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Papers by Robert Akid

Metallurgical and Materials Transactions, Feb 1, 2018

The sensitivity to hydrogen embrittlement (HE) has been studied in respect to precipitation size ... more The sensitivity to hydrogen embrittlement (HE) has been studied in respect to precipitation size distributions in two nickel base superalloys, Alloy 718 (UNS N07718) and Alloy 945X (UNS N09946). Quantitative microstructure analysis was carried out by the combination of scanning and transmission electron microscopy and energy dispersive x-ray spectroscopy reduced sensitivity to HE compared to Alloy 718 when considering high strength conditions despite the absence of intergranular d phase.

Fatigue & Fracture of Engineering Materials & Structures, Jun 7, 2022

The objective of this work was to determine the parameters that affect the mass transport and the... more The objective of this work was to determine the parameters that affect the mass transport and the distribution of species in microcapillaries close to the specimen surface. Local experiments were carried out under static and flow conditions on type 316L stainless steel in 1.7 M NaCl, pH = 3, by means of the electrochemical microcell and the scanning droplet cell technique. The polarisation behaviour of pure iron (used as a model system) in an aqueous environment was calculated adopting a finite element approach and was compared to the experimental results. The corrosion system consists of three parallel electrochemical reactions: the oxygen reduction reaction (ORR), the hydrogen evolution reaction (HER) and iron dissolution.

Austenitic heat-resistant stainless steel Sanicro 25, often used in energy production installatio... more Austenitic heat-resistant stainless steel Sanicro 25, often used in energy production installations, was cyclically strained at room temperature, and initiation and growth of short fatigue cracks have been studied. Cylindrical specimens with pre-existing shallow notch were cyclically strained under constant total strain amplitudes, while the surface relief, crack initiation, and short crack growth were observed, in-situ, using optical and electron microscopes. Persistent slip markings were found to be the principal locations where natural fatigue cracks initiated. The character of the crack paths of short cracks was studied using SEM and EBSD techniques. The results exhibit prevailing transgranular crack growth, ie, along the persistent slip markings within the grains. The growth of the largest cracks in the central part of the shallow notch was measured for different applied strain amplitudes. Because the coalescence of initiated cracks plays a significant role during crack growth, the use of the "equivalent crack" concept gives adequate results in the description of the crack growth rate. The crack growth law of short cracks was established and compared with the Manson-Coffin law of the material.

ECS transactions, Dec 17, 2019

In marine environments corrosion is exacerbated by formation of destructive biofilms containing s... more In marine environments corrosion is exacerbated by formation of destructive biofilms containing sulphate-reducing bacteria, which promote corrosion by forming corrosive species, such as H2S. Corrosion-causing biofilms are often resistant to inactivation by biocides since the biofilm bacteria are protected by a matrix of exopolymeric substances (EPS). Paradoxically, a biofilm of the endospore-forming Paenibacillus polymyxa can actually inhibit corrosion. Sol gel technology and immobilized microorganisms have been combined in a unique coating that inhibits corrosion on aluminium alloys, being low-cost, effective and environmentally friendly. Viability studies show that P. polymyxa endospores can withstand solvent concentration of up to 50% and an acid concentration of up to 1.5M and germinate within a biotic sol-gel coating. Electrochemical impedance spectroscopy and linear polarization resistance tests, in addition to an extended field trial, have shown that the endospore-loaded coatings show resistance to corrosion and biofilm formation relative to coatings without added endospores.

Journal of The Chemical Society-dalton Transactions, 1985

ABSTRACT A series of polytungstate anions [XW12O40]n–(X = P, Si, Fe, Co, or H2; n= 3,4,5,6, and 6... more ABSTRACT A series of polytungstate anions [XW12O40]n–(X = P, Si, Fe, Co, or H2; n= 3,4,5,6, and 6 respectively), spanning a range of reduction potentials, have been studied as sensitizers for the photoreduction of water and O2. [SiW12O40]4– was the most efficient sensitizer for H2 evolution in the presence of colloidal platinum. Saturation kinetics were found with respect to the concentrations of Pt,[SiW12O40]4–, and CH3OH as predicted by a simple kinetic scheme. The maximum rate depended on competition between the natural decay of the excited polyanion and quenching by alcohol. Electron transfer from photoreduced polyanions to O2 was also investigated by flash photolysis. Rate constants depended on the reduction potential of the polyanion and increased by a factor of 3 700 on going from [PW12O40]3– to [FeW12O40]5–, in line with the Marcus equation for adiabatic electron-transfer reactions.

