The key to selecting the proper
inhibitor is knowing the system (Table 1), and in anticipating potential
problems in the system. Any testing should mimic system conditions (Table 2).
Tests can also be designed to examine a particular requirement, such as
flowability, or the effects of the chemical on the reservoir rock.
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Table 1. Tests of pipeline and storage system operating
conditions |
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For
gas pipeline and storage systems, there are tests that need to be performed
that are not concerned with corrosion, but with the system's operating
conditions. Among the tests that should be performed are: |
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Table 2. Some test methods that may be used to evaluate
corrosion inhibitors are briefly described |
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The test can be run at different temperatures and pressures. Film
forming and continuous inhibitors can be evaluated. |
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This method can evaluate the effects of shear stress on corrosion
rates and on the inhibitor film strength. It is useful for simulating
conditions found in turbulent areas of the pipeline. |
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A
corrosion inhibitor may be defined, in general terms, as a substance which when
added in a small concentration to an environment effectively reduces the
corrosion rate of a metal exposed to that environment. The use of chemical
inhibitors to decrease the rate of corrosion processes is quite varied. However
they often play an important role. In the oil extraction and processing
industries, for example, inhibitors have always been considered to be the
first line of defense against corrosion. A great number of scientific studies
have been devoted to the subject of corrosion inhibitors but most of what is
known has grown from trial and error experiments, both in the laboratories and
in the field.
http://www.corrosion-doctors.org/Inhibitors/lesson11.htm
Inhibitors are chemicals that react
with a metallic surface, or the environment this surface is exposed to, giving
the surface a certain level of protection (see corrosion costs
study findings). Inhibitors often work by adsorbing themselves on the
metallic surface, protecting the metallic surface by forming a film. Inhibitors
are normally distributed from a solution or dispersion. Some are included in a
protective coating
formulation. Inhibitors slow corrosion processes by either:
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Increasing the anodic or cathodic polarization behavior (Tafel slopes); |
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Reducing the movement or diffusion of ions to the metallic surface; |
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Increasing the electrical resistance of the metallic surface. |
The scientific and
technical corrosion literature has descriptions and lists of numerous chemical
compounds that exhibit inhibitive properties. Of these, only very few are
actually used in practice. This is partly due to the fact that the desirable
properties of an inhibitor usually extend beyond those simply related to metal
protection. Considerations of cost, toxicity, availability and environmental
friendliness are of considerable importance.
Despite the developments in
corrosion resistant alloys over the past few decades, carbon steel still
constitutes an estimated 99% of the material used in the oil industry. It is
usually the most cost effective option, being a factor of 3 to 5 times cheaper
than stainless steels. Yet its corrosion resistance is poor in aggressive
environments, and the cost savings can only be realized by adding a corrosion
inhibitor to the environment or applying a protective coating to the steel.
Inhibitors are used in a wide range of applications, such as oil pipelines,
domestic central heating systems, industrial water cooling systems and metal
extraction plants.
A particular advantage of corrosion
inhibition is that it can be implemented or changed in situ without disrupting
a process. The major industries using corrosion inhibitors are the oil and gas
exploration and production industry, the petroleum refining industry, the
chemical industry, heavy industrial manufacturing industry, water treatment
facilities, and the product additive industries. The largest consumption of
corrosion inhibitors is in the oil industry, particularly in the petroleum
refining industry. The total consumption of corrosion inhibitors in the
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 areas. Cathodic inhibitors can provide inhibition by three different mechanisms
as:
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Cathodic precipitates |
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Some cathodic
inhibitors, such as compounds of arsenic and antimony, work by making the
recombination and discharge of hydrogen more difficult. 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 the cathodic depolarization caused by oxygen. The
most commonly used oxygen scavenger at ambient temperature is probably sodium
sulfite (Na2SO3). (reference)
Cathodic poisons are used
advantageously as corrosion inhibitors by stifling the cathodic reduction
processes that must balance the anodic corrosion reaction. However cathodic
poisons can also increase the susceptibility of a metal to hydrogen induced
cracking since hydrogen can also be absorbed by metal during aqueous corrosion
or cathodic charging.
