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Coatings basics corrosion
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Corrosion Control
Corrosion Control Methods
• Better Design
• Better Selection of Materials
• Corrosion Control by Coatings
• Corrosion Control by Inhibitors
• Corrosion Control by Cathodic Protection
• Corrosion Control by Anodic Protection
Factors affecting the service Life
Better Design
Weld Design
Avoid
Crevices
Avoid Erosion Corrosion
Better Material Selection
• Material Selection Criteria
• Lowest Corrosion rate
• Also falls within the project cost
• If there is choice to select between steel and SS, you can better select SS
• But if the choice is to select only steel, then one has to make thorough investigations of various steels and find the best for application – based upon the chemistry of the steel
Protection by Coatings
Protection by Coatings
Classification of Coatings
Metallic / Ceramic/Organic
Room Temperature Application/ High
Temperature Application
Zn ( Galvanization), Cr, Sn, Ni, Cd
Aluminising, Al-Cr, Nitriding, carborizing
Method of Application
Hot Dip Method
Slurry method
Electroplating/Electroless Plating
Liquid route ( Cyanide bath)/Sol-gel
PVD/CVD
Thermal Spray / Laser
Coatings by Spray – paint spray
Paint Coatings
• Though appears simplest method of
Corrosion Control, actually not so in
practice.
• It is an art as well as science.
• Requires good knowledge of paint
formulations, paint selection, application
skill, suitable surface preparation method &
experience and inspection methods.
Why paint Coatings ?
• Aesthetics
• Corrosion Protection
• Both
Paint System
• Surface Preparation
• Primer
• Intermediate layer
• Top layer
•A technician is required to properly carry out
above jobs.
•An inspector / supervisor is required to
continually monitor the requirement as per
specifications.
Main Properties of a Coating
• The coating material should be an effective electrical insulator
• It can be applied with no breaks
• The coating must be strong enough to resist the impact of sharp objects.
• It constitutes initially a perfect film which remains so with time.
Surface Preparation
• Surface Cleaning
– Conversion Coatings
– Blasting
• Dry
• Hydroblasting
Paint Application
• Brush
• Rollers
• Air Spray
• Airless Spray
Coating Composition
• Binder ( Resin)
• Solvent
• Pigment
• Additives
Pigments
• Several Functions as part of Coating
– Provide colour & Opacity
• TiO2, C or lamp black
– Reinforce the film structurally
• Iron oxides
– Corrosion resistance
• Zn ( Galvanically)
– Talc & mica act as extenders, reduce permeability
through film as plate like particles block
permeation.
– Lead chromate as inhibitive pigment
Barrier Coating Inhibitive
Primer Organic Rich
Coatings
Inorganic rich coating
Pigment
Role of Pigment
Organic Zinc Rich Coatings • They utilize an organic resin rather than an inorganic
silicate binder for film formation.
• They protect steel galvanically as well as by barrier
protection but do not provide a high level of galvanic
protection as do inorganic Zn rich coatings.
• They are formed by simple solvent evaporation or by
chemical reaction of components.
• Film properties of these coatings are similar to that of free
zinc organic coatings.
• Organic Zn Coatings do not require high level surface
cleanliness while inorganic Zn coating require the best
cleaned surface.
Inorganic Zn-rich Primer
Organic Zn-rich primers
Additives
• Trace or small quantities of these help in
modifying various properties:
– Phenylmurcury, Zinc, Cuprous compounds act
as mild inhibitors
– Co and Mn Napthanates - help in drying
– ZnO - protection of resin from heat and Sun
What is required from a Paint
Coating
• Good Adhesion
• Flexibility
• Impact Resistance
• Resistance to environment
– Chemical
– Moisture
– Sunlight / Rains
Types of Coatings
• Conventional
– Lacquers, solutions of synthetic resins ( vinyl chloride,
rubber and acrylic)
– Water emulsion (latex) Coatings ( acrylics and Vinyls)
– Oil based coatings
– Epoxy Coatings
– Coal tar Epoxy coatings
– Poly-urethanes
– Polyester and Vinyl ester coatings
– Organic Zn rich Coatings
• Special
– Solvent Less Coatings
– Fiber Reinforced coatings
Lacquers • Lacquers are solutions of natural resins ( Shellac or bituminous materials) or
synthetic resins ( vinyls, chlorinated rubber and acrylics).
• Upon application, solvent evaporates and the resin deposits unchanged.
• Films have good water and chemical resistance but poor solvent and heat
resistance.
• Require abrasive blast cleaning.
• Applied in Multiple coats and easy to topcoats for repair.
• Have Good gloss Retention and resistance to weathering.
