Understanding the influence of parameters on intergranular corrosion test results
The test method described in ISO 11846 Method B is one of the most frequently used tests to study the susceptibility of 6xxx-series aluminium alloys to intergranular corrosion. However, this standard allows for a significant variation in several testing parameters, and with the emergence of alloys that contain increasing amounts of recycled material, understanding the influence of these parameters is crucial.
The variations in testing parameters allowed in this method includes the ratio of the test solution volume to the total specimen area, and the test solution temperature. The test solution volume to the total specimen area influences the pH of the electrolyte – corrosivity of the solution changes with time – while higher temperature will affect corrosion morphology.
The difference in corrosion depth, for example with aluminium alloy 6005, could be as much as 50%, depending on the test parameters.
Consequently, understanding the influence of these parameters is key to our ability to ensure a more accurate comparison of the intergranular corrosion (IGC) resistance of the alloys tested. IGC is a type of localized corrosion that reduces the mechanical properties of materials, such as aluminium, leading to structural damage or accidents.
More recycled aluminium content for a more sustainable automotive industry
The automotive industry often uses the ISO 11846 Method B standard to determine whether 6xxx-series aluminium alloys have IGC.
Today, the industry is demanding more recycled content in the alloys they use in their components and systems. And because the concentration of elements such as iron, zinc and copper in these alloys is going to be different than the concentration in standard alloys, this will probably have an effect on the IGC properties of these alloys.
Effects of variations in solution volume-to-area ratio
We recently delivered a paper to the Aluminum Extrusion Technology Seminar & Exposition (ET) in Florida, based on our work to further study the solution volume-to-area ratio and the solution temperature effect on the IGC test results for standard alloy 6005.
In the first segment – the solution volume-to-area ratio – we tested four different ratios. The results of these tests showed that both the weight loss and the level of IGC attacks increased with increasing test solution volume/sample area ratio.
Furthermore, the change in pH of the test solution decreased with increasing solution volume/sample area ratio. Hence, the pH of the solution will increase as the metal dissolution increases, the solution is getting less corrosive and this will eventually slow the corrosion rate.
By increasing the solution volume, the corrosion process will be able to continue and the level of the IGC attacks will increase.
Effects of test temperature variations on IGC properties
In this part of the work, we used three different test temperatures. The weight loss increased only slightly with increasing solution temperature at 5ml/cm2, while at 20ml/cm2, the weight loss increased significantly with an increasing solution temperature. Average maximum corrosion depth decreased with increasing solution temperature, both at 5ml/cm2 and at 20ml/cm2.
Nevertheless, the corrosion depth is significantly higher at 20ml/cm2 as compared to 5ml/cm2 at each of the three temperatures tested.
The increase in temperature results in more extensive corrosion limited to the surface region, rather than extending further into the recrystallized layer.
A more accurate comparison of the IGC resistance of aluminium alloys
This work shows that understanding the influence of the test parameters is important toward ensuring a more accurate comparison of the IGC resistance of the alloys tested, especially when comparing the results from different tests.
Let me conclude in this way, citing ISO 11846:2008, which points out that “test results cannot be regarded as absolute, because they are not applicable to all environments that can be met in service. They are best used in a relative manner, to compare the intergranular corrosion resistance of various heats of solution heat-treatable aluminium alloys.”
