Solid State Phenomena Vol. 227

Abstract: Sulfur-oxidizing bacteria (SOB) of Acidithiobacillus genus, especially of A. thiooxidans and A. ferrooxidans species are considered as very aggressive biological factors that influences deterioration of many materials, including mineral building materials like concrete and stoneware. Biofilms formed by these bacteria play a significant role in microbiologically influenced corrosion (MIC) of various materials in conditions that ensure sufficient moisture. The aim of this work was to assess differences between A. thiooxidans and A. ferrooxidans biofilms formed on concrete and stoneware. Both strains were prone to form biofilms on concrete and stoneware. However, the type of mineral materials strongly influenced metabolic activities of the tested strains, thus providing to formation of biofilms displaying different features. The higher activities of cells were observed in biofilms of A. ferrooxidans bacteria. The higher total biomass attached to the material surface as well as concentration of proteins in liquid culture medium were observed in biofilms grown on concrete samples. The optimal time of biofilm growth on tested materials was 48 hours on concrete, and 72 hours on stoneware. Amongst materials tested, concrete samples were more susceptible to corrosion in the presence of A. thiooxidans and A. ferrooxidans bacteria.

Abstract: This study investigates how sulfate-reducing bacteria (SRB) influence the process of microbiologically induced corrosion (MIC) of carbon steel by measuring corrosion potential using open-circuit potential (OCP) measurements. MIC is mainly influenced by Desulfovibrio vulgaris, formerly known as Desulfovibrio desulfuricans subsp. Desulfuricans, deposited as spirillum desulfuricans, which produces D(-)-lactate dehydrogenase. This strain was recommended by ATCC to be used in the tests described in ASTM. A pure colony of SRB was isolated from the Baram and Sungai Ular areas in Malaysia. An evaluation of SRB growth was performed during the test in the inoculated medium anaerobically at 37 ̊. The results showed that the corrosion potential E_oc increases in the presence of SRB in pure and mixed cultures as compared to the control sample. These results indicate that the SRB caused the metal loss on the carbon steel surface through direct corrosive action of the H₂S generated by the bacteria during their metabolic process of reducing sulfates to the sulfide form.

Abstract: This study defines the interrelationship between turbidities and cell number counting efficiency for the growth of one of the microbiologically influenced corrosion (MIC) species in a medium. The metabolism activities during bacteria growth can accelerate the corrosion process and shorten the reliability of pipelines. Thus, the investigation of MIC species’ development and metabolic activities is significant. An experiment was performed on sulfate-reducing bacteria (SRB) that practiced the medium as the substance to grow. Desulfovibrio vulgaris, a strain of SRB, was cultured in a postgate C medium to measure the bacteria survival using two different measurement methods. The medium was modified to pH 7.5 at 37°C and placed in anaerobic vials. During 24 hours of incubation, samples were retrieved, and the value of turbidity and cell numbers was measured. Based on the SRB growth graph pattern, the amount of bacteria cell numbers was increased parallel to the value of the medium’s turbidity in respect to time. Both values (turbidity and bacteria cell numbers) dramatically increased from hour1 to hour24. The results supported that the turbidity value was positively correlated with bacteria cell numbers.

Abstract: Bacteria of Desulfovibrio genus belong to group of widespread sulphate-reducing bacteria (SRB). D. desulfuricans is considered one among many bacterial species involved in microbiologically influenced corrosion (MIC) of metals, mainly of stainless steels and other alloys. SRB can produce gaseous hydrogen sulphide. This gas is released into the environment leading to formation of metal sulphides that significantly influence electrochemical processes and ultimately enhance the corrosion of materials. Biofilms formed by these bacteria are especially harmful for highly alloyed steels and many alloys. The aim of this work was to compare the character of growth and biofilm formation by three strains of D. desulfuricans (standard soil strain DSM and two wild intestinal strains: DV/A and DV/B) on the surface of NiTi alloy.

Abstract: In order to improve the power production efficiency of biomass-fired boilers, power plants must be operated at higher steam temperatures than nowadays. One of the main factors hindering the rise of the steam temperatures is the corrosive nature of the flue gases and fly ash towards the superheaters. In this study, the high-temperature corrosion resistance of three commercial superheater steels exposed to potassium chloride was compared. The focus was on the effect of pre-oxidation on the protective properties of different steels, whereupon various variables were used during the pre-oxidation.

