Improving Coating Performance Continually
ISO 12944 is an international standard detailing the corrosion protection of steel structures by protective coating systems. In the 2018 update, several changes and additions were made. One of the areas in which changes were made was in the detail of lab testing methods, which is covered in Part 6 of the standard.
Why is lab testing necessary?
Lab testing is used to help in the selection of suitable paint systems, and ISO 12944-6 covers protective paint systems designed for application to uncoated steel, hot dip galvanised steel (covered by ISO 1461), and steel surfaces with thermal sprayed metallic coatings (covered by ISO 2063-1 and ISO 2063-2).
Suggested paint specifications to protect carbon steel over a range of durability periods in the corrosivity categories C2 to C5 are laid out in the tables in part 5 of ISO 12944. The suggested specifications within these tables are representative of several commonly used generic paint chemistry types (e.g. alkyd, acrylic, epoxy, polyurethane etc). The film thicknesses within these data tables were devised by an ISO panel of experts, based on their experiences of protective paint systems in the field.
Specifications based on the film thickness quoted in ISO 12944-5 should be verified by undergoing the range of accelerated test regimes stated in ISO 12944 part 6. Any paint system must meet the test requirements in part 6 to be classified as compliant with the standard. External 3rd party testing of these systems is not mandatory, although many paint manufacturers choose to carry out 3rd party accreditation.
The suggested ‘typical’ specifications in ISO 12944 part 5 are not exclusive and do not prevent the use of older technologies that no longer feature in the ‘typical’ systems. Likewise, ISO 12944 does not exclude the use of new and innovative paint systems that do not feature in part 5. Provided that any of these systems are tested to, and pass the requirements stated in part 6, then they can be deemed to be compliant with the standard.
It should be noted that the standard does not apply to paint systems used for electroplated or painted steel.
What does lab testing consider?
The paint system you should use to protect steel structures from corrosion is dependent upon several factors, including the corrosive environment that is covered in Part 2 of ISO 12944. The standard describes the environmental impact of exposure to the atmosphere and when immersed in water or buried in soil. Corrosivity categories are then used to provide a classification system, and this is used as a guide to select suitable coatings systems for the corrosion protection of steel structures.
Water immersed or buried-in-soil structures include:
- Offshore structures
- Oil pipelines
- Buried tanks
When defining corrosive category, the type of environmental exposure is considered, and includes:
- Humidity
- Salt spray
- Water immersion
A paint system will age because of the environment to which it is exposed (as well as other factors such as steel structure design (ISO 12944-3), surface preparation (ISO 12944-4), and the method of application.
Artificial aging tests carried out in a lab assesses the influence of the characteristics of the paint system (for example, dry-film thickness and binder) on the durability of the paint system. These results can then be considered in the determination of which paint system should be used to meet the performance requirements specified by the application.
How does lab testing work?
Each corrosive category is assigned a durability range of low, medium, high, or very high. This durability defines the duration of the exposure to the environmental influence.
Samples of the coatings are subjected to each of the exposures separately, and measurements of coating thickness and adhesive strength are taken before and after exposure. These measurements are then used to calculate the effect of the environment on the coating tested.
Samples are evaluated for:
- Blistering
- Cracking
- Corrosion
- Adhesion
For paint systems that would be exposed to salt spray and cyclic aging, manual scribing of the samples is used to replicate the effect of these environments. Unless otherwise agreed, each test should be carried out on three test panels.
To ensure that test results are accurate, the standard recommends that a reference is used. This reference should be a paint system:
- that has been used successfully on site for several years;
- the performance of which is well established by lab testing; and
- that is as close as possible to the paint system being tested (e.g. composition, type, and thickness)
The standard provides tables of appropriate test procedures and durations.
What changed in ISO 12944-6 in 2018?
The changes made to the standard in 2018 introduced new testing for very high durability (25+years) in the following corrosive categories:
- C2 – low
- C3 – medium
- C4- high (where cyclic testing has been introduced)
In addition, in category C5 (high and very high corrosivity) the updated standard introduced cyclic testing for high and very high durability.
Why has cyclic testing been introduced?
The aim of cyclic testing is to better replicate in-field conditions, and industry has found that this method of testing is more representative of what happens out of the lab, in the real world.
What should a lab test report show?
The lab test report must reference ISO 12944-6, and contain the following information:
- the test laboratory details
- the date of the tests
- the details of the test (what test was carried out, the duration of the test, and the results of each test)
- a description of the substrate and surface preparation
- details required to identify the paint system
- details of the reference system
- the duration and conditions of drying/curing and conditioning
- classification of the paint system after testing
- photographic evidence
- thickness of layers (for example, zinc and thermal-sprayed metallic coating)
- any deviation from the test methods specified
Summing up ISO 12944-6
In summary, ISO 12944-6 details the standards to which lab testing must adhere, how testing should be carried out, and how assessment of results should be made and reported.
The aim is to enable coating inspectors and applicators to continually improve the coating systems used in all environments and for all durability requirements.
To ensure your painters and inspectors are fully up to date with ISO 12944 and all of its parts and are working to the latest industry standards and best practices, the Institute of Corrosion offers Coating and Inspection Training Courses presented by IMechE Argyll Ruane and Corrodere. For more information, contact us today.