Predicting Internal Oxidation: Building on the Wagner Model

2011 ◽  
Vol 696 ◽  
pp. 1-11 ◽  
Author(s):  
David J. Young
Keyword(s):  
Internal Oxidation ◽  
Binary Alloy ◽  
Reactive Component ◽  
Internal Interface ◽  
Parabolic Kinetics ◽  
Internal Precipitation ◽  
Diffusion Effects ◽  
Multiple Oxidants ◽  
Low Solubility ◽  
Dilute Alloy

Wagner’s 1959 diffusion model of the internal oxidation process provided a method of predicting the rate at which a binary alloy was penetrated by dissolved oxygen as it precipitated the more reactive (but dilute) alloy component. Parabolic kinetics were predicted to depend on oxygen permeability in the unreacted alloy solvent and also, in cases where the reactive component was sufficiently mobile, the diffusion coefficient of the latter. The model has proven very successful, but is restricted to single oxidant-binary alloy systems, in which the precipitated oxide has extremely low solubility. This paper reviews recent results on a number of internal precipitation processes which cannot be described with the Wagner theory. These include formation of low stability carbides and nitrades; internal precipitation driven by multiple oxidants; the templating effects of prior precipitates on subsequently formed corrosion products; cellular precipitation morphologies; internal interface diffusion effects; volume changes in the reaction zone and the effects upon them of simultaneous external scaling.

High Temperature Corrosion of Fe-Cr, Fe-Al, Fe-Si and Fe-Si-Al Alloys in CO2-H2O Gases

2010 ◽  
Vol 654-656 ◽  
pp. 1948-1951 ◽  
Author(s):  
Thomas Gheno ◽  
Huan Li ◽  
Jian Qiang Zhang ◽  
David J. Young
Keyword(s):  
Internal Oxidation ◽  
Oxide Scales ◽  
Al2o3 Layer ◽  
Oxidation Rates ◽  
Cr Content ◽  
Corrosion Rates ◽  
Parabolic Kinetics ◽  
Wet Gas ◽  
Combustion Technology ◽  
Slow Growing

Iron and model alloys containing 2.25, 9, and 20 wt% Cr, 2, 4 and 6 wt% Al, 1, 2 and 3 wt% Si, and dilute Fe-Si-Al ternaries were reacted in dry and wet Ar-CO2 gases at 800°C. External oxide scales grew on Fe according to fast, linear kinetics in dry CO2. Additions of H2O accelerated the reaction until steady-state parabolic kinetics were achieved. High Cr content alloys developed slow-growing chromium-rich oxide scales. Dry CO2 mixtures produced faster rates than wet gas mixtures. Lower Cr alloys developed thicker iron oxide scales, featuring cavities, cracks and poor adherence, and sustained internal oxidation. The presence of H2O led to even higher oxidation rates. Aluminium additions to iron of up to 4 wt% provided no protection, but instead caused internal oxidation. A level of 6 wt% significantly slowed oxidation by forming a continuous Al2O3 layer. Silicon additions had little effect, apart from promoting internal oxidation. However, simultaneous alloying with aluminium and silicon strongly depressed corrosion rates. The effectiveness of different alloy additions is discussed, along with the effects of water vapour and carbon activities, in the context of oxyfuel combustion technology.

Solubility Enhancement of Poorly Soluble Drug Simvastatin by Solid Dispersion Technique

2016 ◽  
Vol 2 (2) ◽  
pp. 91-95
Author(s):  
Neelima Rani T ◽  
Pavani A ◽  
Sobhita Rani P ◽  
Srilakshmi N
Keyword(s):  
Dissolution Rate ◽  
Drug Carrier ◽  
Solid Dispersions ◽  
Aqueous Solubility ◽  
Onset Of Action ◽  
Kneading Method ◽  
Wetting Property ◽  
Poorly Soluble ◽  
Dispersion Technique ◽  
Low Solubility

This study aims to formulate solid dispersions (SDs) of Simvastatin (SIM) to improve the aqueous solubility, dissolution rate and to facilitate faster onset of action. Simvastatin is a BCS class II drug having low solubility & therefore low oral bioavailability. In the present study, SDs of simvastatin different drug-carrier ratios were prepared by kneading method. The results showed that simvastatin solubility & dissolution rate enhanced with polymer SSG in the ratio 1:7 due to increase in wetting property or possibly may be due to change in crystallinity of the drug.

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