T. COUVANT, EDF R&D, France

DOI: 10.1533/9780857097453.1.70

Abstract: Corrosion is one of the major obstacles to extending the lifetime of nuclear power plants within agreed safety requirements. A large variety of the structural metals present in primary and secondary circuits of pressurized water reactors (PWRs) suffer corrosion. Uniform corrosion, flow-accelerated corrosion (FAC), pitting, stress corrosion cracking (SCC), environmentally assisted fatigue and hydrogen embrittlement can all affect the major components of PWRs, despite stringent selection of materials for component manufacture. Remedies can vary: adjusting water chemistry, reducing superficial strains and stresses, replacing materials or changing microstructures. Experience in the field has demonstrated that increasing chromium content is an efficient strategy: to date nickel alloys containing 30% chromium exhibit very good resistance to corrosion such as SCC. It can be shown that tendency to corrosion can largely depend on manufacturing conditions.

Key words: corrosion, austenitic alloys, pressurized water reactors, primary water, cracking.

2.1 Introduction

We begin the chapter with an outline of the history and fundamental prin­ciples of corrosion.

2.1.1 History

Corrosion and its effects have been observed since the first steps in metal­lurgy. Corrosion damage increased with the use of iron over the centuries. In 1830, de la Rive (1801-1873) showed that bimetallic junctions suffered fast corrosion due to impurities present in zinc. Later, Faraday (1791-1867) cor­related the current flow with the associated rate of corrosion. In the 1930s, Wagner (1901-1977) showed that the uniform dissolution of metals did not require separate anodic and cathodic sites but that metal dissolution and the accompanying cathodic reaction can occur randomly with respect to space and time over the surface. In the 1950s, Pourbaix (1904-1998) edited a series of major diagrams giving the domain of stability of many elements as a function of potential and pH.

From the 1950s the importance of corrosion to the economy became increasingly evident. Today, corrosion is one of the major degradations to overcome in order to extend the lifetime of nuclear power plants in agree­ment with safety requirements.