Experiments were conducted to characterize and compare the localized corrosion susceptibility of the granular microstructure of aluminum (Al) alloy AA7050 in the peak aged T6 condition cast by the novel controlled diffusion solidification (CDS) process against the conventional wrought plate counterpart. CDS is a casting technique that involves mixing two precursor melts at specific temperatures and compositions before solidification. This process avoids the problem of hot tearing by causing copious nucleation of the solid phase within the melt before solidification, reducing the amount of solute segregation and, thus dendritic growth creating a consistently equiaxed microstructure.
The effect of the CDS processing route on its relative localized corrosion susceptibility was elucidated by making links between the microstructure heterogeneities and the localized corrosion susceptibility as evaluated in aqueous saline solutions. Microstructures were characterized and compared with the use of the following techniques:
1. Scanning electron microscopy (SEM) to characterize grain size, shape and distribution.
2. Electron backscattered diffraction (EBSD) in SEM mode to characterize grain misorientation and the associated distribution.
3. Auger electron spectroscopy (AES) to characterize the composition of the grain boundary region including the precipitate free zone (PFZ) and the grain boundary precipitates.
4. Atom probe tomography (APT) to define the size, distribution, and composition of the strengthening matrix precipitates along with the grain boundary region (PFZ and grain boundary precipitates).
Electrochemical experiments were conducted to characterize and compare the localized corrosion susceptibility of the two materials (CDS and conventional wrought) exhibited in aqueous saline solutions. Specific techniques include the following:
i. Potentiodynamic polarization measurements of mechanically-abraded surfaces to determine the corrosion potential (Ecorr) and breakdown potential (Eb).
ii. Potentiostatic anodic polarization of mechanically-abraded surfaces to observe the mode and extent of localized corrosion.
iii. Open-circuit potential (OCP) measurements of fracture surfaces to determine the OCP of a surface with a significantly higher grain boundary area fraction relative to the bulk material.
iv. Cyclic acidified salt (sodium chloride (NaCl)) fog testing (ASTM-G85-Annex 2) to validate the relative localized corrosion susceptibility under more realistic atmospheric corrosion exposure conductions.
The CDS casting technique resulted in an entirely equiaxed microstructure. The microstructure was isotropic with an average grain size of 25±1 µm and an aspect ratio of around 1. This grain structure was in stark contrast with the wrought material, which exhibited a granular structure elongated along the rolling direction. The wrought material had a cord length of 56±3.2 µm in the rolling direction, 51±3 µm in the traverse direction and 13.3±1.6 µm in the short traverse direction. The wrought material had an aspect ratio of around 4 in the longitudinal plane (LS), 2.6 in the short transverse plane (ST) and 1.2 in the rolling plane (LT). AES and APT revealed that the CDS material had a higher amount of copper (Cu) segregation into the grain boundary precipitates. Electrochemical testing showed that the wrought material had a Eb of −750 ± 3 mV while the CDS had a higher Eb of −697 ± 4 mV. The Cu segregation into the grain boundary precipitates yielded more electrochemically active grain boundaries, as revealed by the OCP measurements. Despite this fact, localized corrosion of the CDS material initiated as pitting and propagated as a mixed mode involving intergranular corrosion (IGC) and pitting. The localized corrosion mode exhibited by the wrought material was purely IGC: both in initiation and propagation. The difference in corrosion mode was found to be due to the differences in the size of the Fe-based IMPs and the distribution of the Cu secondary phase precipitates: The CDS had large Fe IMP trapped at the grain boundary triple points and clustering of Cu secondary phase precipitates. Conversely, the wrought material had finely dispersed Fe IMP of significantly smaller size than those found in the CDS, and its Cu secondary phase precipitates are evenly distributed along the grain boundaries. These differences in precipitate distribution enhanced susceptibility for pitting in the CDS and reduced the driving force for IGC. The propagation of localized corrosion was markedly reduced in the CDS material: about half of that exhibited by the wrought material (under identical exposure conditions). Cyclic acidified salt fog testing revealed industry acceptable levels of localized corrosion susceptibility in-line with the benchmark alloys that are currently used in automotive applications. / Thesis / Master of Applied Science (MASc)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/20572 |
| Date | January 2016 |
| Creators | Feenstra, Darren R. |
| Contributors | Kish, Joseph R., Materials Science and Engineering |
| Source Sets | McMaster University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
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