There is absolutely nothing that you can do to stop the degrading of the metal in your coins. There is also almost nothing you can do to stop continuing damage to coins that you handle regularly.
The best thing to do, as with many other precious objects, is to minimize the damage.
There is one thing I really hate about collecting antiques etc.. That is losing or breaking something. Imagine, that plate I just dropped survived for five centuries...was treasured by its owners, loved by brides and collectors...but it reached my hands, only to finally be broken. I HATE THAT!!!!
Damage, loss, degrading...that is all part of owning things.
So, how do we minimized damage and store coins of this type.
If you have a very valuable collection...forget display and handling....things will happen even when you are very careful. Sooner or later it will end up on the floor, crushed under foot, or inside the cat or dog.
If you do not want to use a manufactured metal box, or other system:
Valuable coins should be stored in soft, acid free, uncolored, flannel, possibly the flannel that is made for storage of silver flatware. They should not be tightly constrained, flattened or stacked. They should be loosely wrapped or folded in separate pieces of cloth and lightly packed without pressing down in an airtight box, also made from non acidic materials. Large pieces of cloth may be folded and stitched into pockets in rows, with flaps that fold down as well, to hold them inside their pockets. The thread too should be cotton and acid free.
Plastic envelopes, too tight for thin and possibly bent or rumpled coins or coins that are actually glued together(not an uncommon practice for the rarest coins) or cracked.
Little boxes with magnifying tops. Nice to observe through, but putting them in a box and carrying them around will not do them any good, being bounced around inside their rigid little space. Plastic is not necessarily acid free. Careful shopping will get you archival plastics though. Be sure that cloth or paper wrapped coin hoards are stored air tight and dampness free, because paper and cloth can actually attract moisture.
Card stock envelopes with cellophane windows...make sure they are acid free, and the layers of cellophane do not press on less than flat coins that may break when flattened.
You might try learning to fold diamond papers from acid free tissue. Make a little ring of thick, acid free, non fluffy cotton(white or natural) yarn big enough to go around the coin generously. Place the yarn circle in the diamond paper, and your coin inside the ring. Fold up the paper, and store without pressing the papers together, thus pressing the coins as well.
Coins should not touch each other in storage, especially if made of different metals!
There are plastic (coin shaped) coin cases available. These are great for static storage as long as they are acid free, but shipping and carrying them around is still not a good idea, as just the rattling around inside can harm delicate edges. There are also boxes made for standard sized coins with plastic gaskets to keep them still, but these old coins will not fit anything standard. If they can be lined with acid free paper or cloth, I would feel better about long term storage, possibly just wrapped in acid free tissue in a larger than necessary case.
Patterned storage cloth or paper, like paper towel, can, in time, impress that pattern onto the metal of your coin...no pressure...no acid. Acid free tissue also buffers from neighbors.
One of the reasons that life exists on this planet, is that water exists here, and water is a universal solvent...in time...everything will be affected by dampness..even the traces of alloys in your gold will be affected by dampness. Gold like all refined metals ARE NEVER PURE. There is always some sort of impurity in the metal, even the very best. That impurity can be affected by bad atmosphere and solvents and acids. Gold is nearly immutable, as you can see by visiting museums that display thousands of years old jewelry, but IT IS POSSIBLE TO RUIN IT with improper storage with just the right things present to work on it...again, not likely the gold, but the alloy mixed into it. Gold is pretty difficult to dull, but you can do it!
Storage in sealed glass containers would be ideal, but finding such storage would be very difficult, and again, rattling around inside is an issue.
If you had a Bunsen burner or a tiny torch....now remember this is a rather extreme and anally retentive thing to try, but would be effective and a fun experiment.
Buy a large but thin tube in glass, used for chemistry. Lay on a stone or metal surface and heat the tube till it slumps to form a tube that is flattened rather than round. Flatten an end with a heated butter knife while hot, to seal it. Slide your coin into the flattened tube. Heat the area beyond the other side of the coin. With a butter knife while hot, flatten the tube to seal off the coin inside. Heat another knife to red hot, and cut the sealed case off from the tube and cool. Perhaps this would be a fun project to try with a penny to see if it works. You can buy pennies captured in tiny bottles blown around them. You would not necessarily harm the patina as the entire tube would not get that hot. Try it with a badly damaged coin that is beyond recovery and see what happens.
