -Z ([i],AcFi) Constituent i C,S C S C,A C,AF M-K. x lo3 (kg/mol) 228.30 172.22 270.18 485.96 AV, x lo6 (m3/mol) 53.28 39.35 149.82 112.81 AF,, -3.85 . m'/mol AFcsH -15.39 . m3/mol *Computed with molecular weights and molar volumes of the principal constitu-**The terms involving the unhydrated and subject to carbonation C,S and C S ents of Portland cement and parameters used in conjunction with 4 . 2 5 .can be omitted because their contribution to porosity change is negligible.

AIRInternational audienc

Microstructural evolution of anode supported solid oxide fuel cells (SOFC) during medium-term stack testing has been characterised by scanning electron microscopy (SEM). Low acceleration voltage SEM imaging is used to separate the three anode phases (nickel, yttria-stabilised zirconia and porosity). Microstructural quantification is obtained using a software code that yields phase proportion, particle size, particle size distribution and a direct measure of triple phase boundary (TPB) density (lm-2). In addition, an anode degradation model is proposed. The model describes the gradual degradation of the anode due to nickel particle sintering and the concomitant loss of TPB. Fundamental operational and structural parameters of the anode can be used to estimate the TPB length change with time from the degradation rate. The combination of experimental results and modelling allows separating the degradation due to sintering of nickel particles from total stack degradation. Anode degradation occurs principally during the first 500 operating hours. For stack tests carried out over more than 1,000 h, anode degradation was responsible for 18 to 41% of the total degradation depending on initial microstructure.

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It was found that catalytic activity and selectivity of metal catalysts can be altered dramatically and in a reversible manner.

X-ray photoelectron spectroscopy (XPS) was used to investigate the effect of electrochemical oxygen pumping on Pt catalyst films interfaced with an 02--conducting yttria-stabilized zirconia solid electrolyte. It was found that electrochemical oxygen pumping to the catalyst causes spillover of significant amounts of anionic oxygen from the solid electrolyte onto the platinum film surface. The spillover oxygen species has an XPS binding energy 528.8 eV compared to 530.4 eV for chemisorbed oxygen, which is also observed on the surface, and is less reactive than chemisorbed oxygen with the reducing ultrahigh vacuum background. The detection of the anionic oxygen species upon electrochemical pumping confirms the previously proposed explanation of the non-Faradaic electrochemical modification of catalytic activity (NEMCA) or electrochemical promotion in catalysis.Controlled modification of the rate and selectivity of catalytic reactions on metal and metal oxide surfaces has been a longsought goal in heterogeneous catalysi~,l-~ where dopants and metal-support interactions are frequently used to improve catalyst perf~rmance.l-~ It was recently found that the cata1ytic"ll and chemisorptiveI2 properties of metals can be affected in a dramatic and reversible manner by using solid electrolytes, such as 8 mol % Y2O3-doped Zr02 (YSZ), an 02-conductor, or jY-A1203, a Na+ conductor, as active catalyst supports to induce the effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA)&12 or electrochemical promotion in catalysis:13 The metal catalyst, usually in the form of a 1-40-pm thick porous film, is deposited on a solid electrolyte, such as YSZ, and also acts as the working electrode (WE) in a solid electrolyte cell of the type: gaseous reactants, metal catalyst IYSZIM, 0, where M stands for the metal counter electrode (CE) which catalyzes the reaction (e+ CO + 0,) (e.g. Pt, Pd, Ag) 0, + 4e-* 20"(1) and supplies 0 2 -to the catalyst through the gas-impervious YSZ solid electrolyte under the influence of an externally applied voltage or current ( Figure 1). The induced increase Ar in catalytic reaction rate r, e.g. in the case of C2H4 oxidation on Pt,7 is up to 3 X 10s times higher than the rate Z/2F (where I is the applied current and F is the Faraday constant) of the supply of 02-to or from the catalystCl2 and up to 70 times higher than the opencircuit (Z = 0) catalytic rate r,. The NEMCA effect has been studied for over 20 catalytic reactions on Pt, Pd, Au, Ni, and Ag surfaces"'2J4 using 02-,@J1J2 Na+,&"J and H+ l4 conducting solid electrolytes. The resulting dramatic modification in catalytic rates is accompanied by significant variation in activation energies7JJ1 and product selectivity.6.' l A first step in elucidating the origin of NEMCA was the theoretical pr~position~.~ and experimental verification (via a Kelvin probes.9) that the work function e@ of the gas-exposed metal catalyst surface is significantly and reversibly altered under NEMCA conditions and in fact that Ae@ = eAVwR where VWR is th...