Localized Corrosion Damage Morphology and Corrosion Electrochemistry for Al-Zn-Mg-Cu Fastener Galvanic Couples in Marine Environments

Aluminum alloy 7050-T7451 (Al-Zn-Mg-Cu) is often used for aerospace applications because it provides an advantageous combination of strength, stress corrosion cracking (SCC) resistance, corrosion resistance, and fracture toughness. Defects in corrosion protection are enhanced at complex joining/fastener locations which trap electrolyte into tight crevices, leading to occluded local environments where a galvanic cell is established between the Al alloy and a steel fastener. While the danger of galvanic corrosion has been recognized, little research has developed an understanding of the physical, metallurgical, and electrochemical factors governing the corrosion damage at the meso- and macro-scale, as well as characterization of the damage morphology between AA7050 and stainless steel. The overall objectives of this work are to (1) investigate the electrochemical, microstructural, and physical factors that govern galvanic corrosion pit morphology development at the macro-, meso-, micro-, and nano-length scales under conditions representative of galvanic coupling between a rivet and plate in an aerospace structure, (2) explore and compare various laboratory-simulated to field damage modes in order to better simulate field damage in fatigue tests, (3) develop an electrochemical-based framework to explain the observed galvanic corrosion morphology developed in environmental conditions simulating a rivet, (4) understand the source and impacts of various cathodic reaction rates when AA7050-T7451 is coupled to stainless steel, and (5) extend these findings to further understand the effect of an inhibitor, such as chromate, on galvanic corrosion damage evolution.
The macro-scale corrosion (i.e., location and number of corrosion sites) was investigated using in operando X-ray tomography. X-ray tomography on a simulated fastener of AA7050-T7451 and stainless steel revealed multiple corrosion fissures grew simultaneously over the period of exposure under NaCl and MgCl2 droplets. Fissures did not follow obvious clusters of constituent particles, suggesting the presence of a newly developed strong cathode, such as Cu-replating, and a fixed strong cathode, attributed to the stainless steel rivet. Detailed examination was undertaken to understand the precise macro- and micro-cathodes controlling the galvanic corrosion central to damage between AA 7050-T7451 and Type 316 stainless steel, enabling targeted suppression of certain cathodes for mitigation of corrosion damage. It was determined that Cu-replating may have a greater contribution to the total cathodic charge than the stainless steel rivet. This was an interesting observation as no corrosion sites grew when the stainless steel was removed before exposure. This suggests that the stainless steel provides the key driving force for AA7050-T7451 localized galvanic corrosion initiation and enables Cu to replate on the surface. Replated-Cu subsequently supports damage evolution.
The meso-scale corrosion (i.e. damage morphologies) of AA7050-T7451 galvanically coupled to stainless steel was investigated using electrochemical methods guided by anodic and cathodic polarization in rivet-specific solutions. This developed a range of relevant potentials and environmental conditions for likely corrosion susceptibility in environments typical around rivets. It was also found that various corrosion damage morphologies can be developed in AA7050-T7451, associated with the galvanic conditions specific to a fastener. For instance, intergranular corrosion was found to prevail in acidic environments and became further intensified with the addition of Al ions in solution. Intragranular corrosion pits were developed in neutral and alkaline NaCl environments. Deep, elongated fissures in grains oriented in the L-direction were often produced in alkaline NaCl environments following the addition of Al ions, which was attributed to Cu-replating on AA7050-T7451. It was found that AA7050-T7451 was more likely to undergo dealloying of Al2CuMg in alkaline exposure, further promoting Cu to replate The corrosion damage morphologies that were developed in specific cases are significant as they may each affect the fatigue transition differently. Investigation of methods used to study galvanic corrosion were compared to tear-down field damage and other field studies. Zero resistance ammeter resulted in the closest correlation between the simulated fastener corrosion morphology and actual service corrosion from the perspective of damage morphology.
The metallurgical factors and electrochemical framework controlling the path of intergranular and intragranular corrosion were also established in this work. AA7050-T7451 is composed of many different regions with a variety of compositions. This includes the matrix, intermetallic particles, solute-depleted zones, and precipitate-free zones. Each of these regions have different chemical compositions and critical electrochemical potentials. It is rationalized that the difference in critical potential associated with these microstructural regions, combined with Ecouple within a region accounts for the intergranular corrosion. TEM showed that Cu and Al were often incorporated into the strengthening precipitate, MgZn2. Furthermore, Cu-depletion was often observed in the region adjacent to the grain boundary. It was determined that the window of intergranular corrosion susceptibility decreased with incorporation of Cu and/or Al into the MgZn2. An intergranular corrosion framework was proposed: Cu2+ in solution was found to raise the pitting potential of the solute-depleted grain boundary region, closing the window for intergranular corrosion to occur. Cu2+ is often found in the micro-chemistry of a corrosion pit. This also suggests that pitting can be attributed to the coarse secondary phases in AA7050-T7451. The corrosion pathway was also determined using EBSD, which showed that unrecrystallized grains containing low-angle boundaries may be more susceptible to intragranular corrosion damage. This was speculated to be attributed to unrecrystallized grains collecting Cu in the precipitates during the over-aging heat treatments of the AA7050 (T7451) with associated Cu-depletion.
