Evaluation of the pitting corrosion for aluminum alloys 7020 in 3.5% NaCl solution with range of temperature (100-500) °C

Abstract

The effect of homogenous heating variables (temperature and time holding) on the tendency of high strength Al-alloys type 7020 is investigated. The experiments were carried out by heating the specimens at different temperatures (100, 150, 200, 250, 300, 350, 400, 450 and 500) °C for two periods of holding time (one hour and two hours). Microstructure of an alloy was investigated by the optical microscope before and after heating at each temperature and precipitation phase and clearing revealed MgZn 2 . An immersion test was applied on specimens after heating to show the reduction in weight. Polarization technique was used to study the effects of microstructures on their electrochemical behavior in 3.5% NaCl solution. Microscopic and X-ray examination were used to reveal pitting corrosion on the microstructure of non-heated and heated 7020 Al-alloys. The results show that increasing the temperatures of heating from 100°C to 500°C caused the appearance of pitting corrosion on the precipitated phases and increase of pitting corrosion tendency with increase of temperature of heating; increasing time holding of heating from 1 hr. to 2 hrs. increases the pitting corrosion.

Effect of Friction Stir Welding Parameters on Microstructural Characteristics and Corrosion Behavior of Aa7020-O Based on Taguchi Doe Approach

“…For all samples, it can be observed that pitting initiation cites are having correlation with cites of intermetallic precipitates and greatly affected by their distribution and size. Such observation was also observed by Mosleh et al [12], Alsamuraee et al [18], and Bousquet et al [19]. This observation suggests the formation of corrosion cells where the precipitates, of less Ecorr, acts as a sacrificial anode to the Al matrix, of higher Ecorr.…”

“…Precipitation hardening for the T4-condition samples was achieved by natural ageing at room temperature for more than 30 days to reach a stable condition [5]. For the T6-condition samples, precipitation hardening was achieved by artificial ageing at 165±5 o C for 10 hours to obtain optimum strength as reported by Wojcirch [18]. A sample from each experiment trial was cut for microstructure investigation.…”

“…To balance the positive charge produced by reactions 3and 4, chloride ions will migrate into the pit. The resulting HCl formation inside the pit causes accelerated pit propagation [18]. Also, galvanic cell corrosion between oxide film covered surfaces and uncovered pits surface, helps in the acceleration of the corrosion.…”

“…Other investigations [18,19] have related the corrosion severity with the size and density of intermetallics and intergranular precipitation, especially when the predominant corrosion mechanism is pitting corrosion. Where coarse precipitates of high density increase the galvanic corrosion, resulting in a highly localized pitting corrosion and hence, increase the CR.…”

Study of SiC/graphite particulates on the corrosion behavior of Al 6065 MMCs using tafel polarization and impedance

“…Hybrid composite shows lowest I corr due to formation of oxide film and presence of low percentage of graphite particles being anodic to the matrix [8,28,29].…”