Browsing by Author "Robinson, M. J."
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Item Open Access Aluminium-based Coatings for Cadmium Replacement(Cranfield University, 2008-06) Cardilli , Emanuele; Robinson, M. J.Cadmium electroplating is widely used in the aerospace industry for the corrosion protection of high strength steels. Cadmium is also used as compatible coating to reduce the galvanic corrosion generated in the assembly of components manufactured with different materials. However, environmental and safety concerns over the high toxicity of cadmium has led to the investigation of suitable replacements. Aluminium coatings are promising coatings for the replacement of electroplated cadmium. Previous studies have shown that the use of SermeTel 984, a commercial aluminium sprayed coating, is beneficial in eliminating the hydrogen direct embrittlement without increasing the risk of re-embrittlement. However, the coating has shown to be prone to passivation in the mild corrosive environment. The addition of active zinc and magnesium particles are thought to avoid the passivation of the aluminium. A range of modified SermeTel 984 coatings, containing 0.5%, 3%, 10% and 50% of zinc, and SermeTel 984 modified with the addition of 30% and 50% of Mg/Al alloy particles in weight have been evaluated as possible alternatives. Chromium free SermeTel 984 and a SermeTel 984 modified with aluminium alloy 7075 particles together with IVD aluminium coatings have also been included in the project. The sacrificial protection of the new coatings has been evaluated together with the associated risk of hydrogen re-embrittlement and compared with the electroplated cadmium. Zinc modified SermeTel 984, containing 1% of zinc and 5% of zinc electroplated aluminium, IVD aluminium and electroplated Zn-Ni coatings have been studied as alternative coatings to cadmium as compatible coatings. Slow strain rate testing has been performed to study the effect of hydrogen on the re- embrittlement of steel substrate as a result of the corrosion of the aluminium-based coatings in 3.5% NaCl. Linear polarisation testing in 3.5% NaCl has been used to evaluate the self-corrosion rates of the coatings; galvanic coupling measurements have been used in the case of steel substrate to evaluate the sacrificial properties or, in the case of bronze/aluminium assembly, to evaluate the compatibility properties of the coatings. Polarisation behaviour tests have been used to study the anodic or cathodic control of the corrosion mechanisms. Total corrosion, calculated as the addition of self-corrosion and galvanic corrosion, has been calculated to evaluate the duration of the coating in service compared to electroplated cadmium.Item Open Access The application of electrolytic in-process dressing to precision grinding processes(2007-06) Zervos, Charilaos; Stephenson, David J.; Robinson, M. J.Electrolytic In-Process Dressing (ELID) has attracted a lot of attention in recent years as it is a promising method of grinding a wide range of materials, including ones that are difficult to machine, to high quality finishes. The EC funded project Nanogrind undertook the construction of a high precision grinding machine which will incorporate an ELID system, applying the technique on a spherical wheel for the production of components with intricate geometries (e.g. freeforms, aspherics). In this project, the main objective was to understand the basic features of ELID, such as the electrochemical properties of the oxide layer and its effect on the final finish of the produced components (glass, a hard and brittle material was the main focus). Acoustic emission studies, a promising in-process monitoring tool, verified the effectiveness of ELID grinding when the contact area between the wheel and workpiece is increased. Compared to conventional resin bond wheel grinding, ELID ground components were of higher quality with less damage introduced in them. ELID current monitoring also gave excellent results when it was correlated to surface finish achieved across the surface of the testpieces ground. Electrochemical Impedance Spectroscopy was an innovative technique that was applied in order to acquire basic information about the metal bond wheel, such as the charge transfer resistance. Its correlation to the oxide layer thickness gave an indication of the way corrosion rates change with the presence or not of an oxide film on the wheel surface. Finally, sub-surface damage introduced into glass components was evaluated and was compared to the depths of damage predicted by theoretical models, giving further insight into the way the overall processing time of a component should be optimised in order to acquire high quality, damage-free finished components in a time efficient, cost effective way.Item Open Access Control of Magnesium Alloy Corrosion through the Use of Engineered Intermetallics(Cranfield University, 2014-12) Pidcock, Andrew; Robinson, M. J.; Impey, Susan A.The low