In the investigation of coating failures and the characterisation of coating defects the first step is a visual examination, often with the aid of a hand held magnifying lens.  To assist in establishing the cause and mechanism of failure a more detailed examination of paint flakes collected during the investigation is required. A microscope, which is an instrument that enables (through a combination of lenses) the human eye to see enlarged images of small objects is used for this purpose.

Present day light microscopes can provide magnifications of up to 1,500 times. At these high magnifications, however, the depth of field and working distance become very small. Stereo microscopes use two separate optical paths to provide slightly different viewing angles to each eye, resulting in a three-dimensional image. This arrangement provides a greater working distance, a greater depth of field, and a useful magnification of about 100 times. It is this type of microscope that is used routinely to examine paint flakes. When  the stereo microscope is coupled with attachments such as cameras, video cameras and digitizing computers the images observed can be recorded.

The light microscope, irrespective of the quality of the optics and illumination, is restricted (by the wavelength of light) in the size of object that can be examined. To distinguish smaller objects a type of illumination with a smaller wavelength is required. In an electron microscope electrons are accelerated in a vacuum until their wavelength is very short. Beams of these electrons are focused on a sample and an image is formed on an electron sensitive photographic plate. A particular type of electron microscope, the scanning electron microscope (SEM), produces images of the surface of a sample by scanning it with a high energy beam of electrons in a raster pattern. The SEM has a large depth of field and produces a three dimensional image with a wide range of magnifications from 10 times to about 500,000 times. SEM microscopes are used in paint laboratories to provide detailed images  of surface structure. They can, with the use of Energy Dispersive X-Ray analysis (EDX) attachments, also provide information about the elemental composition of the sample.

When the top and bottom surfaces of paint flakes are examined by microscopy for contamination, inclusions and the presence of rust and scale information on the degree of surface preparation can be obtained. When the paint flakes are examined in cross section the number of coats, their colour and individual coating thicknesses can be determined. Voids, vacuoles and the penetration of rust staining, if present, can also be seen.

Examples of the above defects and others are shoen on the following pages.

Blisters

Description: Dome shaped projections or blisters in the dry paint film through local loss of adhesion and lifting of the film from the underlying surface. Many mechanisms can be involved including osmotic gradients associated with soluble salts, soluble pigments, corrosion products, retained solvents and solvents from cargoes. Non-osmotic blistering associated with electro endosmosis, cathodic disbonding, thermal gradients related to cold wall effects and compressive stress.

Bubbles

Description: Bubbles within a paint film appear as small blisters and can be found in excessively thick paint films where trapped air or solvent within the coating is not released before the surface dries.

Cohesive Failure

Description: A failure within a single coating layer. This failure mode is often seen with inorganic zinc coatings whose cohesive strength is poor and with coatings applied at high dry film thicknesses.

Contamination

Description: The presence of dirt, salts, rust, grinding dust etc. on the steel surface, or the painted surface that can lead to poor adhesion and coating failure.

Cracking

Description: The splitting of a dry paint film through at least one coat to form visible cracks which may penetrate down to the substrate. Cracking is generally a stress related failure and can be attributed to surface movement, ageing, absorption and desorption of moisture and general lack of flexibility of the coating. The thicker the paint film the greater the possibility it will crack.

Delamination

Description: Loss of adhesion between coats of paint (Intercoat adhesion failure) generally related to poor surface preparation, contamination between coats; exceeding overcoat times, condensation, application to a glossy surface.

Edge Coverage - Poor Edge Coverage

Description: Coatings have a natural tendency to flow away from edges neccessitating the use of stripe coats to achieve the specified film thickness. They can however be formulated to achieve good edge coverage as illustrated above.

Exudation

Description: The appearance of an oil-like film on the surface of the coating after drying. Migration of a component of the coating e.g. plasticizer, additive, or curing agent that remained unreacted. Also seen within the coating due to incompatibility between two or more raw materials.

Inclusions

Description: Particles of grit and dust embedded within the coating system or on the surface often from contamination of the wet primer or undercoat with grit from other blast cleaning operations.

Incompatibility - Tar (Black)  Resin (Yellow)

Description: Incompatibility between two or more raw materials resulting in separation and a non-homogeneous film.

Multi-Coats

Description: The presence of multiple layers of paint that build up over time when existing systems are continually overcoated during years of maintenance.

Non-Coalesced Film

Description: After application of emulsion paint, the water evaporates and the resin particles (the binder) coalesce to form a continuous film that will not redissolve in the solvent/water that originally carried it. If the coating is applied at too low temperature coalescence may be hindered and a non-coalesced film produced.

Pinholes

Description: The formation of minute holes in the wet paint film during application and drying, due to air or gas bubbles which burst, giving rise to small craters or holes which fail to coalesce before the film has set.

Scale

Description: Formation of layers of rust scale that expand and increase in volume beneath the coating often by penetration of moisture to the steel substrate through defects in the coating system. Can also be the result of painting over a surface without removing the existing scale.

Staining

Description: Discolouration of a coating system generally from contact with a solid or liquid that imparts a discolouration or stain to the coating. The staining may only be present on the surface or may penetrate into the coating.

Surface Profile - Typical Surface Profile

Description: The roughness (anchor pattern) imparted to the steel substrate by blast cleaning. Depending on the abrasive type and size the peak to valley height and the peak density can vary. Rogue peaks can be produced that are larger than the average profile and can lead to rust spotting. The photomicrographs of the surface profile are from a scanning electron microscope at approximately 1,000 times magnification.

Underfilm Corrosion

Description: The presence of rust beneath the coating system that could be from application to a corroded substrate, rust creep from areas of mechanical damage or penetration of moisture to the steel substrate through defects in the coating system.

Undispersed Pigment

Description: Pigment particles that have not been homogeneously dispersed in the binder.

Voids / Vacuoles

Description: Bubbles within the coating often related to air entrainment during mixing, solvent balance, over thickness, application in hot conditions where the surface may dry preferentially and application over a porous substrate.  They can vary considerably in size and extent and if excessive can make the coating more permeable and reduce performance life.

Weed/Algae Fouling

Description: Green algae (enteromorpha) commonly known as grass is found as fouling on the vertical sides of a vessel hull where there is a high light intensity. It will attach to the paint system on the hull when the biocide activity of the anti-fouling coating is too low or the vessel has been stationary for long periods and its presence can have a deleterious effect on the vessels speed and fuel consumption.