Evaluation of Internal Tank Corrosion and Corrosion Control Alternatives
Authors: Titcomb Alan Norman 
Published: October 2008

A summary of the experience gained by the carriage of crude oil, refined petroleum products, and seawater ballast in tankers is presented. This information will prove useful in the selection of corrosion control systems to be used in similar tanks in the offshore petroleum industry. Key factors which affect tank corrosion are identified and their impact discussed. The various corrosion control systems used today are described and their relative effectiveness in preventing general and pitting corrosion under different tank conditions is evaluated. The systems include full and partial coatings, sacrificial anodes, and full scantlings. The results of an economic analysis of corrosion control alternatives used over an assumed 20-year tank life are presented.

INTRODUCTION

The basic objective of the study1 summarized in this paper was to investigate and analyze, on a cost and performance basis, the corrosion control alternatives used to protect the internal surfaces of cargo, cargo-ballast, and ballast tanks of steel ships. The study was conducted under the auspices of the interagency Ship Structure Committee and was carried out by Ne'APort News Shipbuilding. To obtain data on corrosion and on the cost and performance of methods for its control, a survey of ship operators, coating and anode manufacturers, shipyards and contractors, consultant's, and published literature was conducted. No shipboard tests or measurements were made. The study was designed to address several new regulatory, technological, and economic developments which affect the problem of tank corrosion and its control. The scope of the project limited investigation to crude oil tankers and product carriers transporting refined petroleum products only (e.g. gasoline, domestic heating oil, etc.). Chemical carriers and ships carrying edible products were not included.

The same corrosion problems experienced on board ships also exist in many tanks used in the offshore oil industry for the storage and transportation of crude oil or petroleum products, or for ballast tanks in mobile offshore drilling units.

BACKGROUND

Tankers carrying crude oil and refined petroleum products have always experienced corrosion problems. At first the best solution was simply no solution. Steel was relatively cheap and it was easiest merely to let the tank corrode and then repair or replace the steel when it became necessary. However, it gradually became evident that some means of corrosion control was needed, and in the 1950's the subject began to receive widespread attention. Work done at that time gave rise to a better understanding of the problem and its causes. As a result, more effective corrosion control systems were developed which led to ship classification societies reducing the minimum steel scantlings required for ships so equipped. The philosophy was that the reduction in steel weight and cost allowed during initial construction more than offset the initial cost of corrosion control systems and their maintenance or renewal throughout the life of a vessel. An increasing dependence on the ability of a corrosion control system to prevent wastage followed. This basic philosophy survived throughout the sixties and into the seventies.

Today, the factors on which this philosophy was predicated have changed.