Corrosion Basics for Gunsmiths

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Worldwide, corrosion and the steps to mitigate it account for an estimated $400 billion cost to society each year. Vehicles, bridges, buildings, railcars, factories, communication towers, and firearms all fall victim to rust. It’s entropy at work, the natural process of things at a high energy state decaying to a lower energy state, which Mother Nature prefers.

We as firearms owners, enthusiasts, and those engaged in the gunsmithing trade have a keen interest in maintaining our weapons and those of our customers in their as-manufactured, corrosion-free condition, and many owners follow proper procedures in care of their guns. Some owners unknowingly ask for trouble and then wonder why their firearms rust.

Corrosion, otherwise known as oxidation, is an electrochemical reaction in which a minimum of three components are required: An anode, being the metal that is prone to corrode; a corrosive chemical agent called an electrolyte, such as plain water or a salt water solution containing chloride ions that causes a metal to rust; and oxygen to do the, well, oxidizing. Remove any one of these ingredients and corrodible things simply don’t corrode, period.

An example of controlled, desirable corrosion is the electrochemical reaction that takes place in a carbon-zinc battery. The battery is an " A badly corroded firearm demanding attention. electrochemical cell, just like aluminum and stainless steel would be, but carbon and zinc generate a higher voltage (potential) than an aluminum and stainless steel battery and lasts far longer.

Another corrosion process is galvanic corrosion in which two dissimilar metals in the presence of an electrolyte cause corrosion of the more active (corrodible) of the two metals. The most common electrolyte is water, including freshwater, seawater, acids, and alkalis. Even overnight dew condensing on that hammer left beside an outdoor project is an effective electrolyte.

When a galvanic couple forms, one of the dissimilar metals in the couple becomes the anode and corrodes faster than it would alone, while the other becomes the cathode and generally does not corrode, or if corrosion occurs it is in a localized area forming one or more pits. An example of galvanic corrosion is the corrosion of aluminum rivets joining either stainless steel or carbon steel sheet metal, a practice that is not recommended as the anodic aluminum rivets will corrode and the joint will fail.

Another example is a do-it-yourselfer who is "getting tired of the bolts rusting" on a vehicle or piece of yard or snow removal equipment and replacing them with stainless steel fasteners. Instead of the stainless fasteners rusting, the anodic carbon steel sheet metal they hold together corrodes, and quickly. When joining carbon steel components, it is recommended to use zinc plated, or better yet, zinc-rich coated fasteners than stainless fasteners in those applications.

In both basic or galvanic corrosion, the active or corrodible metal combines with oxygen from the atmosphere to form the metal oxide, more commonly known as rust. In the case of iron and steel, the red or orange oxide is Fe2O3, which is two iron atoms and three oxygen atoms that makes up the common iron or steel rust with which everyone is familiar. Iron and steel can also develop another kind of rust, Fe3O4, which is known as black rust or magnetite. This is the rust that constitutes rust bluing.

Black rust is a moderately-performing barrier to oxygen transfer, and thus helps to prevent red rust from forming on firearms as the gun is already "rusted" with black rust. It’s why early gunmakers used rust bluing to finish metal bits—corrosion protection. The more active metals, such as magnesium, zinc, and aluminum also develop "rust"—white rust. An

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k.a. oxidation. Corrodible iron loses electrons through water or another electrolyte. Oxygen binds with the iron ions in the electrolyte in place of the lost electrons and forms red rust, Fe2O3. Remove any one component and the process stops. In this diagram, rust is indicated by the "eggs" on the nails. Boiled water contains no dissolved gases, thus no oxygen.

Salt increases electrolyte conductivity and corrosivity. example of this corrosion is an aluminum vehicle wheel that exhibits fluffy, white, or light gray deposits under peeling clear, silver, gray, or black powder coating films. Fasteners, being zinc electroplated or coated with high-solids metallic zinc particles in a binder to provide corrosion protection, also form white rust.

In the presence of an electrolyte, the anodic zinc combines with oxygen to form white zinc oxide, preferentially to the cathodic carbon steel basis metal. Also present are zinc hydroxides and zinc carbonates, insoluble compounds that act as a barrier to further corrosive electrolyte attack. Since zinc is more galvanically active than steel, and relatively inexpensive, zinc plating and coating is widely used to protect steel in commercial and industrial use.

