PROTECTIVE COATINGS

A.S.Khanna &

Siva Bohm

Department of Metallurgy

IIT Bombay

Methods to Control Corrosion

• Better Material Selection

• Better Design

•Protective Coatings• Cathodic Protection

• Use of Chemical Inhibitors

What we will learn in this course

• Fundamentals of Coatings – 3

• Surface Preparation – 1

• Paint Application Techniques -1

• Paint Failure Mechanisms and Remedial measures – 1

• Maintenance Coatings -1

• Characterization of coatings – 2

• High Performance Coatings

• Underground Pipelines

• Offshore Structures

• Refineries, Chemical Process industry, Petrochemical & Power

Plants

• Nano Modifies Coatings

• Graphene Based coatings

• Green Coatings

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Protective Coating - MM650/1

Prof. A.S. Khanna

Corrosion Science & Eng.

IIT Bombay

&

Prof Siva Bohm

Honorary visiting scientist - IIT Bombay

Principal scientist - Tata Steel Ltd

9

Protective coating? Examples

• Coatings for Bridges, Structures,

• Coil coating - Anti Corrosion Coatings

• Automotive coatings

• Offshore, Pipelines, Power Plants,

• Protective for Ships, Railways,

• Nano Coating (Graphene / Nanocomposite)

10

The Coating System

Organic

e.g. Paints

Hot Dipped

e.g. Galvanised

Thermally

Sprayed

Metallic

Protective Coatings

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Why we need protective coating? Corrosion!

Corrosion is a form of degradation process

• Produces less desirable material properties in a metal

• can result in a loss of function of the component or system

Why is it Important?

Safety Implications

• Structural Failures/Injuries

• Leaking of Harmful Chemicals

Expensive

• Plant Shutdown, Loss of Production

• High Cost of Remedial Work

• Perception of the Company

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Annual Loss Due to Corrosion

13

The Corrosion Reaction

• The Corrosion of Steel

Requires the Simultaneous

Presence of:-

• Moisture

• Oxygen

14

The Rate of Corrosion

• The Rate of Corrosion is

Determined by:-

• The Period of ‘Wetness’

• The Presence of

Contaminants

e.g. Sulphur Dioxide (SO2)

Chlorides (Cl-)

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Chemical/Electrochemical Corrosion

Anodic Reaction:

Fe Fe2++ 2e-

Cathodic Reaction:

½O2 + H2O + 2e- 2OH-

Combined Reaction:

Fe2++ 2OH- Fe (OH)2

Ferrous Hydroxide

Oxidises Fe2O3(H2O)

Hydrated Ferric Oxide – Red Rust

Anode Cathode

Fe2+ OH-

OH-

½O2 + H2O

Flow of Electrons

The Corrosion of Steel Requires the Simultaneous Presence of

Moisture & Oxygen

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India - Annual Loss Due to

Corrosion• Power Plants Rs. 4k Crore

• Chemical & Petrochemical Rs. 7k Crore

• Navy & Shipping Rs. 10k Crore

• Oil Drilling & Offshore Activity Rs. 8k Crore

• Oil/Gas Distribution Rs. 7k Crore

• Aircraft & Aerospace Rs. 4k Crore

• Railways Rs. 3k crore

• Infrastructure Rs. 5k Crore

• other Rs. 2k Crore

Total (3% GNP) Rs. 50k Crore

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The Rate of Corrosion

Corrosion Rates (µm/year)

Place Type of Environment Rate

Delhi Clean and Dry 8

UTTAR

PRADESH

Rural 19

Gujarat Urban 26

Mumbai Industrial 35

Goa Marine 37

Kovalam

- Kerala

Surf Beach - Humid 615

General Rate (2012) = 20 µm – 40 µm

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Exterior Environments

Inland or Marine

Rural, Urban, Industrial or

Marine

Environment Categories C1 to C5

(ISO 9223 & ISO 12944)

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Exterior Environments

Environment

Category

(ISO 12944)

Corrosion

Risk

Typical Steelwork Location

C3 Medium Most rural and urban areas with low

sulphur dioxide, acid, alkali and salt

pollution

C4 High Urban and industrial areas with moderate

sulphur dioxide pollution and/or coastal

areas with low salinity

C5

C5I

Very High

Industrial areas with high humidity and

aggressive atmospheres

C5M Coastal and offshore areas with high

salinity

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Methods of Corrosion Control

Treatment of

Environment

Organic

e.g. Paints

Hot Dipped Sprayed

Metallic

Protective

Coatings

Cathodic

Protection

Attention

to Design

Structural

Steels

Low Alloy

Steels

Stainless

Steels

Material

Selection

Corrosion Control

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Protective Coatings

‘If the surface preparation isn’t correct, the best coating in the world will not protect the steel.’

