75.Describe techniques to study pitting,crevice corrosion and stress corrosion cracking and evaluate them critically. 79.Temper embrittlement is thought to be associated with the segregation of atomic phosphorous to high angle ferrite grain boundaries,that reflect the position of prior austenite grain boundaries.This occurs at temperatures below 550C. (a)From a thermodynamic (energy)viewpoint what drives the segregation, and how would the equilibrium segregation of P change with a rise in the chemical potential of phosphorous? (b)If a homogeneous sample is quenched from 700C to 480C,what will determine the rate at which the concentration of P at the boundary increases? Be as quantitative as you can,i.e.,an equation would help. (c)Solutes with low solid solubilities in the grains are found to segregate more strongly to the grain boundary than solutes with high solubility. Rationalize this as best you can. 82.Consider vaporization of selenium which forms a gaseous oxide,Se02, when a mixture of oxygen and argon gas flows over the tube,as shown in the figure.Selenium vapor and oxygen can diffuse in each other and react. S。+02→S0 The rate of reaction is given by: Ise To kC2XsXo where r is in moles/unit time/unit volume,and C is the molar density of the mixture. The reaction constant,k,is extremely high at elevated temperature,so that the reaction may be considered instantaneous.Determine expressions for the steady-state Se and 02 gradients in the tube,as well as the rate of Se vaporization. 02+Ar MofE5月75. Describe techniques to study pitting, crevice corrosion and stress corrosion cracking and evaluate them critically. 79. Temper embrittlement is thought to be associated with the segregation of atomic phosphorous to high angle ferrite grain boundaries, that reflect the position of prior austenite grain boundaries. This occurs at temperatures below 550C. (a) From a thermodynamic (energy) viewpoint what drives the segregation, and how would the equilibrium segregation of P change with a rise in the chemical potential of phosphorous? (b) If a homogeneous sample is quenched from 700C to 480C, what will determine the rate at which the concentration of P at the boundary increases? Be as quantitative as you can, i.e., an equation would help. (c) Solutes with low solid solubilities in the grains are found to segregate more strongly to the grain boundary than solutes with high solubility. Rationalize this as best you can. 82. Consider vaporization of selenium which forms a gaseous oxide, Se02, when a mixture of oxygen and argon gas flows over the tube, as shown in the figure. Selenium vapor and oxygen can diffuse in each other and react. e e 2 2 S O SO + → The rate of reaction is given by: 2 2 2 Se O se O r r kC X X = = where r is in moles/unit time/unit volume, and C is the molar density of the mixture. The reaction constant, k, is extremely high at elevated temperature, so that the reaction may be considered instantaneous. Determine expressions for the steady-state Se and 02 gradients in the tube, as well as the rate of Se vaporization