Something in the back of my mind told me this wasn't a good idea. I googled something like "steel waste holding tank" and found the following at www.tec-tanks.com
. Caveat: this is a commercial site and they're selling polyprop tanks, but the following is consistent with my recollections.
Although you’ll see aluminium and stainless holding tanks, no metal of any kind should ever used to hold sewage. Urine is the most corrosive material that is possible to put next to any metal. If you are in doubt (ladies will have to take our word for it), notice the dividers between urinal stalls in men’s toilets. If that facility has been open for more than a week, no matter how clean and well-maintained it is, even though the dividers are stainless steel coated with enamel, you’ll see rust stains from the bolts that attach the dividers to the tile.
I'm not sure he's correct about "pee" being the most corrosive material -- hydrochloric acid is probably just a tad worse.