306
Fundamentals of Corrosion
whether enough or more than enough current is being supplied. Potential
measurements, the third method, is of prime importance.
1. Coupon test. A metal coupon is shaped to conform to the contour
of the pipe, weighed, and attached by a braze-connected cable to
the pipe. Both the cable and the surface between the coupon and
the pipe are coated with coal tar. The coupon is allowed to remain
buried for weeks or months, uncovered, cleaned, and weighed. The
weight loss, if any, is an indication as to whether or not the cathodic
protection is complete.
2. Colorimetric test. A piece of absorbent paper soaked in potassium ferricyanide solution is placed in contact with a cleaned section of the
buried pipeline and the soil replaced. After a relatively short time,
the paper is retrieved. A blue ferrous/ferricyanide reaction indicates
incomplete cathodic protection, whereas an absence of blue on the
paper indicates that cathodic protection is complete.
3. Potential measurements. By measuring the potential of the protected
structure, the degree of protection, including overprotection, can
be quantitavely determined. This measurement is the generally
accepted criterion and is used by corrosion engineers. The basis for
this determination is the fundamental concept that cathodic protection is complete when the protected structure is polarized to the
open-circuit anodic potential of the local action cells.
The reference electrode for making this measurement should be placed
as close as possible to the protected structure to avoid and to minimize an error caused by internal resistance (IR) drop through the
soil. Such IR drops through corrosion product films or insulating
coatings will still be present regardless of precautions taken, tending
to make the measured potential more active than the actual potential at the metal surface. For buried pipelines a compromise location is taken directly over the buried pipe at the soil surface because
cathodic protection currents flow mostly to the lower surface and are
minimum at the upper surface of the pipe buried a few feet below
the soil surface.
9.3 Use with Coatings
Insulating coatings are advantageous to use with either impressed current or
sacrificial anodes when supplying cathodic protection. These coatings need
Fundamentals of Corrosion
whether enough or more than enough current is being supplied. Potential
measurements, the third method, is of prime importance.
1. Coupon test. A metal coupon is shaped to conform to the contour
of the pipe, weighed, and attached by a braze-connected cable to
the pipe. Both the cable and the surface between the coupon and
the pipe are coated with coal tar. The coupon is allowed to remain
buried for weeks or months, uncovered, cleaned, and weighed. The
weight loss, if any, is an indication as to whether or not the cathodic
protection is complete.
2. Colorimetric test. A piece of absorbent paper soaked in potassium ferricyanide solution is placed in contact with a cleaned section of the
buried pipeline and the soil replaced. After a relatively short time,
the paper is retrieved. A blue ferrous/ferricyanide reaction indicates
incomplete cathodic protection, whereas an absence of blue on the
paper indicates that cathodic protection is complete.
3. Potential measurements. By measuring the potential of the protected
structure, the degree of protection, including overprotection, can
be quantitavely determined. This measurement is the generally
accepted criterion and is used by corrosion engineers. The basis for
this determination is the fundamental concept that cathodic protection is complete when the protected structure is polarized to the
open-circuit anodic potential of the local action cells.
The reference electrode for making this measurement should be placed
as close as possible to the protected structure to avoid and to minimize an error caused by internal resistance (IR) drop through the
soil. Such IR drops through corrosion product films or insulating
coatings will still be present regardless of precautions taken, tending
to make the measured potential more active than the actual potential at the metal surface. For buried pipelines a compromise location is taken directly over the buried pipe at the soil surface because
cathodic protection currents flow mostly to the lower surface and are
minimum at the upper surface of the pipe buried a few feet below
the soil surface.
9.3 Use with Coatings
Insulating coatings are advantageous to use with either impressed current or
sacrificial anodes when supplying cathodic protection. These coatings need