Step 2 - since the joint size or weld size is normally thin or small compared to size of base materials A & B , after some initial period there will be establishment of electrical or galvanic connection between A & B directly due to presence of electrolyte in the already corroded area in step 1 ( despite having presence of W in between A&B ). Now in Step -2 the effect of surface area of A and B or C/A ratio will become more dominant and will ultimately control the rate of galvanic corrosion.