Robert Akid

Empirical corrosion fatigue life prediction models of a high strength steel

Crack initiation and growth behaviour in plain hour-glass shaped fatigue specimens of quenched and tempered silico-manganese spring steel (BS250 A53) having a mirror image was studied under fully reversed torsional loading conditions in both the laboratory air and the aggressive (0.6 M, aerated NaCl solution) environments. A surface plastic replication technique was used alongwith optical microscopy to monitor the early stages of environment-assisted fatigue. Non-metallic inclusions were observed to play a major role in crack initiation in both the environments. Debonding at matrix/inclusion interfaces and chemical pitting at inclusion sites were major processes in the early developmental stages of air and corrosion fatigue, respectively. A significant influence of microstructure, i.e. prior austenite grain boundaries, on defect development was also noted during air and corrosion fatigue cracking. Corrosion fatigue failure appears to be a multiple stage process namely; pit development, short crack growth, and long crack growth. Corrosion fatigue crack growth rates are predicted by employing models, which incorporate elastic plastic fracture mechanics parameters to characterise the influence of microstructure. Two empirical corrosion fatigue crack growth models, including a superposition model discussing the inert air and environmental terms involved in the corrosion fatigue process, are presented. A reasonable agreement was found between experimental and calculated lifetimes

A comparison between conventional macroscopic and novel microscopic scanning electrochemical methods to evaluate galvanic corrosion

The scanning reference electrode technique (SRET) has been used to examine corrosion in a galvanic ‘Kelocouple’ system, being that of an explosively bonded joint comprising a tri-metallic sandwich of steel/aluminium/aluminium alloy. Quantitative results of local current densities in 4% sea water at room temperature have been obtained. These have been compared with results from two conventional macroscopic techniques for measuring galvanic current, namely zero resistance ammetry and polarisation curves. The SRET results were found to be at least an order of magnitude higher than the average results obtained using the macroscopic methods. Comparison between SRET current densities and corrosion (as obtained by the amount of aluminium in solution) showed good agreement. Whether attack predominated on the aluminium (nearer the steel) or the aluminium alloy (further from the steel) depended on the concentration of the sea water.

Electricity generation from wastewaters with starch as carbon source using a mediatorless microbial fuel cell.

Microbial fuel cells represent a new method for producing electricity from the oxidation of organic matter. A mediatorless microbial fuel cell was developed using Escherichia coli as the active bacterial component with synthetic wastewater of potato extract as the energy source. The two-chamber fuel cell, with a relation of volume between anode and cathode chamber of 8:1, was operated in batch mode. The response was similar to that obtained when glucose was used as the carbon source. The performance characteristics of the fuel cell were evaluated with two different anode and cathode shapes, platinised titanium strip or mesh; the highest maximum power density (502mWm(-2)) was achieved in the microbial fuel cell with mesh electrodes. In addition to electricity generation, the MFC exhibited efficient treatment of wastewater so that significant reduction of initial oxygen demand of wastewater by 61% was observed. These results demonstrate that potato starch can be used for power generation in a mediatorless microbial fuel cell with high removal efficiency of chemical oxygen demand.

Modelling short fatigue crack growth in a heat-treated low-alloy steel

Abstract Fatigue tests have been conducted both in laboratory air and using an intermittent air/corrosion fatigue sequential cycling technique. All tests were performed on smooth specimens made from a quenched and tempered steel and subjected to fully reversed torsional loading. Crack initiation and growth behaviour was recorded using a surface replication technique. Debonding at the metal matrix/ inclusion interface appears to be the main cause of the formation of short cracks. The growth of these short cracks dominates the fatigue lifetime, being significantly influenced by the material microstructure. Empirical models for short and long crack growth were developed by employing elastic plastic fracture mechanics parameters to predict the total fatigue life of a specimen. Later, these crack growth equations derived from air fatigue tests conducted at stresses above the fatigue limit were applied to the crack growth results obtained from intermittent air fatigue/corrosion fatigue tests carried out at sub-fatigue limit stress levels. A reasonable agreement was observed between predicted and experimental crack growth rates

Corrosion fatigue short crack growth behaviour in a high strength steel

Abstract The influence of an aggressive environment (0.6 M, aerated NaCl solution) on short fatigue crack initiation and growth behaviour has been studied. The study involved three major test series, namely: air fatigue, corrosion fatigue, and intermittent air fatigue/corrosion fatigue. The above tests carried out under fully reversed torsional loading conditions at a frequency of 5 Hz, showed that it was the non-metallic inclusions which took part in crack initiation resulting from debonding at metal matrix/inclusion interface and pitting of inclusions in both air and corrosove environments, respectively. Short fatigue crack growth results in these two environments obtained by using plastic replication technique, indicated a large effect of microstructure i.e. prior austenite grain boundaries. The stage/stages at which the environmental contribution was dominant has been discussed by considering the results achieved from intermittent tests. However, the mechanisms involved in corrosion fatigue short crack growth have also been described in the light of results obtained from futher investigations carried out by conducting corrosion fatigue tests under applied cathodic potential conditions and tests on hydrogen pre-charged specimens under air fatigue and uniaxial tension conditions.

