Does my cactus have a disease and can I save it?

Hi, I’m a new member after a bit of advice. I was given this cactus about 15 years ago (it was tiny then) and have sadly mostly neglected it. It’s been repotted a couple of times though I now know it’s in far too big a pot! A few years ago it got a couple of brown and blackish patched which I pried off as advised by a friend. It’s seemed better since though has got the odd patch and grown very tall and thin. Today I noticed it’s got more patches and a weird grey section. Any ideas what I can do? Not entirely sure what it is. Possibly a saguaro cactus, though also thought possibly a torch cactus. Basically, I know nothing and hope someone can help.

 

I've seen worse.
It actually looks ok, mostly.
Firstly I dont think it needs repotting.
If the discoloured area is still firm I'd guess that is more likely scarring from an insect attack. If you want to cut that damaged stem off you can, the stump will probably reshoot.

I wouldn't like to guess the species, but I will say its not a Saguaro, it looks to me to be a thin growing type of Cereus.

 

In a smaller pot it would be more likely to fall over. I tend to use a grittier compost mix then you appear to have.
Overall doesn't look too bad to me.
This is my entry in the battered cacti stakes.
I have no idea what species this is. Most of the damage is due to sunburn, the reason for the shape is that a slug grazed off most of the top and then two years later this regrew. It flowers throughout the summer (white short lived). It has lived in an unheated greenhouse for the past 5 years due to it's tendency to mealy bugs

 

Thank you, I’ll try that. And yes, not quite the exotic, Mexican desert cactus I suggested . I think it might be a blue column cactus which I think is a cereus

 

You can’t keep a good cactus down! Re. My poor neglected specimen, I have some cactus compost so will try adding some of that with some extra grit. Thank you for the advice!