Seawater corrosion

Critical chloride threshold, CT values for initiation of reinforced steel corrosion m
mortar typical of Florida coastal bridge substructures were determined in laboratory tests.
Previous research has reported CT values that vary by more than an order-of-magnitude,
making design life estimation for structures difficult. On this basis, experiments on
piling type specimens focused on [Cl-] contamination in the splash zone and coupling of
this steel to a large surface area submerged anode. The lower portion of simulated piling
was immersed and the region above the waterline periodically sprayed with NaCI
solution. Corrosion potential with respect to height above the waterline was monitored.
A temporary depolarization method for determining as to whether or not corrosion had
initiated is proposed. Chloride distribution at the reinforcement-concrete interface was
determined in piling using energy dispersive x-ray analysis and related to height with
respect to the waterline. Chloride threshold was related to corrosion potential.

The purpose of this research project is to compare the strength and durability
characteristics of rice husk ash-modified concrete with those of normal concrete in
the marine environment. Specimens prepared from concrete mixes with watercementitious
ratios of 0.40 and 0.55, and rice husk ash content of 0%, 10%, and 20%
were tested. The rice husk ash used was obtained from Agrilectric, power plant
located in Lake Charles, Louisiana. The grinding of the ash to particle size of 7-J..tm
to 45-J..tm was done by Process Research ORTECH Inc., Ontario, Canada. Strength
and durability tests were performed, following ASTM (American Society for Testing
and Materials) Standards. The significant fmdings are that the properties and quality
of the rice husk ash-modified concrete are as good, if not better than normal concrete.

In this investigation an ultrasonic technique and spectral analysis
were employed to detect corrosion of reinforcing steel in concrete
under conditions of accelerated testing. Various properties of a
transmitted ultrasonic pulse were determined, and it was concluded
that the transfer function provided the most realistic possibility
for detection of corrosion and cracking. The most meaningful correlations
of the transfer function with corrosion and cracking events
were for situations where the temporal variation of the transfer
function at the particular frequency was small. The appropriateness
of ultrasonic parameters with regard to problems involving corrosion
of reinforcing steel is discussed.

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.

A pH determination technique of concrete pore water by means of leaching OH- ions from powdered samples was developed. This method was then evaluated in terms of sample size, extent of dilution, leaching period, the type of leaching, calcium hydroxide error, and titration scatter. Based upon variations of the above parameters it was concluded that a maximum amount of hydroxide ions was released into solution for samples with the smallest particle size within a 24 hours static leaching period. A 50ml dilution volume proved optimum with regard to filtration and titration. Under the above circumstances calcium hydroxide dissolution did not seem to pose a significant source of error. Furthermore, repeatability of the measurements was good and titration scatter was determined to be about 0.05 pH units. This method was used to determine the pH of samples containing different amounts of fly ash or silica fume additions as well as plain cement (control) samples. A drop in OH- concentration, that is pH, was detected with increasing pozzolanic addition. The results are discussed with the context of the degree of corrosion mitigation afforded by these pozzolans.

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.

The stress corrosion cracking susceptibility of austenitic stainless steels SS304L, SS316L and SS904L was studied in an acidified seawater environment by slow strain rate testing at 24, 38 and 66$\sp\circ$C. Fractographic evidence of SCC susceptibility was obtained using scanning electron microscopy. The degree of susceptibility to SCC for each alloy in these environments is discussed based on the mechanical parameters, fractography and anodic polarization behavior. The results showed that SS904L performed better than SS304L and SS316L in the aforementioned environments.

Tests were conducted using 10 cm. (3.93 in.) lengths of both
epoxy coated and bare steel reinforcing rods in simulated concrete
environments of varying pH and chloride ion levels. Polarization
studies using cathodic scan techniques were used to characterize the
quality of the epoxy coating. In all the environments tested, bars
with more defects required larger currents at a given potential than
did the bars with few defects. Corrosion potential measurements in
passive environments showed significant fluctuations from day to day,
making it difficult to use potential measurement for corrosion characterization.
Galvanic couples between epoxy coated and uncoated bars
indicated that galvanic currents increased as the differences in the
corrosion potentials increased and also as the chloride ion content
of the environment increased from 0 to 0.05 M.

Corrosion experiments have been performed upon steel and
copper in natural sea water, photo-oxidized sea water and
3.5% sodium chloride-distilled water solutions. Anodic
and cathodic polarization scans were conducted, and the
resulting curves were employed to characterize corrosion
behavior in each of the electrolytes. While little difference
as a function of electrolyte was noted for tests upon
copper, variations were apparent in the case of steel.
The effect of environment has been interpreted in terms of
differences in organics adsorbed upon the metal surface
from one electrolyte to the next. Significance of adsorbed
organics upon corrosion rate of steel in sea water is discussed.

The purpose of the thesis is to determine and compare the performance characteristics of marine piles corroded by chloride diffusion and repaired by several different methods for both uncracked and cracked concrete and to determine their structural integrity. The long-term objective is the comparison of the analytical values with those from an on-going experimental evaluation. The time for chloride concentration reach the threshold value that initiates corrosion in the reinforcement has been determined by Fick's law, extended to 2-D and 3-D chloride diffusion, for (a) uncracked concrete, and (b) for cracked concrete with the Simplified Smeared Approach (SSA). The structural integrity of the concrete circular pile is compared before and alter repair, by (a) finite element modeling using ANSYS software with the maximum deflection, and (b) beam strength analysis to find the moment capacity for cracked and ultimate conditions. The overall findings indicate the adequacy of the repair procedures.