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Original article
8 (
6
); 821-827
doi:
10.1016/j.arabjc.2013.04.023

Investigation of the cut-edge corrosion of organically-coated galvanized steel after accelerated atmospheric corrosion test

,
 Department of Chemistry, Cukurova University, 01330 Adana, Turkey

⁎Corresponding author. Tel.: +90 (322) 3386084 2465; fax: +90 (322) 3386070. ryildiz80@gmail.com (Reşit Yıldız)

Received: , Accepted: ,
© The Author(s) 2013
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

The cut edge corrosion of organically coated (epoxy, polyurethane and polyester) galvanized steel was investigated using electrochemical impedance spectroscopy (EIS). Measurements were performed on specimens that had been tested in an accelerated atmospheric corrosion test. The samples were subjected to 10 s fogging and 1 h awaiting cycles in an exposure cabinet (120 and 180 days) with artificial acid rain solution. According to the investigation, the coatings were damaged from the cut edge into the sheet, this distance was about 0.8 cm. These defects were more pronounced at after 180 days in proportion to after 120 days.

Keywords

Organic coatings
Steel
Zinc
EIS
Corrosion
1

1 Introduction

Atmospheric corrosion is mostly due to the presence of salt and gases dissolved in a thin layer on a metal surface. The atmospheric corrosion rate of metals depends on the thickness of the electrolyte layer (Tomashow, 1964). Changes in the thickness of the layer affect the rate of oxygen transport through the electrolyte layer and solubility of corrosion products and, hence, the metal corrosion rates (Nishikata et al., 1995).

The use of organically coated steel products in a wide variety of industries has grown dramatically, recently, which has led inevitably to increasing demands on performance. As a result, it is vitally important, both from a commercial and environmental perspective, to optimize products’ lifetime by minimizing degradation. In organically coated galvanized steel materials used in the construction industry, one of the most frequently reported modes of failure is corrosion at the metallic cut edges (Ryan et al., 1994). Coil coatings have long been used to protect metals and alloys against corrosion. In spite of the fact that such coatings form a barrier against diffusion of ions, water and oxygen are considerably more permeable and, therefore, are able to be present at the metal/coating interface. This can lead to substrate corrosion and coating delamination under the same conditions. Profiled galvanized steel sheet is commonly used for roof and wall cladding. This material is mostly factory-coated in coil; firstly, with a primer layer containing corrosion inhibitors such as strontium chromate and after that with a thicker top coat for additional barrier protection and the desired esthetic appearance. At the cut edge, however, the bare metal is exposed to the atmosphere, and corrosion may occur. There is a considerable interest in limiting this problem as it accounts for a significant proportion of the failures of coil-coated cladding according to recent industry surveys (Cox et al., 1985; Jones and Nair, 1985). One of the failure mechanisms that must be evaluated in organically coated galvanized materials is a form of localized corrosion, occurring at cut edges of the organic coated substrate, where sheets have been cut to size (Ryan et al., 1994). The undermining of the organic coating as a result of anodic zinc dissolution (1) is a frequent, although not exclusive, cause of cut edge corrosion. In this case, cathodic oxygen reduction (2) is localized on the steel. Previous workers have shown that under these conditions the rate of reactions is controlled by a mass transport of oxygen to the metal surface (Jones and Nair, 1985; Berke et al., 1985).

(1)
Zn ( s ) ⇔ Zn ( aq ) + 2 + 2 e -
(2)
1 / 2 O 2 ( g ) + H 2 O ( 1 ) + 2 e - ⇔ 2 OH ( aq ) - Strontium chromate has been clearly seen to reduce the galvanic corrosion of zinc and steel in a model cell arrangement that simulates a cut edge (Howard et al., 1996). EIS has been used in a lot of studies to analyze the general degradation of organic coatings (Bastos et al., 2011; Bastos and Simoes, 2009; Mansfeld and Tsai, 1991; Zubielewicz and Krolikowska, 2009; Mansfeld et al., 1982; Ranjbar et al., 2004; Walter, 1986; Touzain, 2010; Deflorian et al., 2005; Penney et al., 2007; Niknahad et al., 2010; Zin et al., 2005; Gonzalez-Garcia et al., 2007; Rossi et al., 2005), the impedance response of an organic coating with artificially-produced defects (Geenen et al., 1990) and the impedance response of cut-edge corrosion of organically-coated galvanized steel (Howard et al., 1999a,b,c; Elvins et al., 2008; Simoes et al., 2009).

