electrical resistance corrosion probes

Electrical Resistance (ER):

Electrical Resistance (ER) probes utilize the relatively simple principle of an increase in electrical resistance produced by a decrease in the section thickness of a metallic conductor. The increase in electrical resistance of a corroding sensor is measured relative to that of a corresponding shielded reference sensor element. The uncorroded reference element also serves to compensate for the effects of temperature changes on resistance. ER corrosion monitoring has been widely applied in industry over more than four decades.

ER corrosion sensors have been likened to "intelligent" coupons, facilitating a simple corrosion measurement without the need to remove the coupon from service. ER measurements are usually regarded as relatively insensitive; the sensors generally do not respond rapidly to a change in corrosive conditions/change in corrosion rate. Sensitivity can be improved by decreasing the element thickness but only by compromising the overall sensor life. Probe life corresponds to halving of the sensor element thickness by corrosion (cylinder or strip element designs). For wire elements, the life span corresponds only to a quarter diameter loss.

ER probes can be used in a wide range of environments and can be considered for low conductivity and nonaqueous conditions, where electrochemical techniques are generally unsuitable.

Caution: Conductive deposits such as iron sulfide or carbonaceous material on the sensor elements will obviously distort the readings. The former is particularly relevant in sour oil/gas systems and certain forms of microbial corrosion, such as SRB attack.

ER sensors do not distinguish between general and localized corrosion forms.

References/Literature:

G.L. Cooper: "Sensing Probes and Instruments for Electrochemical and Electrical Resistance Corrosion Monitoring", in Corrosion Monitoring in Industrial Plants using Nondestructive Testing and Electrochemical Methods, Eds. G.C. Morand and P. Labine, ASTM STP 908, American Society for Testing and Materials, West Conshohocken (PA), 1986, pp. 237-250.

Links:
Outline of ASTM B826-97 Test Method for Monitoring Atmospheric Corrosion with ER Probes at:
http://www.astm.org/DATABASE.CART/PAGES/B826.htm

Link to a technical article by a commercial supplier of a new corrosion monitoring technology reportedly based on ER fundamentals but with higher sensitivity at:
http://www.cormon.com/catalog/techpres.htm