Advantages Limitaions
Rapid Drying & Recoating High in VOC
Good General Chemical Resistance Poor Solvent Resistance
Good in water immersion Low film build up
Good gloss Retention Blasted surface is necessary
Good Durability Occasional poor adherence
Water Emulsion Latex • Latex coatings are being successfully used to coat
wood and masonry structures.
• Relatively porous nature of structure allows water
vapour to pass through them.
Advantages Limitations
Reduced level of VOC Limited Durability
Easy to apply, topcoat & repair Poor chemical resistance
Fast to dry for recoating Poor wetting of surface
Excellent Flexibility Poor immersion service
Low Cost Best cure above 50 C.
Oil Based Coatings • Coatings based upon drying oil ( linseed oil, tung, soyabean,
fish oil).
• Cure by reaction with oxygen.
• Though complete dry less than in one day, complete curing
takes much longer.
• Alkyd coatings use resin formed by the reaction of polyhydric
alcohols ( glycerin) and polybasic acid ( phthalic acid) followed
by modification with drying oils. These cure much faster than
unmodified alkyds..
• Silicon alkyd coatings were developed by modifying alkyd resin
with silicon ( 30%) to provide greater gloss retention.
• Epoxy ester coatings are another modification of drying oils to
improve performance, particularly chemical resistance.
• Uralkyd coatings are formed with polyurethane. These coatings
are hard.
Epoxy Coatings
• The Most Common two component thermosetting product.
• An epoxy resin is based on a reaction product of phenols,
commonly bisphenol F or Cresol with epichloridehydrin.
• Available in solvent free, with solvent or water containing
formulations.
• The two components are called base and the curing agent. Latter
is used to polymerise the epoxy resin which has major influence
on the mechanical and chemical resistance properties. The most
common curing agents are aliphatic amines, ketamines and
polyamides.
• Epoxy coatings bond well to the abrasive cleaned steel and clean
concrete.Their films are hard and relatively inflexible. They
chalk in sunlight.
Epoxy Coatings
We make the prepolymer using bisphenol A and
epichlorohydrin
Diepoxy molecule
Bisphenol A Epichlorohydrin
Polyurethane Coatings
R-N = C = O + OH--R = R- NH- C - O -R
• Very important Class of Coatings
• Urethanes are perhaps the most common
materials used in our day to day life.
• Soft, flexible to rigid materials are
availbale.
Polyester and Vinyl ester Coatings
• Unsaturated polyester and vinyl ester thermosetting resins are used as
base for a useful group for a high performance corrosion resistant lining
for steel, concrete and other surfaces
• Polyesters and vinyls contain unsaturated groups which are reacted with
unsaturated monomers such as styrene.The combination of resin and
monomer forms a relatively low viscosity material to which when added
some catalysts such as peroxides accelerate curing.
• A great skill is required to ensure proper propotionating and mixing
– Limited pot life
– Blast surface required when used as primer.
– More expensive than epoxies.
• To the final lining product, one can add reinforcements and fillers to
minimise the effect of shrinkage and enhance properties such as lowering
coefficient of friction and increasing tensile strength and decrease
permeation.
• Polyesters and vinyls are applied in large thicknesses from 500 um to 5
mm.
Natural and Synthetic
Rubber Linings • Natural and synthetic rubber linings are used in such industries as
steel, chemical, paper, plating, mining, agriculture and power
• The physical properties of these steel linings allow them to
protect steel from erosion, corrosion, abrasion and chemical
attack.
• These linings are applied to various types of equipments such as
flue gas desulphurisation systems, pipe, tanks, railway tank cars,
rolls and pulleys.
• Typical thickness of Rubber linings are from 3.2 mm to 6.4 mm.
• Surface should be blast cleaned SA21/2 with a profile of 40-50
um.
• The surface before linings must be primed with suitable primer.
R-N = C = O + OH--R R- NH- C - O -R
Chlorinated rubber
Polyester Coating
Vinyl Acetate
Urethane
- CH - CH2-
Epoxy
--- CH -- CH --
O = C - OCH2 - - CH3 Acrylate
Paint Application
1. Brush or Rollars
2. Spray
• Air Spray
• Airless Spray
• Electrostatic Spray
36
2000+ psi fluid pressure
Transfer efficiency = 50 - 60%
Airless, Theory of Operation
• Airless spray atomization
- hydraulic force through
a cats eye shaped orifice
• 2000+ psi fluid pressure
• Fastest and heaviest spray
finish and is measured in
liters per minutes
• Transfer efficiency =
50-60%
• Protective coatings
Click to see spray
Transfer Efficiency
• Transfer efficiency (TE) = (paint deposited on a part)/(total paint
sprayed)
• TE = (Wp/Ws) x 100%
Wp = Weight of wet coating on the part
Ws = Weight of liquid coating sprayed
Electrostatic Technology
High Performance Coatings
1. One those which are functional specific
2. Can be applied in highly aggressive
environment.
