Passing this on:
All wood pressure-treatment chemicals after 2004 contain copper. Copper in contact with the steel in hangers and nails, in the presence of water and oxygen, promotes galvanic corrosion. That corrosion is an electro-chemical reaction. Basically it’s the same thing we see in many other conditions in which different metals are in contact with each other in the presence of moisture (why we look for dielectric unions). The difference is that this time the copper was in the chemical solution forced by pressure into the wood during treatment.
Here’s a paper with photos.
Here’s a link to Simpson Strong Tie’s page on the levels of corrosion resistance they offer for their connectors.