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We report on the use of in-situ optical emission spectrometry to monitor the Al 3+ loss rate during the anodic oxidation of aluminum in phosphoric acid. Three distinct stages were observed, the highest rate being measured during barrier layer growth. The evolution of the loss rate is markedly different from the evolution previously reported for sulfuric acid anodizing. We speculate that this may be related to the different pore morphologies obtained in these electrolytes. Our measurements also indicate that below 2 mA/cm 2 , field-assisted dissolution is the predominant contribution to Al 3+ loss, while direct cation ejection is predominant at higher current densities.Elucidating the mechanisms of initiation and growth of porous anodic oxide films has recently generated renewed research efforts. [1][2][3][4][5] Despite the widespread use of these films for corrosion protection and nanotechnology applications, several fundamental questions remain unanswered. For instance, the development of porous anodic oxide films can be conceptually described by the following sequence of four, possibly overlapping, stages. 6 The first stage (I) is the barrier layer growth stage, during which the oxide is non-porous. The second stage (II) is the pore initiation stage, during which incipient pores appear in the barrier layer. The third stage (III) is the pore widening stage, during which some of the incipient pores that appeared during pore initiation develop into their final morphology. The fourth stage (IV) is the steady-state pore growth stage during which the barrier layer thickness, the pore diameter and the pore spacing remain constant while the pore length increases with time. The fundamental reasons for the transitions between these stages are however not well understood. A necessary condition for pores to initiate in a dense barrier oxide film is that a sufficiently large fraction of the oxidized metal ends up in the electrolyte rather than in the anodic film. 7-10 For anodic alumina, concentrated acidic electrolytes result in a significant fraction of Al 3+ in the electrolyte, about 40%. 7 For anodic films on other valve metals such as Ti, Ta and Zr, using fluoride-containing electrolytes also results in a significant fraction of the metallic cations in the electrolyte and allows growing porous anodic oxides on these metals as well. Early studies on the growth of porous anodic alumina explained the loss of Al 3+ in the electrolyte and the formation of the porosity by considering an electric field-assisted (electro-)chemical oxide dissolution at the pore base, catalyzed by the acidity. 11-13 18 O tracer experiments revealed that no O 2− anions originally present in the anodic oxide film ended up in the electrolyte for anodic alumina formed in sulfuric acid at 10 V. 14, 15 The Al 3+ loss thus proceeds in these electrochemical conditions by direct ejection of Al 3+ cations from the anodic oxide into the electrolyte rather than by field-assisted dissolution. The direct ejection of Al 3+ in the electrolyte has recently been ...

Overall and detailed kinetic descriptions of the pyrolysis of C3Hs have been proposed as a result of a turbulent flow reactor investigation in the temperature range of 1110-1235 K and at atmospheric pressure. The overall reaction was described by a first-order rate expression with an activation energy of 58.65 kcal/mol and a preexponential factor of 3.2 X sec-'. This expression agrees with previously reported rate data. In addition, a kinetic mechanism involving 13 chemical species and 32 elementary reactions has been postulated to describe the kinetics. Experimental data from the present flow reactor experiments and from static vessel and shock tube experiments reported in the literature were used to verify the mechanism. Agreement over the temperature range of 800-1400 K and over the pressure range of 0.1-8.5 atm was obtained by adjusting three rate constants. Previously reported values for these rate constants appear to require reexamination. The reactions in question are the following:The sum of the rate constants for reactions (2a) and (2b) and the rate constant for reaction (23) are best represented by k b + kzb = 10-0.1 T 4 exp (-8300/RT) cm3/mol sec and k23 = 10'4.55 exp (-14,340/RT) cm3/mol sec which differ with the expressions in the literature.

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