Stress corrosion cracking of prestressing steel in sulphide medium

Stress corrosion cracking behaviour of colddrawn and stress-relieved prestressing steel of standard quality conforming to IS. 1785- Part I has been studied in H2S saturated aqueous solution with and w~thout chloride. The steel was found to be highly susceptible to cracking even under open circuit condition at room temperature.The suscept~b~litdye creased slightly with temperature in the range 3O0C- 80°C. 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 3.5PbNaCI- H2S system was found to be 9. Cathodic polar~sationd ecreased the time to failure while anodic polarisation increased the time to failure. Application of tensile stress was,found to increase the corrosion rate and also the solubility of hydrogen. The strain rate was found to have profound influence on the cracking process and the susceptihlity was h~gha t very low stram rates. SEM fractographic studies revealed that the fracture mode was predominantly intergranular. All these observations could be understood in terms of a decohesion mode

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

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

Studies on manganese phosphating of steel

A new manganese phosphating bath which produces coating relatively at lower temperature within a reasonable time by the use of chemical accelerators bas been developed. Bath formulation and operating conditions have been optimized by coating weight determinations. Corrosion resistance property of the resultant coatings has been evaluated in 3% NaCI by electrochemical methods such as potentiodynamic polarisation, linear polarisation and impedance measurements. Results of the electrochemical techniques have heen complemented by salt spray, humidity and immersion tests. Porosity coatin~ has also been studied. Results show that the newer manganese phosphate bath gives good coatin~ at 353 K within 30 minutes. Addition of nitrates of various metals as accelerator produces thicker coatin~. Corrosion studies have shown that the corrosion resistance of the resultant coatin~ are much superior than the conventional coating;

Cathodic protection of concrete structures using magnesium alloy anode

Cathodic protection of concrete structure is gaining more and more importance. Though the impressed current system is being widely used, sacrificial anode system is also gaining ground. It is well known that alloys of zinc, aluminium and magnesium are the candidate materials for use as sacrificial anodes. However, very little work has been done for evaluating their utility as compared with that done for utilizing impressed current system. In the present work cathodic protection of embedded steel reinforcement bars has been achieved by utilizing a suitable magnesium alloy anode, which is manufactured based on indigenous technology (CECRI know-how). The potential of the embedded steel and the current flowing between the anode and the embedded steel were monitored over a period of one year, in conventional M20 (1:2:4 WIC 0.5) concrete. The results indicated that there is a considerable shift in potential in the presence of chloride. The steel was found to be well protected from corrosion, noticed after breaking open the tested concrete. The anode characteristics were evaluated at the end of the test period and the data obtained prove that the cathodic protection system utilizing magnesium alloy sacrificial anode is suitable under specific conditions

Inhibition of corrosion of mild steel by ammonium pyrrolidene dithiocarbamate in 1 m hydrochloric acid

Inhibition of corrosion of mild steel in 1 M hydrochloric acid solution by Ammonium pyrrolidene dithiocarbamate (APDe) was evaluated using weight loss, polarisation and electrochemical impedance techniclues. Uesults obtained through the above mentioned techniques indicated that inhibition efl"iciency (IE) increased with concentration of inhibitor, thus reaching a maximum efficiency of 95% at 1 x to-I M. The rise in temperature did not alter the corrosion rate of mild steel drastically at higher inhibitor concentration. The adsorption of inhibition followed Langmuir isotherm. TIle activation energies generally increased with concentration revealing chemisorption of inhibitor on the mild steel surface. Surface analysis by XRD and {IV-luminescence emission spectra were also carried out to establish the mechanism of corrosion inhibition

Relationship between Chloride/hydroxide ratio and corrosion rate of steel in concrete

Just as water/cement ratio is an important parameter controlling the strength of concrete, alkali/chloride relationship is an important aspect influencing the performance of steel embedded in concrete. In the past, several studies have been cam’ed out to determine the threshold limit for chloride and it has been established that the tolerable limit for chloride increases with alkalinity. However; in practice, the chloride level is invariably in excess of the tolerable limit and thus is bound to accelerate corrosion. No clear relationship between chloride/hydroxide ratio and corrosion rate of steel has been established so far This paper addresses this important aspect by carrying out some systematic long term evaluation studies. Two diflerent concrete mixes (lean/rich) and two difjerent exposure conditions (immersion/ atmosphere) were used in this investigation. Initial sodium chloride content in the concrete was varied in the range 0, 0.01, 0.05, 0.1, 0.5, 1 and 3.5% N by weight of cement. Corrosion rate was evaluated by conventional gravimetric method. The investigation revealed that even with adequate oxygen availability, the corrosion rate of steel embedded in concrete increases steeply with the Cl/OH ratio, and no such relationship is found to exist when the exposure conditions restricts the availability of oxygen

Quality assurance tests for corrosion resistance of steel reinforcement

Nowadays, for enhancement of durability of reinforced concrete (RC) structures in marine environment, different protective systems, namely coating to steel/concrete surface, corrosion resistant rebars, addition of inhibitor admixtures, etc are adopted. The efficiency of each system should be ensured by conducting appropriate quality assurance tests. This paper discusses the various test methods and presents the appropriate recommendations for corrosion resistance of reba

Synergistic effect of citrate ethylene diamine phosphonic acid and Zn2+ on the inhibition of corrosion of mild steel in low chloride media

Synergistic effect of citrate ethylene diamine phosphonic acid (CEDPA) and Zn2+ on the inhibition of corrosion of mild steel in water containing chloride ion of 60 ppm has been evaluated by weight loss, potentiostatic polarisation and electrochemical impedance spectroscopic methods. The weight loss studies have shown that neither the CEDPA nor Zn2+ has any inhibitive action individually but when they are combined together because of synergistic effect they give higher inhibition efficiency with increase in concentration. An optimal critical concentration of 200 ppm of CEDPA and 80 ppm of Zn2+ has been established to the maximum efficiency of 93 +- 2%. Potentiostatic polarization studies have confirmed this system acts as a mixed inhibitor. X-ray diffraction technique, UV-VIS-NIR spectra and luminescence spectra have revealed that this synergistic effect is due to the formation of Fe2+ -CEDPA complex on the metal surface at a critical concentration ratio of 2:5:

Novel resistance probe corrosion monitor for concrete reinforcement

Corrosion monitors using resistance probe as described in literature are not suitable when they are to be used for monitoring of corrosion in reinforcements. The monitor described herein is otthe direct digital display type. Along with the special type of probe, this is a valuable instrument for monitoring of corrosion of reinforcements embedded in concrete. With suitable probes this can be used in other types of corrosion studies also. The instrument being portable is quite useful for field application