When corrosion occurs in a low-pH
solution, some of the reduced hydrogen does not form gaseous hydrogen, but
instead, diffuses into the metal as atomic hydrogen. This can happen during
pickling and electroplating of metal. Substances such as arsenic, antimony,
sulfur, selenium, tellurium, and cyanide ions prevent
the hydrogen atoms from forming hydrogen gas, and are called cathodic poisons.
Cathodic poisons facilitate contamination by keeping hydrogen in atomic form,
in which hydrogen more readily diffuses into the metal. Environments
containing hydrogen sulfide, which contains both hydrogen and a cathodic poison
(i.e. sulfur), are especially dangerous for alloys and metals. Hydrogen
sulfide is often encountered in the petroleum industry�during the drilling and completion of oil
and gas wells, and during the storage and piping of petroleum products
containing hydrogen sulfide.
A chemical that reacts with
dissolved oxygen to reduce corrosion, such as sulfite
and bisulfite ions that combine with oxygen to form sulfate. This is a
redox reaction and requires a nickel or cobalt catalyst. Removal of air from a
mud by defoaming and mechanical degassing is an essential first step before a
scavenger can lower the dissolved oxygen content.
Both anodic and
cathodic effects are sometimes observed in the presence of organic inhibitors
but, as a general rule, organic inhibitors affect the entire surface of a
corroding metal when present in sufficient concentration. Organic inhibitors
usually designated
as 'film-forming', protect the metal by forming a hydrophobic film on the metal
surface.
The effectiveness of these
inhibitors depends on the chemical composition, their molecular structure, and
their affinities for the metal surface. Because film formation is an adsorption
process, the temperature and pressure in the system are important factors.
Organic inhibitors will be adsorbed
according to the ionic charge of the inhibitor and the charge on the surface.
Cationic inhibitors, such as amines, or anionic inhibitors, such as sulfonates,
will be adsorbed preferentially depending on whether the metal is charged
negatively or positively. The strength of the adsorption bond is the dominant
factor for soluble organic inhibitors.
For any specific inhibitor in any
given medium there is an optimal concentration. For example, a concentration of
0.05% sodium benzoate, or 0.2% sodium cinnamate, is effective in water with a pH of 7.5 and
containing either 17 ppm sodium chloride or 0.5% by weight of ethyl octanol.
The corrosion due to ethylene glycol cooling water systems can be controlled
by the use of ethanolamine as an inhibitor.
Precipitation inducing inhibitors
are film forming compounds that have a general action over the metal surface,
blocking both anodic and cathodic sites indirectly. Precipitation inhibitors
are compounds that cause the formation of precipitates on the surface of the metal,
thereby providing a protective film. 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 of this
category are the silicates and the phosphates. 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 and a
saturation index that depends on 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 non-toxic additives are
required.
Volatile Corrosion Inhibitors
(VCI), also called Vapor Phase Inhibitors (VPI), 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 condenser tubes by neutralizing acidic carbon dioxide or by
shifting surface pH
towards 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 these salts condenses and is hydrolyzed by any moisture to
liberate protective ions. It is desirable, for an efficient VCI, to provide
inhibition rapidly while lasting for long periods. Both qualities depend on the
volatility of these compounds, fast action wanting high volatility while
enduring protection requires low volatility.