3. Have high Corrosion resistance
4. High Erosion & abrasion resistance
5. Can dry very fast
6. Can give large thickness in one coat
Solventless Systems
Can achieve very high thickness in
one coat
Can have Drying time of a few
minutes
Life can further enhanced by
addition of glassfalkes
Fibreglass Reinforced Linings • Fibergalss-reinforced plastic (FRP) linings are unstaurated
polyester and vinyl ester thermosetting resin linings into
which fibre glass are incorporated to optimize the lining
performance.
• They are commonly used to control the internal corrosion
of above ground petroleum storage tank bottoms.
• The main reinforcement used with polyester and vinyl ester
resin liner are E glass mat and 250 um glass veil ( very thin
fibre glass mat) which has electrical properties, strength
and durability appropriate for most applications.
• C glass mat is chemical resistant, e.g. resistant to most
acids. A 3000g/m2 is sometimes used instead of choopede
strand mat.
SubstrateSubstrate
Coating
Glass Flakes
Moisture Penetration
Offshore
Structures
Glass Flake Reinforced
Polyester Coating to achieve
750um in one coat
Application
• Underwater:
– Brushes - Using vigorous
circular motion.
– Underwater pump with
round brush - using
vigorous circular
motion
• Once we have made diepoxy prepolymers, we tie
them all together by adding a diamine.
How Epoxies Work • The well known adhesion of epoxies is due to the
strong polar bonds it forms with the surfaces it comes in contact with.
• On dry surfaces the bond between the surface and the epoxy displaces the air, which is a fluid.
• The same is true underwater. As on dry surfaces, the polar bond attraction is strong enough to displace the fluid, in this case the water, and produce an strong bond even underwater.
• Thus, painting underwater is, in theory, no different that painting above the water. The cross linking reaction of epoxies should be independent of the surrounding environment.
POWDER COATINGS • Type I Thermoplastic - are high molecular weight products
that melt, flow and form a film without change in their
chemical composition. Because they are difficult to grind
they are often applied in fluidized beds and used in thick
films. : polyethylene, polypropylene, PVC, nylon.
• Type 2 Thermosets - are lower molecular weight polymers
that cross link during heat curing to form higher molecular
weight. They can be ground finer and thus applied in thinner
films 25-200 um. They chalk in presence of UV light.
• FBE is one of the best known thermoset Coatings used in
large applications for external and internal pipelines
corrosion protection.
• Application methods : Fluidized bed, electrostatic spray, hot
flocking and thermal spray.
Fluoroelastomeric Coatings • Fluoroelastomeric coatings are among the most expensive and
highest performance coating materials in present Day.
• Application, surface finishing, material mixing are of paramount
importance
• Fluoroelastomeric coatings are chemical resistant coatings that can
withstand elevated temperatures and attack by acids and alkalies,
hydrocarbons, gases and solvents and many harsh industrial
chemicals.
• They are generally two component catalysed systems that cure at
room temperature.
• Physical properties can be controlled by varying the amount of
catalysts.
• They are commonly used as heat and acid resistant linings in power
station flues and duct work. And also in tanks, sumps, ditches,
hazardous waste dikes and immersion zones.
The C-F bond is one of the strongest known
(PVDF shown below)
Fluoropolymer Bonds are Less Likely to Break
Fluorine Content of PVDF
Chemical Spot Test
PVDF
Polyester
Tapes & Wrappers • TAPES are finally prepared coating material ready to apply
onsite.
• They may be either thermoplastic or felt impregnated with
petrolatum or petroleum wax.
• Success of tapes depends upon the application. The most
important problem is the improper overlapping that causes
gaps where shielding corrosion may take place.
• The most common application of tapes is for immediate
repair of underground pipelines.
• One of the failure areas of tapes is in above ground araes
such as soil to air pipe transition zones
• Experienced tape applicators or especially with machine
wrapping one can eliminate failure problems.
Thermal Spray Coatings
• The most latest and easy to apply and
provide much longer life.
• Flame spray, Arc Spray, Detonation Gun or
Plasma Spray guns are used.
• They are also called hybrid systems with
internal metal coating with a top seal
coating of epoxy or PU.
• Life of such coatings is 25- 50 years.
Various Thermal Spray Systems Flame Arc
HVOF Plasma
Actual Application
Flame Spray
HVOF
Thermal Spray by Robots
Summary • Paint Coating is one of the most effective and known
methods of corrosion protection at the surface.
• To get an excellent corrosion protection, several factors
which include, proper surface preparation, correct
application technique, suitable environmental
conditions, inspection during surface preparation and
application are required.
• Paint selection is very important. Depending upon the
environment, a suitable system is selected.
• Most modern paint coating systems such as
Floroelastomers, glass flake systems are used for
selective applications are now readily available.
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