I don't want those annoying villagers to hurt my new friend, and so I will give him some metal plates on top of the skin on his limbs and torso. I can only think of using a metal that won't oxidize and also pinning the plates to the bones.
What complications could arise from this? Is there anything else I would need to do to help the poor fella out?
I suggest you search the corridors of your gothic castle until you find one of these. The reason is that years of research and testing went into the design of suits of armour - there is no need to reinvent the wheel.
Note
If the monster is uncooperative, it may try to take the armour off. Rather than bolting it on to its flesh which will likely drive it mad, the suit of armour can simply be welded* at the joins between major components.
*To prevent total immobility, I suggest welding flexible steel cable across the joints to allow movement.
Picture of medieval suit of armour
Mad science knows no limitations. There are no constraints, rules, regulations, considerations, or any of that other namby-pamby stuff regular people worry about incessantly.
You're making science here!
Instead of worrying about oxidation, may I suggest that the metal actually be radioactive? Why? For science. To find out if it could be done, and if it can't because physics itself disallows it (really, the only authority you obey, and then only begrudgingly) then to see what happens when it fails.
Villagers and their pitchforks are irritating, and I too wish there were a more lasting solution to the problem. While I can't solve that for you, I can tell you that they're going to fear, even be awestruck, by your radioactively-glowing metal-plated abomination. Other villagers in other remote mountainous lairs will always be a problem, but these villagers will get what's coming to them. Either today when their faces start melting, or tomorrow when they are plagued by horrible leukemias and cancers.
Additionally, it may even be the case that this Frankensteinian monster develops superpowers that are unrelated to, and unexplained by either his status as a reanimated composite corpse or the depleted uranium body armor you've bolted to his bones. B movie logic, what can you do? Telekinesis, the ability to breathe underwater, possibly even the ability to shoot laser blasts from his nostrils.
Finally, you do need to decide on a goal. The less utility Abominoid 9000 has in accomplishing this goal, the more in character you'll be. Perhaps you intend to drain the Mediterranean. Or colonize Venus with a slave army of uplifted wombats. How does having a Frankenstein monster help you do these things, you ask?
Mad science!
Seriously, what are you some sort of amateur. Don't ask questions like that, they are beneath us. Like bioethics, or hair care. You need to start thinking like a true mad scientist, and focus your attention on what matters. It can be as simple as world domination, or something so lunatic that it can't be put into words. Summoning eldritch beings from another dimension, upgrading humanity to a more perfect species incapable of emotion or free will, even just getting vengeance on those who have wronged or slandered you in the past. Only you can figure out the actual reason (or maybe you can't figure it out... just identify it).
Do it for science. Mad science.
Leave a Gap
Do not put the armor directly on top of the skin. Have struts that extend from the bone and support plates with an air gap between the skin and the plates.
The gap lets the skin breath and sweat escape. This keeps the skin in better condition. It slows down the rust of the plates, and the rate of blood poisoning.
Bonus points you can detach the plates for repair, since you can fit a spanner into the airgap and hold the bolts on the underside.
The only thing worse than permanently being stuck in a suit of armour, is being stuck in a suit of armour that restricts your movement. I know, I know, Frankenstein's monster isn't really know for his gymnastic flexibility. But if the plates fused to his body clang together and stop him from angrily lifting a villager above his head while roaring, he's not going to be all too pleased.
Other, secondary things to consider are:
Muscles change shape when they flex, going from long(ish) and thin to short(er) and bunched up. Steel is a noticeable inflexible material at room temperature, and bonding it to the skin will prevent the flexing of muscles, as they will have no room to bunch up.
It would be a shame if your monster could not utilise its arm strength because it's biceps could not contract.
I'd say metal isn't the 'right' choice here. One of the common issues with 'external' implants is the 'meat'/inorganic interface tending to get infected or damaged. It also can't 'grow' with the creature
Let us ponder the Linothorax - or its modern kevlar equivilent. By varying the 'type' of material' you can have 'hard' chitinous armour, fibrous sections and joints. Maybe have your new friend 'leak' a substance that seals up damaged armour. Essentially layers of materials that are organic acting as a sort of composite armour.
