Role of anaerobic and aerobic bacteria in localised corrosion: field and laboratory morphological study

Abstract

Bacterial evidence was found in the biofilm deposited on the surface of an API X52 steel coupon following exposure to flowing seawater within a pipeline for 60 days. Extensive corrosion pitting was observed on the steel surface after removing the biofilm. Several types of anaerobic and aerobic bacteria were isolated from the biofilm. The anaerobic bacteria were cultured and grown in API RP 38 culture medium and the aerobic bacteria were grown in a general heterotrophic culture medium. The morphological and chemical characteristics of the bacteria were determined. Subsequently, separate specimens of API X52 pipeline steel were inoculated with each bacterial strain in under laboratory conditions. The specimen exposed to the anaerobic bacteria showed several micropits of different sizes on its surface while that exposed to the aerobic bacteria showed freedom from surface pits.     

Mechanism of dislocation shearing of gamma in fine precipitate strengthened superalloy

Abstract

The dislocation structure of a fine gamma precipitate strengthened nickel base superalloy during steady state creep has been studied. Extensive shearing of gamma particles by partial dislocations, creating stacking faults in both gamma and gamma phases, have been observed in specimens under stresses ranging from 120 to 250 MPa. Detailed analysis of the fault nature and dislocation dissociation revealed that a unit matrix dislocation of a2110 dissociated into two partials of a3112 and a6112. Extrinsic stacking faults were created in both gamma and gamma phases, presumably being created by the movement of the leading a3112 partial, while the movement of the a6112 partial eliminates the stacking fault in the matrix, but leaves a partial dislocation loop around the faulted gamma precipitates.     

Adhesion and corrosion resistance of epoxy primers used in the automotive industry

One of the most important factors in corrosion prevention by protective coatings is the loss of adhesion of the coating under environmental influence. Thus, adhesion strength is often used when characterizing protective properties of organic coatings on a metal substrate. In this work, the adhesion of different epoxy primers (pigment-free, zinc-rich and chromate-based) was examined on steel. Both the dry and wet adhesion strengths of organic primers were measured directly by a pull-off standardized procedure, as well as indirectly by the NMP test. The corrosion stability of coated samples was investigated by electrochemical impedance spectroscopy. It was shown that under dry test conditions all the samples showed very good adhesion. However, different trends in adhesion for different primers during exposure to the corrosive agent (3% NaCl solution) were observed. The lowest adhesion values were obtained for chromate-based epoxy primer; however, the change in adhesion of this protective system during immersion in 3% NaCl solution for 25 days was the smallest of all investigated samples. Electrochemical impedance measurements in 3% NaCl solution confirmed good protective properties of pigmented epoxy primers on steel, i.e., greater values of pore resistance and charge-transfer resistance, and smaller values of coating capacitance and double-layer capacitance, were obtained for these protective systems.     

Cathodic Protection and Stress Corrosion Cracking of Mild Steel District Heating Distribution Systems

Abstract

Inadequate design or construction of certain district heating distribution systems has led to severe corrosion problems. These systems were then cathodically protected by means of impressed current installations, and the frequency of leaks was thereby reduced. However, following this, a number of failures due to stress corrosion cracking have been discovered. This has been traced to the alkaline environment round the pipe produced by the cathodic protecion process, combined with subsequent concentration by evaporation, the potential of the pipe then being in the range known to promote caustic cracking when residual stresses are present in the pipe. The only possible solutions to the problem for existing pipelines are either to remove the cathodic protection and revert to the likelihood of general corrosion problems, or to control the applied current so that the pipeline is kept out of the potential range known to promote stress corrosion. In practice this is difficult to achieve and may result in parts of the pipeline being under-protected.     

Erosive abrasive wear behaviour of stainless steel surface produced by plasma transferred arc hardfacing

Abstract

Erosive abrasive wear is caused by high speed impact of particles entrained in a fluid system on the surfaces of components such as boilers and furnaces. Erosive abrasive wear in boilers results from the impact of hard particles such as ash or clinker entrained in flue gases and can lead to serious damage. The life of boiler and furnace components encountering erosive abrasive wear in service, which are most commonly fabricated from carbon steels, can be improved by hardfacing with a wear resistant material. The effects of wear parameters such as particle size, flux and velocity on the erosive abrasive wear behaviour of a stainless steel surface produced by the plasma transferred arc hardfacing have been investigated using an experimental design approach. The wear resistance of the stainless steel surface was found to be twice that of the carbon steel substrate.     

Interaction energy parameters in hypostoichiometric mono-carbides,TiCx and NbCx,evaluated by statistical thermodynamics

Abstract

Interaction energy E(C-C) between nearest neighbour C atoms and E(C-M) between a C atom and M atom in hypostoichiometric monocarbide MC_x (M = Ti or Nb) were evaluated from the available a(C)-T-x relationships by statistical thermodynamic analysis (a(C): carbon activity, T: temperature, x: C/M atom ratio). Estimated E(C-C) values were positive (i.e. the C-C interaction is repulsive) for both TiC_x and NbC_x and E(C-C) varied with T showing a trend of weakening repulsion with rising T. On the other hand, the C-M interaction appeared to be strongly attractive.     

