Corrosion is always a threat to metals and alloys. Corrosion is a very, very important phenomenon. In 1993, the loss of the United States was just over $300 billion!
Each chemical or electrochemical reaction causes an alloy or metal to corrode.
When two kinds of metal (alloy) contact, or when your metal solutions, in different areas have different quality molarity (density), or from a local temperature is different, even when you are not 100% pure metal, so it has other metal impurity and so on... You have corrosion. This means that when two metals come in contact, one of them is more active, and it erodes another metal. For example: zinc and copper contact: zinc corrodes but protects copper.
Gold is a very, very precious metal, it will not be easily react with other elements, it does not react, or react it too slow (I mean you don't think it will happen)
This is why Au (gold) is used as a dental material, since it will not be easily corroded by snot, so it will not react with it, so it will be safe for a long time.
