Corrosion threshold data of metallic materials in various operating environment of offshore wind turbine parts (tower, foundation, and nacelle/gearbox)

This paper outlines corrosion thresholds for different environmental conditions of metallic materials commonly used in the tower, foundation, and nacelle/gearbox of an offshore wind turbine. These threshold values were derived from laboratory corrosion testing employing electrochemical analysis techniques, using the media/solvents that are representative to the operating environment of those wind turbine parts, such as seawater, grease, oils/lubricants, or their combination, at room temperature and at 328K. These values can provide an indication when general/local corrosion or protective film/surface damages have occurred. They can thus be utilised for detecting and monitoring corrosion at certain locations in the wind turbine structure. The presented data have been verified and validated to ensure their repeatability and reliability by means of numerous laboratory tests in accordance to the relevant engineering test standards and an extensive literature/published data review.


Data
The investigated metallic materials commonly used in the foundation, tower and nacelle/gearbox of an offshore WT with their typical environments are listed in Table 1.
The Open Circuit Potential (OCP), Zero Resistance Ammeter (ZRA), Electrochemical Impedance Spectroscopy (EIS) and PotentiodynamicPolarisation Curve (PPC) are the electrochemical analysis techniques utilised in conjunction with the conducted corrosion testing. Value of the data The data generated from laboratory testing following the known/internal standards are threshold ranges or values that can be used to validate and indicate when general/local corrosion or protective film/surface damages on metallic materials on various offshore wind turbine structures in their typical environments. The Nyquist and Bode diagrams could be useful for other researchers fitting such data to equivalent circuits in order to gain insights into the actual mechanism of corrosion. Plant operators, inspection/maintenance companies, WT design industries will benefit from one source database with an open access privilege to assist the work in this field or in the structural health monitoring technology development. The data can be integrated into an operating system such as a SCADA-like system for remote detection and monitoring of corrosion/surface damages through the implementation of a Real Time Remote Sensing (RTRS) technology. The data could help in furthering the understanding of corrosion failure mechanisms of the selected metallic materials used in offshore WT parts, which can be used to consolidate and/or optimise the design of the relevant parts with respect to their material selection and operating conditions. characteristics of each of these techniques and the relationships between their relevant corrosion parameters and outputs. The nomenclatures of these parameters are outlined in Table 3. The corrosion threshold ranges or values for various different environmental conditions of the investigated alloys are therefore essentially of the four mentioned electrochemical analysis techniques'   Electrochemical Impedance Spectroscopy (EIS) [8] Non-destructive Active, Uses Alternating Current (AC) Detect corrosion Inform type of corrosion (localised and uniform/general corrosion) Indirect analysis on corrosion mechanisms e.g. diffusion, passivation or activation Indirect analysis on characteristics of the corrosion products or processes e.g. diffusion, adsorption-desorption or water absorption Potential (E), Unit: Voltage (V) Current density (I), Unit: Amps/ square centimetres (A/ cm 2 ) Table 3 Nomenclatures.
Symbol Significance  Tables 4 and 5 are compiled with regards to the types of corrosion i.e. uniform/general or localised corrosion. The table also includes the selected references that are used to verify the presented data. The extensive literature/published data review indicated a large variability in the methods/procedures of testing and data generation. Therefore, those references Table 4 Corrosion threshold ranges or values for different environment conditions in association with the uniform/general corrosion of the commonly used metallic materials in foundation, tower and nacelle of an offshore WT.

Material/ Alloy
WT Parts General Corrosion *Notes      containing only work performed in accordance to the international standards were considered in the review and for the data verification. In addition, Table 6 represents PPC analysed data (ba and bc) of the metallic materials from the corrosion testing conducted at different environments. These parameters can be used to evaluate the corrosion rate, C.R. (their relationship is shown in Table 2), thus for life prediction.

Experimental design, materials, and methods
Test samples or coupons of an approximately 2.0cm Â 2.0cm Â 0.3cm were prepared from the metallic materials listed in Table 1. They were polished using a 1200-grit paper, subsequently in a dissolution comprised of 10% (V/V) colloidal silica gel (0.06 mm colloidal silica gel) and 90% (V/V)  distilled water. Following the polishing stage, the metallic samples were washed and cleaned with a commercial detergent and fresh water, then with distilled water and by isopropanol, then dried up using hot air (ASTM E3-11) [6]. A minimum of 0.5 cm 2 polished surface area is needed to guarantee a sufficient exposure/contact during the corrosion testing.
The set-up and conditions for the corrosion testing in a substitute ocean water environment (from this point onward is referred to as 'Seawater') are in accordance with ASTM D1141 [5]. Meanwhile, the corrosion testing to simulate the conditions and environments in the nacelle/gearbox follows the ASTM D6547 [3] recommendation when using semi solid lubricants with added corrosion inhibitor (from this  point onward is referred to as 'Grease'), the ASTM D665 [4] when using a mixture of 30% (Wt/Wt) Grease and 70% (Wt/Wt) Seawater, and the ASTM D6547 [3] for testing using oils at RT and at 328K.
Electrochemical corrosion testing was performed using a potentio/galvanostat (GillAC, ACM Instruments) that was controlled by software Gill AC serial n o 600. OCP utilises a two electrodes cell, namely a working and a reference electrode. ZRA, EIS and PPC added a second working (a sacrificial) or counter electrode to construct a three electrodes cell system. Silver/silver chloride potassium chloride saturated (Ag/AgCl Sat. KCl) was used as the reference electrodes and graphite rods as the second working or counter electrodes. The test sample was the other working electrode.
Whilst ZRA and EIS were performed using the same test conditions in all environments, OCP and EIS were conducted using different test conditions depending on the environment. The test conditions  used to generate the reported data are specified in Table 7. The complementary information in a format of Nyquist and Bode diagrams to represent the experimental raw data are also presented in Figs. 1e4.