ABSTRACT

This paper explores the use of remote monitoring systems and web-based data analysis to track corrosion rates in real time. Combining monitoring systems with electrical resistance (ER) or linear polarization resistance (LPR) corrosion probes allows measurement of corrosion rate more frequently. Real time transmission of the field data at more frequent intervals provides a greater number of data measurements. Increased data density enables faster and more accurate evaluation of corrosion rate trends. Interactive, web-based analysis provides the user with a tool that allows tracking of corrosion rates in real time. Additionally, corrosion rate changes can be easily isolated to evaluate the effects of environmental or mitigative factors.

Corrosion probe measurements are affected by many factors, including temperature, sensitivity, environmental changes, etc. Individual measurements may appear inconsistent. Typically, a number of historic measurements are averaged to produce a trend. Data analysis tools provide the user with real-time data trending capability. Interactive graphic data analysis enables the user to view the corrosion rate using a variable number of historic measurements to include in creating a rolling average corrosion rate. The user may also view and compare corrosion rates between any two points in history and view cumulative metal loss to evaluate probe life consumption. Relevant applications include monitoring corrosion rates in soils, tanks, casings, pipelines, atmospheric environments, solutions, and offshore immersed structures.

INTRODUCTION

Corrosion rate monitoring techniques, using coupons and probes of various designs and technologies, have been in use for decades in some form or another. Weight-loss coupons were an early type of direct measurement of metal loss to determine the rate of corrosion in many applications, internal and external. These coupons could be placed where corrosion is suspected to be occurring and the rate of corrosion determined by removing and weighing the coupon after a set period. The amount of metal lost from the coupon can be equated to an average metal loss over a period establishing a rate of corrosion, typically expressed in "mils per year" (MPY). A "mil" is equal to 0.001 in (0.0254 mm). Some of the advantages of using weight-loss coupons to determine corrosion rate are that they are useful in internal, external, submerged, and most other corrosion monitoring applications, and that the inspection of the corrosion on the coupon can yield clues as to the type of corrosion and the underlying causes of the corrosion. Disadvantages associated with weight-loss coupons include the time and labor required for evaluating the metal loss, and the insertion and removal of the coupon from the location being evaluated. In addition, though the total metal loss for the period of use can be accurately determined, any changes in corrosion rate occurring during use remain unknown. The effects of changes in the affected environment, or any mitigative measures taken during the evaluation period remain unknown. Several decades ago, electrical resistance (ER) probes started to be used for measuring corrosion rates. Similar to weight-loss coupons, ER probes measure metal loss, but use the change in electrical resistance due to lost material rather than the change in weight of the coupon in order to evaluate the amount of material lost due to corrosion.1 As metal from the exposed portion of the ER probe is lost due to corrosion, the cross-sectional area of the probe element is decreased resulting in an increase in electrical resistance. The amount of metal loss is proportional to the increased resistance. The change in resistance over a known time period enables calculation of a corrosion rate.2 Some advantages of ER probes over weight-loss coupons are the ability to periodically measure the metal loss without removing the device, elimination of the manual steps required to obtain an accurate measurement of metal loss, and the ability to detect smaller increments of metal loss using sensitive probes measuring very small changes in resistance. ER probes can be bonded to the comparative protected structure, thereby measuring corrosion rate with cathodic protection applied. ER probes are recognized as among the most versatile devices for corrosion detection and evaluation.3

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