MIL-HDBK-1004/10
APPENDIX E (continued)
(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 paragraph (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 insulting flanges, or the equivalent.
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