The following, details basic causes of Coating failure.  It also provides some tips on preventing failures as well as some basics.  For more information on Inspection Procedures to prevent failures, See Inspection Procedures. While these guidelines are presented, they should be performed by a trained inspector or consultant. NOTE: Just because someone is a Certified NACE Coating Inspector does NOT make him qualified to do Failure Analysis.  If you do not have one available, you may contact CCI Inspection Services at (281) 367-6740.

Three Major Causes of Premature Coating Failure

POTENTIAL CAUSES OF COATINGS FAILURES

FAILURE ANALYSIS

• Physical Tests

Common Causes

·      Surface preparation

o     Lack of Pre-cleaning (SSPC SP-1)

o     Insufficient Blast

o     Insufficient Profile

o     Surface Contamination

§      Abrasive dust

§      Non-Visible Contaminants (Salts)

·      Application

o     Poor or improper mixing

o     Insufficient (or too much) milage

o     Wrong coating for service

o     Exceeding recoat Window

o     Atmospheric Conditions

o    Poor Application Procedures

·     

Lack of Inspection - Oversight

Basic Instrumentation

·      Hand Held Microscope

·      Paint Inspectors Gage (Tooke)

·      Moisture Meter

·      pH Pencil/Paper

·      Surface Thermometer

·      WFT Gage

Hand Held Microscope

·      Can be extremely beneficial in assisting the naked eye during inspection procedures.

·      They are usually found in 5x, 10x, 30x, and 100x rates of magnification.

·      The 30x, lighted version with thumb wheel focus is the most useful.

·      They are inexpensive and most helpful in examining cracks, micro-blisters, contamination, pinholes, and pinpoint rust, etc., as well as the back of delaminated films for wood fiber, particulates, dirt, mildew, tannin, etc.

Moisture Meters

·      are essential in determining the moisture content of porous substrates, typically wood and masonry. Excessive levels of moisture affect both initial and long-term adhesion. The instrument is invaluable in ascertaining if a substrate can be coated, or why an existing coating failed. The accepted level of moisture content for wood is 7-14%, Masonry is 10-25%.

PH Pencil / pH Paper /pH Meter

·      Is used to determine the pH of a surface to be coated, or where a failure has occurred.

·      Conventional Coatings typically prefer a PH in the range of 7.0 to 9.0.

·      The PH of concrete is approx. 12.5, properly etched is 9.0, 5.0 would indicate excessive acidity.

·      The pH pencil or pH Paper can be used on bare surfaces, such as galvanized

·      metal or wood, or coatings that

·      have been subjected to

·      chemical contamination.

Surface Temp. Thermometer

WFT Gauge

Destructive Coating Thickness

Tooke gage

• Measurements of individual layers in a multi-coat system
• Rarely used during in-process coatings inspection

Chemical Test Solutions

• “Goof-Off”
• Epoxy Thinner
• MEK
• Solvent Emulsion Solution
• 2% Copper Sulfate Solution
• 1:3 Muriatic Acid Solution
• Sodium Hydroxide

“Goof-Off”

• Commercially available blend of solvents that is designed to dissolve latex based paint, leaving alkyd formulations and other resin systems untouched. It easily ends the question
• “ is this latex or oil-based paint on this surface? ”
• Rubbing Alcohol is equally effective.

Epoxy Thinner

• is utilized to determine if the existing coating is an alkyd. The blend of solvents will easily, and quickly, soften most alkyd formulations. They will also produce a reaction, albeit slower, with some epoxies.

MEK

• is useful in the fact that it will dissolve most coatings except or those urethanes that are recommended specifically for “anti-graffiti” purposes.

Solvent Emulsion Solution

• Is another term for a highly concentrated detergent, normally reduced at 1:5-10 ratio with water. The solution is best applied by low pressure spray and allowed to stand for a specific period of time. The detergent emulsifies dirt, grease, oil, etc., making. it water soluble. Solvent emulsion cleaning is superior to solvent wiping (SP-1) in that high pressure water, in large volumes, is utilized as the “solvent.” Conventional SP-1 techniques are not nearly as effective and can cause environmental problems

2% Copper Sulfate

• solution is utilized in testing for the presence of a passivation layer or stabilizer on galvanized surfaces.

Sodium Hypochlorite

• is the chemical term for bleach. It is an excellent indicator in differentiating mildew from dirt. Mildew will bubble when subjected to sodium hypoclorite. Dirt doesn’t. Bleach does deteriorate with age, keep a fresh supply

Physical Tests

ADHESION
– ASTM D 3359
– ASTM D 4541

ASTM D 3359 - Standard Test Method for Measuring Adhesion by Tape Test

METHOD A - utilizes an X-cut and is most suited for field use.

METHOD B - utilizes a cross-cross pattern series of cuts, is most suited for lab use, and is considered unsuitable for films thicker then 5.0 mils DFT.

ASTM D 3359

• To perform Method A use a sharp blade, and metal straight-edge, to scribe an X-cut in the surface through to the previous layer.
• The cuts should be 1.5 inches long with a smaller angle of 30-45 degrees.
• Check the depth with a lighted microscope.
• Remove 2 laps from a roll of 1 inch semi-transparent, fiber re-inforced tape. Remove another 3-5 inch length of tape.
• After 60 to 120 seconds rapidly remove (no jerking ) the tape at an angle of 180 deg. to itself.

Observe as indicated by the following scale:

ASTM D 3359 method A

• 5A: No peeling or removal.
• 4A: Trace peeling or removal along the incisions or at the intersection.
• 3A: Jagged removal along incisions up to 1/16 th inch on either side.
• 2A: Jagged removal along incisions up to 1/8 th inch on either side.
• 1A: Removal from most of the area of the X under the tape.
• 0A: Removal beyond the area of the X.

ASTM D 4541 - Tensile Adhesion Testing