tailieunhanh - Corrosion of Ceramic and Composite Materials Part 2

Tham khảo tài liệu 'corrosion of ceramic and composite materials part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 16 Chapter 2 layer was determined to be the rate-limiting step during dissolution. The composition of the boundary layer may vary depending upon whether diffusion is more or less rapid than the boundary reaction. The basic equation describing the rate of solution under free convection with density being the driving force is dR vix 1 4 D 3 4C exp 5 R 5 4y where g acceleration due to gravity Ap p -p p p sarurared liquid density and p original v kinematic viscosity x distance from surface of liquid Di interface diffusion coefficient C a concentration parameter 5 effective boundary layer thickness R solute radius The exponential term was introduced as a correction for cylindrical surfaces. Since experimental tests often involve cylindrical specimens these equations have been developed for that geometry. In practical applications the condition relating to the corrosion of slabs is most predominant. However if the sample diameter is large compared to the boundary layer thickness the two geometries give almost identical results. After a short induction period which is of no consequence in practical applications in which molecular diffusion predominates the rate of corrosion becomes nearly independent of time. As a surface corrodes the interface if denser than the corroding medium will be eroded away due to free convection caused by density variation. Use of this equation implies that one has at his disposal data relating to the variation of density and viscosity with temperature. In cases where these data are not available the investigator will need to determine them prior to any calculation of corrosion rates. Copyright 2004 by Marcel Dekker Inc. Fundamentals 17 Hrma has used the work of Cooper and Kingery to discuss further the rates of corrosion of refractories in contact with glass. The following equation given by Hrma describes the corrosion under the condition of free convection due to density difference Jc where jc rate of corrosion c solubility

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