Porosity | EUROLAB

What Is Porosity?

The holiday (porosity) test electrically finds invisible holes, pores and thinned areas in a coating. Low voltage with a wet sponge is used for thin coatings, and a high-voltage spark method for thick coatings. At a discontinuity, current reaches the substrate and the instrument gives an alert.

Purpose of Porosity

The purpose is to verify that the protective coating is continuous. A single pore is enough for corrosion to spread under the entire coating; the test is therefore mandatory for buried pipe and tank coatings.

Which Products Is Porosity Applied To?

Buried and submerged pipe coatings
Storage tank interior and exterior coatings
Concrete surface coatings
Marine and offshore coatings
Coatings in contact with food and drinking water

In Which Sectors Is Porosity Used?

Paint & CoatingMetalAutomotiveConstructionMaritime

Standards Related to Porosity

NACE SP0188 (discontinuity/holiday testing of non-conductive protective coatings)
ASTM D5162 (discontinuity in coatings on metallic substrates)
ASTM G62 (holiday detection in pipeline coatings)
ISO 29601 (porosity assessment of protective paint systems)

Porosity Conditions

Method: Wet sponge for thin coating, high voltage spark for thick coating
Gerilim: From the relation given by the standard according to the coating thickness
Scanning speed: At a speed defined in the standard and over the entire surface
Topraklama: The substrate must be properly grounded

Equipment for Porosity

Low-voltage wet sponge detector
High-voltage holiday detector and brush/coil-spring probe
Coating thickness gauge
Grounding cable and connectors
Marking pen

Sample Requirements for Porosity

The sample must be taken so as to represent the batch or production run.
Sample description, material type, and production direction if any, must be stated.
The standard to be applied and any customer specification must be provided together with the sample.
The test may be destructive; the required number of samples must be confirmed in advance.
Surfaces that will be tested must be protected during transport.

Duration of Porosity

Scanning depends on surface area; a medium-sized tank or pipeline section is generally scanned within one working day.

Porosity Procedure

1Coating thickness is measured and the method decided.
2In the high-voltage method, voltage is calculated according to thickness.
3The substrate is grounded and the instrument verified.
4The entire surface is scanned at the defined speed.
5Points giving an alert are marked.
6Discontinuities are repaired.
7Repaired areas are rescanned and the result is reported.

Porosity Acceptance Criteria

The method standard defines how the test is performed, not what the result must be. The acceptance limit comes from the material or product standard, the customer specification, or the relevant regulation; this page therefore does not give a numerical limit.

Evaluation of Results

Assessment relies on whether a discontinuity is found in the coating. Protective coating specifications usually require zero discontinuities; every point found is repaired and rechecked. Since choosing too high a voltage can puncture a sound coating, the calculation must be made using the standard's relationship.

Porosity Report

Sample description, material/product information, and batch number if any
Sample preparation method and conditioning conditions
Standard and test conditions applied
Instrument used and calibration status
Measurement results and units
Measurement uncertainty (if requested)
A statement that the result applies only to the specimen submitted for testing

Accreditation

EUROLAB holds internationally recognised accreditation obtained from accreditation bodies party to the IAF and ILAC multilateral recognition arrangements, under ISO/IEC 17025 for laboratory testing, ISO/IEC 17020 for inspection and surveillance, and ISO/IEC 17021-1 and ISO/IEC 17065 for system and product certification. This chain ensures the reports and certificates issued are accepted worldwide.

Whether this test can be offered within accredited scope depends on your product and the standard applied; since accreditation is method-based, the scope is confirmed in writing at the quotation stage.

Scope and authorisations

Related Regulations

Harmonised standards applied according to the product family
EU 765/2008 (market surveillance and accreditation framework)
Relevant product regulations and customer specifications

Paint and Coating Testing — What Does This Testing Field Cover?

The section below describes the field that the Porosity test belongs to; the conditions and acceptance criteria of this particular test are given in the sections above. The scope specific to your product is confirmed in writing at the quotation stage.

Paint and coating testing measures the adhesion of a coating to the surface, its mechanical durability and its resistance to environmental effects. Because a coating is a thin layer, the results depend both on the material and on the QUALITY OF APPLICATION; the same paint gives an entirely different result over a different surface preparation.

This dual dependence is the most important knowledge in the field. A coating failure usually arises not from the paint itself but from the surface preparation: oil residue, insufficient roughening, an oxide layer or moisture. Coating tests therefore assess not only the product but also the process, and the results are used for process control.

COATING THICKNESS is a recurring variable in the field. Many durability results relate directly to thickness; a thin area becomes the starting point of corrosion. Thickness measurement is therefore reported routinely alongside the other tests and is usually taken at several points — thickness falls at edges and corners, especially on complex geometries.

Three approaches are used on the adhesion side, each giving different information. The tape test is fast and suitable for field use but is a coarse scale. The cross-cut test shows behaviour at the intersections of layers. The pull-off method produces a numerical strength value and is the most suitable for comparison.

On the mechanical side, flexibility (mandrel bending), impact resistance, hardness, abrasion and stone chip resistance are measured. Stone chipping has a special place in automotive: gravel striking the coating under real road conditions is simulated, and the result is assessed according to whether the coating continues to protect the base metal.

On the appearance side, gloss and colour difference are monitored NUMERICALLY. Colour is assessed with a colour measuring instrument rather than by eye and the colour difference is expressed as a number; this makes it possible to compare different lots and different suppliers.

On the durability side, accelerated ageing (xenon arc, fluorescent UV), humidity, condensation and salt spray are applied. The result is assessed with scales for blistering, rusting, creep from a scribe and loss of adhesion; these scales are defined in separate standards and the report must state which was used.

In industrial processes such as powder coating and electrophoretic coating, the curing cycle (temperature and time) is what determines the result. An under-cured coating looks good but has poor chemical resistance and adhesion; oven profile measurement is therefore part of coating quality control.

Method groups in this field

Thickness
Dry film thickness by magnetic and non-magnetic methods; multi-point measurement and assessment of the distribution.
Adhesion
Tape test, cross-cut test and pull-off adhesion strength.
Mechanical strength
Mandrel bend flexibility, rapid deformation (impact), pencil and scratch hardness, Taber abrasion, stone chipping.
Appearance
Gloss measurement, colour difference calculation, orange peel and surface defect assessment.
Environmental durability
Salt spray, humidity, condensation, xenon arc and fluorescent UV ageing, chemical and stain resistance.
Assessment scales
Scales for blistering, degree of rusting, creep from a scribe and loss of adhesion.

Commonly confused in this field

A coating result depends on the surface preparation; even if the paint itself is good, a poor preparation gives a poor result.
A durability result without a reported thickness cannot be compared.
The tape test is a coarse scale; numerical comparison requires the pull-off method.
Colour cannot be assessed by eye; numerical colour difference measurement is required.
An ageing result without a stated assessment scale cannot be interpreted.
If the curing cycle is incomplete the coating looks good but its durability is poor; the oven profile is part of quality control.

Which decisions these results support

Paint system approval and supplier selection
Process control of the paint line and lot acceptance
Supporting outdoor and automotive durability claims
Detecting defects arising from surface preparation
Monitoring colour and gloss consistency lot by lot

Related Tests

Frequently Asked Questions About Porosity

Does high voltage damage the coating?

It does if the voltage is not correctly calculated for the thickness: excessive voltage punctures a sound coating. Thickness is therefore measured first and voltage set accordingly.

Can it be applied to conductive coatings?

No. The method relies on the coating being insulating and the substrate conductive. Conductive coatings, such as zinc-rich primers, require different approaches.