In this work, the degradation of the paint film by artificial acid rain solution at the cut-edge of the organic coated galvanized steel was studied with EIS. Measurements were performed at the end of the 120th and 180th days. The experimental data were fitted to appropriate equivalent electrical circuits with ZView program.

2

2 Experimental

Specimens of hot dip galvanized steel (HDG), with a zinc coating 20 μm thickness on steel were obtained. Sections of galvanized steels, with dimensions of 5 × 10 cm, were first coated with 10 μm of wash primer (polyvinyl butyral), and then 35–40 μm of epoxy, polyurethane and polyester paint were applied separately as a top coat. Then organic-coated galvanized steels were cut 1 cm from edge. A test cell was prepared, with dimensions of 50 × 75 × 40 cm and made from Plexiglass, the samples were placed at an inclination of 45° relative to the horizon (Fig. 1), then these samples were exposed to the artificial acid rain solution according to test conditions. The experiments were performed in ambient laboratory air and temperature (25 °C). The corrosion tests were performed on these materials. In order to simulate fogging an experimental set up was constructed for the laboratory investigation. Fogging intensity was regulated by introducing clean compressed air into an orifice of a thin tube with flowing artificial rain, hence, forming small drops leaving the orifice.

Experimental test cell set-up.
Figure 1
Experimental test cell set-up.

The samples were exposed to a 10 time concentrated artificial acid rain solution (pH 3.5) to simulate fogging/awaiting test cells. The composition of a 10 time concentrated artificial acid rain solution is as follows (Dehri et al., 1999):

Sulfuric acid (1.84s.g.) 31.85 mg/L
Ammonium sulfate 46.2 mg/L
Sodium sulfate 31.94 mg/L
Nitric acid (1.42s.g.) 15.75 mg/L
Sodium nitrate 21.25 mg/L
Sodium chloride 84.85 mg/L

(pH adjusted to 3.5 with sodium carbonate).

Test conditions were 10 s fogging, 1 h awaiting. After 120 and 180 days the samples were removed from the test cell for EIS. PVC tubes, (0.503 cm2) were glued to the substrates and filled with a 10 time concentrated artificial acid rain solution (Fig. 2). After exposing the substrates into this 10 time concentrated artificial acid rain solution for 2 h, EIS was performed at open circuit potentials in the 100 kHz–10 mHz frequency range using an amplitude of 10 mV (peak to peak) at room temperature with CHI 660 °C. The counter electrode was a platinum wire with a 0.7 cm2 surface area and Ag/AgCl (3 M KCI) electrode was used as the reference.

Three-electrode system used for the impedance measurements on coated galvanized steel.
Figure 2
Three-electrode system used for the impedance measurements on coated galvanized steel.

3

3 Results and discussion

3.1

3.1 The evaluation of epoxy-coated galvanized steel cut-edge impedance diagrams

The Nyquist plot with its Bode diagram as an inset of epoxy-coated galvanized steel exposed to artificial acid rain solution for 120 days is given in Fig. 3. It is clear that two capacitive loops are present in high and low frequency regions. The high frequency capacitive loop is related to the coating resistance (Rpf) and the low frequency capacitive loop is related to the pore resistance (Rpo) (Ranjbar et al., 2004; Dehri and Erbil, 2000; Thompson and Campbeel, 1994). The Nyquist plot of epoxy-coated galvanized steel in artificial acid rain solution after 120 days does not yield a perfect semicircle, generally attributed to the frequency dispersion as well as to the inhomogeneities of surface and mass transport resistance (Zhang et al., 2004; Dehri et al., 2003). A slightly depressed semicircular shape at high frequencies in the Nyquist plot was observed. It indicates that the corrosion of epoxy-coated galvanized steel in artificial acid rain solution is mainly controlled by a charge transfer process (Zin et al., 2005; Nikravesh et al., 2011).

