Thermal spray aluminium is not a premium paint. It is a metallic coating bonded by impact, and it gets specified where a conventional system has no realistic life and the asset is somewhere nobody will return to easily.
A Different Mechanism, Not a Better Paint
A paint system adheres through a combination of mechanical key into the blast profile and chemical interaction at the interface. Thermal spray aluminium has only the first. Molten or semi-molten aluminium particles are propelled at the steel, flatten on impact, and interlock with the profile. There is no chemistry holding it on.
Everything demanding about TSA follows from that single fact. A paint on a marginally imperfect surface may still perform, because chemical adhesion compensates for a weak mechanical key. TSA on a marginally imperfect surface has nothing to compensate with. Contamination, an oxide film, insufficient profile or the wrong profile shape each remove bond directly rather than reducing it gradually.
Paint has two ways to hold on and can survive losing one. Thermal spray aluminium has one, which is why its preparation requirements are not a stricter version of painting practice but a different proposition.
The second consequence is protective rather than adhesive. Aluminium is anodic to steel in seawater, so a TSA coating does not merely act as a barrier. At a scratch, a holiday or a cut edge, the aluminium corrodes preferentially and protects the exposed steel galvanically. A damaged paint film exposes steel to attack; a damaged TSA coating keeps defending the damage. For an asset where inspection and repair are expensive, that difference in failure behaviour is most of the business case.
Two Standards, Two Audiences
ISO 2063 splits the subject in a way that matters for how a project is set up, and the split is often missed.
Part 1, ISO 2063-1:2019, second edition published February 2019, is Design considerations and quality requirements for corrosion protection systems. Its introduction states that it targets designers of components, and it covers planning the corrosion protection system and the constructive design of the components being protected.
Part 2, ISO 2063-2:2017, first edition published September 2017, is Execution of corrosion protection systems. Its introduction states that it targets manufacturers, and it covers carrying out thermal spray corrosion protection in the workshop and on site.
Where the requirements sit in Part 1: coating thickness at clause 5.2.2 with recommended values in Annex C, preparation of the surface to be coated at clause 6.3, sealing of thermal-sprayed coatings at clause 6.5, and thickness testing at clause 6.7.3.
Where they sit in Part 2: surface preparation at clause 6.2, subdivided into masking, blasting and testing of the prepared surface, with further guidance in Annex H; sealing at clause 6.4 and, for on-site work, clause 9.4.6, with Annex I; roughness at 7.4.2 and adhesion strength at 7.4.3.
Adhesion is tested by pull-off. Annex A of Part 2 is normative and specifies adhesion testing using the pull-off test in accordance with ISO 4624, the same method used for paint systems.
The structural point is that design decisions and execution requirements live in different documents aimed at different parties, which is exactly the interface where cathodic protection and coating design come apart. A specification that cites “ISO 2063” without the part has named a two-document family and pointed at neither.
Preparation, and Why the Window Is Short
Part 2 subdivides surface preparation into masking, blasting and testing of the prepared surface, and that third step is the one that distinguishes TSA practice from painting practice.
The prepared surface has to be verified before spraying begins, not merely achieved. Because the bond is purely mechanical, the profile has to be angular and of the character the process requires, which constrains abrasive selection in ways painting does not. Round or worn abrasive produces a peened rather than cut surface, and a peened surface gives poor mechanical key however clean it looks.
Timing then matters more than it does for paint. Blasted steel begins to oxidise immediately, and in a Malaysian coastal environment, with high humidity and airborne chloride, it does so quickly. Flash rust that a painter might accept as light is a bond defect for TSA. The practical consequence is that blasting and spraying are scheduled as a continuous operation on a worked face rather than as separate activities with the surface left open between them, and that weather contingency is planned rather than improvised.
For paint, a blasted surface left overnight is a judgement call. For thermal spray aluminium it is a prepared surface that no longer exists.
