I've carried rounds in duty leather, and the only way to prevent verdigris is to use nickel plated brass. Even at that, the rounds will have to be changed out regularly, since the chemicals used to tan the leather causes all kinds of problems with brass, even when plated. I've seen plain brass cartridges that have been left in leather loops for a number of years that have been eaten all the way through.
Now that my wife and I have taken up Cowboy Action Shooting, the only rounds we can use in our Cowboy rigs are nickel plated. I also use the nickel plated cases for the carbine rounds, for "lost brass" matches, where we have to leave the brass on the ground because of the size of the match, in numbers of shooters. When you have three or four hundred shooters at a match, there isn't time to pick up brass.
Hope this helps.
Fred