Question

a.What are the main types of corrosion concerning aircraft?

b.What is the main difference between corrosion in steel and aluminum?

c.Why is humidity critical for Filiform corrosion?

d.What is meant by stress corrosion and how does it manifest?

Answer 1

Part A:

1:Intergranular corrosion: it occurs in grain boundaries .Since corrosion is a energy phenomenon and grain boundaries are high energy regions. This destroys solidity of metal .

2:Pitting corrosion : it is a wet corrosion process occurs when deposit is cleaned away and where small pits are formed on surface .

3:concentration cell corrosion :it is also a wet corrosion process ,occurs when two or more metals are in CNN ontact with different concentration of same solution like water, moist air .

4:Dissimilar metal corrosion :occurs when dissimilar metals are in contact in presence of electrolyte.

part B: when iron in steel corrodes it forms a rust and this doesn't adhere to iron well and so it doesn't form a protective layer to further layer .so atmosphere always attacks a new layer and this new layer get lost. But in aluminium ,when atmosphere attacks oxide layers are formed and this oxide layer adheres to metals well and so this layer acts as a protective layer and prevents the atmosphere corrosion to penetrate below oxide layer. So aluminium have a self protecting corrosion mechanisms ,where steel doesnot have such mechanism .

part C: filiform corrosion is a localised takes when there is high level of humidity. Humidity is critical for this because it allows water and oxygen to migrate .And occurs a oxygen concentration gradient and corrosion results.the corrosion attack is supported by moisture which penetrates the surface layer and become depleted of oxygen making the area anodic .the concentration of dissolved oxygen become highest at some region and this region become cathodic.so without the support of moisture anode and cathode region not possible. Hence humidity is critical for this type of corrosion.

part D: stress corrosion is a crystalline cracking of metals under tensile stresses in presence of corrosive agents. It is most likely to occur at points of higher stress. When solvent of some kind penetrates into crack of stressed metal and weakens the atomic bond .to occurs this corrosion, residual stresses and corrosive agents are must.