Corrosion Fall 21 Chemistry 2023 fall

CH101

1. Corrosion and its Prevention
Corrosion of Fe to form Fe
2O
3( spontaneous)
Extraction of Fe from Fe
2O
3(non-spontaneous)
Both involve electrochemical reactions

2. Corrosion of Iron
Caused by Electrochemical Reactions
Oxidation and Reduction go together
→Oxidation: Fe → Fe
2+
+ 2 e ( Metal corrodes )
→Reduction happens in the environment such as:
a. 2 H
+
+ 2 e → H
2 ( acidic medium )
b. O
2+ 4H
+
+ 4 e → 2 H
2O ( acidic medium with dissolved O
2)
c. O
2+ 2 H
2O + 4 e → 4 OH
-
( Neutral / basic medium with
dissolved O
2)
d. Metal ion can also be reduced
Cu
2+
+ 2 Fe
2+
→ Cu + 2 Fe
3+
H
+
, O
2, H
2O and impurities (salts) speed up rusting

4. 1.Rusting of Iron in oxygen rich water: Rust Fe
2O
3.xH
2O
2.Step I:Fe → Fe
2+
+ 2e
3. ½ O
2+ H
2O+ 2e→ 2 OH
-
4.Overall: Fe + 1/2 O
2+ H
2O → Fe
2+
+ 2 OH
-
( Soluble )
5.Or 2 Fe +O2 +2 H
2O→2 Fe
2+
+ 4 OH
-
………..(1)
6.Step II: 2 Fe
2+
→ 2 Fe
3+
+ 2e
7. ½ O
2+ H
2O+ 2 e→ 2 OH
-
8.Overall: 2 Fe
2+
+ ½ O
2+ H
2O → 2 Fe
3+
+ 2 OH
-
….(2)
9.Add reaction (1) and (2) to get
10. 2 Fe + 3/2 O
2+ 3 H
2O → 2 Fe ( OH )
3
Fe ( OH )
3transforms into : Fe
2O
3.xH
2Owhich is called rust

5. 1.Dissolvedsaltsfacilitatecorrosion
2.Acidityfavorscorrosion.Aciditycomesfrom
dissolvedCO
2
oxidesofS,N&
Fe
3+
+3H
2O→Fe(OH)
3+3H
+
etc
The factors which speed up corrosion

6. 1.Coatingsofpaintorplasticsprotectthemetal,buttheycancrackorsuffer
otherdamage,therebylocalizingandaccentuatingtheprocess.
2.Passivation:inwhichathinmetaloxidelayerformsonthesurfaceand
preventsfurtherelectrochemicalreactions.(Aluminumreactswith
oxygentoformatightlyboundthinprotectivelayerofA1
2O
3
3.Specialpaintsdesignedforrustpreventioncontainpotassiumdichromate
(K
2Cr
2O
7)andleadoxide(Pb
3O
4)whichcausethesurfaceofirontobe
oxidizedandpassivated.
4.Thepresenceofsmallquantitiesofalloyingmaterialscangreatlyinfluence
theamountofprotectionthatafilmprovides.
Stainlesssteelisanalloyofiron+chromiuminwhichthe
chromiumleadstopassivationthroughCr
2O
3
formationandpreventsrusting
Factors which retard Corrosion

7. 5.UseofasacrificialanodesuchasMgforprotectingFe:
Accordingtothestandardreductionpotentialsofironandmagnesium
Fe
2+
(aq)+2e
-
Fe(s) E
o
=-0.41V
Mg
2+
(aq)+2e-Mg(s) E
o
=-2.39V
Mg(s)Mg
2+
(aq)+2e- E
o
=+2.39V
Mg
2+
(aq)ismuchhardertoreducethanFe
2+
(aq).ItfollowsthatMg(s)
ismoreeasilyoxidizedthanFe(s)
Magnesiumisthesacrificialanode
6.GalvanizingwithZn(seenextslide)
7.TincoatingonSteel
8.SilvercoatingonSteel(Silverspoon)
Factors retarding Corrosion

8. Mg as sacrificial anode
Inert ( graphite )