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https://hdl.handle.net/2142/68644
Description
Title
Corrosion of Aluminum-Iron Eutectic Composites
Author(s)
Berke, Neal Steven
Issue Date
1980
Department of Study
Metallurgy and Mining Engineering
Discipline
Metallurgical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Metallurgy
Language
eng
Abstract
The corrosion of Al-Al(,3)Fe eutectic composites and individual phases, prepared by employing directional solidification or special mechanical processing to vary microstructure, was studied under conditions of preferential corrosion of the Al(,3)Fe second phase in 2.4 M H(,2)SO(,4) and preferential corrosion of the aluminum matrix by pitting in 5.0 M NaCl at pH 1.0. The "iron-like" Al(,3)Fe phase provided the total cathode reaction in both solutions and the corrosion rate depended on the metallographic parameter S(,v), the surface area of Al(,3)Fe in the volume. S(,v) is proportional to 1/L, where L is the mean intercept spacing.
In sulfuric acid the continuity of the Al(,3)Fe phase was of utmost importance. In discontinuous structures the phase rapidly corroded out and the Al to Al(,3)Fe ratio increased rapidly lowering the mixed corrosion potential of the structure. For continuous structures it was found that the Al(,3)Fe phase still persisted at the bottoms of long narrow trenches formed by the corrosion of the Al(,3)Fe. For finer structures S(,v) was larger, and thus the area of Al exposed was larger leading to a faster drop in potential. The results imply that discontinuous structures would suffer less corrosion in composite systems where the reinforcing phase preferentially corrodes.
In 5 M NaCl at pH 1.0 the spacing of the Al(,3)Fe was the dominant factor. Corrosion measured via hydrogen volume collection, cathode polarization, and weight loss was higher for the finer structures where S(,v) was higher. For continuous structures the area of Al(,3)Fe exposed by pitting should be proportional to pit volume while in the discontinuous structures the area would be proportional to the number of Al(,3)Fe particles in the surface of the pit since those in the volume would fall out. Equations were derived assuming random nucleation of pits and random growth of pits. The equations for the continuous eutectic structures predicted an exponential increase in the corrosion rate with time that was larger for finer structures. For the discontinuous structures predicted total pit volume increased with t('3) at longer times and was also dependent on the number of pits nucleated. The equations derived for both structures described the main features of the corrosion behavior with differences being attributable to the nonrandomness of pit nucleation and the preferential growth of pits away from Al(,3)Fe particles.
Further studies of pit nucleation and growth are needed to refine the equations. Other solutions and different structures should also be studied to develop a fuller analysis of corrosion versus microstructure as a basis for alloy design.
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