91
Archeologists take great pains to remove chloride ion (Cl
−
) from iron specimens
attributing to this ion a role in oxidation of the ferrous ion (Fe
2+
) to ferric state
(Fe
3+
). The resulting mineral compound, akagenéite (β-FeOOH) expands inside the
corrosion layers and causes cracking and spalling of the outer surface (Rimmer and
Wang 2010).
Metal abrasion can promote corrosion by exposing fresh metal surfaces to corrosive seawater. Data buoy mooring chains are composed of individual hot-dip zinc
galvanized links, a coating that greatly improves corrosion resistance of the steel
chain. Nevertheless, link to link friction due to wave-induced buoy heave can abrade
the protective zinc coating promoting corrosion of the underlying steel. Continued
abrasion exposes fresh surfaces to the corrosive seawater. This corrosion is of such
concern that chain moorings are replaced at yearly intervals.
Many alloys or single metal formulations of great utility in engineering of marine
structures exhibit high corrosion resistance despite low theoretical redox potentials
such as aluminum (E
o
= −1.662) and titanium (E
o
= −0.9) and various stainless steel
and nickel alloys (the light weight and resistance to corrosion of aluminum and
titanium make them especially useful in instrument and vehicle manufacture).
While these metals and alloys are, in principle, easily corrodible in oxic environments, the oxides formed during initial corrosion impart a thin protective coating to
the metal surface impervious to oxygen, thus preventing further corrosion.
Aluminum and titanium are self-protective, while the cobalt contained in stainless
steel and nickel alloys provides the protective oxide. Nevertheless, these protective
coatings may be easily dissolved away if the surrounding electrolyte (seawater) is
devoid of oxygen and becomes more acidic as is common, for example, in sediment
porewaters or within dense biofouling mats.
Bacterially Mediated Electrochemical Corrosion
Micro-biofouling resulting in the formation of a biofilm that coats metallic structures immersed in seawater actively promoting metal corrosion (Videla and Herrera
2005). This biofilm conforms a barrier to oxygen diffusion promoting localized
anoxia upon the surface. Under such conditions, heterotrophic marine bacteria transition to anaerobic metabolism using a number of alternate electron acceptors
including nitrate, metallic iron and manganese, and nitrate and sulfate as oxidizers
of organic matter (Froelich et al. 1979). This increased demand for oxidants leads to
the loss of the protective metal oxide coatings exposing the bare metal as the metal
oxides in the coating are coopted for bacterial metabolism. Moreover, bacterial
metabolism lowers the prevailing pH due to the production of carbonic, sulfuric,
and nitric acids which further promote the solubilization of metal oxides and attacks
the newly exposed bare metal. Selective or accidental abrasion of parts of the surface can establish redox pairs between the coated and uncoated regions thus promoting anodic corrosion of the sites coated with biofilm relative to the cathodic
oxygen reduction occurring at the recently exposed material.
4.3 Metal Corrosion Considerations Pertinent to Ocean Observing
Archeologists take great pains to remove chloride ion (Cl
−
) from iron specimens
attributing to this ion a role in oxidation of the ferrous ion (Fe
2+
) to ferric state
(Fe
3+
). The resulting mineral compound, akagenéite (β-FeOOH) expands inside the
corrosion layers and causes cracking and spalling of the outer surface (Rimmer and
Wang 2010).
Metal abrasion can promote corrosion by exposing fresh metal surfaces to corrosive seawater. Data buoy mooring chains are composed of individual hot-dip zinc
galvanized links, a coating that greatly improves corrosion resistance of the steel
chain. Nevertheless, link to link friction due to wave-induced buoy heave can abrade
the protective zinc coating promoting corrosion of the underlying steel. Continued
abrasion exposes fresh surfaces to the corrosive seawater. This corrosion is of such
concern that chain moorings are replaced at yearly intervals.
Many alloys or single metal formulations of great utility in engineering of marine
structures exhibit high corrosion resistance despite low theoretical redox potentials
such as aluminum (E
o
= −1.662) and titanium (E
o
= −0.9) and various stainless steel
and nickel alloys (the light weight and resistance to corrosion of aluminum and
titanium make them especially useful in instrument and vehicle manufacture).
While these metals and alloys are, in principle, easily corrodible in oxic environments, the oxides formed during initial corrosion impart a thin protective coating to
the metal surface impervious to oxygen, thus preventing further corrosion.
Aluminum and titanium are self-protective, while the cobalt contained in stainless
steel and nickel alloys provides the protective oxide. Nevertheless, these protective
coatings may be easily dissolved away if the surrounding electrolyte (seawater) is
devoid of oxygen and becomes more acidic as is common, for example, in sediment
porewaters or within dense biofouling mats.
Bacterially Mediated Electrochemical Corrosion
Micro-biofouling resulting in the formation of a biofilm that coats metallic structures immersed in seawater actively promoting metal corrosion (Videla and Herrera
2005). This biofilm conforms a barrier to oxygen diffusion promoting localized
anoxia upon the surface. Under such conditions, heterotrophic marine bacteria transition to anaerobic metabolism using a number of alternate electron acceptors
including nitrate, metallic iron and manganese, and nitrate and sulfate as oxidizers
of organic matter (Froelich et al. 1979). This increased demand for oxidants leads to
the loss of the protective metal oxide coatings exposing the bare metal as the metal
oxides in the coating are coopted for bacterial metabolism. Moreover, bacterial
metabolism lowers the prevailing pH due to the production of carbonic, sulfuric,
and nitric acids which further promote the solubilization of metal oxides and attacks
the newly exposed bare metal. Selective or accidental abrasion of parts of the surface can establish redox pairs between the coated and uncoated regions thus promoting anodic corrosion of the sites coated with biofilm relative to the cathodic
oxygen reduction occurring at the recently exposed material.
4.3 Metal Corrosion Considerations Pertinent to Ocean Observing