General Guidelines for Corrosion Testing of Materials for Marine Applications (EFC 3) PDF

INTRODUCTION

In most practical applications of materials a knowledge of the corrosion properties of the candidate materials is essential for selection purposes. Such knowledge can be derived from service experience, which usually involves long term exposure under frequently ill defined and ill documented conditions, and corrosion testing.

However, because the corrosion mechanism of a system depends on many variables, corrosion testing itself has many pitfalls. Often tests are designed to investigate specific forms of corrosion or specific applications. In this respect uniform corrosion is the least disturbing. However, tests relating to localised corrosion are far from ideal: in many instances only a qualitative description of the corrosion behaviour can be obtained, frequently on the basis of go-no go tests. At best, the results from such tests can be used for ranking purposes. A more quantitative approach, although desirable, is still utopian in most cases of localised corrosion.

This sad situation is a result of the greater - number of variables influencing the localised corrosion processes compared to those involved in the case of uniform corrosion. The following points must usually be considered critically for all corrosion tests:

(i) the purpose of the test

(ii) the test conditions (e.g. environment, specimen size, geometry and preparation, temperature, flow velocity, potential, etc.)

(iii) interpretation of the test results, including a definition of the reliability and the limits of applicability

(iv) translation of the test results into data suitable for engineering practice.

The foregoing hold for all types of corrosion testing. The purpose of the present paper is to discuss marine corrosion testing, in particular with regard to the environmental test conditions. In this context the possible use of synthetic sea water as opposed to in situ conditions will be discussed as well as the problems related to the use of natural sea water in test loops, i.e. recirculated, stored, etc. In most cases, tests in sea water are conducted to obtain information relating to the behaviour of materials that are intended for service in that environment. The results of such tests are then used as a basis for materials selection.

For the tests to yield meaningful results, a knowledge of the environment that exists under actual service conditions is necessary. Quite often the water quality within the plant, under normal operating conditions, differs significantly from that existing at the intake to the plant. In order to conduct realistic corrosion tests these variations must -be taken into account. The bulk environmental conditions can be clean sea water, e.g. around offshore structures and some power stations. In other instances the water is polluted or brackish, while in other cases, e.g. ships, a variety of water qualities will be encountered during service.

Edited by: A.D. Mercer