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Because corrosion has a well-recognized destructive influence on the structural integrity of nearly all engineering structures (aircraft, pipelines, bridges, etc.), considerable effort is expended to ensure adequate life and usefulness of the structures and materials.

Under most circumstances, the reliability of a given structure will tend to decrease over time necessitating a service or maintenance operation once the reliability drops below some given threshold value. However, because of uncertainties about the actual reliability, some service operations may occur too late, resulting in a performance penalty, or, alternatively, may be conducted too soon, resulting in a cost penalty. These situations are illustrated in the figure below.

 

Because of uncertainties about the actual reliability of a given structure, some service operations may occur too late, resulting in a performance penalty, or, alternatively, may be conducted too soon, resulting in a cost penalty.

To aid in determining the optimum timing of service/maintenance operations as well as to provide quantitative performance information, Southwest Research Institute (SwRI) has been actively engaged in developing new tools and devices to help monitor the integrity of engineering structures and systems. SwRI offers expertise and a wide range of experience in solving industrial corrosion problems. Coupled with advanced manufacturing methods and techniques, new devices have been developed and successfully used to:

• Monitor for localized corrosion in process and cooling water systems

• Coating and paint integrity and performance

• Internal corrosion of natural gas pipelines

• Stress corrosion cracking

Multielectrode Array Sensor (MAS) Probe

	Multielectrode Array Sensor (MAS) Probe

The multielectrode array sensor (MAS) probe developed at SwRI is ideally suited for monitoring corrosion rates in process streams (Yang et al., 2004, 2005). 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, Ni-Cr-Mo, and Cu-Ni 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 for corrosion control services to the chemical process and oil and gas companies and provided maintenance and data analysis services to these companies.

 
Recently, SwRI has developed a high-resolution MAS system that has much 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.
 

Localized Corrosion Monitoring

	Simplified model of localized corrosion involving spatially separated anodes and cathodes.

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. SwRI developed a patented multielectrode array sensor (MAS) system consisting of an array of electrodes to monitor localized corrosion.

 

	Conceptual construction of a multielectrode array sensor.

Corrosion is an electrochemical process, whereby metal dissolves (also called anodic reaction) giving up electrons to other environmental species (e.g., oxygen) that get reduced absorbing the electrons (also called 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. Such an exchange of electrons occurs dynamically throughout the metal surface initially, but once a sufficient area becomes anodic, the attack is concentrated at those sites. Electrical methods are far more sensitive in detecting corrosion than other methods, but the electron flow cannot be detected directly in the metal component of interest since it occurs inside the metal.

 

	Several different types of probe configurations are shown. Multielectrode sensors in the side loop of a brine plant.

Our method of measuring the electron flow is to create a bundle of metal electrodes, all insulated from each other, but connected through a network of resistors. The electrode array can be made of the same metal as the component of interest or a mixture of different electrodes as the specific need dictates. It is typically difficult to know beforehand which area will be anodic and which area will be cathodic. In practice, the differences in microstructures among even nominally identical metal electrodes enable some electrodes to serve predominantly as anodes and others predominantly as cathodes. The smaller the individual electrodes, the more representative they are of the anodic and cathodic sites on the metal. But, if they are too small, they may not represent the metallurgy of the actual corroding metal.

 

	Typical responses of the standard deviation of the currents measured from a 25-electrode sensor made of type 304 stainless steel. It clearly shows the following order of increasing corrosiveness: Deionized Water < Saturated KCl < 0.0025 M FeCl3 < 0.25 FeCl3. It also shows that the MAS probe is able to follow the changes in the localized corrosivity of the environment rapidly and reversibly.

Signals from MAS are the large number of current values measured at a given time interval from all the electrodes. However, these signals must be reduced to a single parameter so the sensor can be conveniently used as a real-time sensor for on-line monitoring purposes in industrial applications. The current that was the most anodic or three times the standard deviation of the currents from the different electrodes has been used as the simple one-parameter signal for the MAS system.
 

