2. Coatings: External (outside the barrel..bluing, parkerizing, etc)
There are several categories of external coatings. Some are designed to work against wear and others against corrosion. LOTS OF CLAIMS get made - I'll try to dispel some of them:
Corrosion Resistance Coatings have performance often measured in terms of "hours of salt spray". This is a farce. While the ASTM B-117 salt spray test does give some indication of corrosion resistance it has limitations. For instance, if you replaced the lab-controlled salt solution with actual seawater you get substantially different results due to the microbes present. Do not take the "Salt Spray Resistance" numbers and extrapolate a head-to-head comparison for real world corrosion resistance. Just like everything else that never leaves a laboratory the salt spray cabinet has no idea what is really out there.
Phosphating - either Manganese or Zinc (Zinc being better) - AKA Parkerizing
Long a classic, this is a reasonably good corrosion resistant coating. The bare steel parts are immersed in a phosphoric acid solution (hence the name) and the surface gets a very small amount of the phosphate compound deposited onto it. Due to the bubbling (that's hydrogen coming off) the coating is porous. This porosity is desirable for either future paint application or impregnating with something lubricious (like oil). Because it is so flexible (corrosion resistant, basis for paint, basis for oilling) it is a popular choice to this day.
Black Oxide - AKA Bluing
Another timeless firearm finish - this has less corrosion resistance than Parkerizing. For those who wonder - there is no chemical difference between "bluing" and "black oxide". Processes exist for both "hot" and "cold" application with the "hot" generating a thicker and overall better coating. Unlike the Phosphating acidic process Black Oxide is applied via a basic process (remember your acids & bases?). Essentially, you are oxidizing the iron atoms in a controlled fashion (unlike rust, which is uncontrolled). Doing this results in VERY LITTLE measurable coating thickness. While the black oxide layer won't flake off like red rust it will still permit oxygen to diffuse through it. Thus, it only slows down corrosion - it does not stop it. Cosmetically this coating can be quite attractive but that has more to do with how much polishing is done beforehand than any chemistry in the tank. This process only works on steels (not aluminum/titanium/polymers). While you could do it to a stainless steel there are better ways to protect them.
Passivation of Stainless Steel
Protecting stainless steel is easy. You clean it really well, grit blast it and drop it in a heated nitric acid solution. Unlike the Black Oxide process which goes after the iron the Passivation oxidizes the chromium atoms in stainless steel. The resulting extremely thin layer (no dimensional change is noticed) of chromium oxide is a true barrier to further oxygen diffusion. Thus, passivated stainless steel can be expected to survive a long time in ambient atmospheric conditions without corroding at all. To some degree the stainless steel would do this on its own when exposed to oxygen - the trick is getting a uniform layer before any iron atoms oxidize and form a pore in the surface. Thus, a passivation leyer is partially self-healing, though a good scratch/gouge might later prove to be a point of attack. Obviously, the more chromium content in the stainless the better this works but there MUST be at least 12% free chromium available or it won't work at all.
There are many paintable coatings available for today's firearms. Everything from el-cheapo Krylon to pricy-DuraCoat has been used. The single most critical thing with any of them is SURFACE PREPARATION. With good surface prep (usually cleaning/degreasing and grit blasting) the coating will adhere. Since these coatings are merely barriers to oxygen diffusion (and buffer layers for wear) they have to remain in place to work. If they flake off they won't work. Thus, a properly prepped surface with Krylon over it is better than an improperly prepped surface with DuraCoat. All the latest and greatest whiz-bang organic chemistry on earth won't save you if your coating isn't on your gun. And yes - some of the better paints have interesting corrosion resistant chemicals embedded in them like strontium chromate. You pay the big bucks you get the good stuff. Still, since this coating has some real thickness to it you have to consider the dimensional tolerances of parts especially in dynamic interfaces (like frame/slide rails).
This is what you do to protect aluminum from corroding. Just because aluminum "can't rust" doesn't mean that it can't oxidize. In point of fact, both aluminum and titanium are actually MORE ACTIVE than iron and thus want to oxidize (corrode) even more quickly than iron. There are a couple of kinds of anodizing to know - regular and hard. Regular anodizing is a non-porous corrosion resistant barrier layer, about .0002-.0004" thick, formed of aluminum oxide. This is a GREAT prep for painting. Hard anodizing is much thicker, running about .0015-.004" thick, and usually gets impregnated with something like Teflon afterwards to both improve its wear properties and to seal up those pores. Since it is porous in nature it is NOT as good in corrosion resistance as regular anodizing. Still, a few thousandths of an inch of aluminum oxide (the stuff the grit in sandpaper is made of) can really resist wear. Both regular and hard anodizing is done in an acidic bath (chromic, sulfuric and boric are all popular) with the aid of an applied electrical current. Just like Black Oxide you are oxidizing the aluminum atoms in the surface in a controlled fashion. However, since electricity is forcing the process forward you build up a thicker layer. When calculating the dimensional change for anodizing it should be noted that half of the coating thickness is grown INTO the part and half OUTWARD. Thus, a .002" hard anodize will only close up a hole's diameter by .002" (rather than .004" on the diameter or .002" per side/radius like with a plating).
Wear Resistance Coatings often like to brag about their "hardness". This is misleading. While hardness is a factor in wear resistance many others things play a part (coefficient of friction, surface finish, material of the other side of the wear couple, bearing strength, lubrication, etc...). Wear is a phenomenon that you can't simply throw a coating at and solve the problem. In many cases you just have to accept that wear will inevitably occur and pick the part to get worn out (the sacrificial part in the wear couple). You can perhaps extend the time between repair/replacement but everything wears out eventually. Usually you pick the cheapest part but sometimes you pick the easiest part to replace. I always chuckle when I read of someone getting a wear resistant coating on an AR-15 bolt carrier. Tell me, rifleman, which is easier to replace - the bolt carrier or the UPPER RECEIVER?!?!?!
