Difference between electropolishing and pickling Inconel 718

Can someone tell me the difference made to the surface of Inconel 718 when it is electropolished vs. pickled. We weld 718 and the key to a good weld that doesn't leak is a pristine surface. We were having leaks through weld doing pickling so switched to electropolishing as a cleaning method. Seemed to work for a while then the leaks came back. Switched to a double pickle, one right after the other and so far so good but would really like to understand what is going on at the surface between the two systems. We also did a test with scotchbriting the surface and got very good results. I know the presence of Ti and Al in 718 make it difficult to weld. Is on of these surface prep methods removing more Ti and Al than the other? Anyone have any theories. Thanks