Corrosion Rate after SSHT Before Decomposition
1.5 kg of Al—Mg 2.5% alloy was smelted from 1.462 kg of aluminum (purity 99.99%) and 0.038 kg of magnesium (purity 99.999%) in a graphite crucible in an induction furnace under a protective atmosphere. Magnesium and other alloying elements were wrapped up in Aluminum foil and plunged into already melted Al. The melt was poured out into a steel mould of 150×15×260 size. Before casting the melt was vigorously stirred by graphite rod. The same procedure (besides the alloy's composition) was used for smelting of all the mentioned below Al base alloys.
Casting stress relief annealing was carried out at 350° C. for two hours, cooled down to room temperature and then the strips were rolled in a duo rolling mill to a thickness of 3.5 mm. This annealing procedure is optional and may be performed to reduce internal stress and to homogenize the structure. SSHT of the strips was carried out in an electric batch type furnace with circulating air. The strips were heated up to 415° C., maintained at this temperature for four hours and quenched in water to room temperature. A rolling duo mill having a roll diameter of 300 mm was used for rolling the ingots with different rates of deformation.
The test samples had a size of 30 mm diameter and 2.5 mm of thickness. Aluminum samples were machined directly from the ingots while alloy samples were machined from the strips and later subjected to solid solution heat treatment, and optionally an artificial aging process. The artificial aging process was carried out in a batch furnace at 150-200° C., depending on alloy composition, under an air atmosphere (the specific temperatures used during the artificial aging process for each alloy are presented in Table III below).
The corrosion value, coulombic efficiency and polarization tests were carried out in electrochemical half-cells in 4M KOH at 50° C. The corrosion value at OCV and coulombic efficiency in galvanostatic experiments were measured by weight loss. Here and further all the potentials were measured vs. Hg/HgO reference electrode with IR drop correction. Before each test the sample's working surface was polished by the SiC abrasive paper grit 600, followed by a fine alumina suspension AP-A polishing.
The corrosion rate at OCV for Al and Al based alloys after solid solution heat treatment is as follows:
TABLE I Corrosion rate Alloy composition SSHT (° C.) (mg/cm2 · min) Al 99.99 — 0.81 Al 99.9 — 0.92 Al—Mg 2.5% 415 0.52 Al—Mg 3.8% 415 0.51 Al—Mg 6% 415 0.63 Al—Mg 2.7%—Si 0.7% 415 0.83 Al—Mg 2.1%—Ge 0.6%—Ga 415 0.85 0.3% Al—Si 1.2% 560 0.82
As shown in Table I, performing solid solution heat treatment for Al—Mg alloys having an Mg content of less than 4% results in a significant decrease in the corrosion rate in comparison to pure Al, as well as other Al based alloys. It was further found that the corrosion products of Al—Mg alloys having up to 4% Mg completely dissolve in an alkaline solution, and therefore, the working (corroded) surface of these alloys is smooth and clean. In contrast, it was found that the other alloys, including the Al—Mg alloy with 6% Mg, form a porous layer of corroded product on the working surface of anode. This porous layer can notably increase the anodic polarization, as will be shown below. Additionally, the corroded products may migrate into the electrolyte to form a very fine suspension, further disrupting the efficiency of the anode.