TS-145
Influence of Microstructure on Seawater Corrosion of Al-Mg-Si Alloy A6N01
Chiofi TAKAHASHI*, Kazuyoshi MATSUOKA, Tetsuya SENDA, Noriyuki KOTANI and Fujio YANO.
ABSTRACT
Corrosion behavior of the industrially maufactured and experimentally produced materials of an Al-Mg-Si alloy A6N01 was investigated for a soak in synthetic seawater at 25℃ for one year. Microstructural analyses of the specimens were carried out by EPMA, TEM and XRD. Although the chemical compositions and mechanucal properties were within the limit prescribed in the standard (JIS), a wide variation of the microstructure was observed probably depending on manufacturing process. SEM observations of the surface after cleaning the samples revealed that the corrosion morphologies were categorized into three types including general corrosion, interranular corrosion and pitting with intergranular corrosion. The as-received materials were characterized by the detaled analyses of the second-phase particles and the precipiates in the aluminum matrix. The corrosion mophologies correlate with microstructural features probably because the local-action cell formation depends on the distribution of additive elements in the aluminum matri.
Key Words: Al-Mg-Si alloy, seawater corrosion, microstructure, corrosion morphology, intermetallic compounds, preferemial dissolution, extruions.
1. Introduction
Aluminum alloys are widely used in ship construction for their attractive characteristics; high strength to weight ratio, good formability, good corrosion resistance, and the others. The 6000 series alloys in particular have excellent extrudability to have the advantage for the further reduction in weight offered by using extruded products such as sections, shapes and so on. An international common standard of aluminum alloys for hull construction and marine structure was defined by IACS in 1998 [1]. It defined their grades and temper conditions, and the alloys, 5083, 5086, 6005A, 6061 and 6082, are covered by the Requirements for extruded productS. The alloy grades 6005A and 6061 of the 6000 series alloys, however, should not be used in direct contact with seawater unless protected by anodes and/or palm system. Some restrictions of use of 6000 series alloys have been mentioned in most rules of certification classifications [2], [3]. This is why that sometimes they exhibit the pitting with intergranular corrosion in seawater. A6N01 alloy is one of AI-Mg-Si alloys, developed in Japan for Shinkansen. Its chemical composition, very similar to 6005A alloy, is defined by Japan industrial standard (JIS), and it had been permitted to be used for ship, limited to the upper structure, before revision of NK rule as of July in 1999 [4].
In this paper the relationship between microstructure and corrosion behavior of an Al-Mg-Si alloy A6N01 was investigated. The materials were soaked in synthetic seawater for 6 months and one year, and the relationship between microstructure and corrosion behavior of the industrially and the experimentally extruded materials of Al- Mg-Si and Al-Mg alloys was investigated.
2. Experimental procedure
2.1 Materials
The specimens were prepared from 24 different extruded materials of a 5083 and several kinds of 6000 series aluminum alloys, as given in Table 1. The I-series specimens were taken from the industrially manufactured materials before shipment, which were provided by six aluminum companies in Japan. Therefore each material had been extruded into the different profile, and details of its temper designations had followed the limits as defined by each company. Most of A6N01 products, except for 6I-9 in the T6 temper, were in the T5 temper. All the E-series specimens were experimentally extruded in the same laboratory of a company.
The alloying elements of Mg and Si in 6000 series alloys combine to form the compound Mg2Si with a ratio of magnesium to available silicon of 1.73:1. Al-Mg-Si alloys are indeed considered to be quasi-binary alloys of aluminum and Mg2Si, and it is also known that Mg2Si and the excess of Si over and above that required to form Mg2Si contribute mechanical and corrosion properties of the materials. Thus the amount of the Mg2Si and excess Si (excess Mg for 6E-9) were also calculated as described in Table 1. Table 1 shows that I-series materials of A6N01 have very similar chemical composition to one another, although they were provided by different aluminum companies and the limits as defined by JIS for A6N01 are very broad for each element. There is no significant difference between 6I-series materials, except for Cu and Si content of 6I-6.
