Sea-water corrosion

Experiments were conducted to determine the effectiveness of localized cathodic polarization for reducing corrosion of simulated prestressed concrete piles containing admixed calcium chloride and exposed to a simulated sea water tidal cycle. The specimens contained both continuous and segmented steel tendons, the purpose of the latter being to facilitate measurement of cathodic protection current. Conductive rubber in an impressed current system was used as the anode material. The specimens were initially freely corroded and then cathodically polarized at a constant current ranging from 0.5 to 1 mA/m$\sp2$ which corresponded to potentials (current-on) which ranged from $-$0.500 to $-$1.100 V(sce) in the anode region. The magnitude of impressed current and its distribution along the embedded steel was monitored as a function of exposure time, level of polarization and water levels. Current-on and instant-off potential distribution for both the continuous and segmented tendons were also measured. The level of cathodic polarization was assessed as a function of position along the specimens by the depolarization method. The results were evaluated within the context of marine bridge substructure cathodic protection technology.

The stress corrosion cracking (SCC) tendencies of several engineering alloys were studied in an acidified seawater environment as a function of applied strain rate and electrolyte temperature. The selected alloys included austenitic stainless steels 304L, 316L, 904L and A-286 (an iron-based superalloy at two heat treatments yielding ultimate tensile strengths of 130 and 200 ksi), Inconel 718 (220 ksi ultimate tensile strength) and Hastelloys C-22 and C-276. The slow strain rate test technique was used to evaluate the SCC strain rate dependency of each alloy at extension rates of 4.7 x 10^-6, 4.7 x 10^-4 and 4.7 x 10^-3 mm/sec. The effect of electrolyte temperature was evaluated at 38C and 60C at a single extension rate of 4.7 x 10^-5 mm/sec. Control specimens were tested in a laboratory air environment at an extension rate of 4.7 x 10^-5 mm/sec. Various mechanical parameters of the specimens tested in the corrosive medium were compared with those of control specimens to quantify the degree of cracking. Fractographic evidence of SCC was obtained using scanning electron microscopy (SEM). An attempt was made to correlate SCC tendency with the alloy's passivation kinetics and microstructure. Atmospheric exposure testing was performed in a simulated space shuttle launch pad environment for selected alloys.

Study of selected candidate steels for offshore application was undertaken to observe the effects of cathodic protection and cyclic frequency on corrosion fatigue life. Keyhole Compact Tension Fatigue experiments under constant amplitude sinusoidal loading and stress ratio of 0.5 were performed on 25.4 mm thick specimens in natural sea water and also in air upon three different steels (Y.S. 500-563 MPa). These steels represented different strengthening techniques, namely precipitation hardening, direct quenching--a thermomechanical control process (TMCP), and controlled rolling. Cathodic polarization was in the range between freely corroding and -1.10 Volts (SCE). The tests were performed at a frequency of 0.3 and 1.0 Hz. The results are presented in the S-N and potential versus cycles to initiation format. No effect of frequency (1.0-0.3 Hz) was observed at cathodic protection of -1.10 V (SCE). The steels showed an increase in fatigue life to an optimum potential, and excessive potentials were detrimental. The fatigue life in dry air was greater than in laboratory air (~50% RH).

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.

In order to investigate the cracking tendency of
cathodically polarized SEACURE superferritic stainless
steel, a series of potentiostatically controlled tests were
conducted in flowing natural seawater. U-bend and tensile
specimens were used to determine the threshold potential and
limiting fracture stress.
The microstructure was examined using a metallurgical
microscope and a chemical analysis was conducted across the
grain boundary. These results were then correlated to
hydrogen assisted cracking, by performing SEM and EDAX
analyses on fractured U-bend and tensile samples. Finally a
model was developed which explained the predominantly
intergranular cracking in the static stress condition and
also accommodated the transgranular failure during dynamic
loading.

The influence of hydrodynamic parameters and limiting
current density by cathodic protection on a steel cylinder in
flowing water is investigated. The limiting current density
variations along the circumference of a circular cylinder are
determined for different flow velocities. Several methods
are also used to relate the cathodic current density
distribution to variations in hydrodynamic and diffusional
boundary layer thickness along the circumference of the
cylinder for laminar and turbulent flow regimes. a
comparison of experimental current densities on a cylinder to
that predicated by theory is also examined.

Computerized modeling techniques have been developed for the
analysis of polarization behavior, using the expected
chemical reactions and transport phenomena applicable to the
formation of calcareous deposits. The method uses the ADINAT
finite element heat transfer program and is modeled, as much
as possible, on theory rather than laboratory measurements.
In contrast, the presently available cathodic protection
computer models base their polarization behavior on
empirical data. The thesis presents experimental versus
theoretical data for a simple flat plate configuration under
cathodic protection in natural sea water over a range in
temperatures. The flat plate allows for a simplified
experimental apparatus and simplifies the derivation of
equations for the transport phenomena. The program can,
however, handle complex three dimensional geometries.
Effects of temperature and flow on the formation and
protection of calcareous deposits are also discussed in
light of the experimental results.

'J 'ht' c· f feet of seawater ,_tnd potential on the fatigue crack
initiation hehavior of several microalloyed and thermomPrhanically
processed steels has been investigated. Five
steels with yield strength in t~e range 500 to 750 MPa (73
to 108 ksi) were selected which included a range of processing
histories from conventional quenching and tempering to
precipitation hardening and direct quenching. These steels
were chosen because of their potential utility as materials
of construction for deep water compliant platform hull and
riser systems. Blunt notch compact tension specimens were
exposed to natural flowing seawater under constant load ampli
tnrlc. conditions at both freely corroding and cathodically
protected potentials. Cycles to crack initiation were
characterized by cyclic stress intensity range normalized
relative to the square root of notch radius. Test data are
presented and compared with data from other tests and steels
using appropriate stress analysis methods.

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.

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.