But zinc would not work for firearms because it is so reactive, is not mechanically robust, and is not aesthetically pleasing. Magnesium and magnesium alloys also form white rust—as anyone who has owned a lawn mower with a cast magnesium mower deck can attest, the underside of the magnesium deck reacts with acids in the cut grass and the action of bacteria and fungus to form powdery, white magnesium oxide if not washed away routinely. Like zinc, magnesium is not for guns.

Aluminum readily forms aluminum oxide on its surface in open air, and if the aluminum oxide is removed by chemical or mechanical means, the surface starts to form more aluminum oxide immediately, within minutes. This is why aluminum is popular and widely used as an unfinished architectural material. Anodizing is the controlled electrochemical process of forming aluminum oxide on the surface of aluminum.

Anodizing is an effective corrosion prevention mechanism for aluminum; the layer of anodizing is electrically non-conductive and blocks the electrochemical corrosive attack from an electrolyte, and also blocks the basis metal from oxygen in the atmosphere. In the firearms world, alloy handgun frames and AR upper and lower receivers are "hard" anodized and dyed black, silver, or other colors (e.g., Charter Arms revolvers).

Galvanic Series To understand this more in depth, look at the galvanic series of common metals, with the most active metals (readily corrodible, or anodic) at the top, and the most noble metals (least corrodible, or cathodic) at the bottom. Generally, the closer together dissimilar metals are on this galvanic series, the less probability of corrosion.

A common practice of using metals adjacent or close on the galvanic series is the use of zinc plated or zinc-rich coated fasteners to join carbon steel or aluminum parts, as mentioned previously. The zinc will corrode preferentially before the steel fastener substrate does, because the zinc is the more active (anodic) metal. Notice that the stainless steels, nickel, Inconel, and Hastelloy-C alloys appear twice on the galvanic series, with the parenthetical suffixes of active and passive.

An active alloy is one where free iron particles from machine tool bits, shaping rollers, carbon steel grinding dust contamination, or other ferrous fabrication equipment are present on the surface. In the case of an active stainless steel in the presence of an electrolyte, the microscopic free iron particles corrode and form discreet pits of galvanic corrosion in the stainless steel. These free iron particles initiate corrosion and concentrate it in the area immediately around the iron particle.

The corroding iron particle overcomes the ability of the chromium oxide on the surface of the stainless steel to resist corrosion, and a corrosion pit forms in the stainless steel. Recall that galvanic corrosion involves two dissimilar metals in contact with an electrolyte. This free iron pit corrosion can be problematic to a stainless steel article.

After all fabrication operations are completed, some manufacturers of stainless steel products submerge them in a strong nitric, nitric/sulfuric, or citric acid solution to "pickle" (dissolve) the free iron and thus "passivate" the surface of the stainless steel, making the surface passive to free iron corrosive attack. For example, stainless steel products used in food preparation are customarily passivated.

Again, passive stainless and other alloys are much further down the galvanic series, meaning that they are less prone to corrosion.

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Far This does not mean that this sink is made from a "poor grade of stainless steel," rather, the wrong grade of stainless for the intended use and likely not passivated to remove free iron. Second, the brass sheet corroding under the SS nut. Third, the aluminum sheet in contact with the brass sheet corroding where exposed to air and humidity. The more noble stainless steel is not corroding and is causing rapid corrosion of the two more active metals.

In firearm manufacture and gunsmithing, machining of stainless steel firearm components is a given part of the process. Some firearm manufacturers and gunsmiths incorporate the process of passivation of stainless firearm components following machining operations. A technical discussion of the stainless firearm passivation process is outside the scope of this article, and there is much information on passivation processes on the Internet.

I positively cringe when someone posts a recommendation in a firearm-related Interwebz forum or DIY video to use steel wool to polish out scratches in a stainless steel handgun or rifle.

Those who take steel wool to stainless steel are making the surface of the stainless steel much more active and prone 1 Magnesium 2 Magnesium alloys 3 Zinc 4 Aluminum 1100 5 Cadmium 6 Aluminum 2024-T4 (copper-containing Al alloy) 7 Steel, Malleable Iron 8 Cast Iron 9 410 Stainless Steel (active) 10 304 Stainless Steel (active) 11 316 Stainless Steel (active) 12 Lead/Tin Solder to corrosion.

Should the surface polished with steel wool come in contact with an electrolyte—even humidity—the stainless steel will rust again, and more readily! Never use steel wool on stainless steel! There are plenty of non-metallic abrasives that can be used, including abrasive-containing polymer non-woven meshes (e.g., 3M Scotch- Brite) in a variety of grit sizes as well as old fashioned silicon carbide wet/ dry sandpaper up to superfinishing 4000 grit for high polishing.

