soldering iron tip keeps melting/disintegrating?

Acid flux is one possibility, another is that perhaps you are cleaning the tip with something abrasive. Modern soldering tips are iron plated and will last a long time if used properly. If the plating gets damaged by an abrasive, it will rapidly deteriorate in the manner pictured. A strip of wet sponge, real sponge not plastic foam, is usually best for cleaning. A quick wipe of the hot iron before soldering is usually all that is required.

Weller Soldering Iron Tips

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You need to know the rage of use that you would like for this tool. When buying tools you must know what you are buying them for, what the difference between them is, and how good of a company is the one you are looking at. Soldering irons...well do you want one that you can change the temperature of the tip on??? Do you solder enough that you would like a vacuum system included in the soldering gun to vent the extremely toxic fumes from the solder and flux??? I would also recommend getting a stand that will hold things you will be soldering on. If you just need to make solder connections that will hold things any old heat source will work, soldering gun, torch. If you will be soldering electronic components it is crucial that you know how to do this for the optimal conductivity. The color of the solder is something you should know how to judge (the way light gleams of the joint after you are done, you know). Well, you might want to research this some more, or first ask yourself, "How much am I going to be soldering?"

Maybe you are using acid flux? Normal solder is a lead/tin 60/40 composition, with a rosen core to provide the flux. You don't normally need any additional flux.

I have an old iron like yours, and it is still using the original tip. If you can upgrade to an adjustable temperature controlled iron, the tip should last even longer. Plus you can buy different size tips for different soldering jobs.

There are Wellers and Wellers.

What you appear to have in a model with a straight copper tip, which seems to be a cheap model.

Copper will not last long if you have an acid based flux. But even with rosin cored solder, copper tips corrode rapidly.

If you are soldering PCBs or copper wires etc., you virtually MUST have a plated tip soldering iron.

I have used Weller temp. controlled irons for years, but they have always had plated tips. They last virtually forever, and of course, never change shape, which is important.

Weller Soldering Gun Tips

Tinning your tip often is important, but so is cleaning the tip. Wipe your tip on a dampened sponge to clear off any debris. Another thing: take a break every once in a while and unplug it for a few minutes; it's not good to leave an iron on for an hour.

It seems you may be using a very corrosive flux. The tip shouldn't erode, especially with limited usage. Is there an off chance that you are using a 110 volt iron on a higher voltage outlet?