In fact - something like an upgraded lobster carapace would give you a shiny, metallic look, with a touch of alien-ness.
On the other hand, you need to worry about villagers with giant lobster crackers and butter, rather than pitchforks and torches...
Ideally titanium. They are both biocompatible, corrosion-resistant (titanium a bit more so, but both probably adequate), and titanium is something like 45% lighter than steel (someone mentioned keeping weight down on the poor creature he has suffered enough)
In 2016, researchers discovered a gold-titanium alloy that has 4× the hardness (resistance to denting) of pure titanium. You can't say "titanium-gold alloy" in your fiction without sounding like a fancy man, which may be good or bad for your artistic purposes. The Iron Man guy uses it.
By slapping armour on your construct, you're partially to completely negating one of the great advantages that humans have over other creatures: physical endurance.
Why, might you ask, does adding armour reduce the construct's endurance? Is it the weight? No, it's because the armour covers the skin, and the skin is a human's superpower amongst the other members of the animal kingdom, that allows a human to shed in excess of 1kw of excess heat through sweating. Add armour, reduce the capacity to sweat, and therefore reduce the capacity for cooling.
Even medieval knights in their removable shells of armour compromised their ability to cool themselves by sweating, and coped with that compromise simply because they lived and used their armour in a cool climate, and they rode horses, which reduced their levels of physical effort generating heat.
So... I'd suggest putting your armour beneath the skin. The skin might get damaged but the internal organs would be safer. Better that than overheating.
Make it an Exoskeleton. (Credits to PipperChip)
No need for separate skin. No need for bones. The muscles and organs attach directly to your armor.
You only have to figure out how to make breathing, muscle movement and temperature regulation work. Look to insects and crustaceans for inspiration.
A great advantage compared to e.g. crustaceans is that your Frankenstein’s Monster is artificially made and doesn’t have to grow, so it doesn’t have to moult or anything.
Simulated Magic
Frankenstein's monster was brought to life by electricity. This was a little understood phenomenon at the time, especially by ordinary folk. Equip the monster with metal plates that are electrically charged. When the villagers attempt to lay hands on him, they will receive a powerful shock. This will create fear and awe.
The monster will need rubber boots to avoid premature discharging into the ground. If he needs to recharge, he can be provided with a hand-operated version of the static-electricity generator used to animate him. Either that or he can return to the castle and plug himself in.
Note that it can also be used to potentially trick people into thinking it's an iron golem.
Flee now, or I shall breathe poison death upon you all!
This is a trick used in Advanced Dungeons & Dragons, Dungeon Module A3: Assault on the Aerie of the Slave Lords, Part C: the Catacombs, first area (published 1981).
Muscle memory.
If any of the limbs were sourced from users with armor-preference, it's worth accommodating those preferences if possible. Using armor effectively is difficult enough, to say nothing of using unfamiliar armor.
Oobleck Add a layer of this stuff between metal exoplates and the monster's body.
A non-newtonian fluid that reacts differently according to the energy of the impact. So a hard impact from a hammer will cause it to tense up and resist, compared to a slower movement, which poses minimal threat.
The outside layer of metal is there to turn sharp blades which could cut the oobleck. This could be attached, but realistically a jerkin of chainmail or plate like the samurai had could be more efficient. Organic creatures could not heat metal enough to liquify it by biological processes, and laying a metal carapace via organic methods would be like a snail growing a shell - very slow.
It may be feasible for the creature to create and replenish its own oobleck layer over time too, perhaps substituting oobleck for fat in the sub-dermal skin layer.
Mobility. While weight may not be a problem, having material around joints does restrict range of motion if only by a little.
Thermal restriction. Your skin is now covered and now cannot use sweat to cool down. one suggestion is to teach your frankenstein how to take off and put on armor so they can periodically take off the whole set or sections to cool off. Regardless I think equipping your franky with a titanium wire woven shirt (could be interwoven with normal fibres for comfort) would provide minimal relief by conducting the heat away to exposed areas. I would choose copper threads but copper is not skin friendly for long periods of time.
