Corrosion probes

Electrical re.slstance probe.s. These probes are the next most common type of corrosion probes after coupons. This type of probe measures changes in the electrical resistance as a thin strip of metal gets thinner with ongoing corrosion. As the metal gets thinner, its resistance increases. This technique was developed in the 1950s by Dravinieks and Cataldi and has undergone many improvements since then. 

Multiinformational Prohes Corrosion probes can provide more information than just corrosion rate. The next three types of probes yield information about the type of corrosion, the kinetics of the corrosion reaction, as well as the local corrosion rate. 

Microbiauy induced corrosion (MIC) probes. Devices are available to measure the amount of microbial activity in some environments. MicrobiaUy induced corrosion is known to be an actor in many corrosion-related problems in processing plants. The monitoring devices for MIC are limited in their range and, at present, are available only for a few specific environments. This is an exciting area for development of corrosion probes and monitoring systems. 

Use of Corrosion Probes The major use of corrosion monitoring probes is to measure the corrosion rate in the plant or the field. In addition to corrosion-rate measurements, corrosion probes can be used to detect process upsets that may change the corrosion resistance of the equipment of interest. This is usually equally as important a measurement as corrosion rate since a change in the process conditions can lead to dramatic changes in the corrosion rate. 

Limitations of Probes and Monitoring Systems There are limitations even with the most up-to-date systems. Some of the things which cannot be determined using corrosion probes include ... 

Choices of alternative materials. Corrosion probes are carefully chosen to be as close as possible to the alloy composition, heat treatment, and stress condition of the material that is being monitored. Care must be taken to ensure that the environment at the probe matches the service environment. Choices of other alloys or heat treatments and other conditions must be made by comparison. Laboratory testing or coupon testing in the process stream can be used to examine alternatives to the current material, but the probes and the monitors can only provide information about the conditions which are present during the test exposure and cannot extrapolate beyond those conditions. 

For example, consider localized corrosion. Although data from corrosion probes indicate corrosion rate, it is not possible to tell that localized corrosion is the problem. 

An example of this is in a condenser where the corrosion probe is in a region where the temperature is lower than that at the critical condition of interest. Local scale buildup is another example of this type of situation, as is formation of a crevice at a specific location. 

The data obtained from probes and monitoring systems are most useful when analyzed by a corrosion specialist. Data not taken, analyses not made, or expertise not sought can quickly lead to problems even with the most up-to-date corrosion probes and monitoring system. 

Probe/Insirumentalion Developments The principles of good practice in the design, construction and location of corrosion probes have been reviewed. Specific probe designs which acknowledge hydrodynamic influences and the combined effects of mass and heat transfer have been developed. 

Liening, E. L., Industrial Applications of Corrosion Probes (Case Histories) , Materials Performance, 16, 39-41, 9 September (1977)... 

J. Ian Munro, P.E., B.A.Sc.E.E. Senior Consultant, Corrosion Probes, Inc. Registered Pro-fessional Engineer (Ontario, Canada) Member, National Association of Corrosion Engineers International Member, The Electrochemical Society Member, Technical Association of Pulp ir Paper Industry (Anodic Protection)... 

This is an electronic method used to measure corrosion rate in mils per year. The corrosion probe can be inserted through a packing gland. It is read periodically with a portable instrument that measures the change in electrical conductivity of the probe. It is simple, but perhaps a little less reliable than the coupon. 

Corrosion detection plays an important role in any corrosion control program. Most of the methods employ nondestructive test methods and include hydrogen evaluation, radiography, dynamic pressure, corrosion probes, strain gauges and eddy current measurements. Of these, the methods employed in cooling tower practice are hydrogen evaluation and corrosion probes. 

Corrosion probes detect and measure the amount of corrosion occurring at a given point in a system and can be used to estimate the total amount of corrosion and the type of corrosion anticipated. Probes are available for use in a wide variety of temperature and pressure conditions. 

The geometries discussed above are relevant to laboratory scale experimental cells. The geometries discussed below are more appropriate for situations encountered in industry or practical application involving galvanic corrosion, cathodic protection, or field corrosion probes. 

Rates of the corrosion probe were above 1,000 mpy (25.4 mm/y). (10)Trade name. 

Ellison and Schmeal [19] explored a similar approach by using the corrosion of carbon steel in concentrated sulfuric acid as the mass-transpoit/corrosion probe. A model was proposed to reconcile corrosion rate data from pipe and rotating cylinder geometries based on the premise that the corrosion rate is controlled by the transport of Fe + from the interface. The data were subsequently used by Silverman [21] to construct a more precise model for correlating mass-transfer effects between pipe flow and a rotating cylinder. 

In areas where general corrosion is the expected form, a simple ultrasonic thickness gage can be utilized to determine the extent of corrosion, based on baseline readings made at installation or previous inspections. The entire unit need not be examined. Attention can be focused on those areas most likely to corrode, such as liquid levels, mixing zones, or areas of high turbulence. Corrosion probes, which can be placed in process equipment or pipelines, can monitor corrosion conditions by measuring an actual corrosion current, or other process parameters known to be related to general corrosion rates. These data can be constantly monitored and recorded to predict equipment wear, or as an alert to upset conditions. 

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