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
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 8
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) 9
The Coating System Protective Coatings Organic Metallic e.g. Paints Hot Dipped Thermally e.g. Galvanised Sprayed 10
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 11
Annual Loss Due to Corrosion 12
The Corrosion Reaction • The Corrosion of Steel Requires the Simultaneous Presence of:- • Moisture • Oxygen 13
The Rate of Corrosion • The Rate of Corrosion is Determined by:- • The Period of ‘Wetness’ • The Presence of Contaminants e.g. Sulphur Dioxide (SO 2 ) Chlorides (Cl - ) 14
Chemical/Electrochemical Corrosion Anodic Reaction: Fe Fe 2+ + 2e - Cathodic Reaction: ½O 2 + H 2 O ½O 2 + H 2 O + 2e - 2OH - Combined Reaction: OH - Fe 2 + OH - Fe 2+ + 2OH - Fe (OH) 2 Anode Cathode Ferrous Hydroxide Oxidises Fe 2 O 3 (H 2 O) Hydrated Ferric Oxide – Red Rust Flow of Electrons The Corrosion of Steel Requires the Simultaneous Presence of Moisture & Oxygen 15
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 16
The Rate of Corrosion Corrosion Rates (µm/year) Place Type of Environment Rate Delhi Clean and Dry 8 UTTAR Rural 19 PRADESH Gujarat Urban 26 Mumbai Industrial 35 Goa Marine 37 Kovalam Surf Beach - Humid 615 - Kerala General Rate (2012) = 20 µ m – 40 µ m 17
Exterior Environments Environment Categories C1 to C5 (ISO 9223 & ISO 12944) Inland or Marine Rural, Urban, Industrial or Marine 18
Exterior Environments Environment Corrosion Typical Steelwork Location Category Risk (ISO 12944) 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 C5I Industrial areas with high humidity and aggressive atmospheres C5 Very High C5M Coastal and offshore areas with high salinity 19
Methods of Corrosion Control Corrosion Control Protective Cathodic Treatment of Attention Material Selection Coatings Protection Environment to Design Metallic Organic Structural Low Alloy Stainless e.g. Paints Steels Steels Steels Sprayed Hot Dipped 20
Protective Coatings • 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. ‘If the surface preparation isn’t correct, the best coating in the world will not protect the steel.’ 21
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 22
Selection of the Protective Treatment Interior (Special) Durability/Appearance 25/30 Years Interior (Low Risk) Appearance/Fire Protection Exterior (High Risk) 50/60 Years 120 Years 23
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 Contaminated Steel Surface Substrate for Coating 24
Surface Cleanliness Removal of Scale and Rust EN ISO 8501-1 Mechanical Preparation Abrasive Blast Cleaning Steel Grade B Steel Grade A Preparation Grade St3 Preparation Grade Sa3 25
Protective coatings - Organic Composition of Paints Binder Solvent Pigment Oil or Resin Organic or Water Fine Solid Particles Film Former Dissolves Binder Opacity Cohesion Reduces Viscosity Colour Wet Film Substrate (Steel) Dry Film Substrate (Steel) 26
Typical Longs Paint System Resistance to Environment Finish Coat(s) Aesthetic Appearance ‘Builds’ Film Thickness Intermediate Coat(s) Wets and Adheres to Substrate Primer Coat(s) Corrosion Inhibition Surface Preparation Substrate (Steel) Paint Thickness Quoted in µm (1 µm = 0.001 mm) 27
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 28
Fillers vs Nanocomposites Properties of different fillers for Polymers Dimension Density Filler Form aspect ratio (µm) (g/cm³) 2.5 Glass spheres microspheres 1 2.5 Calciumcarbonate cube 0.2 - 10 1 1.7 Kaolin lamella 0.5 x 5 3 - 10 2,6 Talc lamella 0.5 x 5 3 - 10 2.8 Glass fibres fibre 10 x 200 20 2.5 Carbon fibres fibre 7 x 200 30 1.6 Montmorillonite lamella 0.001 x 0.2 100 - 500 2.4 29
Paint Systems • Application Conditions – Temperature • >3°C Above Dew Point – Humidity • To Suit Drying and Curing Paint Application in appropriate Conditions 30
Comparison of Paint Types Binder Water System Tolerance of Overcoating Comments Resistance Cost Poor Surface After Ageing Black (Based Good Low Fair Very Good May Soften in Hot on Tar with Coatings Conditions Products) of Same Type Alkyds Fair Low Fair Very Good Vinyl Very Good Moderate Poor Good ‘Chalks’ in UV Epoxy Very Good Moderate Very Poor Poor Light Polyurethane Very Good High Very Poor Poor 31
Longs Paint Systems Schematic Representation of a Modern Coating System 50 µm Two Pack Polyurethane Finish Site 150 µm HB Epoxy MIO Undercoat 100 µm HB Zinc Phosphate Epoxy Undercoat Shop 25 µm Sealer Coat 100 µm Sprayed Aluminium Total Steel Substrate 300 µm Blast Cleaned: Sa 3 32
Typical Painting System Acrylic Finish Coat Non Pigmented Epoxy Undercoat Epoxy MIO Epoxy Sealer Coat Zinc Rich Epoxy Primer Shot Blasted Steel Substrate 33
Coating for Tubes, Energy & Power 34
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 E corr, Zn < E corr, Fe Zn => preferential dissolution of Zn & formation of Zn oxides/ hydroxides (protective layer) 35
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 36
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 Surface treatment with Cr – Barrier coating Appliance Electrical Auto Building Oil Chromate (Cr 6+ ) – Inhibit the anodic & cathodic reaction • Cheap! 15 INR / m2 37
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