Damage Tolerance of Thermal Spray Aluminium (TSA) Coatings
TWI Industrial Member Report 1156-2021 [pdf / 2483KB]
By S Paul
Clean energy sources, such as offshore wind, are being increasingly utilised to decarbonise energy production. The economic material of choice for offshore structures is carbon steel. However, this material is particularly prone to corrosion in marine environments thus necessitating the use of corrosion mitigation systems, such as coatings and cathodic protection. A widely used coating system that acts as a barrier in all marine environments and offers cathodic protection to immersed steel structures is thermally sprayed aluminium (TSA). Even though the technology has been around for decades and industrial standards developed by ISO, AWS, NACE, SSPC, DNVGL, ANSI and NORSOK have been established, it is not always clear how these coatings perform over the long periods required today in the presence of damage.
In the work published thus far, the focus has been on the performance of TSA coatings, particularly when immersed in seawater, including the effect of damage. However, the work published has focussed on the effect of single, small-scale damage such as scribes or holidays exposing up to 5% area of the specimen on the corrosion performance. The effect of multiple damage or large-scale damage, such as 20% exposed area, has not been studied in detail. Moreover, most of the published information on damaged specimens was carried out over a short period of time. Thus, long-term data on the performance of damaged TSA in controlled, simulated seawater environments would be beneficial. The damage tolerance typically up to 5% has been evaluated. In cases where a larger damaged area was used, no systematic study has been conducted to understand the protection mechanism. Only short-term data are available for such work. Thus, the mechanistic understanding of the long-term behaviour of TSA when damaged is an important objective. In addition to the dissolution of the aluminium coating, the deposits (if any) formed on the damaged regions also play a role in the corrosion kinetics. The evaluation of the deposits on the defect regions needs to be carried out to understand their role in the corrosion process and thus ascertain if the deposits offer any protection.
Photographs of specimens before testing showing holiday (defect) size and distribution. Specimens were approximately 40mm×40mm. The percentage of defect area are approximately 5% (a), and ~18% (b and c). a) b) c)
- TSA polarises steel to potentials which are more negative than ‑800mV Ag/AgCl at 25°C, even when a defect or holiday (up to ~18%) is present demonstrating the potential of providing cathodic protection to badly damaged coatings immersed continuously in seawater without the presence of external CP.
- TSA coating corrosion rates were calculated to be below 0.01mm/year after 3 months of testing even when a holiday (up to ~18%) was present in TSA-coated steel specimens. The long-term TSA coating corrosion rate was found to be below 0.005mm/year.
- The defect region of the TSA-coated steel was covered with a deposit comprising an inner layer of Brucite [(Mg(OH)2] and an outer layer of Aragonite (CaCO3). This layer reduced the exposed steel area and lowered the corrosion rate of TSA.
- Mg- and Ca-salts (in seawater) play an important role in marine corrosion processes, therefore the use of aqueous solutions of 3.5wt% NaCl as a substitute for seawater for the evaluation of corrosion performance of TSA coatings in marine environments must be avoided, as it could lead to results not representative of seawater exposure.