EIS study of the effect of high levels of SO2 on the corrosion of polyester-coated galvanised steel at different relative humidities

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Abstract

The effect of SO2 on the degradation of polyester-coated galvanised steel at different relative humidities was investigated using electrochemical impedance spectroscopy. Measurements were performed on specimens which had been tested in an accelerated gaseous corrosion test. For this purpose the samples were subjected to SO2 gas for 16 days in atmospheric test cells with adjusted relative humidity (RH) from 60 to 100%. Subsequently, the impedance response of the coated material was measured and evaluated. The results indicated that the coating performance varies with RH. Thus, under condensing conditions, the organic coating and galvanised layer was totally removed, the impedance response being interpreted as the formation of an iron sulphide film on the surface. At lower RH, remarkably, the coating remained effectively intact with the coating resistance varying inversely with RH. This work is relevant to the application of such organic-coated products adjacent to combustion flues where high levels of SO2 occur in association with high humidity.

Introduction

Atmospheric corrosion is a consequence of the dissolution of a metal due to the presence of salts and gases dissolved in a moisture film on the metal surface. The thickness of the film and, consequently, the amount of water present on a metal surface depends predominantly on the thermodynamic activity of water vapour in the atmosphere (relative humidity) as well as the chemical and physical state of the metal surface. It is well established that the atmospheric corrosion rates of metals depend on the thickness of the electrolyte layer [1]. Changes in the thickness of the layer affect, amongst other things, the oxygen gas transport rate through the electrolyte layer and the solubility of corrosion products and, hence, the metal corrosion rates [2].

The presence of chemically active pollutants that are water soluble have a considerable effect on corrosion in humid atmospheres. SO2 is commonly present in urban and industrial regions. Its presence in moisture films enhances the corrosion rate for a number of reasons. SO2 is dissolved in the moisture film and undergoes oxidation largely to sulphuric acid [3]. Thus, many corrosion product films formed in the atmosphere contain large proportions of sulphate ions although species like sulphite, bisulphite and bisulphate may also be produced [4], [5], [6]. Moreover, if the conditions are suitable, generally at high SO2 partial pressure, SO2 may act as a cathodic reductant resulting in the formation of various reduced sulphur anions, e.g.: thiosulphite, di-thionite, sulphite and sulphide [7].

Organic coatings have long been used to protect metals and alloys against corrosion. Although such coatings form a barrier against diffusion of aggressive ions; water vapour, and oxygen and other gases are permeable and, therefore, will be present at the metal/coating interface. Under certain circumstances, this can lead to substrate corrosion and coating delamination [8].

Polyester-coated hot-dip galvanised steel is widely used in the building industry for external cladding, including roofing, because of its good corrosion behaviour [9]. If the coatings have no defects and have appreciable thickness, they will provide effective protection. However, in practical applications many coatings contain small defects (which may form during their production or in usage) which allow the penetration of aggressive species directly onto the metal leading to corrosion. At defects in the organic coating on galvanised steel, zinc will be dissolved anodically with the formation of various zinc salts. The higher the SO2 content of the atmosphere the lower the hydroxide and oxide content of the corrosion products and the higher the sulphate content [5], [10]. Thus, a common reaction sequence during atmospheric corrosion with SO2 gas is the initial formation of zinc hydroxide or zinc oxide followed by the interaction of these products with SO2 to give initially a basic zinc sulphite or sulphate. Eventually soluble hydrated zinc sulphate, which is the thermodynamically favoured end-product, will form.

Electrochemical impedance spectroscopy (EIS) has been widely used as a tool in the investigation of the protective properties of organic coatings for metals. The macroscopic coating resistance and coating capacitance may be determined, as well as the charge transfer resistance and double layer capacitance values associated with the metal–electrolyte interface at local coating defects. A general equivalent electrical circuit model may be used to describe an intact dielectric coating (Fig. 1(a)) or a coating with porosity (Fig. 1(b)) in which electrochemical reactions may proceed, where Rso is the solution resistance, Rpf and Cpf the paint film resistance and paint film capacitance, respectively; Cdl the double layer capacitance and Rct the charge transfer resistance of an electrochemical (corrosion) process [8], [16].

In modelling electrochemical (corrosion) processes, the term Qdl, a constant phase element (CPE), may be substituted for Cdl:Qdl=C×1(iω)αwhere C is a constant depending on the specific analysed system, i=−11/2, α a coefficient ranging between 0 and 1, and ω the angular frequency. The physical meaning of a CPE has been discussed by many authors and can be related to surface heterogeneity. In this case, CPE represents all the frequency-dependent electrochemical phenomena; including double layer capacitances and diffusion processes [11]. Qdl is introduced instead of Cdl in the time constant associated with the corrosion process and is represented as a deviation from a semi-circle.

The aim of this study was to determine the effect of SO2 gas on the atmospheric corrosion of polyester-coated galvanised steel substrates at different relative humidities. Under normal atmospheric conditions SO2 concentrations are in the range 30–300 ppb. However, under certain conditions this concentration can increase greatly. For example, industrial factory environments, close to flue outlets from space-heating equipment and boilers, in ductwork transporting combustion gases, and where the conditions are close to the dew point, SO2 and water vapour concentrations can both be greatly elevated. In any of these situations, organic-coated materials may be used and it is therefore of interest to examine their performance in environments which are severely contaminated by SO2.

Section snippets

Experimental

Test cells of dimension 20×30×50 cm containing appropriate water–glycerol mixtures [5] to control the internal relative humidities at 60, 70, 80, 90 and 100% were prepared. One day later, after equilibration, the substrates (polyester-coated galvanised steel) were placed in the test cells in such a way to make an angle of 45° with the horizontal (Fig. 2). They were then subjected to the SO2 gas environment for 16 days at room temperature (25±2 °C). The rate of gas flow through the chamber was 2 cm3

100% relative humidity

After removal of the substrate from test cell, the organic coating was found to have mostly failed. Further, black-coloured foul-smelling corrosion products were also observed under the defective coating. After electrochemical impedance measurements, the organic coating was removed from the metal and the surface corrosion products removed for analysis. These data showed that the corrosion products contained (by weight) 48.2% Fe, 8.7% Zn, 29.7% S2−, 9.3% SO42− and 4.1% other species not analysed

Concluding remarks

This work presents data on the effect of high concentrations of sulphur dioxide (accelerated atmospheric corrosion test) on the degradation of polyester-coated galvanised steel. While the concentrations used are unrealistic for atmospheric exposures the experiment provides a useful indication for the expected performance of such coatings used in ducting and cladding close to flue outlets where combustion products may impact directly onto the coating. Under such conditions, the humidity and SO2

Acknowledgements

This study [TBAG-1696 (198T029)] was supported by The Scientific and Technical Research Council of Turkey (TUBITAK). The authors are grateful to TUBITAK.

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