Environment-Induced Cracking of High-Strength Al-Zn-Mg-Cu Aluminum Alloys: Past, Present, and Future

Abstract

Environment induced cracking (EIC) during commercial use of aluminum alloys started over 125 years ago (mid-1890’s), some 45 years earlier than previously documented, with earliest failures for Al-Zn-Mg-Cu, 7xxx series alloys occurring a decade later. Needs for lighter, thicker and stronger alloy products, firstly driven by WW1 and WW2 militaristic requirements and subsequently by relentless demands from modern aircraft industry designers, resulted in major in-service EIC in commercial high strength Al-Zn-Mg-Cu alloys in the US and UK during the 1950’s, 1960’s and 1970’s. These were avoidable had research findings from France, Germany and Japan from the 1930’s and 1940’s been implemented. Unprecedented US Government R&D funding during the late 1960’s, 1970’s and early 1980’s led to AA7050 and similar alloys, that essentially eliminated EIC issues during commercial usage for several decades. EIC assessment for the following ‘new-generation’ high-strength alloys relied totally on standard ASTM Test Methods, incapable of providing data directly relatable to the service conditions. Although EIC service issues for the latest generation of 7xxx series alloys remains manageable, the premature appearance of EIC requires a quantitative understanding of EIC initiation under environmental and mechanical conditions directly relatable to intended use, to prevent un-expected failures for future alloys. Directions for future high-strength 7xxx series aluminum alloy development and EIC assessment to provide quantitative date relatable to service conditions and input for structural design and for service life prediction are discussed.

Corrosion behaviour of detonation gun sprayed cermet coatings on AA5083

“…Such a distinct potential plateau in the anodic region of AA5083 indicates a limited passivation behaviour and pitting occurred at −784 mV. Pitting corrosion occurred due to the dissolution of anodic beta phase precipitates (Al 8 Mg 5 ) present at grain boundaries of AA5083 [4]. As observed from the PDP curves, all the coated samples shift the cathodic region to a low current density region and at the same time, the anodic region of all the coated samples showed better passivation behaviour than the substrate.…”

“…Holroyd and G.M. Scamans [4] discussed all the corrosion-related issues on Al–Mg alloys. Despite their excellent properties, such as good weldability, formability, lightweight and moderate strength, 5xxx series alloys are highly susceptible to stress corrosion cracking and intergranular corrosion.…”

Comparison of strength, stress corrosion cracking and microstructure of new generation 7000 series aluminium alloys

“…The anodic dissolution mechanism involved the galvanic cell between matrix and the GBPs during the corrosion process. However, hydrogen generation was accompanied with anodic dissolution owing to galvanic reaction among GBPs, PFZ and adjacent alloy matrix [6,25]. Although there still exist controversy over the exact SCC mechanism in Al-Zn-Mg-Cu alloys, researchers believe that the SCC mechanism of 7000 series aluminium alloys is the combination of anodic dissolution and hydrogen embrittlement [19,26].…”

“…Recently, it was reported that the new generation alloys were designed to surpass the aviation industry specified stress corrosion cracking (SCC) and exfoliation acceptance tests. However, the new generation alloys have the high SCC susceptibility compared with traditional alloys with low Zn and high Cu content [5][6][7]. Particularly, European Aviation Safety Agency (EASA) [8] reported that there is potential SCC risk of the new generation 7000 series aluminium alloys in aggressive environment.…”

Enhanced SCC resistance of 7056 aluminium alloy by Y and Si additions

“…7xxx (Al-Zn-Mg-Cu) aluminium alloys have been widely used in the aerospace field due to their excellent mechanical properties [1][2][3][4][5][6]. However, the susceptibility to stress corrosion cracking (SCC) is a key issue limiting the development of new generation high Zncontaining 7xxx aluminium alloys [7][8][9][10][11][12][13][14][15][16]. At present, lots of investigations are focused on improving the SCC resistance of 7xxx series aluminium alloys by controlling heat treatment [8,[17][18][19][20][21][22][23][24][25][26] and reducing recrystallisation by forming new dispersions [27,28].…”