Papers by Keyword: Intergranular Stress Corrosion Cracking

Abstract: The present study was conducted on sensitized 304 stainless steels in 0.5mol/L Na₂S₂O₃ solution under the slow strain rate testing (SSRT). To monitor the evolution of intergranular stress corrosion cracking (IGSCC) occurring, acoustic emission (AE) technology was applied in this test. Besides, signal characteristics of different AE sources, including the dissolution along susceptible grain boundaries, the plastic deformation ahead of the crack tip and the mechanical fractures of material ligaments, were identified distinctly with fractography and clustering analysis.

Abstract: A number of mechanisms have been proposed to understand stress corrosion cracking (SCC) of metals, e.g. (1) slip dissolution and active pass corrosion based on anodic dissolution of metals, (2) tarnish rupture and internal oxidation based on oxidation ahead the crack tip followed by cracking of the oxides, and (3) hydrogen cracking, etc. If dissolution of metals takes the essential role in the stress corrosion cracking concerned, cracking susceptibility is expected to be significantly affected by dielectric constant of water. Because dielectric constant represents a character of water as a solvent, which determines solubility of metal oxides, and therefore corrosion rate of metals is strongly dependent on dielectric constant of water. K-constant type SCC growth rate tests have been done as a function of physical property (dielectric constant) of water by either manipulating temperature under iso-pressure condition (15MPa) or manipulating pressure under iso-thermal condition (330oC). Intergranular cracking was more enhanced and the crack growth was significantly accelerated under the condition of higher dielectric constant, indicating that dissolution of metal plays important role in the cracking mechanism of 316L stainless steels under the present testing conditions.

Abstract: The effects of cold working and heat treatment on caustic stress corrosion cracking (SCC) of mill annealed (MA) alloy 800M in boiling solution of 50%NaOH+0.3%SiO2+0.3%Na2S2O3 were investigated by means of microstructure examination, tensile test, X-ray stress analysis, SCC testing of C-rings, Auger electron spectroscopy (AES), scanning electron microscopy (SEM) and metallography. The microstructure of alloy 800M under tested conditions was austenite. With a strain of 25% by cold working, the grains of alloy 800M became longer, yield strength (YS) and ultimate tensile strength (UTS) increased, elongation (δ ) decreased, residual stress and the susceptibility to SCC increased. With increasing temperature of heat treatment of alloy 800M with cold working, the grains became bigger , residual stress, YS and UTS decreased and δ increased, the susceptibility to SCC of alloy 800M decreased. In boiling caustic solution, SCC cracks on the surfaces of C-ring specimens polarized potentiostatically at –20mV/SCE initiated from pitting and propagated along grain boundaries. AES analysis indicated that the surface films on MA alloy 800M were enriched in nickel and depleted in iron and chromium.

Abstract: Thermo-mechanical processing of type 304 and type 316L stainless steels done by (a) cold rolling to a reduction in thickness of 20 to 80 percent and (b) solution annealing to obtain a medium size of grains led to a considerable improvement in resistance to both sensitization and intergranular corrosion. The nature of the resultant grain boundaries was examined in a scanning electron microscope using orientation imaging microscopy in electron back scattered diffraction mode. Fraction of random and special grain boundaries were established for each set of thermo-mechanical processing. After appropriate sensitization treatments, the degrees of sensitization of these stainless steels were evaluated by double loop electrochemical potentiokinetic reactivation tests. Standard ASTM tests were used to evaluate susceptibility to intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC). These studies showed that a particular combination of thermomechanical processing led to formation of over 75 percent random grain boundaries in the steels and this imparted resistance to sensitization and to IGC and IGSCC. This opens a new concept in grain boundary (GB) engineering of a high fraction of random GB increasing the resistance to localized corrosion like IGC and IGSCC. Textural studies were carried out with the help of X-ray and MTM-FHM software. It showed significant change of texture in type 304 stainless steel, while no change in the texture of type 316L stainless steel after cold rolling and annealing.