Coating and Paint Integrity Monitoring

To assist in mitigating corrosion, many engineering structures have coatings or paints applied. At present, maintenance cycles for commercial and military aircraft and ground vehicles, as well as engineered structures, are usually based on experience and coating appearance rather than a quantitative determination of coating integrity. SwRI has developed a new embedded sensor design to monitor and quantitatively assess coating integrity and protectiveness. Multiple coating systems have evaluated on carbon steel and aluminum alloy substrates, including: a three-coat polyurethane, a one-coat polyurethane, and a one-coat enamel. In addition, the ability to detect defects in the form of surface contamination by hydraulic fluid, pinholes, and scribes have also been detected. Of additional importance, the embedded sensor was not observed to lead to coating failure nor did it enhance corrosion of the substrate.
 

This sensor can be emplaced in areas not easily inspected for coating integrity (such as lap joints) and can detect the onset of coating degradation prior to significant substrate corrosion. It is currently being employed in support of SwRI's efforts to evaluate corrosion preventive compounds and paint systems for the U.S. Marine Corps and is being considered for possible use by the Federal Highway Administration (FHWA) for bridge systems.

Natural Gas Pipeline Internal Corrosion Monitoring

	SwRI developed a sensor capable of measuring the performance and degradation of coating systems. The sensor can be used in conjunction with laboratory tests for rapid determination of coatings system performance or on actual components.

SwRI is currently engaged in a project funded by the U.S. Department of Energy and Pipeline Research Council International (PRCI) to develop a unique distributed wireless sensor network for detecting the presence of entrapped water and possible corrosion inside natural gas pipelines. The monitoring system consists of a network of tiny wireless computers coupled with suitable sensors for detecting the presence of water, determining the overall environmental corrosivity, and ultimately the corrosion rate. The ultimate sensor package would encompass all the various components and be contained as a sphere less than 1.5" in diameter to enable easy flow and transport down the pipeline. Current efforts are aimed at determining the characteristics of wireless data communication inside pipelines as well as identifying the most effective approach for corrosion rate measurements.

The overall concept of a distributed wireless sensor network is being pursued in other areas as well including aircraft structures and bridges.

SwRI is actively developing a distributed wireless sensor network to effectively monitor natural gas pipelines for internal corrosion that cannot currently be inspected using traditional methods. The monitoring system consists of a corrosion monitoring component coupled with a wireless micro-computer network system and is being evaluated under a wide range of natural gas pipeline conditions.

Stress Corrosion Cracking Monitoring

	SwRI has developed a sensor system to monitor and study stress corrosion cracking at highly sensitive levels using MEMS fabrication methods.

SwRI has developed a sensor system to monitor and study stress corrosion cracking at highly sensitive levels using MEMS fabrication methods. 
Many engineering structures are subjected to the simultaneous conditions of an applied stress (or load) and a corrosive environment. Under these conditions, numerous cases of structural failures from stress corrosion cracking (SCC) that have been reported in:

• Boilers

• Pressure vessels

• Oil and gas production and transmission piping

• Components associated with nuclear power generation, bridges, sea craft, and aircraft

To aid in monitoring for SCC and to provide a method of estimating crack propagation rates, SwRI has developed a sensor system using MEMS (Microelectromechanical Systems) fabrication methods utilizing structural engineering materials or analogues. In its current form, the sensor system consists of small MEMS devices containing both manual and self-actuated stressed members in which crack propagation is monitored using the potential drop method. Current plans are to explore possible application of this technology to monitor for stress corrosion cracking in aerospace, nuclear, and the electronics industries.

For more information about corrosion sensors and integrity monitoring capabilities at SwRI or how you can contract with SwRI, please contact Elizabeth Trillo, Ph.D., at etrillo@swri.org or (210) 522-4210.

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Southwest Research Institute^® (SwRI^®), headquartered in San Antonio, Texas, is a multidisciplinary, independent, nonprofit, applied engineering and physical sciences research and development organization with 12 technical divisions.

April 20, 2009