And indeed, corrosion is a concern for 7075 aluminium due to the higher content of zinc which is more susceptible to corrosion.
So to all the people out there that says it can't possibly be apple's fault, think again. You guys stay in countries with low humidity, helping to reduce the chances of corrosion. But unfortunately I live in Singapore where humidity is high all year round.
I live in Florida with high humidity most of the time. No issues with my Gold iPhone 6s+