Scope of Inhibitor
A corrosion inhibitor is a chemical substance which, when added
in small concentrations to an environment, minimizes or prevents
Corrosion inhibitors are used to protect metals from corrosion,
including temporary protection during storage or transport as well
as localized protection, required, for example, to prevent corrosion
that may result from accumulation of small amounts of an
aggressive phase. One example is brine, in a nonaggressive phase,
such as oil. An efficient inhibitor is compatible with the
environment, is economical for application, and produces the
desired effect when present in small concentrations.
Mechanisms of actions of inhibitors
Inhibitors are substances or mixtures that in low concentration
and in aggressive environment inhibit, prevent or minimize the
Generally the mechanism of the inhibitor is one or more of three
that are cited below:
•The inhibitor is chemically adsorbed on the surface of the metal
and forms a protective thin film with inhibitor effect or by
combination between inhibitor ions and metallic surface.
• The inhibitor leads a formation of a film by oxide protection of the
• The inhibitor reacts with a potential corrosive component present
in aqueous media and the product is a complex
Scope of Inhibitor
Corrosion control by use of inhibitors is extremely useful
in many environments, however,there are certain exceptions
, such as:
(a) equipment and components subjected to turbulent ﬂow.
(b) systems operating above the stability limits of inhibitor.
(c) equipment subjected to high velocity, beyond 4 m/s.
Examples of Application of Inhibitors
•Petroleum Industry. Corrosion phenomena in the petroleum
industry occur in a two-phase medium of water and hydrocarbon.
It is the presence of a thin layer of water which leads to corrosion,
and rigorous elimination of water reduces the corrosion rate to a
negligible value. The inhibitors used in petroleum industry, both in
production and refining are either oil soluble–water insoluble types
or oil soluble–water dispersible compounds.
•Packaging Industry. For transportation of machinery,components
and equipment by sea, vapor phase cyclohexylamine and hexa-
methylamine are used.
•Sour Gas Systems. A major problem is encountered in steel
pipelines in various sour gas environments. Chemical inhibition is
one of the effective methods used to mitigate sulfide induced
corrosion. Inhibitors containing alkyl ammonium ions are found to
suppress corrosion effectively.
•Potable Water Systems. Corrosion is experienced in potable water
transportation pipes of steels and cast iron. Inhibitors, such as
Ca(HCO3)2 and polyphosphates are commonly used to combat
•Engine Coolants. Inhibitors, such as NaCrO4 (sodium chromate),
borates and nitrites (NaNO2) and mercaptabenzothia-zole are
widely used for protection of auto-mobile engines. Chromates are a
Classification of Inhibitors
Inhibitors may be classiﬁed as shown in Fig( 1.1).There are
two major classes: inorganic an organic. The anodic type of
inorganic inhibitor includes chromates, nitrites, molybdates
and phosphates, and the cathodic type includes zinc and
polyphosphate inhibitors. The ﬁlm forming class is the major
class of organic inhibitor as it includes amines, amine salt
and imidazoilnes – sodium benzoate mercaptans, esters,
amines and ammonia derivatives.
The addition of inorganic inhibitors causes suppression of
electrochemical reaction at anodic– cathodic areas. Most of the
times, inhibitors are used in a blended form. These inhibitors only
react at an adequate level of concentration.
(A) Chromate Inhibitors
They are most effective inhibitors, but they are toxic and, hence,
their application is restricted and is not advised. In industrial water,
the threshold concentration is 120 mg/L. Chromate inhibitors
contain either Na2CrO4 or Na2Cr2O7.
Fe → Fe+2 + 2e- (Oxidation of iron)
CrO-4 +8H+3e → Cr+3 + 4H2O (Formation of Cr+3 )
Fig (1.2) : Formation of a mixed
iron oxide and chromium oxide ﬁlm
They are effective inhibitors for iron and a number of metals in
a wide variety of waters. Like chromates, nitrites are anodic
inhibitors and they inhibit the system by forming a passive ﬁlm
with ferric oxide. These are environmentally-friendly inhibitors.
Besides steel, nitrites also inhibit the corrosion of copper.
Nitrites should not be used in open systems as they would
oxidize to nitrates in the presence of oxygen.
Nitrites are not effective if presence of chloride and sulfate .
(C) Phosphate Inhibitors
Phosphate retards corrosion by promoting the growth of
protective iron oxide ﬁlms and by healing the defects in
Molybdenum is an alloying element which is known to
increase passivation of stainless steels of type 316.
Sodium molybdates forms a complex passivation ﬁlm at the
The passive ﬁlm can only be formed in the presence of oxygen.
Silicates are strong anodic inhibitors and passive ﬁlms can
be formed even on the corroded surface. The monomeric silica
does not provide any protection. In waters below pH levels of
6.0 , the silicate used is Na2O.2SiO3 and with a pH greater than
6.0, it is Na2O3.3SiO3 .
Silicate treatment also prevents dezinciﬁcation in brass and
corrosion of copper.