density and high relative strength of Mg alloys means they can offer engineering benefits over steels or Al alloys. However, the susceptibility of Mg alloys to corrosion has limited their exploitation and restricted their use to more benign environments. An Mg-Al intermetallic surface layer is a good candidate for a robust corrosion protection method. This work demonstrates their development by using a novel ionic liquid electroplating process to deposit Al on to Mg substrates that when heat treated diffuses to form discrete intermetallic layers. Examination of three Mg-Al-Zn alloys showed that the amount Mg-Al intermetallic phases in their microstructures was linked to the quantity of Al they contained. Subsequent self-corrosion measurements using electrochemical impedance spectroscopy demonstrated that their performance was connected to the amount of intermetallic present, and in particular the strength of the micro-galvanic couples generated between the anodic and cathodic phases. Measurements of the self-corrosion behaviour of manufactured samples of the Mg-Al intermetallics confirmed that they could provide significant improvements, but it was acknowledged that their noble nature compared to an Mg substrate would encourage galvanic corrosion if a surface layer was damaged. As such the galvanic activity of the Mg-Al-Zn alloys and Mg-Al intermetallics was compared against a pure Mg standard using zero resistance ammetry and the resistance box technique. Galvanic models of alloy self-corrosion and a damaged intermetallic surface layer were also used to assess the potential problem. These measurements demonstrated that the intermetallics could act as strong cathodes, but further discussion on the nature of the behaviour suggested means by which galvanic corrosion might self-limit or self-repair. The galvanic corrosion experiments also revealed how the combination of current flow and a solution saturated with Mg2+ ions could lead to the formation of a highly protective Mg(OH)2 film with promising characteristics.Item Open Access Crevice corrosion of austenitic and duplex stainless steels in seawater(Cranfield University, 1994-01) Masters, G. L.; Robinson, M. J.Duplex stainless steels are finding increasing use in seawater applications, but are prone to attack by crevice corrosion. A mechanism of deaeration and acidification is thought to produce locally aggressive conditions within the crevice. A variety of experimental techniques were therefore used to investigate the crevice corrosion resistance of some commercial duplex stainless steels in seawater and other, more aggressive Cr-containing environments. The established marine grade 316L austenitic stainless steel was used for reference purposes. The investigations provided both quantitative data regarding alloy performance, and qualitative observations regarding the attack mechanism. The crevice corrosion resistance of the duplex stainless steels was shown to be superior to that of 316L, and to increase with (Cr + N) content. Electrochemical polarisation scans demonstrated the importance of dissolved 02 in the seawater in maintaining a protective passive film on these alloys. Active peak current density, a measure of the severity of attack, was shown to increase with decreasing pH. In a simulated crevice solution, the duplex alloys were shown to exhibit both a narrower active range and lower peak current density than 316L. Potentiostatic tests showed weight loss to be an order of magnitude lower for the duplex alloys. Measured weight losses were in agreement with those predicted from polarisation data. The technique suggested that cathodic protection potentials as noble as -600 mV (SCE) may be sufficient to protect duplex stainless steels in seawater. Metallographic observation and electron beam analysis showed that the attack mode is potential dependent, with ferrite and austenite preferentially attacked at more active and noble potentials respectively. Real time tests showed that the internal crevice pH of austenitic stainless steel could fall to as low as 1.3. The rise in corrosion current was found to be a reliable indicator of attack, and was consistent with measured weight loss. Potential shifts were found to be relatively insensitive indicators of breakdown.Item Open Access The effect of marine fouling on the corrosion of steel in the marine environment(1990-04) Maitland, Julia R.; Robinson, M. J.An investigation was made into the corrosion of steel immersed in seawater by electrochemical (LPR scans) and weight loss techniques. Steel samples in the form of studs or a specially designed corrosion probe were immersed in a re-circulating laboratory based flow loop inoculated with marine microorganisms. A paralled biological study into biofilm development on corroding steel was made using viable counts, scanning electron microscopy and radio-respirometric assays. The development of a biofilm on steel studs cathodically protected at -850 mV S.C.E. and -950 