Galvanic corrosion behavior of dissimilar metal joint Zr/Ta/R-SUS 304 ULC in hot nitric acid

Corrosion behavior of an explosion-bonded dissimilar metal joint Zr/Ta/R-SUS 304 ULC in boiling HNO₃ solutions containing Cr⁶⁺ ions has been examined by an immersion corrosion test, polarization curve and galvanic current measurement, and a bent-beam stress-corrosion test. Transpassive dissolution followed by intergranular attacks occurred in the R-SUS 304 ULC part of the joint exposed to boiling 3 kmol·m^-3 HNO₃+1 kg·m^-3 Cr⁶⁺ and 8 kmol·m^-3 HNO₃+2 kg·m^-3 Cr⁶⁺ solutions, while no local attack was observed at the Zr/Ta and Ta/R-SUS 304 ULC interfaces. The corrosion rate of the stainless steel part was independent of the surface area of the Zr part. From the measurement of polarization curves for Zr and R-SUS 304 ULC stainless steel and galvanic current densities for a galvanic couple consisting of these two materials, it was confirmed that the Zr part acts as an anodic site and thus no galvanic corrosion occurs in the stainless steel part. Stress-induced corrosion including failure due to cracking was not observed on the dissimilar metal joint exposed to boiling 3 kmol·m^-3 HNO₃+1 kg·m^-3 Cr⁶⁺. In this solution, stress-corrosion cracking (SCC) occurred on Zr polarized at potentials higher than 1.5 V (vs. Ag/AgCl/3.33 kmol·m^-3 KCl). The corrosion potential of the joint never exceeded the critical potential for SCC of Zr, because it was primarily determined by electrochemical properties of R-SUS 304 ULC stainless steel and retained on the low potential side of transpassive region of the steel.