Tutorial: Corrosion monitoring in real time

12/21/2006


Corrosion in process plants and water distribution is a perpetual problem that costs industry hundreds of billions of dollars each year. Monitoring corrosion has traditionally been a slow and imprecise process, only yielding answers after the fact. New types of instrumentation offer the ability to watch conditions that cause corrosion in real time.

Several manufacturers now offer corrosion monitoring probes and transmitters that use LPR (linear polarization resistance) analysis combined with other methods to determine how corrosive the process is at any given time. The probe has three electrodes, made from the same material as the process equipment and piping. The transmitter takes a series of electrical readings between the electrodes which can translate into the degree of corrosiveness of the process liquid at any given time. A 4-20 mA signal from the transmitter provides the status to the operator in real time.

This information identifies conditions that promote general and localized (pitting) corrosion. Process operators can apply this knowledge in several ways:

  • Identify points in a process when the product is particularly corrosive (e.g., reactor startup or upset) to help mitigate those conditions;

  • Monitor the effectiveness of water treatment or corrosion inhibitors added to the process;

  • Check characteristics of feedstocks; and,

  • Create a corrosiveness history to identify trends in a given process and schedule maintenance.

This type of monitoring does not give you specific information on localized metal loss. By itself, it can't predict where a piping failure will occur. Poor welds, flaws in castings, and other mechanical material issues will still be the weak links in a corrosion chain and can fail in the most closely monitored systems. But careful analysis of history provided by these probes can predict when enough metal has been lost to put weak points at risk if you understand where they are. Traditional methods of checking pipe wall thickness using ultrasonic probing can work in tandem with monitoring methods to verify your understanding of corrosion in specific equipment.

Corrosion is common in:

  • Potable water treatment and distribution;

  • Wastewater treatment;

  • Chemical manufacturing;

  • Pulp & paper;

  • Cooling systems and towers;

  • Petrochem and hydrocarbon processing; and,

  • Many other applications. If corrosion is a problem in your process, it won't be hard to identify.

Click here for a more detailed article on LPR corrosion monitoring from Control Engineering .

Corrosion monitors are available from:

Control Engineering Daily News Desk
Peter Welander , process industries editor





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