H. S. Khatak

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.

Studies on hydrogen permeability of 2.25% Cr–1% Mo ferritic steel: correlation with microstructure

Abstract The influence of microstructure on the hydrogen permeability, diffusivity and solubility in 2.25% Cr–1% Mo ferritic steel was investigated using electrochemical permeation technique. Varieties of microstructures ranging from martensite in water-quenched (WQ) steel to a predominant ferrite structure in annealed steel were characterised using analytical transmission electron microscopy. In the tempered structures, continuous precipitation of a variety of carbides of different morphologies and sizes was also characterised. The hydrogen diffusivity showed a continuous increase as the structure changed from martensite to ferrite and also with increasing extent of tempering. Solubility showed a corresponding decrease. The trends have been understood in terms of the number of reversible traps available for hydrogen in these different structures. Accordingly, martensite structure offered the maximum resistance to hydrogen diffusivity and tempered structure the least resistance due to the annihilation of defects during tempering and reduction in the solute content of the matrix due to precipitation.

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.

Effect of ferrite transformation on the tensile and stress corrosion properties of type 316 L stainless steel weld metal thermally aged at 873 K

This article deals with the effect of the microstructural changes, due to transformation of delta ferrite, on the associated variations that take place in the tensile and stress corrosion properties of type 316 L stainless steel weld deposits when subjected to postweld heat treatment at 873 K for prolonged periods (up to 2000 hours). On aging for short durations (up to 20 hours), carbide/ carbonitride was the dominant transformation product, whereas sigma phase was dominant at longer aging times. The changes in the tensile and stress corrosion behavior of the aged weld metal have been attributed to the two competitive processes of matrix softening and hardening. Yield strength (YS) was found to depend predominantly on matrix softening only, while sig-nificant changes in the ultimate tensile strength (UTS) and the work-hardening exponent, n, occurred due to matrix hardening. Ductility and stress corrosion properties were considerably affected by both factors. Fractographic observations on the weld metal tested for stress-corrosion cracking (SCC) indicated a combination of transgranular cracking of the austenite and interface cracking.

Effect of External Stress on the Behavior of Oxide Scales on 9Cr-1Mo Steel

Tensile specimens of 9Cr-1Mo steel weresubjected to oxidation in air at a temperature of 973 Kfor periods of 25, 47, 70, 97, 120, and 140 hr. Theintegrity of the oxide scale was examined by an in-situ technique which involved the recording of theacoustic-emission activity associated with the breakingof the scale. Specimens were also subjected to oxidationfor the same times after subjecting them to an external stress of 40 MPa. The applicationof the external stress was found to alter the integrityof the scale. Spalling of the scale occurred to a lesserextent when external stress was applied due to partial release of growth stresses becauseof the elongation of the specimen. The scale buckledbefore spalling when no external stress was applied. Onthe other hand, the scale developed wedge-type cracks before spalling when subjected toexternal stress during oxidation. Postoxidationexamination of the stressed specimen revealedsubstantial segregation of silicon and chromium at thegrain boundaries (oxide ridges). The specimens, which wereoxidized without external stress, revealed a reductionin the concentration of chromium on the surface. Thisbehavior was attributed to enhanced spalling in the case of the latter specimen compared to theformer.

Studies on the influence of metallurgical variables on the stress corrosion behavior of aisi 304 stainless steel in sodium chloride solution using the fracture mechanics approach

Stress corrosion data on a nuclear grade AISI type 304 stainless steel in a boiling solution of 5M NaCl+ 0.15M Na2SO4+ 3 mL/L HC1 (bp 381 K) for various metallurgical conditions of the steel are presented in this article. The metallurgical conditions used are solution annealing, sensitization, 10 pct cold work, 20 pct cold work, solution annealing + sensitization, 10 pct cold work + sensi-tization, and 20 pct cold work + sensitization. The fracture mechanics approach has been used to obtain quantitative data on the stress corrosion crack growth rates. The stress intensity factor,K1, andJ integral,J1, have been used as evaluation parameters. The crack growth rates have been measured using compact tension type samples under both increasing and decreasing stress intensity factors. A crack growth rate of 5 X 10-11 m/s was chosen for the determination of threshold para-meters. Results of the optical microscopic and fractographic examinations are presented. Acoustic signals were recorded during crack growth. Data generated from acoustic emissions, activation energy measurements, and fractographic features indicate hydrogen embrittlement as the possible mechanism of cracking.

