Why Steel Needs Protection Wind Mill Offshore Structure - - PowerPoint PPT Presentation

Why Steel Needs Protection

Offshore Structure Wind Mill

Vehicle Additives Industrial/Mar ine Pigments Resin Solven t Strengt hener Coloran ts Inhibito rs Alkyd Epoxy Urethane Polyesters Vinyls Acrylic Siloxanes Benzene Toluene Xylene Thinners Zn Dust Zn Chromate Zn Phosphate Polyaniline C black TiO2 ZnO Glassflake Fibers Mica Thickness UV absorbers Surfactants Wetting Agents Drying Agent High Performance Coatings Solventless PVDF Underwater coatings Fire Retardant Coatings Waterborne Coatings Sol-gel Route Dispersion Method

Barrier Coatings

So how the durability can be enhanced?

Barrier Coating

Inhibitive Primer Organic Rich Coatings

Inorganic rich coating Pigment

Role of Pigment

Inorganic Zn-rich Primer Organic Zn-rich primers

Selection Criteria for Zinc Based Coatings

Organic Zinc Rich Coatings

• If the complete surface

cleaning is not possible.

• It can be applied on partial

cleaned surface ( st2/st3)

• It can not be applied if

humidity is more than 60. Requires dry environment.

• Only for room temp.

application.

• Needs to coat immed.

Inorganic Zinc Rich Coating

• Can be applied on a

Sa21/2 or better cleaned surface.

• Cures only if there is

high humidity.

• Suitable even at high

temperatures up to 450oC

• Can be used as

independent coating with long life.

Durability of Coatings

Steel Substrate

Primer Intermediate layer Top Coat

Concentration of Various Zn Coatings

Zn Coating % Zn on Dry Film Corrosion Effect Durability Inhibited Zn Coating 5-15 Very little ( No Cathodic Protection) 2-3 years Organic Zinc Rich Coating 70-78 Reasonable Cathodic Protection 4-6 years Inorganic Zinc Silicate 82-85% Good Cathodic Protection 6-8 years Cold galvanized coatings ZRC /Zinga 95-96% Excellent Durability 10-12 years Metallized Zinc Coating 100% ( 3 times coating Thickness Very Very Long Life 25-50 years Hot Dip galvanization 100% (Thickness 20-50 µm Long life 35-60 years

CAUSES AND MECHANISM OF PAINT FILM FAILURES

HOW COATINGS WORK ?

Reasons of Coating Failure?

REASONS FOR COATING FAILURE

Pigment Volume Concentration (PVC

5 10 15 20 25 30 20 25 30 35 40 45 50 55 60 PVC %

Properties of coating varies with PVC (Pigment Volume concentration) PVC = (VOL OF PIGMENT) * 100 (VOL OF PIGMENT + VOL OF BINDER)

RUSTING PERMEABILITY BLISTERING

FAULTY DESIGN

PERMEABILITY

Influenced by

– PAINT FORMULATION

• Permeability increases above CPVC & vice versa

– DRY FILM THICKNESS

• Permeability reduces as film thickness increases

– CROSS LINK DENSITY

• Thermoset coatings have high

cross-linking & lower permeability

CROSSLINKED NETWORK POLYMER

FAULTY DESIGN

IMPROPER COATING SELECTION FAILURE OF WRONGLY SPECIFIED TOP COAT APPLIED OVER INTACT SHOP PRIMER

FAULTY DESIGN

USAGE OF INCOMPATIBLE COATING SYSTEMS

COATING SYSTEM FAILURE DUE TO INCOMPATIBILTY BETWEEN TOP AND BOTTOM COATS. BLEEDING:

• TOP COAT SLIGHLTY DISSOLVES

UNDERCOAT.

• STAINING OF TOPCOAT BY COLOUR

DIFFUSION FROM UNDERCOAT.

• INCOMPATIBILITY OF COATING SYSTEMS.

BLEEDING

POOR SURFACE PREPARATION

• 75% failures attributed to faulty surface preparation.
• Service life of coating system is enhanced by 50-100% between

manual vis-à-vis blast cleaned steel.

• Coating fails miserably if there is underneath mill scale, rust, soluble

salts, oils & greases.

• Surface contaminants

– Impairs wetting – Affects adhesion – Promotes blisters / rusting – Causes delamination

The presence of various contaminants at the metal/coating interface has a detrimental effect on the durability, performance and integrity of coating system. The influences

• f

contaminants lie in their abilities to:

• Draw water through the organic

coating to establish a conductive film

• f moisture
• Provide ions to carry the corrosion

currents.

• Provide ions of specific effect on

under film corrosion.

• Insulate the reactive metal site on

substrate from reactive site

• f

coating.

POOR SURFACE PREPARATION--- Continued…..

Purpose of surface preparation The purposes of surface preparation are as follow

• Removal of contaminants
• Increase in surface area

The actual exposed surface per unit area is greatly increased by proper surface

• preparation. As a result, many additional

reactive sites on the metal surface are exposed, allowing for additional chemical or polar adhesion of the primer to substrate. The effective surface area can be doubled or tripled by roughening the surface.

