Sir Random Guy . . . . . . .
ANTEX . . . .ANTEX . . . . why I haven't used those little jewels since the late nineteen sixties, for soldering on fine work or in cramped places.
However . . .I know that I still have at least 3 stored away.
Methinks that your tip just got TOO HOT before receiving solder and the black oxidation that you now presently see, is isolating that new solder that you are attempting to use, from even adhering to that area.
You lucked out in initially getting that small tinned area on the tip.
Two suggested procedures now:
Considering that your iron is being this unit :
Which is a stand alone unit, and not one of the controlled / adjustable heat soldering iron stations.
When the new tip arrives, slip it on the end of the heating element and THEN degrease it from your fingers oil . . .or any prior handling "touchors". . .having touched the tiplet.
Then you use a rosin cored solder and apply an end directly to the tip to accept solder and start tinning, move the solder around to cover even more area .(Applied liquid Rosin flux can assist matters.)
Your previous procedure could just have heated the internal flux and it popped out of an end(s), therewith, leaving no internal flux, so that surrounding . . .then . . . fluxless solder "coilet" wouldn't adhere with the tips original thin tinned coating.
On tinning the NEW tiplet, use this alternate procedure:
Plug in the iron and keep poking solder at the end of the tip, until its external temperature JUST reaches the point where solder will melt and adhere to the tiplet.
Reach over and unplug the iron and keep moving the solder around to cover and tin as much tip area as you can. The stored "thermal flywheel" effect of the irons mass should get your area mostly tinned.
If needing more tinned area. just plug in the iron again to initiate heating to the same degree as experienced before. Repeat.
My procedure for tinning the whole end of an iron is to have it tinned back as far as I was successfull and then take a knife blade and pull back solder from the edge of the tinned area onto an adjoining untinned area BEHIND it.
That is done in fast "touches" so as to not have the knife blade sinking / stealing away all of the tips heat.
Those incremental side by side moves pulls some solder with the blade, such that the slight scraping of the untinned area with the blade, then has that solder flowing into that newly cleaned area. Eventually you will have the whole tip tinned .
ANTEX sez:
The soldering bits are plated with iron and chrome for extra long life.
Soooooooo . . . you can see the chrome plating being used up to the point where the iron tapers, and then the tapered end is iron plated.
On my irons when I had used them up to the point of their depletion of the iron plating, I just treated their tinning as if I now had a copper tiplet and tinned them in a normal manner.
Then I wasn't worried about wire brushing or filing or grinding them down a minute amount, to be able to retin.
Back to that problem tiplet, pull the unit out cold and see if judicious use of either plain steel wool or stainless wool will remove that tip blackening and get the end shiny enough to use the described procedure on it.
Thasssit . . . .
73's de Edd