mV S.C.E. was studied using these three indices of biological activity. The corrosion of steel in the real marine environment and in a bacteria-free system were monitored. Results indicate that steel corrosion followed similar trends in all these investigations. Corrosion rates were initially very high, decreased, stabilised and rose slightly. Attempts to correlate results from weight losses and LPR scans were made. Biofilms on the steel surface were very similar, and not influenced by the use or level of cathodic protection nor period of exposure. Viable counts indicate higher levels of Sulphate Reducing Bacteria were present on corroding than on cathodically protected steel. The development of a biofilm could not be correlated with trends in corrosion rates.Item Open Access The Effect of Nickel Plating on Hydrogen Embrittlement of High Strength Steel.(Cranfield University, 1992-07) Evans, Juliet M.; Robinson, M. J.The microstructure of high strength steel is susceptible to delayed failure caused by the absorption of hydrogen produced either during cathodic charging or electroplating. When 0.8%C and AISI 4340 steel are subjected to constant load testing, a wide range of failure times is observed. By applying Weibull statistics small changes in experimental parameters such as heat-treatment are detected readily and are explained in the terms of hydrogen trapping at microstructural defects. During the electroplating of steel in a double-cell, quantitative measurements are made of the amount of hydrogen permeated. Current densities are measured in the range 2- 40 mAcm2 and it is shown that, although the lowest current density produces the most mechanically sound plate, it also causes the largest amount of hydrogen absorption. The nickel deposit is found to act as a reservoir for reversibly trapped hydrogen allowing diffusion to continue into the steel after the cessation of plating. Permeation measurements were taken on AISI 4340 steel using an electrochemical probe developed from the Barnacle Electrode. The effects of cathodically charging and electroplating with nickel are compared. Exposure of the steel to the atmosphere is shown to have an important influence on the hydrogen content after a period of time due to a limited occurrence of corrosion. Various post- plating treatments are commonly used to remove a damaging concentration of hydrogen and the quantitative effects of such treatments are described. Finally, a mathematical model is proposed which explains the reason for the wide spread of delayed failure times. It is found that if the stress intensity necessary to initiate a crack is known and, provided either the nominal stress or the crack size is known, it is possible to calculate either the allowable defect size or the allowable stress below which cracking is not expected to occur.Item Open Access The effect of sulphate reducing bacteria on the hydrogen absorption of cathodically protected high strength low alloy steel(Cranfield University, 1994-03) Kilgallon, P. J.; Robinson, M. J.The hydrogen embrittlement of two HSLA steels was studied in conditions typical of the marine environment. Double cantilever beam specimens, heat treated to produce the microstructure in the heat affected zone of a weld, were tested in seawater containing sulphate reducing bacteria (SRB) over a range of cathodic protection (CP) potentials and the threshold stress intensities ([Threshold Stress Intensity]) were recorded. The hydrogen concentration absorbed by the steel ([Surface Hydrogen Concentration]) was measured and shown to be higher at more negative CP potentials and significantly increased when SRB were present. An inverse relationship was established between log [Threshold Stress Intensity] and ([Surface Hydrogen Concentration]. It was concluded that crack propagation occurs by a single mechanism whether or not SRB are present. Three point bend specimens of both steels were machined from welded plate. Corrosion fatigue tests were carried out in seawater with and without SRB. The presence of active SRB caused increased crack growth rates. Sediment samples were collected from the River Mersey and the base of a North Sea platform. In addition, SRB were added as an inoculum to artificial seawater. SRB numbers were enumerated and their activities assessed by measuring the concentrations of sulphide generated. Hydrogen permeation tests were performed on steel held at a range of CP potentials and exposed to each environment. Measurements were also carried out in seawater containing chemically prepared sulphides. Hydrogen absorption was shown to be enhanced when SRB were present and to be related to the total sulphide (TS) concentration in the environment. High hydrogen concentrations were produced by chemically prepared sulphides and the nature and thickness of the sulphide film appeared to be important in determining the extent of hydrogen absorption. Chemically produced sulphide gave sustained levels of absorbed hydrogen, but