First, let me state, that most all data is recoverable. Its just a matter of whats required to read it. If there was some evidence on those that the FBI needed to put some big corprate fruad guy in jail, best believe, the data would be recovered. The technology that they use though is not at the average persons finger tips. This is why your best bet is to hand them to a professional. Rates for that sort of work can be a lot more than you might expect, and estimates will vary from company to company. They will have tools that are far more sensitive that what you likely have.
You could always try cleaning them in a way that doesn't harm them any further, so what you get, if nothing, then hand them to a professional.
If you do decide to go all out though, and your a DIY'er at heart, which I am guilty of being, then your best bet for recovering the data would be to clean them in a drying solution. Near pure Alcohol, or Acetone, those would be your best bets (or at-least as far as I know). Afterwards you can let them sit in a bag of rice for a few days, or a week even.
At this point, try sticking them in, see if you can get a read. Its been a while since I played with disk reading software, but I know that there are much slower reads that are far better at recovering data.
If you can't get a read, you need to make sure your connecting correctly. The reason its important to ensure that you have a good connection, is because you want to know if there is a problem with the data or not. This is basic troubleshooting.
At this point you will need to get...
- a soldering iron,
- solder for the iron,
- and some good quality flux.
Truthfully, you probably will greatly benifit from any electonics experiance you have had in the past, I was fortunate enough to have a dad who was an electronics major when he was in college.
If you have ever soddered somthing with a shitload of pins before, like DRAM, you will probably be much better at this starting out. What you want to do is individually cover each copper connection with a thin coat of solder. You want it to be thin enough that it will still fit in your SD slot, yet you need enough coverage on the connector that you can see almost no rust.
The most important part is to ensure you don't short circut the card. In other words, soder on one connection can absolutly 100% not touch another connection. (I told you experiance helps)
If your diligant enough, smart enough, and patient enough, you can do it with no experiance soldering, but you should practicing on somthing else first.
If you still cannot get your data back at this point, and you feel confidant you correctly repaired the connecors, then your data is corruptted. If you fixed the connection correctly, depending on the software you use, the program that you try reading the card with might give you a helpful
"data corupted" error message. If thats the case, you can come back here and enquire about the "Error" that you are getting.
But, I am willing to bet, at some point, you'll find that the cards are readable.
Best of luck to you.