Corrosion of heat exchanger tubes and carbon steel piping at a chemical processing plant
had been attributed to a high level of microbiological activity in the cooling water. Two
electrochemical biofilm activity sensors with integrated data acquisition and data analysis
capabilities were installed in the plant?s cooling system to augment the coupon-based corrosion
monitoring activity. Those sensors provided the plant with an on- line measurement and early
detection of biofilm activity on metallic surfaces. Sensor response was correla ted with coupon
examinations, determinations of biocide residuals, and determination of the numbers and types of
microorganisms. Results from the plant monitoring activity are described. These results
emphasize the necessity to integrate the various types of field and laboratory data to monitor and
effectively control microbiologically influenced corrosion (MIC).
Since the mid 1990's, a large DuPont chemical plant began using fresh pond water for cooling in the form of a closed recirculating system, circulating approximately 1/3 of the pond's volume (140MM gallons) through the system each day. Previously, it was a once-through system, and mild corrosion and some macro fouling occurred. However, soon after the switch to the recirculating system, corrosion, macro- fouling, and microbiologically influenced corrosion (MIC) increased significantly. Accelerated corrosion and MIC of carbon steel (CS), frequent occurrences of pitting corrosion and MIC of austenitic stainless steel, and accelerated corrosion of yellow metals (admiralty brass, copper/nickel and Monel®) have resulted in substantial increases in maintenance and new equipment replacement costs site wide.
The cooling water is a calcium-dissolving, aggressive water that contains a high level of biological activity. Hence, at least a portion of that corrosion had been attributed to a high level of microbiological activity in the cooling water. Prior methods of control had relied upon a general treatment of the cooling water with biocides, i.e., chlorination. High levels of corrosion persisted despite that treatment and control approach.
Corrosion monitoring was initiated in 1998 based upon corrosion coupon racks and online corrosion monitoring probes in side streams of the cooling water supply. To further assess the corrosivity of the system, information on the cooling water chemistry was also obtained from chlorine residuals, pH, microbial counts, etc. A supplemental approach to monitoring the microbiological activity on metallic surfaces was proposed to assess the effectiveness of biocide treatment on control of MIC, and to provide guidance for subsequent treatments. Two electrochemical BIOGEORGE? biofilm activity monitoring probes were installed upstream and downstream of a process unit where additional biocide was locally added to supplement the biocide that was already being added to the site cooling water system. In support of these biofilm-monitoring probes, numerous microbiological test methods were also used to characterize and quantify organism levels in the cooling water, as well as on deposits (scale, corrosion products, and biofilm) formed on corrosion coupons.
