Corrosion - Chatt

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29 Νοε 2013 (πριν από 3 χρόνια και 9 μήνες)

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Corrosion (Rusting)

The annual cost of corrosion to the U.S. economy is $350 billion!


The Corrosion Process

Cell Reaction
:
2 Fe
(s)

+ O
2

(g)

+ 2H
2
O
(l)



㉆攠
2+

(aq)

+ 4OH


(aq)

How Can Adjacent Pieces of Iron Be Chemically Different?

1. Defects (due to compression or chipping)

2. Impurities (e.g.
-

Sn)
*

3. Differences in [O2]

*

Sn is less active than Fe and will therefore act as the cathode
.

How Can Rusting Be Prevented?


1. Protective coatings

paint


*

Sn will promote rusting if the coating is damaged.

Fe galvanized with Zn or Sn
*

2. Alloying Fe with Cr to form
stainless steel
.

3. Cathodic protection

a) Attaching a more active metal (e.g.
-

Mg) to Fe.

sacrificial anodes

Anode half
-
reaction:

Mg


Mg
2+

+ 2e
-

Cathode half
-
reaction:

O
2

+ H
2
O + 4 e

-



4 OH
-


b) Connecting the Fe object to the


terminal of a power source.

4. Electroplating