J.M. Sykes

The effect of phase compositions on the pitting corrosion of 25 Cr duplex stainless steel in chloride solutions

Abstract The effect of annealing temperature on pitting resistance of 25% Cr duplex UNS S32550 was investigated. CPT and pitting potential were determined for this alloy after annealing at 1020, 1060, 1100 and 1140 °C. The higher values of CPT and pitting potential were found after annealing at the lower temperatures. Pitting was always observed preferentially in the ferrite phase. The results can be partially explained by the changes in chemical composition of ferrite and austenite phases.

Spray deposition of steam treated and functionalized single-walled and multi-walled carbon nanotube films for supercapacitors

Steam purified, carboxylic and ester functionalized single-walled carbon nanotube (SWNT) and multi-walled carbon nanotube (MWNT) films with homogeneous distribution and flexible control of thickness and area were fabricated on polymeric and metallic substrates using a modified spray deposition technique. By employing a pre-sprayed polyelectrolyte, the adhesion of the carbon nanotube (CNT) films to the substrates was significantly enhanced by electrostatic interaction. Carboxylic and ester functionalization improved electrochemical performance when immersed in 0.1 M H(2)SO(4) and the specific capacitance reached 155 and 77 F g(-1) for carboxylic functionalized SWNT and MWNT films respectively. Compared with existing techniques such as hot pressing, vacuum filtration and dip coating, the ambient pressure spray deposition technique is suggested as particularly well suited for preparing CNT films at large scale for applications including providing electrodes for electrochemical supercapacitors and paper batteries.

A galvanostatic pulse technique for investigation of steel corrosion in concrete

Abstract A galvanostatic pulse technique was used to study corrosion of steel in mortar immersed in simulated sea water and steel exposed to 0.3 M NaOH + 0.6 M NaCl, both as aqueous solution and set with agar gel. Transients were analysed to provide impedance information. Diffusion impedance was identified for corroding steel embedded in mortar and in gel but not in solution. Three time constants were evident for steel embedded in mortar while only two time constants were identified for corroding steel in alkaline chloride gel and solution. The extra time constant for steel cast in mortar appears to be associated with the dielectric characteristics of the mortar.

Metastable pitting in 25 Cr duplex stainless steel

Abstract Evidence of metastable pitting in conventional austenitic stainless steels at room temperature is now common. Because of the small currents associated with individual events, most studies have been carried out on micro electrodes (typically 50 mm diameter). Here in contrast, with high chromium duplex stainless steels (DSS), metastable pitting is clearly observed on larger electrodes in tests at elevated temperatures in neutral NaCl solutions. Below the Critical Pitting Temperature (CPT), large metastable current peaks are observed across a range of low potentials, with less activity at higher potentials. Above the CPT, stable pits initiate at the same low potentials where metastability occurs below the CPT, usually preceded by one or two metastable events. There are evident differences between the growth of metastable pits—and the transition from metastable to stable pitting—in high-chromium DSS from that in single phase austenitic steels. A proposed model to explain the metastability in these highly alloyed duplex steels is described.

Quantitative EELS analysis of zirconium alloy metal/oxide interfaces.

Zirconium alloys have been long used for fuel cladding and other structural components in water-cooled nuclear reactors, but waterside corrosion is a primary limitation on both high fuel burn-up and extended fuel cycle operation. Understanding the processes that occur at the metal/oxide interface is crucial for a full mechanistic description of the oxidation process. In this paper we show that reliable quantification of the oxygen content at the metal/oxide interface can be obtained by Electron Energy Loss Spectrometry (EELS) if enough care is taken over both the preparation of Transmission Electron Microscopy (TEM) samples and the methodology for quantification of the EELS data. We have reviewed the accuracy of theoretically calculated inelastic partial scattering cross-sections and effective inelastic mean-free-paths for oxygen and zirconium in oxidized Zr-alloy samples. After careful recalibration against a ZrO₂ powder standard, systematic differences in the local oxygen profile across the interface in different zirconium alloys were found. The presence of a sub-stoichiometric oxide layer (a suboxide) was detected under conditions of slow oxide growth but not where growth was more rapid. This difference could arise from the different corrosion resistances of the alloys or, more likely, as a result of the transition in oxidation behaviour, which refers to a sharp increase in the oxidation rate when the oxide is a few microns thick.

