I also use Tecre parts. In my own experience, humidity is what causes this "rusting", also known by book and button collectors as "foxing".
I made over a hundred 2.25" magnets as favors for my own wedding five years ago. Several friends and family still have them on the fridge. Some of these are fine, and some have foxed - largely dependent on the humidity in the home. Those who don't run air conditioning on a regular basis have the most foxing on their magnets.
One friend has a coastal weekend home where she doesn't run air conditioning during the week. In that house, paper and magazines curl overnight, and the alphabet magnet set that I gave her foxed quickly and thoroughly.
I've lived in Austin, Houston, Seattle, and New Orleans. Foxing in Texas wasn't too bad because of rare rain and good air conditioning. In Seattle, it was terrible, because the humidity is constantly high, and we didn't have air conditioning (only heating). I had to buy a dehumidifier, and most of my own buttons and fridge magnets still foxed after a few months. In New Orleans, we have AC again, which reduces the indoor humidity somewhat. Fridge magnets I made last year are still in good condition.
So, I believe it's mostly dependent on the humidity where the products are made and where they are kept. I try to make items in conditions of 50% humidity or less so that moisture won't be trapped inside, and I store Items in sealed containers if I know I might not sell them for a few months.
I think vintage pins have less foxing because the images were printed on an older style of photo paper and then covered with celluloid. If anyone knows of 8.5"x11" laser printer paper that doesn't wick moisture into a button I'd love to know! (Wishful thinking, I'm sure.) :)