The Role of Metallurgical Features in the Microbially Influenced Corrosion of Carbon Steel: A Critical Review.

Microbially influenced corrosion (MIC) is a potentially critical degradation mechanism for a wide range of materials exposed to environments that contain relevant microorganisms. The likelihood and rate of MIC are affected by microbiological, chemical, and metallurgical factors; hence, the understanding of the mechanisms involved, verification of the presence of MIC, and the development of mitigation methods require a multidisciplinary approach. Much of the recent focus in MIC research has been on the microbiological and chemical aspects, with less attention given to metallurgical attributes. Here, we address this knowledge gap by providing a critical synthesis of the literature on the metallurgical aspects of MIC of carbon steel, a material frequently associated with MIC failures and widely used in construction and infrastructure globally. The article begins by introducing the process of MIC, then progresses to explore the complexities of various metallurgical factors relevant to MIC in carbon steel. These factors include chemical composition, grain size, grain boundaries, microstructural phases, inclusions, and welds, highlighting their potential influence on MIC processes. This review systematically presents key discoveries, trends, and the limitations of prior research, offering some novel insights into the impact of metallurgical factors on MIC, particularly for the benefit of those already familiar with other aspects of MIC. The article concludes with recommendations for documenting metallurgical data in MIC research. An appreciation of relevant metallurgical attributes is essential for a critical assessment of a material's vulnerability to MIC to advance research practices and to broaden the collective knowledge in this rapidly evolving area of study.

An Investigation into the Surface Integrity of Micro-Machined High-Speed Steel and Tungsten Carbide Cutting Tools.

The performance and lifespan of cutting tools are significantly influenced by their surface quality. The present report highlights recent advances in enhancing the surface characteristics of tungsten carbide and high-speed steel cutting tools using a novel micro-machining technique for polishing and edge-honing. Notably, the main aim was to reduce the surface roughness while maintaining the hardness of the materials at an optimal level. By conducting a thorough analysis of surfaces obtained using different techniques, it was found that the micro-machining method effectively decreased the surface roughness of the cutting tools the most effectively out of the techniques investigated. Significantly, the surface roughness was reduced from an initial measurement of 400 nm to an impressive value of 60 nm. No significant change in hardness was observed, which guarantees the maintenance of the mechanical properties of the cutting tools. This analysis enhances the comprehension of surface enhancement methodologies for cutting tools through the presentation of these findings. The observed decrease in surface roughness, along with the consistent hardness, exhibits potential for improving tool performance. These enhancements possess the capacity to optimise manufacturing processes, increase tool reliability, and minimise waste generation.

Microbial Communities of Orange Tubercles in Accelerated Low-Water Corrosion.

The rapid degradation of marine infrastructure at the low tide level due to accelerated low-water corrosion (ALWC) is a problem encountered worldwide. Despite this, there is limited understanding of the microbial communities involved in this process. We obtained samples of the orange-colored tubercles commonly associated with ALWC from two different types of steel sheet piling, located adjacent to each other but with different levels of localized corrosion, at a seaside harbor. The microbial communities from the outer and inner layers of the orange tubercles and from adjacent seawater were studied by pure culture isolation and metabarcoding of the 16S rRNA genes. A collection of 119 bacterial isolates was obtained from one orange tubercle sample, using a range of media in anaerobic and aerobic conditions. The metabarcoding results showed that sulfur and iron oxidizers were more abundant on the outer sections of the orange tubercles compared to the inner layers, where Deltaproteobacteria (which include many sulfate reducers) were more abundant. The microbial communities varied significantly between the inner and outer layers of the orange tubercles and also with the seawater but overall did not differ significantly between the two steel sheet types. Hence, we saw similar microbial communities in orange tubercles present, but different levels of localized corrosion, for two different types of colocated steel sheet piling. Metallurgical analysis found differences in composition, grain size, ferrite-pearlite ratio, and the extent of inclusions present between the two steel types investigated.IMPORTANCE The presence of orange tubercles on marine steel pilings is often used as an indication that accelerated low-water corrosion is taking place. We studied the microbial communities in attached orange tubercles on two closely located sheet pilings that were of different steel types. The attached orange tubercles were visually similar, but the extents of underlying corrosion on the different steel surfaces were substantially different. No clear difference was found between the microbial communities present on the two different types of sheet piling. However, there were clear differences in the microbial communities in the corrosion layers of tubercles, which were also different from the microbes present in adjacent seawater. The overall results suggest that the presence of orange tubercles, a single measurement of water quality, or the detection of certain general types of microbes (e.g., sulfate-reducing bacteria) should not be taken alone as definitive indications of accelerated corrosion.