Degradation mechanism of coating due to soluble salts

 Osmotic blistering  Under film corrosion Oxygen diffusing through the organic film and dissolving in the water layer formed at the interface is the controlling factor which determines underfilm

• corrosion. The structure and thickness of the coating are the main parameters in

the corrosion of the steel substrate. The kinetics of steel corrosion is governed by the osmotic pressure, ionic conductivity and oxygen solubility of the aqueous electrolyte solution and by the solubility in water of the corrosion products.

POOR SURFACE PREPARATION- ADHESION FAILURES

SUBSTRATE SURFACE CONTAMINATION

INTERCOAT CONTAMINATION RUST SCALES

CONDENSATION

ADHESION FAILURES - BLISTERING

ENTRAPPED SOLVENTS HYDROGEN EVOLUTION

( CATHODIC PROTECTION )

OSMOTIC BLISTERING

BALLAST TANK

BLISTERING

STRESS INDUCED FAILURE

• STRESS IS FORCE APPLIED THAT

TENDS TO STRAIN OR DEFORM THE OBJECT.

• COATINGS FACE MULTIPLE TYPES OF

STRESSES IN AN EXPOSED ENVIRONMENT.

• COATINGS FAIL DUE TO ITS

INABILITY TO RELIEVE STRESS

• STRESS IS TRANSMITTED THROUGH

OUT A MUTICOAT SYSTEM & FINALLY RELEIVED AT ITS WEAKEST LINK – EXAMPLE – DELAMINATION

• a means for stress dissipation

DELAMINATION CRACKING & FLAKING

MECHANICAL STRESS

• VARIOUS EXTERNAL STRESSES ENCOUNTERED

– Expansion / contraction due to temperature fluctuations. – Expansion / contraction due to fluctuation in humidity (e.g. in case of wood). – Vibration and movement due to operation of machines. – Flexing due to people walking. – Post forming operation. – Volume expansion due to absorption of water & chemicals. – Impact of falling object.

CRACKS SPREADING FROM IMPACT AREA

INTERNAL STRESS

• Stress exerted due to

– Cross linking , Solvent evaporation ,Reduction in polymer mobility during cross linking – Shrinkage to relieve internal stress . ALLIGATORING

• Very large cracking resembling the

skin of an alligator

• Internal stresses due to
• Shrinkage of surface faster than

body of coating.

• Excess Film thickness
• Limited paint flexibility
• Hard topcoat over soft

undercoat.

WEATHERING STRESS

• STRESS DUE TO

– Heat/Cold , Thermal cycling

STRESS CRACKING & PREMATURE COATING FAILURE DUE TO THERMAL CYCLING.

APPLICATION RELATED

• Wrong mixing ratio - ‘mismixed’
• Lack of adequate induction time
• Too short / too long over coating interval
• Too thick / too thin application DFT
• Inappropriate surface temperature
• Too much distance between spray gun & substrate
• Inappropriate control on application environment
• Over baking / under baking (stoved coatings)

APPLICATION RELATED - MIXING RATIO

• Excess hardener

– Hardener exudes to surface, reacts with humidity, condensed water leading to amine blush

• n surface.
• Less hardener

– Too soft undercoat ; if over coated undercoat swells due to solvent penetration

• Sagging also occurs

APPLICATION RELATED SURFACE TEMPERATURE

• Too high surface temperature

– Too fast solvent evaporation leading to poor flow out,

• range peel & pinholes.
• Too low surface temperature

– If below dew point , condensation occurs on substrate, may lead to peel off problem during over coating. – Slow curing of 2K coatings – Poor coalescence of latex coating leading to powdery deposition

APPLICATION RELATED FILM THICKNESS

• Too high DFT

– Solvent popping, run down, slower curing – Susceptible to cohesive failure

• Too low DFT

– Inadequate barrier protection – High permeability

APPLICATION RELATED RELATIVE HUMIDITY

• High relative humidity

– Possibility of amine blush, low gloss – Retarded drying - under curing – Chances of flash rusting – Condensation on substrate

• Low humidity

– Under curing of IOZ - cohesive splitting after over- coating

APPLICATION RELATED FAILURES

• Too high gun distance (> 20 inches) leads to

– Faster solvent evaporation – High wet film viscosity – Inadequate wetting – Poor flow out – Pinholes

ORANGE PEEL

• Resembles the skin of an orange
• Occurs due to

* Improper spraying technique * Improper low pressure * Improper thinning solvent

APPLICATION RELATED FAILURES

SAGGING :

• MASS SLIPPAGE OF FILM
• EXCESS FILM BUILD
• IMPROPER MIXING
• IMPROPER THINNER

BRUSH MARKS :

• IMPROPER VISCOSITY
• IMPROPER THINNER
• INADEQUATE MIXING
• NON-ADHERENCE TO POTLIFE

CONCLUSION