The present work is aimed to demonstrate the differences on the electrochemical behaviour of pipe flow electrodes with different lengths. This work compares the CO2 cathodic kinetics, taking place on the surface of two turbulent flow systems: pipe flow electrodes of two different lengths and a RCE, both systems are mounted in the same flow loop. It is demonstrated that the limiting current density measured on pipe flow electrodes, is dependent on the electrode length. It is also demonstrated that the limiting current density measured on the surface of a RCE, is equivalent to the limiting current density measured on a long pipe flow electrode, if the electrode length effect is taken into account. These results are of particular interest in the design and operation of pipe loop experiments.
The quantitative description of CO2 corrosion in steel pipelines has been commonly described by means of semi-empirical methods based on statistical studies of the phenomenon 1,2. These methods have proven to be helpful. However, their lack of theoretical foundations, does not allow the user to make deeper kinetic studies.
The influence of turbulent flow on the corrosion of steel in CO2 containing environments, has been a research topic of interest, for the past few years. Some studies have developed different concepts intended to understand and predict, in a more accurate way, the corrosion of steel in CO2 containing environments 3,4. The control of the hydrodynamic conditions, can be a key aspect to be considered during the design of laboratory tests and the corresponding analysis of results. Several laboratory hydrodynamic systems have been used in the study of corrosion and flow 5, such as: rotating electrodes, annular flow, impingement jets, pipe sections, etc.
Hydrodynamic systems involving electrodes located in pipe sections are also very popular in laboratory testing, this is due to the extensive information available in the literature for them. On the other hand, a pipe testing section can be regarded as a very reliable system, if the study of the corrosion phenomena taking place inside pipelines is the main objective.
Several hydrodynamic systems can be studied at the same time in experimental set ups known as flow loops. In a flow loop the testing solution is forced, by an external pumping device, to circulate trough the different hydrodynamic systems involved in the design of the set up. In this way, constant environment composition and physicochemical conditions are assured, in all the hydrodynamic systems and therefore, equivalencies can be made. However, the use of a flow loop could be expensive and its operation troublesome.
On the other hand, the rotating cylinder electrode (RCE) is a hydrodynamic system that has become a popular laboratory testing device, due to its relative ease of use and the availability of numerical expressions which can describe its behavior 6,7. However, the usage of the RCE has become controversial as it seem that the results obtained in this system are not comparable with the results obtained in other hydrodynamic systems or in real life conditions 8.
