SwRI: Multielectrode Array Sensor (MAS), Sensors and Corrosion Monitoring, Corrosion Testing and Research

	Through-wall pitting in pipe
	Schematic of the multi-electrode array sensor showing operating concept
	Multielectrode Array Sensor Probes constructed using a variety of materials

Corrosion is an electrochemical process whereby metal dissolves (anodic reaction) giving up electrons to other environmental species, such as oxygen, that get reduced absorbing the electrons (cathodic reaction). The anodic and cathodic reactions occur simultaneously at different locations on a metal surface exchanging the electrons through the metal, which is a good electrical conductor. In practice, this internal flow of electrons cannot be measured or monitored.

Multielectrode Array Sensor Probe

The multielectrode array sensor (MAS) probe, developed at Southwest Research Institute (SwRI) is ideally suited for monitoring corrosion rates in process streams. Multiple discrete elements or electrodes are used to replicate the material of interest. The MAS probe measures corrosion rates by measuring the current flow between coupled electrodes. The electrodes can be manufactured from a wide range of alloys and product forms.

SwRI has used this method to monitor the corrosion of:

• Carbon steels

• Stainless steels

• Ni-Cr (nickel-chromium)

• Ni-Cr-Mo (nickel-chromium-molybdenum)

• Cu-Ni (copper-nickel) alloys

Probes for the SwRI MAS systems can be designed to work at temperatures up to 300 °C (572 °F) and pressures of 13.8 MPa (2,000 psi). The design of the MAS probe also allows the real-time measurement of corrosion rates in both the liquid and vapor phase. SwRI has installed MAS systems to perform corrosion control services for chemical process and oil and gas companies and has also provided maintenance and data analysis services to these companies.

Recently, SwRI has developed a high-resolution MAS system with greater sensitivity to measure corrosion rates. Improved resolution allows the measurement of a large range of corrosion rates spanning from passive corrosion rates of stainless steels and nickel-chromium-molybdenum alloys [~10^-5 mm/yr (4 ?10^-4 mpy)] to localized penetration rates for these alloys [up to 10 mm/year (400 mpy)]. The revised system is capable of simultaneously monitoring 54 channels or 6 probes with 9 electrodes.

Existing corrosion monitoring technologies, such as linear polarization resistance, electrochemical noise, and electrical resistance methods, are not ideally suited to monitor localized corrosion in real-time. This problem becomes especially acute when thin electrolyte films are involved such as in atmospheric corrosion. The SwRI-developed MAS system consists of an array of electrodes to monitor localized corrosion.

Key features of the MAS System

• U.S. Patent No. 6,683,463 and U.S. Patent No. 6,987,396

• Near real time measurement of uniform and localized corrosion rate

• Detection in a wide variety of environments such as under salt films, high resistivity environments, under bio-films or in the atmosphere

• Probes can be manufactured from virtually any engineering alloy

• Probes using thermally aged materials or actual weld material have been produced to examine the effects of fabrication processes

• High temperature and high pressure compatible

• Effects of galvanic coupling can be evaluated in process streams

High-resolution multielectrode array sensor system		MAS packaged for atmospheric corrosion monitoring
Corrosion monitoring in crude oil		Monitoring in chemical plant piping
Atmospheric corrosion rate of carbon steel in a tropical marine environment. Corrosion rate inside a sheltered location is significantly lower then the corrosion rate observed outside		Thin film MAS element can be used to monitor corrosion in unusual locations and restricted locations.

For more information about multielectrode array sensor capabilities at SwRI or how you can contract with SwRI, please contact Garth Tormoen at gtormoen@swri.org or (210) 522-2645, or Elizabeth Trillo, Ph.D., at etrillo@swri.org or (210) 522-4210.