• Protective coatings are the most common form of corrosion control.

• High performance coatings can give very high durability steel structures if applied to properly prepared surfaces.

• Over 50% of coating failures are caused by poor or inadequate surface preparation.

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Selection of the Protective Treatment

• Life of the Structure/Coating

– Very Long 20 Years or More

– Long 10 to 20 Years

– Medium 5 to 10 Years

– Short Less Than 5 Years

• Environment & Design (Size and Shape)

• Access for Maintenance

• Facilities for Shop and Site Treatments

• Costs

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Selection of the Protective Treatment

Interior (Low Risk)

Appearance/Fire Protection

50/60 Years

Interior (Special)

Durability/Appearance

25/30 Years

Exterior (High Risk)

120 Years

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Purpose of Preparation

• Removal of Millscale, Rust

and Contaminants

– Oil and Grease

– Organic Deposits

• Bird Droppings

• Slime/Algae

– Chemical Deposits

• Soluble Salts

• Urban Pollution

– Old Coatings

• Provide a Satisfactory

Substrate for CoatingContaminated Steel Surface

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Surface Cleanliness

Removal of Scale and Rust

Mechanical Preparation

Steel Grade B

Preparation Grade St3

Abrasive Blast Cleaning

Steel Grade A

Preparation Grade Sa3

EN ISO 8501-1

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Protective coatings

- Organic Composition of Paints

Wet Film

Substrate (Steel)

Dry Film

Substrate (Steel)

PigmentFine Solid Particles

Opacity Colour

Binder Oil or Resin Film Former Cohesion

SolventOrganic or Water Dissolves Binder Reduces Viscosity

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Typical Longs Paint System

Resistance to Environment Aesthetic Appearance

‘Builds’ Film Thickness

Wets and Adheres to Substrate Corrosion Inhibition

Paint Thickness Quoted in µm (1 µm = 0.001 mm)

Finish Coat(s)

Intermediate Coat(s)

Primer Coat(s)

Surface Preparation

Substrate (Steel)

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Classification of Paints

Based on Pigment – Primers

- Zinc / Calcium Phosphate, Chromate, Molybdate

- Metallic Zinc or Aluminium Powder

Based on Binder – Intermediate and Finish Coats

- Drying Oil Type

- Alkyds

- One Pack Chemical Resistant

- Acrylated Rubber, Vinyl

- Two Pack Chemical Resistant

- Epoxy, Polyurethane, Coal Tar Epoxides

- Bituminous

- Asphaltic Bitumens, Coal Tar Pitches

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Fillers vs Nanocomposites

Properties of different fillers for Polymers

Filler FormDimension

(µm)aspect ratio

Density

(g/cm³)

Glass spheres microspheres 2.5 1 2.5

Calciumcarbonate cube 0.2 - 10 1 1.7

Glass fibres fibre 10 x 200 20 2.5

Carbon fibres fibre 7 x 200 30 1.6

Kaolin lamella 0.5 x 5 3 - 10 2,6

Talc lamella 0.5 x 5 3 - 10 2.8

Montmorillonite lamella 0.001 x 0.2 100 - 500 2.4

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Paint Systems

• Application Conditions

– Temperature

• >3°C Above Dew Point

– Humidity

• To Suit Drying and Curing

Paint Application in appropriate Conditions

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Comparison of Paint Types

Binder Water

Resistance

System

Cost

Tolerance of

Poor Surface

Overcoating

After Ageing

Comments

Black (Based

on Tar

Products)

