slug

I dont think it is.
Apologies for sounding like a barrack room lawyer but...
The point is that the paint has flaked off. The paint is not just there to look good it is there to form a barrier against corrosion, otherwise why paint anything. IMO the paint is not up to the job and is not fit for purpose.
Once the paint has gone then you will naturally get corrosion.

So, in a nutshell the effect is corrosion the cause is flakey paint.