Advance Chemistry (Corrosion)

8/19/2019 Advance Chemistry (Corrosion)

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where M is a metal.

Electrochemical corrosion of iron

Corrosion often begins at a location (1) where the metal is under stress (at a bend

or weld) or is isolated from the air (where two pieces of metal are joined or under a

loosely-adhering paint lm.) The metal ions dissole in the moisture lm and theelectrons migrate to another location (!) where they are ta"en up by adepolari#er.

$%ygen is the most common depolari#er& the resulting hydro%ide ions react with the

'e! to form the mi%ture of hydrous iron o%ides "nown as rust.

7hich parts of the metal serve as anodes and cathodes can depend on many

factors, as can be seen from the irreular corrosion patterns that are

commonly observed. /toms in reions that have underone stress, as miht

be produced by formin or machinin, often tend to have hiher free

eneries, and thus tend to become anodics.


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In contrast to anodic sites, which tend to be locali$ed to specific reions of

the surface, the cathodic part of the process can occur almost anywhere.

ecause metallic oxides are usually semiconductors, most oxide coatins do

not inhibit the flow of electrons to the surface, so almost any reion that is

exposed to O% or to some other electron acceptor can act as a cathode.

#he tendency of oxyen0deprived locations to become anodic is the cause of

many commonly0observed patterns of corrosion

Rusted-out cars and bathroom stains

/nyone who has owned an older car has seen corrosion occur at "oints

between body parts and under paint films. + salts which precipitate as reenish

stains.


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Control of corrosion

)ince both the cathodic and anodic steps must tae place for corrosion tooccur, prevention of either one will stop corrosion. #he most obvious stratey

is to stop both processes by coatin the ob"ect with a paint or other

protective coatin. ?ven if this is done, there are liely to be places where

the coatin is broen or does not penetrate, particularly if there are holes or

screw threads.

/ more sophisticated approach is to apply a sliht neative chare to the

metal, thus main it more difficult for the reaction M → M%+ + % e– to tae

place.

Sacrificial coatings

One way of supplyin this neative chare is to apply a coatin of a more

active metal. #hus a very common way of protectin steel from corrosion is

to coat it with a thin layer of $inc; this process is nown as alvani$in.#he

$inc coatin, bein less noble than iron, tends to corrode selectively.

=issolution of this sacrificial coatin leaves behind electrons which

concentrate in the iron, main it cathodic and thus inhibitin its dissolution.

#he effect of platin iron with a less active metal provides an interestin

contrast. #he common tin0plated can 2on the riht3 is a ood example. /s

lon as the tin coatin remains intact, all is well, but exposure of even a tiny

part of the underlyin iron to the moist atmosphere initiates corrosion. #he

electrons released from the iron flow into the tin, main the iron more


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anodic so now the tin is actively promotin corrosion of the iron@


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Corrosion gallery

Corrosion of a nail

#he nails are immersed inaar which forms a moist

solid el. #he aar also

contains phenolphthalein

and hexacyanoiron2III3

1e2CA3– which forms a deep

blue color 2:prussian blue:3

in the presence of 1e%+. #he

blue colors are clearly

associated with those parts

of the nail that have been

stressed, thus facilitin the

anodic release of 1e%+ from

the metal. #he the pin

color shows the cathodic

reions that have been

made alaline by the

reaction

O% + % 4%O + 5e– → 5 O4–

#his clearly shows the

separation between the

anodic and cathodic


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processes in corrosion.

8Illustration from ( of 7est

IndiesB lin9

Water distribution main

If you live in the older part of a city

where the mains are -&0'&& years

old, the water you drin may well

have passed throuh a pipe in this

condition@ )evere corrosion lie this is

more common in areas where the

water is acidic. )uch water comesfrom mountain snowmelt and runoff,

and usually ac!uires its acidity from

dissolved atmospheric carbon dioxide.

7aters from rivers, laes, and

especially roundwaters from wells

have usually been in sufficiently lon

contact with carbonate0containin

sediments to have been neutrali$ed.7ater0wors administrators lie to

mae the water slihtly alaline and

slihtly supersaturated in calcium

carbonate in order to maintain a thin

coatin of solid carbonate on the

interior of the pipe which acts to

protect it from corrosion.

http://wwwchem.uwimona.edu.jm:1104/courses/iron.htmlhttp://wwwchem.uwimona.edu.jm:1104/courses/iron.html


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Corrosion of reinforcing bars in

concrete

/ll lare concrete structures contain

steel reinforcin bars 2:rebars:3 thathelp ensure structural interity under

varyin load conditions and especially

durin earth!uaes. Intrusion of

water, even in the form of fo or

mists, can lead to serious corrosion

damae, as seen in this picture of this

column which supports a hihway

overpass.

Corrosion at metallic

joints

#he picture shows two steel

structural members "oined

by cast iron flanes which

have been bolted toether.

1or some reason, one of the

pieces has become moreanodic than the other,

leadin to extensive

corrosion of the upper part.

Bacterial-assisted corrosion

#his as pipe was buried in a red soil

that contained iron pyrites 21e).3 #he

bacteriumthiobacillusferrooxidans derives its enery by oxidi$in 1e%+ to the

more soluble 1e>+, transferrin the

electrons to O%. It also oxidi$es the

sulfur, producin sulfuric acid. #he

resultin chemical coctail has eaten a

lin

http://soils1.cses.vt.edu/ch/biol_4684/Microbes/Thiobacillus.htmlhttp://soils1.cses.vt.edu/ch/biol_4684/Microbes/Thiobacillus.html


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hole into the pipe.

#hese alvani$ed bolts were usedto "oin wooden beams toether.

)ubse!uent movement of the

beams due to varyin load

conditions abraded the $inc coatin.

/ lac of oxyen near the centers

of the bolts also liely contributed

to the corrosion by preventin the

formation of aprotective oxide

film.

Pitting corrosion

What you need to know

Mae sure you thorouhly understand the followin essential ideas which

have been presented above. It is especially imortant that you now theprecise meanins of all the hihlihted terms in the context of this topic.

• ?lectrochemical corrosion of metals occurs when electrons from atoms at the

surface of the metal are transferred to a suitable electron acceptoror depolarizer . 7ater must be present to serve as a medium for the transport of

ions.


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• #he most common depolari$ers are oxyen, acids, and the cations of less active

metals.

• ecause the electrons flow throuh the metallic ob"ect itself, the anodic and

cathodic reions 2the two halves of the electrochemical cell3 can be at widely

separated locations.

• /nodic reions tend to develop at locations where the metal is stressed or is

protected from oxyen.

• Contact with a different ind of metal, either direct or indirect, can lead to

corrosion of the more active one.

• Corrosion of steel can be inhibited by galvanizing, that is, by coatin it with $inc,

a more active metal whose dissolution leaves a neative chare on the metalwhich inhibits the further dissolution of 1e%+.

• Cathodic protection usin an external voltae source is widely used to protect

underround structures such as tans, pipelines and piers. #he source can be asacrificial anode of $inc or aluminum, or a line0operated or photovoltaic power

supply.


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Advance Chemistry (Corrosion)