The aim of this research was to identify the effect of alloying additions and heat treatment on the localized corrosion mechanisms of AA7xxx alloys that have been grain refined with an Al-5Ti-1B master alloy. The major alloying elements, Zn (3.5 – 6.5 wt. %), Mg (1.5 – 2.5 wt.%), Cu (0 – 3 wt. %), and Ti (0.04 – 0.25 wt.%), were varied and ten AA7xxx aluminum alloys were cast, heat treated to the T4, T6, T79, T76, and T73 tempers, and studied extensively.
Casting high integrity near net shaped AA7xxx components by introducing Ti and B into the melt to facilitate a non-dendritic microstructure produced a unique microstructure due to the phenomena of athermal nucleation, unconstrained growth, and solute field impingement. The three phenomena listed above create three microstructural regimes; dendritic area rich in Ti, interdendritic region lean in Ti, and grain boundary area rich in eutectic phases.
The breakdown potential, Eb, in a detreated 0.5 M NaCl(aq) was determined for each alloy and heat treatment after 1 h at the open circuit potential. The effect of alloying element composition and heat treatment on Eb was examined and an empirical expression was created. It was determined that a semi-logarithmic relationship exists between Eb and the Zn/Cu weight ratio. Zn and Cu have opposing effects on Eb, where, Zn-depletion from the solid solution into the strengthening precipitates increased the Eb while Cu-depletion decreased the same. Cu-lean alloys have a continually increasing Eb from T4 to T6 and T7x temper conditions, while, Cu-rich alloys have no significant change in Eb with temper condition. The critical Zn/Cu weight ratio was determined to be approximately 5.
Corrosion initiation mechanisms of a Cu-lean and Cu-rich alloy were studied using the dual beam FIB-SEM platform after 5 min of potentiostatic polarization in detreated 0.5 M NaCl(aq) at potentials of interest. The initiation mechanism of the Cu-rich alloy (alloy 3) was determined to be dealloying of the S-phase (Al2CuMg) along the grain boundary leading to intergranular corrosion. The surface of the Cu-lean alloy (alloy 6) after potentiostatic polarization above Eb showed three mechanisms of corrosion; corrosion domes, corrosion rings, and interdendritic corrosion. Corrosion rings were identified as the initiation mechanism, corrosion domes were determined to be metastable sites and not a precursor to further corrosion, interdendritic corrosion was determined to be the propagation mechanism. Interdendritic corrosion is the selective dissolution of the interdendritic region leaving the dendritic regime intact after immersion in a corrosive solution which is unique to these alloys grain refined with the Ti-B master alloy. Corrosion domes were shown to cause an active-passive region in the potentiodynamic polarization curve.
Modified EXCO experiments were used to study the propagation mechanism in each of the alloys at each temper. All alloys and tempers were susceptible to interdendritic corrosion, however Cu-lean alloys showed less surface damage after 96 h of immersion in the modified EXCO solution. Alloys with a Zn/Mg weight ratio < 2 were susceptible to intergranular corrosion in the T6 temper, however, in the T4 and T7x temper the primary mode of corrosion propagation is interdendritic corrosion. Due to the bimodal distribution of the grain size, intergranular corrosion was only observed in the small grain areas. The extent of attack was more severe in the small grain regions with intergranular corrosion present than areas without leading to the conclusion that intergranular corrosion is more detrimental than interdendritic corrosion. / Thesis / Master of Applied Science (MASc)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/22835 |
| Date | January 2018 |
| Creators | Kramp, Jordan |
| Contributors | Kish, Joey, Shankar, Sumanth, Materials Science and Engineering |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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