When "how" is more important than "how much", and the role played by modern coatings guidance
Protecting Offshore Wind's most vulnerable zone
As offshore wind farms continue to develop as an increasing part of our energy mix, and operating as they do in extremely demanding environments, the durability of corrosion protection systems has become a critical factor in asset reliability, maintenance costs, and lifecycle performance. Nowhere is this challenge more apparent than in the splash zone, the area where steel foundation structures are subjected to the combined effects of seawater immersion, wave action, atmospheric exposure, UV light, abrasion and mechanical abrasion.
Historically, glass flake reinforced coatings have played a prominent role in protecting these high-risk areas. Their long track record has established them as one of the industry’s preferred solutions for extending coating life and reducing maintenance interventions. However, recent discussions within industry standards and specifications have raised an important question: is coating performance determined simply by the amount of glass flake present, or by how that glass flake is engineered and formulated into effective coating systems?
Recent work examining offshore coating guidance suggests that the industry may need to focus less on glass flake quantity and more on coating optimisation supported by real-life and laboratory performance assessments.
The science behind the barrier
The effectiveness of glass flake coatings has always been linked to their ability to impede the movement of moisture and corrosive species through the coating film.
When moisture attempts to penetrate a conventional coating, it can migrate through microscopic pathways towards the steel substrate. By incorporating lamellar glass flake particles into the coating matrix, these pathways become substantially longer and more complex. Instead of moving directly through the coating, moisture molecules must navigate around thousands of overlapping plate-like particles, which themselves are impervious to the passage of moisture.
This phenomenon is commonly known as the “tortuous path” effect.
The result, when correctly formulated, is a significant reduction in permeability and therefore ultimately a slower rate of corrosion initiation. Even relatively small additions of glass flake have been shown to deliver notable improvements in moisture vapour transmission resistance, demonstrating why the technology has remained central to offshore corrosion protection for decades, and has been incorporated into numerous resin chemistries, such as epoxies, polyesters and vinyl esters.
For offshore wind structures where maintenance access is difficult and costly, improving barrier performance can translate directly into longer maintenance intervals and lower lifetime costs.
The evolution of Glass Flake Technology
Traditional glass flakes were produced using a “bubble” manufacturing process, creating comparatively thick particles with lower aspect ratios. While effective for their time, these particles offered inherent limitations in barrier efficiency.
Modern production techniques use advanced spinning technologies capable of producing considerably thinner flakes with far higher aspect ratios. Some modern particles can be manufactured at sub-micron thicknesses while maintaining excellent uniformity and consistency.
Table 1
| Characteristic | Traditional Glass Flake | Modern Glass Flake |
|---|---|---|
| Manufacturing method | Bubble process | Spun process |
| Particle thickness | 3-18 μm | <3 μm |
| Aspect ratio | Lower | Significantly higher |
| Barrier performance | Good | Excellent |
| Optimisation flexibility | Limited | High |
This development is important because aspect ratio, the relationship between particle diameter and thickness, has a direct influence on barrier performance. Higher aspect ratio particles create much longer and more complex diffusion pathways, allowing equivalent or even superior protection to be achieved with lower overall glass content, and at the same coating dry film thickness.
In simple terms, today’s glass flake particles work harder than their predecessors. The relative permeability of the binder type itself must also be considered and is why a system type approach, considering both binder and filler, to formulating individual coating products is necessary.
Why more glass is not always better
However, coating optimisation is far more complex than applying a minimum glass flake percentage requirement.
For many years, specifications have often associated high-performance splash zone coatings (as well as internal coatings of other equipment such as storage tanks and pressure vessels) with a glass flake contents exceeding 20% by mass. Whilst the origins of this number, as well as how it is defined, often lack clarity, and despite its undoubted good intention of improving product performance, this requirement originates from a period when glass flake particle technology was very different from what is available today.
