Steel--Cathodic protection

The effect of substrate finish and composition, flow rate and exposure duration on the composition, morphology and protective properties of calcareous deposits formed during cathodic polarization of several steels in seawater has been investigated. The current density behaviour of the scale formation in conjunction with morphology and composition characteristics indicated that the substrate finish and composition did not influence the nucleation or growth of the deposit. The results strongly suggest a relationship between electrolyte velocity, current density behaviour and morphology. With increased flow and consequent higher current density requirements, results indicated a low nucleation rate of CaCO3. The current density behaviour and composition of the calcareous deposit revealed a growth progression of a Mg rich film formed in the first few minutes, followed by an increasingly uniform surface coverage by CaCO3 in the form of aragonite.

Knowledge of the reversible potential for corrosion reaction under condition of deep sea exposure and how this is influenced by parameters such as temperature and pressure is of importance to the understanding and design of deep sea cathodic protection systems, since this potential is used as the cathodic protection criterion. Also, if the polarized potential is more negative than the reversible potential of the hydrogen reaction, then hydrogen embrittlement may occur. Thermodynamic analysis methods are used to calculate Gibbs free energy change and reversible potential of reactions involved in cathodic protection of steel for temperatures and pressures typical of deep sea. Based on the assumptions employed it is found that the reversible potential for the iron reaction becomes less negative with depth for the first 1000m and remains approximately constant beyond this.

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 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.

The efficiency of cathodic protection of steel in seawater
is partially due to calcareous deposits. To better understand
deposits, expericents are conducted testing the effects of variables
such as tecperature, velocity, and applied current, on
film formation and protection capabilities. It is the intent
of this thesis to investigate differences in using recirculating
seawater rather than once-through seawater in these experiments.
It is believed that these differences were caused by an increase
in dissolved organics in the recirculating bath. Data is in
the form of current needed to maintain a potential of -1.0v SCE
since differences in current help describe differences in film
properties.

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.