Codes and Standards

V. CRITERIA USED FOR EVALUATING CATHODIC PROTECTION

Criteria have been developed to determine that a structure has been made completely cathodic, or in other words that it is fully protected from corrosion. These criteria are set forth in National Association of Corrosion Engineers (NACE) Standard RP-01-69 (use latest revision) titled, "Recommended Practice -- Control of External Corrosion on Underground or Submerged Metallic Piping Systems." These criteria are also contained in Section I, Corrosion, of Part 192 (Transportation of Natural and Other Gas by Pipeline: Minimum Federal Safety Standards), Title 49 of the Code of Federal Regulations, which was prepared following the passage by the Congress of the Natural Gas Pipeline Safety Act of 1968.

Probably the most used criterion is the one based on a simpte measurement of the electrical potential between the pipeline and adjacent earth or water. The wording of this criterion is as quoted below from the NACE Standard RP-01-69:

A positive indicator for steel and cast iron structures is "A negative (cathodic) voltage of at least 0.85 voit as measured between the structure surface and a saturated copper-copper sulphate reference electrode contacting the electrolyte. Determination of this voltage is to be made with the protective current applied."

This is all based on the fact that when a pipeline is under cathodic protection, direct current flows from the conducting environment onto the pipeline as shown by Figure 3. This current flow through the environment and coating resistance forces the pipeline to assume a negative electrical polarity with respect to the environment. The question, then, is just how negative the pipe should be to serve as an indication that full protection has been attained. The value of -0.85 volt is used for steel pipe as measured using a standard copper-copper sulphate reference electrode to contact the environment. If other types of reference electrodes are used, the values will be different.

Figure 4 illustrates how the protective potential is measured. The reference electrode (used to assure stable and repeatable readings) is normally placed on the earth surface directly above the pipeline as shown. This position  tends to be a neutral zone where there is the least net concentration of cathodic protection current flow between the electrode and the pipe surface. When working with submerged pipelines, suitable submersion electrodes may be lowered to a position just above the pipeline.

Potentials on steel pipe which are less negative than -0.85 volt to copper-copper sulphate electrode indicate less than full cathodic protection. On the other hand, potentials more negative than -0.85 volt to copper-copper sulphate electrode indicate wasted energy -- since once corrosion is stopped at -0.85 volt, there is no real need to carry more negative potentials at a given point as far as corrosion control at that point is concerned. In actual practice, however, it is usually necessary to maintain more negative potentials at drainage points of cathodic protection current along a pipeline in order to maintain the minimum of -0.85 volt at locations remote from the drainage points. This is primarily a result of attenuation -- voltage drops caused by cathodic protection current on the pipeline flowing through the longitudinal resistance of the pipeline steel in order to return to the drainage point. In this respect, large diameter coated lines are much easier to protect cathodically than are small diameter coated pipes because the larger cross sectional steel area in a large pipe means lower longitudinal electrical resistance with resulting lower attenuation.

Where there are more negative than necessary cathodic protection potentials on coated pipelines, gaseous hydrogen is generated at coating defects in the steel surface. Hydrogen bubbles may cause mechanical J lifting of paint around defects, increasing the current requirements for cathodic protection. This effect is most likely to occur in environments of iow resistance.

Avoiding coating damage by excessive cathodic protection is best accomplished by avoiding over-protection in the first place although coatings that are resistant but not immune to this effect can be selected.

Normally, cathodic protection design engineers will strive to keep the polarization potential on their protected pipeline below the hydrogen over-voltage potential, which is the point at which free hydrogen starts to evolve. The "polarization potential" is the potential measured be* tween the pipe and adjacent earth immediately (within a fraction of a second) after cathodic protection current flow to the pipe is interrupted. The reading must be taken very quickly because the polarization potential decays very rapidly at first. Although its rate depends upon environmental conditions, free hydrogen evolution on steel pipe can be looked for when the polarization potential approaches a value in the order of - 1.2 voit as measured to a copper-copper sulphate reference electrode.

Other accepted criteria for steel (and cast iron) structures which may be used are given below based on the NACE Standard RP-01-69 and are supplemented by explanatory notes as appropriate.

1) A minimum negative (cathodic) voltage shift of at least 300 millivolts, produced by the application of protective current. (Notes: The "voltage shift" is from cathodic protection current ON to the value read immediately -- within a fraction of a second -- after turning the cathodic protection current OFF. Does not apply to ail structures; not applicable to structures in contact with dissimilar metals. Used where -0.85 volt to copper-copper sulphate electrode not readily attained. Not always feasible to simultaneously interrupt all cathodic protection current sources on a protected section of pipeline.)

2) A minimum negative (cathodic) polarization voltage shift of 100 millivolts measured between the structure surface and a saturated copper-copper sulfate reference electrode contacting the electrolyte. (Notes: This is a measurement of the cathodic protection-OFF decay in the polarization potential. This decay must be to a point at least 100 millivolts less negative than the polarization potential measured immediately -- within a fraction of a second -- after first turning the cathodic protection current OFF. Full decay may take excessively long on some pipelines. Not always feasible to simultaneously interrupt all cathodic protection current sources on a protected section of pipeline.)

3) A structure-to-electrolyte voltage at least as negative (cathodic) as that originally established at the beginning of the Tafel segment of the E-Log-I curve, (Notes: The E-Log-I curve is developed, using specific techniques, by applying increasing increments of cathodic protection current to an initially unprotected pipeline and measuring the pipeline.to-reference electrode potential at each value of applied current. These potentials are plotted against the logarithm of applied current. Typically, starting from the minimum applied current value, the plot will appear as initial and final straight line portions connected by a curved section or "break." It is the indicated pipeline-to-reference electrode potential at the begining of the final straight line portion that is used as the criterion of protection. Effective, but a slow procedure.)

4) A net protective current from the electrolyte into the structure surface as measured by an earth current technique applied at predetermined current discharge (anodic) points of the structure. (Notes: Simplest evidence of compliance is a definite indication that reference electrodes placed on each side of the pipeline opposite the previously anodic spot are electrically positive with respect to a reference electrode placed directly above the pipeline at the previously-anodic spot, This indicates that direct current is flowing onto the pipeline from the environment on each side.)

While the criteria discussed above are the accepted working tools used by pipeline corrosion engineers to evaluate the effectiveness of their cathodic protection systems, the ultimate criterion is whether or not the development of pipeline corrosion leaks has been effectively stopped.