AbstractCopper is abundantly used throughout marine infrastructure, both in its pure and alloyed form.Pure copper is used in electrical cables in offshore infrastructure due to it desirable electrical properties. Copper itself is not corrosion resistant to seawater, therefore extensive sheathing is employed to protect the copper from being exposed to seawater. However, these cables are
susceptible to damage, leading to the exposure of copper to seawater. Faults can arise due to a number of reasons involving physical, climatic and/or chemical effects. One particular fault includes water-treeing caused by the dielectric breakdown of cable insulation. This form of fault often results in small faults in the μm range. Though this seems small, such a fault can ultimately lead to cable failure due to the corrosion of the copper core in seawater. Therefore, an effort is made here to investigate a technique which could prevent the corrosion of copper exposed to seawater in this manner, subsequently improving the lifetime of a defective cable. In particular in this thesis, the anodic passivation of copper microelectrodes (consistent with μm sized faults) in model seawater is investigated as a corrosion protection technique.
Anodic passivation involves the use of electrochemical methods, where polarising a given metal to positive applied potentials encourages the growth of a protective film. This can be achieved by performing voltammetric or chrono methods. In the case of copper, a partially passive CuCl film forms. The anodic passivation of copper has proven to be complex, however, where dynamic current phenomenon is observed during voltammetry to high positive potentials. This phenomenon refers to current oscillations/noise, which are primarily linked to the formation and dissolution of CuCl.
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.
Initial results for copper microelectrodes in 0.5MNaCl(aq) using voltammetry found that CuCl forms initially at approximately –0.11 V vs. saturated calomel electrode (SCE), and is the dominant product observed throughout voltammetry to +5.00 V vs. SCE. The continual presence of CuCl is confirmed by performing in situ Raman spectroscopy. At 0.20 V vs. SCE, the formation of Cu(II) is observed which coincides with the onset of dynamic current phenomena—chaotic current oscillations. 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 μA in magnitude) in addition to baseline current oscillations (around 5-6 μA). 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 arelinked to the their ability to act as surfactants.
The anodic passivation of copper-nickel alloys is also owed to the formation of CuCl. 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 μA) 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|
|Supervisor||Jonathan Dawes (Supervisor) & Frank Marken (Supervisor)|
- Anodic Polarisation
- Copper Alloys