Is marine sediment the source of microbes associated with accelerated low water corrosion?

Accelerated low water corrosion (ALWC) is a form of microbiologically influenced corrosion (MIC) associated with the degradation of marine structures around the low tide water level. A better understanding of the role of microbes in this degradation and the source of these microbes is required to improve the prediction and mitigation of the costly failures occurring due to ALWC. The microbial communities present in a sediment sample and on an ALWC tubercle on adjacent steel sheet piling from a tidal estuary were studied using culture-based isolation and metabarcoding. A total of 43 pure cultures were isolated from the sediment using a variety of culture conditions. Phylogenetic analysis of their 16S rRNA genes placed them in the Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria (Alphaproteobacteria and Gammaproteobacteria). 16S rRNA gene metabarcoding of the sediment and tubercle revealed similar microbial groups at varying relative abundances. No Deltaproteobacteria were isolated from the sediment but they were present in both samples according to metabarcoding and their high abundance (49.3%) in the tubercle could indicate an important functional role. Although some sediment isolates and operational taxonomic units from the metabarcoding have previously been associated with surface colonisation or biofilm formation in MIC, there was no strong evidence for the notion that the sediment adjacent to ALWC was the source of tubercle microbes. Further isolation strategies and functional investigations of representative bacteria at different stages of corrosion are being carried out for additional laboratory-based corrosion studies.

A comparison of the antifouling performance of air plasma spray (APS) ceramic and high velocity oxygen fuel (HVOF) coatings for use in marine hydraulic applications.

Maritime hydraulic components are often exposed to harsh environmental conditions which can lead to accelerated deterioration, reduced function, equipment failure and costly repair. Two leading causes of maritime hydraulic failure are biofouling accumulation and corrosion. This study examined the antifouling performance of three candidate replacement high velocity oxygen fuel (HVOF) coatings relative to the performance of the current baseline air plasma spray (APS) ceramic coating for protection of hydraulic actuators. Following 20 weeks immersion at tropical and temperate field exposure sites, the control APS ceramic accumulated significantly greater levels of biofouling compared to the HVOF coatings. More specifically, the magnitude of growth of real-world nuisance hard fouling observed on in-service hydraulic components (eg calcareous tubeworms and encrusting bryozoans) was significantly greater on the APS ceramic relative to HVOF coatings. Possible explanations for the observed patterns include differences in surface topography and roughness, the electrochemical potential of the surfaces and the colour/brightness of the coatings.

Stigmergy co-ordinates multicellular collective behaviours during Myxococcus xanthus surface migration.

Surface translocation by the soil bacterium Myxococcus xanthus is a complex multicellular phenomenon that entails two motility systems. However, the mechanisms by which the activities of individual cells are coordinated to manifest this collective behaviour are currently unclear. Here we have developed a novel assay that enables detailed microscopic examination of M. xanthus motility at the interstitial interface between solidified nutrient medium and a glass coverslip. Under these conditions, M. xanthus motility is characterised by extensive micro-morphological patterning that is considerably more elaborate than occurs at an air-surface interface. We have found that during motility on solidified nutrient medium, M. xanthus forges an interconnected furrow network that is lined with an extracellular matrix comprised of exopolysaccharides, extracellular lipids, membrane vesicles and an unidentified slime. Our observations have revealed that M. xanthus motility on solidified nutrient medium is a stigmergic phenomenon in which multi-cellular collective behaviours are co-ordinated through trail-following that is guided by physical furrows and extracellular matrix materials.

Imaging of various optical fiber Bragg gratings using differential interference contrast microscopy: analysis and comparison.

Differential interference contrast images of various optical fibers and optical fiber Bragg gratings (FBGs), written with the phase mask technique, are presented to provide information about the resultant refractive index variations present in each case. Use of different fiber types using two distinct phase masks producing four Type I FBGs and a Type In FBG allowed similarities and differences in these FBG images due to variations in the Talbot diffraction patterns produced to be studied.

Effect of embedded optical fibres on the mechanical properties of cochlear electrode arrays.

