Electrochemistry of Copper and Copper Alloys in Marine Environments: Chaotic Corrosion Phenomena

Abstract

Copper is abundantly used throughout marine infrastructure, both in its pure and alloyed form.

Anodic passivation involves the use of electrochemical methods, where polarising a given metal to positive applied potentials encourages the growth of a protective film.

In this thesis, the aim is to develop an understanding of the anodic passivation mechanism for copper microelectrodes in model seawater. The mechanism is investigated primarily with voltammetry to distinguish potential-dependent processes. The effects of electrode diameter, pH, salinity, temperature and the presence and type of gas on anodic passivation is explored.  In addition to pure copper microelectrodes, the effects of alloying copper with nickel is also investigated. In particular, copper-nickel alloys constantan and monel are studied.

It is hypothesised that the Cu(II/I) process is linked to the onset of current oscillations, where Cu(II) causes instabilities in the CuCl film leading to current noise/oscillations. Current oscillations proceed to 5.00 V vs. SCE, where large current spikes are observed (up to 100 _A in magnitude) in addition to baseline current oscillations (around 5-6 _A). The use of a polymer of intrinsic microporosity (PIM) revealed mechanical processes associated with aspects of the current oscillations. The use of PIM revealed a link between the large current spikes and the expulsion of particulate CuCl, hindered by the presence of PIM. This process is referred to as colloidal dissolution. For pure copper, no evidence for anodic gas evolution is observed (high potentials can cause water splitting), and thus not considered in proposed mechanisms for pure copper.

The effects of the presence and type of gas in the electrolyte has a profound effect on the anodic passivation of copper, particularly on the current oscillations observed up to 5.00 V vs. SCE. More specifically, the frequency and presence of large current spikes change depending on the presence and type of gaseous solutes in the electrolyte (aqueous 0.5 M NaCl).  Vacuum-degassing, and saturation of the electrolyte with O2, He and H2 suppresses current spikes, whereas saturation with Ar, N2 and CO2 enhance current spike events (and thus colloidal dissolution). Saturation with air gives a mixture of effects owing to N2 and O2.

It is proposed that colloidal dissolution events occur due to gas bubble nucleation, where bubble nucleation causes strain in the CuCl film. The release of said strain then leads to film breakdown and the expulsion of CuCl particulates. Furthermore, the effects of different gases are linked to the their ability to act as surfactants.

The anodic behaviour of the alloys constantan and monel proves to be similar at low positive applied potentials (up to 1.80 V vs. SCE), and significantly different at potentials > 1.80 V vs. SCE. The difference being a transition from low-level current oscillations similar to copper (5-6 _A) to high current oscillations in the mA range. This transition occurs at 1.80 V vs. SCE. The potential-dependence of the high current noise is linked to gas evolution due to the formation of a stoichiometric oxidant NiO2, and thus a Ni(IV/II) process. A mechanism is proposed with support from Pourbaix diagrams and scanning electron microscopy (SEM) imaging, based on gas evolution and rapid breakdown of a CuCl film.

Overall, this thesis highlights the complexity of anodic passivation for copper and coppernickel microelectrodes and shows that film formation and dynamic current phenomena are sensitive to changes in electrolyte environment and alloying. New hypotheses are given for the partial passivation of copper and copper-nickel alloys, and novel approaches are used to do so, including the use of PIM and recession analysis by SEM.

Date of Award	1 May 2020
Original language	English
Awarding Institution	University of Bath
Supervisor	Jonathan Dawes (Supervisor) & Frank Marken (Supervisor)