Chapter 7 Electrochemistry

§7.13 Corrosion and protection of metals

1) Corrosion:

Destruction of materials due to the chemical, electrochemical and physical attack of the media.

Railway bridge in Boston

White marble of Jinshui Bridge, Beijing

Stone Sculpture before the Capitol, Washington D.C.

1. General introduction

One-fifth of the iron and steel produced annually in the world is used to replace rusted metal.

Since corroded metal often loses its structural integrity and attractiveness, corrosion of metal probably results in disaster and has great impact on national economics and safety.

Brass sculpture before the Capitol, Washington D.C. Brooklyn Bridge,

New York

Warship of Pacific Fleet, Russia

2) Why does metal undergo corrosion?

Only few elements exist in free element state in natural circumstance. Corrosion of metal, i.e. conversion of element to stable compound, is thermodynamically favored.

Naturally occurring copper single crystals, Museum of Natural Sci., Washington, D.C. USA

Naturally occurring copper sheet

Naturally occurring gold

2. Classification of corrosion

1) Based on materials:

Corrosion of metals; Corrosion of non-metals (wood, plastic, concrete, stone, etc.)

2) Based on Media:

natural corrosion; industrial corrosion (with surface solution containing acid, base, H2S, etc.)

3) Based on mechanism:

chemical corrosion (2Fe + O2 = 2FeO);

electrochemical corrosion; biochemical corrosion

4) Based on uniformity:

general corrosion; local corrosion

5) Other kinds:tension corrosion; contact corrosion;

friction corrosion; external current corrosion;

Concussion corrosion

3. local corrosion

Local corrosion is initiated du

e to the ununiformity of metal a

nd / or solution.

1) The ununiformity of metal:

2) The ununiformity of metal surface

3) The ununiformity of solution

(1) multiphase texture;

(2) crystal boundary;

(3) crystal facet;

(4) impurities;

(5) enrichment;

(6) tension and deformity

1) The ununiformity of metal:

Corrosion of crystal boundary

intergranular corrosion

Trans-crystal corrosion

transgranular corrosion

(1) multiphase texture (2) crystal boundary;

Crystal plane (111) (100) (110) (120)

Relative corrosion rate 1 0.9 0.55 0.32

(3) crystal facet;

(4) impurities; (5) enrichment;

Fe

Fe = Fe2+ + 2e

Fe2+

Fe2+

Fe2+OH

Cl

Cl

ClCl

Fe2+

Impurity

O2

O2 + 2H2O + 4e 4OH

Mechanism of Pitting corrosionpH can

attain 3.5 ~ 4

Tension corrosion

Anodic region

Cathodic region

Reactions:

Anodic reaction: Fe Fe2++2e¯

Cathodic reaction: 2H2O + 2e¯ H2+2OH ¯

Nails in a solution with phenolphthalein and K3[Fe(CN)6]

(6) tension and deformity

Where is anodic region and cathodic region?

These two beakers contain 1.0 mol·dm-3 Cu(NO3)2 solutions, a

copper electrode, and a salt bridge. If the two copper

electrodes were connected together, would a current flow

between them?

3) The ununiformity of solution

(1) Concentration difference of metal ions;

(2) Concentration difference of media ions;

(3) Accumulation of H+ in pit or cracks;

(4) Concentration difference of dissolved oxygen

2) The ununiformity of metal surface

(1) Smoothness of the surface;

(2) Micropore in protective layer;

(3) Corrosion products

Different aeration corrosion

Waterline corrosion

Crack corrosion

(4) Concentration difference of

dissolved oxygen

O2

O2

O2

O2O2

Where does corrosion take place and

where does rust form?

4 Theoretical consideration of electrochemical corrosion

Zn + 2 HCl ZnCl2 + H2

Why does Zn of 99.5 % purity dissolve in dilute HCl in 1 min,

while that of 99.999% purity does not dissolve even after 8 h?

anode reaction:

Zn Zn2+ + 2e

Cathode reaction:

2H+ + 2e H2 Conjugation reaction

2H+ + 2e H2

H2 2H+ + 2e

Zn Zn2+ + 2e

Zn2+ + 2e Zn

re Zn2+/Zn

re H+/H2

lg jcorr

/ V

lg j

corr

Conjugation reaction

Corrosion current

Corrosion / stable /

mixed potential

re Zn2+/Zn

re H+/H2

2H+ + 2e H2

H2 2H+ + 2e

Zn Zn2+ + 2e

Zn2+ + 2e Zn

lg jcorr

/ V

lg j

corr

Positive shift of the metal or increase of the hydrogen evo-lution overpotential can both hinder the corrosion of the metal.

Copper wire Why does copper wire accelerate corrosion of iron nail?

metal a

Fe 0.7

Cu 0.87

value of a in Tafel equation

5. Corrosion protection

1) Application of coatings: (1) metal coating:

electroplating, chemical plating

Zn (anodic protective layer)

Sn (cathodic protective layer)

(2) non-metal coating:

paint (polymer coating); anticorrosion oil; porcelain enamel; plastic; glass (packaging of IC); inherent oxide layer, etc.

Coating forms a barrier layer to inhibit corrosive species from reaching metal surface.

coating

Corrosive species

metal

Both thick inorganic coating and organic coating were applied to protect the cable and steel structure of Brooklyn Bridge, New York, USA.

When Al contacting with the air, a thin inherent layer of aluminum oxide forms on its surface. Being stable in the air, water and even some dilute acidic solution, this thin oxide layer inhibits further corrosion of the metal. With potential of naked aluminum of – 0.6 V, the oxide-coated aluminum becomes more stable even than the common metals, such as iron, zinc, etc.

2) Alloying

Stainless steel:

containing chromium and nickel, both of which form inherent oxide film that change steel’s reduction potential.

To be stainless steel, the chromium content needs to be at

least 10.5%.

The corrosion rate of stainless steel at general corrosion

may be as low as 1 cm for 106 years.

3) Electrochemical protection

Let the potential of iron kept at the stable zone of iron.

Cathodic protection:

with sacrificial anode

with auxiliary anode

Anodic protection:

set the metal at stale zone of Fe3O4. passivation potential

Fe2+

Fe2O3

Fe

pH

/ V

2 4 6 8 10 12 140

Fe3O4

Fe3+

FeO22

Cathodic protection: with sacrificial anode

magnesium / aluminum / zinc alloys

Cathodic protection: with auxiliary anode:

Pipeline

4) Inhibitor

(1) Inorganic inhibitor:

Anodic inhibitor

cathodic inhibitor

C / mol·dm-3

Cor

rosi

on r

ate

mm

/ h

NO2

CrO42 SiO3

2 HPO42

CO32

10-5 10-4 10-3 10-2

0.5

1.5

2.5

react with corrosion product or ions in solution to form inorganic deposition coating.

silicate, phosphate, chromate, nitrite, etc.

Self-assembled monolayer of alkanethiols

(2) Organic inhibitor:

small molecules, usually containing N, S, O, P atoms, can readily adsorb onto metal surface.

metal

1) Cu does not react with dilute sulfuric acid, but why do

es the solution gradually turn blue upon exposure of the

system to the air?

2) Why can Au dissolve in NaCN solution when the air w

as purged.

3) Annihilation can reduce corrosion rate of metal, why?

Discussion: