Transcript INHIBITOR - Universiti Sains Malaysia

INHIBITOR
IS A CHEMICAL SUBSTANCE THAT,
WHEN ADDED IN A SMALL
CONCENTRATION TO AN ENVIRONMENT,
EFFECTIVELY DECREASES THE
CORROSION RATE
In the oil extraction and processing
industries inhibitors have always been
considered to be the first line of
defense against corrosion
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The efficiency of inhibitor can be
expressed by a measure of this
improvement :
Inhibitor efficiency (%) =
[CRuninhibited – CRinhibited]
x 100%
CRunhibited
or
[Rp,inhibited – Rp, uninhibited]
x 100%
Rp,inhibited
CR : corrosion rate ; Rp = polarization
resistance
Inhibitor efficiency of TransCinnamaldehyde (TCA)
TCA
ppm
0
250
Rp
Corrosion Corrosion Efficiency,
Ω. cm2 current ,
Rate,
mA.cm-2
%
mm.y-1
14
1.55
18.0
0
35
0.62
7.2
60
1000
143
0.152
1.76
90
5000
223
0.097
1.13
94
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Inhibitors are chemicals that interact with
a metallic surface, or the environment this
surface is exposed, giving the surface a
certain level of protection.
Inhibitors often work by adsorbing
themselves on the metallic surface by
forming a film
Inhibitors slow corrosion process by:
 - Increasing the anodic or cathodic
polarization behavior (Tafel slopes)
 - Reducing the movement or diffusion of
ions to the metallic surface
 - Increasing the electrical resistance to
the metallic surface
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CLASSIFICATION OF INHIBITOR
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Passivating (anodic) inhibitors
Cathodic inhibitors
Organic inhibitors
Precipitation inhibitors
Volatile corrosion inhibitors
Passivating (anodic) inhibitors
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Passivating inhibitors cause a large anodic
shift of the corrosion potential, forcing the
metallic surface into the passivation
range.
There are two types of passivating
inhibitors:
- oxidizing anions, such as chromate,
nitrite and nitrate that can passivate steel
in the absence of oxygen.
- nonoxidizing ions, such as phosphate,
tungstate and molybdate that require the
presence of oxygen to passivate the steel
The reduction
potential of
chromate ion
to solid Cr2O3
is possible to
increase the
corrosion
potential of
steel into its
passivation
region but not
for molybdate
and tungstate.
INHIBITING MECHANISM OF NITRITES
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The formation of ferric oxides with the
participation of nitrite ions takes place
according to the following reactions
(Joseph et al):
Formation of a lower oxide:
NO2-+8H++6e = NH4++2H2O
9 Fe(OH)2 = 3Fe3O4+6H2O+6H++6e
2H2O = 2H++ 2OH9Fe(OH)2+NO2- = 3Fe3O4+NH4++2OH-+6H2O
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Formation of a higher oxide:
NO2-+8H++6e = NH4++2H2O
6Fe(OH)2 = 2Fe3O4+4H2O+4H++4e2Fe3O4+H2O=3(γ-Fe2O3)+2H++2e2H2O = 2H++ 2OH6Fe(OH)2+NO2-=3(γFe2O3)+NH4++3H2O+2OHSodium nitrite is more effective in suppressing the
aggressive properties of chlorides than are benzoate
and chromate. In the presence of sulfate, nitrate is
slightly less effective than are chromate and
benzoate.
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Protective properties of sodium nitrite as
function of sodium chloride concentration
CNaCl
CNaNO2 CNaNO2
%mass %mass CNaCl
T, hr
0.5
0.20
0.4
20.5
0.5
0.35
0.7
49.5
0.5
0.5
0.45
1.00
0.9
2.0
Pot.
mV
-11
State of
metal
surface
corrodes
+271 protected
43.0 +285 protected
122.0 +300 protected
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Chromate-base inhibitors are the leastexpensive inhibitors and were used until
recently in a variety of application (e.g.
recirculation cooling systems of internal
combustion engines, refrigeration units
and cooling towers). Sodium chromate,
typically in concentrations of 0.04 to
0.1% was used for this applications. At
higher temperatures or in fresh water with
chloride concentration above 10 ppm,
higher concentration are required. If
necessary, sodium hydroxide is added to
adjust the pH to a range of 7.5 – 9.5. If
the concentration of chromate falls below
a concentration of 0.016% corrosion will
be accelerated.
Cathodic Inhibitors
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Cathodic inhibitors either slow the
cathodic reaction itself or selectively
precipitate on cathodic areas to increase
the surface impedance and limit the
diffusion of reducible species to these
area.
Cathodic inhibitors can provide inhibition
by three different mechanisms: 1. as
cathodic poisons; 2. as cathodic
precipitates, and 3. as oxygen scavenger.
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Some cathodic inhibitors, such as
compounds of arsenic and antimony, work
by making recombination of hydrogen more
difficult. These substances are very
effective in acid solutions but are ineffective
in environments where other reduction
processes such as oxygen reduction are the
controlling cathodic reactions.
Other cathodic inhibitors, ions such as
calcium, zinc, or magnesium, may be
precipitated as oxides to form a protective
layer on the metal.
Oxygen scavengers help to inhibit corrosion
by preventing cathodic polarization caused
by oxygen. Examples of this type of
inhibitors are sodium sulfite and hydrazine.
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They remove dissolved oxygen from
aqueous solutions;
2 Na2SO3 + O2(dissolved ox.) = 2Na2SO4
N2H4 + O2 = N2 + 2H2O
These inhibitors will work effectively
in solutions where oxygen reduction
is controlling the cathodic process
but will not effective in acid solution.
Organic Inhibitors
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Both anodic and cathodic effects are sometimes
observed in the presence of organic inhibitors, but
as general rule, organic inhibitors effect the entire
surface of corroding metal present in sufficient
concentration.
Organic inhibitors, usually designated as film
forming, protect the metal by forming hydrophobic
film on the metal surface. Their effectiveness
depends on the chemical composition, their
molecular structures, and their affinities for the
metal surface. Because film formation is an
adsorption process, the temperature and pressure
in the system is the important factors.
Organic inhibitors will adsorbed according to the
ionic charge of inhibitors and the charge of the
surface.
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Cationic inhibitors, such as amines, or anionic
inhibitors such as sulfonates, will be adsorbed
preferentially depending on whether the metal is
charge negatively or positively.The strength of
adsorption bond is the dominant factor for
soluble organic inhibitors.
These materials build up a protective film of
adsorbed molecules on the metal surface, which
provides a barrier to the dissolution of the metal
in the electrolyte. Because the metal surface
covered is proportional to the inhibitors
concentrate, the concentration of inhibitor in the
medium is critical. For any specific inhibitor in
any given medium there is an optimal
concentration.
Precipitation Inhibitors
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Precipitation-inducing inhibitors are film forming
compounds that have general action over the
metal surface, blocking both anodic and cathodic
sites indirectly. Precipitation inhibitors are
compound that cause the formation of
precipitates on the surface of the metal, thereby
providing protective layer. Hard water that is
high in calcium and magnesium is less corrosive
than soft water because of the tendency of the
salts in the hard water to precipitate on the
surface of the metal and form a protective film.
The most common inhibitors in this category are
the silicates and the phosphates, i.e. sodium
silicate is used in many domestic softeners to
prevent the occurrence of rust water. In aerated
hot water systems, sodium silicates protect steel,
copper and brass.
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However the protection is not always reliable and
depends heavily on pH and a saturation index
that is influenced by water composition and
temperature. Phosphates also require oxygen for
effective inhibition. Silicates and phosphates do
not afford the degree of protection provided by
chromates and nitrites; however they are very
useful in situations where nontoxic additive are
required.
Volatile Corrosion Inhibitors
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Volatile corrosion inhibitors (VCIs), also vapor
phase inhibitors (VPIs), are compounds
transported in a closed environment to the site of
corrosion by volatilization from a source. In
boilers, volatile basic compounds, such as
morpholine or hydrazine , are transported with
steam to prevent corrosion in condencer tubes by
neutralizing acidic carbon dioxide or by sifting
surface pH toward less acidic and corrosive values.
In closed vapor spaces, such as shipping
containers, volatile solids such as salts of
dicyclohexylamine, cyclohexylamine, and
hexamethylene amine are used. On contact with
the metal surface, the vapor of this salt condenses
and hydrolyzed by any moisture to liberate
protective ions.
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It is desirable, for an efficient VCI, to provide
inhibition rapidly and to last for long periods.
Both qualities depend on the volatility of these
compounds, fast action wanting high volatility,
whereas enduring protection requires low
volatility.
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The majority of inhibitor applications for aqueous,
or partly aqueous, systems are concerned with
four main types of environment:
Aqueous solution of acids as used in metalcleaning processes such as pickling for removal of
rust or mill scale during the production and
fabrication of metals or in the postservice
cleaning of metal surfaces.
Natural waters, supply waters, and industrial
cooling towers in near-neutral pH range (5 to 9)
Primary and secondary productions of oil and
subsequent refining and transport process.
Atmospheric or gaseous corrosion in confined
environments, during transport, storage, or any
other confined operation.