The Localized Corrosion and Stress Corrosion Cracking of a 6005A-T6 Extrusion Profile.

Jijun Ma,Jun Gao,Jun Wang,Qingwei Yang,Chengxiong Zou,Jinshan Li,Haisheng Wang,Jian Tang,Bin Tang,Quanmei Guan,Jing Sun,Hongchao Kou,William Yi Wang

doi:10.3390/ma14174924

Abstract

In the present work, the localized corrosion and stress corrosion cracking (SCC) behaviors of a commercial 6005A-T6 aluminum extrusion profile was studied comprehensively. The velocity of crack growth in self-stressed double-cantilever beam (DCB) specimens under constant displacement was estimated, which also provides insight into the local microstructure evolutions at the crack tips caused by the localized pitting corrosion, intergranular corrosion (IGC), and intergranular SCC. Characterizations of local corrosion along the cracking path for a period of exposure to 3.5% NaCl were revealed via optical microscope (OM), scanning electron microscope (SEM), and electron backscatter diffraction (EBSD). The typical features of the pits dominated by the distribution of precipitates included the peripheral dissolution of the Al matrix, channeling corrosion, intergranular attack, and large pits in the grains. The discontinuous cracking at the crack tips indicated the hydrogen-embrittlement-mediated mechanism. Moreover, the local regions enriched with Mg2Si and Mg5Si6 phases and with low-angle grain boundaries presented better SCC resistance than those of the matrix with high-angle grain boundaries, supporting a strategy to develop advanced Al–Mg–Si alloys via interfacial engineering.

Highlights

With the quick development of the high-speed railway and the operation of the China Railway High-speed service for more than a decade, one of the greatest challenges is the management/maintenance of these trains in environmental conditions [1,2,3]
Based on our previous investigations of the local corrosion features of the 6005A-T6 alloy [2], we found that the Mg/Si ratio is a critical indicator in the development of novel, advanced, corrosion-resistant Al alloys, since the Si–Al interface and primary α-Al matrix remained unattached, presenting much better intergranular corrosion (IGC) resistance than that of the 5083-H111 and 6082-T6 alloys
We found that those main cracking paths are almost all along the precipitationfree zones (PFZs), while the pits occur with a lot of discontinuous precipitates nearby

Summary

Introduction

With the quick development of the high-speed railway and the operation of the China Railway High-speed service for more than a decade, one of the greatest challenges is the management/maintenance of these trains in environmental conditions [1,2,3]. In order to support the foundations for interactive corrosion risk management, it is essential to reveal the foundations of environmentally induced cracking caused by corrosion [4,5]. Through increasing the concentration of Cu in Al–Mg–Si alloys, their strength can be improved [9], and the decomposition sequence can be changed by enhancing the number of possible phases and the complexity of the precipitation sequence [9,10,11]. The size and the morphology of precipitates play important roles in improving both the strength and the corrosion resistance of Al alloys [9,12,13,14,15]. The contributions of precipitation, high-angle grain boundaries (HAGBs), low-angle grain boundaries (LAGBs), and the hydrogen-assisted cracking were experimentally and theoretically evaluated, paving a path to the development of advanced, SCC-resistant Al–Mg–Si alloys

Materials and Methods

Microstructure Characterizations

The Crack Growth of the Pre-Cracked Self-Stressed DCB Specimen

Characterizations of the Localized Corrosion and SCC Microstructures

Pit-to-Crack Transformations and IGSCC

Conclusions