P. Muraleedharan

Photocatalytic Inhibition of Microbial Adhesion by Anodized Titanium

Biofouling is one of the concerns in the use of titanium for seawater cooled condensers of power plants. Earlier studies have shown that anodized titanium and its alloys with a thin film of anatase (TiO2) on its surface can inhibit attachment of Pseudomonas sp. when illuminated with near-UV light (350 - 380 nm). In the present study, a comparison of the photocatalytic inhibition of microbial attachment on titanium surfaces anodized at different voltages was carried out. Thin films of anatase of varying thickness were produced on titanium grade-2 by anodizing in dilute orthophosphoric acid solution at 30 V, 50 V and 100 V. The photocatalytic efficiency of these anodized surfaces was measured by the methylene blue degradation method. The anodised surfaces were exposed to liquid cultures of Gram-negative Pseudomonas sp., Gram-positive Micrococcus sp. and to a mixed algal culture. Photocatalytic inhibition of microbial attachment was maximum on the titanium surface anodized at 30 V, followed by the surface anodized at 50 V and then at 100 V. The photocatalytic inhibition of microbial attachment was also found to be dependent on the cell wall characteristics of the organism. The Gram-negative Pseudomonas sp. with a lipoproteinaceous outer membrane was the most susceptible to the photocatalytic effect, while the Gram-positive Micrococcus sp. with peptidoglycan cell wall showed moderate susceptibility and the algae with siliceous cell wall showed no susceptibility at all.

Influence of Surface Characteristics and Microstructure on Adhesion of Bacterial Cells onto a Type 304 Stainless Steel

A study was carried out to understand the influence of the surface characteristics/microstructure of a type 304 stainless steel on bacterial adhesion by exposing solution-annealed, sensitized and air-oxidized stainless steel specimens in a culture of Pseudomonas sp. in dilute nutrient broth. Epifluorescence microscopy of the exposed surfaces revealed that the pattern of adhesion as well as number density of bacterial cells was different depending on the metallurgical condition of the substratum. Among the specimens with different microstructures, the sensitized specimens had the highest bacterial density, followed by the solution annealed and the oxidized specimens. The same trend was shown by the total viable counts on the various surfaces, estimated by a plate count technique. The study assumes significance in the context of the widely reported observation of preferential attack of the welded region during microbiologically influenced corrosion of fabricated components.

Biofouling control of titanium by microroughness reduction

Abstract The effects of repeated pickling and polishing on biofouling of titanium surfaces were investigated using a confocal laser scanning microscope (CLSM) and standard microbiological methods. Titanium surfaces (i) once pickled (control), (ii) repeatedly pickled (five pickling cycles of 5 min duration) and (iii) polished (surface finish of 1000 grit silicon carbide paper) were exposed to seawater. A decrease of one order of magnitude in bacterial and algal attachment was recorded on the repeatedly pickled and polished titanium surfaces compared to the control surfaces. Statistical analysis of the data was carried out using MYSTAT software involving Tukey-Kramer Multiple CLSM used as an effective tool to measure the microroughness of the test surfaces as well as to quantify the biofilm forming tendency on these surfaces. The results indicated that microroughness reduction leads to significant decrease in biofouling.

Microbiologically influenced corrosion of AISI type 304 stainless steels under fresh water biofilms

The influence of biofilms formed by microorganisms on the corrosion behaviour of stainless steel (SS) in various fresh water environments was studied using electrochemical techniques. Biofilm characterization studies showed an algal-dominated biofilm in the normal dark and light exposures, whereas, in the continuous dark exposures, bacterial-dominated biofilm was observed. The open circuit potential (OCP) and the breakdown potential (Eb) of the specimens with biofilms showed a positive shift compared to those of the freshly polished specimens. Though ennoblement of breakdown potential showed protective nature of biofilm, ageing of the algal-dominated biofilm was found to initiate localised corrosion of the substratum due to the heterogeneities in the biofilm, creating anodes and cathodes. In the case of specimens containing 123-day old algal-dominated biofilm, steady increase in current density was observed during polarization starting from OCP which is indicative of initiation and steady propagation of crevice corrosion. Mikrobiologisch beeinfluste Korrosion von nichtrostenden Stahlen vom Typ 1.4301 unter Frischwasserbiofilmen Der Einflus von durch Mikroorganismen gebildeter Biofilme auf die Korrosion von nichtrostendem Stahl in verschiedenen Frischwasserumgebungen wurde mit elektrochemischen Techniken untersucht. Studien zur Charakterisierung des Biofilmes zeigten einen durch Algen dominierten Biofilm bei normal dunkler und Licht-Auslagerung, wahrend bei dauernd dunkler Auslagerung ein durch Bakterien dominierter Biofilm beobachtet wurde. Das Freie Korrosionspotential und das Durchbruchspotential der Proben mit Biofilm zeigten eine positive Verschiebung verglichen mit den frisch polierten Proben. Obwohl die Veredelung des Durchbruchpotentials die schutzende Natur des Biofilms zeigte, wurde bei einer Alterung des durch Algen dominierten Biofilms ortliche Korrosion des Substrates durch die Heterogenitat des Biofilmes, die zur Bildung von Anoden und Kathoden fuhrte, initiiert. Im Falle der Proben, die einen 123-Tage alten, durch Algen dominierten Biofilm besasen, wurde eine stetige Zunahme der Stromdichte wahrend der Polarisation vom Freien Korrosionspotential aus beobachtet, was ein Indikator fur die Einleitung und Ausbreitung von Spaltkorrosion bedeutet.