Research and development over recent decades have demonstrated that coating performance depends on several interrelated factors,
- glass flake particle size
- particle thickness, these two determining its
- aspect ratio
- resin chemistry
- other fillers present, and Critical Pigment Volume Concentration (CPVC),
- adhesion promoter levels
- application characteristics
Increasing glass flake loading indefinitely does not necessarily improve performance. In many cases, excessive glass content can actually reduce coating effectiveness, negatively affect application properties, or push formulations beyond their optimum pigment volume concentration.
A coating containing less than 20% by weight but engineered around modern high-aspect-ratio particles, and fully optimised, may offer the same or better performance than coatings simply meeting this legacy compositional requirement.
Table 2
| Factor | Influence on performance |
|---|---|
| Glass flake thickness and aspect ratio | Increases diffusion path length |
| Resin chemistry | Determines durability and adhesion |
| Glass flake content | Must be optimised, not maximised |
| Adhesion promoter levels | Enhances substrate and filler bonding |
| Application quality | Critical to ensuring performance as-intended |
| Qualification testing | Verifies real-world performance |
Performance must remain the ultimate measure
This shift in understanding presents an important challenge for standards development. Modern performance standards and specifications may often identify required generic chemistries, mandate coating film thicknesses, impose compositional limits on some ingredients as well as defining minimum performance requirements.
However, what happens when these different needs conflict with each other? Examples include:
- Performance requirements being met with alternate chemistries and scheme thicknesses.
- Performance requirements being met despite compositional requirements not being observed.
- Compositional requirements which are incompatible or non-optimised when used with specific chemistry types.
More importantly, many of the established qualification tests for offshore coatings do not directly require a specific glass flake loading. Instead, they focus on measurable outcomes such as:
- Corrosion creep
- Cathodic disbondment resistance
- Adhesion retention
- Cyclic ageing performance
- Water immersion resistance
The offshore sector has long relied on rigorous qualification testing because it reflects real-world performance more closely than compositional requirements alone and because of the challenges in using real life performance via track record, not least of which is the impact imposing field-based performance history has on recent innovation.
Data presented from multiple qualified coating systems demonstrates strong performance across adhesion retention, corrosion resistance and long-term durability, reinforcing the argument that testing, combined wherever available with proven field experience should remain the primary basis for coating selection.
This in itself presents a contradiction in that longer, field-based performance evidence may no longer be fully aligned with modern laboratory testing for a variety of reasons such as;
- Product may no longer be available.
- Pre-qualification testing requirements may no longer be the same.
- Acceptance criteria may have been
This is why the continued development of pre-qualification testing requires a considered approach, with several modern standards accepting testing results to earlier versions of the document. An example of this is the NORSOK M-501 system, which permits the acceptance of some test results as far back as 2004, provided that all other requirements of the current edition are met and the minimum coating scheme thicknesses mandated in the current edition are also observed.
This pragmatic approach permits innovation whilst acknowledging the extensive performance history of established coating technologies and at the same time avoiding the unnecessary requalification of systems that have already demonstrated long-term durability and compliance in offshore service environments.
What this means for Offshore Wind
Generally, the offshore wind industry has access an ever-increasing range of products including glass flake coatings, which are typically subjected to extensive and relevant pre-qualification testing as well as an increasing body of field performance evidence, which is likely to grow as decommissioning accelerates. Additionally, corrosion and coatings professionals have an increasing library of evidence (field or laboratory based) on which to base their decisions. The industry is able to draw on its own coating experiences, history and guidance, as well as those in other industries preceding it. Major coating failures in the splash zone are rare and generally attributable to other factors.
This is allowing the industry to focus more attention in other areas such as:-
- Improving the design of structures to allow effective coating.
- Ensuring a focus on quality control throughout the fabrication and coating application processes.
- Understanding how protective coatings can be used synergistically with other means of corrosion protection.
- Questioning what the industry does now is also suitable as we move into a period of longer design life and new foundation structure challenges (floating wind).
For assistance with offshore wind coating specification writing, pre-qualification testing program design and review, as well as third-party coating inspection for your offshore wind projects please contact fergus.macKinnon@safinah.com
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