Coatings basics corrosion

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.