TiN - AKA IonBond
Popular on Deagles, drill bits and general issue pimp gear this is NOT the tin coatings your grandpa was used to seeing on steel. This is TiN (titanium nitride) not tin (latin = stannum) - VERY different animals. TiN is deposited via a vapor deposition method (either physical or chemical) in a heated vacuum chamber. The result is VERY, VERY THIN usually gold-colored ceramic coating of exceptional hardness. Given its rigidity (like most ceramics) it is vulnerable to chipping. Also, as the PVD/CVD process isn't the greatest for adhesion if pretreat cleanliness isn't maintained poor QC will lead to flaking problems right quick.
Chrome plating is a time-tested wear coating. It is deposited from a chromic acid bath, catalyzed with sulfuric acid, onto steel parts. You need to have special tooling, called anodes, made from lead that roughly conform to the shape of the surface to be coated. The part is made cathodic using a DC power supply and the current causes the chrome in solution to "plate-out" on the steel part. While surface prep is always important the inherent rigoroursness of forward-current electroplating makes for good ahesion even on very smooth (non-grit blasted) surfaces. Thus, you can get the surface nice and smooth first in the steel and then plate chrome over it (and polish the chrome, too). After plating most heat treated low-alloy steels will need to be "baked" to remove the hydrogen they picked up during the processing. If not, the hydrogen will "embrittle" the steel which is bad news. Interestingly, chrome is deposited as one crystal structure but changes (usually during the "bake") to another shortly after deposition. The resulting specific volume change and almost total lack of ductility cracks the chrome. While EXCELLENT process control (even some aerospace-grade platers don't know all the tricks to it) can limit the severity of the cracking and prevent them from going all the way through the chrome plating you must figure that some through cracks exist. Thus, chrome by itself is not considered a barrier to corrosion.
There are three types of electroless nickel - hi-phos, mid-phos and lo-phos. The "phos" indicates the "phosphorus" content of the plating - the higher the "phos" the harder the plating. Electroless nickel, unlike chrome, doesn't require any electrical current to "throw" the plating onto the part. Instead, the nickel hypophosphite plating solution autocatalytically decomposes and "drops" nickel out of solution onto the steel part. Since this is not the greatest method for good adhesion the pre-treat cycle is critical. Done right you can take a ballpien hammer to the nickel on a corner and it won't chip - do it wrong and it'll come off during normal assembly. Sometimes the "right" surface prep includes such nasty chemicals as hydrofluoric acid (wanna give your EH&S guy a heart attack just ask him about THAT chemical). Just as with chrome you have to bake heat treated steels afterwards but this process also hardens, and cracks, the nickel plating. Since it is inherently cracked (the harder the more cracks) it is not considered to be an effective barrier against corrosion. Still, with the right "phos" level it can be about as hard as chrome plating.
Carburizing - AKA Case Hardening
This is not so much a finish as a heat treatment process. Back in the old days (think mid to late 1800's) the carbon steel frames on guns could rust pretty easily. The best "bluing" process they had back then was a sort of "plum" bluing that was essentially controlled red rust - really not that great of a process. As such, they were looking for a way to prevent red rust from "spalling" off of their guns and so borrowed a trick from the old sword makers - carburizing (a form of case hardening). Basically, they would take the part and pack it with ground up charcoal or (hopefully animal) bones and then put it into a furnace. The high temps would make the carbon in the charcoal/bone mobile and allow it to diffuse into the surface of the steel. When it cooled the surface was now much harder (since it had more carbon) while the core of the part was still resilient (having the original lower carbon content). The hard surface could still rust, but because the steel matrix was so bloody hard it resisted "spalling" or "flaking off". Thus, you could take some steel wool to the rust and then oil it and be fine - unlike the car bodies we've all watched bubble up with rust over the years. Obviously being a harder surface gave it wear resistance, something useful for all them holster-draws the cowboys were doing. In modern times we don't do the "packing" method anymore but instead use a carbon-rich heat treating atmosphere (some mixture of cracked natural gas) to accomplish the same thing.
Salt Bath NitroCarburizing - AKA Tennifer & Melonite
This is a subset of case-hardening. Regardless of the fancy-pants marketing name slapped on it the process is the same. You take a steel part and immerse it in a molten salt bath that contains cyanide (2 more things your EH&S guy will just love - molten salts and cyanide). The high temperature from the molten salt (around 1,000 F IIRC) makes the nitrogen and carbon mobile and they diffuse into the surface of the steel. Both nitrogen and carbon almost equally strengthen steel so having them both do it gets the steel harder faster (and time at temp is money to a heat treater). Just like carburizing this does NOT prevent corrosion, it just keeps down on the ugly red rust flaking off. Its advantages over "carburizing" include a faster heat-up time, quicker achieving of hardness and greater case-depth uniformity.
NOTE - Stainless steel parts that get EITHER carburized OR nitrocarburized are, metallurgically speaking, no longer stainless. The reason is because the carbon combines with the chromium to form chromium carbides. This decreases from the available "free" chromium content needed to form the tightly adherent chromium oxide film. Lacking that, the steel becomes just a very expensive low-alloy steel. Why firearms makers bother to make parts from stainless steel and then nitrocarburize them is completely beyond me. They'd get virtually identical performance from a low-alloy steel like 4140 that got nitrocarburized and it would cost a LOT less. For you welders out there - this same phenomenon can happen when welding certain grades of stainless steel. That's why there are "L" grades of the 300-series stainless steels, the "L" indicates low carbon content so as to avoid making the weld-zone non-stainless.