Alloy design and creep strength of advanced 9%Cr USC boiler steels containing high concentration of boron

Abstract

The research and development project of National Institute for Materials Science (NIMS) for advanced ferritic heat resistant steels for 650°C ultra super critical (USC) plants, revealed that the addition of >0·01 mass% boron to a 0·08C–9Cr–3W–3Co–V–Nb–,0·003N steel remarkably improves the long term creep strength. Boron enriched in M₂₃C₆ carbides near prior austenite grain boundaries suppresses the coarsening of carbides during creep deformation, leading to excellent microstructural stability and creep strength. If creep strength was further improved by the addition of nitrogen, it was found to enhance precipitation of fine MX. Addition of excess nitrogen to the high boron containing steel was found to reduce creep rupture lives and ductility. This results from a decrease in the amount of effective boron, which is dissolved in M₂₃C₆ and suppresses its coarsening, resulting from the formation of coarse BN at normalising temperature. The highest creep strength is obtained with steel of the following composition: 0·08C–9Cr–3W–3Co–0·2V–0·05Nb–0·008N–0·014B (mass%), which has an improved creep strength compared to P92. The 10⁵ h extrapolated creep rupture strength at 650°C is ～100 MPa. This steel also shows good creep ductility even in the long term. In conclusion, high boron bearing 9Cr–3W–3Co–V–Nb steel combined with the addition of 0·008 mass% nitrogen is a promising candidate for thick section components in the 650°C USC plants as it shows superior creep strength without impaired creep ductility.     

Effect of the chemical modification of the precursor of ZrO2 films on the adhesion of organic coatings

ZrO₂ films were deposited on low carbon steel by a sol-gel process as a chemical pretreatment before the application of a polyester paint. The films were obtained by a dip-coating technique using solutions of Zr(OBuⁿ)4 containing complexing agents (acetylacetone or acetic acid). These additives modified the alkoxide at the molecular level, so a new precursor was formed in solution. This new compound shows slower rates of hydrolysis and condensation than Zr(OBuⁿ)4, which allows the stability of solutions and the morphology of the gel to be controlled. Moreover, the length and the temperature of the thermal treatment influence the structure of the gel by reducing the amount of organic residues. The behavior of the films in promoting the adhesion of organic coatings was evaluated by measuring the detachment of the paint from a cross-scratch of samples which had been exposed for different times in a salt fog chamber. Tests using electrochemical impedance spectroscopy were also carried out. According to the results, most of the samples pretreated with zirconia layers showed a good performance, even better than commercial chemical treatments, when acetic acid was used as a modifier of the alkoxide precursors.     

Improvement of wear resistance of hot working tool steel by hardfacing Part 2 – Case study

Abstract

The performance and service life of the die components are limited because of different reasons such as thermal and mechanical fatigue cracking, wear, plastic deformation, etc. To minimise these damages, the dies are normally subjected to a variety of surface engineering processes. In the present study, the dimensional loss and lifetime of hardfaced and unmodified H11 steel hot forging dies were monitored during service and compared. For this purpose, various experimental tools such as XRD, SEM, optical metallography and microhardness test were employed. Microhardness profiles from surface to the depth of the dies after service were determined to study the variations in mechanical properties. It was observed that the hardfacing with Stellite 21 and Inconel 625 alloys leads to an increase in wear resistance and the working life of the dies in industrial scale. In the case of hardfaced dies, the work hardening of surface produces a self-protecting layer which has a strong metallurgical bonding to the substrate and thus enhances more resistance to further wear, while hardness reduction during service in unmodified H11 steel die causes a reduction in wear resistance and plastic deformation of surface layer.     

Effect of treatment variables on size refinement by phosphide inoculants of primary silicon in hypereutectic Al–Si alloys

Abstract

The effects of phosphorus containing inoculant identity/process history, addition level, addition temperature, and contact time on number density N _A of primary silicon particles in small volumes of cast Al–20 wt-%Si are reported. Inoculation replaces the coarse, branched primary silicon otherwise obtained in the upper part of these castings with a uniform distribution of small polyhedral primary silicon particles. The inoculants tested increased in effectiveness in the sequence: die pressed and heat treated Al–Fe–P; die pressed Al–Fe–P; extruded Al–Cu–P; and Al–Fe–P prepared by a proprietary route. This last inoculant gave a maximum N _A at 200 ppm addition level in this volume of melt for a contact time of 10 min at 800°C, and at 10 min contact time for an addition level of 100 ppm at this temperature. An addition temperature of 850°C produced a small reduction in N _A, compared with 750 or 800°C. The significance of these findings is discussed in the context of previously published work and possible mechanisms leading to less effective inoculation at high addition levels and extended contact times.