Toilet waste pipe rotting away? What's the best fix?

I like to think I am somewhat handy, but a little bit of knowledge can be dangerous.

A toilet on 1st floor has 3" copper waste pipe. The house is 45 years old? The bathroom is over a crawl space, so the waste line below is accessible. There's been a crack / split in the waste pipe. I had put non-hardening plumbers putty over the crack some time ago and stuck a plastic tray in the cawl space to catch any water that would come out of the crack when it flushes. I forgot about this for 1 - 2 years now. Checked recently and saw the tray was full (for a while, it seemed the water would evaperate as fast as it would collect in the tray. But the putty fell off and water has been building up in the tray. Here's a shot of the pipe last night:


So what is the right / cost effective thing to do? I took the toilet off the horn and took a picture: The rotting / crack / split is about 3" long from this view (I broke off a little tab of metal which made it wider, but would ahve gotten in the way during a fix anyway? :


I bought a waxless seal, hoping that it would extend below the crack, but the crack is about 3" long and the seal isn't that long. I bought epoxy putty - I was thinking I could squeeze it into the cracks from the in and outside... although the waxless seal may get in the way? Or would I want to use radiator clamp type rings to hold a piece of rubber over the crack from the outside? Or call a plumber to replace the toilet horn and some of the downpipe? Or do that myself with a roubber coupling to join the copper to PVC. Is that a DIY task? Any idea of the cost / number of hours for the plumber to do that? The toilet is off the ground at this point, so part of the work is done.

So is this crack (which has been there for years, but not sure if it's been growing) 'normal'. is the pipe rotting away and indicative of a bigger problem? THis crack is at the 'front' of the pipe / toilet - during a flush, does the front of the waste pipe take the full force of the water on it's way down the pipe / this is where it wears away from years of flushes? We are in NY - water is hard here, but we've had a water softener for 10 years now.

thank you!

thanks!

I wouldn't bother patching.
I would replace.

There may be something in the water, maybe clorine tablets in the tank, or even urine.

Thanks. Do you think it's a DIY task? And / or what the hours would be? Everthing on top / below is accessible. I don't want to spend loads of time and still have a leaker if it's only a couple? hours of a pro (never had a plumber or most any other pro work at my home so I am totally oblivious to time it takes a expert to do things and what it costs.

oh, and you mention this:

maybe clorine tablets in the tank, or even urine

you meant that to explain the rotting away? Does copper pipe normally rot away over 10s of years?

A copper toilet drain pipe.
What happens when you pee in your toilet at night, and you don't flush.

Copper can desolve over time.

pee and don't flush!? You been watching me? Gross as it may be, I (foolishly?) routinely don't do that in my bedroom bathroom - save water / lower water and sewer bill? I'm thinking the urine is staying in the bowl. How's that affecting the waste line till you flush? very acidic water at that point? and for the second or 2 of that very acidic water over time does that? Rather than 4 - 8 x as much volume of less acidic water (i'm not flushing 4 - 8 times that I pee?) that single slug of more acidic water is that much worse? Or just any water / pee over time is enough to do it?! Wow! I am thinking metals / copper are 'better' than cheaper / newer plastics? But what about water lines that are copper? no pee in them (hopefully?!) so tap water is OK? Old houses - when did indoor water lines start and what materials were they!?? they have copper water lines and are OK, right? but nothing lasts forever?

When you pee, it doesn't stay in the bowl, it drips down the drain.
Try pouring water into your toilet.
It only goes as high as the bend in the trapway, and then drips over the dam, so to speak.
When you don't dilute the pee with water, you have corrosive liquid on your waste line.

I never use copper drains in bars, they perforate very quickly, from the lemons and limes. Plastic lasts longer. You can always put chewing gum over the holes, but eventually, the pipe that the gum is stuck to won't even be there.

If you want to save water, you should think about getting something that flushes with less water.

DWV copper is super thin to begin with.

Urine, feces, food matter all have acidic bases.

Countless times I've seen in older homes that have cistern water is a thin line ate right through the bottom of the pipe in a straight line.

Why?

Users of the toilet would urinate in it 4-8 times a day and not flush to save water....that urine would trail over the trap level and eat right through the copper as Terry described.

I would replace as mentioned with PVC and be done with it.