Layer | Fill | Outline |
---|
Map layers
Theme | Visible | Selectable | Appearance | Zoom Range (now: 0) |
---|
Fill | Stroke |
---|---|
Collaborating Authors
Results
ABSTRACT Electrochemical noise (the spontaneous fluctuations in current or potential associated with corroding electrodes) has been studied for about 37 years, but it is only relatively recently that a reasonably sound theoretical basis for the technique has been derived. It is now clear that the technique provides a method, the determination of electrochemical noise resistance (Rn) that provides a reasonable estimate of the corrosion rate. It also seems probable that it is able to provide information about the type of corrosion occurring, although the optimal technique for doing this is not yet established. Furthermore, there are indications that although the estimate of corrosion rate is relatively robust, indicators of corrosion type are very susceptible to extraneous influences. However, as an estimator of corrosion rate, electrochemical noise measurement is relatively poor compared to conventional techniques, but its potential ability to identify the type of corrosion occurring remains its major advantage. INTRODUCTION The measurement of electrochemical noise, EN, for the study of corrosion processes is now relatively well-established. In this paper the properties of the technique and the methods of analysis will be reviewed, and its capabilities compared with those of other electrochemical methods. MEASUREMENT OF EN Initial measurements of EN recorded the fluctuation in potential, usually measured relative to a conventional reference electrode. This is known as the electrochemical potential noise, EPN. Subsequently it was appreciated that it was also possible to measure the fluctuation in current or electrochemical current noise, ECN. This can either be done by maintaining a constant potential with a potentiostat, or by coupling two electrodes through a zero resistance ammeter (also know as a current amplifier). This then led to the now conventional three-electrode method wherein the ECN is measured between two nominally identical working electrodes, while the EPN is measured with a third electrode, which may either be a reference electrode or a third electrode that is nominally identical to the two working electrodes. It is important to appreciate that there are a number of artefacts that can be introduced during the measurement process, including instrument noise, aliasing and quantization noise. These are detailed in. An important characteristic of the measurement process is the sampling rate of the measurement. This is frequently of the order of 1 sample per second, as is it often found that the power present in the EN signal is very small at higher frequencies. It is also much simpler to obtain reliable measurements when the sampling frequency is well below the power line frequency, since this presents a major noise source at higher frequencies. In addition, the only well-established theoretical analysis depends on the properties at very low frequencies. Despite this, it may be useful to make measurements at higher frequencies in some situations, and this probably merits more careful examination. Specimen area has a somewhat counter-intuitive effect on measured EN. Workers who are used to normal measurements of electrochemical potential and current tend to assume that electrochemical noise can be treated in the same way, so that the potential noise, measured as the standard deviation of potential, öE , so that it has units of volts, is independent or area, while the standard deviation of current, öI , is proportional to the specimen area, A. However, this is not what is expected for ?normal? noise behaviour; instead óE is expected to be proportional to A-1/2, while öI is expected to be proportional to A1/2. Note that there are some situations, notably wh
ABSTRACT To understand Cathodic Protection (CP) better, electrochemical studies and surface chemistry were used to establish what is actually happening when metal specimens are cathodically protected at a series of potentials in aerated 3.5% NaCl solution. It was found that films are formed and that these significantly affect the current demand and corrosion rate of the test specimens. The specimen with the least corrosion rate and most coherent film was at -1300 mV (vs Ag/AgCl/3.5% NaCl), but this is a potential where there can be significant hydroxide formation (OH-) which can have detrimental side effects. INTRODUCTION Cathodic Protection (CP) is a technique used worldwide to minimize corrosion of buried or immersed metallic structures. Various international standards/specifications set out the application and control of CP but do not give consideration to the electrochemistry and surface chemistry of the metallic structure/electrolyte interface [1] [2] [3] [4]. The ISO Standard [5] is more definitive, describing CP in terms of what is actually happening electrochemically at the metal/electrolyte interface of a cathodically polarized structure. It defines CP as electrochemical protection achieved by decreasing the structure potential to a level whereby the corrosion rate of metal is significantly reduced but still fails to provide intimate direction as to the nature and role of the interface or influence of surface films. In concept, cathodically protecting a metallic structure involves applying a current that will flow from the anode through the surrounding electrolyte (water, soil) to the structure surface. The applied current shifts the potential in a more negative direction. If at a sufficiently negative potential, the amount of current applied is great enough, then it will overpower the current discharging from previously anodic sites causing the entire surface to become cathodic and protected [6] [7]. However, this is not always the case; for example some pipelines having reportedly adhered to protection criteria such as -850 mV Cu/CuSO4 OFF, have been found through metal loss to be not fully protected some years later [8]. This has led to the reasoning that there is more to CP than just applying a certain amount of current and checking to ensure that the pipeline meets the potential value according to set criteria. Electrochemical Analysis of CP When applying CP for the first time to a structure it is well known that a higher current and lower structure/electrolyte voltage is observed. Over a period of a week or more the current decreases and the voltage increases. Within the CP industry it is stated that the surface has passivated, but does the industry really understand what this means? By implication, the application of CP is thought to give rise to a passive film on the steel surface probably from a combination of the steel?s corrosion products, hydrogen gas and calcareous deposits [9] [10]. In order to understand CP and the nature of the passive films it is important to know what is actually happening electrochemically at the surface of the metallic structure when CP is applied under the test conditions. The electrode potential/electrolyte pH diagram, the Pourbaix diagram, is used to explain the anodic and cathodic behavior of steel in electrolytes of different pH and categorizes the domains of corrosion, passivation and immunity, see Figure 1. As CP is increased and the potential becomes more negative, the surface electrochemistry will move to the region of immunity, position B on the Pourbaix diagram, where steel is more thermodynamically stable and corrosion does not occur. Over time however, the pH will increase as a result of the generation of alkalinity due
- Well Completion > Well Integrity > Subsurface corrosion (tubing, casing, completion equipment, conductor) (1.00)
- Production and Well Operations > Production Chemistry, Metallurgy and Biology > Corrosion inhibition and management (including H2S and CO2) (1.00)
- Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers > Materials and corrosion (1.00)