Grain-Boundary Chemistry and Intergranular Stress Corrosion of Iron Alloys in Calcium Nitrate

Intergranular corrosion and stress-corrosion measurements have been made on Fe alloys in calcium nitrate at 60°C. Tests were conducted on material with grain-boundary chemistries ranging from 0.23 monolayers of S with no P to 0.31 monolayers of P with no S. Grain-boundary chemistries were measured with Auger electron spectroscopy (AES). Subcritical crack-growth rate tests were conducted using compact-tension (CT) samples with acoustic emission (AE) monitoring on selected samples. Intergranular corrosion (IGC) and stress corrosion occurred in all materials tested within the passive regime at + 750 mV_SCE. Intergranular corrosion rates of about 10^–5 mm/s and crack velocities of 10^–2 mm/s were observed independent of grain-boundary chemistry. A stress corrosion cracking (SCC) threshold of $5MPam$ was determined, again independent of grain-boundary chemistry. Acoustic emission was detected during crack growth, but the results correlated best with fracture of ligaments or inclusions behind the crack tip rather than discontinuous crack jumps. Short atomic length or less than 10 μm jumps would not be detected by the AE monitoring equipment and therefore cannot be ruled out.