mabagal wrote:If it doesn't look like the other ones, just replace it already.
I would agree with this, assuming there is also no damage to the wheel rim and stud. It doesn't seem like a biggie at this point from the images I have seen.
To be honest, and not to sound disrespectful, as I know many of my customers sometimes worry about all kinds of things, and especially if they are not mechanically minded, but this kind of issue is really a non issue and offers little or nothing to worry about if this is the extent of it.
If there are no other components affected, then simply change the nut.
On another note here, and I could be wrong as I can't see clearly with no focus on the target in the picture, but that doesn't appear to be weld from my perspective. From here it looks to be a transparent, or translucent bulge. If so, then that could simply be resin or some kind of glue. Either way, it still shouldn't be an issue.
From your picture, that looks like a narrow taper, possibly a 19mm nut. I'm guessing the car is a small car, and likely Japanese, less likely Korean. I imagine its quite an old car from the size of the nut and wear to the nut surface. I also imagine its been quite a long time since that nut has been removed judging from the corrosion to the side surface, and the fact you mentioned the wheels were difficult to remove.
If I am right at all, then I would suggest you also remove the rear wheel drums [as its likely to have them] and inspect the rear brakes. You will possibly need to do some cleaning and adjustment back there. I would also strongly suggest you remove the rear shoes and clean the backing plates, and lubricate those backing plates with brake grease. You can also double check your handbrake cables and levers at the same time. If I am right so far, then I can give you further pointers on what to look out for.