Localised Corrosion of Ni-Base Superalloys in Seawater

Abstract

Nickel Superalloys are used extensively within the oil and gas industries. Theses alloys have good resistance to a wide variety of corrosive environments in industrial processes such as chemical and petrochemical processing, marine engineering, oil/gas production and transport, and nuclear reactors. Similarly to general grade steels they are readily available, but nickel-base alloys are preferred due to their superior corrosion resistance. Within oil and gas they are often utilised as sub-sea connectors. The geometry of these connectors makes the alloys susceptible to a crevice corrosion, despite the alloys good resistance to general corrosion This project focuses on determining the corrosion properties of two nickel alloys; Inconel 718 (718) and Custom Age 625+ (625+); within environments appropriate to the oil & gas industry; notably chloride-rich electrolytes. The work produced will aid in determining whether these alloys are likely to suffer crevice corrosion when in service as sub-sea connectors. Electrochemical techniques, including potentiodynamic polarisation (PDP), and a modified Tsujikawa-Hisamatsu Electrochemical (THE) technique were used to obtain data on the crevice corrosion behaviour, the critical crevice temperature (CCT), and the crevice potential (ECREV), of the materials at different temperatures. Heat treatments were additionally employed to manipulate the microstructures of the alloys, so that the effect of the precipitates in the microstructure on the corrosion resistance could be investigated. Microstructures which contained the combination of precipitates found in these alloys (γ’, γ�, δ-phase) had the highest corrosion resistance, with alloy 625+ being more resistant to localised corrosion than alloy 718. Three chloride concentrations were tested in combination with the microstructural conditions and temperature effects to assess if chloride concentration had a significant role in the crevice corrosion behaviour. Although the chloride concentration could affect the CCT, the temperatures at which these alloys underwent crevice corrosion are unlikely to be of concern in their current in-service deployment. Crevices which had formed were observed using Scanning Electron Microscopy (SEM) and Laser Scanning Confocal Microscopy (LSCM). Both alloys suffered crevice corrosion through an intergranular attack pathway for heat treatments where there was no precipitation, or where the gamma precipitates were dominant. When the delta phase was the dominant precipitate, it was the matrix which provided a corrosion pathway.

Date of Award	31 Dec 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Dirk Engelberg (Supervisor)