Seawater

Experiments have been performed where cylindrical 1018 steel specimens were polarized to -0.900 V.(SCE) in natural sea water at 24 and 3C and with rotation speeds of 0 and 0.83 Hz. The nature of the calcareous deposits which formed was characterized, first, by monitoring current density during the experiments and, second, by post-test SEM investigation of deposit morphology and thickness and by EEDS analysis of deposit composition. The dependence of cathodic current density and film composition upon temperature and exposure time has been established, and the observed trends have been interpreted in terms of (1) the temperature dependence of the solubility limit and relative nucleation and growth rates for Ca and Mg rich phases and (2) possible electrical conductive behavior on the part of Mg rich deposits.

The fatigue response of selected high strength low
alloy steels, that are under consideration for future
offshore structural applications, was investigated. Freely
corroding and cathodically polarized experiments were
conducted under conditions relevant .to these applications.
Cathodic polarization experiments involved testing of A537
Direct Quenched (A537 DQ) steel at different potentials in
the -0.80 V to -1.10 V (SCE) range. The damage to freely
corroding high strength steel was more than on structural
steel for calculations based on the Wirsching wide band
spectrum. Polarization to -0.80 V (SCE) improved the
fatigue life of A537 DQ steel over the freely corroding
life. A definite conclusion with regard to the influence of
different levels of cathodic polarization on the fatigue
life of the steel investigated can not not be drawn based on
the results from this study due to data variability.

Experiments were conducted to investigate frequency dependence of sea water corrosion fatigue of notched 1018 steel under constant deflection, reverse bend test conditions. Frequencies of 1850, 1200, and 600 cycles per minute (CPM) were studied. Results indicate that frequency effects are not observed at high stresses, apparently due to the mechanical damage component of the fatigue process dominating over corrosion considerations. At low stresses reduced frequency resulted in decreased cycles to failure. Acidification of the crack tip solution is proposed as possibly responsible for this behavior. For specimens subjected to equal stress levels in the frequency dependent region, greater cathodic protection is required to mitigate corrosion fatigue for 600 CPM specimens (-0.86 volts standard calomel electrode) (V SCE) than for 1850 CPM specimens (-079V SCE).

Laboratory experiments have been performed to characterize the effects of initial current density and selected variables (initial current density, temperature and surface treatment) upon the cathodic polarization behavior of API 2H Grade 42 steel in natural sea water. The procedure involved galvanic coupling of a cylindrical steel specimen to a larger diameter aluminum sacrificial anode ring through an external resistor, which offset the otherwise impractically small anode/cathode surface area ratio and permitted the desired initial current density to be realized. In the initial polarization stage the change in potential versus current density data with time was found to be linear with a slope equal to the product of the total circuit resistance and cathode surface area and with the vertical intercept corresponding to the anode open circuit potential. Lower temperature or increased flow resulted in reduced polarization and a relatively high current density, but data for experiments employing a particular resistance conformed to straight lines with the same slope R[ext] x A[c] (external resistance times cathode surface area). Some experiments exhibited concave behavior at longer test duration. An explanation for the linear interdependence of potential and current density and the following concave behavior is presented. The laboratory experiments were compared with offshore structure survey results. It is projected that the potential-current density behavior of galvanic cathodic protection (CP) systems of different geometries can be quantitatively interrelated through this slope parameter. Implications of the data are discussed within the context of cathodic protection design, rapid polarization and system performance for offshore structures.