Question

Metallic materials used in aeronautical industry can be susceptible to pitting. Select a metal that is known to pit and describe environmental conditions and corrosion-influencing factors that can cause pitting of this selected metal when used in the aeronautical industry.

Describe the relevant mechanism of atmospheric corrosion that takes place in that scenario.

Answer 1

Pitting is very destructive types of corrosion. If not properly protected,metals such as aluminium, steel and magnesium used in aircraft are susceptible to this.

Pitting is a form of extremely localised attack that resulting in small holes. Pitting can start at precipitates at the surface of certain alloys when the particle has a different electrochemical potential from the surrounding metal matrix. Pitting can also occur in surface regions where the corrosion protective layer is absent. When surface protection is used small gaps in the layer can occur because of incorrect application or in-service damage by abrasion, erosion or some other event. The corrosion forms as a hole at the gap in the protective layer, which then develops into a wider cavity below the surface.

Pitting requires an incubation period before it becomes visible, which can be months or years depending on the type of metal and electrolytic fluid. Once started, however, the pit tends to penetrate the metal at an ever increasing rate forming a large cavity beneath the surface. Most pits grow downwards from horizontal surfaces, such as upper wing and horizontal stabiliser surfaces. Pits are less likely to develop on vertical surfaces, and only rarely grow upwards from the bottom of horizontal surfaces.

Therefore, the inspection of aircraft components for damage caused by pitting corrosion shoud focus on upper horizontal surfaces. However, pits are difficult to detect by visual inspection because their surface opening is very narrow and is often covered with corrosion products, even though the underlying metal is severely corroded.