those generated biogenically decayed rapidly unless the TS concentration was maintained in the solution.Item Open Access The effect of Zinc-Cobalt electroplating on the hydrogen embrittlement of high strength steel(Cranfield University, 2001-07) Hillier, Elizabeth M. K.; Robinson, M. J.Electroplated cadmium coatings are widely used in the aerospace industry to provide corrosion protection for components such as fasteners, which are manufactured from high strength steels. Due to environmental and safety concerns with regard to the use of cadmium, there is a requirement to investigate suitable replacements. Any alternate coating, in addition to effective corrosion protection, must not have deleterious effects on the steel substrate. Hydrogen is known to be absorbed during electroplating and there are concerns that this could cause hydrogen embrittlement of high strength steel fasteners. This study has investigated the effect of zinc-cobalt electroplating on the embrittlement of high strength steels used for aerospace applications, due to promising corrosion properties. i Permeation measurements have been used to measure the hydrogen uptake during electroplating, and slow strain rate testing performed to establish the effect of hydrogen on the embrittlement of the steel substrate. The influences of bath composition, temperature and pH have been studied, plus the addition of other iron group elements to the plating bath to minimise the risk of embrittlement. Hydrogen that has passed into the metal substrate has been located by a decorative mapping technique that also showed the high concentration present in the electroplated coatings. Corrosion tests have been executed to assess the performance of the electroplated coatings that have been tested. The factors that affect the ernbrittlement of zinc-cobalt electroplated high strength steels and the methods, including modulated multi-layer coatings and baking, that could be used for its control are described. 10Item Open Access An electrochemical investigation of erosion corrosion of duplex stainless steel in sea water containing sand particles(2001) Hussain, Essam A. M.; Robinson, M. J.Passive metals, like stainless steels, are usually protected from corrosion by a thin oxide film on the surface. If the film is stable it will reform spontaneously when it becomes damaged. Erosion-corrosion of passive metals proceeds by the repeated removal and repair of the oxide film. Erosion corrosion behaviour studies were carried out on UNS 31803 grade of duplex stainless steel in seawater using a water jet impingement apparatus. Electrochemical polarisation scans and optical film thickness measurements were utilised to measure the rate of repair of the passive film while varying the fluid flow velocity in seawater with and without the addition of sand particles. The value of the passive current density revealed the severity of the attack, as the fluid velocity was changed on the surface of the material. This demonstrated the pitting of the ferrite phase at 380 mV and breakdown of the austenitic phase at 880 mV. With the presence of sand particles, the passive current density was significantly enhanced and increased up to 7.5 jiA/cm2 for 3 grams and the value stayed constant as the sand concentration increased to 4 grams. Both the current and potential versus time measurements supported the polarisation results. In general, the passive film produced an interface colour of the first or second order. Modelling of the erosion-corrosion behaviour under particle impingement was characterised by developing a relationship between passive film colour thickness, sand particle energy from the water jet impingement system and the electrochemical scans. It was possible to show that the mean charge passed for a single particle impact of 0.01 impact was 6 x.10*12 coulombs and the corresponding absolute current was 6x 10'12 Ampere. Overall, the conclusion drawn from this work was that the UNS31803 duplex stainless steel would exhibit a good resistance to erosion corrosion if used in ship power plants and offshore piping giving a depth of attack between 0.07 and 0.08 mm/year.Item Open Access Evaluation of aluminium–based coatings for cadmium replacement(Cranfield University, 2003-12) Chalaftris, George; Robinson, M. J.Cadmium electroplating is widely used in the aerospace industry for the corrosion protection of aircraft components like fasteners manufactured from non corrosion resisting high strength steels. However, environmental and safety concerns over the high toxicity of cadmium and its compounds have led to the investigation of suitable replacements. Alternatives to cadmium should offer effective corrosion protection and have no detrimental effects on the steel substrate. Hydrogen is known to be absorbed during processes like electroplating, thus causing hydrogen embrittlement on high strength steel fasteners. Another source of hydrogen is the water reduction occuring in parallel with the