Corrosion Flashcards
How much does corrosion cost the US economy a year?
276 billion $
How much of the annual world wide production of steel is used to replace material lost to rust/ corrosion?
15%
What types of damage occur at the surface of steel?
dissolution (loss of material - corrosion)
Formation of surface scale (oxidation) with no strength
How does corrosion directly cost economies money?
- Component replacement
- Painting and other preventative measures
- Use of expensive corosion resistant materials
How does corrosion indirectly cost the economy money?
- Plant shutdown
- Loss of product
- Loss of efficiency
- contamination of product
How much does corrosion cost the average industrialized economy?
3.5% of its GDP
What kind of reaction is corrosion?
REDOX (electrochemical)
What happens to a metal in a REDOX reaction?
Metal atoms are oxidised (lose electrons)
Where do metals oxidise?
At the anode
Where do the ‘free’ electrons transfer to?
the Cathode
Does iron want to be an oxide or pure iron?
It is energeticaally favourable for Fe to be an oxide
What is the REDOX reaction for iron?
- Oxidizing iron supplies electrons to the edge of the droplet to reduce oxygen from the air
- electrons move through the iron to the outside of the droplet
- in the droplet the hydroxide iron can move to react with the iron(II) ions moving from the oxidation region.
- Iron(II) hydroxide is precipitated
- rust is produced by the oxidation of the precipitate
What allows the oxidation of iron to continue?
The electrochemical cell action
What is produced at the cathode during an electrochemical reaction?
H2
What can be done to protect against corrosion?
Coat them in polymer (polymer coated steel pipes: HDPE top and intercoat, with an epoxy primer)
Galvanic protection
How does galvanic protection work?
Connect metal you dont want to be oxidized to a more reactive metal.
Connected to the cathode
It is easier to replace the sacrificial rod than the metal pipe (for example)
The sacrificial rod acts as an anode, whereas the pipe acts as the cathode
this forms a voltaic cell (this is because magnesium has a lower standard potential than steel)
How does hot dip galvanized coating work?
zinc is corroded in preference to steel and sacrifices itself
corrosion products from the zinc are deposited on the steel resealing it from the atmosphere and therefore stopping corrosion
If damage to a galvanized coating reveals the bare steel then the steel can still be protected as they provide both barrier and sacrificial protection
What are the advantages of galvanizing?
- Zinc weathers at a very slow rate, giving long and predictable life
- coating corrodes sacrificially, allowing protection even if the coating is damaged
- It can prevent sideways creep, this can underimne the coatings
- Hot dip galvanizing is very versatile and economically efficient
How long does a galvanized coating last?
> 40 years
Also easy to inspect
How does cathodic protection work?
- Connect a wire to a direct current source to the metal to be protected
- positive terminal is attatched to an inert (normally graphite) protectie anode burried in the soil
- redirects current to form a sacrificial anode to the soils-water electrolyte, rather than on the anode are on the pipe line or other-metallic structure
How does the cathodice series work?
The lower the more inert, the less reactive, less prone to corrosion
Galvanizing metal should be from lower in the series
