AA7075-T6 is widely used in aerospace structures, and often SS316L fasteners are extensively used, especially for repairs because they are relatively cheap and easy to acquire. Separately, these materials are very efficient at performing their functions. However, when placed in intimate contact (such as in a fastener/hole configuration) and exposed to marine atmospheres, a galvanic couple forms which exacerbates the localized corrosion of the AA7075-T6. In service, the AA7075-T6 is coated with a primer and organic topcoat. Thus any defects in the coating serve as anodes in the galvanic couple with an extremely unfavorable cathode:anode area ratio. In addition, the interior of the hole is often poorly coated or suffers mechanical damage during operations. The overall goal of this research is to evaluate the efficiency of the mitigation strategy of coating the cathodic material with a sol-gel coating, thereby effectively ionically isolating the stainless steel fasteners from the AA7075-T6. Titanium fasteners are also under study, as they are expected to have very limited galvanic coupling with the AA7075-T6. This characteristic made them a positive baseline to which we continually compared the stainless steel corrosion.

Experimentally the interactions between the SS316 and Ti-6Al-4V bolts, washers, and nuts and the coated AA7075-T6 were assessed using a configuration designed by NAVAIR [1]. It consists of a plate with eight holes and eight fasteners. Details can be found elsewhere [1]. In the present work, the fasteners were spray coated with low quality sol-gel coating to serve as a baseline of damage accumulation. Around four of the holes for the fasteners, two scribes were made through the coating in order to assess the effects of coating defects on the corrosion morphology. These NAVAIR plates underwent ASTM B117 [2] testing for 1,344 hours. Once the plates were taken out of the testing environment, they were stripped of the primer, coating, and the fasteners were removed. The plates were then cut into sections dividing the hemispheres of the holes. The exposed holes were polished down to a 1 micron mirror finish, and Keller’s Etchant was used to highlight the grains. Cross-sectional images of the holes were acquired using the Hirox RH 8800 optical microscope. The primary focus was assessment of damage within the holes as these are the locations of highest stress. Corrosion damage is known to serve as an initiation site for fatigue cracks [3].

After the samples were taken out at 1,344 hours, it was observed that the corrosion damage in the hole in which the stainless steel fastener had been was far more severe than that in the hole in which the titanium fastener had been (see Figure 1). The most intense pitting on all of the holes occurred near the head of the fastener, indicating that the head of the fastener is throwing cathodic current not only to the surface of the plate, but also inside the hole. In the case of the stainless steel fastener, the damage was primarily long intergranular fissures, whereas the Ti-6Al-4V caused more pitting with a smaller amount of intergranular attack.

Using polarization curves generated in relevant simulants of the different localized sites as boundary conditions, FEM modeling results will be presented that rationalize the type, location, and extent of the damage observed for the different noble metal fasteners as well as the expected reduction in that damage if a high-quality sol-gel coating were applied to them.

References

1. Feng, Z., & Frankel, G. S. “Galvanic test panels for accelerated corrosion testing of coated al alloys: Part 1 – Concept” Corrosion 2014; 70(1), 95–106. https://doi.org/10.5006/0907
2. ASTM B117, “Standard Practice for Operating Salt Spray (Fog) Apparatus” (West Conshohocken, PA: ASTM International, 1994)
3. Burns JT, Larsen JM, Gangloff RP. “Driving forces for localized corrosion-to-fatigue crack transition in Al-Zn-Mg-Cu” Fatigue Fracture Engineering Material Structures 2011; 34:745–73. doi:10.1111/j.1460-2695.2011.x