Electronic – Why does solder turn dull grey on the iron and exhibit poor wetting

This looks to me like a combination of too high heat and crappy tip coating. Unfortunately this is common with unregulated soldering irons. They usually get much hotter than a regulated iron because they always put out the same power, and that power has to be enough to solder against a ground plane or something else that acts like a heat sink. A regulated iron would just crank up the power when needed, but a unregulated iron is stuck with a fixed power, so gets too hot, sometimes way too hot, when just sitting in the holder. I've seen a unregulated 25W soldering iron get hot enough to visibly glow in a dark room.

The excessive heat can not only damage whatever you're soldering, but it speeds up the oxidation reaction that eventually ruins tips. Solder oxidizes fairly quickly, even at normal soldering temperatures. You may have tinned the tip regularly, but I'm sure that didn't last long. Then the tip coating starts to oxidize. Once that's worn off, you're left with copper. Copper is actually eaten away by soldering itself. It migrates into the tin/lead mix, which is one reason solder works so well on copper.

People that buy unregulated irons are shopping on immediate price, so manufacturers make them as cheap as possible. That probably includes the cheapest possible tip coating that looks good out of the box.

So in short, it looks like you got what you paid for.

First question: How long have you had the tip? You could probably easily and cheaply replace it. If the iron doesn't have replaceable tips, get rid of it!

If you've not had the tip long, then a few pointers to keep in mind:

• When the iron is not being used, keep some solder on the tip. This prevents oxidation and corrosion.
• When using the iron, keep a little solder on the tip when the iron is in the stand. Wipe clean before using. Avoid excessive wiping on a wet sponge as this can cause temperature fluctuations which expand and contract the metal, and stress the tip. Consider using brass shavings instead of a wet sponge.
• Never use sandpaper or abrasives to clean the tip. Also avoid dipping it into flux to clean. (Except for re-tinning which should be infrequent.)
• Use a good quality solder. Old solder can have contaminants which leave behind residue and basically make soldering more difficult. (By "old" I mean solder that's been around collecting dust, not necessarily that it has a short shelf life.)

Finally, if you've got a cheap iron, it may be too hot or cold and not melting solder at the correct temperature. Consider getting a temperature controlled unit. I could recommend a Hakko FX-888D for ~ $100 US, see also this tutorial.

Some additional soldering tip pointers:

• http://www.inlandcraft.com/uguides/tipcare.htm