What is stress corrosion cracking

Most of the metals are prone to stress corrosion cracking.Stainless steel is more prone to stress corrosion cracking compared to other metals and alloys.Stress corrosion cracks are generally subsurface cracks which are not detected early enough to take corrective measure.
Three conditions are essential for the stress corrosion cracks to occur:
1.The metal temperature must be high, above 70 deg celsius.
2.Tensile stresses must be present in the structure.
3.The atmosphere in which the structure is working must be salty.
Let us look at the LOX and LN2 tanks erected by us for the shore based steel plant.
In the mornings when the atmospheric temperature was around 40 deg.C the metal temperature can be as high as 140 deg.C.
Where the outer stainless steel wall was anchored to the inner carbon steel wall by wedges with fillet welds the stress distribution in the fillet calculated showed high tensile stresses at the stainless steel wall,
Since the tank was erected near the sea moist air saturated with salt condensed inside the tank when the atmospheric temperature fell in the night and deposited salt especially at the fillet joints.In course of time the concentration of salts reached sufficiently high to initiate cracking.These are the cracks we discovered on the steel surface.Stress corrosion cracks generally are buried but in the inner tank where fillet weld was made the cracks were extensive and also cracks can be seen on the inner surface of the tank.How we repaired forms the subject of next blog.