inhibitor
TRANSCRIPT
Inhibitor IntroductionTypes and compositionApplication
IntroductionAn inhibitor is a substance that when added in
small concentrations to an environment, decreases the corrosion rate
Corrosion inhibitors are additives to the fluids that surround the metal or related object.
The effectiveness of a corrosion inhibitor depends on fluid composition, quantity of water, and flow regime.
A common mechanism for inhibiting corrosion involves formation of a coating, often a passivation layer, which prevents access of the corrosive substance to the metal.
Introduction An inhibitor can be considered as a retarding
catalystThere are numerous inhibitor types and
compositionsMost inhibitors have been developed by
empirical experimentation Inhibition is not completely understood but it
is possible to classify inhibitors according to their mechanism and composition
Introduction 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
Inhibitors: types and compositionAdsorption-type inhibitorsHydrogen-evolution poisonsScavengersOxidizersVapor-phase inhibitors
Adsorption-type inhibitorsUsually designated as film forming protect
the metal by forming a hydrophobic film on the metal surface.
The effectiveness of this inhibitor depends on the chemical composition, their molecule structure and their affinities for the metal surface
These are organic compounds which adsorb on the metal surface and suppress metal dissolution and reduction reactions
Adsorption-type inhibitorsEx: organic amines (pyridine,
diethylamine, methylamine, etc.)
Chemisorption of an organic compound is facilitated by the presence of polar groups in the molecular structure, which can readily attach to the metal surface
Hydrogen-evolution poisonsRetard the hydrogen-evolution
reactionEx: arsenic and antimony ionsAs a consequence, 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.
Scavengers
These substances act by removing corrosive reagents from solution
Ex: sodium sulfite(Na2SO3) and hydrazine (N2H4)
It remove dissolved oxygen from aqueous solutions
O2HNOHN
SO2NaOSO2Na
22242
42232
Scavengers Work very effectively in solutions
where oxygen reduction is the controlling corrosion cathodic reaction
But will not be effective in strong acid solutions
Oxidizers Used to inhibit the corrosion of
metals and alloys that demonstrate active-passive transitions
Metals:iron and its alloy and stainless steels
Substance: chromate, nitrate and ferric salts.
Vapor-phase inhibitors
These are very similar to the organic adsorption-type inhibitors
Possess a very high vapor pressure (usually about 10-2 to 10-7 mmHg)
These materials can be used to inhibit atmospheric corrosion of metals without being placed in direct contact with the metal surface
Continue…Are placed in the vicinity of the
metal to be protectedTransferred by sublimation and
condensation to the metal surface
Only effective in closed spaces such as inside packages or on the interior of machinery during shipment
Continue…Dicyclohexylamine nitrite (DAN)
has been found especially effective for steel surfaces
But increases attack on copper alloy
Nitrobenzoate ( a universal vapor-phase inhibitor): protect ferrous, copper and other alloy system
Application Inhibitors find greatest use in
recirculating systems.Ex: recirculating cooling water,
automotive coolantsOnce-through sytems usually
consume too much of the inhibiting chemical to be economically feasible
ApplicationThe effectiveness of inhibitors is diminished
by ◦ Increasing solution corrosivity, ◦ Increasing concentration and ◦ Increasing temperature
Many inhibiting compounds are toxic (chromates, arsenic) and recent environmental regulations have limited their use
Application Many inhibitors are effective for more than
one type of alloy but◦ pH◦ temperature◦ and other condition
are unique for eachAn inhibitor for one metal may be corrosive
to others
Table 5.1: Effectiveness of inhibitors in near neutral pH water
Metal Chromates
Nitrites
Benzoates
Borates
Phosphates
Silicates
Tannins
Mild steel
E E E E E RE RE
Cast iron E E IE V E RE RE
Zinc E IE IE E RE RE
Copper E PE PE E E RE RE
Aluminium
E PE PE V V RE RE
Pb-Sn solder
E A E RE REE-effective, IE-ineffective, RE-reasonably effective, PE-partially effective, V-variable, A-aggressive
Application Three types of environment find greatest
use for inhibitors1) Natural, supply and industrial cooling waters
by nearly neutral (pH 5 to 9) rangeEx:
◦ organic phosphonate, acrylic copolymer as the main active
ingredient. ◦ It has a good corrosion and scale inhibition effe
ct on the
high hardness cooling water ◦ applicable to carbon steel, copper, copper alloy
and
stainless steel, etc.◦ widely used in the airconditioning cooling water
in hotels, ◦ restaurants, etc..
