The two most widely used epoxy coatings were selected as representatives of organic coatings in this study: One coating was a clear primer of epoxy polyamine (clear coating) (EPON Resin 828 and EPIKURE Curing Agent 3164), and the other was a yellow primer of epoxy polyamide (pigmented coating) (DeSoto 513 × 419, PPG). In order to achieve faster degradation results, the commercial ground steel Q-panels of (75 × 150)mm with 0.8 mm thickness were used as the metal substrate, instead of an aluminum substrate. The coatings were prepared by using a film applicator (BYK), with the dry film thickness of (47.9 ± 4.8)µm and (34.0 ± 3.9)µm for clear coating and pigmented coatings, respectively. The coatings were cured at 80°C for 2 h in a forced air-drying oven (VWR) after 24-h curing at room temperature.

Three solutions, deionized water, 0.5 wt% sodium chloride, and a mixed salt solution, were chosen as the fog spray solutions. The mixed solution containing 0.05 wt% NaCl (sodium chloride) and 0.35 wt% (NH₄)₂SO₄ (ammonium sulfate) was prepared according to ASTM G85.A5. Before the accelerating experiment of fog spray, the margins of coating samples were sealed by the anticorrosion insulation tape (3M). The cyclic accelerating test was carried out in the Q-FOG cyclic corrosion tester according to ASTM G85.A5. For each spray-fog solution, 1 hour of fog spray and 1 hour of drying were executed alternately between 25 °C and 35 °C for all coating samples in the cyclic corrosion tester. Three samples for each epoxy coating were taken out periodically during the 35 days of immersion time. The pendulum hardness and pull-off adhesion for the coating samples were measured successively with a pendulum hardness tester (BYK) and an adhesion tester (DeFelsko) according to ASTM D4541.

Three different locations of each pair of coating samples were tested by ENM and EIS through a ParaCell electrochemical testing cell and a Reference 600 + Potentiostat (Gamry Instruments). A testing area of 2.85 cm² could be obtained for each coating sample by means of the ParaCell electrochemical testing cell. After each pair of coating samples was installed, the electrochemical cell was placed in a Faradaic cage to avoid electromagnetic disturbance from external sources. The ambient temperature during the measurements was maintained at 25 °C, and the testing solution was 3.5 wt% NaCl solution. For the ENM testing, the current and potential signals were recorded using a zero-resistance ammeter (ZRA) and a potentiometer with the frequency of 2Hz (ESA410 Data Acquisition, Gamry). After the ENM testing, the same coating sample was under the EIS measurement through the typical three-electrode system with a working area of 2.85 cm². In addition, the SKP technique (RHC040, KP Technology) was adopted in this study. The Kelvin probe used for this study was 1 mm diameter, and the scanning area of the coating surface was 4000 µmand t µm with the resolution of 200 µm. The samples were kept in the SKP chamber with 95% humidity and room temperature. The coating preparation and testing protocol of this study are summarized in Figure 1.

Although the quality of coatings can be presented in many aspects, the surface appearance is the most direct and visualized way to evaluate the durability of coatings. For the fog solutions of deionized water and 0.5 wt% NaCl, no significant degradation was observed on the coating samples during the 35 days exposure. However, two epoxy coatings experienced a significant gradual deterioration in the exposure of the mixed salts. Some corrosion defects can be observed on the surface of the clear coating after 3 days of exposure while the defects appeared after 15 days for the pigmented coating.

For the gloss change of the coatings, as shown in Figure 2, the gloss of two epoxy coatings degraded less in the fog atmosphere of deionized water and 0.5 wt% NaCl solution than that in the mixed salt fog. It means that the mixed salt solution of 0.05 wt% NaCl + 0.35 wt% (NH₄)₂SO₄ is more corrosive to the coating degradation than deionized water or 0.5 wt% NaCl solution. For this fog spray testing, both epoxy coatings cannot endure the fog corrosion of mixed salt under a cyclic accelerating atmosphere. It demonstrates that the synergistic effect is achieved by adding sulfate solution in an atmosphere of chloride solution, which obviously accelerates the coating degradation and the underneath steel corrosion (Morcillo, 1999; de la Fuente et al., 2007).

The hardness of coatings is the most important factor affecting the abrasion and scratch resistance of coatings. The pendulum damping test has been found to have a good sensitivity in detecting the coating hardness, where hardness is defined as the resistance to deformation (Dashtizadeh et al., 2011; Boumaza et al., 2016; Samad et al., 2018). As for both epoxy coatings studied in this work, there is no significant change of the pendulum hardness as a function of time under the accelerating fog atmosphere of deionized water and 0.5 wt% NaCl solution. However, for the mixed salt solution, the pendulum hardness of clear coating degraded rapidly in the very beginning of fog spray and was kept at a low value after 10 days; the pendulum hardness of pigmented coating began to drop sharply after 20 days of fog spray (Figure 3).

