Electrostatic powder coating originated in the USA in the 1950s. For years, it has been extensively used for painting processes in the automotive industry. Compared to traditional coatings, it provides even, uniform and reproducible coatings with less waste [1
]. The electrostatic coating process starts by electrostatically charging the powder. At the spray gun exit, charged particles repel each other, and spread evenly [2
] on the object which is oppositely charged or grounded. The electrostatic attraction between the powder particles and the object allows the powder to adhere to the surface, forming an even coating [3
]. The application of the electrostatic powder coating is followed by a high-temperature curing process, which provides a smoothly-coated surface. The powder is charged using either a corona or tribo-charging spray gun [3
]. In the case of corona-charging, the powder particles pass through an ion-rich area and become charged based on their permittivity. Tribo-charging is based on the frictional charging of powder that is transported through a pipe of a specific material, such as polytetrafluoroethylene (PTFE), metal or other powder particles [1
]. The powder particles are more extensively spread and have a higher transfer efficiency at higher charge [6
]. Since tribo-charging generates less charge on powder particles, corona-charging is generally used for electrostatic powder coating [1
]. Today, thanks to the commercial success and wide application of powder coatings, extensive research is carried out in this field of study, in particular in the powder coating chemistry [5
Compared to liquid paints, powder coatings feature several advantages including the absence of volatile organic content, the reduction in coating material loss (up to 68%), the reduction of dust formation (from 40% to 84%) [1
], high utilization rates, fast curing, the minimal health risks involved, and the elimination of hazardous wastes [7
]. However, a major disadvantage of electrostatic coating technology is that the curing of the powder coating requires a significant amount of heat energy. For this reason, the future of these technologies will include the development of low temperature curable (LTC) powder coatings, whose application will enable energy saving solutions and also the ability to coat heat-sensitive substrates [8
]. The second disadvantage is the lack of coating uniformity in Faraday-cage areas. Aside from several challenges needing improvement, powder coating is an environmentally-friendly coating process with good performance properties.
There are numerous methods to evaluate the corrosion performance of powder coatings. Mafi et al. [5
] investigated the protective behaviour of powder coatings in 3.5% w/w NaCl solution using electrochemical impedance spectroscopy (EIS) and open circuit potential (OCP) measurement, as well as differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA). Bhadu et al. [9
] studied the properties of two polyester-based powder coatings using electrochemical impedance spectroscopy in 3.5% w/w NaCl solution, the salt spray test, and scanning electron microscope (SEM) analysis. Both papers [5
] showed better protective properties of epoxy–polyester powder coating compared to polyester powder coating, which is attributed to the higher cross-linked density of the epoxy-based powder.
In this paper, the protective properties of two epoxy–polyester and two polyester powder coatings with rough and smooth surface textures were evaluated in different aggressive media, that is, in 4% w/w NaOH solution using the EIS method, evaluated in a humid and warm atmosphere using humidity chamber testing and in a marine environment using the salt spray chamber testing. The structure, morphology, and uniformity of powder coatings were analysed by SEM and EDX analysis. The results were discussed regarding the chemical composition of the binder and coating thickness, which was strongly affected by surface textures. The coatings were provided by four different manufacturers, and are commonly used in switch-gear cabinet production.
2. Materials and Methods
Mild steel plates with the dimensions 100 mm × 150 mm × 3 mm were chemically pre-treated using iron phosphating in a bath and a 0.5−2 µm thick Fe-phosphate coating was obtained. Subsequently, the epoxy–polyester and polyester powder coatings with different surface textures, used for indoor and outdoor exposure, were applied to the pre-treated samples using a PG1 electrostatic spray gun, with spraying parameters of 98 kV, 100 µA, and 17 kHz. The powder-coated samples were then cured in an oven for about 20 min at 185–200 °C. Table 1
presents technical data for the tested powder coatings.
