Anatolie G. Carcea

Anodic Kinetics of NiCr[Mo] Alloys During Localized Corrosion: I. Diffusion-Controlled Dissolution

Artificial pit electrodes have been used to measure the pitting dissolution kinetics in the anodic limiting current region for a range of Ni-22Cr-xMo (wt %) alloys with x=0, 3, 6, 9 and 13. Most of the data were obtained at 25―90°C using I M NaCl solution. Type 316 stainless steel and pure nickel were used for comparison. At high temperatures the anodic limiting current density for the NiCr(Mo) alloys (x = 0, 3, 6, 9) is nearly the same as that of stainless steel, other things being equal; at intermediate temperatures the NiCr(Mo) alloys give higher i lim values. Changing the NaCl concentration between 0.1 and 1 M causes a 20% change in i lim at a given pit depth, and the reasons for this are discussed. Comparison of pure Ni and Ni-22Cr shows a 35―40% higher i lim for the latter at a given pit depth, and the reasons for this are discussed. Literature values of solubility and diffusivity, and a 1-D finite difference model, are used to rationalize ― in part ― the limiting current densities and associated average diffusivities in terms of concentration-dependent diffusivities.

The Study of Stress Corrosion Cracking on Alloy 600 C‐Ring Samples by Polychromatic X‐Ray Microdiffraction

Microscopic strains associated with stress corrosion cracks have been investigated in stressed C-rings of Alloy 600 boiler tubing. Polychromatic X-ray Microdiffraction (PXM) was used to measure deviatoric strain tensors and the distribution of dislocations near cracks that had been propagated in electrochemically-accelerated corrosion tests. Stress corrosion cracking (SCC)-generated intergranular cracks were produced in two Alloy 600 specimens after 6h and 18h tests. The diffraction patterns and resultant strain tensors were mapped around the cracked area to a one micron spatial resolution. The strain tensor transverse to the crack growth direction showed tensile strain at the intergranular region just ahead of the crack tip for both specimens. Both cracks were found to follow grain boundary pathways that had the lowest angle of misorientation. Dislocation distributions within each grain were qualitatively obtained from the shapes of the diffraction spots and the effect of “hard” and “soft” grains on the crack pathway was explored for both 6h and 18h specimens.

Apparent Inversion of the Effect of Alloyed Molybdenum for Corrosion of Ordinary and Enhanced 316L Stainless Steel in Sulfuric Acid

An aging assessment of the OPG waste resin storage system predicted the potential for premature failure of the carbon steel resin liners. Consequently, resin liners made of 316L stainless steel with a minimum content of 2.5% molybdenum were selected to replace the carbon steel liners. The 2.5% Mo 316L stainless steel was specified to enhance pitting resistance in the spent resin environment. With the additional Mo, one would expect that a brief electrochemical corrosion test will reveal the superiority of such alloy over conventional 316L steel. This study reports a contrary experience