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ABSTRACT Quality control in district heating systems to keep uniform corrosion rates low and localized corrosion minimal is based on water quality control. Side-stream units equipped with carbon steel probes for online monitoring were mounted in district heating plants to investigate which techniques would be applicable, and if on-line monitoring could improve the quality control. Water quality monitoring was applied as well as corrosion rate monitoring with linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS), electrical resistance (ER) technique, mass loss and a crevice corrosion cell for localized corrosion risk estimation. Important variations in corrosion rate due to changes in make-up water quality were detected with the continuous monitoring provided by ER and crevice cell, while LPR gave unreliable corrosion rates. The acquisition time of two-three days for EIS measurements was too long for real time data, and reliable mass loss data could only be obtained after 6 months exposure. It was furthermore found that localized corrosion events detected by the carbon steel crevice corrosion cell correlated with oxygen peaks of even a few hours duration. INTRODUCTION The water in the Danish district heating systems is generally of high quality with low oxygen concentration and conductivity, and high pH resulting in low corrosion rates for both uniform and localised corrosion of carbon steel. However, the corrosion resistance of carbon steel is highly affected by environmental changes such as pH decrease, increasing conductivity, increasing oxygen content, formation of deposits and microbial growth. In case of corrosion failures only periodical water analyses are available, hence it is difficult to correlate corrosion attacks with operation of the plant and the subsequent changes in the water quality. A three-year Nordic research project supported by Nordic Innovation Fund ~ has focused on on-line monitoring of corrosion rate and water parameters in district heating systems in order to provide better quality control. If the risk of corrosion is detected at an early stage, it might be possible to correct the problem and avoid a failure. In specially designed side-stream units on-line monitoring of oxygen, pH, conductivity and temperature was applied concurrently with monitoring of corrosion rate with electrochemical techniques (linear polarization resistance LPR, electrochemical impedance spectroscopy EIS) electrical resistance (ER) technique, mass loss and a crevice corrosion cell for localized corrosion risk estimation. Units were installed at seven different plants.
- Water & Waste Management > Water Management (1.00)
- Materials > Metals & Mining > Steel (1.00)
- Energy > Oil & Gas > Upstream (1.00)
ABSTRACT Effects of film formation on carbon steel in hydrogen sulfide media may corrupt corrosion rate monitoring by electrochemical techniques. Electrochemical data from hydrogen sulfide solutions, biological sulfide media and natural sulfide containing geothermal water have been collected and the process of film formation in sulfide solutions was followed by video. It can be shown that capacitative and diffusional effects due to porous reactive deposits tend to dominate the data resulting in unreliable corrosion rates measured by electrochemical techniques. The effect is strongly increased if biofilm in combination with ferrous sulfide corrosion products cover the steel surface. Corrosion rates can be overestimated by a factor of 10 to 100 with electrochemical techniques ? both by linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS). Oxygen entering the system accelerates corrosion rates, but this effect may not be detected if rates are already overestimated. It is concluded that electrochemical techniques can be used for corrosion rate monitoring in some hydrogen sulfide media, but care must be taken when choosing the scan rates, and it is important to realize when direct techniques like electrical resistance or mass loss should be used instead. INTRODUCTION Corrosion Monitoring Carbon steel in oil and gas pipelines and subsea construction can be exposed to anaerobic sulfide media from either inorganic or biologically produced sulfide species. When steel is exposed in a hydrogen sulfide environment, protective or non-protective ferrous sulfides can form and localized corrosion can occur. In the case of microbially influenced corrosion (MIC) in an anaerobic environment with sulfatereducing bacteria (SRB), SRB introduce aggressive hydrogen sulfide to the environment and form a biofilm in combination with other microorganisms. The resulting deposits are combined ferrous sulfide and biofilm. In order to obtain corrosion rates for on-line monitoring or mechanistic corrosion studies, reliable techniques for corrosion rate estimation are necessary. The linear polarization resistance technique (LPR) is a simple fast technique and thus can be readily applied to commercial corrosion monitoring. Electrochemical impedance spectroscopy (EIS) is primarily a laboratory technique which is especially useful for monitoring film-covered metals. With EIS the responses from electrochemical reactions with different time constants can be separated. However, with respect to corrosion of carbon steel in hydrogen sulfide containing media, limitations of electrochemical techniques have been found. This is due to solid material bridging the probe or formation of surface films affecting the electrochemical response.
- Europe > Iceland (0.29)
- Europe > Norway (0.28)
- North America > United States (0.28)
- Well Completion > Well Integrity > Subsurface corrosion (tubing, casing, completion equipment, conductor) (1.00)
- Production and Well Operations > Production Chemistry, Metallurgy and Biology (1.00)
- Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers > Materials and corrosion (1.00)