Application2)Primary and secondary production of crude
oil and subsequent refining processesThe main ingredient :is two-imidazoline, soluble in oil, gas, but also dispersed in
water. This unique feature makes it be the preferred
component of gas corrosion inhibitor.
Application3) Acid solutions for pickling to remove rust
and mill scale during the production and fabrication of metal parts or for postservice cleaning of such parts
Ex: The pickling operation makes use hydrochloric acid; that presents many benefits, inclusive of a better finishing and appearance, higher efficiency, and a reduced iron build-up.
Table 5.2: Inhibitors used for industrial application
Environment Sample inhibitor
Waters
- Potable water CaCO3 deposition, silicates, polyphosphates, zinc salts
-Recirculing cooling water
Chromate, nitrate at 300-500 ppmCalcium polyphosphates at 15-37 ppmSilicates at 20-40 ppm
-Automotive coolants
Nitrite, benzoate, borax, phosphate, sodium mercaptobenzothiazole, benzotriazole
-Steam condensates
Neutralizers: ammonia, morpholine, cyclohexamine, benzylamine, long chain aliphatic amines such as octdecylamine at 1-3 ppm
-Brines and seawater
Refrigeration brines: chromates, 2000-3300 ppmDiluted seawater: sodium nitrite, 3-10%Hot desalting brines: mixed chromate and phosphate, 50-100 ppm
Continue…
Environment Sample inhibitor
Acid pickling solution
-Sulfuric acid Phenylthiourea, di-ortho-tolyl-thiourea, mercaptans, sulfides, (0.003-0.01%)
-Hydrochloric acid Pyridine, quinoline, various amines, decylamine, phenylthiourea, dibenzylsulfoxides
Oil production and refining
-Primary and secondary recovery
Fatty imidazolines, various amines including primary amine, diamines, amido-amines, oxyethylated primary amines, alkyl pyidines, quaternized amines
-Refining Imidazoline and derivatives
Cathodic inhibitorOn addition of the corrosion inhibitor, Ecorr
decreases and icorr decreases.Displacement of the corrosion potential in the
negative direction indicates mainly retardation of the cathodic process (cathodic control)
Cathodic inhibitorCathodic inhibitors are generally less
effective than the anodic type. They often form a visible film along the
cathode surface, which polarizes the metal by restricting the access of dissolved oxygen to the metal substrate.
The film also acts to block hydrogen evolution sites and prevent the resultant depolarizing effect.
Cathodic inhibitorExample:
◦Zinc hydroxide◦Zinc phosphate◦Calcium carbonate◦Calcium phosphate
Anodic inhibitorOn addition of the corrosion inhibitor, Ecorr
increases and icorr decreases.Displacement of the corrosion potential in the
positive direction indicates mainly retardation of the anodic process (anodic control)
Anodic inhibitors build a thin protective film along the anode, increasing the potential at the anode and slowing the corrosion reaction.
The film is initiated at the anode although it may eventually cover the entire metal surface.
Because the film is not visible to the naked eye, the appearance of the metal will be left unchanged.
Anodic inhibitor
Example:◦ Chromate, molybdate, and nitrite -- catalyze the
reaction between the metal and oxygen to form a passivating film. Chromate and nitrite are the only anodic inhibitors that function in the absence of oxygen.
◦ Orthophosphate -- also catalyzes the reaction between steel and oxygen to form a passivating gamma iron oxide film. Oxygen must be present in water for orthophosphate to function as an anodic inhibitor.
◦ Polyphosphate -- exhibits some anodic properties but functions primarily as a cathodic inhibitor.
Anodic inhibitor
Mixed inhibitorOn addition of the corrosion inhibitor, icorr
decreases but Ecorr changes very little (either positive or negative).
Little changes in the corrosion potential suggests that both anodic and cathodic processes are retarded.
Mixed inhibitor They are typically film forming compounds that cause
the formation of precipitates on the surface blocking both anodic and cathodic sites indirectly.
Examples:◦ Sodium silicate, for example, is used in many
domestic water softeners to prevent the occurrence of rust water. In aerated hot water systems, sodium silicate protects steel, copper and brass. However, protection is not always reliable and depends heavily on pH.
◦ 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 non-toxic additives are required.
Polarisation resistance, Rp
The efficiency of an inhibitor can also be studied by recording polarisation curves or by polarisation resistance measurements.
corrca
cap iR
)(3.2
Inhibition efficiency, p
It is important to be able to evaluate inhibitors and determine the conditions under which they are effective.
The inhibition efficiency can be expressed by,
100
%
inhibitedR
duninhibiteRinhibitedR
p
ppp