88
Payload capacity and power limitations common to small mobile platforms such
as profiling floats, buoyancy gliders, AUVs, wave gliders, and other lightweight
mobile platforms have driven the development of low power, low weight, compact,
flush-mounted sensors that can be incorporated into the hull without compromising
hydrodynamic vessel properties. Doppler velocity and Doppler navigation loggers,
ADCPs, optical instruments such as nephelometers and fluorometers, and a variety
of other instruments configured for such applications are now widely available.
4.3 Metal Corrosion Considerations Pertinent to Ocean
Observing
4.3.1 Fundamentals of Electrochemistry
Metal corrosion depends on the susceptibility of individual metals to oxidation,
susceptibilities that have been well established through experimental assessment.
To such an end, all redox half cells, including those where metals participate in the
reaction, are compared to a standard half cell, the hydrogen reference electrode
consisting of a platinum-black electrode immersed in a 1 N HCl solution and bathed
in a stream of hydrogen gas bubbles. This electrode is defined, by convention to
have a standard half-cell potential of 0 V at standard temperature and pressure (STP:
1 atm pressure and 273.16° K). Electrical potentials established between this absolute reference cell and all other half cells at standard conditions (STP as above and
electrolyte concentrations at 1 M) are referred to as standard half-cell potentials.
Regardless of the sense of the reaction, equations are written as reductions. The
equation for the standard hydrogen half-cell reduction is
2
2
0
2
0
H
H
V
+
−
+
=
=
e
E
(4.1)
If the electrode in question undergoes corrosion when in contact with the reference
half cell, the resulting potential will be negative, the hydrogen half cell will constitute the cathode, the target half cell will operate as the anode. On the contrary, if the
hydrogen gas is oxidized, the potential of the test half cell will be of a positive sign.
Potentials of a wide range of half cells have been experimentally determined and are
tabulated for ease of use as the electrochemical series (Vanýsek 2010). Standard
potentials of the electrochemical series (E
0
) range from a low of −3.04 V for the
highly corrodible metal lithium to 2.87 V for the highly corrosive fluorine gas, a
surprisingly low span of less than 6 V. Platinum (often used as a cathode due to its
comparable inertness) exhibits a standard reduction potential of 1.18 V while gold
exhibits a potential of 1.692 V. Zinc, in keeping with its easily corrodible nature,
exhibits a correspondingly low standard reduction potential (−0.762 V). Potentials
of half cells under circumstances other than standard conditions may be calculated
using the exponential Nernst equation (Whitfield and Turner 1981; Millero 2013).
Pertinent conditions for oxic seawater are pH ca. 8 ([H
+
] = 10
−8
mol.kg
−1
),
pO 2 = 0.21 atm or 358 μmol.kg
−1
.
4 Environmental Constraints to Instrumental Ocean Observing: Power Sources…
Payload capacity and power limitations common to small mobile platforms such
as profiling floats, buoyancy gliders, AUVs, wave gliders, and other lightweight
mobile platforms have driven the development of low power, low weight, compact,
flush-mounted sensors that can be incorporated into the hull without compromising
hydrodynamic vessel properties. Doppler velocity and Doppler navigation loggers,
ADCPs, optical instruments such as nephelometers and fluorometers, and a variety
of other instruments configured for such applications are now widely available.
4.3 Metal Corrosion Considerations Pertinent to Ocean
Observing
4.3.1 Fundamentals of Electrochemistry
Metal corrosion depends on the susceptibility of individual metals to oxidation,
susceptibilities that have been well established through experimental assessment.
To such an end, all redox half cells, including those where metals participate in the
reaction, are compared to a standard half cell, the hydrogen reference electrode
consisting of a platinum-black electrode immersed in a 1 N HCl solution and bathed
in a stream of hydrogen gas bubbles. This electrode is defined, by convention to
have a standard half-cell potential of 0 V at standard temperature and pressure (STP:
1 atm pressure and 273.16° K). Electrical potentials established between this absolute reference cell and all other half cells at standard conditions (STP as above and
electrolyte concentrations at 1 M) are referred to as standard half-cell potentials.
Regardless of the sense of the reaction, equations are written as reductions. The
equation for the standard hydrogen half-cell reduction is
2
2
0
2
0
H
H
V
+
−
+
=
=
e
E
(4.1)
If the electrode in question undergoes corrosion when in contact with the reference
half cell, the resulting potential will be negative, the hydrogen half cell will constitute the cathode, the target half cell will operate as the anode. On the contrary, if the
hydrogen gas is oxidized, the potential of the test half cell will be of a positive sign.
Potentials of a wide range of half cells have been experimentally determined and are
tabulated for ease of use as the electrochemical series (Vanýsek 2010). Standard
potentials of the electrochemical series (E
0
) range from a low of −3.04 V for the
highly corrodible metal lithium to 2.87 V for the highly corrosive fluorine gas, a
surprisingly low span of less than 6 V. Platinum (often used as a cathode due to its
comparable inertness) exhibits a standard reduction potential of 1.18 V while gold
exhibits a potential of 1.692 V. Zinc, in keeping with its easily corrodible nature,
exhibits a correspondingly low standard reduction potential (−0.762 V). Potentials
of half cells under circumstances other than standard conditions may be calculated
using the exponential Nernst equation (Whitfield and Turner 1981; Millero 2013).
Pertinent conditions for oxic seawater are pH ca. 8 ([H
+
] = 10
−8
mol.kg
−1
),
pO 2 = 0.21 atm or 358 μmol.kg
−1
.
4 Environmental Constraints to Instrumental Ocean Observing: Power Sources…