Cathodic protection

An experiment was designed to provide a simulated steel fatigue crack under cathodic protection in seawater for study. Calcareous deposits were found to form in the simulated crack at -0.800, -0.900, -1.000 and -1.100 volts (SCE) with cycle frequencies of both 0.5 and 1.0 Hertz. By X-ray and SEM analysis, calcium carbonate deposits were shown to grow in quantities sufficient to block and become compressed by this simulated crack. Increased fluid velocities from crack pumping were shown to negatively affect crystal nucleation and growth. Due to kinetic barriers to crystal growth, aragonite and brucite were determined to be the most likely deposit type; excluding calcite, dolomite and magnesite. The deposits formed at -1.000V were shown by current requirements and SEM to provide the best protection as a coating.

Electrochemical conditions within corrosion fatigue cracks
are thought to exert an important influence upon
propagation rate. For the case of cathodically polarized
steel in sea water, it has been observed that pH of
electrolyte adjacent to steel increased and calcareous
material formed on the steel surface. As a consequence of
the latter process the electrochemistry within
cathodically polarized cracks may be unique compared to
other electrolytes. The objective of this study has been
to measure the potential and pH within simulated fatigue
cracks of ABS DH 32 steel under cathodic protection. In
addition, the calcareous deposits which formed within the
simulated cracks were observed and analyzed. Based upon
these data, potential, pH profiles and calcareous deposit
thickness and composition were measured. This, in turn,
permitted conditions within the simulated crack to be
better understood.

The influence of hydrodynamic parameters and cathodic
polarization on the formation of calcareous deposits on a
steel plate in flowing seawater is investigated. Current
density variations on the plate are determined for a
combination of applied potentials and seawater
velocities. Equations are developed to relate the
cathodic current density distribution to variations in
hydrodynamic and diffusional boundary layer thickness
along the plate for laminar and turbulent flow regimes. A
comparison of experimental current densities on a bare
plate to that predicted by theory is examined. Effects of
flow on the formation and properties of calcareous
deposits are discussed in light of recent developments in
marine cathodic protection.

It is well recognized that the electrochemistry modifications
within cathodically protected fatigue crevices in
seawater has pronounced effect on crack growth rate. This
study focuses its attention on potential distribution within
simulated fatigue crevice as a function of electrolyte velocity,
fatigue frequency and cathodic protection potential.
Results of salt water electrolyte and seawater electrolyte
are compared to bring about the effect of calcareous deposits,
Finally, a mathematical model based on mass transfer
laws is developed to rationalise potential distribution
trends within fatigue crevice and are compared with the
observed behaviour.

To investigate environmental conditions which influence the
type and formation of calcareous deposits, AISI 1010 steel
specimens were exposed in sea water at constant velocities
and controlled potentials. Chemistry and thickness of the
deposits which formed as a result of cathodic polarization
of steel in sea water were determined. These variables were
found to be a function of velocity and controlled potential
with film thickness decreasing as velocity increased for all
potentials considered. This behavior is interpreted in
terms of variations in pH at the metal surface. A mass
transfer analysis is presented which projects the pH at the
metal surface as a function of velocity and controlled potential.
Scanning electron micrographs are also included
to give information regarding the nature and type of deposits.
The experimental results are discussed within the
frame of accepted concepts of sea water cathodic protection.

Field and laboratory ambient and simulated deep seawater sacrificial anode cathodic protection experiments were performed by coupling steel specimens to Al-Zn-Hg anode through an appropriately sized external resistor and thereby permitting a particular level of cathodic protection from freely corroding to overprotection to be simulated. The effects of sea current velocity, surface preparation, initial current density, temperature and hydrostatic pressure upon cathodic polarization and characteristics of calcareous deposits were investigated in the context of slope parameter and steady-state potential and current density trend. The results revealed that a linear relationship between cathode potential and current density is applicable for design of sacrificial anode cathodic protection systems and analysis of cathodic protection survey data from existing structures both in shallow and deep waters. It was also found that for cathodically polarized steel specimens in ambient (shallow) seawater steady-state cathode potential and current density varied according to a sigmoidal trend that indicates the importance of calcareous deposits in such exposures and demonstrated the utility of rapid polarization. On the other hand, no sigmoidal trend was apparent for field and simulated deep water tests; but instead steady-state current density was constant for potential range between -0.80 and -1.05 v (Ag/AgCl). This disclosed that no particular benefit could be derived from employing rapid polarization in cold water exposures. SEM, EDX and X-ray diffraction analysis revealed that the calcareous deposits formed upon specimens exposed at 5C and ambient pressure or 8.96 MPa laboratory experiments exhibited two layer structures--an outer layer of CaCO3 as aragonite and inner layer of Mg(OH)2 as brucite. The morphology and coverage of the deposits depended on the design slope parameter (initial current density). Field testing results indicated that deposits were also composed of CaCO3 and Mg(OH)2 where the former was calcite. Because of the poor coverage of the deposits formed in the deep water condition, limited current density reduction was noted. These results suggest that a different cp design approach and strategy should be considered for deep, cold seawater compared to that commonly used in shallow water environments.

A procedure has been devised to characterize the adhesion of calcareous deposits formed upon steel in association with cathodic polarization in seawater. This involved bending the substrate in-situ and characterization of deposit debonding in terms of visual appearance, changes in the cathodic current density and the net charge transferred in association with film disruption. Specific attention has been focused upon the influence of exposure time, applied potential, flow rate, temperature, substrate surface finish and electrolyte type upon deposit adhesion. Presence of a Mg-rich, inner film as well as the impingement of CaCO3 particles enhanced the adhesion of calcareous deposits. Electrostatic forces, Van der Waals attraction, and chemical alteration of precipitates could be important factors contributing to deposit adhesion.

The Center for Ocean Energy Technology at Florida Atlantic University is developing an ocean energy turbine system to investigate the feasibility of harnessing Florida's Gulf Stream current kinetic energy and transforming it into a usable form. The turbine system has components which are prone to marine corrosion given the materials they are made of and to the harsh environment they will be exposed to. This study assumes a two-part system composed of a coating system acting as a barrier and sacrificial anode cathodic protection which polarizes the metal structures to a potential value where corrosion is significantly reduced. Several configurations (varying in anode quantity, size and location) were considered in order to cathodically protect the structures with various coating qualities (poor, good and excellent). These cases were modeled and simulated via Boundary Element Method software and analyzed so as to assess the most appropriate design.