Introduction

Corrosion is the destructive attack of a metal by its reaction with the environment

What is Corrosion?

Note that the word “corrosion” refers to the degradation of a metal by its environment.

According to ‘ASTM' corrosion glossary, corrosion is defined as "the chemical or electrochemicalreaction between a material, usually a metal, andits environment that produces a deterioration of the material and its properties”.

What is rusting ?What is white rust ?

Other definitions include Fontana's description that corrosion is the extractive metallurgy in reverse,2 which is expected since metals thermodynamically are less stable in their elemental forms than in their compound forms as ores.

Physical Processes of Degradation

Physical degradation processes include the following:

1. Fracture2. Fatigue3. Wear4. Erosion or cavitation erosion5. Radiation damage

Red rust

Waterline

Environmentally assistedprocess

Example

Fracture Stress-corrosion cracking Stress-corrosion cracking of bridgecables, of landing gear on aircraft

Fatigue Corrosion fatigue Vibrating structures, such as aircraftwings, bridges, offshore platforms

Wear Fretting corrosion Ball bearings in chloride-contaminated oil

Table 1.1 Physical degradation processes and their environmentally assisted counterparts

Environmentally Assisted Degradation Processes

Cavitation erosion

Cavitation corrosion Ship propellers, pumps, turbine blades,fast fluid flow in pipes

Radiation damage

Radiation corrosion Increased susceptibility of stainless steelsto dissolution or to stress-corrosioncracking [1, 2]

Electrochemical Reactions

Corrosion is an electrochemical process. That is, corrosion usually occurs not by direct chemical reaction of a metal with its environment but rather through the operation of coupled electrochemical half-cell reactions.

Half-Cell Reactions

Anodic Reactions

The loss of metal occurs as an anodic reaction. Examples are

Fe(s) → Fe2+(aq) + 2e− (1)Al(s) → Al3+(aq) + 3e− (2)2Cu(s) + H2O(1) → Cu2O(s) + 2H+(aq) + 2e− (3)

The following reaction is also an anodic reaction:

Cathodic Reactions

In a cathodic reaction?

2H+(aq) + 2e− → H2(g)

Another common cathodic reaction is the reduction of dissolved oxygen to hydroxyl ions, a reduction reaction which occurs in neutral or basic solutions.

O2(g) + 2H2O + 4e− → 4OH−(aq)

Coupled Electrochemical Reactions

At anodic sites: Fe Fe+2 + 2e–

At cathodic sites: 2 H+ + 2e– H2

Overall reaction: Fe + 2 H+ Fe+2 + H2

The reason that two different electrochemical half-cell reactions can occur on the same metal surface lies in the heterogeneous nature of a metal surface.

The heterogeneous nature of a metal surface showing various types of imperfections

Kesimpulan :

1.Rx oksidasi2.Rx reduksi3.Media elektrolit4.Jalan untuk perpindahan

A Note About Atmospheric CorrosionThe need for the presence of an electrolyte as a condition for corrosion to occur is illustrated by the phenomenon of atmospheric corrosion, i.e., the corrosion of metals in the natural outdoor atmosphere.

Fig. 2.7 The continuation of reactions initiated in Fig. 2.6. (a) The precipitation of ferrous hydroxide on the iron surface. (b) The conversion of ferrous hydroxide to a hydrated ferric oxide

The Faraday and Faraday’s Law

Faraday’s law states that the mass (w) of metal corroded is given by

19 19 23

2

1 1,6 10 1 1,6 10 6,0225 10 96450=96500 C/mol

Fe Fe 2

massa (g) Fe yang teroksidasi bila teramati arus sebesar 1 A selama 1 jam?

C=1 C/s 60 mnt x 60 s/mnt =

massamol

ArC

i C i tt

e C mol e C

e

Berapa

3600 C 3600 C 0,037 mol e

96500 C/mol

1Banyak besi yang teroksidasi : 0,037 mol = 0,018 mol

2

W=

e

i t eitAr unsur w

nF FAr

en