EFFECT OF HEAT TREATMENT ON THE CORROSION BEHAVIOR OF Al-10%WT Mg ALLOYS

Abstract

In the present investigation, the effect of heat treatment on the electrochemical behavior of Al-10% wt Mg alloy in 3.5% wt NaCl is studied by different electrochemical techniques such as polarization technique (Tafel plot) and electrochemical impedance spectroscopy. It has been shown that the corrosion resistant of the alloy decreased with the ageing treatment at 250 °C for 24 h. However, this latter increased with naturally ageing. The SEM (electron microscope) and Energy Dispersive EDS characterization have shown that the alloy with the two treatments of ageing became susceptible to the pitting corrosion.Keywords: Al-Mg alloy; ageing treatment; corrosion; microstructure IntroductionThe addition of selected elements to pure aluminum enhances its; mechanical, physical, and chemical properties.The addition of magnesium to aluminium gives good hardness, good weldability and favorable corrosion resistance. Alloys in which magnesium is the primary alloying element are used in ship hulls, passage boards and other products exposed to marine environments because of their excellent corrosion resistance [1,2]. Non-heat treatable Al-Mg alloys (5xxx series) have better formability but; on the other hand, they are not precipitation hardenable, and that may limit their use in certain areas, and heat treatment provides alloys with interesting physical and mechanical properties [3]. Increasing the percentage of Mg in the aluminium matrix gives an alloy with good hardness, but very susceptible to intergranular corrosion and stress corrosion [2]. However, a percentage greater than 3%wt Mg in the aluminium α matrix favours the precipitation of the β (Al3Mg2) phase on the grain boundary and the latter is anodic with respect to the aluminium matrix and corrodes preferentially [4]. The corrosion behavior of Al-Mg alloys is controlled by the morphology and distribution of its precipitates on the grain boundary [2,3]. It was discovered that the kind, size, and distribution of the intermetallic phases located along the grain boundaries are regarded as the main factors responsible for the susceptibility to stress corrosion cracking [5]. A thin layer of invisible oxide forms immediately when the aluminium surfaces are exposed to the atmosphere, which protects the metal from further oxidation. Therefore the alloy becomes passive. But this behaviour can be interrupted by the existence of pitting corrosion, which produces a severe problem [2,6]. In the present work, the electrochemical behaviour of the Al-10%wtMg alloy was investigated at two different ageing treatments by means of polarisation curves, electrochemical impedance spectroscopy, characterised by SEM and EDS.

MORPHOLOGICAL, STRUCTURAL, MICROHARDNESS AND ELECTROCHEMICAL CHARACTERIZATIONS OF ELECTRODEPOSITED Cr AND Ni–W COATINGS

“…Corrosion behavior of the coatings was also studied through EIS test. As reported, increasing Rp signifies improvement of corrosion resistance [16,18]. Among the samples, the Ni-Cr coating depositing at 3 A.dm -² shows the maximum Rp value due to high value of Cr content of alloy.…”