Near surface pH measurements in aqueous CO₂ corrosion

Abstract

Corrosion of carbon steels in a carbon dioxide (CO₂) corrosive environment is an important issue and in some cases the steel surface can be covered by a protective corrosion product film. Research has shown that under certain conditions, once a truly protective film has precipitated on the steel surface, the corrosion rate can decrease by an order of magnitude. Over many years, both quantitative and qualitative research has been carried out to further understand the initiation and growth of the protective film. However, a main limitation is in the correlation of film properties with bulk solution conditions. Research has shown that serious errors in predicting/reasoning can be made by operating with bulk instead of surface water chemistry conditions. This paper uses a pH sensor design to be used for real time surface pH measurement. The study shows a simultaneous electrochemical and surface pH analysis at two varying conditions of pH (pH 6 and pH 6.6) where a characteristic difference in the morphology and hence protectiveness of a corrosion product film is anticipated.

Introduction

The formation and development of corrosion product films on a metal surface is essential to corrosion science. A “protective film” can cause a significant reduction in the corrosion rate by blocking the underlying steel from further dissolution. However, local defects or damage in the corrosion product film/scale covered surface can lead to severe localised corrosion by exposing the underlying metal to the aggressive corrosion environment [1]. Over the years, intensive research [[2], [3], [4], [5], [6]] has been carried out offering insight into the various complex processes occurring in CO₂ corrosion. These studies show that CO₂ corrosion product films can be affected by many factors such as temperature, pressure, flow rate and pH value. Among these parameters, it has been shown both experimentally and computationally that pH has a strong influence on the corrosion rate [3,[7], [8], [9]]. The most important effect of pH is indirect and relates to how pH changes conditions for formation of iron carbonate (FeCO₃) scales and can have a profound effect on how effectively the film is able to limit corrosion kinetics on the underlying steel.

The formation of FeCO₃ on a metal surface in a CO₂ environment takes place when the concentrations of iron ions (Fe²⁺) and carbonate ions (CO₃²⁻) exceed the solubility product (K_sp) of FeCO₃ in a solution [10]:

$$F{e}_{\left(aq\right)}^{2+}+C{O}_{3\left(aq\right)}^{2-}\to FeC{O}_{3\left(s\right)}$$

The nature, morphology and protection offered by the FeCO₃ precipitation process is dependent on the environmental conditions which influence the formation of CO₂ corrosion product films primarily by changing FeCO₃ supersaturation (SS) [11]. The supersaturation level which is deemed to be the driving force behind the precipitation process [7,8] is defined as:

$$S{S}_{FeC{O}_3}=\frac{\left[F{e}^{2+}\right]\left[C{O}_3^{2-}\right]}{K_{sp}}$$

Where [Fe²⁺] and [CO₃²⁻] (in mol/m³) are the concentrations of ferrous and carbonate ions, respectively. K_sp (in mol²/m⁶) is the solubility product of FeCO₃, which is a function of ionic strength and temperature [12].

Solution pH affects the supersaturation of FeCO₃ due to its impact on the carbonate ion concentration. At high solution pH, high concentrations of CO₃²⁻ result in fewer Fe²⁺ ions being required to exceed the solubility product of FeCO₃ and generate significant levels of precipitation [2]. Through consideration of the equilibrium and dissociation chemical reactions that take place in a CO₂ corrosive environment, referenced in Nordsveen et al. [13], the CO₃²⁻ concentration and concentration of other ions in the system as a function of pH is determined and is shown in Fig. 1. The values are calculated for a temperature of 80 °C and atmospheric total pressure.

Fig. 1 shows that the concentration of CO₃²⁻ increases by over an order of magnitude with increasing pH meaning much less Fe²⁺ ions are needed to reach the critical SS required for substantial crystal nucleation and growth. The supersaturation of FeCO₃, for a constant Fe²⁺ ion concentration (0.001 M), is calculated for varying solution pH and is included in Fig. 1. This calculation is used to clearly indicate the indirect impact of pH on the SS of FeCO₃. Furthermore, the figure is annotated to indicate the CO₃²⁻ ion concentration and SS value at both pH 6 and pH 6.6.