Claustrophobia. Again you can train your man made guy but I would pay attention to field of view and breathability if you include a helmet.
Thermal attack. Fire, boiling oil, etc will get on your monster. While there may not be much to be done about being engulfed in it, but giving your guy a handheld shield could help them deflect incoming thrown oil or hot water, etc.
I'd suggest a couple things.
1a. The titanium thermal conductive shirt. (kinda) I was thinking of having it interlaced in the 1. layer
Interlayer/standoff A very coarse threaded gambeson, but with a twist. Its more of a fine net kind of garment filled with brass wool, such that wind can pass though. Chainmail sleeves and coif are also attached to make this an modified arming doublet. (brass because its not armor and it needs to be corrosion resistant.) this is to allow air to pass between the plates and skin. (also this layer includes similar brass filled gambeson and chainmail for pants.
plate armor. Your normal knights armor. Nothing fancy. I would say better neck protection with a more pronounced throat guard etc and a full coverage helmet with a flip down visor. probably more easily removable in a state of panic so claustrophobia won't make him go crazy but that might be a double edged sword in regards to head protection (take off helmet in panic, gets attack to the head.)
edit: as for fire, I think quick detach plates would be ok to say ditch plates soaked in oil. Leather floats that inflate like airbags can also be an option for quicksand. buoyancy is key in quicksand, that and making sure you move slowly. (leather can be sewn well to be somewhat airtight and probably inflated by a miniscule black powder charge
If the plates are "pinned" to the bone, it seems possible to first insert the pin, and then attach metal plates on top. Allowing more flexible options.
Biocompatibility is the first thing that comes in mind. It has to be non-toxic for sure. But sticking a metal in the flesh is a bit more demanding. A lot of metals, while non-toxic, would cause inflamation when burried in the flesh. In modern surgery, titanium, stainless steel, gold are often used. These metals are used for implants and artificial joints. You can also use other metals with a coating of the above metals/materials.
Similarly there is the consideration of the physiology. The metal plates/armor should not hinder muscle, bone, and skin function. This is more of the design of the plate, not just purely the material choice. armadillos seems to provide a mid way option between strength and flexibility. Their armors are not completely rigid like turtles'. So maybe a flexible metal would be nice. You may want to choose memory alloy. They are flexible, and can endure some bends and dents. Most importantly, they can return to original shape once heated. Giving the armour some way of recovery.
Then comes physical strength. As for the purpose of armour, it should be strong, and light. Which is also the main consideration when building an airplane. So like and airplane, Aluminium or Titanium alloys would be good choices.
I think Titanium is an obvious choice using currently available materials. It is biocompatible, corrosion resistant, strong, and light. But if advancement of technology is allowed, some futuristic, bio-compatible, memory alloy would be more attractive option.
I think a combination of plates and chain mail integrated together, made from some X material (where X = the lightest of the strongest material (mithril for example) that also doesn't conduct heat efficiently.
You have to think about what the villagers will attack your friend with - arrows, spears, pitchforks, rocks, fire.
Plates to protect large areas like the front and back of the legs, the chest, the back and use chain mail for the areas that need to flex - the sides, neck, etc.
I assume you're making your creature as strong as possible. Adding metal place won't make it stronger, so it will be slower. I assume you accept that tradeoff.
I assume it's far from being invincible, otherwise you wouldn't need any extra protection. Metal plates are great to protect against arrows or swords, which villagers happen not to have (normally). An armor will give you quite a good protection against bone breaking from blunt weapons such as big sticks, but still, the impact energy will have potential to damage internal organs.
There is, however, one weapon that will be quite accessible to your villagers, and metal armor might amplify the damages instead of preventing them: the heat. Treated with torches, or boiling water - the metal will absorb heat very quickly, and then slowly give it back to the monster's body, slowly frying it. Think about cooling in water, and then it comes:
Not only water, mud or quicksand makes also 'great' job. You created something much heavier than water, it will sink in a pond, in a swamp or anything similar. Swamps are terribly prevalent in wild forests, and it's quite impractical to be very heavy if you're going to run through them in the night.
Maybe you should really think over your idea...
