Uniform corrosion proceeds over the entire surface area of the material exposed to the environment leading to a general slow thinning accom­panied by a release of corrosion products. Uniform corrosion is usually relatively easy to measure and predict. In the primary circuit of PWRs, released ions are transported and may be activated when they reach the reactor pressure vessel (RPV), which is a major problem to be overcome.

Therefore, uniform corrosion is minimized combining appropriate materi­als (nickel alloys and stainless steels), surface finish, passivating treatments and alkaline pH.

Flow-accelerated corrosion or flow-assisted corrosion (FAC) is a mecha­nism in which the passive layer dissolves in fast flowing water, without any mechanical erosion. As a consequence, the underlying metal continuously corrodes to recreate the protective oxide. FAC rate decreases when the flow velocity decreases and when the pH increases. FAC stops as soon as oxygen is dissolved in water. FAC affects carbon steel piping of the secondary cir­cuit where water or wet steam circulates.

Pitting is a localized corrosion forming holes at the surface of the metal, induced by the local depassivation of an area, which becomes anodic while a large area becomes cathodic. The acidity inside the pit is sustained by the spatial separation of the cathodic and anodic half-reactions, which creates a potential gradient and the transport of anions into the pit. The presence of surface defects, such as scratches and local changes in chemical compo­sition promote pitting which causes little loss of material but it may lead to deep corrosion in a component. Pitting mainly affects materials such as austenitic stainless steels exhibiting a good resistance to uniform corro­sion thanks to their good passivation. However, the presence of chlorides and oxygen at relatively low temperature (typically 80°C) may weaken the passive layers and enhance pitting via an autocatalytic process: Cl — ions start to concentrate in the pits for charge neutrality and promote the reaction of positive metal ions with water to form a hydroxide corrosion product and H+ ions. The increasing acidity within the pits accelerates the process.

Stress corrosion cracking (SCC) is a progressive failure affecting metals subjected to a tensile stress (residual or applied) while they are exposed to a corrosive environment. SCC occurs in specific and limited conditions in terms of water chemistry, material and loading. SCC usually involves a long incubation period prior to initiation, followed by a slow crack exten­sion stage and transition in a fast crack propagation stage leading to failure. Stress concentrations, cold work and irradiation promote SCC. The mate­rial most susceptible to SCC is Alloy 600 (in both primary and secondary waters). However, stainless steel becomes susceptible to SCC in primary water under specific conditions: polluted environments (oxygen plus chlo­rides), high level of cold work and irradiation. The susceptibility of nickel alloys to SCC strongly decreases when the chromium content of the mate­rial increases, especially above 20%. Basically, SCC results in the oxide ingress, usually at grain boundaries, which locally weaken the material. The oxide penetration is enhanced by the presence of strain and is affected by precipitation. If local stresses are sufficient to fail weakened grain boundar­ies, a crack extension occurs.

Environmentally assisted fatigue occurs under the combined actions of low frequency cyclic loading and oxidation. Therefore, ingredients are very similar to those involved in SCC mechanisms in the sense that a synergy operates locally between oxidation and mechanics. The major difference with SCC is the nature of the loading: cyclic loading strongly promotes strain localization in shear bands. The movement of dislocations (defects allowing the non-reversible deformation of the metal) enhances oxide ingress and failure, especially when shear bands emerge at the surface, breaking the pas­sive layer. Therefore, strain rate is one of the key controlling parameters of the mechanism.

Last, it should be noted that one of the corrosion products is able to embrittle metals. Indeed, as reported before, the cathodic reaction (reduc­tion of water) produces hydrogen which partly enters into the metal and interacts with the microstructure. The entry of hydrogen is limited by the growth of the passive layer; also its transport and interactions with the metal strongly depend on temperature. Hydrogen embrittlement is the process by which a localized accumulation of a sufficient level of hydrogen can eventually lead the metal to fracture under residual or applied stress. Therefore, situations limiting the transport of hydrogen (low temperature, presence of traps) promote such embrittlement. Other mechanisms of introducing hydrogen into metals exist, such as manufacturing (welding) and irradiation.