The Reaction of Solid Iron with Molten Aluminum

As one of the series of studies on hot-dip aluminum coating on steel, the mutual diffusion between molten aluminum and solid steels was studied. Holes with 10mm diameter were bored in the center of 16mm∅×15mm cylindrical steel specimens (0.02, 0.11, 0.45, 0.83% C steel, 2% Cr-1% Mo Steel, 13 Cr and 18-8 stainless steel), into which various aluminum alloys rods (99.99% Al, 98% Al, 98% Al+5% Si and 98% Al+5% Si+0.2% Be) with nearly the same diameter as the hole were tightly inserted respectively. These combined specimens were heated in vacuum at 680, 720, 760 and 800°C for 6 hours, then cooled, and the phenomena of mutual diffusion between the molten aluminum alloys and solid steels were investigated. The composition of aluminum and its alloys except 99.99% Al described above is the same as that of the molten bath used in the previous research on aluminizing.In the case of 99.99% Al and 98% Al, aluminum diffuses deepest into every steel, and the lower the carbon in steel, the deeper grows the tongue of Fe-Al alloy layer into the steel. With molten Al+Si and Al+Si+Be alloy, the tongue does not grow into each steel and an inhibiting action of Si and Be against diffusion of aluminum into steel, instead of which the tongue grows remarkably into molten aluminum is recognized. Moreover many lumps of Fe-Al alloy exist in aluminum and remarkable dissolution of iron into molten aluminum is also recognized.Generally, the diffusion constant of 98% Al to each steel is larger than that of other aluminum or its alloys and that of Al+Si+Be is minimum. The diffusion depth is minimum in 13 Cr and 18-8 steel. In the reaction of each carbon steel with molten aluminum, the activation energy increases rapidly with the increase of carbon content from 0.02 to 0.11%, but is nearly constant regardless of the more increase of carbon content. With 99.99% Al and 98% Al, the activation energy is about 40, 000cal/mol, with Al+Si alloy about 20, 000cal/mol and with Al+Si+Be alloy even much smaller.From these investigations the inhibiting action of Si and Be on the diffusion of aluminum into steel was comfirmed, but it is necessary to care that Si or Be accelerates the dissolution of iron into molten aluminum bath.