AN IMPROVEMENT TO THE ANTICORROSIVE PROPERTIES OF EPOXY POWDER COATING BY ZINC PHOSPHATE AND ZINC ALUMINUM PHOSPHATE PIGMENTS

Epoxy powder coatings have been widely used to protect metal substrates from corrosion [1,2]. Different kinds of organic and inorganic pig-ments have been used to improve the anticorro-sion performance of epoxy powder coatings. Chromates due to their excellent anticorrosion performances have been widely used to im-prove the protection behavior of the epoxy coatings. However, chromates are toxic and carcinogenic [3,4]. Environmental regulations have become stricter in recent years. Attempts have been carried out to use environmentally friendly alternatives. Zinc phosphate is one of these alternatives. New modifications are performed and new generations of ZP have been produced [3-5].The anticorrosive perfor-mance of zinc phosphate and zinc aluminum phosphate (the first and second generation of phosphate-based pigments respectively) in epoxy powder coatings was studied using elec-trochemical impedance spectroscopy (EIS). It is well known that the inhibitive performance of various anticorrosive compounds could be successfully investigated by EIS. EIS offers a powerful method to examine the corrosion phenomenon of the metallic substrates by providing qualitative and quantitative data [6-8]. One of the most important requirements of protective coatings is resistance against cathodic disbondment. The combination of cathodic protection and organic coatings is used in order to ensure the long-term protection of steel structures continuously exposed to an electro-lyte [8]. As a result, the anticorrosive perfor-mance and cathodic disbonding resistance of organic coatings are two vital parameters for a variety of organic coating systems [9]. The disbonding is referred to the loss of bond at the edge of holidays in the coating due to the high-pH environment beneath the film as a net result of cathodic reaction [10,11]. In this study, an attempt is made to improve the adhesion properties as well as the anticorrosion resist-ance of an epoxy powder coating on the surface of mild steel. ZAP and ZP were used to improve the anticorrosion performance as well as coating resistance against cathodic delamination. Fur-thermore, the role of pigment type on the adhesion of the epoxy powder coating was investigated using pull-off test.

EXPERIMENTAL

Preparation of Epoxy Powder Coatings

Epoxy powder coatings were prepared using ZP and ZPA pigments according to Table 1.

Table 1: The formulation of epoxy powder coating used in this work

The compounds listed in Table 1 were blended in an extruder and then grinded to make a fine powder. The ZP and ZAP pigments were supplied by Heubach Co. Epoxy powder coatings were applied to the ST 37 steel samples. Samples were cured at 180 °C for 20 min. The dry film thickness (DFT) of the coatings was about 30±5 µm.

Characterization

The anticorrosion performance of the epoxy powder coatings was studied by electrochemical impedance spectroscopy (EIS). To this end, an area of 1 cm² of each sample was exposed to 3.5 wt% NaCl solution up to 63 days. The test was carried out at open circuit potential (OCP) within a frequency range of 10 mHz to 10 kHz (with the perturbation of ±10 mV). FRA software was used to analyze the information obtained from EIS measurement. EIS was carried out in a three-electrode electrochemical cell including refer-ence electrode (saturated Ag/AgCl), auxiliary electrode (Pt), and working electrode. In order to evaluate the cathodic disbonding of coated substrates, an artificial hole with a diameter of 1 mm was drilled at the center of each sample. The test was carried at the cathodic potential of -1.38 V vs. Ag/AgCl using a magnesium sacrificial anode. A pull-off test method was employed according to ASTM D 4541-02; studs with a diameter of 25.4 mm were fixed to the surface of the coated panels with a cyanoacrylate adhesive. The fixture was pulled at 5 mm.min^-1 using an Instron machine (the universal testing instrument) until the powder coating material was detached from the substrate. For each test, four replicate samples were employed, with the average value cited.

RESULTS AND DISCUSSION

Two different coating systems studied herein are presented in Table 2.

The anticorrosion performances of the epoxy powder coatings, modified with ZP and ZPA pigments, were studied by EIS. The Nyquist plots of the samples are shown in Figure 1.

Table 2: Differentcoatings studied herein

Figure 1: Typical Nyquist diagrams of ZP_b (A) and ZAP_b (B) during different immersion times in a 3.5 wt.% NaCl solution

Using electrical model shown in Figure 2, the Nyquist plots were fitted by AUTOLAB FRA software (Table 3) [6].

In Figure 2, R_s and R_ct represent electrolyte resistance and charge transfer resistance respectively; C_dl stands for double layer capacitance and R_c is coating resistance.

The values of coating resistance (R_c) is signify-cantly decreased after 63-day exposure time. Moreover, R_ct and C_dl are observed after 63 days of sample immersion. This observation can clearly show the electrolyte diffusion to the coating/metal interface.

Figure 2: Equivalent electrical model of the coated mild steel

The order of R_c and R_ct of the samples is ZAP_b > ZP_b, while the order of C_dl of the samples (after 63 days) is ZP_b > ZAP_b.

Table 3: Characteristic parameters evaluated from the impedance diagram for coating systems

The results show that the anticorrosive perfor-mance of ZAP-modified samples is greater than that of ZP-treated samples. When ZP and ZAP pigments are exposed to aqueous solutions, the Zn²⁺ and phosphate ions will be extracted from pigments in the coating matrix (Equation 1) [5].

Furthermore, when electrolyte reaches coating/ metal interface, the following electro-chemical reactions happen [4].

The corrosion products of steel substrate (Fe²⁺ and OH^-) react with Zn²⁺ and PO₄^3- ions and following reactions happen [8,9].

As a result, the reaction of corrosion products and pigment extracts produces insoluble species, namely Zn(OH)₂ and Fe₃(PO₄)₂, and reduces the corrosion of substrate according to the two following mechanisms:

1. Pore plugging of organic coating and de-creasing the diffusion of corrosive species to the metal surface;

2. the passivation of steel substrate due to the formation of a passive layer on the metal surface and hindering the access of aggressive species to the metal surface.

By considering the mentioned mechanisms, the solubility of the phosphate pigments is the most important parameter in determining the anti-corrosive performance of phosphate pigments.

The solubility of ZP and ZAP are completely different. The results obtained from the ICP-OES analysis of ZP and ZAP pigments in a 3.5 wt.% NaCl solution are shown in Table 4 [5].

Table 4 shows the higher solubility of zinc and phosphate ions of ZAP compared to ZP; also, the results show the better anticorrosion perfor-mance of ZAP-loaded coating compared to the sample containing ZP. It is quite clear that pore plugging and the formation of a passive layer is more probable in the coating containing ZAP compared to the one modified by ZP because of the more solubility of ZAP. This indicates that the permeation of the corrosive electrolyte into the coating containing ZPA is considerably lower compared to the coating containing ZP.

The passive layer and pore plugging can hinder corrosive electrolyte permeation into the surface of metal. This can be responsible for the increase in mild steel corrosion resistance against a 3.5 wt% NaCl solution. Therefore, the film formation at metal/coating interface containing ZAP is more effective than the one created in the coating reinforced with ZP.

Table 4: The result of ICP-OES analysis of the pigment extracts in a 3.5 wt.% NaCl solution