Hydrogen diffusion and trapping in a low copper 7xxx aluminium alloy investigated by Scanning Kelvin Probe Force Microscopy

Abstract

A B S T R A C TThe susceptibility to hydrogen embrittlement (HE) of the 7046 aluminium alloy (AA 7046) was investigated. Samples of AA 7046 corresponding to different ageing temperature/time couples were hydrogenated by cathodic charging in a H 2 SO 4 solution. Scanning Kelvin Probe Force Microscopy (SKPFM) combined with global hydrogen amount measurements allowed apparent hydrogen diffusion coefficients (D app ) to be measured: the decrease of the D app values with the increase of the ageing duration was attributed to hydrogen trapping by hardening η' and η precipitates for the aged alloy. Additional SKPFM measurements were carried out on hydrogen charged samples after desorption at 25°C and combined with SEM observations of the fracture surfaces after tensile tests. Results showed that hydrogen could be trapped at the grain boundaries leading to brittle intergranular fracture. However, hardening precipitates could act as efficient trapping sites and reduce hydrogen trapping at the grain boundaries. Conclusion is that the most critical microstructural parameters for HE of AA 7046 correspond to the grain boundaries while ageing could contribute to improve the resistance to HE of the alloy by a well-controlled precipitation.

Influence of hydrogen on electrochemical behavior of Ni-based superalloy 718

“…Mechanical tests performed on H-free and H charged tensile samples showed that hydrogen in grain boundaries induced intergranular rupture and hydrogen trapped at carbides and matrix/precipitates interfaces induced cleavage [2]. The fracture surface of the outgassed sample has shown only cleavage that reinforced the fact that hydrogen content after desorption treatment was mainly related to hydrogen trapped at matrix/precipitates interfaces and thus responsible for a large part for the OCP value ac cording to the work of Oger et al (33]. These results have shown that hydrogen was able to modify the electrochemical behavior of alloy 718 in relation with diffusible and trapped hydrogen content.…”

“…Indeed, the precipitates which act as trapping sites distort the lattice and/or create interfaces at which the hydrogen atoms will distort the lattice less. In this sense, the influence of hydrogen on OCP should give indica tion about the interaction between hydrogen and precipitates/ matrix interfaces in subsurface as recently shown by Oger et al [33]. Further, analysis of the OCP curves requires to consider H desorption during OCP measurements at 25 °C, as shown in Fig.…”

Stress Corrosion Cracking in an Extruded Cu-Free Al-Zn-Mg Alloy

“…This fits well with our observations. In recent years, the hydrogen distribution within Cu-free Al-Zn-Mg alloys has been studied and it has been suggested that the bulk precipitates are important in hydrogen trapping [41][42][43]. Motionless dislocations have also been demonstrated as important hydrogen trapping sites [44].…”

Hydrogen - dislocation interactions in a low-copper 7xxx aluminium alloy: About the analysis of interrupted stress corrosion cracking tests

“…These local defects can then evolve into cracks according to different types of damage mechanisms such as preferential dissolution of the grain boundaries for alloys susceptible to intergranular corrosion, stress concentration at the crack tip, with mechanisms assisted or not by hydrogen [1][2][3]. Concerning the 7xxx series aluminium alloys, many studies highlighted the effect of hydrogen during SCC mechanisms [4][5][6][7][8]. Classically, SCC behaviour of aluminium alloys is studied by slow strain rate tests (SSRT) in aqueous environment in order to exacerbate the role of the corrosive environment as well as the adsorption and the diffusivity/transport of the absorbed species [9][10][11].…”