Appendix D to Part 192 - Criteria for Cathodic Protection and Determination of Measurements
49:3.1.1.2.8.18.8.1.17 : Appendix D
Appendix D to Part 192 - Criteria for Cathodic Protection and
Determination of Measurements
I. Criteria for cathodic protection - A. Steel, cast
iron, and ductile iron structures. (1) A negative (cathodic)
voltage of at least 0.85 volt, with reference to a saturated
copper-copper sulfate half cell. Determination of this voltage must
be made with the protective current applied, and in accordance with
sections II and IV of this appendix.
(2) A negative (cathodic) voltage shift of at least 300
millivolts. Determination of this voltage shift must be made with
the protective current applied, and in accordance with sections II
and IV of this appendix. This criterion of voltage shift applies to
structures not in contact with metals of different anodic
potentials.
(3) A minimum negative (cathodic) polarization voltage shift of
100 millivolts. This polarization voltage shift must be determined
in accordance with sections III and IV of this appendix.
(4) A voltage at least as negative (cathodic) as that originally
established at the beginning of the Tafel segment of the E-log-I
curve. This voltage must be measured in accordance with section IV
of this appendix.
(5) 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.
B. Aluminum structures. (1) Except as provided in
paragraphs (3) and (4) of this paragraph, a minimum negative
(cathodic) voltage shift of 150 millivolts, produced by the
application of protective current. The voltage shift must be
determined in accordance with sections II and IV of this
appendix.
(2) Except as provided in paragraphs (3) and (4) of this
paragraph, a minimum negative (cathodic) polarization voltage shift
of 100 millivolts. This polarization voltage shift must be
determined in accordance with sections III and IV of this
appendix.
(3) Notwithstanding the alternative minimum criteria in
paragraphs (1) and (2) of this paragraph, aluminum, if cathodically
protected at voltages in excess of 1.20 volts as measured with
reference to a copper-copper sulfate half cell, in accordance with
section IV of this appendix, and compensated for the voltage (IR)
drops other than those across the structure-electrolyte boundary
may suffer corrosion resulting from the build-up of alkali on the
metal surface. A voltage in excess of 1.20 volts may not be used
unless previous test results indicate no appreciable corrosion will
occur in the particular environment.
(4) Since aluminum may suffer from corrosion under high pH
conditions, and since application of cathodic protection tends to
increase the pH at the metal surface, careful investigation or
testing must be made before applying cathodic protection to stop
pitting attack on aluminum structures in environments with a
natural pH in excess of 8.
C. Copper structures. A minimum negative (cathodic)
polarization voltage shift of 100 millivolts. This polarization
voltage shift must be determined in accordance with sections III
and IV of this appendix.
D. Metals of different anodic potentials. A negative
(cathodic) voltage, measured in accordance with section IV of this
appendix, equal to that required for the most anodic metal in the
system must be maintained. If amphoteric structures are involved
that could be damaged by high alkalinity covered by paragraphs (3)
and (4) of paragraph B of this section, they must be electrically
isolated with insulating flanges, or the equivalent.
II. Interpretation of voltage measurement. Voltage (IR)
drops other than those across the structure-electrolyte boundary
must be considered for valid interpretation of the voltage
measurement in paragraphs A(1) and (2) and paragraph B(1) of
section I of this appendix.
III. Determination of polarization voltage shift. The
polarization voltage shift must be determined by interrupting the
protective current and measuring the polarization decay. When the
current is initially interrupted, an immediate voltage shift
occurs. The voltage reading after the immediate shift must be used
as the base reading from which to measure polarization decay in
paragraphs A(3), B(2), and C of section I of this appendix.
IV. Reference half cells. A. Except as provided in
paragraphs B and C of this section, negative (cathodic) voltage
must be measured between the structure surface and a saturated
copper-copper sulfate half cell contacting the electrolyte.
B. Other standard reference half cells may be substituted for
the saturated cooper-copper sulfate half cell. Two commonly used
reference half cells are listed below along with their voltage
equivalent to −0.85 volt as referred to a saturated copper-copper
sulfate half cell:
(1) Saturated KCl calomel half cell: −0.78 volt.
(2) Silver-silver chloride half cell used in sea water: −0.80
volt.
C. In addition to the standard reference half cells, an
alternate metallic material or structure may be used in place of
the saturated copper-copper sulfate half cell if its potential
stability is assured and if its voltage equivalent referred to a
saturated copper-copper sulfate half cell is established.
[Amdt. 192-4, 36 FR 12305, June 30, 1971]