Elimination of intergranular corrosion susceptibility of cold-worked and sensitized AlSl 316 SS by laser surface melting

Susceptibility to intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC) due to sensitization is one of the major problems associated with austenitic stainless steels. Thermal exposures encountered during fabrication (welding, hot working, etc.) and elevated temperature service may lead to sensitization of components of austenitic stainless steels. Laser surface melting (LSM) is an in-situ method to increase the life of a sensitized component by modifying the surface microstructure without affecting the bulk properties. In this paper, the results obtained in the attempt to improve IGC resistance of coldworked and sensitized 316 SS by LSM are presented. Type 316 SS specimens cold worked to various degrees ranging from 5 to 25% reduction in thickness and sensitized to different degrees by exposing at 898 K for different durations were laser surface melted using continuous wave (cw) CO2 laser. ASTM standard A262 practice A, optical metallography, and ASTM standard G108 were used to characterize the specimens before and after LSM. Influence of prior deformation on the desensitization behavior was evaluated for the laser melting conditions adopted during the investigation. Complete dissolution of M23C6 due to laser melting and suppression of re-precipitation due to rapid quenching result in a desensitized homogenous microstructure, which is immune to IGC. Under identical laser melting conditions, the extent of desensitization decreases with an increase in the degree of cold work, and hence, higher power levels and an extended interaction time must be adopted to homogenize the sensitized microstructure with prior cold work.

Stress corrosion crack growth studies on AISI type 316 stainless steel in boiling acidified sodium chloride solution

Abstract Stress corrosion cracking (SCC) data for AISI type 316 stainless steel in solution-annealed and sensitized (at 923 K for 20 h) conditions are presented in this paper. The tests were conducted in an environment of boiling 5 M NaCl + 0.15 M Na 2 SO 4 + 2.5 ml/1 HCI using a fracture mechanics approach. Stress parameters, K I and J I , were calculated along with the crack growth rates (d a /d t ). The tests were conducted under both increasing stress intensity and decreasing stress intensity conditions. The sensitized stainless steel was found to have lower values of K ISCC and J ISCC and higher plateau crack growth rates than the solution-annealed material. The value of K I for the transition of the cracking mode from transgranular to intergranular in the sensitized stainless steel corresponded to the value of K I above which the plateau region started. In both conditions of the material, the initiation of stress corrosion cracking and the initial crack propagation were transgranular. The sensitized material exhibited a transition to an intergranular cracking mode while the solution-annealed material did not. A large number of secondary cracks were observed in the solution-annealed material.

Evaluation of the stress corrosion resistance of cold rolled aisi type 316 stainless steel using constant load and slow strain rate tests

The effect of cold rolling (5 to 15% reduction in thickness) of an AISI Type 316 stainless steel on stress corrosion cracking (SCC) has been studied in boiling magnesium chloride of 45% concentration (boiling point = 428 K) using both constant load and slow strain rate methods. The constant load test results indicated increasing SCC susceptibility with increasing degree of prior deformation, whereas the ranking by the slow strain rate technique was in the reverse order. The reasons for this discrepancy have been discussed. A modified procedure of slow strain rate testing, involving the interruption of the test after a chosen duration of time and measuring average crack growth is suggested as an alternative.

Surface Alloying of Nitrogen to Improve Corrosion Resistance of Steels and Stainless Steels

Abstract It is well known that the addition of nitrogen to steels and stainless steels enhances the passivity and localized corrosion resistance, in addition to improving the mechanical properties. Selective alloying of surfaces of steels and stainless steels with nitrogen could also enhance the corrosion resistance and improve the mechanical properties without affecting the bulk properties. Techniques like ion implantation, laser alloying, nitriding, etc. can be effectively used to introduce very high levels of nitrogen. In addition, these techniques can also produce modified surfaces with novel microstructures to further improve the properties. The surface alloying methods also provide an opportunity to selectively nitrogenate the surface of finished components in order to obtain better properties. The review highlights the techniques, modifications and the properties obtained further.