Corrosion

Question 1

How much does corrosion cost the US economy a year?

Answer 1

276 billion $

Question 2

How much of the annual world wide production of steel is used to replace material lost to rust/ corrosion?

Answer 2

15%

Question 3

What types of damage occur at the surface of steel?

Answer 3

dissolution (loss of material - corrosion)

Formation of surface scale (oxidation) with no strength

Question 4

How does corrosion directly cost economies money?

Answer 4

• Component replacement
• Painting and other preventative measures
• Use of expensive corosion resistant materials

Question 5

How does corrosion indirectly cost the economy money?

Answer 5

• Plant shutdown
• Loss of product
• Loss of efficiency
• contamination of product

Question 6

How much does corrosion cost the average industrialized economy?

Answer 6

3.5% of its GDP

Question 7

What kind of reaction is corrosion?

Answer 7

REDOX (electrochemical)

Question 8

What happens to a metal in a REDOX reaction?

Answer 8

Metal atoms are oxidised (lose electrons)

Question 9

Where do metals oxidise?

Answer 9

At the anode

Question 10

Where do the ‘free’ electrons transfer to?

Answer 10

the Cathode

Question 11

Does iron want to be an oxide or pure iron?

Answer 11

It is energeticaally favourable for Fe to be an oxide

Question 12

What is the REDOX reaction for iron?

Answer 12

• 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

Question 13

What allows the oxidation of iron to continue?

Answer 13

The electrochemical cell action

Question 14

What is produced at the cathode during an electrochemical reaction?

Answer 14

H2

Question 15

What can be done to protect against corrosion?

Answer 15

Coat them in polymer (polymer coated steel pipes: HDPE top and intercoat, with an epoxy primer)

Galvanic protection

Question 16

How does galvanic protection work?

Answer 16

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)

Question 17

How does hot dip galvanized coating work?

Answer 17

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

Question 18

What are the advantages of galvanizing?

Answer 18

• 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

Question 19

How long does a galvanized coating last?

Answer 19

> 40 years

Also easy to inspect

Question 20

How does cathodic protection work?

Answer 20

• 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

Question 21

How does the cathodice series work?

Answer 21

The lower the more inert, the less reactive, less prone to corrosion

Galvanizing metal should be from lower in the series