You are only allowed to ask one question at a time so I edited down your list to a single question. Corrosion is the breaking down of a solid structural material (usually a metal) into weaker chemicals. It is a slow process and naturally occurs over a long period of time. It can be caused by chemicals such as salt or oxygen or it can be caused by microbes as well. The most common and well known type of metal corrosion is the rusting of iron. The oxygen in the air will react with exposed iron over time to form iron oxide, or rust. Rust is not a structurally stable material, so care must be taken to prevent the iron from rusting to maintain structural integrity in buildings and cars. Metals that are most likely to be affected by corrosion are magnesium, zinc, aluminum, iron, tin, and copper. Gold and platinum are two metals that are least likely to corrode over time.