These abrasives do not promote corrosion, providing they were not previously used on iron or steel. 13 Lead 14 Tin 15 Nickel (active) 16 Inconel nickel-chromium alloys (active) 17 Hastelloy C (active) 18 Brasses 19 Copper 20 Bronzes 21 Copper-Nickel Alloys 22 410 Stainless Steel (passive) 23 Monel nickel-copper alloys 24 Silver Solder 25 Nickel (passive) 26 Inconel nickel-chromium alloys (passive) 27 Chromium-Iron (passive) 28 304 Stainless Steel (passive) 29 316 Stainless Steel (passive) 30 Hastelloy C (passive) 31 Silver 32 Titanium 33 Palladium 34 Graphite 35 Gold / Platinum Another caution regarding stainless steel firearms is to remove them from your work area where you routinely grind carbon steel.

The grinding dust is rich in steel and iron oxide. If that grinding dust falls on a stainless steel firearm, the more minute particles will become trapped in polishing marks in the stainless steel, setting up future corrosion trouble by activating the stainless surface. Wiping or blowing off ferrous grinding dust is ineffective, due to dust particle size and slight magnetization of the dust or the stainless steel firearm components.

A reminder: Most stainless steels used in firearms are martensitic, because those SS alloys are generally easier to machine. Speaking of machinability, high machinability steels generally contain molybdenum disulfide, and sulfur is another corrosion promoter. High machinability steels readily corrode and need to be protected.

If you use rubberized abrasive bits in a Dremel or Foredom tool for polishing work on stainless steel and carbon steel firearms, it is highly recommended that you maintain two segregated sets of these polishing bits, one for stainless work and another for the carbon steel work so that fine, free iron particles em-

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Light surface rust can be removed using bronze wool or a credit card and a penetrating oil such as Kroil. Bronze wool and credit card plastic are softer than firearm bluing and will not harm bluing as long as the rusty oil slurry is wiped off and not rubbed around. Contrary to what is often published, 0000 steel wool WILL remove or thin bluing. bedded in a bit used on carbon steel is not used on a stainless firearm.

The same applies to having two sets of abrasive polishing cloths or papers—keep them separate, or if need be, downgrade polishing materials from use on stainless to carbon steel only and never back. Another dissimilar metal situation with potential for corrosion is set up when an aluminum alloy, carbon steel, or stainless steel firearm is brought in contact with graphite. Looking at the galvanic series, graphite is more noble than titanium but more active than gold or platinum.

Graphite is a particular crystalline structure of carbon, just like carbon black or diamond are also just carbon atoms with different crystal structures. Graphite is a Pitting corrosion to a stainless steel handgun frame, likely caused by contact with either rubber or carbon fiber reinforced grip panels that had absorbed moisture or due to the handgun being stored in a vehicle or under some other high humidity conditions. popular dry lubricant; the purer it is, the more electrically conductive it becomes.

A graphite-containing dry or wet lubricant when used on aluminum alloy, carbon steel, and stainless steel (either passive or active) will cause those metals to form corrosion pits under the right circumstances with the right electrolyte. To avoid a potential corrosion situation, NEVER use graphite on firearms! Carbon itself in some forms can promote corrosion. There are many firearm accessory products made of reinforced carbon fiber, such as grips, mounts, brackets, etc.

Despite all of the excellent properties of carbon fiber reinforced (CFR) components, there can be issues with using CFR articles and metals together. The carbon fibers in CFRs cause this material to be highly electrically conductive. Therefore, when a more active metal is physically and thus electrically connected to a CFR article, that metal component is more susceptible to galvanic corrosion, examples of which are carbon steel or unpassivated stainless steel.

This situation becomes worse when a large surface area of a carbon fiber reinforced component like a grip plate is coupled to small and anodic carbon steel grip screws. Those screws will develop corrosion quickly in the presence of moisture in any form unless protected with a corrosion-inhibiting product such as Corrosion X, Eezox, Cortec Bullfrog, or WD-40 Specialist Long-Term Corrosion Inhibitor, among others.

When in contact with a carbon fiber composite article aluminum and unpassivated stainless steel are likely to be prone to galvanic corrosion as well. Galvanic corrosion with carbon fiber components has been an issue for decades, but there is no real universal solution to the problem, aside from providing some sort of non-conductive barrier between the CFR article and the adjoining Owners have been reporting occurrences of pit corrosion under grip frames.

