I have no idea where this should go, so move it needs be Its come up a lot of times and since Ill be using it on some of my own parts in preparation for anodising I decided to throw this up here. Consider this a test on my behalf as I needed to figure out submersion times for other work. The chemicals are dangerous but can be bought easily. I wont go into the chemistry of it here, you can read up on it if you want to, its pretty basic but the chemicals pretty nasty but like everything if your careful its ok. One thing, nitric acid will eat steel containers after a while so pick your storage vessels/containers with care. You will need caustic soda(drain cleaner) and nitric acid (70%) This works great for cleaning alloy castings. The idea is, the caustic soda will erode the alloy very slowly, and also any gunk that is sitting on the alloy. But, it will not erode the copper and other stuff in the alloy so this stays on the surface as 'smut', its like black power and is pretty hard to get off after the caustic soda stage. Thats where you need the nitric acid, this will erode everything on there but the aluminium, leaving you with a clean new looking part, and extremely close to the original dimensions where machined faces are concerned. The benefits of this method is total cleaning, that is, the solution can get everywhere unlike media blasting. Its also safe for cleaning complex parts that contain oil ways or air ways where trapped media could be a disaster. There is also no work/scrubbing involved doing this, just dipping times. Anyway, onto the job. The tin pictured here is about 1.5litres. I added to that around a half inch of caustic soda. The gearbox bracket on the left is fairly dirty and grubby as you can see, Add the caustic soda to the water stirring all the time, it will get hot but wont boil, leave your part in slowly, it will fizz like hell and give of hydrogen which is flammable so do it outside, Give it around 20min in there or until its all dark grey/black, lift out and rinse off well with water, use gloves... Rinsed, lots of smut present on part, Next onto the acid, the acid I bought was 70%, this needs to be diluted with an equal amount of water, ALWAYS add the acid to the water. It will also get warm when you do this. Mix in a high density polyethylene container, you'll see HDPE marked on some containers, use those ones as the plastic will hold the solution for 3 or 4 yrs. Dont use steel as It will oxidise it and will not contain it, aluminium will be fine too actually, And left into the Nitric solution, do not get nitric acid or caustic soda in your eyes, its not good at all and will keep burning. The reaction is minimal here this time as Nitric wont oxidise Aluminium, only the other components in it = smut on surface. 15min is all it needs, And out, still looks dirty but this rinses off easy as the smut has now been removed chemically from surface and is just sitting there, Rinse off well and pour nitric acid solution back into your container, it can be re-used, the caustic soda can go down the drain, Once rinsed it will look like this> Not bad hey? And again, The chemicals used here are dangerous, use with extreme care and keep everyone away, wash down work, workplace, and tools with loads of water when your done Brian.