corrosion of the coating, and thus causing re–embrittlement of the steel substrate. This study has investigated the effect of the application of two aluminium–based coatings, SermeTel CR984–LT and Galvano–Aluminium Alcotec, on the hydrogen re–embrittlement of high strength steels used for aerospace applications. Slow strain rate testing has been performed to study the effect of hydrogen on the re–embrittlement of the steel substrate as a result of the corrosion of the aluminium–based coatings in a variety of environments, such as NaCl solution, salt spray and marine atmosphere. Permeation measurements have been used to measure hydrogen uptake by unplated steel membranes potentiostatically charged at the potentials of the aluminium–based coatings, so as to simulate the amount of absorbed hydrogen due to their corrosion. Corrosion tests have been utilised to evaluate the performance of SermeTel CR984–LT and Galvano–Aluminium Alcotec.Item Open Access Factors Affecting The Exfoliation Corrosion Of Aluminium Alloys(Cranfield University, 1996-12) Jackson, N; Robinson, M. J.Aluminium alloys suffer from localised attack termed exfoliation corrosion which is intergranular in nature where the attack proceeds around the elongated grains present in the microstructure. The formation of a more voluminous corrosion product than the original material results in the lifting of grains giving the characteristic of exfoliation corrosion - lamellar attack. This results in surface degredation in the form of pitting, flaking or blistering. The precipitates present within the microstructure and the grain shape can alter how susceptible the material is to exfoliation corrosion. The distribution of precipitates present can be altered by heat treatment which affects the exfoliation corrosion susceptibility. A study on the effects of heat treatment and grain shape on exfoliation corrosion has been completed for two AlCuMg alloys (2014 and 2024) and two aluminium - lithium alloys (8090 and 2091). A MASTMAASIS salt - spray cabinet was used to determine the exfoliation corrosion susceptibility of all the alloys. A four - point bend test was developed to determine the depth of penetration and compared to depths measured metallographically. The results for the 2XXX series alloys suggest that as the grain aspect ratio increases the exfoliation susceptibility increases. Although the aspect ratio did not differ greatly between each alloy, the different phases present in the two alloys seem to contribute more markedly to the susceptibility. Heat treatment in the 2024 alloy suggested that the peak - aged condition gave a higher susceptibility to corrosion than the under - aged and over - aged conditions. Results indicated that the heat treatment has a greater effect on exfoliation corrosion than the grain aspect ratio, showing that the grain boundary microstructure is an important factor in the exfoliation process. The four - point bend test tended to underestimate the depth of attack by virtue of measuring the whole area of the specimen whereas metallographic measurements only took into account the attacked areas. End grain testing suggested that the attack can not only proceed longitudinally but also "short - circuit" down the transverse path of the elongated grains in the 2XXX series alloys. For the aluminium - lithium alloys the over - aged condition showed more resistance to attack than the under - aged and peak - aged conditions but the under - aged was more resistant to attack for the 2091 alloy. Stress corrosion cracking showed trends similar to exfoliation corrosion.Item Open Access Flexible liners for corrosion protection of pipelines(Cranfield University, 2012-02) Allison, Crispin; Robinson, M. J.Flexible plastic liners are sometimes installed into new and existing oil and gas pipelines to prevent corrosion of the pipe wall. A practical difficulty of this method is that the plastic liners are permeable to gases, which can collect and form an annular space between the liner and the pipe. If the operating pressure in the pipe decreases then the collected gas can cause the liner to collapse and block the pipe. One method for overcoming this problem is to insert vents at intervals along the liner to allow the gas to escape into the pipe during depressurisation. However, there is concern that this arrangement might lead to excessive corrosion beneath the vent where the pipe wall is exposed. The rate of corrosion is expected to be controlled by the vent size but this principle needs to be confirmed by experiment. The work described in this thesis is aimed at investigating this corrosion by experiment for a range of conditions typical of oil and gas production. A novel crevice corrosion cell was designed, consisting of an X100 carbon steel plate and a sheet of transparent Perspex, separated by a thin gasket. A small hole in the Perspex simulated a liner vent and allowed carbon dioxide to reach the steel surface. Tests were carried out in 3.5% NaCl solutions saturated with carbon dioxide at 1 bar partial pressure. Corrosion rates along the length of the annular space were measured using the Linear Polarisation Resistance (LPR) technique on pairs of insulated X100 electrodes set into the plate. The corrosion rates within the annular space have been shown to be small compared to those in the bulk solution and to diminish rapidly with distance from the vent. Mathematical modelling, based on the transport of carbon dioxide, is described to explain these findings and support the experimental work. The effectiveness of the LinerVentTM, installed over the vent, in a turbulence pipeline was demonstrated. The benefit of applying cathodic protection within the annular space was also demonstrated. The results are discussed in terms of the fundamental corrosion principles and their practical implicationsItem Open Access Flow accelerated preferential weld corrosion of X65 steel in brine(Cranfield University, 2014-04) Adegbite, Michael Adedokun; Robinson, M. J.; Impey, Susan A.Preferential weld corrosion (PWC) remains a major operational challenge that jeopardizes the integrity of oil and gas production facilities. It is the selective dissolution of metal associated with welds, such that the weld metal (WM) and / or the adjacent heat-affected zone (HAZ) corrode rather than the parent metal (PM). Corrosion inhibition is conventionally used to mitigate this problem however several indications suggest that some corrosion inhibitors may increase PWC. Furthermore, it is not possible to detect systems that are susceptible to PWC and or to understand the apparent ineffectiveness of some corrosion inhibitors at high flow rates. Consequently, the aim of this research is to assess the suitability of submerged jet impingement method to study flow accelerated preferential weld corrosion, which is critical to safe and economic operations of offshore oil and gas facilities. In this research, a submerged jet-impingement flow loop was used to investigate corrosion control of X65 steel weldment in flowing brine, saturated with carbon dioxide at 1 bar, and containing a typical oilfield corrosion inhibitor. A novel jet-impingement target was constructed from samples of parent material, heat affected zone and weld metal, and subjected to flowing brine at velocities up to 10 ms- 1 , to give a range of hydrodynamic conditions from stagnation to high turbulence. The galvanic currents between the electrodes in each hydrodynamic zone were recorded using zero-resistance ammeters and their self-corrosion rates were measured using the linear polarisation technique. At low flow rates, the galvanic currents were small and in some cases the weld metal and heat affected zone were partially protected by the sacrificial corrosion of the parent material. However, at higher flow rates the galvanic currents increased but some current reversals were observed, leading to accelerated corrosion of the weld region. The most severe corrosion occurred when oxygen was deliberately admitted into the flow loop to simulate typical oilfield conditions. The results are explained in terms of the selective removal of the inhibitor film from different regions of the weldment at high flow rates and the corrosion mechanism in the presence of oxygen is discussed.Item Open Access Hydrogen concentration measurements using a gel-filled electrochemical probe(Cranfield University, 1993-12) Allcock, Bryan W.; Robinson, M. J.;A novel gel-filled electrochemical hydrogen probe was developed and used to measure hydrogen concentrations in carbon-manganese steels. The results were compared with those from an electrochemical permeation technique and a volumetric method. The probe was used to determine the distribution of hydrogen in 5mm steel plates cathodically charged on one side to represent the wall of a pipe or pressure vessel used in hydrogen service. The concentration measurements obtained by the three techniques were in good agreement with each other and with those predicted from diffusion equations and this permitted the precise boundary conditions on the charged metal surface to be determined. Surface reaction kinetics were investigated to model the hydrogen distribution and these were solved using solutions to Fick's diffusion equations. After long charging times the hydrogen concentration on the efflux surface of the plate approached that on the influx side, indicating that an almost uniform hydrogen distribution had been established. Rather than rapid loss of hydrogen from the free surface, as had been assumed previously, it was clear that there was a large resistance to hydrogen transport across the metal/air interface. Microstructural damage was examined both optically and using the scanning electron microscope. Separate investigations were carried out to help understand the effect that reversible and irreversible trapping had on the diffusion of hydrogen through the steel.Item Open Access Hydrogen embrittlement of high strength