Dealloying Behavior in Ni-Cr-Fe Alloys As a Precursor to SCC in Boiling Caustic Environments

Abstract

Experiments have shown that the susceptibility of Ni-Cr-Fe Alloys to dealloying, and subsequently stress corrosion cracking (SCC), highly depends on alloy composition and electrochemical potential. Dealloying is defined as the selective dissolution of the more active elements in an alloy (e.g., Cr and Fe in a Ni-based alloy). A dealloying process results in a remnant nano-porous layer on the surface that could be a precursor to SCC by a mechanism known as film-induced cleavage (FIC) where cracks propagate through this layer with sufficient velocity to promote energy transmission into the underlying metal resulting in cleavage fracture. Presence of a threshold amount of a more noble element could theoretically suppress this phenomenon by surface diffusion. This was investigated further by previous researchers, and the percolation theory of materials was hypothesized that states that the more noble element in an alloy creates a percolating mass that would limit the ability to retain a connected pathway of the reactive elements from the surface given it reaches a certain threshold of the mass of the material. This percentage was estimated to be between 50-60% for most alloying systems and termed the parting limit. In this study, percolation theory and dealloying is further explored by exposing several Ni-based and Fe-based alloys with varying percentages of Ni to boiling caustic solutions (50% NaOH) and evaluating their dealloying behavior.As mentioned, dealloying highly depends on electrochemical potential and is believed to occur above a threshold potential known as the dealloying critical potential. The dealloying critical potential is defined as the condition at which a transition occurs from a “passivated” alloy surface to the sustained formation of a bi-continuous porous structure. It can be identified by running an anodic polarization scan and initially matching anodic peaks to a pH-potential diagram (Pourbaix diagram) to evaluate the potential dissolution behavior. Bench-top electrochemical measurements were conducted to find the dealloying critical potential and make a correlation between this potential and alloys’ composition. The figure attached shows the anodic polarization scan results for all alloys included in the study in 50% NaOH at 140°C. It was observed that for the samples with high Ni content (i.e., Alloy 600 and Alloy 690) there was only a single active peak, which indicates transitioning to passivation and inhibition of dealloying. All other materials were susceptible to dealloying. Electron microscopy techniques were used to analyze all the samples used in the figure after exposure to 50% NaOH at 140°C at open circuit potential (OCP) and during a potentiostatic hold at the dealloying critical potential. Scanning electron microscopy (SEM) imaging followed by energy dispersive X-ray spectroscopy (EDS) revealed varying degrees of dealloying, with low Ni-containing alloys showing greater severity.The role of dealloying as a precursor to SCC was also evaluated by utilizing U-bend samples for Alloy 690, Alloy 800, and SS 304. The findings were in-line with previous dealloying result; the alloys with Ni content below the parting limit, SS304 and the Alloy 800, both underwent SCC after 48 hours of exposure to 50% NaOH solution at 135 °C, while the Alloy 690 sample (~60Ni) did not show any signs of cracking. A peculiar phenomenon was also noted for alloys containing Mo where it is seen that their dissolution current is approximately two folds higher than their counterparts with approximately the same Ni content. For example, when comparing SS 304 and SS 316 or Alloy C-276 and Alloy G-35. A possible explanation for that is the electrochemical behavior of Mo in caustic conditions where it is shown in its pourbaix diagram that dissolution occurs over a wider range of pH and potential in comparison to the other alloying elements. This earlier dissolution can possibly create more terrace sites on the surface that could accelerate the dissolution of iron and chromium. However, further studies are needed before a conclusion can be reached. Further work has also been conducted on Alloy 800 under the same conditions to evaluate the possibility of controlling the geometry of the dealloyed layer. The results so far indicate that the time of exposure and the electrochemical potential are critical in controlling the geometry of the dealloyed layer. Parallel studies are being conducted on SS 304 to evaluate the effect of Pb on extending the dealloying regime. The results so far have shown that the addition of Pb would shift the OCP more positively towards a region of passivity. It was also noted that dealloying kinetics is hindered in comparison to pure caustic conditions. However, SCC may still be possible with dissolved Pb present. Figure 1