WHAT IS RUST CORROSION? Rust is the product we get when iron goes through the corrosion process. Iron and steel are not found free in nature. Both are extracted from iron ore in a blast furnace or electro oven. Large amounts of energy are introduced into the iron ore in the extraction process, when this stored up energy is released, the iron has a tendency to corrode back to its natural form "iron oxide" or RUST. Metals that are found free in nature are stable and are classified as being more noble but even metals found in nature have varying electrical potential Gold has a potential of 1.498 v whereas Antimony has a potential of only 0.152v once we add man made metals and Alloys to this a very basic scale of a metals nobility would look some thing like this.
THE SAXON RELIC EXAMPLE
This Saxon gold brooche is over a thousand years old with no signs of corrosion Gold is found free in nature you can pan for it, a completely natural and stable metal its free! The Iron Hammer Axe ia also over 1000 years old Iron is not found free in nature therefore as soon as it is exposed to natures Air and Water it corrodes and disintegrates back to the very earth it came from. Iron and steel require a barrier surface protection on all surfaces to stop the natural corrosion process. Hence we can say Gold is more noble( stable) than Iron/Steel.
AUTOMOTIVE METALS NOBILITY TABLE
GOLD THE MOST NOBLE
HIGH GRADE STAINLESS STEEL
50/50 LEAD TIN SOLDER
LOW GRADE STAINLESS STEEL
STEEL & CAST IRON
Gold being the most NOBLE / CATHODIC + (positive) Magnesium being the least NOBLE most ANODIC - (negative) The less noble metal will be consumed by the more noble metal when they are connected by an electrolyte (water). For example sacrificial anodes are made of zinc when used to protect steel because the zinc will be sacrificed to corrosion and the steel will stay in tact, this is known as cathodic protection. There are other less known metals and alloys within this scale but for the sake of clarity these have been omitted.
WARNING ! THINK VERY CAREFULL AT THE CONSTRUCTION STAGE HOW CONNECTION OF DISIMILAR METALS IS GOING TO EFFECT CORROSION. AS SOON AS THEY ARE CONNECTED ELECTROLYTIC CORROSION IS INEVITABLE. THE USE OF ISOLATING NON CONDUCTIVE COATINGS, SEALERS ,ADHESIVES AND FASTENERS WILL PREVENT ELECTROLYTIC CORROSION