§7.13 Corrosion and protection of metals

1) Corrosion:

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

White marble of Jinshui Bridge, Beijing

Stone Sculpture before the Capitol, Washington D.C.

1. General introduction

2. Classification of corrosion

1) Based on materials:Corrosion of metals;

Corrosion of non-metals

2) Based on Media:

natural corrosion; industrial corrosion.

air, water, earth

3) Based on mechanism:

chemical corrosion;

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 due to the ununiformity

of metal and / 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:

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

4 Theoretical consideration

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

/ V

lg j

Conjugation reaction

Corrosion current

Corrosion / stable /

mixed potential

lg jcorr

corr

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.

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.

2) Alloying

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

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.

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

does the solution gradually turn blue upon exposure of

the system to the air?

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

was purged.

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

Discussion: