Experimental Studies Relate Effect Of Cathodic Protection With Certain Generic Coating Systems
Authors: Simpson V.P. , Robinson Ronald C. 
Published: October 2008

Applications of protective coatings and cathodic protection are recognized corrosion-control methods for marine steel structures. With the growing use of tall offshore structures for deep waters, corrosion control has become critical and difficult to provide throughout the life of the structures.

Typically, coatings have not been used in the submerged areas of offshore steel structures because of a lack of confidence in their ability to perform in conjunction with cathodic protection. With short life expectancy, they appeared too costly. Attempts to cathodically protect bare steel structures have not been completely successful because of excessive current requirements, shielding problems, limitations of anode throwing power, and weight.

Since 1975, an experimental program has been conducted by Ameron to evaluate the performance of coatings both with and without cathodic protection in simulated marine exposure. Numerous marine coating systems have been tested over a 5-year period using protection levels of -850, -1100 and -1200 mv to Cu/CuS04 electrode. The tests are described and results are presented and discussed.

The results indicate that certain coatings are compatible with cathodic protection, with 2 specific systems offering relative insensitivity to long-term overvoltages of -1200 mv (Cu/CuSO4 electrode). Lower protective current densities required for coated steel coupled with more uniform protection offer potential benefits.

The design of a cathodic protection system can be simplified and the systems efficiency improved by using the long-term compatibility data developed in this program. Problems associated with cathodic protection of offshore steel structures can be reduced by use of coatings for the submerged portion.

INTRODUCTION

During the past decade there has been a dramatic increase in the size and complexity of both oceangoing deep draft vessels and offshore structures. Nowhere have these developments been more evident than in the production and exploration platforms recently designed and installed throughout the world. It is expected that the design of the structures will continue to provide engineering challenges as the search for petroleum expands into the deeper and more hostile ocean environment.

The corrosion protection of offshore structures has continued to increase in sophistication with certain basic systems partially standardized in Europe and North America. 1, 2 Above the tidal zone, high performance, well-recognized protective coating systems have been specified and successfully used for several decades. These typically comprise inorganic zinc-rich primers, epoxy or vinyl mid coats and, occasionally, chlorinated rubber, acrylic or urethane topcoats. 1,3 In the tidal zone area there has been increasing use of monolithic epoxy cladding materials which offer both erosion and corrosion protection. For the submerged areas the use of cathodic protection has been generally accepted.

Either impressed-current or sacrificial-anode systems are used to protect the. bare steel that is totally immersed. 2,4 Impressed current cathodic-protection systems have not been as popular as sacrificial-anode systems because of the former's need for very careful design, and electrical power coupled with constant monitoring when in service in order to assure continued optimum performance. 3,5 Cathodic-protection systems will probably remain the preferred method of corrosion control for the submerged portions of these offshore structures.