THE EFFECT OF NITROGEN ON SENSITIZATION AND STRESS CORROSION CRACKING OF AISI 304 STAINLESS STEELS (SLOW STRAIN RATE, POLARIZATION CURVES, THERMODYNAMIC MODELING TESTS, AQUEOUS, ELECTRON-OPTICS)

Sensitization in the heat affected zones of AISI 304 stainless steel (SS) welded piping of boiling water reactors (BWR) has caused intergranular stress corrosion cracking (IGSCC) failures. New grades with reduced carbon levels, to prevent IGSCC, and nitrogen additions to improve strength, have been proposed as alternative materials, although no systematic study of the effect of nitrogen on SCC of these SS in BWR environments has yet been conducted.; This investigation involved both theoretical modeling and experimental studies. Thermodynamic and kinetic modeling of sensitization were carried out. Electrochemical potentiokinetic reactivation tests and analytical electron microscopy were performed to study sensitization behavior. Slow strain rate tests and scanning electron microscopy were used to study SCC resistance. Anodic polarization tests were also performed.; The results of the sensitization studies show that nitrogen up to 0.16 wt.% increases the chromium concentration at the grain boundary (X(,Cr)('i)) and decreases the width of the chromium depleted zone adjacent to the grain boundary, thereby lowering the degree of sensitization (DOS). However, carbon additions increase the DOS. The results of the SCC studies show that (1) nitrogen up to 0.16 wt.% increases the IGSCC resistance, but nitrogen of 0.25 wt.% causes TGSCC, (2) nitrogen up to 0.16 wt.% increases the critical potential (E(,SCC)) for SCC, while carbon additions decrease it, (3) the potential ranges at which IGSCC and simultaneous IGSCC, TGSCC and pitting occurred correspond to the passive range and the potentials above the breakdown of passivity on the polarization curves respectively.; The mechanism by which nitrogen retards sensitization is by increasing X(,Cr)('i) and hence retarding the growth of the carbides. The mechanism of IGSCC is film rupture followed by anodic dissolution of the chromium depleted areas along the grain boundaries. There is a good correlation between the SCC and sensitization studies, for example, E(,SCC) and the fracture mode at potentials above it are dependent on the DOS. Nitrogen of 0.25 wt.% is not beneficial because it causes discontinuous precipitation of chromium carbides and promotes a planar array of mobile dislocations.