Technical report | Fatigue Crack Initiation from Pitting Introduced during the Anodising Process

Abstract 

This report discusses four examples of Australian Defence Force aircraft components manufactured from aluminium alloys that exhibited surface pitting arising from the anodising process. In three of the examples, the anodising pits acted as initiation sites for fatigue cracks, which ultimately led to failure of the components. The information presented in this report highlights the issue of anodising pitting and the associated potential for fatigue cracking which can extend to entire batches of anodised components.

Executive Summary

Fatigue cracking is by far the most common cause of failure for aircraft components. The presence of a surface defect that acts as a crack initiation site considerably reduces the fatigue life of the component. Surface defects that arise as a result of pitting during the anodising process are of concern because pitting will be present on the entire surface of the component, increasing the likelihood that fatigue crack growth will occur.

Anodised aluminium alloy components are used extensively throughout the Australian Defence Force's (ADF) aircraft fleet and in most cases the anodising process will have been performed correctly. Anodising remains an effective corrosion protection method for aluminium alloy components, however it needs to be recognised, particularly by original equipment manufacturers (OEMs), that process control is crucial so that the benefits of anodising are not lost due to the formation of excessive etch pits. This is especially so given that anodising is a batch treatment and therefore problems with a single batch can extend to numerous parts. This could potentially lead to a shortage of spares should an entire batch need to be rejected, or more importantly, could result in an entire batch of components developing cracking while in service.

This report presents examples of components from ADF aircraft that exhibited surface pitting which formed during the anodising pre-treatment process. In three of the examples, fatigue cracking had initiated from the surface pits leading to failure of the components.

Awareness of the problems that can be associated with anodising may assist in identifying defective components and thereby minimise the impact of anodising pitting defects on the ADF fleet.