AEP-61 performance requirements for underwater hull paint systems

What it is

AEP-61 is the NATO Allied Engineering Publication that sets out how an underwater hull paint system (UHPS) is qualified. It is a paint manufacturer's specification, not a company's: the document addresses the manufacturer who submits a coating system for testing and the navy "qualifying activity" that runs the test programme and decides whether the system qualifies. A UHPS in this document means the anticorrosive base coat or coats applied to the hull, an optional tie-coat, and the antifouling topcoat that resists marine biofouling such as barnacles and tubeworms.

What the paint system has to achieve

The document fixes two service-life targets. Anticorrosive protection has to run at least 15 years in seawater-immersion corrosion category Im2 of ISO 12944-2. Antifouling protection is banded into short-life (2-3 years), intermediate-life (4-7 years) or long-life (8-12 years) classes, chosen according to the vessel's operating rate and speed profile. Within that service life, general topcoat re-application and localized repair are treated as ordinary minor maintenance rather than a failure of the system.

Basic product requirements follow from those targets. The anticorrosive coating has to use two or more coats, in colours distinguishable from each other and from the antifouling topcoat, at a minimum nominal dry film thickness of 250 micrometres excluding any zinc-rich primer or tie-coat. Application conditions are fixed at 85% relative humidity or below, a substrate temperature at least 3°C above the dew point, and an application temperature between 5°C and 35°C. Shelf life has to be at least 18 months for the anticorrosive paint and 12 months for the antifouling paint, and the system has to demonstrate cathodic-protection compatibility.

The qualification test programme

Qualification runs through laboratory tests, natural-exposure tests, and ship trials, each set out in a table with its own duration and standard. Laboratory work includes 1,440 hours of salt spray, 720 hours of water condensation, 4,200 hours of sea-water immersion, re-coatability and repairability testing, and six months of cathodic-protection compatibility testing against an initial 6 mm holiday. Natural-exposure testing immerses panels for one to several years depending on the antifouling type and the service-life class being sought. Ship trials move from hull patches, at least 2 metres wide and run for two to five years depending on vessel type and service-life class, to full-hull application. For trials on non-navy vessels, the document requires the work to be certified and verified by an independent third party the navy activity and the manufacturer agree on together.

Acceptance criteria

Each test carries a numeric pass mark. Pull-off adhesion after exposure has to retain 50% of its original value with no adhesive failure, against thresholds of 4 MPa (or 3 MPa where the system uses a zinc-rich primer) before testing. Scribe-line delamination has to stay under 3 millimetres. No rusting, blistering, cracking or peeling is permitted after most tests. Biofouling acceptance depends on the antifouling type: under 10% colonised surface for static immersion, no more than 25% fouled area for dynamic-flow exposure, and 5% (or 15% for non-biocide systems) fouled area after a ship trial's high-pressure fresh-water wash. Cathodic disbondment has to produce a holiday diameter under 20 millimetres. Each component paint is also identified against a fixed schedule of physicochemical tests: density, solids content, pigment content, drying time, flash point, pot life, VOC content, and biocide release rate and content.

Biocides and environmental limits

AEP-61 states plainly that application of paint containing tributyltin (TBT) "is now banned internationally", and that vessels already carrying TBT-containing antifouling coatings would need those coatings removed or sealed by the end of 2007. Separately, its environmental-impact requirements for qualification state that organotin biocides "are prohibited from use as antifouling agents" in the antifouling paint submitted. Antifouling paints are classified by biocide content into four bands, from high-release-rate copper systems through to biocide-free systems, and by mechanism into ablative/self-polishing, diffusion, or non-releasing types. Beyond these classifications and the organotin ban, the document does not set its own numeric limit on biocide release rate; where a release rate matters, it is a value the manufacturer reports against ISO 15181, not a threshold AEP-61 sets.

How it sits alongside AEP-59

AEP-61 states that a UHPS qualified under it is intended to be applied in accordance with AEP-59 (Application Process for Optimum Paint and Coating Systems' Performance) to reach its intended service life: AEP-61 sets the qualification bar for the paint system, AEP-59 governs how it is applied. The specification is covered by STANAG 4698, which is the agreement by which NATO nations commit to it. Its normative references also draw in around fifty ISO and ASTM test methods, among them ISO 12944 (corrosion protection of steel structures), ISO 20340 (offshore protective paint systems), ISO 4624 (pull-off adhesion), ISO 4628 (coating degradation assessment) and ASTM D 6990 (biofouling resistance).

What AEP-61 does not cover

The document sets no NATO-wide qualified-products list, accreditation scheme or certificate: it describes a navy qualifying activity's own test-and-decide process, not a body that certifies against it. It sets no obligation of its own on who has to use it or when; that is a matter for whatever tender, contract or national implementation invokes it.

Standards it references

• AEP-59Binds