How to increase durability with Avantguard protective coatings
By challenging the standards for corrosion protection, Avantguard activated zinc primers are significantly reducing application and maintenance costs on a wide range of industrial assets, from power stations and storage tanks to bridges and offshore windfarms.
Greece’s largest ever energy production investment, the Ptolemais V power block, is due to go online in 2022. Located in Greece’s Western Macedonia region, the 660MW lignite-fired thermal power unit will produce affordable electricity for the region, bringing significant economic and environmental benefits. A few years ago, an asset like the Ptolemais V power block would almost certainly be protected with an anticorrosive coating system based on an inorganic zinc or zinc-epoxy primer – the standard primer types for steel in very high corrosive environments. However, Terna S.A., the contractor responsible for Ptolemais V, chose a three-coat system based on Hempel’s Hempadur Avantguard, a relatively new primer type based on patented activated zinc technology.
This choice is no longer unusual. Over the last few years, Avantguard primers have become increasingly common on specification lists across a range of industries, from ZPMC’s super-post-panamax port cranes in Malta to British Petroleum’s new gas processing facility in Oman. They are used at Koole Terminals’ downstream development in the Netherlands, Decal Spain’s steel storage tanks at the Huelva terminal on the Atlantic coast and Iberdrola’s 350 MW Wikinger windfarm, located off the German island of Rügen in the Baltic Sea.
In all these cases, Avantguard primers were chosen as they provide longer system lifetime, extended maintenance intervals and higher application efficiency than equivalent zinc-rich coating systems. In fact, the anti-corrosion performance of Avantguard based systems is so improved that they are challenging some of the requirements in the traditional standards for corrosion protection.
Challenging the standard
The industry standards that define the design, testing and durability of anti-corrosive coating systems are usually based on the performance of conventional coating technology. When Avantguard systems are tested under these test methods – including salt spray or cyclic corrosion tests (as described in ISO 12944 and NORSOK M-501) – they significantly outperform conventional zinc epoxies, either by meeting the test requirements for a longer time or by meeting the test requirements at lower film thicknesses.
For instance, properly designed Avantguard-based systems can meet the anti-corrosion test requirements of ISO 12944 part 6 or NORSOK M-501 even after extending the duration of the test by 50% and in some cases even 100%. Also, Avantguard systems with overall thicknesses below those required by the standards show that they can still protect the steel and stay below the limits required in the standard. This allows for the design of coating systems that require less paint overall, improving the environmental footprint of steel protection.
The advantages of this improved performance are obviously many. On a storage tank, windfarm, processing facility or other industrial asset, Avantguard primers enable extended maintenance intervals and can increase the lifetime of an anti-corrosive coating system by up to 50%. As a result, operational costs related to maintenance can be lowered by as much as 30% across the asset’s lifetime.
Avantguard primers also outperform traditional zinc coatings when it comes to application efficiency. In order to ensure strong galvanic protection, zinc primers are typically formulated with a high concentration of zinc pigments (80% or higher). However, this leads to very poor film characteristics, such as low adhesion values, high potential for mud-cracking and high viscosity. As a result, they require extensive surface preparation and conditions must be ideal during application. If these criteria are not met, the performance of the coating can be severely compromised. Conventional zinc-rich coatings are also slow to dry, which delays application of the next coat in the system and can lead to bottlenecks and project delays.
Avantguard products do not suffer from these issues. They ensure good coating stability, edge retention and film formation, even in high temperatures and humidity (up to 95% relative humidity). They can also tolerate up to 25% higher dry film thicknesses, which significantly reduces the likelihood of application defects and improves final coating quality. In addition, Avantguard primers can be formulated with a recoat interval of just 45 minutes at 20°C, 50% quicker than most standard zinc primers at the same temperature. When used with other fast-curing products, they enable applicators to coat more sections in one shiftfor higher productivity in both new construction and maintenance situations.
How do Avantguard systems achieve these results? The answer lies in how Avantguard coatings utilise zinc within the film.