Studies regarding corrosion mechanisms in zirconium alloys

Understanding the key corrosion mechanisms in a light water reactor primary water environment is critical to developing and exploiting improved zirconium alloy fuel cladding. In this paper, we report recent research highlights from a new collaborative research programme involving 3 U.K. universities and 5 partners from the nuclear industry. A major part of our strategy is to use the most advanced analytical tools to characterise the oxide and metal/oxide interface microstructure, residual stresses, as well as the transport properties of the oxide. These techniques include three-dimensional atom probe (3DAP), advanced transmission electron microscopy (TEM), synchrotron X-ray diffraction, Raman spectroscopy, and in situ electro-impedance spectroscopy. Synchrotron X-ray studies have enabled the characterisation of stresses, tetragonal phase fraction, and texture in the oxide as well as the stresses in the metal substrate. It was found that in the thick oxide (here, Optimized-ZIRLO, a trademark of the Westinghouse Electric Company, tested at 415°C in steam) a significant stress profile can be observed, which cannot be explained by metal substrate creep alone but that local delamination of the oxide layers due to crack formation must also play an important role. It was also found that the oxide stresses in the monoclinic and tetragonal phases grown on Zircaloy-4 (autoclave testing at 360°C) first relax during the pre-transition stage. Just before transition, the compressive stress in the monoclinic phase suddenly rises, which is interpreted as indirect evidence of significant tetragonal to monoclinic phase transformation taking place at this stage. TEM studies of pre- and post-transition oxides grown on ZIRLO, a trademark of the Westinghouse Electric Company, have used Fresnel contrast imaging to identify nano-sized pores along the columnar grain boundaries that form a network interconnected once the material goes through transition. The development of porosity during transition was further confirmed by in situ electrochemical impedance spectroscopy (EIS) studies. 3DAP analysis was used to identify a ZrO sub-oxide layer at the metal/oxide interface and to establish its three-dimensional morphology. It was possible to demonstrate that this sub-oxide structure develops with time and changes dramatically around transition. This observation was further confirmed by in situ EIS studies, which also suggest thinning of the sub-oxide/barrier layer around transition. Finally, 3DAP analysis was used to characterise segregation of alloying elements near the metal/oxide interface and to establish that the corroding metal near the interface (in this case ZIRLO) after 100 days at 360°C displays a substantially different chemistry and microstructure compared to the base alloy with Fe segregating to the Zr/ZrO interface.

Cathodic disbonding of chlorinated rubber coatings from steel

Cathodic disbonding of chlorinated rubber coatings from steel has been examined under free corrosion conditions. Two coatings, a clear lacquer and a primer containing iron oxide pigment, on steel panels were exposed to 0.5 M NH4Cl, LiCl, KCI, CsCl or SrCl2 solutions. In some tests a double-cell was used in which the defect and the surrounding coated areas could be exposed to different solutions. There were pronounced differences between the pigmented and unpigmented coatings. With alkali metal chlorides the rate of disbonding of unpigmented lacquer showed a strong dependence on cation mobility, but with the pigmented coating it did not. It is proposed that the controlling process changes from ion transport along the interface to oxygen diffusion. Without cathodic polarisation the disbonding eventually ceases.

The active—passive transition in low alloy steels in carbonate solutions

Abstract A rotating ring—disc electrode has been used to study film growth on low alloy steels in carbonate—bicarbonate solution. Soluble Fe(II) ions have been detected during film growth initiated by potential steps to active, pre-passive and passive potential regions. Film growth in the pre-passive region has two distinct stages. Initial passivation during Stage 1 is followed by reactivation and extensive dissolution during Stage 2. Stage 2 film growth may be attributed to either the transformation of metastable passive film into a poorly protective film or the local break-down of the passive film at bainite laths or prior austenite grain boundaries.

The effect of low-frequency cyclic stresses on the initiation of stress corrosion cracks in X60 line pipe steel in carbonate solutions

Abstract The initiation and early stages of growth ( μ m) of stress corrosion cracks in X65 line pipe steel have been studied with superimposed low frequency cyclic stresses. The effects of frequency, cyclic amplitude and the level of background tensile stress have been examined.

Effect of Copper on Active Dissolution and Pitting Corrosion of 25% Cr Duplex Stainless Steels

Abstract Electrochemical studies were made of the influence of Cu on pitting behavior and active dissolution of 25% Cr duplex stainless steels (DSS) in chloride-containing neutral and acid solutions. Alloys with increased Cu content showed higher pitting potentials (Ep) in 1 M hydrochloric acid (HCl) and 3.5% sodium chloride (NaCl) solutions. In HCl, the current density in the active state also was lower for the Cu-containing alloys. However, the critical pitting temperature (CPT) in ferric chloride (FeCl3) was not improved significantly by addition of Cu. Pitting in all environments took place preferentially in the ferrite phase.