Heater Water valve leaking

Hi

My Ford Explorer 2002, is leaking from the heater water valve. I'm planing to replace it very soon. But what is the yellow stuff coming out from it? It is corrosion?? How i could treat it to prevent this corrosion going further??

How many galons of antifreeze and water would it take after reflusing the old antifreeze??

Please see the attached foto https://www.dropbox.com/s/ah8gdtbw1ljuqgk/20150305_152031.jpg?dl=0

Thanks for joining, it's great to have you here.

Tony:wavey: