Based on the Ellingham Diagram Ca oxidation has a more negative Gibb's than Al oxidation across all temperature ranges. Logically, one would assume that Ca would dross out on the mixture first. However, all material I've come across indicate that Al acts as a sort of "oxidation shield" for Ca in Pb-Ca-Al alloys. Does anybody know why this is?
In contrast to CaO, Al2O3 is corrosion-resistant (CaO reacts with H2O and CO2 from the air and can be easy removed). I think that your Ca really first oxidizes, but then, for example, corroded in the sample preparation by contact with water; The next layer of Al2O3 is retained and comes to the front. This aluminum oxide layer protects remaining CaO from corrosion with water.
Thank you Mr. Verlotski, I'm actually working with the alloying of the metals. Excluding the Elingham diagrams, the data I have from actual runs does match the literature. I apologize if I was unclear on this.
Hi Emmanuel,
Al react with O2 to form Al2O3, which is a stable (thermodynamically) passive (protective) layer and it prevents the further corrosion underneath. A good example is Cr in stainless steel; Cr (12-18%) react with O2 and form Cr2O3 passive film over the surface and it prevent further corrosion.Hence people call it as stain less -steel, even though Fe is readily active to corrosion.
Similar discrepancy is observed in the case of Mg and Ca. According to the Ellingham diagrams Mg should oxidize first. In practice however, it is observed that Ca oxidizes first even in molten alloy containing both elements. In fact, this has open up a possibility to melt magnesium without protective gasses.
The simple explanation is that Ellingham diagrams are plotted considering only binary systems, i.e. oxygen-magnesium, oxygen-calcium. When the actual system involves all 3 together (oxygen, calcium and magnesium) the activities changes, and the respective positions of the free energy plots of magnesium and calcium.
This may be the explanation in your case as well.
The concept of "oxidation shield" is to prevent calcium to be in contact with air. If you do this shield, in this case, you eliminate the calcium oxidation Gibb's free energy out of the story and this is the technology art. Now, based on patent EP0079765 B1by Raymond David Prengaman (free on Google), in making the Pb-Ca-Al alloy without using of inert atmospheres or flux covers, this concept "oxidation shield" workout as "melting lead, heating the molten lead at a temperature from 549 to 660°C and stirring into the heated molten lead an eutectic calcium-aluminum alloy having an average content of 73% by weight calcium and 27% by weight aluminum". The calcium-aluminum eutectic can be alloyed at temperatures as low as 549°C formed by simply melting aluminum and thereupon adding the calcium". In this case, aluminum in the eutectic alloy protects physically the calcium from oxidation during alloying and only about 1.6% of calcium dross out as a result of surface oxidation. Therefore, there is no need to think about Ellingham Diagram as much as how to minimize the alloying kettle top surface area and the process time. It is the melting and casting process trick-like to overcome the calcium is in contact with air.
Best regards
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