the side and insufficientc urrent flowst ot he top and bottom. Because of
these factors cathodic protection is usually provided with coated surfaces.
2.6T esting forC ompletenessofP rotection
There ares everal ways that the effectiveness of protection can be checked.
The first two methods arequalitative and do notprovide data about whether
enough or more than enough currenti sb eings upplied. Potential
measurements, the thirdm ethod, is of prime importance in practice.
2.6.1C oupon Tests
Am etal coupon is shaped to conform to the contour of the pipe, weighed,
and attached by abrazed-connected cable to the pipe. Both the cable and the
surfacebetween the coupon andpipe are coated with coal tar.The coupon is
allowed to remain buried for weeks or months, recovered, cleaned, and
weighed. The weight loss, if any,i sa ni ndicationo fw hether or not the
cathodic protection of the pipeline is complete.
2.6.2C olormetric Tests
Ap iece of absorbent paper soaked in potassium ferricyanide solutioni s
placed in contactw ith ac leaned section of the buried pipelinea nd the soil
replaced. After arelatively short time, the paper is retrieved. Ablue ferrous
ferricyanide reaction indicates incompletec athodic protection, whereas, an
absence of blue on the paper indicates that the cathodic protection is complete.
2.6.3P otential Measurements
By measuring the potential of the protected structure, the degree of
protection, including overprotection, can be quantitatively determined.
This measurement is the generally accepted criteriona nd is used by
corrosion engineers. The basis for this determination is the fundamental
conceptthat cathodic protection is complete when the protected structure is
polarizedt ot he open-circuita nodicp otential of local action cells.
The reference electrode for making this measurement should be placed as
closelya sp ossible 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 wills till 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, ac ompromise location is takend irectly over the buried pipe
at the soil surfaceb ecause cathodic protection currents flow mostlyt o
Corrosion of Linings and Coatings
52
CAT8247—CHAPTER 2—6/10/2006—12:05—SRIDHAR—XML MODEL B–pp. 43–54
these factors cathodic protection is usually provided with coated surfaces.
2.6T esting forC ompletenessofP rotection
There ares everal ways that the effectiveness of protection can be checked.
The first two methods arequalitative and do notprovide data about whether
enough or more than enough currenti sb eings upplied. Potential
measurements, the thirdm ethod, is of prime importance in practice.
2.6.1C oupon Tests
Am etal coupon is shaped to conform to the contour of the pipe, weighed,
and attached by abrazed-connected cable to the pipe. Both the cable and the
surfacebetween the coupon andpipe are coated with coal tar.The coupon is
allowed to remain buried for weeks or months, recovered, cleaned, and
weighed. The weight loss, if any,i sa ni ndicationo fw hether or not the
cathodic protection of the pipeline is complete.
2.6.2C olormetric Tests
Ap iece of absorbent paper soaked in potassium ferricyanide solutioni s
placed in contactw ith ac leaned section of the buried pipelinea nd the soil
replaced. After arelatively short time, the paper is retrieved. Ablue ferrous
ferricyanide reaction indicates incompletec athodic protection, whereas, an
absence of blue on the paper indicates that the cathodic protection is complete.
2.6.3P otential Measurements
By measuring the potential of the protected structure, the degree of
protection, including overprotection, can be quantitatively determined.
This measurement is the generally accepted criteriona nd is used by
corrosion engineers. The basis for this determination is the fundamental
conceptthat cathodic protection is complete when the protected structure is
polarizedt ot he open-circuita nodicp otential of local action cells.
The reference electrode for making this measurement should be placed as
closelya sp ossible 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 wills till 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, ac ompromise location is takend irectly over the buried pipe
at the soil surfaceb ecause cathodic protection currents flow mostlyt o
Corrosion of Linings and Coatings
52
CAT8247—CHAPTER 2—6/10/2006—12:05—SRIDHAR—XML MODEL B–pp. 43–54