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Automated corrosion sensors as on-line real time process control tools

Final Report Summary - CORRLOG (Automated corrosion sensors as on-line real time process control tools)

In developed countries, direct corrosion losses reach approximately 3-5 % of the gross national product. This immense number represents the loss by 50 times higher than direct annual losses caused by fires. Corrosion expenditures could be radically reduced if monitoring of corrosion became a natural part of the decision making processes. However, for the time being, corrosion measurements in process plants, soils, or atmospheric conditions are usually a part of expert-conducted surveys, the results of which are presented to end-users in written reports. This project is based on the need for implementing corrosion sensors as process control tools, because the information on actual corrosivity of the environment is crucial for effective corrosion protection. Implementation of on-line and real time monitoring enables operators to take immediate counter measures if corrosion is accelerating, and thus decrease the corrosion costs. Properly executed corrosion monitoring system providing more precise knowledge on the actual corrosivity that would be available to the wide professional public will help to mitigate losses incurred due to substitution of damaged devices, loss of serviceability, ecological impacts, etc.

The aim of CORRLOG project was to develop small, independently working sensor devices for reliable, rapid, and reproducible monitoring of corrosion. It included two dimensions. The first dimension was related to optimisation of the direct accessibility of the data for end-users, i.e. the minimum need for interpretation and optimised simplicity of communication between the sensor device and the end-user. This part is to a large degree independent of the actual process system in which the sensor is applied (atmospheric conditions, water, soil, etc.). The second dimension was related to the accuracy and reliability of the sensors in their individual application platform. The project was focused on development of indoor and outdoor atmospheric corrosion sensors and fluid or soil media corrosion sensors for internal or external corrosion detection.

The innovative objectives of the project consisted of:
(1) high sensitivity and reliability of the sensor device, i.e. short time to respond;
(2) small size for sensors and measuring device and battery driven device;
(3) simple operation demanding no special skills of the personnel;
(4) GSM-reading based remote control; and
(5) detection of localised corrosion.

The concept of the measuring device is simple and yet highly effective. The electronic unit measures and registers the changes over time in the electrical resistance of a thin metal track applied on an insulating substrate. If the metal corrodes, the cross-sectional area of the track decreases and electrical resistance increases. In practice, two such elements are built into a probe. One element is exposed to the corrosive environment and corrodes whereas the other element is shielded and thus protected from corrosion. The resistance of both elements is measured at the same time and resistivity changes due to the varying temperature are compensated. Based on the initial cross sectional area of the exposed element, the cumulative metal loss at the time of reading is determined. There is no need to remove the probe from the environment; hence, the technique is online. The technique can be used practically for any kind of environment. Recent developments of the ER technique involving better means for temperature compensation and higher resolution of the resistance measurements have now turned the technique into a (at least semi-) real time technique where the time needed to create sufficiently high increase in resistance has been reduced to hours or even minutes.

The development of an aqueous corrosion sensor with optimum reliability and sensitivity for the desired end use has been the objective throughout the project. Using knowledge on the ER sensing principle, different geometrical arrangements of sensing elements, and different sealing materials were tested to study the effect of fluctuating temperature and strain on performance. A new testing procedure in non-corrosive media has been developed for this purpose and knowledge on design parameters obtained. Based on the defined acceptance criteria including temperature compensation and sensitivity, two final prototype designs were chosen, one for a flush-mounted probe, and one for an insertion probe. Correlation between electrical resistance responses monitored by the existing logger and the corrosion rate actually occurring on the sensor element was tested in a well-defined laboratory test and found to be in very good agreement. Design specifications based on end-user requirements were identified and a number of insertion steel prototypes manufactured and tested in preliminary field tests. The probes proved to be successful and the results are in agreement with coupons (weight loss) mounted in the same systems.

At the initiation of the project it was discussed how sensor material should be manufactured to best simulate the actual construction material. Based on these results the conclusion is that for the systems in question (geothermal water, district heating water, oil and gas, soil) the difference in corrosion behaviour between iron foil and carbon steel is low, and iron foil is a suitable sensor material to fulfil the needs of the end-users. The practical manufacturing of the sensor elements was also studied and the final recommended technique is the use of metal foils. Chemical or electrochemical plating was also studied e.g. for zinc-iron samples, but strong limitations on sensitivity could be the result, as the electrical resistivity of plated layers are in some cases extremely high. Machining of samples to low thickness also turned out to be unsuitable and impractical. So in practice the sensitive pipeline prototype sensor designed in this project for energy systems (oil, gas, geothermal water) is sensitive to changes in corrosion rate, but not too sensitive to temperature fluctuations, and iron foil is an adequate material to be used for the corrosion sensors used in this industry. The insertion probe is now available at Metricorr and will be marketed for district heating water applications primarily. The flush-mounted probe can be manufactured on request.

In this work an already patented idea (Metricorr) was incorporated. This idea makes it possible to monitor formation of protective scale and corrosion rate simultaneously. This is a strong tool for e.g. providers of corrosion protection systems, who can optimise their protection and document its function. Another already patented idea on measuring electrochemical parameters were also tested for these systems, and gave useful information on corrosion status for especially the more trained personnel. The data treatment is incorporated in the available software from Metricorr. The Metricorr logger was at the start of the project already in function, but has during the course of the project been adjusted and optimised to implement knowledge gained in the project. Traditionally, it was developed for cathodically protected pipelines, but the changes makes it suitable for all systems, where the potential measurement option can be used to follow open circuit potential, and the spread resistance option to follow scale formation.

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