Reference: Barlo, T., "Cathodic Protection Parameters Measured on Corrosion Coupons and Pipes Buried in the Field", Corrosion '98, NACE Paper No. 661.


Review: This paper presents the results of an extensive field study, some important correlations, analysis, and limitations.

The paper's Abstract contains the following accurate description of its contents.

"The results from a field study on operating pipelines using coupons of different sizes (49 and 4.2 cm2) in two widely different soil environments are presented, and correlations between the cathodic protection parameters of on-potential, instant-off potential, IR drop, and polarization on the coupons and pipes are considered. Factors controlling the relationships are analyzed and some limitations in the use of coupons are discussed."

In his Introduction Dr. Barlo points out that:

" few studies have adequately demonstrated how well a coupon replicates the level of protection on the pipeline to which it is attached."

This paper is a careful attempt to fill that gap. References to other literature is provided when appropriate to support the data and/or analysis in this paper.

An important conclusion arising from this study:

" neither the environment nor the area of the coupon were highly significant factors in establishing agreement between the pipe and coupon criteria parameters."

Details of the pipeline, coupons, test bed configuration, test procedures, data collection, and analysis as well as graphical presentations constitute the bulk of the paper.

The Summary section of this paper is a comprehensive summary of the paper as well as a summary of present knowledge related to the use of coupons to monitor cathodic protection systems. Selected conclusions from Dr. Barlo's summary appear below.

" size and shape of the holiday appear to have little effect on the instant-off potential achieved on the coupon over an extended period of time greater than a few months."

"The polarization resistance may exceed the geometrical resistance of the coupon in time, and, thus, control the protection level on the coupon."

" the instant-off potential of the coupon can be expected to accurately assume the local instant-off potential of the pipe to which it is connected."

"For the same conditions, the level of polarization on the coupon may not be the same as that on the pipe because of differences in the depolarized potentials of the coupon and pipe."

"Despite their simplicity, coupons can provide considerable useful information regarding the level of protection of a buried pipeline."

"Coupons can be relied upon to give an accurate indication of the level of protection, especially with respect to the -850 mV (Cu/CuSO4) polarized potential."