Triple Activation is key
In zinc-rich coatings, the zinc acts as a sacrificial element. As zinc is less noble than iron, corrosive elements such as water or oxygen corrode the zinc instead of the iron, leaving the steel intact. This process, known as the galvanic effect, is dependent on the transfer of galvanic current. But, in conventional zinc epoxies, current transfer can be an issue. A normal zinc epoxy coating is around 60-80 microns thick, yet research shows that only the zinc in the first 20-30 microns provides galvanic protection – the rest is left unused.
Avantguard is different. It combines the elements used in conventional zinc epoxies with two new substances – glass spheres and a Hempel proprietary activator.
Zinc epoxy without activated zinc technology
Avantguard: Zinc epoxy with activated zinc technology
The combination of these elements and the right choice of binders, pigments, fillers and additives enhances current transfer to ensure all the zinc is activated throughout the coating. This leads to an improvement in the galvanic effect, the barrier effect and the inhibition effect – what Hempel calls ‘Triple Activation’.
- Galvanic protection
All of the zinc is activated through the lifetime of the coating. This stops steel corrosion more effectively and reduces creep corrosion if the coating suffers mechanical damage.
- Barrier effect (low water permeability)
If mechanical damage occurs, the compounds produced by the unique zinc activation process fill any space within the film, sealing it and enhancing the coating’s water barrier properties.
- Inhibition effect
Ions migrating from the environment to the steel surface are captured within the coating, reducing the concentration of corrosive agents that can reach the surface of the steel.
Improving mechanical strength
The unique combination of hollow glass spheres and the zinc activation process have another positive effect: Improved mechanical strength. Industrial steel structures are often exposed to severe mechanical stress, such as extreme temperature fluctuations or mechanical damage from impact or abrasion. In zinc-rich primers, this can result in the formation of micro-cracks, which eventually lead to real cracks and subsequent corrosion. This makes the zinc-rich primer the weakest mechanical point in a traditional zinc protective coating system.
If a crack forms in an Avantguard coating, the glass spheres absorb most of the initial impact and stop it from developing. In addition, the sub-products formed during the zinc activation process then occupy the space left by the micro-crack, preventing it from developing further. This significantly reduces rust corrosion and ensures that the coating maintains its anti-corrosive performance for longer, helping extend the lifetime of the entire coating system and protecting the structure for longer.
The glass spheres in the coating absorb the impact of the initial crack and stop it from propagating. (The picture on the right shows an enlargement of the crack seen on the left.)
The hollow glass spheres form at the base of a crack and stop it from becoming larger. (The picture on the right shows an enlargement of the crack seen on the left.)
without activated zinc technology
Zinc epoxy with activated zinc technology
In order to assess mechanical strength, steel panels with a centred weld were applied with a single coat system. After curing, the panels were exposed to the NACE TM0304 thermal cycling test for 150 cycles (2 hours of exposure at 60°C and 2 hours of exposure at -20°C).
In the impact test, steel panels were applied with a single coat system. After curing, the panels were exposed to the impact test at heights of 25 cm, 50 cm and 100 cm.
Avantguard coatings offer one more benefit over standard zinc coatings – greater environmental performance. Longer asset lifetimes and extended maintenance cycles are generally more sustainable as they lead to reduced emissions and energy use in repair processes. In addition, Avantguard primers can enable specifiers to reduce the number of coats or thickness of the overall coating system to achieve a certain durability. This can lower VOC emissions during application by as much as 30%.
There is an increased need for more sustainable and long-lasting coating solutions that help protect the environment. The latest revision of ISO 12944 included the possibility to reduce the amount of material required (the thicknesses of the coating system) if a new technology demonstrates that it can meet the standard’s performance requirements, as is the case with Avantguard. It is foreseeable that more technologies like Avantguard will appear in the near future – technologies that reduce the amount of coating required while still providing the necessary protection and durability for industrial assets. As it is, the use of Avantguard coating systems will continue to grow, due to the many benefits for asset owners, contractors and applicators, as well as the environment.