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4.3.4 Corrosion Protection
Preventive Corrosion Protection
In consequence of the multiple pathways for marine corrosion and its severe economic impact, a primary precaution for those operating electromechanical equipment in the seawater is its prevention. Avoidance of galvanic pairs with direct
contact of dissimilar metals immersed in the seawater electrolyte is foremost among
these precautions. Protective measures can be as simple as the use of nylon washers
between metal parts. Shoulder washers incorporate the traditional circular flat
washer continuing to an orthogonal cylindrical inset lining thus isolating throughbolts from contact with the piece.
Additional to insulation of potential galvanic pairs, certain rules of thumb may be
followed which help to ameliorate corrosion problems (Brown 1969; Horne 1969).
Metal selection for small but important structural components such as bolts, rivets, or
hinges should be such that they constitute cathodes with reference to the overall structure in order to prevent their corrosion which can otherwise result in catastrophic
failure of the assembly. The structural material constituting the anode will tend to
corrode but corrosion will be generalized and structural integrity will be extended.
Small cathodic pieces may be painted or embedded in epoxy to seal out the seawater
electrolyte interrupting the electrolytic redox cell preventing corrosion of adjoining
anodic couples. Paint or polymeric protective coatings may be used but may prove to
be counterproductive if used unwisely. For many applications, large structural parts
composed of self-protecting metals may be left uncoated since otherwise the coating
might fail at points of greatest stress or flexion, points that may be critical to structural
integrity and where corrosion will concentrate. Slow generalized surface corrosion
may prove preferable to intense localized hot-spot corrosion.
RMS Titanic Rusticles
Spectacular video imagery of the wreck of the Royal Mail Ship Titanic obtained
by Ballard and collaborators in 1986 aboard the research submersible Alvin
deep in the north Atlantic highlighted the discovery of rusticles, thick coatings
and chords of a mineral/biofilm accretion rich in reddish ferric oxihydroxides
hanging from rails and other surfaces. Projections of total collapse of the wreck
within this current century highlighted in the public eye the role of bacteria in
mediating metal corrosion. In fact, a new bacterium species named Halomonas
titanicae was discovered and described by researchers from Spain and Canada
in rusticles samples subsequently recovered from the wreck. The metabolic
strategy for iron oxidation of H. titanicae remains to be elucidated. Video
imagery of the wreck and its rusticles can be viewed at https://oceantoday.noaa.
gov/titanicwrecksite/. Accessed September 16, 2017.
4.3 Metal Corrosion Considerations Pertinent to Ocean Observing
4.3.4 Corrosion Protection
Preventive Corrosion Protection
In consequence of the multiple pathways for marine corrosion and its severe economic impact, a primary precaution for those operating electromechanical equipment in the seawater is its prevention. Avoidance of galvanic pairs with direct
contact of dissimilar metals immersed in the seawater electrolyte is foremost among
these precautions. Protective measures can be as simple as the use of nylon washers
between metal parts. Shoulder washers incorporate the traditional circular flat
washer continuing to an orthogonal cylindrical inset lining thus isolating throughbolts from contact with the piece.
Additional to insulation of potential galvanic pairs, certain rules of thumb may be
followed which help to ameliorate corrosion problems (Brown 1969; Horne 1969).
Metal selection for small but important structural components such as bolts, rivets, or
hinges should be such that they constitute cathodes with reference to the overall structure in order to prevent their corrosion which can otherwise result in catastrophic
failure of the assembly. The structural material constituting the anode will tend to
corrode but corrosion will be generalized and structural integrity will be extended.
Small cathodic pieces may be painted or embedded in epoxy to seal out the seawater
electrolyte interrupting the electrolytic redox cell preventing corrosion of adjoining
anodic couples. Paint or polymeric protective coatings may be used but may prove to
be counterproductive if used unwisely. For many applications, large structural parts
composed of self-protecting metals may be left uncoated since otherwise the coating
might fail at points of greatest stress or flexion, points that may be critical to structural
integrity and where corrosion will concentrate. Slow generalized surface corrosion
may prove preferable to intense localized hot-spot corrosion.
RMS Titanic Rusticles
Spectacular video imagery of the wreck of the Royal Mail Ship Titanic obtained
by Ballard and collaborators in 1986 aboard the research submersible Alvin
deep in the north Atlantic highlighted the discovery of rusticles, thick coatings
and chords of a mineral/biofilm accretion rich in reddish ferric oxihydroxides
hanging from rails and other surfaces. Projections of total collapse of the wreck
within this current century highlighted in the public eye the role of bacteria in
mediating metal corrosion. In fact, a new bacterium species named Halomonas
titanicae was discovered and described by researchers from Spain and Canada
in rusticles samples subsequently recovered from the wreck. The metabolic
strategy for iron oxidation of H. titanicae remains to be elucidated. Video
imagery of the wreck and its rusticles can be viewed at https://oceantoday.noaa.
gov/titanicwrecksite/. Accessed September 16, 2017.
4.3 Metal Corrosion Considerations Pertinent to Ocean Observing