Nyquist and Bode (as inset) plots of epoxy-coated galvanized steel at the cut-edge after exposure to the artificial acid rain solution for 120 days (solid lines show fitted results).
Figure 3
Nyquist and Bode (as inset) plots of epoxy-coated galvanized steel at the cut-edge after exposure to the artificial acid rain solution for 120 days (solid lines show fitted results).

The related electrochemical equivalent circuit used to model the epoxy-coated galvanized steel–artificial acid rain solution interface is shown in Fig. 4, where Rs represents the solution resistance, Rpf corresponds to the first loop of Nyquist plot, and Rpo represents the pore resistance. Rpo includes charge transfer resistance (Rct), diffuse layer resistance (Rd), and the accumulated species resistance at the metal/solution interface (Ra). CPE represents a constant phase element to replace a double layer capacitance (Cdl) in order to give a more accurate fit to the experimental results (Zin et al., 2005; Jorcin et al., 2006). The impedance parameters obtained by fitting the Nyquist plot to the equivalent circuit are listed in Table 1. In Fig. 3, high and low-frequency loops were attributed to coating capacitance and double layer capacitance, respectively. The total capacitance of the capacitors is equal to the sum of the double layer capacitance and coating capacitance.

Equivalent electrical circuit for defective organic-coated metals. Rs: solution resistance, Rpf: coating resistance, CPE1: coating capacitance, Rpo: pore resistance, CPE2: double layer capacitance.
Figure 4
Equivalent electrical circuit for defective organic-coated metals. Rs: solution resistance, Rpf: coating resistance, CPE1: coating capacitance, Rpo: pore resistance, CPE2: double layer capacitance.
Table 1 Fitting parameters Rs, CPE1, Rpf, CPE2, Rpo, CPE3, and Rdiff for the epoxy coated galvanized steel at the cut-edge after exposure to the artificial acid rain solution test for 120 and 180 days.
Epoxy coating Rs (kΩ cm2) Rpf (kΩ cm2) CPE1 (F cm−2) Rpo (kΩ cm2) CPE2 (F cm−2) Rdiff (kΩ cm2) CPE3 (F cm−2)
120 days 0.91 2500 1.4 × 10−09 10,500 5.14 × 10−07 – –
180 days 0.90 11.5 1.33 × 10−08 24.6 2 × 10−05 1000 8.2 × 10−05

The Nyquist diagram obtained at the sample exposed to artificial acid rain solution for 180 days is shown in Fig. 5. By increasing days (from 120 to 180 days), the significant influence of exposure time on the size of the impedance spectra was observed. As it is seen from the inset of Fig. 5 there are 3 time constants, two capacitive loops in high and middle frequency regions and followed a straight line in low frequencies. The high and middle frequency capacitive loops are related to Rpf and Rpo, respectively. The low frequency straight line implies that the corrosion of epoxy-coated galvanized steel is diffusion controlled. The representative equivalent circuit model is given in Fig. 6. It is known that, the anodic dissolutions of galvanized steel and the cathodic oxygen reduction reactions take place simultaneously on the metal surface. This was represented as Rdiff (Rdiff = diffuse layer resistance, CPE3 = diffuse layer capacitance) (Zhang et al., 2004; Özcan et al., 2002; Bonora et al., 1996). The diffusion process may be due to either the transportation of corrosive ions and soluble corrosion products at the metal/solutions interface or the diffusion of dissolved oxygen to the galvanized steel surface (Worsley et al., 2001). However, the second one is more probable.

