Metal-Rich Coating Development for Fracture Mitigation in Al-Mg Alloys
Authors: McMahon Matthew E. , Burns James T. , Scully John R. 
Published: February 2020

The AA5xxx-series is a lightweight family of aluminum alloys utilized in marine vehicles [1]. Specifically, AA5456-H116 is solid solution strengthened by a supersaturation of the Al matrix with 5 wt. % Mg. However, mildly elevated temperatures can lead to precipitation of the highly anodic β phase (Al3Mg2) on the grain boundaries. This process is termed sensitization and is often quantified by the ASTM G-67 Nitric Acid Mass Loss Test (NAMLT)). In saline environments β dissolution can lead to severe intergranular stress corrosion cracking (IG-SCC). This behavior is governed by a coupled anodic dissolution process (of the β phase and under certain conditions the (α) Al matrix), which by the hydrolytic acidification mechanism catalyzes formation of an aggressive, locally acidified crack tip chemistry. Occluded acidification enables crack tip hydrogen (H) production everywhere the Al matrix discharges protons. This newly evolved H ingresses into the fracture process zone, thereby enabling embrittlement of the inter-β phase ligaments and increasing the likelihood for grain boundary decohesion. Such H effects enable crack propagation between discrete β phase particles by a hydrogen-assisted mechanism to drive crack growth though IG-SCC [2]. Mild cathodic polarization (to avoid β and α breakdown) has been shown to effectively eliminate IG-SCC in highly sensitized AA5083-H131 (NAMLT ≤40 mg/cm2) tested in 0.6 M NaCl [2]. The aim of the current study is to evaluate the efficacy of this mitigation strategy under more aggressive conditions; specifically, for AA5456-H116 (which has a higher bulk Mg content), at a higher NAMLT level (65 mg/cm2), as well as more aggressive solution chemistries that more accurately simulate marine splash and spray as well as alternate immersion service conditions. This assessment will be conducted first using potentiostatic control with an infinite cathode, followed by utilization of pure sacrificial anode materials as well as by newly discovered chemical inhibition mechanisms which inform development of metal-rich coatings for IG-SCC mitigation.
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) [3]. 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 [3]. 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.

Acknowledgments

This research was financially supported by the Office of Naval Research with Dr. Airan Perez as the Scientific Officer.

References

[1] N.J.H. Holroyd, G.M. Scamans, Environmental Degradation of Marine Aluminum Alloys—Past, Present, and Future, Corrosion. 72 (2016) 136-143.
[2] C.B. Crane, R.. Kelly, R.P. Gangloff, Crack Chemistry Control of Intergranular SCC in Sensitized Al-Mg, Corrosion. 72 (2016) 242–263.
[3] M.E. McMahon, J.R. Scully, J.T. Burns, Mitigation of intergranular stress corrosion cracking in Al-Mg by electrochemical potential control, JOM. (2017) Accepted.