steel electroplated with zincâ  cobalt allo(Elsevier Science B.V., Amsterdam., 2004-03-03T00:00:00Z) Hillier, Elizabeth M. K.; Robinson, M. J.Slow strain rate tests were performed on quenched and tempered AISI 4340 steel to measure the extent of hydrogen embrittlement caused by electroplating with zincâ  cobalt alloys. The effects of bath composition and pH were studied and compared with results for electrodeposited cadmium and zincâ  10%nickel. It was found that zincâ  1%cobalt alloy coatings caused serious hydrogen embrittlement (EI 0.63); almost as severe as that of cadmium (EI 0.78). Baking cadmium plated steel for 24 h at 200 à °C gave full recovery of mechanical properties but specimens plated with zincâ  1%cobalt and then baked still failed in 89% of the time of unplated controls. It was shown that hydrogen uptake and embrittlement could be controlled by depositing thin layers of cobalt or nickel at the steel/ coating interface. For example, the least embrittlement was caused by zincâ  10%nickel (EI 0.037) due to a nickel rich layer with very low hydrogen diffusion coefficient that formed during the initial stages of electroplating. Similarly, a 0.5 m nickel layer was effective in lowering the embrittlement caused by zincâ  1%cobalt to that of zincâ  10%nickel. Furthermore, a 0.5 m cobalt layer deposited before a zincâ  1%cobalt coating gave virtually 100% recovery of mechanical propertItem Open Access Hydrogen Embrittlement Susceptibility of Super Duplex Stainless Steels(Cranfield University, 2010-04) Alsarraf, Jalal; Robinson, M. J.This thesis describes the metallurgical and environmental factors that influence hydrogen embrittlement of super duplex stainless steels and presents a model to predict the rate at which embrittlement occurs. Super duplex stainless steel has an austenite and ferrite microstructure with an average fraction of each phase of approximately 50%. An investigation was carried out on the metallurgical and environmental factors that influence hydrogen embrittlement of super duplex stainless steels. Tensile specimens of super duplex stainless steel were pre-charged with hydrogen for two weeks in 3.5% NaCl solution at 50º C at a range of applied potentials to simulate the conditions that exist when subsea oilfield components are cathodically protected in seawater. The pre-charged specimens were then tested in a slow strain rate tensile test and their susceptibility to hydrogen embrittlement was assessed by the failure time, reduction in cross-sectional area and examination of the fracture surface. The ferrite and austenite in the duplex microstructures were identified by analysing their Cr, Ni, Mo and N contents in an electron microscope, as these elements partition in different concentrations in the two phases. It was shown that hydrogen embrittlement occurred in the ferrite phase, whereas the austenite failed in a ductile manner. An embrittled region existed around the circumference of each fracture surface and the depth of this embrittlement depended on the hydrogen charging time and the potential at which the charging had been carried out. The depth of embrittlement was shown to correlate with the rate of hydrogen diffusion in the alloy, which was measured electrochemically using hydrogen permeation and galvanostatic methods. A two-dimensional diffusion model was used to calculate the hydrogen distribution profiles for each experimental condition and the model could be employed to provide predictions of expected failure times in stressed engineering components.Item Open Access Hydrogen re-embrittlement susceptibility of ultra high strength steels(Cranfield University, 2005-09) Figueroa-Gordon, Douglas J.; Robinson, M. J.300M ultra high strength steel has been widely used for over forty years as a structural material in aerospace applications where a high strength is required. These parts are generally protected from corrosion by electroplated cadmium sacrificial coatings. However, there are concerns over this coating material due to its high toxicity and alternative coatings including Zinc-14%Nickel and SermeTel®1140/962 have been considered. It is known that applying electrodeposited coatings causes atomic hydrogen to be absorbed by the steel substrate producing delayed failure by direct hydrogen embrittlement. Hydrogen is also absorbed when a sacrificial coating undergoes corrosion in service and this process is known as re-embrittlement. The effect of electroplated Zinc-14%Nickel and aluminium based SermeTel®1140/962 sacrificial coatings in causing hydrogen embrittlement and re-embrittlement of 300M steel have been compared to that of conventional electroplated cadmium. AerMet®100 ultra high strength steel has been also considered as alternative replacement for the conventional 300M. Hence, the hydrogen embrittlement and re-embrittlement susceptibilities of AerMet®100 were studied when coated with cadmium, Zinc-14%Nickel and SermeTel®1140/962. In addition, two alternative alloys