Question

a.If you see a white, powdery deposit on a surface, what does it mean to the component?

b.Why does intergranular corrosion reduce the strength of a component and does this

affect airworthiness of an aircraft?

c.What are the problems to detect small, sub-laminar tears on a large aircraft?

Answer 1

a)

Aircraft structures are mostly made up of Aluminum or titanium materials owing to their less density that enhances fuel efficiency. When aluminium corrodes, unlike steel structures, it usually first shows as a whitish or grey “dulling” of the aluminium surface. So basically this is a corrosion attack on the structure. This corrosion may spread uniformly entire the exposed surface, or limit to a region like stress corrosion cracking. In case of steels,  It is a corrosion product of zinc formed under certain specific conditions of exposure. the reasons may be due to the combined effect of service weather and local site conditions like dust and pollution, moisture trapped between sheets during transport or in service. Aircraft is painted to form this kind of rust carefully and frequent quality inspections are done to find any powdery deposits on the surface.

b)

Yes, Intergrannular corrosion affect airworthyness of flight. Unlike uniform corrosion, which is propagated on the surface, Intergranular corrosion is the highly localized effect on grain boundaries. It weakens the grain boundaries, thereby if any crack present within the material, there is a high probability that it propagates through grain boundaries which leads to catastrophic failure of materials. This type of attack results from local differences in the composition of the material. In steels, grain boundary precipitation, notably chromium carbides in stainless steels. At high temperatures, chromium attacks with carbon, it forms chromium carbide and deposits at grain boundaries. Now grain boundaries are rich in carbon while other areas are not. Aluminium 7000 series alloys are highly susceptible to intergranular corrosion. It obviously makes aircraft unfit for flying since it leads to catastrophic failure of the corroded components owing to crack propagation along the grain boundaries.

c)

To detect small, sub-laminar tears on aircraft, we need to adapt Non-destructive testing methods like visual inspection, ultrasonic etc. No matter how perfect our test is some small minute manage to escape from our eye. to adapt spray or die penetration or X-ray, doing on all aircraft body would be difficult since aircraft is large in size and have complicated profiles. So a lot of factors are involved that makes a very small crack go undetectable on the aircraft structure.