Incorporating optical fibres in cochlear electrode arrays has been proposed to provide sensors to help minimise insertion trauma and also for the delivery of light in optical nerve stimulation applications. However, embedding an optical fibre into an electrode array may change its stiffness properties, which can affect the level of trauma during insertion. This report uses measurements of buckling and deflection force to compare the stiffness properties of a range of cochlear electrode arrays (Nucleus straight array, rat array, cat array and guinea pig array) with custom arrays containing an embedded optical fibre. The cladding diameters of the optical fibres tested were 125 µm, 80 µm and 50 µm. The results show that the stiffness of the optical-fibre-embedded arrays is related to the diameter of the optical fibre. Comparison with wired arrays suggests optical fibres with a diameter of 50 µm could be embedded into an electrode array without significantly changing the stiffness properties of the array.

Infrared neural stimulation fails to evoke neural activity in the deaf guinea pig cochlea.

At present there is some debate as to the processes by which infrared neural stimulation (INS) activates neurons in the cochlea, as the lasers used for INS can potentially generate a range of secondary stimuli e.g. an acoustic stimulus is produced when the light is absorbed by water. To clarify whether INS in the cochlea requires functioning hair cells and to explore the potential relevance to cochlear implants, experiments using INS were performed in the cochleae of both normal hearing and profoundly deaf guinea pigs. A response to laser stimulation was readily evoked in normal hearing cochlea. However, no response was evoked in any profoundly deaf cochleae, for either acute or chronic deafening, contrary to previous work where a response was observed after acute deafening with ototoxic drugs. A neural response to electrical stimulation was readily evoked in all cochleae after deafening. The absence of a response from optical stimuli in profoundly deaf cochleae suggests that the response from INS in the cochlea is hair cell mediated.

Measurement of forces at the tip of a cochlear implant during insertion.

An optical fiber-based sensor has been developed to measure the forces at the tip of an electrode array during insertion into the cochlea. The sensor, utilizing optical fiber Bragg grating technology, was incorporated into a custom-designed Pt-banded electrode array for guinea pigs. In vivo experiments were undertaken in which forces at the tip of the array were measured in real time during the insertion. Data were obtained for maximum insertion forces of up to 254 mN. Histology was performed on the excised cochleae with the sensors fixed in position to evaluate the level of insertion trauma. The insertion experiments demonstrated a clear correlation between the applied force and collateral tissue damage.

Infrared Neural Stimulation: Influence of Stimulation Site Spacing and Repetition Rates on Heating.

A model to simulate heating as a result of pulse repetitions during infrared neural stimulation (INS), with both single- and multiple-emitters is presented. This model allows the temperature increases from pulse trains rather than single pulses to be considered. The model predicts that using a stimulation rate of 250 Hz with typical laser parameters at a single stimulation site results in a temperature increase of 2.3°C. When multiple stimulation sites are used in analogy to cochlear implants, the temperature increases further depending upon the spacing between emitters. However, when the light is more localized at multiple stimulation sites the temperature increase is reduced.

Modeling of the temporal effects of heating during infrared neural stimulation.

A model of infrared neural stimulation (INS) has been developed to allow the temporal characteristics of different stimulation parameters and geometries to be better understood. The model uses a finite element approach to solve the heat equation and allow detailed analysis of heat during INS with both microsecond and millisecond laser pulses. When compared with experimental data, the model provides insight into the mechanisms behind INS. In particular, the analysis suggests that there may be two broad regimes of INS: the process tends to be limited by the total pulse energy for pulse lengths below 100 µs, while the temperature gradient with respect to time becomes more important above 100 µs.

Stigmergy: A key driver of self-organization in bacterial biofilms.

Bacterial biofilms are complex multicellular communities that are often associated with the emergence of large-scale patterns across the biofilm. How bacteria self-organize to form these structured communities is an area of active research. We have recently determined that the emergence of an intricate network of trails that forms during the twitching motility mediated expansion of Pseudomonas aeruginosa biofilms is attributed to an interconnected furrow system that is forged in the solidified nutrient media by aggregates of cells as they migrate across the media surface. This network acts as a means for self-organization of collective behavior during biofilm expansion as the cells following these vanguard aggregates were preferentially confined within the furrow network resulting in the formation of an intricate network of trails of cells. Here we further explore the process by which the intricate network of trails emerges. We have determined that the formation of the intricate network of furrows is associated with significant remodeling of the sub-stratum underlying the biofilm. The concept of stigmergy has been used to describe a variety of self-organization processes observed in higher organisms and abiotic systems that involve indirect communication via persistent cues in the environment left by individuals that influence the behavior of other individuals of the group at a later point in time. We propose that the concept of stigmergy can also be applied to describe self-organization of bacterial biofilms and can be included in the repertoire of systems used by bacteria to coordinate complex multicellular behaviors.

