Abstract
A comparative study of crevice corrosion resistance has been conducted in elevated temperature seawater for five different Ni-Cr-Mo alloys. The crevice initiation and propagation behavior of Alloy 625 has been compared to four higher grades of Ni-Cr-Mo alloys, namely Alloy 276, Alloy 59, Alloy 2000 and Alloy 686. Each of these alloys demonstrated a susceptiblity to crevice corrosion in elevated temperature seawater, although the crevice corrosion resistance of each of the higher grade alloys was superior to Alloy 625. A relative ranking is presented for each of these alloys for elevated temperature seawater crevice corrosion resistance.
Crevice corrosion experiments were conducted using fluoroelastomeric (FKM) gasket crevice formers, and it is shown that two apparently comparable FKM gasket materials evoke dramatically different crevice corrosion initiation behavior from Ni-Cr-Mo alloys under test. No elevated temperature seawater crevice corrosion initiation was observed for any of the Ni-Cr-Mo alloys in this study while using crevice formers from one of these FKM materials, even under polarization conditions that led to severe corrosion of the boldly exposed (uncreviced) surfaces. Improvement to the crevice initiation behavior of Alloy 625 using the inhibitive FKM gasket material persisted for several weeks in elevated temperature seawater. A residue deposited by the inhibitive FKM gasket onto the surfaces of Ni-Cr-Mo alloy specimens is characterized, and the extents of crevice inhibition afforded by this gasket material are explored. The relative improvement in Alloy 625 seawater crevice corrosion resistance afforded by the unspecified properties of this FKM gasket material are more dramatic than the relative improvements in crevice corrosion resistance observed for higher grades of Ni-Cr-Mo alloys evaluated in the present study.