The galvanic current interactions under atmospheric conditions were conducted using coupled multi-electrode arrays constructed of AA7050-T7451 coupled to Type 316 stainless steel. It was found that under a NaCl thin films of 70 µm, the net galvanic current density increased by at least one order of magnitude when compared to full immersion kinetics. To investigate the current interactions inside a rivet geometry, a coupled multi-electrode array constructed in a fastener geometry revealed that anodic currents were often higher at the mouth of the crevice and inside the crevice. In many of the exposures, AA7050 net anodic sites were often found to abruptly switch to net cathodic sites. This suggested that dealloyed S-phase (Al2CuMg) and/or Cu-replating on the surface was the source of the increased cathodic kinetics of AA7050-T7451 electrodes. When AA7050-T7451 was coupled to stainless steel, the AA7050-T7451 electrodes were still significant cathodes supporting ORR. In some cases, AA7050-T7451 electrodes contributed 90% of the total net cathodic reaction. Anodes were dictated by local sites where AA7050 initiated pitting. It is speculated that SS enables initiation, but propagation largely is supported by replated Cu. The CMEAs were also tested under atmospheric wet/dry cycle conditions and resulted in a 4-fold increase in current density, attributed to strong spikes in current on the onset of wetting/drying as the water layer became more concentrated. The cathodic reaction rates under thin films were also studied utilizing an array constructed with an embedded, sintered Ag/AgCl electrode used as both the counter and reference electrode. It was found that cathodic ORR reaction rates increased by at least one order of magnitude in atmospheric thin films or droplet environments as compared to bulk, full immersion conditions. Under droplet conditions, electrodes on the edge of the droplet experienced increased ORR cathodic kinetics when compared to electrodes under the center of the droplet. Under a continuous thin film, AA7050 electrodes exhibited equivalent cathodic kinetics as a function of position. Lastly, the controlling factors in constant potential holds which simulate galvanic couples between Type 316 stainless steel and AA7050-T7451 were investigated and it was found that ORR kinetics of planar and micro-electrodes of pure Cu did not follow mass transport control under Cottrell diffusion kinetics at a constant potential in a droplet.
These findings were extended to further understand the effect of an inhibitor, such as chromate, on galvanic corrosion damage evolution and electrochemistry. Damage morphology studies showed that while the fissure density was reduced with the addition of soluble chromate, aggressive environments created by application of high anodic potentials or galvanic coupling to stainless steel induced deep fissure formation. In one case, fissures were over 37 times deeper in chromate solutions than chromate-free solutions. The growth of corrosion fissures in chromate-containing environments were speculated to be caused by the fully protonated H2CrO4 which exists in strongly acidic solutions. Evidence suggests H2CrO4 is present in active pits, but does not inhibit corrosion because H2CrO4 is not charged and cannot compete with Cl- and adsorb or interact with the corroding surface. Electrochemical results showed that additions of Na2CrO4 reduced cathodic ORR kinetics by one order of magnitude for Type 316 stainless steel and by less than one order of magnitude on Cu. In contrast, additions of Na2CrO4 increased the cathodic kinetics of replated Cu on AA7050-T7451, which is detrimental to corrosion inhibition. The addition of chromate was determined to significantly inhibit the corrosion of Al2CuMg, which minimizes the possible formation of porous Cu-rich phases and Cu-replating on the surface. Therefore, inhibition of Cu2+ release through passivation of Al2CuMg would be a viable route to lower cathodic reaction rates.
This thesis contributed to the scientific understanding of dissimilar, metal-based crevice corrosion in complex precipitation-hardened aerospace alloys, clarified approaches that enabled laboratory simulation of corrosion damage modes seen in the field, helped inform the attributes that can be controlled and regulated in developing new corrosion damage mitigation strategies, investigated the galvanic current interaction under atmospheric conditions, and extended these findings to understand the effect on galvanic corrosion-induced damage morphology of the legacy inhibitor: chromate.