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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