Mitigation of Environmental-Assisted Cracking in Martensitic Stainless Steel via ZnNi Plating in Sodium Chloride Solution

The impact of galvanic coupling on the environmental-assisted cracking (EAC) resistance of a martensitic stainless steel (P675) was evaluated by the introduction of a ZnNi electrodeposit during fracture mechanics testing. The influence of ZnNi galvanic coupling on EAC behavior was studied under both constant stress intensity K (dK/dt = 0 MPa√m/h) and rising K (dK/dt = 4 MPa√m/h) conditions in 0.6 M NaCl. For constant K, the ZnNi anode was galvanically coupled to the steel via a zero-resistance ammeter, and both small and large area ratios of ZnNi to P675 steel were examined across different exposure durations. The results showed that even a small anode-to-cathode area ratio (∼1:1,000) was effective in reducing the crack growth rate, depending on exposure time. Shorter exposure durations (e.g., 90 min) led to reduced crack propagation, whereas longer exposures (up to 800 min) further mitigated EAC under small-area galvanic coupling conditions. This improvement was attributed to the electrochemical potential generated by the galvanic coupling, which remained within the immunity well throughout the immersion time. In contrast, the large area ratio (∼6:1) did not initiate EAC, even when the potential was outside the immunity well. This unexpected suppression of cracking is attributed to Zn2+ ions released from ZnNi, which appear to counteract the electrochemical driving force for crack initiation and propagation. To validate this, ZnNi coatings with small area ratios (∼1:1,000) were directly applied to the P675 steel surface, and fracture mechanics testing was conducted under rising K conditions. The results demonstrated that ZnNi continued to suppress EAC even under increasing crack-driving force, confirming that controlled galvanic coupling can effectively enhance crack resistance in martensitic stainless steel.