Transcript Coating and Inhibitors

Coating and Inhibitors
• Introduction to Corrosion Monitoring
• What is Corrosion Monitoring?
• The field of corrosion measurement, control, and prevention covers
a very broad spectrum of technical activities. Within the sphere of
corrosion control and prevention, there are technical options such as
cathodic and anodic protection, materials selection, chemical dosing
and the application of internal and external coatings. Corrosion
measurement employs a variety of techniques to determine how
corrosive the environment is and at what rate metal loss is being
experienced. Corrosion measurement is the quantitative method by
which the effectiveness of corrosion control and prevention
techniques can be evaluated and provides the feedback to enable
corrosion control and prevention methods to be optimized.
A wide variety of corrosion measurement techniques exists, including:
Non Destructive Testing Analytical
Chemistry
• Ultrasonic testing
• Radiography
• Thermography
• Eddy current/magnetic flux
• Intelligent pigs
Operational Data
• pH
• Flow rate (velocity)
• Pressure
• Temperature
Corrosion Monitoring
• Weight loss coupons
• Electrical resistance
• Linear polarization
• Hydrogen penetration
• Galvanic current
Analytical Chemistry
• pH measurement
• Dissolved gas (O2, CO2, H2S)
• Metal ion count (Fe2+, Fe3+)
• Microbiological analysis
Fluid Electrochemistry
• Potential measurement
• Potentiostatic measurements
• Potentiodynamic measurements
• A.C. impedance
Classification of corrosion protection methods
Active corrosion protection
Passive corrosion protection
Permanent corrosion protection
Temporary corrosion protection
Protective Metallic Coatings
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Metallic coatings provide a layer that changes the surface properties of the workpiece
to those of the metal being applied. The workpiece becomes a composite material
exhibiting properties generally not achievable by either material if used alone. The
coatings provide a durable, corrosion resistant layer, and the core material provides
the load bearing capability. The deposition of metal coatings, such as chromium,
nickel, copper, and cadmium, is usually achieved by wet chemical processes that
have inherent pollution control problems. (corrosion costs study)
Alternative metal deposition methods have replaced some of the wet processes and
may play a greater role in metal coating in the future. Metallic coatings are deposited
by electroplating, electroless plating, spraying, hot dipping, chemical vapor deposition
and ion vapor deposition. Some important coatings are cadmium, chromium, nickel,
aluminum and zinc.
Plating and surface treatment processes are typically batch operations, in which
metal objects are dipped into and then removed from baths containing various
reagents to achieve the desired surface condition. The processes involve moving the
object being coated through a series of baths designed to produce the desired end
product. These processes can be manual or highly automated operations, depending
on the level of sophistication and modernization of the facility and the application.
• Organic Protective Coating
• Many paints, coatings and high performance
organic coatings have been developed as a
need to protect equipment from environmental
damage. Of prime importance in the
development of protective coatings was the
petroleum industry that produced most of the
basic ingredients from which most synthetic
resins were developed.
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Metal Cladding
The corrosion resistance of a substrate can be improved by metallurgically bonding to the susceptible core alloy a surface
layer of a metal or an alloy with good corrosion resistance. The cladding is selected not only to have good corrosion
resistance but also to be anodic to the core alloy by about 80 to 100 mV. Thus if the cladding becomes damaged by
scratches, or if the core alloy is exposed at drilled fastener holes, the cladding will provide cathodic protection by corroding
sacrificially.
Cladding is prevalently applied at the mill stage by the manufacturers of sheet, plate or tubing. Cladding by pressing, rolling
or extrusion can produce a coating in which the thickness and distribution can be controlled over wide ranges and the
coatings produced free of porosity. Although there is almost no practical limits to the thickness of coatings that can be
produced by cladding, the application of the process is limited to simple shaped articles that do not require much subsequent
mechanical deformation.
Aluminum Cladding
Among the principal uses are aluminum cladding in the aircraft industry, lead and cadmium sheathing for cables, leadsheathed sheets for architectural applications and composite extruded tubes for heat exchangers. The thickness of the
cladding is usually between 2% and 5% of the total sheet or plate thickness, and since the cladding is usually a softer and
lower strength alloy, the presence of the cladding can lower the fatigue strength and abrasion resistance of the product. In
the case of thick plate where substantial amounts of material may be removed from one side by machining so that the
cladding becomes a larger fraction of the total thickness, the decrease in strength of the product may be substantial.
A clad finish being soft in nature is subject to damage during manufacturing and while in service. Caution must be exercised
while polishing or cleaning, since it is sensitive to harsh chemicals and abrasive materials.
Cladding and Weld Overlaying
Compared to carbon and alloy steels, all corrosion resistant alloys are expensive. In many cases, corrosion resistance is
required only on the surface of the material and carbon or alloy steel can be clad with a more corrosion resistant alloy.
Cladding can save up to 80% of the cost of using solid alloy. Cladding of carbon or low alloy steel can be accomplished in
several ways including roll bonding, explosive bonding, weld overlaying and “wallpapering”. Clad materials are widely used in
the chemical process, offshore oil production, oil refining and electric power generation industries. The use of clad steel is
not new. Corrosion resistant alloy clad steel has been available for over 40 years. Almost any corrosion resistant stainless
steel or nickel alloy can be bonded to steel. The steel can be clad on both sides or on one side only. The hot roll bonding
process is used to produce over 90% of clad plate products.
