Stress-Corrosion Cracking of High-Purity Iron in Ammonium Nitrate Solution

Sites of preferential corrosion of floating-zone-refined iron (99· 996%) in 5N-NH₄NO₃ at 75° have been investigated by transmission electron microscopy, and susceptibility of the iron to stress-corrosion cracking has been examined in constant strain rate tests. Electron microscopical observations have shown that individual dislocations or dislocation walls do not undergo any preferential dissolution. Instead, the corrodent dissolves the iron crystals predominantly along the {111} and {lID} planes. In polycrystalline material, quenched rapidly from above the A₃ point, both segregated and pure grain boundaries were attacked preferentially. The Iron was prone to stress-corrosion cracking under these conditions. Preferentialcorrosion of pure grain boundaries is attributed to their higher free energy and smaller activation energy for dissolution, as compared with those of grains.