Sealing Is Part of the Coating, Not a Finish
An as-sprayed TSA coating is not a solid metal film. It is built from flattened particles, and it contains interconnected porosity as a consequence of how it forms. ISO 2063 treats sealing as its own requirement in both parts, at clause 6.5 of Part 1 and clauses 6.4 and 9.4.6 of Part 2 with Annex I, which tells you it is a design decision as well as an execution step.
The reason it matters is that porosity gives electrolyte a path toward the steel. Sealing fills those pores and completes the barrier, so an unsealed coating is an unfinished coating rather than a coating that has merely skipped its cosmetic layer. Where TSA is specified and then handed over unsealed, what has been delivered is not the system the design assumed.
The same applies to the colour or topcoat layers some specifications add above the seal. They sit within the qualified system or they do not, and a topcoat chosen for appearance after the fact can behave differently in service from the one the specification assumed.
Where It Earns Its Cost
TSA is more expensive to apply than a paint system, requires a more capable applicator, and is less forgiving of site conditions. It is specified where those costs are smaller than the alternative.
The last item is the one that rarely reaches the capital cost comparison. A maintenance recoating offshore is not only materials and labour. It is scaffolding or rope access, a vessel or a shutdown window, permits, confined space provision where applicable, and people working at height over water. An asset that needs three fewer campaigns over its life has avoided three sets of that exposure, and the only place that shows up is a whole-life calculation nobody ran at tender.
Where a splash zone member, a hot line or a hard-to-reach structure is on the scope, it is worth evaluating TSA against the recoating cycle rather than against the initial price of paint. Our industrial painting and coating service covers both routes, and the materials supplied under it carry approvals appropriate to the service specified.
Frequently Asked Questions
Why is surface preparation stricter for thermal spray aluminium?
Because the bond is purely mechanical. Molten aluminium particles flatten on impact and interlock with the blast profile, with no chemical adhesion at the interface. A paint system can tolerate a marginal surface because chemistry compensates for a weak key. TSA cannot, so contamination, oxide, insufficient profile or the wrong profile shape remove bond directly. ISO 2063-2 treats testing of the prepared surface as a distinct step within clause 6.2.
Is sealing optional?
No. An as-sprayed coating is built from flattened particles and contains interconnected porosity that gives electrolyte a path toward the steel. Sealing fills that porosity and completes the system. ISO 2063 treats it as a requirement in both parts, at clause 6.5 of Part 1 and clauses 6.4 and 9.4.6 of Part 2, so an unsealed coating is unfinished rather than merely uncoated on top.
Which part of ISO 2063 applies to me?
Part 1, ISO 2063-1:2019, is aimed at designers and covers design considerations and quality requirements, including coating thickness, preparation, sealing and thickness testing. Part 2, ISO 2063-2:2017, is aimed at manufacturers and covers execution in the workshop and on site, including surface preparation, roughness, sealing and adhesion strength. A specification citing ISO 2063 without the part has pointed at neither.
How is TSA adhesion verified?
By pull-off testing. Annex A of ISO 2063-2 is normative and specifies adhesion testing using the pull-off test in accordance with ISO 4624, the same method applied to paint systems. As with paint, the failure mode matters alongside the number, since failure at the substrate interface points at preparation while failure within the coating points elsewhere.
Sources: ISO 2063-1:2019, Thermal spraying, Zinc, aluminium and their alloys, Part 1: Design considerations and quality requirements for corrosion protection systems, second edition February 2019, including coating thickness at 5.2.2 with Annex C, preparation of the surface to be coated at 6.3, sealing at 6.5 and thickness testing at 6.7.3, and the introduction’s statement that the document targets designers of components · ISO 2063-2:2017, Part 2: Execution of corrosion protection systems, first edition September 2017, including surface preparation at 6.2 covering masking, blasting and testing of the prepared surface with Annex H, sealing at 6.4 and 9.4.6 with Annex I, roughness at 7.4.2, adhesion strength at 7.4.3, and normative Annex A specifying pull-off adhesion testing in accordance with ISO 4624 · ISO 4624, Paints and varnishes, Pull-off test for adhesion
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