Hello friends, I wish to bring up this topic about reaction of Sodium Silicate with molten aluminium.

Many of us might have come across the point that Sodium Silicate is corrosive to Zinc, Aluminium, Tin, lead etc in moist environment. But we also know that Sodium Silicate is the most commonly used binder in CO2 sand castings. I now want to know that is the Sodium Silicate based binder sand mixture suitable to be used only on Ferrous castings or we can use it on Non Ferrous too with the help of some other ratio of mixing?

I also want to share here an interesting observation which I had made a few years ago when I was working with Refractory paints used on Sand Moulds to prevent metal sticking and porosity etc. At that time, we were making one such Forging lubricant which was Graphite and water based. At that time a renowned caster had asked me if our application could help them get rid of blow holes on one of their castings which used sand core and was Gravity Die Cast Aluminum Alloy LM24. We had made several attempts on coating the sand core with various ratio coatings of our Graphite lube. But it didnt give us desirable results. Then we started experimenting with a new Slurry/Coating which we made using Sodium Silicate instead of our other binder. This gave us disastrous results. Since the coating was water based, even after sufficient baking, on pouring of Molten LM6, the sand core/coating reacted heavily with the molten metal, giving lot of scary bubbling even seconds after pouring and risering was complete. On release we found huge pores, rather caters on the casting.

This above incident scared me away from experimenting with Sodium Silicate on Aluminium Casting applications. But months later, when we ourselves required a suitable coating for our own Gravity Casting process, we began usage of the same Sodium Silicate on graphite based lubricants, but on a much milder dilution ratio, which was supervised by a very experienced technician.

So, now my question is, will Sodium Silicate binder based sand core cured with CO2 gas be suitable for usage in Gravity Die casting of Aluminium alloy LM6?

I would really appreciate if learned members or guests can throw some light on this topic.