Topic: Corrosivity Of Some BPs and Gun Metals

Retrieved: 12/12/2014
Last Post: 03/26/2006

The following is from an e-mail exchange between Dutch Bill and me some years ago. I obtained Bill's permission to post this on Cas City and did so on the NCOWS Forum because of the many folks there that shoot valuable old guns with various BPs. It was well received there. Many of the shooters frequenting this board do not frequent the NCOWS Forum, maybe don't even read it, so thought I would post this information here also.

At the time this was written I was getting interested in Hodgdon's 777 FFG for use in some of my old guns. All have steels with much less resistance to corrosion than modern steels, such as used in Rugers and Rossis. While the metals used in the barrels have excellent mechanical properties, the presence of abundant granular impurities and other artifacts of steel making in the late 1800s and early 1900s result in the need for much greater attention to preventing corrosion, especially in all-weather hunting, than do my few post WW2 guns.

I had been using Pyrodex and some Goex in them for years but, wanted the least corrosive BP available. Swiss and Hodgdon's 777 FFG seemed the best choices. I do clean my old guns at the end of each shooting day, sometimes even during a days shooting, for example, between trap or sporting clays rounds (25 shots per barrel), LRR rounds (usually 10 shots), etc. Still, some of these old guns have pitted bores, most likely largely because of inadequate or no cleaning after shooting early nitro loads having corrosive primers, but, granular impurities are clearly areas of locallized corrosion on one barrel of one gun. In the most pitted of these old barrels I no longer shoot any BP and am rather a fanatic about cleaning the bores of those old guns after each day's use -- especially the one that is my all-time favorite hunting hammer double and sporting clays gun.

Bill had just the information I wanted. Perhaps this information will be valuable to others here.

I saw your posting on the SASS Wire regarding questions on the relative corrosiveness of Triple Seven.

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Any powder using potassium nitrate as an oxygen source is capable of rusting/corroding various metals under certain conditions. These powders produce potassium carbonate as a primary solid product of combustion. Under certain atmospheric conditions this potassium carbonate will cause rusting of ferrous metals or leaching of brass alloys.

When the R.H. is 30%, or less, the deposits of potassium carbonate are non- hygroscopic. Without sufficient moisture the potash will not form an electrolyte solution on the surface of the metal. Above 30% R.H. the potassium carbonate, or potash, begins to pick up moisture from the air.

As the R.H. rises the amount of water picked up by the potassium carbonate increases. Once "damp" it forms an electolyte. This causes a uniform surface rusting of ferrous metals. On brass it will leach copper from the alloy which gives the green colored deposits on the surface of the brass. Under the microscope the surfaces of the metal show a uniform removal of surface metals without signs of any pitting. I have used my computer microscope to watch this uniform metal removal over periods of time.

When you have a powder's combustion residue with any traces of a chloride, such as potassium chloride, you will then see scattered pitting of the surface in addition to the uniform surface metal leaching or rusting.

When Hodgon formulated Triple seven they greatly reduced of amount of potassium perchlorate compared to the amount found in Pyrodex, about 17% in Pyrodex, based on the patent covering Pyrodex. I ran Triple Seven against Pyrodex and the Swiss BP on mild steel plates and on sheet brass and sheet copper.

The extent of surface leaching was the same with all three powders.

All produce potassium carbonate as a primary solid product of combustion. The Swiss use a chloride-free grade of potassium nitrate so there is no evidence of pitting. Triple seven produces a few widely scattered pits in the surface of the metals. Pyrodex produces extensive pitting of the various metals' surfaces. Essentially, the extent or degree of surface pitting simply reflects the chloride content of the respective powders.

But your Aug. 4, 2003, 8:03 AM posting regarding the different grades of steels is correct. Some steels being more corrosion resistant. The mild steel used in some muzzleloading rifle barrels is one of the least corrosion resistant steels. In most cases, the harder steels are more corrosion resistant.

So the whole issue is clouded by what sort of steel is involved and differences in climate produces varying results with the same powder.

Regarding your comments on Du Pont and Goex. Mixed bag with them.

Ever since WWI the U.S. black powder industry was at the mercy of their raw material suppliers as far as ingredient purity goes. From the early 1900's until 1986 almost all of the domestically produced potassium nitrate was made by the conversion of synthetically produced sodium nitrate. Generally, this potassium nitrate would be 0.4 to 0.5% sodium nitrate. This gave a powder with an affinity for moisture greater than if the potassium nitrate is free of sodium nitrate. In 1986 the largest producer of potassium nitrate in the U.S. began a process wherby potassium chloride is reacted with nitric acid to yield potassium nitrate and free chlorine. This process gives a 99.0% conversion effeciencty. The plant produced mainly fertilizer grade potassium nitrate. This was used at Goex's Moosic, PA plant. This source contained about 0.4% sodium nitrate and a fraction of a percent of unconverted potassium chloride. This company ceased operations in 2000 which ended potassium nitrate production in the U.S. GOEX was then forced to use imported potassium nitrate. The new source is of a higher purity compared to the old source. This source appears to be free of any sodium compounds and chlorides. In most respects the powder presently being produced by GOEX is superior to the Du Pont it replaced.

The thing that must be pointed out is that any powder based on the use of potassium nitrate will yield a combustion residue capable of rusting/corrosion under specific conditions of climate.

So your analyses of differences in steels is 100% correct but throw in differences in climate conditions as an additional variable in this.

Regards, Bill K. (aka. Dutch Bill)

This explains why my muzzle loader grew brown "fur" and my other arms have not had a similar experience. The muzzle loader steel was apparently more susceptible to rust.