|
System |
Inhibitor |
Metals |
Concentration |
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Acids |
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HCl |
Ethylaniline |
Fe |
0.5% |
|
.. |
Mercaptobenzotriazole
|
.. |
1% |
|
.. |
Pyridine
+ phenylhydrazine |
.. |
0.5% + 0.5% |
|
.. |
Rosin
amine + ethylene oxide |
.. |
0.2% |
|
Sulfuric |
Phenylacridine |
.. |
0.5% |
|
Phosphoric |
Sodium
iodide |
.. |
200 ppm |
|
Others |
Thiourea |
.. |
1% |
|
.. |
Sulfonated
castor oil |
.. |
0.5-1.0% |
|
.. |
Arsenic
Oxide |
.. |
0.5% |
|
.. |
Sodium
arsenate |
.. |
0.5% |
|
Water |
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Potable |
Calcium
bicarbonate |
Steel,
cast iron |
10 ppm |
|
.. |
Polyphosphate |
Fe,
Zn, Cu, Al |
5-10 ppm |
|
.. |
Calcium
hydroxide |
Fe,
Zn, Cu |
10 ppm |
|
.. |
Sodium
silicate |
.. |
10-20 ppm |
|
Cooling |
Calcium
bicarbonate |
Steel,
cast iron |
10 ppm |
|
.. |
Sodium
chromate |
Fe,
Zn, Cu |
0.1% |
|
.. |
Sodium
nitrite |
Fe |
0.05% |
|
.. |
Sodium
phosphate monobasic |
.. |
1% |
|
.. |
Morpholine |
.. |
0.2% |
|
Boilers |
Sodium
phosphate monobasic |
Fe,
Zn, Cu |
10 ppm |
|
.. |
Polyphosphate |
.. |
10 ppm |
|
.. |
Morpholine |
Fe |
variable |
|
.. |
Hydrazine |
.. |
O2 scavenger |
|
.. |
Ammonia |
.. |
neutralizer |
|
.. |
Octadecylamine |
.. |
variable |
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Engine
coolants |
Sodium
chromate |
Fe,
Pb, Cu, Zn |
0.1-1% |
|
.. |
Sodium
nitrite |
Fe |
0.1-1% |
|
.. |
Borax |
.. |
1% |
|
Glycol/water |
Borax
+ MBT |
All |
1% + 0.1% |
|
Oil field brines |
Sodium silicate |
Fe |
0.01% |
|
.. |
Quaternaries |
.. |
10-25 ppm |
|
.. |
Imidazoline |
.. |
10-25ppm |
|
Seawater |
Sodium
silicate |
Zn |
10 ppm |
|
.. |
Sodium
nitrite |
Fe |
0.5% |
|
.. |
Calcium
bicarbonate |
All |
pH dependent |
|
.. |
Sodium
phosphate monobasic + Sodium nitrite |
Fe |
10 ppm + 0.5% |
In choosing between possible
inhibitors, the simplest tests should be done first to screen out unsuitable
candidates. The philosophy of initial screening tests should be that poorly
performing candidates are not carried forward. An inhibitor that does poorly in
early screening tests might actually do well in the actual system, but the user
seldom has the resources to test all possible inhibitors. The inhibitor user
must employ test procedures that rigorously exclude inferior inhibitors even
though some good inhibitors are excluded. (reference)
Inhibitor selection begins with the
choice of physical properties. Must the inhibitor be a solid or liquid? Are
melting and freezing points of importance? Is degradation with time and
temperature critical? Must it be compatible with other system additives? Are
specific solubility characteristics required? This list can be extensive, but
is important because it defines the domain of possible inhibitors. It must be
the first step of the inhibitor evaluation for any new system. These physical
measurements are those routinely done as part of minimal quality acceptance
testing.
The challenge in inhibitor
evaluation is design of experiments that simulate the conditions of the real
world system. The variables that must be considered include temperature,
pressure, and velocity as well as metal properties and corrosive environment
chemistry. System corrosion failures are usually localized and attributed to
micro conditions at the failure site. Adequate testing must include the most
severe conditions that can occur in the system and not be limited to macro or
average conditions. Examples of micro environments are hot spots in heat
exchangers and highly turbulent flow at weld beads.
Test specimens should be the same
metal as that to be protected; even very small
differences in metal chemistry can make major differences in inhibitor
performance. Inhibitor performance can vary greatly on different metals
and thus inhibitor rankings based on one metal are not universal. Much less
obvious are differences between the "same" metal. These nonchemical differences
include grain size and orientation, residual stresses, and surface condition.
Surface preparation should, to the extent possible, provide a surface
comparable to that in the system that is being modeled. Except in special
tests, minimal cleaning includes a solvent wash to degrease the sample. More
vigorous cleaning such as bead blasting or acid activation can markedly affect
inhibitor response even while improving reproducibility of tests. Many
experimenters activate test specimens in acid when doing electrochemical
measurements. The purpose is to remove any protective or passive oxide layer so
that metal solution equilibrium is reached rapidly. Methods for preparing specimens can be found
in ASTM G 1, Practice for
Preparing, Cleaning, and Evaluating Corrosion Test Specimens.