• Vapor Deposition (VD)
• Vapor deposition refers to any process in which materials in a vapor
state are condensed through condensation, chemical reaction, or
conversion to form a solid material. These processes are used to
form coatings to alter the mechanical, electrical, thermal, optical,
corrosion resistance, and wear properties of the substrates. They
are also used to form free-standing bodies, films, and fibers and to
infiltrate fabric to form composite materials. Vapor deposition
processes usually take place within a vacuum chamber. There are
two categories of vapor deposition processes:
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• Physical vapor deposition (PVD)
• Chemical vapor deposition (CVD)
• Electroless plating
• Electroless nickel (EN) plating is a chemical reduction process
which depends upon the catalytic reduction process of nickel ions in
an aqueous solution (containing a chemical reducing agent) and the
subsequent deposition of nickel metal without the use of electrical
energy. Due to its exceptional corrosion resistance and high
hardness, the process finds wide application on items such as
valves, pump parts etc., to enhance the life of components exposed
to severe conditions of service ,particularly in the oil field and marine
sector. With correct pretreatment sequence and accurate process
control , good adhesion and excellent service performance can be
obtained from EN deposited on a multitude of metallic and nonmetallic substra6tes.
Electroless Plating
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Due to its unique properties of excellent corrosion resistance, combined
with a high wear resistance and uniformity of coating, EN finds extensive
applications in a number of fields. Some of the prominent areas of
application are :-Oil & Gas: Valve components, such as Balls, Gates, Plugs etc. And other
components such as pumps, pipe fittings, packers, barrels etc.
Chemical Processing: Heat Exchangers, Filter Units, pump housing and
impellers, mixing blades etc.
Plastics: Molds and dies for injecting and low and blow molding of plastics
components, extruders, machine parts rollers etc.
Textile: Printing cylinders, machine parts, spinneret's, threaded guides etc.
Automotive: Shock Absorbers, heat sinks, gears, cylinders, brake pistons
etc.
Aviation & Aerospace: Satellite and rocket components, rams pistons,
valve components etc.
Food & pharmaceutical: Capsule machinery dies, chocolates molds, food
processing machinery components etc.
Corrosion Inhibitors
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Corrosion Inhibitors
• The use of chemical inhibitors to decrease the rate of
corrosion processes is quite varied. In the oil production
and processing industries, 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. However,
most of what is known has grown from trial and error
experiments, both in the laboratories and in the field.
• Rules, equations or theories to guide inhibitor
development or use are very limited. A synergism, or
cooperation, is often present between different inhibitors
and the environment being controlled, and mixtures are
the usual choice in commercial formulations.
Classification of inhibitors
• 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. 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:
• Increasing the anodic or cathodic polarization behavior (Tafel slopes);
• Reducing the movement or diffusion of ions to the metallic surface;
• 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.
• Inhibitors have been classified differently by various
authors. Some authors, for example, prefer to group
inhibitors by their chemical functionality. However, by far
the most popular organization scheme consists in
regrouping corrosion inhibitors in a functionality scheme
as follows:
• Passivating inhibitors
• Cathodic inhibitors
• Organic inhibitors
• Precipitation inhibitors
• Volatile corrosion Inhibitors
Passivating Inhibitors
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
Non oxidizing ions such as phosphate, tungstate and molybdate that require
the presence of oxygen to passivate steel.
These inhibitors are the most effective and consequently the most widely used.
Chromate based inhibitors are the least expensive inhibitors and were used until
recently in a variety of applications, e.g. recirculation-cooling systems of internal
combustion engines, rectifiers, refrigeration units, and cooling towers. Sodium
chromate, typically in concentrations of 0.04-0.1% was used for these applications.
At higher temperatures or in freshwater with chloride concentrations above 10
ppm higher concentrations 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. Therefore it is essential that periodic colorimetric
analysis be conducted to prevent this from occurring.
• In general, passivation inhibitors can
actually cause pitting and accelerate
corrosion when concentrations fall below
minimum limits. For this reason it is
essential that monitoring of the inhibitor
concentration be performed.
Cathodic inhibitors
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:
• Cathodic poisons
• Cathodic precipitates
• Oxygen scavenger.
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).
Organic Inhibitors
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 Inhibitors
• 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
• 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.
Electrochemical Tests to Monitoring inhibitor’s
effectiveness
• While all laboratory corrosion tests require accelerating
corrosion processes, only electrochemical tests can
directly amplify the impact of corrosion processes. The
main reasons why this is possible is that all
electrochemical tests use some fundamental model of
the electrode kinetics associated with corrosion
processes to quantify corrosion rates. The amplification
of the electrical signals generated during these tests has
permitted very precise and sensitive measurements to
be carried out.
• Potentiodynamic polarization methods
• Linear polarization resistance (LPR)
• Electrochemical impedance spectroscopy (EIS)
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Attributes of a Corrosion Engineer
Knowledge of corrosion
Knowledge of corrosion resistant characteristics of materials
Knowledge of corrosive characteristics of chemicals
Information on physical and mechanical properties of materials
Information on availability and cost
Information on fabrication techniques
Knowledge of special requirement of what is being produced
Proficiency in planning, executing, and interpreting test
programs
• Ability to get along with others
• Common sense
Corrosion Engineer
• Work closely with the engineering staff in working out
new designs or modifying existing ones to reduce the
opportunity for corrosion.
• Close contact with the maintenance engineers.
• Work with the production department.
• Collaborate with accounting department in establishing
the actual cost of corrosion.
• Work closely with the purchasing department.
• Assist the sales department by helping discover any
deficiencies in products.
• Keep management informed of his need and his
accomplishments so that he may be provided with the
support he will require to be effective.