Stress-Assisted Corrosion of Aluminum 6061 in Basic Solution

Caustic stress corrosion cracking has only been superficially examined in aluminum alloys. In caustic (basic) environments, the inherent, protective oxide layer becomes soluble on aluminum and allows for further degradation. The purpose of this study is to observe the effect of a strong base solution (sodium, hydroxide, sodium hypochlorite, sodium silicate, pH ~12) on the strength and stress-strain behavior of Aluminum 6061. The stress-strain behavior is also used to gain insight into the mechanisms of stress corrosion cracking. Hydrogen embrittlement is hypothesized to have played a major detrimental role. An unidentified substance formed around the plastic region only on samples submersed in the solution. This could have implications of caustic corrosion inhibition in aluminum alloys. As the field of materials science progresses, complex questions arise about how to take advantage of acquired knowledge to better the nation’s economy, defense, and technological advancements. The study of materials science has origins from all over the world and timeline, and has a massive interconnection with the history of humankind. In fact, the names of the Bronze Age, Stone Age, and Iron Age hint at how whole eras in human history can be defined by how it was affected by the understanding of materials science. The present study does not attempt to define an age. However, it does attempt to answer a few small, but important questions about material science that have been brought about only recently. In 1928, The Silver Bridge, so-named for the shiny aluminum paint, crossing the Ohio River around the Point Pleasant area was built with an innovative “eye-bar” design (Ballard 1929). Engineers were not sure how the new design would distribute the loads placed on it, so stronger steel replaced the original mild steel in the design to account for error in load distribution. The strong design and stronger material led engineers to believe that the bridge would McNair ScholarS JourNal ▪ VoluMe 12 95 stand for centuries. However, 9 years later, in 1967, disaster struck. Residents reported hearing a loud boom and watched as the bridge collapsed “like a deck of cards,” killing and injuring several people (Shermer 1968). Engineers could not diagnose the failure with any certainty because their calculations suggested that stress and rust alone could not have possibly caused the bridge to fail. Some residents of the area even put the fault on the Curse of Chief Cornstalk, a Native American Chief who had lost a battle to the white men many years ago (LeRose 2001). After extensive dissection of the failed structure, it was determined that an internal crack had propagated through the structure during either manufacture or assembly, allowing environmental corrosion to accelerate through the material over the years until the eyebars failed. While the bridge was still standing, internal corrosion defects, such as a crack, could not have possibly been detected without the aid of modern science. Material that has consistently shown favorable resistance to corrosion is aluminum and aluminum alloys. Oxygen reacts spontaneously on the surface of aluminum to make aluminum oxide (4Al + 3O2 → 2Al2O3) in our atmosphere. Although this is a degrading reaction, the oxide layer is so thin that it has little compromising affect on overall strength, yet blocks oxygen from un-oxidized aluminum underneath. This process of the product of a corrosive reaction preventing further corrosion is called “passivation.” Using a computer simulation, Campbell et al. (1999) estimated that a stable, 4 nanometer-thick aluminum oxide passivation layer forms on aluminum in a few nanoseconds in our atmosphere. However, when placed under a tensile (pulling) stress, the protective oxide layer is deformed, revealing un-oxidized aluminum. The aluminum oxide layer is also soluble in certain basic (pH greater than 7) environments, leading to many other ways to undermine the oxide layer’s protective quality. From chemistry, a pH of 7 is neutral, acidic environments are lower than 7, and basic environments (the kind that this study uses) have a pH greater than 7. This is where the present study comes in. An aluminum alloy was placed in a corrosive environment while under a slowly increasing tensile load or “strain rate.” The corrosive environment was a solvent of the oxide layer, and the strain rate (rate of extension) was slow enough to allow for the reaction to occur. This deformation of an aluminum alloy in a caustic environment satisfies the parameters for stress corrosion to occur: a tensile stress and a corrosive environment, while undermining its protective oxide layer. Designing for prevention of failure continues to evolve with newer experimental methods and further study of failed materials. The failure of The Silver Bridge has impacted economic and militaristic applications