The application of electrochemical noise resistance to evaluate the corrosion resistance of AISI type 304 SS in nitric acid

The paper presents the application of noise resistance to evaluate the corrosion behaviour of sensitized AISI type 304 SS in nitric acid of varying concentration (4 N, 12 N, 16 N) and temperature (298 K, 323 K, 348 K). Electrochemical noise data was acquired from a three identical electrode configuration in the required conditions at open circuit potential. The noise resistance was evaluated as the ratio of the standard deviation of the potential to that of the current noise after removing the DC component. The inverse relationship between noise resistance and corrosion rate was exploited to qualitatively assess the corrosion behaviour of AISI type 304 SS in nitric acid. Noise resistance decreased with increase in concentration implying an increase in corrosion rate with increase in nitric acid concentration. An increase in temperature from 298 K to 323 K and 348 K decreased the noise resistance in 4 N and 12 N nitric acid implying higher corrosion rates at higher temperatures. The corrosion rates were similar at 323 K and 348 K in these concentrations. The simultaneous measurement of current and potential noise facilitated the evaluation of the frequency dependence of the noise data to determine the spectral noise resistance (R<SUB>sn</SUB>) and the DC limit of the spectral noise resistance R<SUP>0</SUP><SUB>sn</SUB>. The results from R<SUB>sn</SUB> and R<SUP>0</SUP><SUB>sn</SUB> also indicated higher corrosion rates at higher concentration and temperature. Also R<SUB>n</SUB> and R<SUP>0</SUP><SUB>sn</SUB> correlated well in 4 N and 12 N nitric acid at 323 K and 348 K while disparity was observed at room temperature in 4 N and 12 N nitric acid

Surface alloying of nitrogen to improve corrosion resistance of steels and stainless steels

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

Evaluation of susceptibility to stress corrosion cracking (SCC) in sodium chloride solution of weldments of cold rolled type 316 stainless steel

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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

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On the pitting corrosion resistance of nitrogen alloyed cold worked austenitic stainless steels

Pitting corrosion studies were carried out on cold worked (5%, 10%, 15%, 20%, 30% and 40%) nitrogen-bearing (0.05%, 0.1% and 0.22% N) type 316L austenitic stainless steels in neutral chloride medium. Potentiodynamic anodic polarisation study revealed that cold working up to 20% enhanced the pitting resistance, and thereafter a sudden decrease in pitting resistance was noticed at 30% and 40% cold working. Increase in nitrogen content was beneficial up to 20% cold work in improving the pitting corrosion resistance, beyond which it had a detrimental effect at 30% and 40% cold working. The role of nitrogen in influencing the deformation band width and dislocation configuration is explained based on the results of transmission electron microscopy investigations. Scanning electron microscopy observation of the pitted specimens indicated decreasing size and increasing density of pits, along the deformation bands with increasing nitrogen for 40% cold worked specimens. The macrohardness values increased as the cold working increased from 0% to 40%. X-ray diffraction studies revealed the increased peak broadening of austenite peak {0 2 2} with increase in cold working. The relationship between pitting corrosion and deformation structures with respect to nitrogen addition and cold working is discussed

Determination of corrosion types for AISI type 304L stainless steel using electrochemical noise method

Electrochemical noise measurements were made on AISI type 304L SS in 0.1% NaOH, 5% H2SO4, and 0.1 M FeCl3 to study passivation, uniform, and localised corrosion, respectively. Current noise and potential noise were monitored simultaneously, using a three-electrode configuration, under open circuit condition. The experiments were carried out in a closed cell at ambient temperature, without deaeration. The time records revealed salient features of the nature of corrosion process. The localization index and standard deviation of current noise as a function of time for AISI type 304L SS in 0.1 M FeCl3 depict localised corrosion as the nature of attack and the electrode surface showed significant pits. However, the localization index for AISI type 304L SS in H2SO4 tends towards mixed corrosion although the surface exhibited uniform corrosion. The power spectral density plots of the potential noise were analyzed to correlate to the nature of attack on the electrode surface. An attempt was made to study the corrosion behaviour of AISI type 304L SS in 0.1% NaOH, followed by addition of chloride ions. A pit initiation transient was observed in the potential time record. Noise resistance appeared to be a promising parameter to monitor changes in the corrosivity of the environment and the slopes of the potential power spectral density plots of the potential noise, at the higher frequency region, reflected the activities at the surface

Morphological differentiation observed in manganese oxidizing bacterial colonies

Bacteria play a vital role in bringing about Mn(II) oxidation in the natural environment. A study was conducted to identify the potential threat offered by these bacteria in bringing about biomineralisation of manganese dioxide on titanium surfaces exposed to seawater. During the study it was observed that the bacteria such as Pseudomonas and Bacillus formed brown colonies on agar plates amended with Mn2+ indicating their ability to oxidize Mn(II). These colonies showed distinct morphologies when grown on plates containing Mn(II) while they formed normal colonies in the absence of Mn.(II).Hence it is possible that these morphologically distinct structures produced by the bacterial colonies assist these bacteria to perform this function of Mn-oxidation