Effect of sulfur on stress corrosion cracking of Ni-base alloys in high-temperature water

Stress corrosion cracking (SCC) behaviors of nickel-base alloys 600 and 690 were studied in high-temperature sulfur containing solutions using reverse u-bend test specimens at 340degC. Effects of sulfur valence, sulfur concentration, alloy composition, and heat treatment were investigated. All cracked alloys showed intergranular SCC (IGSCC). SCC susceptibilities of sulfur species and alloys increased as follows: S2O32- (+2) < SO42- (+6) < HS (-2) < S4O62- (+2.5), Alloy 690 TT < Alloy 600 TT < Alloy 600 HTMA < Alloy 600 MA. Effects of the sulfur valences were not related to SCC susceptibilities of the alloys. Various heat treatments of alloys 600 tended to increase the SCC resistance. From the result of deposits and the surface-oxide-layer analyses indicated the reduction of SO42-, S4O62- and S2O32- ions to lower-valence sulfur species without reducing agents. These reduced sulfur species lead to the SCC and anodic dissolution of the alloys. Different reducing processes are suggested for each sulfur species. (author)