Nyquist and Bode (as inset) plots of epoxy-coated galvanized steel at the cut-edge after exposure to the artificial acid rain solution test for 180 days (solid lines show fitted results).
Figure 5
Nyquist and Bode (as inset) plots of epoxy-coated galvanized steel at the cut-edge after exposure to the artificial acid rain solution test for 180 days (solid lines show fitted results).
Equivalent electrical circuit for defective organic-coated metals. Rs: solution resistance, Rpf: coating resistance, CPE1: coating capacitance, CPE2: double layer capacitance, Rpo: pore resistance, CPE3: diffuse layer capacitance, Rdiff: diffuse layer resistance.
Figure 6
Equivalent electrical circuit for defective organic-coated metals. Rs: solution resistance, Rpf: coating resistance, CPE1: coating capacitance, CPE2: double layer capacitance, Rpo: pore resistance, CPE3: diffuse layer capacitance, Rdiff: diffuse layer resistance.

In the evaluation of Table 1, the Rpf values showed a sharp decrease from 2500 kΩ cm2 for 120 days to 11.5 kΩ cm2 after 180 days of exposure, indicating a more corrosive process. It is apparent from Table 1 that the CPE values tend to decrease, as the Rpf values increased. This decrease in the CPE can be attributed to the decrease in local dielectric constant or an increase in the thickness of the electrical double layer (Lavaert et al., 2002).

3.2

3.2 The evaluation of polyurethane-coated galvanized steel cut-edge impedance diagrams

The Nyquist diagrams of the electrochemical impedance data for the polyurethane-coated galvanized steel exposed to artificial acid rain for 120 and 180 days are shown in Figs. 7 and 8, respectively. The related Bode diagrams are added to Figs. 7 and 8 as the inset. In Figs. 7 and 8 it is clear that all the impedance spectra obtained in the artificial acid rain solution consist of two capacitive loops. The related electrochemical equivalent circuit used to model the polyurethane-coated galvanized steel/artificial acid rain solution interface is shown in Fig. 4. The fitted parameters are given in Table 2. As it can be seen from Table 2, the coating resistance and pore resistance decreased with an increase in exposure time. The decrease in the Rpf and Rpo values was attributed to the same corrosion process occurred on the metal surface and pores (Dehri et al., 1999; Ranjbar et al., 2004; Dehri and Erbil, 2000). CPE values obtained from EIS measurements are in good agreement with each other.

Nyquist and Bode (as inset) plots of polyurethane-coated galvanized steel at the cut-edge after exposure to the artificial acid rain solution test for 120 days (solid lines show fitted results).
Figure 7
Nyquist and Bode (as inset) plots of polyurethane-coated galvanized steel at the cut-edge after exposure to the artificial acid rain solution test for 120 days (solid lines show fitted results).
Nyquist and Bode (as inset) plots of polyurethane-coated galvanized steel at the cut-edge after exposure to the artificial acid rain solution for 180 days (solid lines show fitted results).
Figure 8
Nyquist and Bode (as inset) plots of polyurethane-coated galvanized steel at the cut-edge after exposure to the artificial acid rain solution for 180 days (solid lines show fitted results).
Table 2 Fitting parameters Rs, CPE1, Rpf, CPE2 and Rpo for the polyurethane coated galvanized steel at the cut-edge after exposure to the artificial acid rain solution test for 120 and 180 days.
Polyurethane coating Rs (kΩ cm2) Rpf (kΩ cm2) CPE1 (F cm−2) Rpo (kΩ cm2) CPE2 (F cm−2)
120 days 0.89 130 2.3 × 10−09 250 5.96 × 10−06
180 days 0.95 67 9.6 × 10−09 121.7 7.95 × 10−06

3.3

3.3 The evaluation of polyester-coated galvanized steel cut-edge impedance diagrams

The representative Nyquist and Bode (as inset) plots of polyester-coated galvanized steel obtained in artificial acid rain solution are given in Figs. 9 and 10, respectively. As it is seen from Figs. 9 and 10 the Nyquist plots obtained in artificial acid rain media contain two capacitive loops at high and low frequency regions. EIS data were fitted according to the electrical equivalent circuit diagrams given in Fig. 4. The calculated impedance parameters are given in Table 3. As it is shown in Table 3, the Rpf and Rpo values decreased after 120 days in the artificial acid rain media which may be due to the formation of some defects on the coating leading to the access of aggressive ions to the metal/organic substance interface. Rpf and Rpo values of polyester-coated galvanized steel in artificial acid rain media decreased from 11.6 and 146.1 kΩ cm2 to 7.8 and 61.5 kΩ cm2, respectively, with increasing exposure time.

