"pot metal" identification ?

[jeffscarstrucks]

I know this will sound kind of basic when it comes to identifying metal types but I have run into this issue several times over the years. I am involved in the restoration of a large aluminum boat and have done many repairs and fabrication projects on it. Today the window frame of a forward room was being refinished when a break was discovered. No prob, I'll take it home and fix it in 3 minutes, right? Nope. It looks, feels, drills and grinds like aluminum but when you hit it with some heat it almost burns up. I leaned on it a little and stuck some al filler in it and even though it would melt, it sat on top of the parent piece and would not fuse at all. I've battled some old dirty al castings before but this was not the same. I wrote it off as "pot metal" which is a term that I don't fully understand. I know it is a cheap casting of tin and other low quality fillers but back to my original question: How can you tell the difference between this and a 30 year old cast aluminum piece in the field before you spend time on it or alter a repair plan to suit the situation? Thanks, JEFF

[paulrbrown]

Yesterday 11:22 PM
jeffscarstrucks
you are confusing Tin with Zinc, Tin is a very expensive metal which used to be used to coat steel cans to prevent corosion. Zinc is what most people refer to as 'Pot Metal'. Zymac is another metal found in marine castings, it is ok for fresh water, but salt will eat it up right now. Probably the best way to ID zinc is looking for the peculiar corrosin it gets, if you have seen it, you know what I mean. Zinc is also what is used to galvanize steel, and melts and fumes so well when heat is applied, making welding so much fun....I just remembered, putting Muriatiac acid on zinc will cause it to disolve very fast, you can strip galvanizing off bolts in 5 to 10 seconds. So just dab a bit on and if it is aluminum, it will not react with a foaming vengence as zinc does, it is also great for removing rust from steel, but neutralize with baking soda, 1lb to 1 gal water. Hope this helped, Paul

[jeffscarstrucks]

Thanks Paul, This is a great answer. I have dealt with the zinc issue as it is related to galvanized material but never put the two together in terms of castings. I did try to clean the joint with Alclean (a phosphoric acid cleaner by Arcair) and it didn't react like it normally does on a heavily oxidized Al surface. It really didn't do anything so I'll try it on a galvanized piece to see if it should have the same effect as muratic will. This boat is going back in the water in a couple of weeks and to the British ****** Islands the first of December so my part is about done. All of these parts will be epoxy-primed and painted so the salt environment shouldn't be an issue in the end. I'm glad you mentioned the salt corrosion issue; I passed it along to the painter this morning! The corrosion that is particular to zinc castings-Is it an almost white looking substance that makes the metal look "grainy" inside when you break it apart. Once again, thanks for taking the time to answer, JEFF