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Damage Tolerance of Thermal Spray Aluminium (TSA) Coatings

Overview

Clean energy sources, such as offshore wind, are being increasingly utilised to decarbonise energy production, and the economic material of choice for offshore structures is carbon steel. However, this material is 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 offers cathodic protection to immersed steel structures is thermally sprayed aluminium (TSA). However, even though the technology has been around for decades and industrial standards have been established, today, it is not always clear how it performs over the long periods required in the presence of damage.

To address the above knowledge gap, TWI carried out a systematic study to assess the effect of damage on the performance of TSA-coated steel in simulated marine immersion conditions.

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, this work has centred 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, and 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.

Objective

The main objective of the project was to understand the corrosion behaviour of TSA coatings in seawater when damaged and their tolerance to increasing levels of damage.

Approach

Twin-wire arc-spray process was used to coat S355 steel with Al (1050). The coated steel specimens were exposed to synthetic seawater in a bath kept at 25°C with no applied potential. The open circuit potentials were monitored against a standard Ag/AgCl reference electrode. The coated steel specimens had various levels of holidays (~5, 10, 15 and 18%) drilled on the front surface while the back surface was coated in lacquer. Various characterisation techniques, such as SEM, EDX and XRD, were used for microstructural characterisation and phase determination after testing. The data were analysed and used to develop an understanding of the corrosion mechanisms.

 

Results

The TSA coating polarised the specimens below the potential of carbon steel. The defect areas in all the specimens were protected by the TSA ( two examples in Figure 1).

The defect regions were covered by a calcareous matter (Figure 1).  The formation of calcareous matter is beneficial, especially when the coatings are damaged in seawater immersion service.  These deposits comprise an inner layer of Brucite [(Mg(OH)2] and an outer layer of Aragonite (CaCO3).  In the current project, this deposit reduced the exposed steel area and lowered the corrosion rate of TSA.  The mechanism of formation of the calcareous deposits is shown in Figure 2.

The deposition of CaCO3 and Mg(OH)2 (from seawater) on a cathodically protected surface occurs as a consequence of increased pH near the metal-electrode interface (holiday/defect in our experiments). The formation of calcareous matter is an added benefit offered by these anodic coatings. However, these calcareous deposits only form in seawater, or in simulated seawater containing the minerals commonly found in seawater. Thus, the use of 3.5wt% NaCl solution to test these coatings to simulate marine environments is likely to give misleading data.

For the specimens containing one defect (with varying area), the measured potentials reached values around ‑900mV within a few hours of immersion.  The specimens with the smaller defect areas remained more active in the initial stages, but at the later stages its activity declined further and the potentials became very similar to that of specimens with larger defect areas.  The corrosion rates calculated from the electrochemical data for TSA specimens with one defect (defect area from 4.6-18.3%) showed scatter in the early stages of exposure.  However, the scatter decreased with exposure time and the corrosion rate decreased below 0.02mm/year after a month of exposure.  The scatter was minimised after a few months of testing and the specimens showed corrosion rates below 0.005mm/year after 250 days.

For the specimens with a different size and number of defects (but with the same approximate total area: 9-10%), the corrosion rate showed similar trends for all the specimens. The variation in the corrosion rate is not as significant as with the specimens with different defect areas. Instead, the corrosion rates are similar and reduce to below 0.01mm/y within 3 months of testing. With the stabilisation of the corrosion process, the corrosion rate decreased, reaching values ~0.005mm/year after 250 days. Assuming the consumption of the Al coating occurs at a rate of 0.005mm/y acting on the average coating thickness of 0.3mm, these values would give an estimated coating service life of ~60 years. It must be noted that this corrosion rate is not constant and is likely to change further as the calcareous deposits form or mechanical damage of the coating occurs in service.

Outlook

The values of estimated coating life are based on constant immersion. However, in the splash zone the alternate immersion and seawater splash might lead to different corrosive environments. The combined effects of UV, temperature fluctuations, biofouling and so on experienced during service conditions were not simulated in this work. In service, the actual rate of coating consumption will be somewhat different. Furthermore, the corrosion rate during the initial stages of exposure is significantly higher, which needs to be a design consideration. Hence, the values quoted for coating corrosion life in the splash zone are considered a rough estimate. If the coatings are to be used in splash and tidal zones, tests should be carried out in specimens with defects in alternate immersion conditions.

Offshore structures operate at temperatures not limited to 25°C. The North Sea water temperature rarely exceeds 16°C. The tests carried out here should be repeated at different temperatures (preferably at 5-15°C) to get more realistic data for UK North Sea structures. Appropriate guidance should be sought before applying these methods during design, fabrication and service to ensure that components are fit-for-service. Where possible, quantitative electrochemical methods should be explored. As damage to coatings in offshore environments is not uncommon, repair methods should be evaluated. Further research is also needed into the current use of sealants in TSA in various applications. These aspects are currently being studied in follow-on programmes at TWI.

Figure 1. Photographs of specimens after 420 days of testing in ASTM D1141 synthetic seawater
Figure 1. Photographs of specimens after 420 days of testing in ASTM D1141 synthetic seawater
Figure 2. Schematic showing the mechanism of deposit formation in TSA-coated steel containing defect
Figure 2. Schematic showing the mechanism of deposit formation in TSA-coated steel containing defect
Avatar Dr Shiladitya Paul Research and Product Development Programme Manager, Surface, Corrosion and Interface Engineering

Dr Shiladitya Paul is an innovative technology specialist with experience in the industrial development and application of specialised materials, coatings, and corrosion mitigation methods. His chapters in the Encyclopaedia of Aerospace Engineering (Wiley, 2010) and the ASM Handbook (ASM International, 2013) are widely considered as authoritative reference works providing critical scientific concepts on materials, coatings and corrosion, and their application to engineering practice, and he has published over 100 papers/articles on materials, coatings and corrosion. Shiladitya is active in the scientific community as a peer reviewer and member of the editorial team for several journals, and on a number of international, scientific and industrial committees, including holding the positions of Vice Chair of the European Federation of Corrosion WP9) and member, and judge, of the Journal of Thermal Spray Technology (JTST), Best Paper Award committee. He is a registered Chartered Engineer (CEng) in the UK and a Fellow of the Institute of Materials, Minerals and Mining (FIMMM).

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