PROTECTIVE COATINGS A.S.Khanna & Siva Bohm Department of - - PowerPoint PPT Presentation

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

8

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

11

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

12

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-)

15

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

16

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

17

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

18

Exterior Environments

Inland or Marine Rural, Urban, Industrial or Marine Environment Categories C1 to C5 (ISO 9223 & ISO 12944)

19

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

20

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

21

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.

22

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

23

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

24

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 Coating

Contaminated Steel Surface

25

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

26

Protective coatings

• Organic Composition of Paints

Wet Film Substrate (Steel) Dry Film Substrate (Steel) Pigment Fine Solid Particles Opacity Colour Binder Oil or Resin Film Former Cohesion Solvent Organic or Water Dissolves Binder Reduces Viscosity

27

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)

28

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

29

Fillers vs Nanocomposites

Properties of different fillers for Polymers

Filler Form Dimension (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

30

Paint Systems

• Application Conditions

– Temperature

• >3°C Above Dew Point

– Humidity

• To Suit Drying and Curing

Paint Application in appropriate Conditions

31

Comparison of Paint Types

Binder Water Resistance System Cost Tolerance of Poor Surface Overcoating After Ageing Comments

Black (Based

• n Tar

Products) Good Low Fair Very Good with Coatings

• f 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

32

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 Finish Site Shop 50 µm 150 µm 100 µm 25 µm 100 µm Total 300 µm

Schematic Representation of a Modern Coating System

33

Typical Painting System

Acrylic Finish Coat Non Pigmented Epoxy Undercoat Epoxy MIO Epoxy Sealer Coat Zinc Rich Epoxy Primer Shot Blasted Steel Substrate

34

Coating for Tubes, Energy & Power

35

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

• xides/ hydroxides (protective layer)

36

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

37

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

Electrical Appliance Building Auto Chromate (Cr6+) Oil

38

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 O4

2-  2Cr3+ + 2H2O

+ 6OH- Zn2+ + Cr O4

2-  ZnCr O4↓

Cr3+ +OH- + Cr O4

2-  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-

• rthophosphate

39

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
• rganic coating.

40

Coil Coatings ( Roofs & Walls)

Coatings Surfaces Interfaces

Protective coatings for Construction

Adhesion Science Polymer Chemistry Degradation Metallurgy Coatings Application

Differentiated Products

Topcoat ( 50 to 200 mm) Pre-treatment (~1 mm) Steel substrate Primer (~5 m m) Zinc Metallic coating (~20 m m) Backing coat (~10 m m)

41

Pre finished steel

42

Uses of coil coated steel . . .

43

The coil-coating process…

50m

Scale:

“Coil-coating is estimated to be >12% cheaper and more environmentally friendly than post-painting”

‘Coil-Coatings: an overview’, Surface Coatings International

44

Automotive structures

Body in White Rear Axle (wheels) Sub frame (supports the engine)

45

Zn layer (50 µm) CRS (3mm) Sub frame le

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)

E- coat/paint (few µm) Cast iron Rear axle

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,...

• r 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,...

• r 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

48

Metallic coating: galvanizing = sacrificial protection

Substrate

• xide

Effet barrier

Substrate Coating Electrolyte Mn+ ne- O2 OH-

Zn, Zn-Al, Zn-Mg

Galvanisation (10 µm)

Coating

49

Conversion layer : in between Zn and first paint layer

Phosphatation

50

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

51

Etching of surface Oxyde Etching of metal Change of interfacial Ph Layer formation

Zn

e- Zn2+ 2H+ e- H2 Zn(OH)2 3 Zn2+ Zn3(PO4)2 x 4 H2O H2PO4

• + H+

H3PO4 H PO4

2- + 2H+

PO4

3- + 3H+

Zn2+ H2O H+ pH pH

> Zn2+

3NO2

• 3NO3
• Phosphatating bath

H3PO4, Zn2+

Mechanism

Phosphatation: How does it form?

52

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)

53

Automotive Coatings

35-45 mm

Clear coat

12-18 mm

Base color coat

15-40 mm

Primer surface

18-25 mm

Electro 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

54

Metal Coatings – Hot Dip

BS EN ISO 1461 Molten Metal Metal Alloy Layer Steel Substrate

55

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

56

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

57

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.

58

Thank you