Abstract: An investigation was conducted to synthesize βNiAl coating on the nickel based superalloy Inconel 625 in the low activity chemical vapor deposition process (CVD). The deposition was carried out for 8 hours at 1050°C using the BPXpro3252 IonBond company equipment. Surface morphology and cross-section microstructure of the diffusion coating were studied and compared using an optical microscope, an X-ray diffractometer and a scanning electron microscope (SEM) equipped with an energy dispersive spectroscope. It was found that 29 μm thick aluminide coating consisted of two layers: an outer one and the inner interdiffusion one. The outer layer consisted of the βNiAl phase. The inner one consisted of the βNiAl phase with chromium, molybdenum and niobium carbides (M₂₃C₆ and MC type) inclusions. Outer layer hardness was about 564 HV0.002 while interdiffusion layer hardness was about 725 HV0.002. Thermal diffusivity of Inconel 625 superalloy with and without coating was measured using a NETZSCH model 427 laser flash diffusivity apparatus. The thermal diffusivity measurements were conducted in the argon atmosphere at the temperature interval 20 - 1200 oC. Thermal diffusivity of the uncoated Inconel 625 Ni-base superalloy at the room temperature is about 2 mm²/s, while for the coated superalloy thermal diffusivity is about 2.8 mm²/s. The increase of the temperature from 20 to 1200 oC leads to the increase of the thermal diffusivity of the coated sample from 2.8 to 5.6 mm²/s. Cyclic oxidation tests for both coated and uncoated superalloys were performed at 1100°C for 1000 h in the air atmosphere. The aluminized samples exhibited a small mass increase and the α-Al₂O₃ scale was formed during the oxidation test.

Abstract: In the paper new type of thermal barrier coatings characterized by good oxidation and hot corrosion resistance were presented. Bond coats were formed by overaluminizing of an MeCrAlY type coating deposited by low pressure plasma spraying (LPPS). The outer ceramic layer of yttria stabilized zirconia (Metco 6700) was deposited by plasma spray physical vapour deposition (PS-PVD). Rene 80 nickel superalloy was used as base material. The research showed that double-layer bond coat was formed with external NiAl phase layer and inner MeCrAlY layer. The outer ceramic layer was characterized by columnar structure similar to that obtained in the EB-PVD process. The presence of secondary reaction zone (SRZ) was noted.

Abstract: The paper presents results of research into isothermal oxidation test of thermal barrier coatings characterized by high oxidation resistance and hot corrosion. Bondcoats were deposited by overaluminizing of MeCrAlY–type coating deposited by LPPS method. The outer ceramic layer of yttrium oxide stabilized zirconia oxide (Metco 6700) was deposited by plasma spray physical vapour deposition (PS-PVD). For comparison purposes additionally LPPS-sprayed were MeCrAlY bondcoats, which were not subsequently aluminized.. The isothermal oxidation test at 1100^oC for 1000h shown that thickness of the TGO layer in overaluminized bondcoat was significantly thicker in comparison with conventional LPPS-sprayed MeCrAlY bondcoats.

Abstract: The paper presents results of research into thermal barrier coatings characterized by higher oxidation resistance. Bondcoats were formed by overaluminizing of the MeCrAlY coating, deposited by low pressure plasma spraying (LPPS). The outer ceramic layer of yttrium oxide stabilized zirconia oxide was deposited by plasma spray physical vapour deposition (PS-PVD). For comparison purposes additionally formed were MeCrAlY bondcoats, which were not subsequently aluminized. The research showed that during CVD overaluminizing there was formed an additional layer built of the β-NiAl phase, which protects the base material from oxidation. Preserved below it increased chromium content ensures resistance to hot corrosion. The outer layer was characterized by columnar structure, similar to that obtained in the EB-PVD process. The isothermal oxidation tests showed that thickness of the TGO layer into overaluminized bondcoat significantly thicker in comparison with conventional LPPS-sprayed MeCrAlY bondcoats.

Abstract: The paper presents results of tests into heat resistance and specific heat capacity of nickel-based superalloys: Rene 80, Mar-M-200+Hf i Mar-M-247+Hf. Heat resistance tests were carried out with the use of a cyclic method. Each cycle involved a 23-hour period of heating at 1100oC, followed by 1-hour air cooling. The DSC method was applied to determine specific heat. Scale microstructure was examined with the use of an electrone microscope. The tested alloys have been proven to enjoy relatively good resistance to corrosion. The Mar-M-200+Hf alloy enjoys the highest specific heat capacity, while the lowest value of specific heat has been recorded for the Mar-M-247+Hf alloy.