IG-SCC is found to be mitigated in highly sensitized AA5456-H116 alloy by bulk solution electrochemical potential (Ebulk) control in a variety of environments using potentiostatic control, galvanic coupling at cathodic potential, and by added protective ion content. Specifically, fracture mechanics testing quantified the da/dt vs. K behavior in full immersion environments of 0.6 M NaCl (pH 5.6), saturated (5.45 M) NaCl (pH 6.2), 2 M MgCl2 (pH 3.4), and saturated (5 M) MgCl2 (pH 2.4). For each environment, testing at Ebulk from -0.8 VSCE to -1.1 VSCE resulted in at least a 2 order of magnitude decrease in IG-SCC growth rates as compared to IG-SCC severity at OCP (-0.8 VSCE) . The degree of mitigation was traced to the breakdown potentials for both the Al-matrix (Epit (α)) and the β (Epit (β)) in the bulk environment.
Significantly, these results suggest local pitting of the Al matrix proximate to the crack tip more prominently catalyzes metal ion hydrolysis to enable additional hydrogen ingress and embrittlement . Cathodic polarization established at the crack tip reduces pitting, which in turn slows hydrogen evolution. Zinc- and magnesium-rich coatings are potential candidates to inhibit Al matrix breakdown and mitigate IG-SCC. Therefore, the protective ability of magnesium and zinc on AA5456-H116 was assessed in galvanic coupling conditions. The magnitude of this potential depended on sacrificial anode pigment characteristics and exposure time, and both potential-based and chemical effects on the overall galvanic protection were evident. Therefore, the effect of solution ion content was tested by adding Zn2+ or Mg2+ions to dilute and saturated NaCl bulk fracture testing solutions. Ion protective ability depends on several factors. Moreover, several Mg, Zn, and Al primers demonstrated efficacy in supplying the necessary galvanic couple potential, which may protect against IG-SCC through galvanic couple and chemical effects.
Overall, mitigation of IG-SCC in highly sensitized AA5456-H116 in aggressive full immersion environments suggests that electrochemical potential based protection is a viable strategy with opportunity for use in alternate immersion environments. These findings are pertinent to inform the development of coating systems that aim to protect marine structures via metal-based pigments by sacrificial anode-based cathodic protection.
This research was financially supported by the Office of Naval Research with Dr. Airan Perez as the Scientific Officer.
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