What is it about?
The Aluminum (Al)- Magnesium (Mg) 5000-series alloys are currently used on Naval warships to reduce weight while also maintaining a high structural strength, which enables more fuel efficiency and faster travel. However, these alloys are utilized with different Mg contents, such as ~ 5 wt. % in AA5083 and ~5.5 wt. % Mg in AA5456, as well as with different heat treatments. The problem is, these alloys undergo intergranular stress corrosion cracking (IG-SCC) with time in service, and the effects of these different Mg percentages and different heat treatments on the severity of IG-SCC is not fully understood. Therefore, this work is a comparative study to show the effect of increased Mg content, as well as the effect of different treatments, on the eventual IG-SCC behavior to better understand which alloys are more advantageous for future use.
Featured Image
Why is it important?
The impact of this paper lies primarily in the fact that: 1) A hydrogen-based failure model is successfully utilized to explain effects related to heat treatment and composition on the IG-SCC severity demonstrated experimentally 2) The alloy AA5456-H116 is, on average, shown to be less susceptible to IG-SCC than AA5083-H116 3) The Nitric Acid Mass Loss Test, which is used by the Navy and in research to estimate an alloy's IG-SCC susceptibility after time in service, is called into question by evidence of significant differences in nitric acid corrosion morphology between different heat treatments
Perspectives
Personally, I'm captivated by the junction that this research unveiled. On one hand, the work shows definite differences in IG-SCC behavior for the AA5083 (-H116, -H131, and -SHTQ tempers) and the AA5456-H116. But then the test for grading alloy susceptibility in the first place, the Nitric Acid Mass Loss Test, is called into question, which could flip the tables since this proxy is how alloys of different composition and heat treatment are grouped together. That leads to the question: Do these fracture data actually mean anything if the initial susceptibility grouping was flawed? So the paper finishes by assessing the corrosion morphology caused by the nitric acid, and the differences observed suggest that this could indeed be the case. The significance: this same test is still being used to grade Al-Mg structural susceptibility and fitness for service today, which could have significant implications for the Littoral Combat Ship and the CG-47 Class Cruiser.
Matthew E. McMahon
University of Virginia
Read the Original
This page is a summary of: The Effect of Temper and Composition on the Stress Corrosion Cracking of Al-Mg Alloys
, CORROSION, December 2016, NACE International,
DOI: 10.5006/2317.
You can read the full text:
Contributors
The following have contributed to this page
