The purpose of corrosion monitoring has changed dramatically from the early days of weight loss coupon exposure. 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 'spot' 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.
Contrary to popular belief, though, corrosion occurs neither continuously nor at a uniform rate. The occurrence of corrosion can have a significant adverse effect upon the availability and reliability of plant. Of greater benefit to the plant engineer or process control specialist, therefore, is general and localized corrosion data that can be made available on-line, and in real-time. 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 methods of corrosion measurement in use today, and discuss their relative merits in affording the user a better understanding of the relationship between corrosion and process variables.
Corrosion Measurement and Monitoring:
The term “Corrosion Monitoring” is often used to describe a broad range of techniques used to evaluate the degradation of metallic materials. These techniques can be divided into two distinct groups, namely those providing indications of the cumulative damage sustained (off-line, retrospective) and those providing indications of the prevailing corrosion rate (usually on-line and continuous). The techniques described below are applicable with virtually all metallic materials including, for example, carbon steels, stainless steels, duplex and super duplex steels, copper and nickel alloys and the more noble metals commonly found in chemical process equipment such as titanium and zirconium. Generally, if a metallic material can corrode then its corrosion behavior can be measured or monitored using an appropriate technique. However, the following practical points must be considered in relation to the type of measurement or monitoring technique that will be used:
a) Fabrication of the required sensor configuration from the selected material - can the required phase of the metal be formed into an electrode ? will the electrode manufacturing process alter the heat treatment of the metal ?
b) For a given metallic 'mixture' (e.g. solder) or de-alloyed material , the exact composition may be indeterminate and so may present slight errors in calculations of corrosion rates.
A good and experienced probe supplier is an important ally in the selection of probe design.
Although all techniques can be applied successfully for almost all metallic materials, the selection of the correct technique is really determined by the type of corrosion to be monitored. When choosing a corrosion measurement or monitoring technique, it is important to consider the why, where and how - some of these issues are considered more fully toward the end of this article and are summarized here:
1. Why do I need to know about corrosion in my process ? Hopefully, your response is that you need "to work out what is causing the corrosion and reduce or prevent it". However, a deeper consideration of corrosion and its consequences will generally help you to work out your measurement needs such as the frequency of measurement required, the level of corrosion detail needed. Some thoughts may include:
a) Corrosion was the cause of an unplanned shutdown - I need to understand what in the process caused the corrosion.
b) We replaced some of our piping last turnaround but it has corroded through again - we need to know what to replace it with next time.
c) We found some pitting corrosion last inspection but we don't know if it was caused by a process upset or something else.
d) Our equipment is halfway through its planned design life but we still have 75% of our corrosion allowance left - we need to monitor to be assured we can extend the equipment lifetime.
e) We are designing a new process and we need to be sure we have selected the correct materials.
2. Where do I need to measure the corrosion?
a) For existing plant with a history of inspection and maintenance data, the location of the corrosion probe can usually be worked out easily. However, if an existing probe access port is not available you may need to spend time compiling a technical case to justify introduction of the new measurement equipment.