- GALVANIC CORROSION + ELECTRO-CHEMICAL SERIES FOR METAL + ELECTROCHEMICAL ACTION
        

Figures, Description (How it occurs), Causes, Example

°Appear as a white, powdery deposit. 

°Start on surface then extend vertically into the material that decrease the material strength 

Its is started from the intergrabular corrosion.

My additional reference based on this link:
http://www.lambdatechs.com/pitting.html

Based on the picture those corrosion are the the brown spot example:-

~ the damage is greater than the sum of the damage from both cyclic stress and corrosion.
~ causes by combine effect of cyclic stress and corrosion.

Extra notes - https://cwst.com/shot-peening/reasons-to-shot-peen/exfoliation-corrosion/

Example figures of exfoliation corrosion on a Cessna wing flap;

Intergranular corrosion (IGC), also known as intergranular attack (IGA), is a form of corrosion where the boundaries of crystallites of the material are more susceptible to corrosion than their insides 

the grain boundaries of the metal only with smaller or no indication on the surface 

High strength aluminium alloy like 2024 & 7075 can develop the corrosion if it have been improperly heat treated and exposed to corrosive environment

https://en.m.wikipedia.org/wiki/Intergranular_corrosion