Nyquist and Bode (as inset) plots of polyester-coated galvanized steel at the cut-edge after exposure to the artificial acid rain solution for 120 days (solid lines show fitted results).
Figure 9
Nyquist and Bode (as inset) plots of polyester-coated galvanized steel at the cut-edge after exposure to the artificial acid rain solution for 120 days (solid lines show fitted results).
Nyquist and Bode (as inset) plots of polyester-coated galvanized steel at the cut-edge after exposure to the artificial acid rain solution for 180 days (solid lines show fitted results).
Figure 10
Nyquist and Bode (as inset) plots of polyester-coated galvanized steel at the cut-edge after exposure to the artificial acid rain solution for 180 days (solid lines show fitted results).
Table 3 Fitting parameters Rs, CPE1, Rpf, CPE2 and Rpo for the polyester coated galvanized steel at the cut-edge after exposure to the artificial acid rain solution test for 120 and 180 days.
Polyester coating Rs (kΩ cm2) Rpf (kΩ cm2) CPE1 (F cm−2) Rpo (kΩ cm2) CPE2 (F cm−2)
120 days 0.90 11.6 1.53 × 10−7 146.1 2.84 × 10−5
180 days 0.92 7.8 1.75 × 10−7 61.5 1.25 × 10−4

The appearance of the test specimens exposed to acid rain solution was given for 0, 120 and 180 days in Fig. 11. The cut edge corrosion was observed moving toward the inner parts of the metal at the 120th day of epoxy (A′), polyurethane (B′) and polyester (C′). More degradation was observed at the inner parts of the metal at the end of the 180th day (A″) for epoxy. Blisters were observed at the inner parts of the metal at the end of the 180th day (B″) more intensely for polyurethane. The corrosion on the surface of the metal was observed thoroughly at the end of the 180th day (C″) for polyester. As it is seen in Fig. 11 the polyester coating was more damaged than the polyurethane and epoxy coatings.

0, 120 and 180 days during the appearance of test specimens exposed to artificial acid rain solution. Epoxy 0 day (A), Epoxy 120 days (A′), Epoxy 180 days (A″), Polyurethane 0 day (B), Polyurethane 120 days (B′), Polyurethane 180 days (B″), Polyester 0 day (C), Polyester 120 days (C′), Polyester 180 days (C″).
Figure 11
0, 120 and 180 days during the appearance of test specimens exposed to artificial acid rain solution. Epoxy 0 day (A), Epoxy 120 days (A′), Epoxy 180 days (A″), Polyurethane 0 day (B), Polyurethane 120 days (B′), Polyurethane 180 days (B″), Polyester 0 day (C), Polyester 120 days (C′), Polyester 180 days (C″).

4

4 Conclusions

The cut edge corrosion behavior of three organically coated (epoxy, polyurethane and polyester) galvanized steels was studied using EIS techniques in artificial acid rain solution for long exposure days. From the results obtained, the following points can be emphasized;

  1. By increasing days in atmospheric conditions, artificial acid rain solutions accelerated the damage and corrosion rate of the organic-coated galvanized steel.

  2. The experimental data were fitted using the ZView program and appropriate values for each equivalent circuit element have been identified for organic-coated galvanized steel after exposure to artificial acid rain solutions in atmospheric corrosion tests.

  3. After exposure to artificial acid rain solutions for 120 and 180 days in a test cell, the organic-coated specimens experienced corrosion, and the corrosion process is confirmed through localized impedance measurements.

  4. According to the Nyquist and Bode plots, after 120 days the Rpf decreased significantly indicating the defects on the organic coatings. The distance of this defect from cut edge to that into the sheet was about 0.8 cm.

  5. The surface photographs showed that the degradation of coatings (that is defective coating areas near to the cut edge) increased with increasing the exposure.

Acknowledgement

This study was supported by Cukurova University Research Found (FEF2010D10).

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