Corrosion Monitoring for Deep Underground Environment

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The surface of materials forms interface between bulk and environment, and exhibits various properties that are different from that of bulk material. Surface thin oxide films on metals and alloys isolate the internal materials from environment to prevent environmental degradation, and exhibit various chemical and physical function as interaction between environment. Using aqueous solution process, surface of materials is modified by various interactions to produce unique surface layer, exhibiting new chemical composition, ordered structure, functions.In the present work, modification of Ni and Ni-Cr alloys, Fe-Cr alloys by light irradiation, and electronic properties of passive film which is highly related with the light induced modification are discussed using photo electrochemical response and electrochemical impedance spectroscopy.Properties of passive films on Ni and Ni-18Cr formed in acid solution are characterised in order to discuss semiconductor properties of passive films. Mat … More erials were polarised in deaerated 0.1 kmol m-3 H2SO4 at various potentials for up to 24 hours. Photo current generated by irradiation of monochromated light, and capacitance by means of electrochemical impedance were measured for a series of potentials in less noble direction from the film formation potential. The passive films were also characterised using XPS.Passive film on Fe-18Cr formed in the neutral borate buffer solution was studied using an electrochemical impedance spectroscopy and a photo electrochemical response. The passive film is composed of inner oxide and covering hydroxide layer, both which exhibit n-type semiconductor property with band gap energy, Eg, of 3.4-3.5 eV and 2.4 eV, respectively. Electronic energy band structure model as bi-layer semiconductor was proposed based on the experimental results, considering the film structure presumed by XPS. The change of energy band structure by photo irradiation is also discussed which results in photo current transient and change in capacitance of space charge layer. The difference between passive film on Fe-Cr alloys formed in the neutral and acid solution was also discussed in terms of film growth kinetics and corrosion process. Less