Keep in mind that castings are porous unless the porosity is sealed by vacuum impregnation of a methacrylate resin, an expensive process. The alternative is to warm the frame and repeatedly apply a proven corrosion inhibiting product to fill up the porosity much the way oil fills the pores of an Oilite bronze bushing.

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Such studies are fraught with procedural issues that can invalidate the results, including suspending samples above other samples as in the photo, where the corrosive liquid can drip on samples below; using a hand spray bottle to apply a corrosive electrolyte, such spray bottles are not consistent applicators of the fluid sprayed; and uneven or inconsistent application of the maintenance fluid under test, leading to false failures of products applied thinner than others. Let the observer beware! metal.

In the case of grip panels, there are companies that offer thin polymer sheet isolators in the shape of grip panels to physically block the contact of CFR grip panels with the grip frame. In the absence of a polymer isolator, firearms with carbon fiber grips, stocks, and other accessories should be disassembled on a regular basis and the fasteners and metal surfaces in contact with the CFR items should be protected with a corrosion-inhibiting product as mentioned above.

Corrosion-inhibiting products generally work by conveying barrier protection to the corrosion-susceptible metal. This can be as simple as a thin layer of a hydrophobic material such as a grease-based product. The more sophisticated products block oxygen transfer by means of a molecule-thin oxygen barrier consisting of one of a number of proprietary ingredients. If oxygen cannot contact the more anodic (corrosion-prone) metal, then that metal can’t corrode.

Similar oxygen-blocking technology is that used in vapor corrosion inhibiting (VCI) paper and polymer films, such as those made by Cortec and Daubert-Cromwell. An example of VCI paper known to many firearms enthusiasts is the light brown paper that Smith & Wesson handguns were once wrapped in, inside their boxes or plastic cases. VCI products are impregnated with an oxygen-blocking compound that in the case of paper is impregnated with a solvent.

For polymer VCI products such as films and bags, the oxygen blocking agent is usually thinly coated to the dull side of the polymer film. VCI-containing polymer film products are typically tinted a color, such as blue, green, or yellow, indicating the type of corrosion inhibitor, the intended use of the film or bag, or the relative "strength" of the inhibitor. Today, new firearms are frequently packed in a VCI bag at the factory. The problem with volatile corrosion inhibitor products is, well, their volatility.

Unless kept in a tightly-sealed bag, the active ingredients dissipate. VCI products need to be replaced on a regular basis; for brown VCI paper, that’s when it no longer feels damp to the touch, indicating the solvent carrier has evaporated and dispersed. Another potential source for often-seen corrosion of a handgun grip frame beneath rubber grips. The grip frame can be either blued carbon steel, aluminum, or stainless steel.

Any of these materials can develop corrosion from the rubber itself or the rubber in conjunction with a lubricant or firearm care product that either degrades the rubber or contains water or water-based compounds that can act as an electrolyte. For example, Ballistol is a well-known product used in gun care, and one of its selling points is that it readily absorbs water and mixes with water. Ballistol contains mineral oil and some butyl alcohol which is soluble in both oil and water.

The butyl alcohol acts as an emulsifier for any water with which Ballistol comes in contact. In my opinion, that is not an advantage. Rubber and wood are somewhat porous and can absorb and trap moisture from the air or from a product like Ballistol which will act as the electrolyte that can cause corrosion. Rubber grips should be removed periodically and washed with a detergent and allowed to dry thoroughly, and the grip frames should be treated with a coating of a corrosion-inhibiting product as mentioned above.

Wood grips used to be finished on the inside surfaces as well as the outside, however, today as a cost-saving measure the wood grips that I’ve seen are sealed only on the outside. Wood grips on new guns should be removed and their inner surfaces sealed with a few coats of lacquer or shellac to prevent moisture absorption. Another cringe-worthy practice advocated by firearm forum participants is using automotive or industrial maintenance fluids for firearm care.

Many times I’ve read contributions of forum members stating that they lubricate their firearms with a leading brand of synthetic engine oil and "have never had a problem." They should modify their statements to include the word "yet" because problems will arise eventually from multi-purposing products intended for a much different service.

Staying with engine oil for the moment, that product customarily contains additives that are designed to attract and keep fine particulate matter in suspension, and to absorb the moisture that condenses out of the air in the crankcase ventilation systems of engines, so that when the engine warms up next time the absorbed moisture can evaporate out of the oil. Engine oil being engine oil and doing what it is designed

to, I’m not certain that I want dirt particles suspended in a moisturegathering lubricant in my firearms, let alone those of others. Moisture gets absorbed in motor oil used to lube a firearm but never has the opportunity to have that moisture evaporate; guns don’t attain 195 degrees Fahrenheit for lengths of time. If you’re wired to be cheap and collect your "gun oil" from what eventually drips out of an upturned quart bottle or five gallon jug, maybe it’s worth the risk to you, but not to me.