Once a coin has started to degrade, such as copper, even sealed, it will continue to degrade because of the chemicals produced by the degradation process. Isolate already degraded coins from your nice clean collection. Do not count them out. Again, after a disaster, you may have the only examples left of some rare coin. Just keep such a coin isolated, and seek a professional to help restore the coin. A cleaned coin will lose most of its value, but if you really like the coin or it is so incredibly rare that it does not matter to you or collectors, it may be worth it if nothing else will preserve it. Examples of incredibly rare coins would be Edward V of England, or Edward VIII(the idiot) of England. Edward VIII only had a proof coin in England(as I remember) with 6 coins total in the world. Never issued, but they are there. One recently sold for 516,000 pounds. Four are in museums.
Many of the storage systems available on the open market today are really meant for modern coins and bars of precious investment metal and do not take into consideration the unique issues that your nearly thousand year old William I coin may have.
You can buy aluminum storage boxes that will pretty much seal your collection off from the elements. Remember though, even fairly good containers will not last for too long submerged in water, or buried for a long time. Disaster scenarios are by definition pretty invasive. If your collection is valuable, historically significant or just valuable to you, buy or make the very best storage that you can.
Even though my collection will not be the best coins on the market, they will be historically valuable, and worth preserving.
You may have noticed that historical sites and museums are not always as secure as they seem. In times of war or strife, your collection could end up being the last survivor of historical items of their type, if you have stored them properly.
I had a friend years ago from one of the Baltic States. Latvia perhaps? Her house was filled with amber of all descriptions. I commented on the fact that in times of strife, she would have something to sell for food etc.(I think we were discussing her time in the second world war at the time.)
She just turned dark and said: "in the second world war, I could not even get a loaf of bread for my best piece of amber."
This brings me to another subject.
Also, a little warning from a non expert.
Do not put all your eggs into one basket. Investing in precious metals and old coins should be FUN and that should be your primary motive. Buy what you love, and let investment be secondary. Metals can go up OR DOWN in value, no matter what they say on TV.
It is very much like jewelry. You are always buying at retail, unless it is at auction or estate sales etc.. When you go to sell your three carat diamond ring, or your William I silver coin, why would a store or investor buy from you at a profit from your retail purchase, when they can just as easily buy the same thing at wholesale. Typically, when I was in the jewelry business, a seller might get 15 cents on the dollar for their diamonds or gold in cash. They might get 30 cents on the dollar if they left it on consignment. If you open your own store, you might get close to the value you paid.
Remember that anyone who buys from you, in order to realize a profit on their purchase, will have to raise the price and resell it.
That being said, your gold or silver may have at least SOME value when all else has none during war or other hard times. The most valuable things will be food, fuel, tools and your labor.
Buying something now and planning on your grandchildren getting a profit is a good strategy. Over time you are likely to have a rise in value. In my case, I expect that my grand-nephew's children might get a couple of years of education from it, or perhaps a down payment on a house.
Here is a warning scenario. There is a film somewhere on Youtube or somewhere. Excavators found a shoe filled with 420 coins.
So, you have just purchased your Henry I coin and paid $1000.00 for it. The following day someone opens up a floor board in their 12th century house in Kent(they do exist in England.) and there are 600 silver coins JUST LIKE YOURS. The following day, the prices on Ebay or other sites are now half what they were.
You have to at least be able to say to your skeptical friends: "I just love the coin and I had to buy it no matter what the price, because I wanted it."
Prices can vary wildly for the same coin. It may be in nearly perfect condition, with a soft gray patina all over it. You pay a lot for it. Your mother-in-law comes to visit. She decides to help with the housework. They are cleaning silver, and while your wife or husband is making tea, she decides to clean the pile of coins on the coffee table.
Do not rely on investment unless you plan to seal up and pack all your collections away, and even then you have to pray that no one in England finds a treasure trove!