4.3
Corrosion and Corrosion Prevention
Passivity
In passivity or passivation, a metal or an alloy exhibits a much higher corrosion resistance
than its position in the electrochemical series would indicate. True passivity usually results
from exposure of metal to oxygen of the air or to oxidizing solutions. Two theories attempt
to explain the phenomenon of passivity:(i) It is assumed the passivity is the result of formation of a highly protective but very thin
and invisible film on the surface of a metal or alloy, making its solution potential more
noble.
(ii) 2nd theory assumes that a metal becomes passive because of chemically absorbed layer of
O2 on the surface. The layer is considered to be one molecule thick.
Pourbaix Diagrams
From what is said in the previous section, the rate of electrochemical corrosion seems to
depend on a lot of factors: the concentration and nature of the solution, the nature of the
corrosion products, the solution pH, the potential of the two electrodes, the quantity and
direction of the applied external potential difference (if any), etc.
Increasingly anodic
0. 8
IV
II
Passivity
0. 4
Corrosion
Fe++
Anodic
Protection
Increasingly cathodic
0
Protection by
Alkalisation
III
Fe++
Increasing
above 10-6
-0.4
1
I
-0.8
V
VII
Fe++
Decreasing
below 10-6
VI
Cathodic
Protection
-1.2
Corrosion
(FeO2H)
Immunity
-2
0
2
4
6
Increasingly acidic
8
10
PH of Soln
12
14
16
Increasingly alkaline
Fig 4.6 Simplified Pourbaix diagram for Fe-H2O System at 25oC showing domains of
corrosion behaviour
This information can be summarized in an equilibrium diagram of electrochemical reaction
for any metal/solution system. Such a diagram is termed a Pourbaix diagram. A simplified
Pourbaix diagram for the iron water system is shown in fig. 4.6
Under the conditions marked "corrosion", corrosion takes place with the corrosion products
being as shown in the figure. In the region marked immunity, no corrosion takes place due to
low concentration of the Fe2+ ions. In the passivity regions, iron is rendered passive as an
electrode due to the formation of an adherent layer of Fe2O3.
Description of boundary lines:
(i) Fe
(ii) Fe++
Fe++ + 2e- - Iron atoms form soluble iron II (Fe++) ions.
Fe+++ + e-
(iii) Fe++ + 3OH(iv) Fe+++ + 3H2O
- Iron ions are oxidized to form Iron III ions.
Fe(OH)3 + e- - Insoluble Iron III hydroxide precipitates.
Fe(OH)3 + 3H+ - Insoluble Iron III hydroxide precipitates.
(v) Fe + 3H2O
Fe(OH)3 + 3H+ + 3e- - Insoluble Iron III hydroxide precipitates.
(vi) Fe + 2H2O
FeO2H- + 3H + 2e- - Ferrite ions go into solution.
(vii) FeO2H- + H2O
hydroxide.
Fe(OH)3 + e- - Soluble ferrite ions precipitate as Iron III
In the regions of the diagram where solid compounds are formed, it is possible that the metal
may be protected from attack by a coating (Passivation)
The area marked “Immunity” shows at that potential and pH, metallic iron is the stable state
so that corrosion will not occur.
The area marked corrosion is where the ions Fe++ and Fe+++ are stable, while the small area at
the extremely alkaline PH is corrosion due to formation of the ion FeO2-.
Corrosion Prevention
The point marked I in fig. 4.6 represents the conditions when iron (electromotive force = 0.44V) is placed in pure water (pH = 7). The point is in the "corrosion" region and hence
under normal conditions, corrosion will take place. The 3 arrows show the 3 ways in which
we can move from this point to the non-corrosion regions:
(i) Cathodic protection
Connecting a metal that is anodic with respect to iron e.g., zinc, to iron in the same solution.
This causes the net potential to be lowered into the "immunity" region (shown by arrow 1).
This method is termed cathodic protection. The more anodic metal is corroded in place of
iron i.e., it is used sacrificially. The same effect may be achieved by applying an external DC
voltage to move the conditions into the immunity region.
Cathodic protection is applied in several industrial settings. Boiler tubes, for example, may
be protected against corrosion by attaching plates of zinc inside the boiler. Buried pipelines
may be protected by applying an external DC voltage to the line.
(ii) Applying a potential of opposite sign to move into passivity
the external potential may also be applied to move the conditions into the "passivity" region.
This forces the formation of the protective layer which greatly slows down the rate of
corrosion. This method of corrosion prevention (shown in fig. 14.3 by arrow 2) is termed
anodic protection and again may be used for buried pipelines.
(iii) Increase alkalinity e.g inhibitors
increasing the alkalinity (pH) of the solution moves the conditions in the direction shown by
arrow 3. This is termed corrosion control by control of pH. The treated water fed into boilers
has NaOH added to it to bring the pH to the range 11-12 to prevent corrosion of boiler tubes.
(iv) Protection by design
Proper selection of material for any particular corrosive environment and a sound engineering
design are the best means of controlling and preventing corrosion:•
•
•
•
Use of dissimilar metal contacts should be avoided and if it is inevitable that they be
used, then they should be as close as possible to each other in the galvanic series.
If possible, dissimilar metals should be insulated
Proper design should avoid the presence of crevices between adjacent parts of a
structure, even in the case of the same metal
Whenever possible, the equipment should be annealed to reduce residual stresses to the
lowest practical level..
Other methods that may be used to prevent corrosion include:i)
ii)
The use of inhibitors e.g., chromates and phosphates for iron. These stifle the
corrosion reactions occurring at the anode by forming a sparingly soluble compound
with the newly produced metal ion.
use of protective coatings to ensure that the metal and environment do not come into
contact. The protective coatings may be paints, vanishes, lacquers, metallic coatings,
oil, grease, etc.
When metal ions go into solution at the anode in electrochemical corrosion, the compounds
formed may either stick to the electrode or form a precipitate. In the cases where a
precipitate is formed, the corrosion products do not protect the electrode from further
corrosion. The rate of corrosion is then controlled by how fast the cathode can absorb
electrons. The reaction is said to be under cathodic control. When the reaction produces
adherent products, some protection is provided to the electrode and the rate of reaction
(corrosion) is controlled by how fast diffusion through the protective layer can take place.
Thus it is the events happening at the anode, which control the rate of reaction and the
corrosion is said to be under anodic control. If the adherent corrosion products prevent
further corrosion altogether, the electrode is said to have been rendered passive or immune.