GifloM2000 and CSS-42LTM were also taken into consideration and only the extent of hydrogen re-embrittlement was assessed when coated with cadmium and SermeTel®1140/962, respectively. Slow strain rate tests, SSRT, were carried out for plated, plated and baked as well as plated, baked and corroded tensile specimens. The time to failure values were compared using a Weibull distribution, statistical ttests and embrittlement indices. Differences in hydrogen susceptibility of the high strength steels considered might depend upon their intrinsic hydrogen transport characteristics. These properties were studied and compared in terms of hydrogen diffusivity and solubility.Item Open Access Hydrogen transport and embrittlement in 300 M and AerMet100 ultra high strength steels(Elsevier Science B.V., Amsterdam., 2010-05-01T00:00:00Z) Figueroa-Gordon, Douglas J.; Robinson, M. J.This paper describes how hydrogen transport affects the severity of hydrogen embrittlement in 300 M and AerMet100 ultra high strength steels. Slow strain rate tests were carried out on specimens coated with electrodeposited cadmium and aluminium-based SermeTel 1140/962. Hydrogen diffusivities were measured using two-cell permeation and galvanostatic charging methods and values of 8.0 × 10−8 and 1.0 × 10−9 cm2 s−1 were obtained for 300 M and AerMet100, respectively. A two-dimensional diffusion model was used to predict the hydrogen distributions in the SSR specimens at the time of failure. The superior embrittlement resistance of AerMet100 was attributed to reverted austenite forming around martensite laths during teItem Open Access Influence of corrosion damage on the initiation of fatigue cracks in high strength stainless steels(Cranfield University, 2011-04) Rezig, E.; Irving, Phil E.; Robinson, M. J.This research project investigates the influence of corrosion flaws on the initiation of fatigue cracks in high strength stainless steels, and more particularly in 15-5 precipitation hardening high strength stainless steel. Susceptibility of 15-5PH to localised corrosion was examined and pit-like corrosion flaws produced by pitting and crevice corrosion were introduced in fatigue specimens in order to measure the influence of these surface flaws on fatigue crack initiation. From the results and observations made during the experiments, models of crevice corrosion propagation and initial stage of fatigue were developed. Experimental testing revealed that 15-5 precipitation hardening stainless steel is more prone to crevice corrosion than pitting, and that crevice corrosion is thought to be the most likely cause of any pit-like flaws in this material. The first results of the modelling of the propagation of crevice corrosion in 15-5PH stainless steel showed that the initial growth across the metal surface was proportional t3/ 8 . It follows that the depth growth rate in that initial stage was proportional t5/ 8 . All pre-corroded fatigue specimens failed from fatigue cracks which initiated from crevice corrosion flaws during fatigue testing. After failure the shape and size of corrosion flaws where the cracks initiated were measured and their largest Kt values determined by finite element analysis. No general pattern linking total fatigue life and stress concentration factor value was found. However, it was shown that stress concentration factor Kt has an influence on the initiation and early crack growth behaviour, but has no effect on the life of longer cracks. In addition, it appeared that not considering the small excursions found at the bottom of corrosion flaws in the assessment of the stress concentration factor Kt underestimates the values of Kt.Item Open Access The influence of sulphate-reducing bacteria on hydrogen absorption by steel during microbial corrosion(1990-05) Parker, C. H. J.; Robinson, M. J.; Seal, K. J.During the anaerobic corrosion of steel, hydrogen sulphide (H2S) evolution by the Sulphate-Reducing Bacteria (SRB) promotes hydrogen absorption by enhancing cathodic hydrogen evolution and inhibiting the combination of hydrogen atoms. While the effects of hydrogen damage are well known, it is unclear whether surface SRB films influence hydrogen absorption any differently to the separate effects of H2S. Also, for particularly hydrogen sensitive metals in soured environments, there is some doubt regarding what constitutes a safe but effective level of cathodic protection. Using the electrochemical technique of Devanathan and Stachurski, special apparatus were used to monitor hydrogen permeation through steel foils exposed simultaneously to cells and cell-free dialysates in batch cultures of either Desulfovibrio vulgaris (Woolwich) or Desulfovibrio desulfuricans (Norway 4). Further experiments were performed using applied potentials of -850mV (SCE) and -lOOOmV (SCE), and polarisation scans were performed on replicate steel specimens under the same conditions. Changes in pH, Eh, cell numbers and H2S levels were monitored during SRB growth.