Introduction What is Corrosion? Corrosion is the destructive attack of a metal by its reaction with the environment Note that the word “corrosion” refers to the degradation of a metal by its environment.

What is rusting ? What is white rust ? According to ‘ASTM' corrosion glossary, corrosion is defined as "the chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties”. 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. What is rusting ? What is white rust ?

Physical Processes of Degradation Physical degradation processes include the following: Fracture Fatigue Wear Erosion or cavitation erosion Radiation damage

Red rust Waterline

Environmentally Assisted Degradation Processes Table 1.1 Physical degradation processes and their environmentally assisted counterparts Environmentally assisted process Example Fracture Stress-corrosion cracking Stress-corrosion cracking of bridge cables, of landing gear on aircraft Fatigue Corrosion fatigue Vibrating structures, such as aircraft wings, bridges, offshore platforms Wear Fretting corrosion Ball bearings in chloride-contaminated oil

Cavitation erosion Cavitation corrosion Ship propellers, pumps, turbine blades, fast fluid flow in pipes Radiation damage Radiation corrosion Increased susceptibility of stainless steels to dissolution or to stress-corrosion cracking [1, 2]

Half-Cell Reactions 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 : Rx oksidasi Rx reduksi Media elektrolit Jalan untuk perpindahan

A Note About Atmospheric Corrosion The 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 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