Formation of calcareous deposit influences applied potential and current for long-term protection of offshore structures. A model is proposed to predict the applied potential-time history with an aim to establish mathematical correlation between applied potential on cathodically protected surface and time by computer simulation technique, following principles of electrochemical kinetics. Comparative studies between experimental and numerical analysis data show good agreement. The model opens up a method of determination of rate constant of kinetics of calcareous deposit formation. Effect of slope parameters on potential and current decay is also revealed.
Cathodic protection for the submersed portion of steel offshore structure with or without coating is the most technically appropriate and economically viable corrosion protective method. The fundamental criteria for cathodic protection is to cathodically polarize the structure to a potential of -0.80V vs. silver-silver chloride electrode or more negative potential. 1' 2. Protection of an offshore structure such as a petroleum production unit is normally accomplished by galvanic anode. Historically cathodic protection (CP) design procedures have been carried out through trial and error method using a single long-term current density 2. During the last 10 to 15 years, CP was based on rapid polarization technique whereby a high initial current was applied for formation of protective calcareous deposit, which gives rise to a low maintenance current density (3-4). Recently a technique based on of slope parameters is being applied for sacrificial cathodic protection of offshore structures 5-8.
Advent of newer methods have developed the CP design systems in many respects including lower mass of anode consumption but still at times cathodic protection systems fail due to over protection or under protection 9,10. One of the major problems that people face while designing the cathodic protection is the applied potential or current change from the position of the anode over the surface of the structure. While the distributed anode arrays on space frames such as production platforms tend to provide relatively uniform polarization, the unidimensional pipe gives rise to a problem of potential attenuation with increasing distance. Computer aided mathematical models have been developed to estimate current and potential distribution using Laplace's equation V20=0 7,1~-13.
Formation of calcareous deposit is a very important aspect for cathodic protection of offshore structure. It not only influences potential and current distribution but the kinetic aspect of its formation determines applied potential and current for long term protection of marine offshore structure 8. In the present investigation attempts have been made to f'md a mathematical correlation between cathodic polarized protection potential and time. The complexity of physical, chemical and biological phenomena occurring on cathodically protected surface does not permit a simple relation to exist between them. Thus with the aid of suitable numerical interpolation methods and computer simulation a relation is proposed.
EXPERIMENTAL PROCEDURES
Low carbon steel rod of 1 cm diameter and 10 cm lorig of which 2 cm was immersed in sea water collected from the Bay of Bengal, Digha region where an offshore structure for petroleum digging is to be hoisted shortly. The water was allowed to flow in with a uniform velocity controlled by a pump and electrical control unit to a rectangular refractory container. The water was drained out through a hole at the other end and collected in a reservoir from where the water was pumped into the refractory vessel. Thus a wat
