Hydrogen embrittlement and corrosion evaluation of Alloy 725 alloyed with B and Cu

Abstract

The need to achieve materials with higher strength and better corrosion resistance results in producing new alloys. Nickel base alloys are among the materials which have been subjected to several studies related to producing new alloys. Nickel based superalloys, in particular, drew a lot of attention. Nickel superalloys are durable materials in mechanical loadings while having good performance against corrosive media, simultaneously. In this work the effect of heat treatment and chemical alloying with boron and copper on hydrogen embrittlement (HE) and corrosion resistance have been investigated. The role of sulfur segregation at grain boundaries (GB) on HE of nickel is also studied. HE studies have been done in micro-scale by in-situ electrochemical micro-cantilever bending test. The cantilevers made either inside the grains to investigate the role of alloying elements and precipitates on HE, or on two grains to study the effect of GBs on the HE. By heat treatment of Alloy 725 in three conditions, the different effects of GB on HE is studied. One sample is solution annealed (SA) in order to remove the effect of any precipitates, another sample is aged (AG) based on the heat-treatment standard guideline normally used in industries and the last sample is over-aged (OA) to reach coarser precipitates. All the single-crystal cantilevers were made on three samples which were cracked under the H-charging condition, however, crack propagation was more severe in the OA sample. Bi-crystal micro-cantilevers bent under H-free and H-charged conditions revealed the significant role of the GB in the HE of the beams. The results indicated that the GB in the SA sample facilitated dislocation dissipation, whereas for the OA sample, it caused the retardation of crack propagation. For the AG sample, testing in an H-containing environment led to the formation of a sharp, severe crack along the GB path. The effect of boron and copper alloying on HE is also investigated in this thesis. Three samples were used. The standard Alloy 725 (Mod A) was used as a control group, comparing with the one alloyed with 250-350 wt%.ppm B (Mod B) and <100 wt%.ppm B + 2.3 wt% Cu (Mod C). Cross-sectional view of the bent beams taken by scanning electron microscopy (SEM) showed the superior resistance of Mod B against HE by facilitating the GB dislocation transfer/generation. While bending Mod A sample in hydrogen environment leads to form a sharp intergranular cracking, Mod B showed some nano-voids/cracks mostly in dislocation slip bands and rarely in GB path. However, a reduction of strength was observed in loaddisplacement (L-D) curves of Mod B. The addition of Cu, although not participated in GB segregation, compromised the lost strength of Mod B. In Mod C, after bending in H-charged condition, the nano-voids were formed in GB, but no load drop in L-D curves nor crack propagation in post-deformation observations was detected. The micro-alloying proposed in this study could be an important contribution to the future developing of H resistant alloys via GB segregation engineering. Sulfur is among the elements that have a detrimental effect on the mechanical properties of the metals if segregated at the GBs. The effect of co-segregation of S and H is investigated in this thesis on pure Ni. A pure Ni GB shows completely plastic behavior with no fracture observed in the experiments. Electrochemical H-charging of the sample with no S present in the GB leads to a crack formed at the notch tip, which propagates by means of the mixed plastic–brittle fracture mode. Cantilever testing of the H-charged GB with S results in a clear brittle fracture of the GB. The co-segregation of S and H shifts the sudden drop in the load–displacement curves to smaller values of displacement. Finally, intergranular corrosion passive layer properties of Mod A, B and C is investigated in this thesis. Intergranular corrosion test showed continuous corrosion at the GB in Mod A while Mod C remains completely intact without any corrosion attack. Mod B corroded in the areas around the Mo-rich boride phases formed due to abundant of B element in this alloy. Mod C showed the least defect density in the passive layer while the passive layer of Mod B thicker. The incorporation of Mo in Mod C was proposed to be the responsible for less defect density of Mod C compared to Mod B. Mod A on the other hand had the most defect density and shows higher passive current density in potentiodynamic polarization test.