Organic inhibitors are used in the oil industry to control oil and
gas well corrosion. Most common types are long chain (C18) . The
most common types of organic inhibitors are shown below:
Primary amine, RNH2 (4) Polyethoxylated compounds
Secondary amine, R2NH (a) Amines
Tertiary amine, R-N(CH3)2
(2) Diamines (x and y vary between 2 and 50)
R – NHCH2CH2CH2NH2
(3) Amides (b) Diamines
R – CONH2
(x + y+ z varies between 3–10)
Organic inhibitors react by adsorption on a metallic surface.
Cationic inhibitors (+), like amines, or anionic inhibitors (−),
like sulfonates, are preferentially adsorbed depending on the
charge of the metal surface (+)or(−). At zero point of charge,
there is no particular preference for an anodic or cathodic
Oxygen, even in very small amounts, may cause serious corrosion
in feedwater lines, stage heaters, economizers, boiler metal, steam
operated equipment and condensable piping. It must, there-fore, be
removed from the closed system. The solubility of oxygen varies with
both pressure and temperature. Oxygen is the main cause of
corrosion. It reacts by consuming electrons at the cathode causing
cathodic depolarization and enhancing the rate of corrosion.
Chemicals which eliminate oxygen from the closed systems are
called scavengers. Ammonium sulﬁte (NH4)2SO3, and hydrazine
(N2H4) have been successfully used over the years to eliminate
oxygen. Oxygen scavengers remove oxygen as shown below:
)NH4)2SO3 + 1/2 O2→ (NH4)2SO4
1. Org . molecule(aq) + nH2O(ads →( Org . molecule(ads) + nH2O(soln(
2. N2H4+ O2 → N2 ↑+2H2O
3. Na2SO3 + 1/2 O2 → Na2SO4
4. NH4HSO3 + 1/2 O2 → NH4HSO4 (Ammonium hydrogen sulfate)
• In reaction in equation 1 best at temperature (10 ċ)
• In reaction in equation 2 increase the total dissolved
solid content .
• In reaction in equation 3 the rate of rection is slow at
temperature below (15ċ)
4Fe3O4 + O2 → 6Fe2O3
6Fe2O3 + N2H4 → 4Fe3O4 + 2H2O + N2 ↑
Table (1.2) : Advantages and disadvantages of sodium sulﬁte and hydrazine
■ Does not reduce ferric oxide to
■May decompose to form
■ Reacts less rapidly compared to
■ Rapid reaction
■ Contributes no solids
■Reduces ferric oxide to
■ More expensive than sodium
■ Toxic and ﬂammable
■ Less dosage for scavenging
compared to sodium sulﬁte
Inhibitor Application Techniques
Basically, there are three well-known inhibitor application
(1) Continuous injection
(2) Batch treatment
(3) Squeeze treatment
1 . Continuous Injection
As the name suggests, inhibitors are injected in the system to
achieve inhibition objectives through the system. Normally the
inhibitor is injected into the system by means of an electric or gas
driven chemical pump. The inhibitor is added at the point of
turbulence to achieve uniform mixing. This method is used for
municipal water supplies, cooling towers and oil wells, to minimize
scaling and corrosion problems. In the continuous injection
method, a constant supply of chemicals is maintained at a
2. Batch Treatment
This is a periodic treatment in which a large quantity of
chemicals is used for an extended period of time. It is
commonly used to treat ﬂowing oil wells. Batch treatment is
also called slug treatment. For batch treating, the tube
displacement method is employed. Several barrels of inhibitor
are pumped into the tubing at the top. The inhibitor is
displaced to the bottom of the tubing with the ﬂuids in the oil
well. The well is closed for a speciﬁc period before operation.
The batch is used mainly to treat water with biocides.
3 Squeeze Treatment
Continuous treatment of oil wells by inhibitors is achieved
by this method. The liquid inhibitor is pumped down through
the tubing into the oil producing geological formation under
low pressure which acts as a chemical reserve. In oil wells, 1–2
drums of inhibitor is mixed with 10–20 bbl of water (1bbl
(British barrel) = 36 gallons), and is pumped into the well
followed by pumping in over-ﬂush ﬂuid (50–75 bbl). The
inhibitor is absorbed by the formation.
Inhibitor Efficienc y and Inhibitor Concentration
(1) The efﬁciency of corrosion inhibition can be expressed as
Einh = (CR o – CRI) / CR O
Einh = efﬁciency of a corrosion inhibitor
CRo = corrosion rate with zero inhibitor
CRI = corrosion rate in the presence of an inhibitor
(2) The quantity of inhibitor required for a ﬂuid to be inhibited
can be obtained by the relationship:
Qinh =( Vfluid / 1*106) *Cinh (ppm)
Qinh = quantity of inhibitor, kg
Vfluid = volume of ﬂuid to be inhibited , liters
Cinh = inhibitor concentration, ppm
Calculate the dosage of sodium chromate required to be added
to 500 000 liters of
water, if the concentration of sodium chromate is 5 ppm.
QNn2CrO4= ( VNa2CrO4/ 1*106) *CNa2CrO4 (ppm) 06 ) *5 (ppm)
The Table (1.3) in the next page state some corrosive systems
and the inhibitors used to protect them.