Does chemical reaction occur between steel and aluminum molten? If yes, how much is it? How can the amount of reaction be reduced?

I would expect problems if the steel impeller is in direct contact with molten aluminium. If you have a look at phase diagrams, you'll find intermetallic compounds between Fe and Al, and molten Al is rather reactive. Take this as an example: Difference between cold and warm chamber high pressure die casting machines is that the piston etc. used to press the melt into the mould is immersed in the liquid melt in the warm chamber machines - and while you'll find cold chamber machines for aluminium, to my knowledge you won't find warm chamber ones to process it, one major reason being the difficulty of finding a material that can sustain the working conditions in contact with liquid aluminium. However, I am talking about production machines here: For lab/research equipment, there might be solutions like coatings that would give you not eternal, but sufficient lifetime. To suggest which these might be, I leave to the experts. Wish you luck with your setup and experiments.

Dear Dirk

thanks alot

but , can you tell me which materials are suitable for coating ?!

Dear Esmaeil,

It is a general practice to melt aluminum alloys in graphite crucible at lab scale (R&D) because of its inertness to molten aluminum. Nevertheless, a congruent melting phase Al4C3 may form. Therefore, a carbon coating will work for your stainless steel impeller.

Dear Krishna

thank you

Dear Esmaeil,

This is the classic problem with the processing of cast particulate reinforced aluminum MMCs. Molten alumium reacts with all metals. For short times, one may be able to get away with direct contact. However, in your case, you have a multiple issues: long times in contact with molten alumium, abrasive particles and movement that will wear away your coating. With any coating, you will have a CTE mismatch with your impeller and you will not end up with a crack free coating. Softer or more elastic coatings with abrade away quickly. Your only choice is really to go with a ceramic impeller which will increase cost, limit impeller designs which are practical, and require great care in preheating, etc. I think alumina is your best bet. If you must try coating, I'd say that boron nitride is your best bet. There are some ZYP products in an aerosol that provide very good coatings. There is also a BN "Hardcoat" that can be brushed or sprayed on that may be more durable. In all cases, you will need to be careful to limit the speed of your impeller...this is not going to work for a "high shear" impeller. Regards

Dear Eric

thanks alot

Most kinds of stainless steel are severely embritled by liquid aluminium. Because chromium is able to resist corrosion by molten Al fairly well, the stainless steel type AISI 446, having high Cr content (~25%), performs better than less enriched grades. Chromium electroplating can be used to protect stainless steels from molten Al. However, hard ceramic particles suspended in the molten Al could easily deteriorate this protective layer due to erosion. Grey cast iron resists better to liquid Al than most stainless steels or low-carbon steels. This is explained by a graphite protective layer left when Fe is initially leached by the liquid Al. Another protective technique involves previous coating of steel or iron with Al by hot dipping followed by oxidation to form a surface layer of aluminium oxide which protects from corrosion by the Al melt (*).

Reference:

(*) Wang Deqing, Shi Ziyuan, Zou Longjiang, A liquid aluminum corrosion resistance surface on steel substrate, Applied Surface Science, 214(1-4), 2003, 304-311.