90
does not occur to any large extent but rather the electrochemical reduction of oxygen occurs (Eqs. 4.2 and 4.3). This is because reducible metal ions in seawater solution are present only at vanishingly small concentration, in the nano- (10
−9
) and
picomol.kg
−1
(10
−12
) ranges. Alkaline metal ions on the other hand are very abundant with a sodium ion concentration, for example, of 0.469 mol.kg
−1
, but these
metals are not susceptible to reduction at the prevailing Eh. Metallic lithium, sodium
or potassium, for instance, when plunged into water will explode in flame. The ion
in solution will only return to the metallic state with commensurate energy input.
Some lithium cell phone batteries for example have exhibited the unsettling property of exploding in flame.
4.3.3 Metal Corrosion in Seawater
Electrochemical Corrosion
The diversity of metals in use in ocean observing instruments and platforms requires
careful consideration of measures to prevent or ameliorate corrosion. Single metal
pieces exposed to seawater are subject to galvanic corrosion of their own since different parts of the metal structure in contact with seawater are subject to different
ionic environments, such as stagnant water versus moving water, and different
degrees of coating with biofilm (see below) thus effectively creating galvanic cells.
Contact of dissimilar metals submerged in the seawater medium establishes more
powerful galvanic couples and corrosion of the least noble of the metals in contact
ensues rapidly. Steel through-bolts, for example, are commonly used to secure aluminum frames, but such composite structures can easily fail due to corrosion of the
aluminum (despite aluminum’s self-protecting properties).
Various types of metal corrosion are recognized including generalized surface
pitting, stress corrosion cracking, and crevice corrosion. The latter is often due to
chemical changes of stagnant waters among and under fouling organisms covering
the metal structure (Brown 1969). Dezincification is a particularly insidious form of
corrosion attacking brass, alloys of copper and zinc. Since the zinc is selectively lost
and the copper remains in place, the piece appears intact but in fact has lost much of
its integrity and is fragile and subject to failure.
Sound Brass
A sound brass body, in the nautical sense of being undamaged, is elastic and
will ring like a bell when struck with a metal piece, or deform if struck too
vigorously. An unsound piece subject to dezincification and microscopically
resembling fine pumice will return a dull sound more like striking wood, or it
may indeed shatter if struck too vigorously.
4 Environmental Constraints to Instrumental Ocean Observing: Power Sources…
does not occur to any large extent but rather the electrochemical reduction of oxygen occurs (Eqs. 4.2 and 4.3). This is because reducible metal ions in seawater solution are present only at vanishingly small concentration, in the nano- (10
−9
) and
picomol.kg
−1
(10
−12
) ranges. Alkaline metal ions on the other hand are very abundant with a sodium ion concentration, for example, of 0.469 mol.kg
−1
, but these
metals are not susceptible to reduction at the prevailing Eh. Metallic lithium, sodium
or potassium, for instance, when plunged into water will explode in flame. The ion
in solution will only return to the metallic state with commensurate energy input.
Some lithium cell phone batteries for example have exhibited the unsettling property of exploding in flame.
4.3.3 Metal Corrosion in Seawater
Electrochemical Corrosion
The diversity of metals in use in ocean observing instruments and platforms requires
careful consideration of measures to prevent or ameliorate corrosion. Single metal
pieces exposed to seawater are subject to galvanic corrosion of their own since different parts of the metal structure in contact with seawater are subject to different
ionic environments, such as stagnant water versus moving water, and different
degrees of coating with biofilm (see below) thus effectively creating galvanic cells.
Contact of dissimilar metals submerged in the seawater medium establishes more
powerful galvanic couples and corrosion of the least noble of the metals in contact
ensues rapidly. Steel through-bolts, for example, are commonly used to secure aluminum frames, but such composite structures can easily fail due to corrosion of the
aluminum (despite aluminum’s self-protecting properties).
Various types of metal corrosion are recognized including generalized surface
pitting, stress corrosion cracking, and crevice corrosion. The latter is often due to
chemical changes of stagnant waters among and under fouling organisms covering
the metal structure (Brown 1969). Dezincification is a particularly insidious form of
corrosion attacking brass, alloys of copper and zinc. Since the zinc is selectively lost
and the copper remains in place, the piece appears intact but in fact has lost much of
its integrity and is fragile and subject to failure.
Sound Brass
A sound brass body, in the nautical sense of being undamaged, is elastic and
will ring like a bell when struck with a metal piece, or deform if struck too
vigorously. An unsound piece subject to dezincification and microscopically
resembling fine pumice will return a dull sound more like striking wood, or it
may indeed shatter if struck too vigorously.
4 Environmental Constraints to Instrumental Ocean Observing: Power Sources…