Microstructural characteristics of the oxide films formed on Alloy 690 TT in pure and primary water at 325 °C

Highlights

• Oxidation behavior of Alloy 690 TT is influenced by water chemistry.

• The porous, Cr-depleted inner layer formed in oxygenated water is not protective.

• Increase in pH promotes the dissolution of Cr species.

• Higher solution conductivity leads to a thicker oxide film in primary water.

• Sample surface status affects the thickness of inner oxide layer.

Abstract

Microstructural characteristics of the oxide films formed on Alloy 690 TT after the exposure in oxygenated pure and primary water at 325 °C were investigated. Change in water chemistry from pure to primary water was found to vary the film structure from duplex to triple layer, and lead to change in the morphology of NiO on the surface. The increased pH and solution conductivity of primary water also accelerate the dissolution of Cr species and enhance the electrochemical corrosion. The Ni-rich and Cr-depleted inner layer formed under both water chemistries is porous and not protective. The related oxidation mechanism is discussed.

Introduction

Nickel-based Alloy 690 (Ni–30Cr–10Fe) is a technologically important material which is widely used as structural material for steam generator tubes in pressurized water reactors (PWRs). The higher chromium content than Alloy 600 (Ni–16Cr–9Fe) endows an improved corrosion resistance [1], [2], and failures of Alloy 690 tubes have rarely been reported. However, its long-term corrosion behavior under service conditions is still a highly concerned issue. For example, corrosion leads to nickel release in the water, which can be activated into ⁵⁸Co in the nuclear core under neutronic flux, and as such increases the global radioactivity of the primary circuit of PWR [3]. Corrosion is also the primary cause for the initiation and growth of stress corrosion cracking [4].

Characteristics of the oxide films formed on an alloy, including chemical composition, microstructure and thickness, etc., determine the protect ability of the oxide film and thus plays a crucial role in the corrosion behavior [5], [6], [7], [8]. A couple of material and environment relevant factors influence the oxide film characteristic. In addition to the chemical composition and microstructure of the alloy, water chemistry and surface finishing condition are two key factors that can vary the characteristic of the oxide film. According to McIntyre et al. [9], [10], the boiler chemistry has a strong effect on the surface oxide composition of chromium-containing alloys such as alloys 600, 690 and 800. It is well-known that PWR primary water operates with H₃BO₃ as a chemical shim for excess thermal neutron absorption to control the chain reaction, while LiOH is used to adjust the pH value [11]. Compared with pure water in the secondary circuit, addition of H₃BO₃ and LiOH increases the pH value and solution conductivity. Besides, oxygen could be introduced into both primary and secondary circuits occasionally by aerated feed water [12], or by adding of oxygen or H₂O₂ during the shutdown process of PWRs to stabilize the oxide and decrease the radiation activity of the coolant [13]. Consequently, an off-water chemistry is formed temporarily. It is therefore worthy to investigate the influence of changes in water chemistry on microstructure and composition of the formed oxide film.

Surface finish is also an important factor influencing corrosion behavior of an alloy. According to Perez et al. [14], surface finish produces a subsurface deformation layer with high dislocation density, phase transformation and recrystallization, which would impact corrosion mechanisms. In general, studies in laboratories use ground or polished specimens revealed the effects of surface finish on corrosion resistance [15], [16], [17]. However, under normal service conditions, the surface of components is not polished, and it is of great importance to study the corrosion on such as-machined surfaces.

Our previous work [18] has reported that surface finish (ground, mechanical polished and electropolished) has obvious effect on the microstructure of oxide films formed on Alloy 690 TT in oxygenated primary water. Corrosion on as-machined surface of such a component as heat-exchange tube, however, has not yet been studied. As such, the objective of this work is to obtain a better understanding of the microstructural characteristics of the oxide films grown on the as-received commercial Alloy 690 TT tube specimens after the exposure in pure and primary water both containing 2 ppm O₂ at 325 °C. The related oxidation mechanism, and effects of water chemistry and surface finish on oxidation are discussed.