Investigation on influence of sodium- or strontium-modification on corrosion-resistance of Al–11.7%Si alloy

Weight loss test in 4% sulfuric acid aqueous solution bath and salt spray test with 5% NaCl aqueous solution were used to evaluate the corrosion resistance of Al–11.7 wt%Si alloys with no modification, sodium modification, or strontium modification. SEM and optical microscopy were used to, respectively, observe the surface and depth corrosion morphologies of the alloys. The results showed that the corrosion-resistance of the alloy with no modification was the best, followed by the Sr-modified alloy, with the Na-modified alloy showing the worst results. Up to 360 h duration in 4% sulfuric acid aqueous solution bath, all the three alloys showed uniform corrosion characteristics. However, after 360 h duration, pitting corrosion became the typical characteristic of the modified alloys and it resulted in a considerable increase in percentage weight loss. After 480 h, the depth of the pitting holes in the Sr-modified alloy was larger than that in the Na-modified alloy, but the transverse corrosion channels in the former were far less than those in the latter. After this duration, the well-developed transverse corrosion channels in the Na-modified alloy became interconnected inch by inch, which led to severe exfoliation corrosion, so the percentage weight loss of the Na-modified alloy rose abruptly.