Matakuliah : S0372 – Kimia Teknik Sipil
Tahun
: Ganjil 2007/2008
IRON AND STEEL CORROSION
PERTEMUAN 11
IRON AND STEEL CORROSION
1. Antecedent.
2. Influence of pH.
3. Corrosion in Ground Water.
4. Grey Cast Iron.
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ANTECEDENT
• Iron if there are any in aggresive electrolyte environment
hence will be dissolve like equation of the following reaction :
Fe  Fe2+ + 2e
Amount of dissolve will as according to current of katodik. Its
reaction of this him take place without many polarization
because its reaction very quickly. Speed of reaction of
corrosion determined by reaction of katodik which [is] its
reaction shall be as follows :
2H+ + 2e  H2
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Its exit gas of H2 will very quickly if there is in sour
environment. Become iron very dissolve in acid with
reaction, the following :
Fe + 2H+  Fe2+ + H2
But on the contrary will react very tardy if in environment
of basa :
Fe3+ + 3OH-  Fe (OH )3
The forming of Fe(OH)3 represent sediment which forming
endue covert till corrosion will be pursued, till NaOH can
be barreled by steel.
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INFLUENCE OF PH
a. At pH 4.
In situation of sour environment as according to equilibrium
diagram of E-Ph (iron – H2O) will not be formed by passive
oxide.
Reaction : 2H+ + 2e  H2
Will walk very quickly because in sour environment (giving
high current strength and low tension). Become will give two
reaction :
Fe  Fe2+ + 2e (very quickly)
2H+ + 2e  H2 (very slowly)
Fe + 2H+  Fe2+ + H2
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At addition of oxygen will be formed by reaction:
2H+ + 2e  H2
Will be quickened by polarization and with addition of oxygen
hence its reaction:
O2 + 4H+ + 4e  2H2O
Forming of gas of H2 will be able to change, its result will
give current of korosi big.
b. 4 < pH < 8 (seeing diagram of E - pH of Fe – H2O)
When there no O2, under is potential of “P” hence there will
be current of corrosion very big, because next to nothing
polarization.
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c. 8 < pH < 11
In environment which is basa mean reaction of
anodik will very tardy caused by in small potential
area.
Potential of “P” pasiva will be keep handy in
reaction of alkalis because in alkali environment will
be difficult once incidence gas of H2. With oxygen
in environment of alkalin will easy to happened
pasiva.
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CORROSION IN GROUND WATER
• Pregnant ground water some element in number which vary consist of
CO3-, HCO3-, Cl-,SO4, and SiO2 and kation Ca2+, Na+, K+ and pittance
Mg2+ dan Al3+.
• Concentration water which enough is 8 or 12oF to form sediment of yg
functioning and flimsy as enduing is covert, colour of sediment is turning
white or turn white brass.
• If water concentration only 8oF, hence formed sediment do not become
to endue impervious and homogeneous covert. If just a few is of acid
contents or happened oxygen cell hence will happened cell of
corrosion.
• If water concentration is high, for example 30 – 40oF, hence chalk
sediment will be thick. I too thick hence will harm at usage of heat,
because that sediment have the character of isolator till pursue transfer
of heat. As a result will happened too hot temperature, which in a
moment will happened explosion like at steaming kettle by boiler stone.
danger of corrosion will start if boiler stone sediment arise also iron rust
of yg tan.
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GREY CAST IRON
• Grey cast iron if hitting ground water and also its land;
ground will happened corrosion which its event very loo
like with steel. But for time old ones resilience of
corrosion grey cast iron is better than steel, especially
if that cast iron do not get mechanic style.
• This matter because of rate and graphite The high, till
its product of him will fastening each other till form to
endue impervious covert.
• But this coat don't have high strength till grey cast iron
will remain to hold up corrosion during functioning as
object which not accept style like at coverr, etcetera.
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