What Is Corrosion?
Coating corrosion testing is performed in which scratched and unscratched coated panels are exposed to salt fog or cyclic corrosion environment; This is the experimental group where it is evaluated in terms of blistering, rust and undercutting.
Purpose of Corrosion
The aim is to verify that the coating system provides the required performance for the targeted corrosion category and durability range.
Which Products Is Corrosion Applied To?
- Steel structure protective coating systems
- Automotive paint systems
- Marine and coastal structures coatings
- Galvanized paint systems
In Which Sectors Is Corrosion Used?
Standards Related to Corrosion
- ISO 12944-6
- ISO 12944-9
- ISO 9227 (tuzlu sis)
- ASTM D1654
- ISO 4628 serisi (derecelendirme)
Corrosion Conditions
- Environment
- Continuous salt fog (ISO 9227 NSS) or cyclic corrosion (ISO 12944-9 / ASTM D5894)
- Time
- Defined in ISO 12944-6 according to corrosion category and resistance range
- Scratch
- Artificial scratches defined in the standard appear on the panel.
- Assessment
- Propagation of blistering, rust, cracking and scratching according to ISO 4628
Equipment for Corrosion
- Salt fog and cyclic corrosion chambers
- scratch removal kit
- ISO 4628 rating tables
- Adhesion test kits
Sample Requirements for Corrosion
- Coating system and layer thicknesses must comply with the specifications
- Panel edges and back side must be protected
- The coating must be fully cured
- Thickness and appearance should be recorded before testing.
Duration of Corrosion
2 weeks to 6 months depending on duration.
Corrosion Procedure
- 1The coating system and target corrosion category are determined.
- 2Panel thicknesses are measured and scratches are made.
- 3Panel edges are protected.
- 4Exposure is applied in a defined environment and time.
- 5The panel is rated according to ISO 4628.
- 6The spread from the scratch is measured and adhesion is retested.
Corrosion Acceptance Criteria
The acceptance criteria are the swelling, rust and spreading limits given according to the corrosion category and durability range in ISO 12944-6.
Evaluation of Results
The duration of continuous salt fog cannot be translated into service life and modern specifications consider cyclic corrosion tests to be more representative. If panel edges are not protected, corrosion starting from the edge will invalidate the result.
Corrosion Report
- Applied standards
- Coating system and layer thicknesses
- Hedef korozyon kategorisi
- Environment and duration
- ISO 4628 ratings and scratch propagation
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 authorisationsRelated Regulations
- ISO 12944 corrosion protection specifications
- Automotive OEM specifications
Paint and Coating Testing — What Does This Testing Field Cover?
The section below describes the field that the Corrosion 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 Corrosion
How many hours of salt fog correspond to which lifespan?
There is no direct cycle. ISO 12944-6 defines test periods for specific corrosion categories and durability ranges; These times are criteria for system selection, not an estimate of field life.