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PAINT COATING FAILURES:
Understanding complex causes
Rob FrancisR A Francis Consulting Services
Ashburton, Vic, Australia
• There are over 50 types of paint failures
• Only cover a few• Example application problems• Problems with humidity & dew point• Problems due overcuring• Problems with colour and appearance• Problems with paint over galvanizing• Osmotic blistering
Outline
Causes of coating failures
0%
20%
40%
60%
80%
100%
1993 2000
2 211 11
68
46
19
41
Perc
enta
ge
Year
Incorrectspecification
Applicationerror
Environmentchange
Faulty paint
Source: M Weston, JPCL, January 2000 Can be overlap of causes
4/32
Design & FabricationInaccessible regions difficult to prepare and paint
Crevices and weld defects difficult to prepare and coat
Sharp edges
Rough welds
Weld spatterPorosity
Can’t blame applicator Importance of good specification,
good fabrication, good inspection
Application Problems• Can range from mainly aesthetic
issues– Runs & sags
• Through to critical durability issues– Misses & thin areas
• Usually obvious with thorough visual inspection and thickness measurement
Usually fault of applicator
Importance of good QC/ Inspection
Runs & SagsApplication error
Coating too thick or excess thinner
Misses & thin areas
Measurement of film thickness (DFT) is a typical specification requirement
Application problem but owner not willing to pay for inspection
Importance of visual inspection
• Arises when temperature drops below the Dew Point
• Dew Point depends on relative humidity• When RH is high, Dew Pt is only 1 or 2 degrees
below air temperature• Problems
• On blasted surface: → Flash rusting• On uncured paint: → Blush, bloom, spots• On cured paint: Permeates film → Blistering
• Usually Application/ Inspection issue
Dew or condensation
Flash rusting
From condensation on a freshly blasted surface→ Poor adhesion, blistering of coating
Epoxy bloom or blush
• Reaction of epoxy with moisture and CO2 to produce an oily (blush) or milky (bloom) surface film.
• Usually caused by low temperature/ high humidity leading to condensation
Intercoat DelaminationPoor adhesion of top coat due to epoxy bloom from condensation or over-cured mid coat
Overcoat Limits
No temperature given but usually temperature dependent
Many coatings overcure if left too long before reocating/ overcoating
Limits should be in Data SheetData sheets rarely
account for Australian summer conditions
Problems with colour• Usually specification problem• Owners often expect colour and gloss to be
retained for coating life• Often aesthetic (surface) issue
– Will usually still provide protection• UV light causes breakdown of binder and/or
pigment• Chalking from binder breakdown
– Can be more severe with heavy rain• Fading from pigment breakdown
– Red, yellow, orange show most problems– Metal/ mineral pigments more stable
Chalking and Fading
Chalking and FadingUV Radiation most
important
Rainfall also important with chalking
PigmentsProperty Organic Inorganic
Colour brilliance & clarity UV light stability Heat stability Opacity Cost
Colour brilliance of organic pigments means they are widely used despite
poor UV resistance
Mineral pigments
Colorbond palette shows stable colours
Failure over GalvanizingCoating failure due to build up of zinc
corrosion products
Mechanism
• Water, oxygen and CO2 penetrate film• React with zinc forming white corrosion products• Coating disbonds from the surface
Common Causes• Surface preparation Chromate from HDG quench bath Surface has oil, grease, contamination Surface not roughened by whip blast Surface not coated immediately
• Wrong coating Alkyd coating suffers saponification
• But failure can still occur Best not to specify paint on galvanizing
0
20
40
60
80
100
A2 B2 G2 A1 B1 E1 G1 Zinc E2 H2 C2 H1 D2 D1 C1
Perc
enta
ge
Acrylic Latex Cating System (1: single coat, 2: two coat)
Zinc loss
White rust
Investigation
• Acrylic latex coatings over HDG• Exposed to severe marine environment (NZ)• Half systems perform better than HDG alone• Half systems perform worse than HDG alone
Christian & Thomson, ACA Conference 21, Brisbane,1981, p39,
Better performance Worse performance
Osmotic blistering
• Requires water and soluble species (with concentration gradient) separated by permeable membrane (paint film)
• Soluble species draws water through coating to attempt to equalise concentration on both sides of coating creating blister
Osmotic blistering
Osmotic blistering factors• Osmotic gradient
– Deionised water/ condensate worst– Fresh water bad– Salt water OK– Dry no problem
• Soluble species– Salts (or soluble pigments or hydrophilic solvents)
• Coating type and thickness– All coatings permeable to some degree– Less permeable better– Thicker coating better– Inorganic zinc silicate absorbs salts
Source: AS/NZS 2312.1 clause 4.3
Osmotic blistering causes• Major risk with
– Heavy condensation/ fresh/ demineralized water– Old corroded steel not properly cleaned– Deck cargo steel
• Should specify for critical applications:– White metal (Sa3) blast – Water wash– Low chloride level– Thick, impermeable coating
• Simple application problems• Problems with humidity & dew point• Problems with overcuring• Problems with colour and fading• Problems with paint over galvanizing• Osmotic blistering
Conclusions
PAINT COATING FAILURES:OutlineCauses of coating failuresDesign & FabricationSlide Number 5Application ProblemsRuns & SagsMisses & thin areasDew or condensationFlash rustingEpoxy bloom or blushIntercoat DelaminationOvercoat LimitsProblems with colourChalking and FadingChalking and FadingPigmentsMineral pigmentsFailure over GalvanizingMechanismCommon CausesInvestigationOsmotic blisteringOsmotic blisteringOsmotic blistering factorsOsmotic blistering causesConclusions
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