Pull-off adhesion can be used to evaluate the pull-off strength of a coating/metal system. During the exposure, the pull-off adhesion of clear coatings exhibited a slight decline under the fog spray of deionized water and 0.5 wt% NaCl solution, while it decreased rapidly in the mixed salt fog. After 15 days of the fog spray, about 75% adhesion was lost in the mixed salt fog for the clear coating. As for the pigmented coating, the pull-off adhesion was decreased by around 200psi under the fog atmosphere of deionized water and 0.5 wt% NaCl solution during the exposure period (Figure 4). Under the fog spray of mixed salt solutions, the adhesion decreased gradually and almost lost the total adhesion at the end of the exposure. Therefore, the pull-off adhesion demonstrates clearly that the destroying power of the mixed salt fog is significantly stronger than that of the deionized water or 0.5 wt% NaCl solution fog.

EIS is the most commonly used and powerful tool that can evaluate the anticorrosion ability of organic coatings by using a system of electrochemical measurement and validation. The coating samples were picked up periodically for EIS measurement during the exposure test. The impedance modulus of EIS spectra at 0.01 Hz (|Z|0.01) is studied representing the corrosion resistance of the coatings. This value is not influenced by the testing conditions and is not generated by modeling. This is the impedance modulus of the coating at 0.01 Hz. This analysis has been widely used by other researchers since 20 years ago (Bierwagen et al., 2000; Thu et al., 2001; Hinderliter et al., 2008). Here, the EIS data at a frequency of 0.01Hz (|Z|_0.01) were chosen to present the corrosion resistance of the coating after the exposure of different salt fogs. The |Z|_0.01 values of the two coatings dropped significantly at the beginning of the fog spray and then reached a stable stage (Figure 5). Thereinto, the |Z|_0.01 value was maintained over 10⁶Ω for two epoxy coatings after the fog spray of deionized water or 0.5 wt% NaCl solution, which means that both epoxy coatings can hold a good capability of anticorrosion after the 35-day fog spray of the two solutions (Shreepathi et al., 2011). However, the |Z|_0.01 values of coating samples decreased quickly to be lower than 10⁵Ω or even less within 3 days in the mixed salt fog spray. It means that the coating after the mixed salt fog spray has largely lost its ability for corrosion protection, for example, the epoxy coatings cannot endure the long-period corrosion by the fog atmosphere of 0.05 wt% NaCl + 0.35 wt% (NH₄)₂SO₄.

Electrochemical noise measurement is a non-destructive and non-intrusive technique capable of monitoring basic changes in an electrochemically active coating system. The common method of collecting ENM data is by measuring the current fluctuations between two nominally identical electrodes at their open circuit potential (OCP) while connecting through a ZRA. There are many methods for the quantitative analysis of ENM data (Homborg et al., 2014; Jamali and Mills, 2016). The basic quantitative method of noise data can be obtained through the standard deviation method (SDM) of potential and current fluctuations in the time domain. To transfer the time domain data to the frequency domain, the majorly used methods include fast Fourier transform (FFT), Fourier power spectrum (FPS), and maximum entropy method (MEM) algorithms.

In order to understand the anticorrosion behavior of the epoxy coatings under the fog spray of different solutions, the electrochemical noise data of the two coating samples were acquired and analyzed using the SDM and FPS methods. Based on the results illustrated in Figures 6, 7, the deterioration trend of two epoxy coatings obtained by the SDM and FPS methods showed a similar variation as those by EIS as a whole. Similarly, the anticorrosive ability of two epoxy coatings dropped quickly at the initial stage of fog spray, and the corrosion resistance kept at a rather higher level in deionized water or 0.5 wt% NaCl solution than that in the mixed salt solution after the 35-day fog spray.

As shown in Figures 5–7, it also found that the R _SDM values of two epoxy coatings were roughly equivalent to the impedance results of EIS, while the R _FPS values were generally higher by one or two orders of magnitude than those by the EIS and SDM methods. It seems that the FPS method was not always feasible to process the ENM data for assessing the anticorrosion performance of organic coating samples after the dry–wet cycle exposure of salt fog.

