Rengaswamy, N.S. (1984) Failure of cold-drawn and stress-relieved prestressing steel effect of tensile stress- strain rate, temperature, pH and polarisation on corrosion cracking in H2S and NH4NO3 environments. PhD thesis, Indian Institute of Science.

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The present study is thus concerned with the stress corrosion cracking behaviour of cold-drawn and stress-relieved prestressing steel (0.85% C, 0.8% Mn, 0.27% Si, 0.29% Cr, 0.04% S and 0.02% P) in (i) H2S saturated aqueous solution with and without chloride and (ii) 1 M ammonium nitrate solution at varying pH and temperature. The stress corrosion studies mainly involved constant load tests in UNISTEEL Stress Corrosion Testing Machine and constant strain rate tests in MONSANTO Tinsometer. In order to understand the mechanism of cracking, electrochemical studies were carried out with a WENKING potentiostat and a Voltage Scan Generator with provision to record the E vs i, E vs t and i vs t variations. The fractured specimens were examined in a JEOL JSM 35 CF Scanning Electron Microscope. Hydrogen sulphide medium: In this medium, the steel was found to be highly susceptible to cracking, the susceptibility decreasing slightly with temperature. The threshold stress was found to increase with pH of the medium-from 15% of proof stress at Ph 2.6 to 40% of proof stress at pH 6.5. A limiting pH of 7, above which stress corrosion cracking did not occur was obtained for water – H2S system and the corresponding value in 3.5% NaCl-H2S system was found to be 9. In potentiostatic studies, the time to failure was found to decrease with cathodic potential up to a certain value of over potential and then remained essentially constant. Anodic polarisation on the other hand was found to increase the time to failure. Hydrogen permeation studies showed that the tensile stress increases the permeation current from 1.8 to 3.6 /uA/cm2 when the stress was increased from 0 to 75% of proof stress. The stress rate was found to have profound influence on the cracking process and at very low strain rates, the susceptibility was very high. The fracture mode was predominantly intergranular. These data in H2S medium indicate that the mechanism involves the cracking of FeS film, the liberation of hydrogen at the crack tip, its absorption and stress-induced diffusion leading to accumulation at weak grain boundaries consequently resulting in cracking by decohesion. Ammonium nitrate medium: In this medium, the steel was not found to be susceptible to cracking at room temperature. The susceptibility increased with temperature the time to failure decreasing from 240 minutes at 40°C to 26 minutes at 106°C at 90% of proof stress (60°C was found to be the critical temperature below which the time to failure increased steeply with decrease in temperature). Potentiostatic studies carried out at 106°C under stressed condition revealed that stress corrosion cracking could occur at controlled anodic potentials in the active-passive zone and not at controlled cathodic potentials. In the pH range of 6 to 9, the time to failure remained almost independent of pH. At a pH of 10, no failure occurred indicating the limiting pH value around 10. In all the above studies, the current-time behaviour showed that stress corrosion failure was associated with typical current-decay behaviour. The strain rate had no influence on the cracking process. The fracture mode was predominantly intergranular. These data in ammonium nitrate medium indicate that the mechanism involves the following stages: i) film formation by anodic passivation; ii) stress-induced film rupture; iii) stress-induced anodic dissolution and crack initiation; iv) repassivation; v) stress-induced film rupture and anodic dissolution at the tip of the crack and vi) crack propagation by stress concentration effects at the tip of the crack.

Item Type: Thesis (PhD)
Uncontrolled Keywords: corrosion cracking; polarisation
Subjects: Corrosion Science and Engineering
Depositing User: Dr. N Meyyappan
Date Deposited: 12 Jun 2012 07:23
Last Modified: 12 Jun 2012 07:23

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