In chemical process and petrochemical plants corrosion problems may be inevitable, but costly and damaging equipment losses are not. With the continual drive to improve production rates whilst protecting both product quality, safety and the environment, corrosion must become a variable that can be continuously monitored and assessed. This 'new' millennium has seen the introduction of new 'real-time' measurement technologies and vast improvements in methods of electronic data handling. The 'replace when it fails' philosophy is receding into a distant memory; plant management today is embracing new technology, and rapidly appreciating the value it has to offer.
This paper makes reference to use of a real-time electrochemical corrosion monitoring system on a plant running a mostly organic process. Much of the plant is constructed of carbon steel, 304L and 316L. Decades of debottlenecking and other process modifications have produced corrosion problems. Whereas the economics of the process won't justify rebuilding the plant with more corrosion resistant materials, undoing all the process improvements is not an option. The monitoring system has enabled the plant to see, for the first time, changes in corrosion behavior caused by specific variations in process parameters. Process adjustments have been identified that reduce corrosion rates while maintaining acceptable yields and quality. The monitoring system has provided a new window into the chemistry of the process, helping chemical engineers improve their process.
The purpose of corrosion monitoring has changed dramatically from the early days of weight loss coupon exposure.1 As new technologies have evolved, so the accuracy of data and relevance of the information to process control has improved.
Corrosion measurement methods in use today still include weight loss analysis, although this is regarded as more of a retrospective status check than a means of 'monitoring'. Off-line measurement methods that interact with the process environment include such general corrosion measurement techniques as electrical resistance and linear polarization resistance. These systems are able to operate in 'stand-alone' mode providing historical corrosion data via battery-powered, field-mounted instruments. The flexibility of installing these systems in remote locations is somewhat offset by the fact that data is available only periodically and there is an overhead in personnel time to download data. Post processing of the corrosion data is often performed in computerized spreadsheet or workbook format.
Of greater benefit to the plant engineer or process control specialist is general and localized corrosion data that can be made available on-line, and in real-time.2,3 The ability to interface corrosion data with a plant or process control system effectively takes corrosion to the higher level of being 'another process variable'. This, then, affords the opportunity to both look at corrosion data in real-time and to correlate corrosion activity with changes in other process variables. Enabling the process operator to view all data through the same process control system interface means that he/she is now able to see immediately the effect that process changes have on corrosion activity. The control room engineer also, therefore, has the ability in the longer term to achieve optimum production rates while protecting plant integrity (e.g. minimizing downtime for repair of damage and increasing equipment service life).
There are many positive aspects to the scenario of on-line, real-time corrosion monitoring for process control. In the following discussion we review the subject corrosion measurement technology, a