Good Low Fair Very Good

with Coatings

of Same Type

May Soften in Hot

Conditions

Alkyds Fair Low Fair Very Good

Vinyl Very Good Moderate Poor Good

Epoxy Very Good Moderate Very Poor Poor ‘Chalks’ in UV

Light

Polyurethane Very Good High Very Poor Poor

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Longs Paint Systems

Steel Substrate

Blast Cleaned: Sa 3

Sprayed Aluminium

Sealer Coat

HB Zinc Phosphate Epoxy Undercoat

HB Epoxy MIO Undercoat

Two Pack Polyurethane FinishSite

Shop

50 µm

150 µm

100 µm

25 µm

100 µm

Total

300 µm

Schematic Representation of a Modern Coating System

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Typical Painting System

Acrylic Finish Coat

Non Pigmented Epoxy

Undercoat

Epoxy MIO

Epoxy Sealer Coat

Zinc Rich Epoxy Primer

Shot Blasted Steel Substrate

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Coating for Tubes, Energy & Power

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Basic Corrosion Protection Using Zinc

• A zinc coating creates an excellent protective barrier between the steel substrate and the environment

• Zinc coatings act in two ways:

– Barrier effect : physically sealing off the steel surface

with a coating with better corrosion resistance

– Cathodic protection/ deposition of zinc salts

Ecorr, Zn < Ecorr, Fe

Zn => preferential dissolution of Zn & formation of Zn oxides/ hydroxides (protective layer)

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Corrosion Protection Using Zinc

& conversion Coatings

• The duration of protection is controlled by:

– coating thickness,

– nature of the zinc coating (use of alloys),

– nature of the external atmosphere

• Additional protection for a metallic coated steel

can be provided by:

– Passivation / conversion treatments such as

chromates, oxides and phosphates

– Organic coatings

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Why is chromate so attractive?• First patented in 1923

• Excellent corrosion protection:

– Self healing effect: if damaged to the metal surface

(scratch or defect), Cr(VI) is released and migrates

through the protective coating and is reduced to form a

Cr(III) layer

– Barrier coating

– Inhibit the anodic & cathodic reaction

• Cheap! 15 INR / m2

Surface treatment with Cr

ElectricalApplianceBuildingAuto

Chromate (Cr6+)Oil

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Traditional Chromate vs alternative

• Chromic acid = cost effective.

• Active and passive protection of the

zinc layer.

• Pickling attack: Zn + 2H+ Zn2+ + H2↑

• Deposition reactions:

3H2 + 4H+ + 2Cr O42- 2Cr3+ + 2H2O

+ 6OH-

Zn2+ + Cr O42- ZnCr O4↓

Cr3+ +OH- + Cr O42- Cr(OH)Cr O4↓

Cr3+ +3OH- Cr(OH)3↓

• Pickling attack:

• Zn + 2H+ Zn2+ + H2↑

• Deposition reactions:

• Zn2+ + 2 H3PO4

Zn(H2PO4 )2

• Zn(H2PO4)2 ZnHPO4

+ H3PO4

• 3 ZnHPO4

Zn3(PO4)2↓ + H3PO4

Alternative zinc-aluminium-

orthophosphate

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Alternative Cr free systems!

• pH controlled precipitation:

• Titanium/zirconium based processes (H2ZrF6, H2TiF6)

• No redox reaction like Cr, but (Zr IV)

• Formation of a very stable oxide layer of TiO2 / ZrO2

precipitates after hydrolysis of the fluorides

Coupling agents: Silanes, Rn – Si – X(4-n)

• X = methoxy or ethoxy groups and provides the

linkage with the metal

• R = non hydrolysable organic which binds to the

organic coating.

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Coil Coatings ( Roofs & Walls)

Coatings

Surfaces

Interfaces

Protective coatings for

ConstructionAdhesion Science

Polymer Chemistry

Degradation

Metallurgy

Coatings Application

Differentiated

Products

Topcoat( 50 to 200 mm)

Pre-treatment(~1 mm)

Steel substrate

Primer(~5 mm)

Zinc Metallic coating (~20 mm)

Backing coat(~10 mm)

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Pre finished steel

42

Uses of coil coated steel . . .

43

The coil-coating process…

50mScale:

“Coil-coating is estimated to be >12%

cheaper and more environmentally

friendly than post-painting”‘Coil-Coatings: an overview’, Surface Coatings International

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Automotive structures

Body in White

Rear Axle (wheels) Sub frame (supports the engine)

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Zn layer

(50 µm)

CRS (3mm)

E- coat/paint

(few µm)

Cast iron

Sub frame

Body in

White

Body in

White

Rear axle

Body in

White

Body in

White

The sub frame & rear axle

systems

• Non visible parts: functional demands only

• Because of their position in the car, they are subject to

very aggressive corrosion conditions (thick Zn layer)

Zn layer

(50 µm)