Role of Nonmetallic Inclusions in Fatigue, Pitting, and Corrosion Fatigue

Abstract A study was conducted of the effect of nonmetallic inclusions on the fatigue and corrosion fatigue resistance of a high-strength steel in 0.6 M sodium chloride (NaCl) solutions. Results indicated that angular calcium-aluminate inclusions played an important role in introducing cracks during air fatigue cycling; however, sulfide inclusions appeared to be the main contributors to sites for corrosion pits and subsequent crack initiation. Conventional consideration based on the stress intensification caused by such defects was insufficient to describe the role of nonmetallic inclusions in fatigue crack development in air and by corrosion pits during corrosion fatigue. However, it was considered that the interaction between geometric discontinuities (i.e., nonmetallic inclusions and cyclic loading) resulted in plasticity localization and, thus, facilitated crack development. Similarly, enhancement of localized dissolution resulting from plasticity localization contributed to corrosion pit development ...

Anticorrosion/antifouling properties of bacterial spore-loaded sol-gel type coating for mild steel in saline marine condition: a case of thermophilic strain of Bacillus licheniformis

This work reports the performance of a sol–gel type coating encapsulated with biofilm of inoculums of protective thermophilic strain of Bacillus licheniformis endospores isolated from the Gazan hot springs-Saudi Arabia for the inhibition of marine fouling and corrosion protection of S36-grade mild steel in 3.5 wt% NaCl medium. In order to improve its anticorrosion properties, the hybrid sol–gel coating is further doped with zinc molybdate (MOLY) and zinc aluminum polyphosphate (ZAPP) pigments. Marine fouling study was conducted at the Arabia Gulf water of Half Moon Bay, Al-Khobar, Saudi Arabia for 10 weeks on the coated samples with and without the bacterial endospores. The assessment of fouling results reveals that the bacterial endospores possess antifouling potentials since it performed better compared to its abiotic counterpart within the immersion period of study due to their foul-releasing effect. Improved corrosion and fouling resistant in the presence of the bacterial endospores could be attributed to their multi-layered hydrophobic and antibiotic coating surface after bacterial encapsulation. Spores accumulation in the sol–gel coating altered the surface wetness thereby preventing the diffusion of corrosion molecules and ions through the bulk of the coating to the metal surface; this is evident in the trend of electrochemical coating resistance and capacitance. Confocal laser scanning fluorescence and scanning electron microscopies were employed to probe bacterial viability and surface micro-cracks inherent in the coating, respectively. This is the first report of axenic thermophilic strain of Bacillus licheniformis isolated from the Arabian Gulf with inhibiting potentials against corrosion and fouling of industrial steel.

Heteropolytungstates as catalysts for the photochemical reduction of oxygen and water

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.

Electrochemistry of deformed smooth surfaces and short corrosion fatigue crack growth behaviour

AbstractThe polarisation characteristics for a deformed smooth surface of a 0·2% carbon steel in an artificial sea water have been determined under static and cyclic loading conditions. The influence of strain level and loading frequency on anodic and cathodic Tafel constants, corrosion current density, and corrosion potential is described. Based on these data, the conditions for corrosion fatigue testing which corresponded to a maximum synergism between surface deformation and anodic dissolution were determined. Corrosion fatigue tests, which were conducted under both constant potential and constant current density conditions have shown that a process of metal dissolution plays a determining role in the short corrosion fatigue crack growth behaviour. An experimentally based criterion is proposed involving the development of a short corrosion fatigue crack, of characteristic size, which is associated with the spacing between the major microstructural barriers. This criterion is a function of both shear st...

Electrochemical Investigation of the Corrosion of Different Microstructural Phases of X65 Pipeline Steel under Saturated Carbon Dioxide Conditions

The aim of this research was to investigate the influence of metallurgy on the corrosion behaviour of separate weld zone (WZ) and parent plate (PP) regions of X65 pipeline steel in a solution of deionised water saturated with CO2, at two different temperatures (55 °C and 80 °C) and at initial pH~4.0. In addition, a non-electrochemical immersion experiment was also performed at 80 °C in CO2, on a sample portion of X65 pipeline containing part of a weld section, together with adjacent heat affected zones (HAZ) and parent material. Electrochemical impedance spectroscopy (EIS) was used to evaluate the corrosion behaviour of the separate weld and parent plate samples. This study seeks to understand the significance of the different microstructures within the different zones of the welded X65 pipe in CO2 environments on corrosion performance; with particular attention given to the formation of surface scales; and their composition/significance. The results obtained from grazing incidence X-ray diffraction (GIXRD) measurements suggest that, post immersion, the parent plate substrate is scale free, with only features arising from ferrite (α-Fe) and cementite (Fe3C) apparent. In contrast, at 80 °C, GIXRD from the weld zone substrate, and weld zone/heat affected zone of the non-electrochemical sample indicates the presence of siderite (FeCO3) and chukanovite (Fe2CO3(OH)2) phases. Scanning Electron Microscopy (SEM) on this surface confirmed the presence of characteristic discrete cube-shaped crystallites of siderite together with plate-like clusters of chukanovite.