Current Techniques in Failure AnalysisCurrent Techniques in Failure Analysis

bybyDwight G. WeldonDwight G. Weldon

WeldonWeldon LaboratoriesLaboratories

Causes of FailureCauses of Failure

Bad application Defective coating Inappropriate specification Unanticipated environmental excursion

Begin at the microscopeBegin at the microscope

Many coating failures are due to simple things, such as the paint being too thick or too thin, or applied over dirt.

A stereo microscope with magnification up to 30-40X is invaluable. If it doesnt solve the problem, it may at least give clues as to what may have happened.

Some time at the microscope might save many hours of expensive laboratory work.

DryDry--sprayed zincsprayed zinc--richrich

Too much aggregateToo much aggregate

Zinc primer Zinc primer insufficient profileinsufficient profile

Analytical TechniquesAnalytical Techniques

Or, when the microscope isnOr, when the microscope isnt t enough!enough!

Some common analytical Some common analytical techniques include:techniques include:

Infrared Spectroscopy Gas Chromatography (GC and/or GCMS) Scanning Electron Microscopy Energy

Dispersive X-ray Spectroscopy (SEM-EDS)

Infrared SpectroscopyInfrared Spectroscopy

Identifies a sample based on its absorption of infrared light at various frequencies.

An extremely useful tool. It is often the technique of choice following the microscopic observations.

Infrared SpectroscopyInfrared Spectroscopy--How it worksHow it works

The molecules making up a sample are in a constant state of motion, and vibrate at various frequencies.

These frequencies depend on the type of atoms, and the type of chemical bonds holding them together. For example, a hydrogen atom single-bonded to a carbon atom will have a stretching vibration near 2900 cm-1, and an oxygen atom double bonded to a carbon atom will have a vibration in the 1650-1750 cm-1 region.

Example of Infrared Vibrational Example of Infrared Vibrational ModesModes

Infrared Spectroscopy Infrared Spectroscopy -- How it worksHow it worksContinuedContinued

The frequency of light which corresponds to these molecular vibrations occurs in the infrared region of the electromagnetic spectrum.

When infrared light is focused on or through a sample, the sample will absorb those frequencies of light which match the frequencies of its internal molecular vibrations. This results in a spectrum.

Infrared Spectroscopy Infrared Spectroscopy -- How it worksHow it worksContinuedContinued

The infrared spectrum is highly characteristic of the type of molecules making up the sample. Because different molecules have their own, unique infrared spectrum, the spectrum is sometimes referred to as a fingerprint.

Uses of Infrared SpectroscopyUses of Infrared Spectroscopy

Identification of coating type Identification of some pigments Determination of degree of cure for some

coating types (inorganic zinc-rich primer, urethanes, possibly others).

Determination of mix ratio for multi-component coatings

Contamination Blushes/exudates Batching variations

IR spectra of generic coating typesIR spectra of generic coating types

Degree of cure of urethaneDegree of cure of urethane

Mix ratio of epoxy coatingMix ratio of epoxy coating

Limitations of Infrared Limitations of Infrared SpectroscopySpectroscopy

Usually qualitative, not quantitative. Limited information on inorganic materials. Cannot identify minor (

Gas Chromatography (GC)Gas Chromatography (GC)

Analyzes volatile and semi-volatile materials by using a gas to flush them through a special column at an elevated temperature. A fancy, expensive oven.

GC: How it worksGC: How it works

The column is kept at an elevated temperature, with a flow of gas (typically helium) running through it. The end of the column is connected to a sensitive detector.

A syringe is used to inject a small amount of sample (either a liquid or a gas) into the column.

Since the components of interest are volatile or semi-volatile (solvents and monomers), their vapors will be swept through the column by the helium carrier gas.

GC: How it worksGC: How it worksContinuedContinued

The inside of the column is either packed with or lined with a stationary phase. One common type is a high-boiling modified silicone.

Different solvents will have either a greater or a lesser attraction to the stationary phase inside the column. Those which have the least attraction will go through the column the fastest.

When the different solvents pass through the column they will be detected by the detector, resulting in a series of peaks eluting at different times. This is called a chromatogram.

GC: How it worksGC: How it worksContinuedContinued

The solvents can be identified (sometimes with difficulty) by the time in which it took them to exit the column (retention time), or by using a mass spectrometer as a detector (GC-MS).

The height or area of a peak is proportional to the amount of that particular solvent, allowing quantitative analysis to also be performed.

GCGC

Example of gas chromatogramExample of gas chromatogram

Uses of Gas ChromatographyUses of Gas Chromatography

Detect and identify solvents and monomers at ppm (sometimes ppb) levels.

Detect residual amounts of solvents in dry paint chips (via solvent extraction or analysis).

Analyze the liquid trapped inside blistered paint for solvents.

Limitations of Gas ChromatographyLimitations of Gas Chromatography

Compounds must be volatile or semi-volatile.

Dry paint chips for residual solvent analysis should not be too old.

Identification of unknowns based solely on retention time can sometimes be difficult. This problem can be overcome with a mass spectroscopy detector.

Scanning Electron Microscopy Scanning Electron Microscopy Energy Energy Dispersive XDispersive X--ray Spectroscopy (SEMray Spectroscopy (SEM--EDS)EDS)

An SEM provides high magnification (1000X and above) images with good depth of field. It allows one to see features not observable with a stereo zoom microscope.

When interfaced with an EDS detector, the elemental composition of very small objects can be determined.

SEM image of a zincSEM image of a zinc--rich primerrich primer

SEM image of crystal growth inside SEM image of crystal growth inside a coating void due to permeation of a coating void due to permeation of

tank contentstank contents

SEMSEM--EDS: How it worksEDS: How it works

An electron gun, typically consisting of a tungsten filament and associated apparatus, produces a beam of high energy electrons.

Electromagnetic lenses are used to focus this beam of electrons on a small sample mounted in an evacuated chamber.

The collision of electrons with the surface of the sample creates scattered electrons, which are collected and result in the magnified image.

SEMSEM--EDS: How it worksEDS: How it worksContinuedContinued

When the beam of energetic electrons collides with the atoms making up the sample, in addition to producing scattered electrons for imaging, x-rays are also generated.

Different chemical elements produce x-rays of differing energies, which allows the elements to be identified.

Elements can be rapidly and simultaneously detected. Normal detectors can detect any element of atomic

number 11 (sodium) and higher. Light element detectors can go down to atomic number 6 (carbon).

Example of SEMExample of SEM--EDS spectrumEDS spectrum

Applications of SEMApplications of SEM--EDSEDS The examination of samples at several hundred

times magnification and higher allows one to see inclusions, residues, phase separations, etc.

Not only can the elemental composition of the sample be determined, but the composition of inclusions, residues, and other small particles can be determined as well.

Determine the composition of a failing surface (identifying the failure plane).

Detecting salts or other contaminants in corrosion products.

Limitations of SEMLimitations of SEM--EDSEDS

Identifies elements, not compounds. Very limited ability to detect organic

compounds. Relatively poor detection limits (about

0.1%). Elemental analysis is only semi-

quantitative.

Solving Failures Solving Failures

Sometimes coatings fail for very simple reasons; sometimes they fail for very complicated or subtle reasons.

However, with good background information and proper samples, most coating failures can be solved.