Cathodic protection

The present research focused on the behavior of arc sprayed zinc and zinc-aluminum coatings on concrete specimens exposed to specific relative humidity environments (100, 85, 60 and 25% RH) and with specific chloride contents (0.0, 3.0, 5.9 and 11.8 kg/m^3). All specimens experienced a decline in current output with time. Anode wastage and formation of oxidation products were mainly responsible for this lack of protection in 100 and 85% RH, whereas for 60 and 25% RH, drying of the concrete and long-term polarization of the anode were the key factors. Validity of the DC measurements was verified with slope parameter analysis on selected specimens. In addition, Electrochemical Impedance Spectroscopy was performed on the galvanic anode and an equivalent circuit model was obtained for the zinc-concrete interface.

Pretensioned concrete members require a certain bond strength between the steel and concrete to maintain prestress force. Previous studies have indicated that when reinforcing steel in concrete is cathodically polarized, a chemical change of the cement near the steel-concrete interface occurs; and this can reduce the bond strength. In the present research experiments were conducted with concrete specimens that contained either a non-stressed seven wire steel tendon or a single strand through the longitudinal direction. The specimens were cathodically polarized with current densities ranging between 50 and 2500 mA/m^2 of steel. Upon achieving a pre-determined charge density transfer, the steel was pulled relative to the concrete until the bond was broken. Results indicate that a total charge density transfer of up to 14000 A*h/m^2 of steel, may introduce an average 16 percent decrease in ultimate bond strength. This and other data were evaluated in order to assess if cathodic protection, as utilized for corrosion control, is likely to compromise structural integrity of pretensioned concrete members and structures.

Development of a baseline design protocol for cathodic protection retrofits of offshore structures is becoming an increasingly important topic in light of the large number of structures operating beyond their original cp system design life. One of the critical steps in this development is determination of the total anode mass which is required to continue sufficient cathodic protection for which structure current demand must be established. Three current demand determination methods were investigated including the Gaussian ammeter, the Potential method, which utilizes the driving potential and anode resistance in its current calculation, and the IR Drop method. To this end, three anodes were deployed at the Naval Research Laboratory, Key West, Fl., and current output determinations were made upon these. Each of the three current measurement methods was evaluated on the basis of accuracy, reliability and feasibility in field use.

Twenty-four test cells arranged in a pipe flow setup were assembled to
investigate the effect of seawater velocity on the polarization behavior of
galvanically polarized mild steel. Each 1023 steel pipe specimen of 10.8 em ID
was coupled via a current limiting resistor to a mercury activated aluminum
anode and exposed to a velocity of either 0.03, 0.09 or 0.30 m/s. The resistors
were sized such that polarization was controlled according to one of six slope
parameters. Steady state potential and maintenance current density values were
determined, and a steady state potential vs. current density curve was
established for specimens in each velocity. Some specimens experienced a rise in
cathode potential and current density after an apparent steady state had been
reached. This was probably related to the influence of velocity on the
protectiveness of the calcareous deposit. Of the specimens that experienced a
rise in steady state potential and current density, a few were later observed to
decrease in potential and current density and reach steady state. Steady state
current density vs. velocity plots of specimens at steady state potentials of -0.78,
-0.88 and -0.98 V showed that current density was directly proportional to
velocity as well as relatively insensitive to potential. Ficks' first law was utilized in conjunction with an empirically derived dimensionless correlation that
characterizes the behavior between fluid velocity and mass transfer of molecular
species from the bulk solution to the cathode surface in turbulent seawater pipe
flow. Calcareous deposit porosity constants were calculated and it was surmised
that as velocity increased by a factor of three, the porosity of the deposits near 0.78
and -0.89 V increased by multiples of about two on average. Porosity at the
above potentials increased with decreasing potential by a factor of a little over
two. SEM micrographs were made and EDX analyses were performed on the
calcareous deposits of selected specimens.

Qualification criteria for cathodic protection of pre-tensioned tendon wires in concrete were studied with regard to the risks of embrittlement due to chromium microalloying and existing corrosion damage. The selected materials included two microalloyed (with 0.23 and 0.24 w% chromium) and one non-microalloyed, high carbon prestressing steels. The slow strain rate testing technique was used to evaluate the effects of polarization to -0.90 and -1.30 VSCE upon strength and ductility of the steels. Fractographic analysis was performed using scanning electron microscopy. Based upon statistical analysis, an attempt was made to relate the remaining strength of the corroded wire to the extent of corrosion damage for different corrosion morphologies.