Effect of phase mask alignment on fiber Bragg grating spectra at harmonics of the Bragg wavelength.

Effects of fabrication conditions on the double-peak structure observed in fiber Bragg gratings at harmonics of the Bragg wavelength were investigated, showing that slight variations in the alignment of the phase mask can affect the grating spectra significantly. A single peak occurs only when the incident beam direction is perfectly normal with respect to the fiber.

Modeling of light absorption in tissue during infrared neural stimulation.

A Monte Carlo model has been developed to simulate light transport and absorption in neural tissue during infrared neural stimulation (INS). A range of fiber core sizes and numerical apertures are compared illustrating the advantages of using simulations when designing a light delivery system. A range of wavelengths, commonly used for INS, are also compared for stimulation of nerves in the cochlea, in terms of both the energy absorbed and the change in temperature due to a laser pulse. Modeling suggests that a fiber with core diameter of 200 µm and NA=0.22 is optimal for optical stimulation in the geometry used and that temperature rises in the spiral ganglion neurons are as low as 0.1°C. The results show a need for more careful experimentation to allow different proposed mechanisms of INS to be distinguished.

Variations of the growth of harmonic reflections in fiber Bragg gratings fabricated using phase masks.

The growth of reflectance peaks from optical fiber Bragg gratings has been studied to determine the relative importance of grating features when writing with the phase-mask technique. Measurements of spectra for two different fiber types using two distinct phase masks allowed the contribution from grating features of half the phase-mask periodicity and of the phase-mask periodicity at the Bragg wavelength to be determined. The dominance of the latter periodicity was ascribed to either the small fiber core diameter that limited the extent of the Talbot diffraction pattern, or the enhanced ±2 diffraction orders of a custom-made phase mask used.

Analysis of transmission mode of a matched fiber Bragg grating interrogation scheme.

A detailed investigation has been undertaken into the transmission mode of the matched fiber Bragg grating interrogation scheme with respect to its use in optical fiber sensor applications. Analytical and numerical models of the scheme have been developed. Experimental studies presented include the effect of the spectral characteristics of the gratings on system performance, results of strain and compression calibrations, a scheme to correct for intensity fluctuations, and the correction of temperature-induced shifts by collocating sensor and reference gratings. The results are in good agreement with a simplified model of the transmission mode. The analysis provides quantitative relationships between key sensor design parameters, such as sensitivity and measurement range as a function of grating bandwidth.

Fabrication of a pi-phase-shifted fiber Bragg grating at twice the Bragg wavelength with the standard phase mask technique.

A pair of reflection peaks/transmission dips, at twice the Bragg wavelength, were observed in spectra of a Type I fiber Bragg grating written with the standard phase mask technique. The occurrence of two peaks/dips, rather than one, is attributed to the interleaved refractive index modulations along the fiber core, with the periodicity of the phase mask that has been observed previously in images of gratings that cause destructive interference in a reflected wave at the Bragg condition owing to the pi phase difference between the grating phases. Thus the standard phase mask technique produced an alternative type of pi-phase-shifted grating at twice the design Bragg wavelength.

Directional dependence of spectra of fiber Bragg gratings due to excess loss.

The reflectance spectra of chirped fiber Bragg gratings can depend substantially on the direction from which the measurement is taken. The measured difference between forward and backward reflectance spectra measured in a linearly chirped grating was shown to be due to the measured excess loss. Simulation using the popular transfer-matrix model demonstrated that the observed asymmetric behavior could be obtained only when excess loss has an asymmetric spectral shape about the local Bragg wavelengths. Application of cladding mode excess losses to the result of a transfer-matrix model accounted for the experimental observation.

Quantitative investigation of the refractive-index modulation within the core of a fiber Bragg grating.

A comparison is made between the modeled and experimentally determined microscopic images of a type I Bragg grating produced in the core of an optical fiber using the ultraviolet irradiation of a phase mask. The simulated image of the refractive-index distribution, which assumes a linear relationship between the irradiation intensity and the refractive-index change, is in good agreement with the measured image.