Theoretical and Applied Fracture Mechanics, Apr 1, 2021

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Fatigue & Fracture of Engineering Materials & Structures, Feb 1, 1996

— There is an increasing demand for an understanding of the effects of complex load history on fa... more — There is an increasing demand for an understanding of the effects of complex load history on fatigue lifetime since it is realised that constant‐amplitude S‐N endurance data may not be appropriate for the prediction of the lifetime of components subject to variable amplitude loading. However the amount of damage accumulated within a component during its lifetime is a function, not only of its' load history, but of the synergistic effects of stress and operating environment. In this respect the ability of a material to resist the damaging effects of electrochemical corrosion can play a major role in determining the ultimate lifetime of a component; the effect of corrosion being dependent upon the specific combination of material/environment, the rate of application of load and the duration of metal/environment contact. For conditions where applied loads are below the fatigue or endurance limit the corrosion processes can lead to the development of defects which, under benign conditions, would not normally form or if developed would attain a non‐propagating condition.The paper presented here includes the results of studies derived from two‐stage intermittent air/environment fatigue cycling and cumulative damage corrosion fatigue loading of a medium carbon steel in two microstructural conditions (i) normalised and (ii) quenched and tempered. From the results of these studies it is shown that a simple damage accumulation model is insufficient to satisfactorily predict ultimate lifetime and consideration should be given to processes such as multiple crack initiation, pitting, Stage I and Stage II environment‐assisted crack growth.

Journal of Raman Spectroscopy, Dec 1, 2008

We have extended our unenhanced (non-SERS) Raman spectroscopic investigations to include a study ... more We have extended our unenhanced (non-SERS) Raman spectroscopic investigations to include a study of the corrosion of an iron electrode in carbonate and phosphate buffer solutions. The measurements have been supported by electrochemical investigations (via cyclic voltammetry), enabling oxidation and reduction reactions to be systematically followed at variable applied potentials. In a carbonate buffer (pH = 9.4) the surface oxidation led to the formation of a 'green rust' (a hydrated hydroxy-carbonate), followed by the a-and b-forms of FeOOH and an underlying magnetite layer formed on the cathodic (reduction) cycle. In a phosphate buffer (pH = 7.7) the surface was passivated by hydrated phosphates of iron [identified as FePO 4 • xH 2 O and Fe 3 (PO 4 / 2 •8H 2 O]. The formation of oxides (Fe 2 O 3 and Fe 3 O 4) were inferred from voltammetry, but spectral identification was more difficult because of broad, ill-defined spectra. Despite the challenges of using unenhanced Raman spectroscopy, we believe that the effort was worthwhile, the reactions identified being more likely to be relevant to real electrochemical environments.

Fatigue & Fracture of Engineering Materials & Structures, Apr 19, 2002

This paper presents experimental data for a 0.2%C steel/artificial seawater system showing the in... more This paper presents experimental data for a 0.2%C steel/artificial seawater system showing the influence of shear loading on corrosion response, via measurements of electrochemical variables, e.g. anodic/cathodic Tafel slopes and polarization resistance. Based on the results of these tests, several corrosion fatigue tests were conducted at different stress levels under potentiostatic control. Analysis of the results shows there to be a dependence of corrosion rate on the ratio of applied/yield strain and test frequency. In addition, the corrosion current associated with corrosion fatigue (CF) damage appears to be dependent upon the crack size, which in turn shows a relationship with fatigue crack growth rate. This paper sets out to determine the influence of stress on electrochemical parameters, i.e. free corrosion potential, Ecorr , polarization resistance, Rp , anodic, τa and cathodic, τc Tafel constants. Based upon these results, it is found that a simple linear relationship between stress and corrosion damage does not exist. Furthermore, analysis of the corrosion current fluctuations during corrosion fatigue crack growth shows a minimum current coincident with the point at which a crack is growing at its slowest rate.

