UNIVERSITÀ DEL SALENTO
FACOLTÀ DI INGEGNERIA
CORSO DI LAUREA MAGISTRALE IN
MATERIALS ENGINEERING AND NANOTECHNOLOGY
Electrochemical Technologies
Preventing corrosion and protection from corrosion
16
Preventing corrosion and protection from
corrosion methods
Preventing corrosion
To stop corrosion or keep it below a given limit, it is possible to intervene on the four partial
processes which form part of this electrochemical mechanism.
Anodic reaction
• Eliminating the driving force by selecting a material which is more noble than the cathodic
process (Ea>Ec). Noble materials can also be used in the form of a deposit or coating (such as
plating, for instance).
• Eliminating the driving force by bringing the potential of the material into the immunity
zone (E<Eeq) with cathodic protection.
• Slowing down the anodic reaction by means of passivation phenomena which bring the
material into conditions of passivity.
• Slowing down the anodic reaction by bringing the material into conditions of passivity with
the application of anodic protection.
• Slowing down the anodic reaction with passivating inhibitors added to the environment,
which encourage the formation of passivation films.
1
Cathodic reaction
• Eliminating the driving force by removing oxygen and maintaining a pH equal to or higher
than neutrality (in the absence of H2S).
• Increasing the overvoltages of the cathodic process in the presence of oxygen. The physical or
chemical removal of oxygen (with degassification or an oxygen scavenger,
respectively) is always preferable.
• Increasing the overvoltages of the hydrogen release process in acid solutions using filming
inhibitors.
• Decreasing the overvoltages of the cathodic process of hydrogen release in acid environments
by cathodic alloying in the case of Ti, Cr and Ta alloys. In these specific instances, increasing
the velocity of the cathodic process brings the material into conditions of passivity.
Environment
Corrosion can be slowed, but not stopped completely, by increasing ohmic resistance, for
example by using insulating coatings (thick paints and plastic coatings).
The metallic material
It is usually impossible to intervene directly on the metal to block or slow down its progression
towards corrosion except in the specific instance of corrosion by electrical interference, as in
the case of pipelines running alongside the tracks of electrical transportation systems such as
train and underground lines. introduction of insulating joints on the pipeline
Preventing corrosion and protection from
corrosion methods
- alloying - electrodeposition
intrinsic - displacement
- thermal treatments - chemical reduction
of metallic - plating
material - spray
metallic
coating not metallic - chemical or electrochemical reaction (oxidation,
By controlling the phosphating, chromatization)
physical-chemical - paints, glass and ceramic coatings
electrolytic
properties
conductor - chemical & physical (oxygen, pH, anions,
of water hardness, addition of inhibitors, T)
other metallic - homogenization
environment
materials
- elimination of intermetallic contacts
insulation - elimination of dead spaces
- elimination of shielding dead spaces
- elimination of rubbing
cathodic with external sources of e.m.f.
Electrical with less noble soluble anodes
protection
with external sources of e.m.f.
anodic
cathodic coupling
2
Preventing corrosion and protection from
corrosion methods
Corrosion rate depends on:
A) "available" work (driving force)
B) reaction resistance
The methods of prevention or protection act
on the factors determining A and B so that
A or B
Hydraulic analogy summarizing the
corrosion process.
Hydraulic circuit with:
- a pump P
- 4 taps: A (anode), C (cathode), E (electrolyte), M (metal).
Hydraulic circuit: water circulation stops if:
- pump head = 0;
- at least one of the four taps is closed.
Corrosion system: corrosion rate = 0 if:
- "available" work = 0
- the anodic or cathodic process or that the ohmic drop in the electrolyte occur with a
dissipation such that current flow is negligible
(dissipation in the metal are negligible the tap M can not be closed)
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Preventing corrosion and protection from
corrosion methods
Control of corrosion processes
blocking water circulation
It is better to well close one tap, rather than
poorly close three.
It is better to close an already partially closed tap,
rather than fully open ones.
corrosion control should focus
on only one of the possible options:
- cancel the driving force - cathodic overvoltage
- anodic overvoltage - resistivity of the environment
and possibly on that which already controls the corrosion process.
In the case of corrosion, the situation is more complicated:
In the hydraulic circuit: head of the pump or closing a valve
opening (or closing) of the other taps.
In the case of corrosion: driving force, or intervention on a valve
(often) closing or (more rarely) opening of the other.
(Sometimes driving force closing taps stop corrosion
Corrosion = 0 eliminating the contact between metal and electrolyte solution
Hydraulic analogy: remove all water from the circuit there is no circulation
3
Corrosion inhibitors
Inhibitors
Inhibitors are substances which, when added in small quantities to aggressive environments,
can slow down or stop corrosion processes.
These substances are extremely numerous and of very different natures.
They work by modifying the surface layer of the metallic material to be protected following
adsorption or reaction processes which lead to the separation of products on the surface
to be protected.
The consequent increase in reaction resistance results:
- from the inhibition of the cathodic process (e.g. the overvoltage of hydrogen or preventing
oxygen from reaching the metal surface) and/or the anodic process
- from the establishment of passive conditions.
Classifying inhibitors
chemical nature (organic, inorganic inhibitors, etc.)
use (inhibitors for boiler feed waters, for pickling, desincrustation, packing)
conditions of use (inhibitors in a solution or in the vapour phase)
mechanism with which they work (cathodic, anodic and mixed inhibitors).
safe and unsafe. The correct functioning of these inhibitors can only be guaranteed if they
are present in quantities above a threshold value; they may even accelerate corrosion
when their concentration is insufficient.
Corrosion inhibitors
Anodic inhibitor η anodic icorr Cathodic inhibitor η cathodic icorr
Anodic inhibitor
Cathodic inhibitor
Mixed inhibitor η cathodic and η cathodic icorr
"unsafe"
Mixed inhibitor
Inhibitor
Inhibitor