The dry-film thickness (DFT) of the cured powder coated samples was measured with a non-destructive magnetic induction method using an Elcometer 456 instrument (Elcometer Limited, Manchester, UK), according to ISO 2808 [10
]. Measurements were performed on ten different locations per sample. The coating adhesion was determined using the Zehntner cross-cut device (Zehntner GmbH Testing Instruments, Sissach, Switzerland), according to ISO 2409 [11
The 240-h salt spray and humidity chamber tests were used as accelerated laboratory tests to predict the corrosion performance of the powder coatings [5
]. The salt chamber testing was conducted in 5% neutral NaCl solution in an Ascott cabinet, model S450 (Ascott Analytical Equipment Limited, Staffordshire, UK), according to ISO 9227 [14
]. The humidity test was conducted according to ISO 6270-2 [15
]. The samples were periodically examined according to ISO 4628 [16
] in order to evaluate the degradation of coatings [17
]. The accelerated testing was performed on two samples per each powder coating.
The corrosion/stability tendency in the NaOH solution was assessed by measuring the open circuit potential (OCP) [5
]. The open circuit potential of the polyester and epoxy–polyester powder-coated samples was measured against the saturated calomel electrode (SCE) as the reference electrode [5
] in 4% w/w NaOH solution, pH = 12, at room temperature (23 ± 2) °C.
The protective properties of the powder coatings were investigated by electro-chemical impedance spectroscopy [9
], with a VersaSTAT 3 Potentiostat/Galvanostat (AMETEK Scientific 131 Instruments, Princeton applied research, Berwyn, PA, USA). The measurements were carried out after one hour and after 100 h of immersion in 4% w/w NaOH solution, pH = 12, at room temperature (23 ± 2) °C. The impedance spectra were performed at open circuit potential (OCP) with a 10 mV sinusoidal amplitude. The frequency range was from 100 kHz to 100 mHz. A three-electrode cell including a coated metal sample as the working electrode, a saturated calomel electrode (SCE) as the reference electrode and graphite sticks as the auxiliary electrodes were used in the experiments [12
]. The surface of the working electrode was 1 cm2
. The EIS measurements were repeated three times for each sample. The Solartron Z-View 2.2 software was used to interpret data.
The surface morphology and the microstructure of the powder coatings were observed by an Olympus GX51 (Olympus Corporation, Tokyo, Japan) inverted metallurgical microscope and a Tescan scanning electron microscope (SEM) (TESCAN Brno, Brno, Czech Republic) equipped with Oxford Instruments energy dispersive spectroscopy (EDX) (Oxford Instruments, Belfast, UK). The energy used for analysis was 20 keV.
The tests performed in this study showed that the powder coatings exhibited very good properties in humid and marine environments as well as in an alkali medium, but also displayed some failures. Extremely good protective properties were displayed in the epoxy–polyester coatings, which are intended for indoor use, while the worst performance was delivered by polyester powder coating B, intended for outdoor use, which lost of adhesion after exposure to the humid and warm atmosphere (ISO 6270-2). A possible cause of this adhesion loss was the distinct relief surface of coating B, for which insufficient coating thickness was determined (DFTmin = 40.4 µm), which led to an early penetration of water to the steel surface. The protective efficiency of an organic coating is generally achieved through the barrier mechanism, which confirms the strong influence of coating thickness on corrosion protection behaviour. Taking into account that the size of the particles in most powders used for electrostatic spraying is 30–50 µm, the coating thickness should exceed 50 µm to obtain a satisfactory protective film.
According to the EIS results, the coating resistance for all the tested coatings at the beginning of exposure showed an acceptable value (106
) for protective coatings [19
], but after 100 h in 4% w/w NaOH, this decreased for all tested coatings. The worst coating resistance was established for polyester coating sample C. The established difference in corrosive durability between the epoxy–polyester powder coating and the polyester powder coating determined by EIS can be related to the better corrosion properties and the higher cross-link density of the epoxy-based binder [5
], as well as the instability of polyester to alkaline hydrolysis [23
]. The cross-sectional examination showed that all tested coatings were fully cured with no cracks and pores and with good adhesion to the substrate.