Bridge Corrosion A look at the corrosion of

Bridge Corrosion A look at the corrosion of steel in Bridges

Steel Life Cycle Iron ore Blast furnace - reduction to iron Steelmaking furnace (add carbon) Oxidation of iron - rust Structural steel

How a bridge cable is made • Cables are made of thousands of individual steel wires bound tightly together. Steel, which is very strong under tension, is an ideal material for cables; a single steel wire, only 0. 1 inch thick, can support over half a ton without breaking.

Suspension Bridges • The towers are built up from reinforced concrete or steel, with a gap for the main roadway. Then the anchorages are installed to hold the cables. Very thin steel cable is spun from between the anchorages until it can be gathered together to form two large cables. Other cables are then dropped down from the main cables and the sections of prefabricated roadway are attached to them.

Corrosion • This is the last stage of a steel product’s life cycle. • When steel is exposed to water and oxygen, it rusts • Forms iron oxide

Ways to prevent corrosion • Protective coatings plating with another metal galvanized steel painting

Preventing Corrosion • Use corrosion resistant metal ex: stainless steel Only problem? Stainless steel is an alloy of chromium and is therefore a heck of a lot more expensive than regular steel! Gateway Arch, St. Louis

Preventing Corrosion • Anodic/Cathodic protection a technique used to control the corrosion of a metal surface by making it work as a cathode. This is achieved by placing in contact with the metal to be protected another more easily corroded metal to act as the anode. Aluminum cathodes on steel

Anodic region • • The negative part of the cell to which current flows Oxidation occurs (gaining electrons) Rust forms The more reactive metal! Zinc anode

Cathodic region • The positive part of the cell to which current flows from • No rust forms • Less reactive metal • The area turns pink in phenolphthalein indicator (its basic!) Steel (Iron) pipe cathode

What do you think? • If you were to use anode/cathode protection for a piece of copper wrapped in zinc, which metal would be rusted? • (remember the more reactive metal will be the anode and rust!)

Corrosion Impact • Approximately 20% of all the iron and steel produced is used to repair or replace corroded structures. (and this cost billions of dollars each year in repair and maintenance costs!)