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Bridge%20Corrosion

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Cables are made of thousands of individual steel wires bound tightly together. ... Gateway Arch, St. Louis. Preventing Corrosion. Anodic/Cathodic protection ... – PowerPoint PPT presentation

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Title: Bridge%20Corrosion


1
Bridge Corrosion
  • A look at the corrosion of steel in Bridges

2
Steel Life Cycle
Iron ore
Blast furnace - reduction to iron
Steelmaking furnace (add carbon)
Oxidation of iron - rust
Structural steel
3
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.

4
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.

5
Corrosion
  • This is the last stage of a steel products life
    cycle.
  • When steel is exposed to water and oxygen, it
    rusts
  • Forms iron oxide

6
Ways to prevent corrosion
  • Protective coatings
  • plating with another metal
    painting
  • galvanized steel

7
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
8
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
9
Anodic region
  • The negative part of the cell to which current
    flows
  • Oxidation occurs (gaining electrons)
  • Rust forms
  • The more reactive metal!

Zinc anode
10
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
11
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!)

12
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!)
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