Why Does Rust Occur? How Corrosion Attacks Steel Fasteners

Key things to remember

The essentials from this document in plain words. Check the full document before you rely on a figure.

• ✓Rust needs both damp and oxygen. Above 60% relative humidity a thin film of moisture forms on metal and acts as an electrolyte; dirty or salty surfaces can corrode below 60%, because dirt absorbs moisture.
• ✓Corrosion works like a torch battery: current flows from the more reactive metal (the anode) to the more inert one (the cathode), and rust forms where current leaves the anode.
• ✓When two metals touch in the damp, the more reactive one corrodes. A steel screw in copper rusts faster; a steel screw in zinc does not rust, but the zinc corrodes.
• ✓Galvanic series, most inert to most reactive: gold, silver, stainless steel (passive), copper, tin, stainless steel (active), lead, steel, cadmium, aluminium, zinc-coated steel, zinc, magnesium.
• ✓The further apart two metals sit on that list, the faster the more reactive one corrodes. Without special protection, only combine metals of the same or almost the same inertness in damp conditions.
• ✓A small anode against a large cathode is the high-risk combination: the corrosion is concentrated on the small part.
• ✓Corrosion is fastest in polluted industrial and city air, then in salty coastal air; metals last longest in the countryside. Seawater spreads corrosion; fresh water concentrates it near the contact point.

Who it helps: Anyone choosing a fastener material or finish to go into a different metal, outdoors or near the coast.