# Comparison of Two Intergranular Corrosion Tests on EN AW-6016 Sheet Material
###### Abstract  
Two different intergranular corrosion tests were performed on EN AW-6016 sheet material, an ISO 11846:1995-based test with varying solution amounts and acid concentrations, and a standard test of an automotive company (PV1113, VW-Audi). The average intergranular corrosion depth was determined via optical microscopy. The differences in the intergranular corrosion depths were then discussed with regard to the applicability and quality of the two different test methods. The influence of varying test parameters for ISO 11846:1995 was discussed as well. The determined IGC depths were found to be strongly dependent on the testing parameters, which will therefore have a pronounced influence on the determined IGC susceptibility of a material. In general, ISO 11846:1995 tests resulted in a significantly lower corrosion speed, and the corrosive attack was found to be primarily along grain boundaries. intergranular corrosion; Al-Mg-Si wrought alloys; (standardized test norms) corrosion test +
Footnote †: journal: Applied  
19 April 2021 28 May 2021 7 June 2021  
**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. **Published maps and institutional affiliations. **  
**C1. **  
**1. Introduction**  
Wrought aluminum alloys from the 6 xxx alloy group are extensively used aluminum materials in automotive and other engineering applications [1]. They can be used in the form of extruded profiles, forged parts such as suspension arms, or sheets for car bodies [1, 2]. They have substantial mechanical properties while having good corrosion resistance. Alloys from the 6 xxx group belong to the heat-treatable alloys and depend on proper thermal treatment to achieve their mechanical properties and corrosion resistance. Intergranular corrosion (IGC) is the preferred corrosion along grain boundaries while leaving the majority of the grain intact [3, 4, 5]. It is sometimes difficult to discover but can significantly reduce the stability of a material by reducing its mechanically effective cross section. The IGC susceptibility of an alloy is dependent on its composition and thermal treatment [4, 6]. For the ease of reading, the term \"grain boundary\" is used in the following to refer to said grain boundaries and the region close to the grain boundary that have a differing chemical composition compared to the grain center [2]. In 6 xxx Al alloys, compositional inhomogeneity along grain boundaries can lead to IGC [4]. The precipitation of Si on the grain boundaries especially promotes IGC due to the difference in the electrochemical potential of Si and Al with Si acting as a cathode leading to an increased dissolution of Al [7]. The type and shape of the surface near grain boundaries also affect the extent of IGC in 6 xxx alloys. If high-angle grain boundaries are dominant, IGC increases even further [8, 9]. Both effects are influenced by composition, plastic deformation, and temperature. Proper thermal treatment can minimize the effect of IGC by influencing the precipitation of Si and the shape of grain boundaries [4]. The EN-AW 6016 aluminum alloy, whose main alloying elements are Si and Mg (see Table 1), is a standard alloy used in the European automotive sector for body sheet parts [10, 11]. This alloy includes excessive Si above the Mg:Si ratio of 1.73:1 used for the formation of Mg\({}_{2}\)Si precipitates [12]. The additional Si enhances the mechanical propertiesof an Al-Mg-Si alloy. However, excessive Si is most likely precipitated as a pure Si phase which can have a negative effect on the IGC resistance of an Al-Mg-Si alloy [13; 14; 15]. There exists a variety of different IGC tests, some developed by industrial manufacturers [16; 17] and others developed by standardization organizations [18; 19]. In general, IGC tests often include the exposure of the material to an acidic solution. Experience and observation of materials during a lifetime need to be considered when designing IGC tests.