My other knives are stored open in felt lined jewelry type boxes.
Danger, danger, Will Rogers!
The dyes in felt and velvets, felt especially, are not always stable. I was once shown a wonderful Bali-Song Cutlery Custom knife with real ivory inserts. What a wonderful piece.... until you turned it over. The ivory on the other side was stained green. It had been stored for years in a display case with a green felt lining and the ivory had absorbed the green dye from the felt. Such damage is unrepairable. The piece went from a thousand dollars to a few hundred at most. Ivory is very succeptible to this, but wood, bone, horn, and many other materials can be similarly damaged. Make sure that any felt, velvet, or other material that you use for long-term storage is "museum grade." Do a search here on Bladeforums.com for "museum grade felt" and you'll find a couple of threads in the past where we've discussed this and listed some sources.
How are they for protecting the knives from corrosion etc.
Unless it's dry nitrogen filled (a very expensive, museum-grade trick), The case you store the knife in will afford little if any protection from corrosion.
Humidity control is nice, but beware of desiccants. If over-used, products like calcium cloride and silica gel will suck the life out of materials like ivory, bone, horn, and wood. Even products like G-10 fiberglass need a bit of moisture to stay in top condition. And where do you think the water goes when you use a chemical desiccant product? The answer is that it is absorbed into and bound up by the desiccant. But what goes in can come out. Sudden changes in temperature or humidity can actually cause chemical desiccants to release moisture back into the space they're supposed to be protecting.
Many years ago, there was a post on rec.knives from a man who had been out of the country for a couple of years. Before leaving, he'd carefully put his treasured collection of rather valuable knives into one of those sealed ammunition boxes with several of those familiar chemical desiccant packs. He returned to find his whole collection hopelessly ruined by rust. What happened? What was the culprit? After some discussion, we concluded that the culprit was the desiccant packs. To properly use them, they need to be baked at 250F for 16 hours and then immediately placed into the sealed container they will protect. He hadn't done that. He put desiccant pack that were already saturated with water into his sealed container. Then, he put that container into an unheated storage. Temperature changes caused the desiccant packs to release that moisture into the sealed container where it rusted the knives. Then, other temperature changes caused the same packs to reabsorb the moisture causing the woods, bone, ivory, and other materials to crack and crumble. The desiccant packs that were supposed to protect his treasured collection ruined it instead.
What can you do to protect display pieces from rust? The best approach is wax. Just as car wax seals and protects the finish on a car, wax can seal and protect the finish, including bare metal, on a knife. The best product is Renissance wax which is the choice of the world's top museums.
Finally, there is the issue of security. A promenient display of valuables, knives especially or even more so guns, can make your home a target for a break in. It's best to make sure that your display is not visible from the outside of your house through the windows, especially from the street, and that the display is not visible from the entry hall of the house where you might have delivery people, solicitors, etc. Such people can get cash payments from robbers for tips, "I delivered a package to this house at 123 S. Wabash Street and in the living room I was a big cabinet full of guns." Guess who gets robbed next week? And you'll never suspect that that package delivery guy was the cause. It's a nice little extra income for him.