The anticorrosion performance of a coating layer can be demonstrated through the surface potential of the coating–metal interface (Leng et al., 1998c; Nazarov et al., 2008; Rohwerder et al., 2011). Through SKP, the delamination reaction can be analyzed as a function of the water activity, the type and activity of cations and anions, the electrolyte concentration, and the oxygen partial pressure in the atmosphere (Stratmann et al., 1996; Leng et al., 1998b; Nazarov et al., 2008; Rohwerder et al., 2011; Upadhyay and Battocchi, 2016; Nazarov et al., 2018). Based on the SKP results (Figure 8), the SKP work function of two epoxy coatings showed a slight decline as a function of time during the fog spray exposure of deionized water or 0.5 wt% NaCl solution, while it decreased rapidly in the mixed salt fog. In addition, while using deionized water or 0.5 wt% NaCl solution, the work function can keep up at a region of -100mV ∼ -700mV with a mean value of about -420mV for the single solution, while the work function varied from -200mV to -1050mV with a mean value of about -700mV for the mixed salt solution. It means that the surface status of two epoxy coatings during the fog spray of a single solution (deionized water or 0.5 wt% NaCl solution) was more stable than that in the mixed salt solution, and the coating resistibility to delamination and corrosion would decrease faster in the fog spray of the mixed salt solution.

Coating degradation is an electrochemical process essentially; thus, the monitoring techniques and data analytical methods deduced from the electrochemical theory or process are used constantly to evaluate the coating degradation behavior.

Compared the EIS (|Z|0.01) results with the ENM results deduced by SDM and FPS, the ENM values obtained by using different data-processing methods of electrochemical signals differed much with each other. For a perfect coating, the results of SDM agreed well with those of EIS, and the results of FPS showed slightly higher than those from EIS and SDM. Meanwhile, for a coating with some defects or badly damaged sites on the coating surface after the salt fog spray, the results of SDM and FPS were much higher than those of EIS as a whole. It means that although the ENM method has been widely utilized for assessing the effectiveness of a coating layer and to distinguish the onset of corrosion due to its high sensitivity to the changes at the coating–metal interface (Conners et al., 2000; Deyá et al., 2013; Jalami and Mills, 2016), the derived results from ENM are not always satisfactory to process strong signals of electrochemical noise when some significant defects exist on a coating sample.

As an analytical technique for localized information, the SKP is an efficient tool to measure the metallic potential under a coating layer in the atmosphere (Stratmann and Streckel, 1990; Stratmann et al., 1991) and further to monitor the formation and development of cathode and anode zones and their developing change (Leng et al., 1998a; Wapner and Grundmeier, 2004; Reddy and Sykes, 2005; Oehler et al., 2012). Generally, the testing areas of EIS and ENM are relatively larger than those when using the SKP, and SKP can provide localized information on the status of the coating surface. Nevertheless, they all demonstrated that the coatings degraded faster in the mixed salt fog.

The presence of chlorides and sulfates at the metal–coating interface is well known to have a joint destructive effect on the integrity of most coating systems. In our coatings, some visible defects appeared on the surface of the two coatings in the first 3 days in the mixed salt solution, while no visible defects were observed even after 35-day fog spray in deionized water or 0.5 wt% NaCl solution. Osmotic blistering and under-film corrosion are deemed to be two primary degradation mechanisms when water-soluble mixed contaminants present at the metal–coating interface (Bastidas et al., 1998; Morcillo, 1999). One of the reasons is that the presence of water-soluble salt at the metal–coating interface can accelerate the absorbing rate of water molecules through decreasing the critical relative humidity of the condensation of the water vapor. Another reason is that the water-soluble salt is able to take catalytic and accelerating effects by participating in corrosion reaction (Morcillo et al., 1997; de la Fuente et al., 2007). Consequently, the loss of adhesion, cathodic disbondment, scribe creep, and a decrease in the adhesion/cohesion strength can be observed clearly for a coating in a short time exposure in the fog atmosphere of the mixed salt solution. This also shows again the powerful synergistic destruction of mixed salts on a metal/coating system.

For a complicated coating system, any variation of a coating parameter is not independent. It will be correlated with other environmental factors, which can be deduced by the correlation method. Correlation analysis is based on Pearson or Spearman product moment coefficients (Zheng et al., 2012). The corresponding correlation results can be presented in covariance correlation matrices; the strength of the relationship between any pair of variables can be estimated through correlation analysis (Evans, 1996; Reimann et al., 2017).

From Tables 1, 2, based on the correlation analysis of the results from different characterization methods, it can be concluded that 1) the EIS results are very well consistent with SDM data of ENM in assessing the coating degradation; 2) the high correlation of gloss, hardness, adhesion, and SKP show significant agreement with each other, which means the degradation characteristics of the coatings are similar as deducing from different measurements; 3) even for a bad coating sample with defects or badly damaged sites, the coating degradation traits deduced from EIS, SDM, FPS, and SKP changed consistently with each other, although the actual values of each parameter were much different for the same coating samples.