CRS (3mm)

E- coat/paint

(few µm)

Cast iron

Sub frame

Body in

White

Body in

White

Rear axle

Body in

White

Body in

White

The body in white system• The coating system for automotive materials is

complex:functional & esthetics demands

Cold Rolled Steel (CRS); 0.6-2mm

Electrogalvanised (EG); 6-8 µm,...

or Hot Dip Galvanised (HDG); 8 – 10 µm

Phosphatisation layer; 1 µm

Electro-coat (10-30 µm)

Filler, also called Primer-surfacer;

Top-Coat; (often two layers, basecoat + clearcoat)

• Degradation of car bodies may be caused by salt (deicing salt, sea) , humidity,

temperature but…

The most direct cause of corrosion initiation is impact (scratches, stone

pebbles, sand)

Cold Rolled Steel (CRS); 0.6-2mm

Electrogalvanised (EG); 6-8 µm,...

or Hot Dip Galvanised (HDG); 8 – 10 µm

Phosphatisation layer; 1 µm

Electro-coat (10-30 µm)

Filler, also called Primer-surfacer;

Top-Coat; (often two layers, basecoat + clearcoat)

Metallic & organic coatings

- Metallic coating layer: HDG or GA(Zn-Fe) or Zn-Ni or EG

trend towards HDG

- Ph: trication system (Zn-Mn- Ni)

- E coat: mixture of binders, pigments & additives, BIW (< charged) in a

charged polymer particles solution

- Filler: leveling & stone shipping resistance

- Top coat: chemical, UV & scratch resistance

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Metallic coating: galvanizing =

sacrificial protection

Substrate

oxideEffet barrier

Substrate

Coating

ElectrolyteMn+

ne-

O2

OH-

Zn, Zn-Al, Zn-Mg

Galvanisation (10 µm)

Coating

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Conversion layer : in between Zn

and first paint layerPhosphatation

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How does the Ph layer look like?

Example of phosphated Galvanised

steel:

Crystal Size: 15-25 µm

Mass:1,5 - 5,0 g/m2

Example of phosphated Bare

steel:

Crystal Size: 7-11 µm (spherical)

Mass:1,5 - 3,0 g/m2

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Etching of surface

Oxyde

Etching of metalChange of

interfacial Ph

Layer formation

Zne-

Zn2+

2H+

e-

H2

Zn(OH)2

3 Zn2+

Zn3(PO4)2 x 4 H2O

H2PO4- + H+

H3PO4

H PO42- + 2H+

PO43- + 3H+

Zn2+

H2OH+

pH

pH

>

Zn2+

3NO2-

3NO3-

Phosphatating bath

H3PO4, Zn2+Mechanism

Phosphatation: How does it form?

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First paint layer: Cataphoretic or

electrocoat paint (20 mm)

+ -

+ rinsing+ curing (180°C)

+

+ +

+ +

Complex formulation adjusted to the application requirements. Main compoenents are

•Resins (or binders) most important component of ecoat and are polymers, Plasticizers

(film flexibility)

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Automotive Coatings

35-45 mmClear coat

12-18 mmBase color coat

15-40 mmPrimer surface

18-25 mmElectro coat

Phosphate

Zn

Steel

stone chip protection, leveling the

roughness of the electrocoated layer, UV

resistance

Chemical, UV and scratch resistance, Colour

Corrosion

resistance

& adhesion

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Metal Coatings – Hot Dip

BS EN ISO 1461

Molten Metal

Metal

Alloy Layer

Steel Substrate

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Metal Coatings - Sprayed

Compressed Air Gun

Droplets of Molten Metal

Steel Substrate

Overlapping Platelets

Steel Substrate

Standard: BS EN 22063

Design: BS EN ISO 14713

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Handling, Storage &

Transportation

• Minimise Damage to

Coatings

• Lifting Devices

• Separation and

Packing

• Avoiding Water and

Dirt Retention on Site Nylon Strops to Lift

Coated Beam

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Summary & Conclusions

• Corrosion Occurs as the Result of Chemical/Electrochemical

Reactions Between Steel and its Environment.

• Corrosion Requires the Presence of Both Moisture and Oxygen.

• The Most Common Form of Corrosion Control is Protective

Coatings.

• Two Types - Organic & Metallic

• The Right Coating Applied Properly to a Good Surface Will

Contribute to a Long Life to First Maintenance.

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Thank you