Brand new soldering iron tip turns black, solder won't stick

As pointed out in the comments and other answer, you need to clean the tip.

There are two options for cleaning, depending on what you have, or what came with the Iron.

1. A Compressed Cellulose sponge, which has been wetted with water. You want it to be damp, but not soaking wet. If it is soaking it just cools the tip down and doesn't help clean it. If it is dry, the sponge will burn, putting more crap on the tip.

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2. A Brass Wire cleaning sponge. These are not the same as steel wool. Steel wool is an abrasive which will damage the tip (as will sand paper). The tips are made internally of copper which is great for heat transfer, but will be damaged/dissolved by the tin in the solder. To allow the tip to work, it is plated with Iron which will withstand the soldering process, and is key to ensuring the tip can be used. This plating is thin and can be easily damaged by abrasives, or scratching against things. The brass wire sponges are not abrasive, they are like the scrubbing pads people sometimes use for washing up. They look like this:

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For both cases you need to do the same thing, basically just drag the tip across the sponge a few times (may only take a couple, may take a dozen, depends on how much grot is on there) at a sort of medium pace (like washing up really). You should see the tip start to go shiny and silver. Once it is, put some solder on, and then again wipe on the sponge. Finally put some more solder on (tin the tip) when not in use.

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So why did it happen so quick? I can think of a couple of reasons:

1. There was some coating on the tip to protect it when sitting on a shelf for ages. Not sure if this would be done - if it was tinned, that should be enough, but you never know.

2. If it is not a temperature controlled iron, then who knows what temperature the tip is at - ideally it should be around 360-380°C, but the non-controlled ones can be anywhere, even as much as 450+°C. The higher temperature will cause the tip to oxidise from stuff in the air much faster. Hopefully you should be able to clean it off on a sponge. Then once clean always leave it