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