Biomineralisation of manganese on titanium surfaces exposed to seawater

A 2-year long study was carried out to isolate and characterise various bacterial species present in the biofilm formed on titanium surfaces exposed to seawater and to assess the manganese oxidizing potential of the marine isolates. The amount of manganese present in the biofilm was also measured using atomic absorption spectrometry (AAS). The results showed that titanium was susceptible to biofouling. More than 50% of the culturable marine bacterial isolates were capable of bringing about oxidation of Mn(II). All these manganese oxidizing bacteria were heterotrophic. Autotrophic manganese oxidizing bacteria such as Leptothrix was not isolated in the present study. The AAS results confirmed that the manganese content in the biofilms increased with increasing exposure time. Hence, the study indicates that the titanium surfaces when exposed to seawater were colonised by a large number of heterotrophic bacteria, which have the ability of bringing about biomineralisation of manganese.

Heat treated anodised titanium surfaces showing enhanced photocatalytic inhibition of microbial fouling

Abstract Titanium, an otherwise perfect condenser tube material in sea water applications, is challenged by the problem of severe biofouling. Anatase, one of the two commercially important crystalline forms of titanium dioxide, possesses excellent photocatalytic activity (PCA). It has been shown in the earlier studies by the authors that anodisation of titanium produces anatase type of TiO2 capable of photocatalytic inhibition of microbial adhesion under near UV light illumination. The present study investigates the influence of anodising voltage and anodising time on the photocatalytic inhibition of Pseudomonas sp., a frequent coloniser of natural biofilms formed on titanium surfaces. The effect of heat treatment of anodised surfaces on PCA was also studied. The results showed that heat treatment resulted in a significant enhancement of PCA. The surface oxide was characterised using glancing incidence X-ray diffraction and atomic force microscopy and the results indicate a marked increase in the cystallinity of the anatase film on the heat treated anodised surfaces. Attempts have also been made to understand the mechanism underlying the photocatalytic inactivation of the bacterial cells on TiO2 surfaces by studying their growth characteristics.

Analysis of intergranular corrosion attack on stainless steels by means of atomic force microscopy and optical microscopy. Part 1: Influence of heat treatment

The aim of this study is to quantify intergranular corrosion attack of heat treated low carbon austenitic stainless steel caused by segregation processes of P (0.2 at% P) by different methods. After nitric acid chromate test the intensity of intergranular corrosion attack was investigated by weight loss measurements as well as determination of depths of grooves by both the optical microscopy and AFM measurements. Generally, the largest attack of corrosion is noticed on samples heat treated at 550 °C. Results of weight loss include the corrosion of matrix, inhomogenities and grain boundaries. The depths of corrosion attack were determined by optical microscope on cross sections. This method can not distinguish between attacks at grain boundaries and attack of inhomogenities. Whereas, measurements of depths by line profiling of AFM-surface images is related to grain boundary attack directly. The weight loss measurements as well as the optical microscopy and AFM-measurements were compared for tested heat treatment conditions. It is concluded that the use of the AFM offers an advantageous method for quantifying intergranular attack. Analyse des interkristallinen Korrosionsangriffs an nichtrostenden Stahlen mit Hilfe der Atomkraftmikroskopie und der optischen Mikroskopie. Teil 1: Einflus der Warmebehandlung Am Beispiel eines phosphordotierten (0,2 at% P) austenitischen Cr-Ni- Stahls mit niedrigem Kohlenstoffgehalt und verschiedenen Warmebehandlungszustanden werden Methoden zur quantitativen Erfassung des interkristallinen Korrosionsangriffes als Folge von Segregationsprozessen getestet. Nach dem Salpetersaure-Chromat-Test wurde die Intensitat des interkristallinen Korrosionsangriffs durch Ermittlung des Gewichtsverlustes, sowie durch Messung der Tiefe der Korrosionsangriffe mit optischer Mikroskopie und durch AFM-Untersuchungen bestimmt. Der starkste interkristalline Angriff wurde nach der Warmebehandlung bei 550 °C beobachtet. Der Gewichtsverlust nach dem Kochtest entspricht der Summe des Angriffes an der Matrix, sowie an Inhomogenitaten und Korngrenzen. Mit der optischen Metallographie werden Angriffe erfast, die sowohl durch Auflosung an Korngrenzen als auch an Inhomogenitaten entstehen. Demgegenuber wird mit Hilfe der AFM-Topographieuntersuchungen die direkte Zuordnung der Messtelle im Profil zu den Graben von Korngrenzen in der Hohentopographie ermoglicht. Es wird die Schlusfolgerung gezogen, das AFM-Untersuchungen eine aussichtsreiche Methode fur die Quantifizierung der interkristallinen Korrosionsangriffe darstellen.

Microbiologically Influenced Corrosion

Abstract Austenitic stainless steels are susceptible to microbiologically influenced corrosion (MIC) when it is used in contact with natural waters. This is due to the changes in the chemistry of the environment at the metal surface because of the settlement and activities of microorganisms. After introducing the mechanism of microbial mediation in the corrosion of metals and alloys, the paper summarizes the work carried out in our laboratory in the area of MIC of austenitic stainless steels. The thrust of our work was in understanding the changes in the electrochemical behaviour of a type 304 stainless steel in the presence of a natural biofilm as well as the influence of metallurgical characteristics on microbial adhesion and MIC.