ASTM International eBooks, Mar 4, 2009

The initial development and growth of defects from 'engineered' surfaces, i.e. fine abrad... more The initial development and growth of defects from 'engineered' surfaces, i.e. fine abraded, polished, shot peened etc., often dominates the resulting component lifetime, particularly for materials of high strength and limited ductility. When subject to the conjoint action of stress and environment this lifetime is impaired and reductions in fatigue strength are often observed resulting from a reduction in defect development time, often termed 'initiation', and enhancement in defect growth rate. A number of factors exist which influence the rate at which defects, such as pits/cracks, develop. Included in these are; physical and chemical material surface condition, the nature of the loading mode, test frequency and electrochemical micro-climate at the metal/solution interface. Based upon corrosion experiments conducted under cyclic and static stress, using low and high strength steels and stainless steels in chloride environments, the following events; surface film breakdown, pit development and growth, pit/crack transition and environment-assisted stage I and stage II crack growth have been observed. Included in these experiments is that of the Scanning Reference Electrode a technique adapted to evaluate stress-assisted localised corrosion, a process considered to be of primary importance during the early stages of stress corrosion and corrosion fatigue cracking; particularly for actively corroding systems.

Progress in Organic Coatings, 2017

Corrosion Science, Jul 1, 2019

The corrosion resistances of a number of tempered states of two Cr-Mo low-alloy steels, the novel... more The corrosion resistances of a number of tempered states of two Cr-Mo low-alloy steels, the novel "HT10″ (martensitic) and the commercial ASTM A182 grade "F22″ (bainitic), were evaluated in CO 2-saturated brine. The tempering heat treatments soften both steels, due to a decrease in dislocation density, and the precipitation of alloy carbides, but also decrease their uniform corrosion rates. Such a decrease is considerable if the steels are tempered at higher temperature for longer time. For the novel HT10, we discuss how the tempering-induced changes to the microstructure contribute to decreasing the cathodic reaction kinetics, and thus the general corrosion rate.

Corrosion Science, Aug 1, 2010

A novel hybrid sol-gel/polyaniline coating has been developed for application onto an AZ31 magnes... more A novel hybrid sol-gel/polyaniline coating has been developed for application onto an AZ31 magnesium alloy for corrosion protection. Electrochemical Impedance Spectroscopy in 3.5 wt% NaCl and diluted Harrison's solutions, along with salt spray tests showed that the coating possesses excellent corrosion resistance. The hybrid coating was modified by doping with silica nanoparticles (for scratch resistance) and cured at a low temperature of 75°C. Whilst conventional sol-gel methods tend to limit the coating thickness values up to 10 lm, the new hybrid sol-gel/polyaniline system presented here allows thick coatings to be deposited, in this case, around 50-60 lm.

Corrosion Science, Apr 1, 2008

A rapid cure silane sol-gel coating containing encapsulated corrosion inhibitors that can be appl... more A rapid cure silane sol-gel coating containing encapsulated corrosion inhibitors that can be applied to a mild steel substrate to form a crack-free coating has been developed. The benefit of this system is that it appears to emulate the protection mechanism found with traditional chrome (VI) based systems, but without the environmental disadvantages, namely that it is non-toxic and non-carcinogenic. The high corrosion resistance performance of this coating is derived from the combination of the hydrophobic nature of the sol-gel coating and the presence of the encapsulated rare earth corrosion inhibitor which can be released at defects within the coating resulting in cerium hydroxide precipitation which hinders the reduction reaction at cathodic sites. The proposed mechanism for this protection is based upon an evaluation of the barrier properties of the coating using electrochemical impedance spectroscopy and long-term immersion/salt spray tests.

WIT Press eBooks, Dec 20, 2012

Aluminium-based alloys show good resistant to uniform (general) corrosion. However they are much ... more Aluminium-based alloys show good resistant to uniform (general) corrosion. However they are much more susceptible to various types of localised corrosion, in particular intergranular corrosion (IGC), where localised attack occurs along the grain boundaries leaving the grains themselves largely unaffected. In order to estimate the progress of such corrosion in a given sample, it is possible to generate a numerical model. While there has been much effort spent in the development of electrochemistry-based models, the use of grey and black-box models remains largely unexplored. Cellular Automata (CA) models that have recently been exploited to model the progression of uniform corrosion may be developed to address localised corrosion, more specifically IGC. A probabilistic approach is chosen because it enables the simulation of complex interactions by replacing the chemical processes by a discrete proxy.