Corrosion inhibitors are generally
described by terms such as oil soluble, water soluble,
oil soluble-water dispersible, etc. Such terms are generalizations, not
rigorous descriptions. An oil soluble inhibitor, for example, in reality
partitions between the liquid hydrocarbon phase and the water phase as do all
other inhibitors; all that can be assumed is that it likely partitions more to
the oil phase.
Partitioning of a single compound between
two phases is clearly defined. Many commercial corrosion inhibitors, however,
are not single compounds but complex mixtures of many compounds, each with its
own unique partitioning coefficient. Thus a commercial corrosion inhibitor has
no unique partitioning coefficient but rather
one for each of the multiple components. Organic inhibitors are generally more
soluble in aromatic hydrocarbons than aliphatic ones and more soluble in long
chain aliphatics than short chain ones. The result is
that partition coefficients must be measured for each
"oil" of interest.
Corrosion
rates are most commonly reported as penetration rates. The usual way of reporting
protection
efficiency is in terms of percent protection. Although this reporting
method is useful for comparing inhibitor performance, it obscures the actual
number of interest the inhibited corrosion rate.
Film persistency tests are more
complex than constant concentration experiments. The test metal is exposed to
an inhibited test solution for a fixed period of time, then
the corrosion rate is determined in a similar solution containing no inhibitor.
Test variables include inhibitor concentration in the initial filming solution
and the number of rinse solution repetitions. A typical experiment might film
for one hour with 1000 ppm inhibitor, rinse one time for an hour, and finally
measure the corrosion rate in a third solution. Film, rinse, and corrode
solution are the same composition except for inhibitor in the filming step.
Metal loss can be determined
gravimetrically, volumetrically, or radiometrically;
all are a direct measure of corrosion. Of these, gravimetric or weight loss
methods are most used for inhibitor testing. Volumetric methods are associated
with inspection or monitoring techniques such as ultrasonic inspection and
electric resistance (ER) probe monitoring, although both are sometimes used in
long-term inhibitor evaluations. Radiometric methods are used as monitoring
methods such as in thin layer activation but could be used for inhibitor
evaluation. The corrosion wheel test used to evaluate
oilfield inhibitors is an example of weight loss testing.
Coupons from weight loss experiment
should be examined visually for localized corrosion seen as pits or edge
attack. Analysis can be as simple as "none, some, or lots" or as detailed
as counting and depth measurement. ASTM G 46 (Practice for Examination and Evaluation
of Pitting Corrosion) provides a complete procedure for evaluating pitting attack.
Electrochemical testing has two
major benefits, one major limitation, and one lesser limitation. The benefits
are short measurement time and mechanistic information. The severe limitation
is the requirement for a conductive corrosive environment. Less troublesome
from a testing perspective is the requirement for a corrosion model. Rapidity
of measurement makes these techniques useful in characterizing inhibitor
performance. Corrosion rates can be determined electrochemically in minutes
while weight loss methods can take days. With the near instantaneousness of
electrochemical methods, changes of inhibitor performance with time are readily
measurable. Questions about inhibitor persistence and incubation time are thus
experimentally accessible and experiments concerned with velocity effects
become less cumbersome.
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A
corrosion inhibitor is a chemical compound that, when added in small concentration, stops or slows down corrosion of metals and alloys.
A
typical good corrosion inhibitor will give 95% inhibition at concentration of 80 ppm,
and 90% at 40 ppm. One of the mechanisms of its effect is formation of a passivation layer, a thin film on the surface of the
material that stops access of the corrosive substance to the metal.
Some
corrosion inhibitors are hexamine, phenylenediamine, dimethylethanolamine, sodium
nitrite, cinnamaldehyde, and others.
The suitability of any given chemical for a task in hand depends on many
factors, from the material of the system they have to act in, to the nature of
the substances they are added into and their operating temperature.
Corrosion
inhibitors are commonly added to coolants, fuels, hydraulic
fluids, boiler water and
many other fluids used in industry.
Retrieved from "http://en.wikipedia.org/wiki/Corrosion_inhibitor"