TY - GEN
T1 - Extended Point Defect Model for nickel-chromium alloys
AU - Urquidi-Macdonald, Mirna
AU - Normand, Bernard
AU - Mendy, Henry
AU - MacDonald, Digby D.
PY - 2011
Y1 - 2011
N2 - A Point Defect Model (PDM) has been developed to opitimize the chromium content in conferring passivity on nickel-chromium alloys in aggressive aqueous media. The model assumes a passive film of defective nickel oxide and a porous, precipitated outer layer of a mixture of Ni(OH) 2 and Cr(OH) 3. The measured impedance is composed of the impedance of the barrier layer, of the outer porous, precipitated hydroxide layer, and that of the electrical double layer. Experimental and theoretical preliminary results (is this paper), in this research work and carried out by other authors confirmed that a Ni-Cr (14% w) alloy in a 1 M H 2SO 4 electrolyte-at four voltages within the passive region-forms a ptype barrier layer of several nm (in this work it is calculated to be from 7 to 12 nm thick and a porous, precipitated film of 59 to 72 nm thick after about 4 hours of polarization). The model assumes and the results of the model confirmed along the XPS results that the outer layer is composed mainly of a mixed hydroxides of nickel and chromium. In this research work, the original PDM had been expanded to provide a mechanistic description of the passive film dissolution, including the formation of the precipitated outer layer and a mechanistic description of the electrical double layer. The Faradaic impedance of the barrier layer is controlled by the reactions that are responsible of the formation and dissolution of the inner layer of the passive film and the generation and annihilation of the point defects, and it is coupled in series with the impedance of the outer layer. The outer layer impedance (including that of the double layer) strongly depends on the reactions at the barrier layer/outer layer interface and on the double layer capacitance. Accordingly, for first time, a mathematical expression is obtained to describe the outer layer impedance in terms of the reactions occurring at the base of the pores in the porous-outer-layer; i.e., at the inner/outer layer interface.
AB - A Point Defect Model (PDM) has been developed to opitimize the chromium content in conferring passivity on nickel-chromium alloys in aggressive aqueous media. The model assumes a passive film of defective nickel oxide and a porous, precipitated outer layer of a mixture of Ni(OH) 2 and Cr(OH) 3. The measured impedance is composed of the impedance of the barrier layer, of the outer porous, precipitated hydroxide layer, and that of the electrical double layer. Experimental and theoretical preliminary results (is this paper), in this research work and carried out by other authors confirmed that a Ni-Cr (14% w) alloy in a 1 M H 2SO 4 electrolyte-at four voltages within the passive region-forms a ptype barrier layer of several nm (in this work it is calculated to be from 7 to 12 nm thick and a porous, precipitated film of 59 to 72 nm thick after about 4 hours of polarization). The model assumes and the results of the model confirmed along the XPS results that the outer layer is composed mainly of a mixed hydroxides of nickel and chromium. In this research work, the original PDM had been expanded to provide a mechanistic description of the passive film dissolution, including the formation of the precipitated outer layer and a mechanistic description of the electrical double layer. The Faradaic impedance of the barrier layer is controlled by the reactions that are responsible of the formation and dissolution of the inner layer of the passive film and the generation and annihilation of the point defects, and it is coupled in series with the impedance of the outer layer. The outer layer impedance (including that of the double layer) strongly depends on the reactions at the barrier layer/outer layer interface and on the double layer capacitance. Accordingly, for first time, a mathematical expression is obtained to describe the outer layer impedance in terms of the reactions occurring at the base of the pores in the porous-outer-layer; i.e., at the inner/outer layer interface.
KW - Double-layer
KW - Outer layer-Helmholtz layer impedance
KW - Passive film
KW - Point defect model
UR - https://www.scopus.com/pages/publications/84867305347
M3 - Conference contribution
AN - SCOPUS:84867305347
SN - 9781618393630
T3 - 18th International Corrosion Congress 2011
SP - 1958
EP - 1969
BT - 18th International Corrosion Congress 2011
ER -