Stress Corrosion Cracking (SCC)

SCC in aluminium alloys

High strength aluminium alloys contain various alloying elements, and copper, magnesium, zinc and silicon all increase the risk of SCC in alloys of this type. Consequently, for aluminium alloys with any of these principal alloying elements there will always be a need to understand the SCC resistance. Corrosion occurs when there is electrochemical attack on the surface of metals resulting in a loss of material at the corrosion site, often referred to as pits. Under the influence of tensile stress, the corrosive attack can be highly localised, leading to fine cracks in the metal structure, and where the corrosion can also be greatly accelerated. This is the phenomenon of Stress Corrosion Cracking, and also why SCC is one of the most disastrous forms of corrosion. In aluminium alloys SCC usually occurs along, or adjacent to, grain boundaries where there is the most significant difference in electrochemical properties due to local changes in composition.

2A05.60 Stress Corrosion Cracking resistance

As well as the chemical composition effects, the heat treatment condition also has a strong influence on the SCC resistance in aluminium alloys. For this reason the 2A05.60 alloy is typically used in a T7 temper, following a heat treatment cycle that was specifically designed to enhance resistance to stress corrosion cracking in the A205 cast metal. The most recent development to optimise the heat treatment cycle for additive manufacture parts in A205 also focussed on this requirement.

Aluminium Materials Technologies Ltd have followed the international standard for stress corrosion cracking resistance testing, Material Analysis for Round Stress Corrosion using Constant Strain ASTM G49-85(2011), and had numerous AM samples tested. Specimen parts were subjected to constant stresses ranging from 20%, 40%, 60% and 80% of nominal yield stress, 420MPa. To create a corrosive environment all samples were alternately immersed in 3.5% NaCl and stressed in accordance with ASTM G47-20.

Tests of this kind can be deemed to have been successful if the specimens have not failed, meaning broken, following the exposure under these conditions for a full 30 day test. Evidence of surface pitting corrosion is normal for these types of alloy, and hence more detailed analysis is required to confirm resistance to SCC. Cross-section analysis of all unbroken samples needs to confirm that there has been no intergranular cracking in order to show resistance to stress corrosion cracking.

The image shows the condition of one specimen after 30 days at the maximum stress level of 80% of Yield Stress. As expected, for aluminium alloys tested in such an aggressive environment, the surface was heavily corroded, but Aluminium Materials Technologies Ltd are very pleased to announce that all the specimens were unbroken after 30 days of testing. Furthermore, in a form of enhanced test programme the samples were left under the same test conditions for a further 10 days. Even after this extreme test condition only some samples at the 80% of Yield Stress level had eventually broken, but not all, and none at any of the other lower stress levels.

This is a remarkable achievement for our A20X product, and additive manufacturing enables us to continue to reveal even more benefits of using this unique alloy.

Mike Bond

Managing Director, Aluminium Materials Technologies Ltd

Contact Us for details about the full report, and to know more about the extraordinary performance of the A20X alloy.

2A05.60 30 day SCC sample_unbroken

2A05.60, the additive manufactured version of the A205 casting alloy, sample under Stress Corrosion Cracking conditions tested at 80% of Yield Stress for 30 days. The sample is corroded but remained unbroken.