Electronic – Are some soldering iron tips unsuitable for lead-free solder

I don't do much lead-free work at all, so I'm asking this question out of ignorance. I have been using leaded solder as long as I have been soldering, and I have never had an issue with a soldering iron tip. The plated part of the tip always stays shiny, I'm OCD about cleaning the iron, careful with the temperatures used, and never leave my iron on long without using it.

I went to borrow an iron from a lab, and the tip was dark grey and rough. It would not accept any solder unless I tried to rub off some of this material, and even then it would only be a small spot that oxidized very quickly. I concluded that the tip was ruined, and went to get another iron with a better tip, but all the irons in that lab had the same condition, or worse. I got a brand new tip and installed it, and the tip lasted for about 15 minutes before it had a perfect coating and could not be cleaned. This whole time, I had been using the available solder: a 99.3% Tin, 0.7% Copper blend with a rosin core.

To further the ancedote, I came across another lab that used the same iron, but only had lead based solder. The tips in this lab ranged from perfect to moderately abused, but all would have been able to easily melt solder. Many irons were set to 450 degrees C, and their tip looked just fine. I'm not sure how much I would trust the temperature though, the iron is just about the cheapest "temperature controlled" iron I've seen.

Taking another new tip and using lead based solder, I was able to end up with a tip that I would classify "used, in good condition" after a soldering session. My conclusion is that the lead free solder is either more corrosive or less protective than normal lead solder. Which (finally) leads to my questions:

• Is there a lead free metallurgy that is more forgiving to soldering iron tips?
• Is there something inherently wrong in my handling of the soldering iron?
• What would I need to solder with this composition?
• Is there any hope for these tips that have been used with the lead free stuff? (My gut says no)

The soldering iron is a Stahl Tools SSVT Variable Temperature Soldering Station.

The solder is Elenco LF-99: 99.3% Sn, 0.7% Cu; Rosin core.

EDIT:

The other oxidized irons used were used to assemble simple kits; nothing very fancy or exotic as far as I know. They were handled by novices, which made me originally believe that the students had destroyed the tips. When I ended up with an oxidized tip that was otherwise clean, I thought it was less likely that the students had killed the iron. As I was using the lead-free solder, applying solder never seemed to clean the tip. I wasn't setting the iron to temperatures that were excessively high in my opinion (400 degrees C), but I don't know what the actual temperature of the iron was.