Back in the 1980s, the practice of coating bullets with molybdenum disulfide was popularized by proponents who claimed higher velocities and enhanced accuracy. Some used commercial products intended for bullet lubrication, others "found" Mo S2-containing products at work and used them. Moly’s popularity dropped off when it was found to build up in rifling and cause a drop in accuracy.

A second observation was made: Rifling seemed to become corroded as evidenced by rusty orange deposits or stains on cleaning patches. Sulfur just being sulfur and absorbing water vapor from the air forms an acid. Recommendation: Use gun lubricants and other firearm and reloading products designed for their intended use. Corrosion of firearms inside gun safes has been attributed to the use of imported drywall sheeting as the fireproof insulation material in construction.

Formaldehyde and other chemicals and high levels of iron pyrites, found naturally in gypsum manufactured in China or elsewhere using raw Chinese gypsum, have been alleged to cause corrosion of firearms. Certain bacteria that feed on iron pyrites generate vapor byproducts that can be corrosive to steel in an enclosed space.

Some safe manufacturers emphasize that they only use domestic drywall, other manufacturers of gun safes emphasize that non-carpeted steel-only safes offer the best corrosion protection because gypsum and moistureabsorbing carpeting is avoided altogether. I cannot comment one way or the other because of lack of personal experience or controlled standardized testing results.

It is recommended that some means of heat and/or desiccation be employed inside a safe to ensure that the temperature of enclosed firearms stays well above the dew point of the safe location and interior. Use of desiccation reduces the amount of water vapor (humidity) in the safe interior atmosphere, particularly in humid summer months.

The colorchanging silica gel desiccators give a visual indication of their saturation and loss of efficacy, and some of them can be regenerated by oven baking at a recommended temperature for several hours. One long acquaintance of mine didn’t want to spend the money on silica gel desiccators so he recalled the deliquescent properties of calcium chloride and put containers of it in his two gun safes.

Chlorides, be they sodium, potassium, calcium, or magnesium are extremely efficient promoters of corrosion in humid environments! Right after the first June, July, and August with calcium chloride in his safes he called, asking about removing rust from his guns. Don’t be that (cheap) guy!

Speaking of humidity, dew point, and condensation, a reminder is made for gunsmiths and their customers to refrain from immediately uncasing firearms that have spent time inside a vehicle interior or trunk during cooler months and especially wintertime. Uncasing a cold firearm in a humid environment can and will result in a microscopically thin layer of moisture condensing on the firearm surface.

Usually the residual oil on the metal surface, or bluing, anodizing, or the chromium layer on the surface of stainless steel will effectively block oxygen transfer, but I have personally seen blued firearms that developed fine rust in a matter of hours or overnight from being very cold and then introduced into a humid environment. Let the cased firearms acclimate for several hours or overnight before bringing them out, and apply oil or a corrosion inhibitor to their metal bits frequently.

Readers may have seen corrosion test videos on the Interwebz where the video producers compare the anti-corrosive performance of various maintenance fluids. The betterproduced of these videos go to great lengths to describe the methodology employed for the test, in an effort to imply some measure of validity.

While these videos may demonstrate some degree of relative corrosion inhibition performance using the materials and methodologies as tested, it is recommended that viewers consider these videos to be anecdotal information at best.

In order to perform truly valid comparative studies using sodium chloride as the electrolyte, one would use standardized corrosion exposure equipment constructed in accordance with ASTM B117, ISO 9227, or similar standard practices using 5% pure sodium chloride, and not iodized table salt which adds to the salt’s corrosivity. Those so-called 5% Neutral p H Salt Fog test standards also require precise control of the exposure zone temperature and fog atomizer pressure, and for control of the fog condensation rate.

Uniform application of the tested products to standardized steel test panels is also required for test validity. Such a study is outside the scope of this article and could well be the subject of its own future article. Suffice to say that Interwebz videos of performance testing of corrosion inhibiting products should be taken with a grain of salt (sorry!). An article such as this one can only scratch the surface of the subject.

Corrosion is one of those topics where a lot of knowledge of the subject can avoid significant cost, a little knowledge can be slightly costly, and no knowledge whatsoever can be incomparably expensive. AG