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| Author(s): |
Fabio Domingos Pannoni
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | São Paulo. |
| Institution: | Universidade de São Paulo (USP). Escola Politécnica (EP/BC) |
| Defense date: | 2002-03-15 |
| Examining board members: |
Stephan Wolynec;
Idalina Vieira Aoki;
Isolda Costa;
Antonio Carlos Guastaldi;
Zehbour Panossian
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| Advisor: | Stephan Wolynec |
| Abstract | |
This work deals, basically, with the study of the influence of different alloy elements on the initial steps of the atmospheric corrosion process. Thirty alloys had been prepared in a partial factorial design experiment; different copper, nickel, phosphorus and molybdenum levels had been added to a carbon steel base alloy, with the objective of searching the individual and synergic influence of the elements in the bare optical properties, in the thin film growth, and in the atmospheric corrosion resistance. The optical study had revealed, for each alloy without film, different optical constants, and, therefore, alloys can be differentiated through reflection ellipsometry. The chemical composition changes causes, during oxidation, different ´DELTA´ and ´PSI´ path. As this path is a function of the thickness and structure of the formed film, can be affirmed that copper and molybdenum exerts constitutional changes in the thin oxide film, while nickel and phosphorus reveal to be less effective in this changes. This work proposes that molybdenum and copper, in a less sharp way, promote an intense water molecules dissociation, leading to an increase of the ionic current in the oxide layer. The ionic current increase leads, according to Fromhold-Cook Theory, to the film thickness increase. The equation that best describes the oxidation process is a logarithmic type one. It was not possible to discern which from the two logarithmic equations (direct or inverse) show thebest fit with the experimental data. We could not observe, in none of the alloys studied, the introduction of a new phase in the oxide film during the experiment. The alloy elements are incorporated in the oxide phase without the formation of a new phase. There´s no indication of complete cessation of film growth at any condition. ) The results gotten in this work had allowed the development of a new steel family destined to the automobile industry; the steelmakers COSIPA and the USIMINAS are commercializing with success this (patented) steel family to all local car manufacturers. (AU) | |