Experiments and analyses were performed to better define the limits of concern regarding hydrogen embrittlement in association with application of cathodic protection to prestressed concrete. To accomplish this, prestressed concrete specimens were locally corroded to different levels by anodic polarization and then polarized to -1.30 V SCE. A procedure of examination was developed using strain gauges to determine the level of prestrain. Relatively few brittle failures of wires resulted due, at least in part, to a relatively low prestrain of the pretensioned tendons. A model was developed which, coupled with data from parallel research, permitted definition of the minimum cross section for brittle failure as a function of the magnitude of prestrain and corrosion morphology of the wire. These results were tabulated in a format that can be used during field inspection to identify structural elements for which fracture could occur upon application of cathodic protection.

Hollow, cylindrical mortar specimens of 0.4 water-cement ratio were prepared without reinforcement and exposed to flowing natural sea water for periods up to one year. Direct currents of 2, 10 and 50 mA were impressed between a mixed metal-oxide titanium substrate electrode positioned within each of these two zones, with a different electrolyte supply and exhaust for the cylinder core and exterior surface. Linear expansion of the specimens was evaluated as a function of exposure duration from the output of embedded strain gages and from dimensional measurement of cylinder length and diameter. It was found that expansion of specimens exposed to direct current exceeded baseline ones (no current). Also, the expansion was anisotropic in that different magnitudes and trends were apparent for the diameter versus length directions. The expansion under free exposure (no current) was determined to be a function of specimen size and of the direction of measurement relative to the cast specimen face.

Experiments were conducted to evaluate occurrence of any deterioration of prestressing steel tendon to concrete bond as a consequence of cathodic polarization. Pretensionned concrete specimens were cathodically polarized with current densities ranging from 50 to 5000 m^2 of steel while exposed to a constant flow of natural sea water. The concrete and steel dimensional changes were monitored by strain gages mounted on the tendons and embedded in the concrete. Contractions of the steel of 25 to 50 percent of the initial tensioning were recorded after 17 to 36 MC/m^2 were transferred to the tendons on specimens polarized at the highest currents. This corresponds to 54 to 114 years of polarization at 10mA/m^2 if bond loss was solely dependent on the charge transfer. It was noticed that the smaller the current, the more the charge that was transferred before steel contraction began. These results imply that cathodic polarization should impose no threat to the prestressing steel-to-concrete bond on typical structures over their expected lifetime.

Cathodic protection is currently recognized as the most practical mean for arresting corrosion of reinforcing steel tendons in existing concrete structures, however, its appropriateness in the case of prestressed concrete is questioned because prestressing steels are relatively susceptible to environmental cracking (hydrogen embrittlement). For the purpose of studying embrittlement tendencies a series of experiments using the slow strain rate technique were performed. The susceptibility to environmental cracking was compared for different steels corresponding to ASTM grades 270 and 250 polarized at $-$900 and $-1300$ mV (SCE) in deaerated saturated Ca(OH)$\sb2$ solutions. The influence of different notch and pit geometries was studied to simulate the behavior of corroded tendons and investigate the transition between smooth and severely notched specimen behavior. Also evaluated was the evolution of the mechanical properties of tendons after excessive polarization. The different cracking processes are discussed based on test data, fractography and exposures conditions associated with concrete structures.

A series of experiments was performed on prestressed concrete specimens for the purpose of studying the tendency of disbonding between the embedded steel and the concrete due to an impressed cathodic current. The procedure first involved impressing an anodic current until active potentials were recorded along the tendon length. Subsequently, cathodic polarization of the prestressed tendons was affected using a current density of 1 mA/cm^2. The strain variation of the concrete specimens was monitored during these two procedures using gages placed on the top or bottom surface. It was found that for a current density of 1 mA/cm^2 during the 30 day monitoring period the concrete relaxed by an amount equivalent to an 80 percent loss of bond between the tendons and concrete. The implications of this with regard to cathodic protection of prestressed concrete structures and components in actual service are discussed.