PAINT COATING FAILURES:

Transcription

1 PAINT COATING FAILURES: Understanding complex causes Rob Francis R A Francis Consulting Services Ashburton, Vic, Australia

2 Outline 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

3 Causes of coating failures Percentage 100% 80% 60% 40% 20% 0% Year Source: M Weston, JPCL, January Incorrect specification Application error Environment change Faulty paint Can be overlap of causes

4 Design & Fabrication Inaccessible regions difficult to prepare and paint Crevices and weld defects difficult to prepare and coat 4/32

5 Sharp edges Rough welds Porosity Weld spatter Can t blame applicator Importance of good specification, good fabrication, good inspection

6 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

7 Runs & Sags Application error Coating too thick or excess thinner

8 Importance of visual inspection Misses & thin areas Measurement of film thickness (DFT) is a typical specification requirement Application problem but owner not willing to pay for inspection

9 Dew or condensation 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

10 Flash rusting From condensation on a freshly blasted surface Poor adhesion, blistering of coating

11 Epoxy bloom or blush Reaction of epoxy with moisture and CO 2 to produce an oily (blush) or milky (bloom) surface film. Usually caused by low temperature/ high humidity leading to condensation

12 Intercoat Delamination Poor adhesion of top coat due to epoxy bloom from condensation or over-cured mid coat

13 Overcoat Limits Many coatings overcure if left too long before reocating/ overcoating Limits should be in Data Sheet Data sheets rarely account for Australian summer conditions No temperature given but usually temperature dependent

14 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

15 Chalking and Fading

16 Chalking and Fading UV Radiation most important Rainfall also important with chalking

17 Pigments Property 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

18 Mineral pigments Colorbond palette shows stable colours

19 Failure over Galvanizing Coating failure due to build up of zinc corrosion products

20 Mechanism Water, oxygen and CO 2 penetrate film React with zinc forming white corrosion products Coating disbonds from the surface

21 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

22 Investigation Better performance Zinc loss White rust Worse performance Percentage A2 B2 G2 A1 B1 E1 G1 Zinc E2 H2 C2 H1 D2 D1 C1 Acrylic Latex Cating System (1: single coat, 2: two coat) 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,

23 Osmotic blistering

24 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

25 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 clause 4.3

26 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

27 Conclusions Simple application problems Problems with humidity & dew point Problems with overcuring Problems with colour and fading Problems with paint over galvanizing Osmotic blistering