Experimental evidence of the influence of pH on FeCO₃ supersaturation and hence the protectiveness of the developed film is clearly shown in a study by Sun et al. [14]. The work demonstrated that under conditions of pH 6 at 80 °C in a 0.54 bar CO₂ saturated 1 wt% NaCl brine with an addition of 50 ppm Fe²⁺, a low bulk SS_FeCO3 was obtained resulting in the formation of a porous, detached and poorly protective FeCO₃ film forming on the carbon steel surface. Higher pH values of 6.6, under the same operating conditions, resulted in a higher bulk SS_FeCO3, faster precipitation kinetics and formation of a more protective FeCO₃ film. The obtained corrosion rate results for varying bulk SS_FeCO3 is shown in Fig. 2. Other similar studies have also demonstrated that protective FeCO₃ formation is accelerated/enhanced at higher solution pH [7,8,15,16].

One main limitation within most experimental studies in literature is that the characteristics of FeCO₃ precipitation is studied as a function of bulk solution properties. It is well understood amongst researchers [[17], [18], [19]] that the local surface chemistry conditions can be very different than those in the bulk solution due to electrochemical surface processes that result in metal dissolution (corrosion) and deposition. This has been further illustrated with the aid of mathematical modelling based on thermodynamic, kinetic and transport theories providing a numerical quantification of surface conditions [13,20,21]. However, difficulties with probe design and development limit the accurate measurement of properties at the near surface region of a corroding surface.

For varying research topics, direct [22,23] and indirect measurement [24,25] methods have been designed to measure the surface pH. The most recent and relevant study was carried out by Han et al. [17] where a mesh capped probe was designed for direct pH measurements on an actively corroding surface. The probe design consists of a flat sensor pH probe with a tip which is adhered to a mild steel mesh allowing the surface pH to be monitored during the corrosion of the mesh. The design was initially suggested by Romankiw [22] for electrolysis systems. The study monitored the surface pH for varying temperature (25 °C and 80 °C) and solution pH (pH = 4, 5, 6 and 6.6) in a CO₂ corrosion environment. Higher surface pH deviation from the bulk was observed at higher temperatures as well as at lower bulk pH conditions. The observed results provided an experimental observation that coincides with the theoretical predictions. However, certain aspects that are believed to be essential to corrosion science was not considered within the research article. The study was limited to a series of short tests and did not take into account the formation of FeCO₃ that may take place on the steel mesh under the conditions of high temperature and pH studied. The duration of the experiments conducted were noted to be 10 h. Previous research [8] has shown that the formation of FeCO₃ may be expected for some of the conditions studied by Han et al. [17] however it may have not been sufficiently “protective” within the short duration of exposure to disrupt the stable measurements obtained and cause a change in surface pH. In order to study the effect of a protective film on the surface pH, the study artificially deposited inert sand (100–500 μm in size) and glass beads (50–80 μm in size) on the mild steel mesh surface to simulate a deposit layer of varying porosity. Both were deposited in the form of a layer of 5 mm in thickness on the mild steel mesh resulting in diffusion affected corrosion reactions. The meshed pH electrode measured the surface pH under the artificially deposited layer (under-deposit pH). The results showed high values of surface pH and was observed to be higher for the less porous glass bead deposit (30% porosity) compared with the more porous sand particle deposit layer (40% porosity). Within the research study by Han et al. [17], the unique experimental method provided an effective and extensive analysis of the measurement of surface pH under an array of conditions. However, it may be argued that the lack of a link with the corrosion rate limits the realistic implications of the study.

The focus of this work is to design and employ a flexible surface pH probe based on that by Han et al. [17] and Romankiw [22]. The probe shall be used to study the surface pH of a metal mesh under conditions of high pH and temperature over a period of 12 days. The research aims to further understand the development of the surface pH as a function of the protectiveness of the corrosion product scale. The study is carried out in conjunction with electrochemical measurements and surface characterisation techniques to provide a link between pH, the mild steel corrosion rate and the protective properties of the corrosion product layer.