Open-cell aluminum foams have several benefits, which include cost effectiveness, the means for high-impact absorption, a low density, and a high surface area. They are commonly used in aircraft components, where the need to perform in marine environments may occur.
When exposed to high-chloride environments, corrosion can ensue, reducing their lifespan. For example, pitting corrosion, a localized surface defect, can compromise the structural reliability of open-cell aluminum foams. Because open-cell aluminum foam is used in advanced engineering functions, it is important to comprehend the way in which corrosion affects it.
To do this, researchers Ho Lun Chan, Kevin Guo, and Vilupanur Ravi at California State Polytechnic University (Pomona, California, USA) studied the exposure of open-cell aluminum foams of different pore densities in simulated marine environments.1
Additionally, they examined the microstructure of the foams, and completed compression tests to determine mechanical behavior. Both of these tests were performed both pre- and post-corrosion. The goal of these trials was to discover the differences between the open-cell aluminum foams and their bulk counterparts.
Corrosion Tests
The research team studied open-cell aluminum foams with constant relative densities between 6 to 8%, and with pore densities of 10, 20, and 40 pores per inch (PPI). Furthermore, they performed the same studies on bulk alloys for comparison (Figures 1 and 2). The foams were both immersed in simulated seawater containing 3.5 wt% sodium chloride (NaCl), and exposed to salt spray environments containing 5 wt% NaCl. Each corrosion test lasted for four days and was completed three different times to ensure repeatability.
The tests revealed that the 40 PPI foam experienced higher mass loss in both tests, which is attributed to the larger pore sizes. Also, in both tests, the bulk alloys exhibited lower mass losses.
Microstructural Analysis
The researchers then examined the microstructures of the test coupons, pre- and post-corrosion using backscattered electron imaging (BEI) and secondary electron imaging techniques in a scanning electron microscope. They also acquired local chemical compositions through energy dispersive spectroscopy.
The team found Al-Fe-Si intermetallic precipitates in sizes ranging from 10 to 100 μm on the foam surface and on the bulk alloy in sizes ranging from 0.5 to 1 μm. Figure 3 displays the microstructure of a foam before and after corrosion using BEI. Figures 3(a), (c), and (e) show pre-corrosion foams. Figures 3(b) and (d) show one day of immersion, and Figure 3(f) shows three days of immersion, in 3.5 wt% NaCl and nitric acid (HNO3) cleaning. The highlighted areas of Figures 3(a) and (b) designate the local microstructure shown in Figures 3(c) and (d).
Compression Tests
To explore the effect of corrosion on mechanical behavior, compression tests were performed on pre- and post-corrosion coupons with the longest dimension oriented parallel to the compression axis. The tests were executed on foams exposed to four days of salt fog corrosion before HNO3 cleaning.
Researchers noted the results suggest that the compressive strength and the Young’s Modulus of open-cell aluminum foams remain unchanged after four days of exposure to salt spray.
Conclusions
Researchers studied open-cell aluminum foams with different pore densities under different marine conditions with the objective of learning more about the corrosion behavior of aluminum foams in the hope that a deeper understanding would lead to expanded utilization in advanced engineering applications.
Corrosion tests proved that, in both immersion and salt fog environments, bulk alloys exhibited lower mass loss than foams. Microstructure studies concluded that corrosion-susceptible Al-Fe-Si precipitates were present on bulk alloys and on foams. And finally, compression tests indicated that the compressive strength and the Young’s Modulus of the foams remain unchanged after four days of exposure to salt spray. The team notes that further work must occur in order to understand the impact of corrosion on mechanical behavior.
Reference
1 H.L. Chan, K. Guo, V. Ravi, “Corrosion Studies of Open-Cell Aluminum Foams in Simulated Marine Environments,” CORROSION 2020, paper no. 15195 (Houston, TX: NACE International, 2020).