Corrosion

Discussion in 'Gunsmithing' started by pokute, May 13, 2015.

  1. pokute

    pokute Sincere as a $5 funeral

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    In the last couple of days, I've been trying to find reliable information about enhancing erosion resistance in firearm chambers. I was spurred to do this by the discovery that an FA revolver that I recently purchased has a replaceable threaded carbide forcing cone.

    What I discovered was an interesting accumulation of first and second hand presumptions, some better than others, and none based on a comfortable understanding of the physical-chemical processes that exist where the lead meets the iron.

    So, in the interest of disseminating vital fundamental knowledge, here goes my take on the whole thing. For the sake of brevity I will refer to the affected parts of the gun as the bore. I do this because one could saw off the barrel quite short without affecting the processes that I want to discuss.

    Definitions and fundamental processes:


    • Erosion is the observable effect of corrosion. Corrosion may occur very slowly, and after years of apparently benign inaction, erosion may be observed.
    • Bore erosion is essentially oxidation, though not entirely "rusting", which is the term we use to describe the unwanted production of red Iron Oxide on the surface (and yes, it must always form on an exposed surface - More on this shortly) of Iron and it's compounds and mixtures.
    • Oxidation is a chemical process, and to a first approximation, chemical processes double in rate for a temperature rise of 10 degrees Centigrade. This is what we call a "rule of thumb", and it is a very rough approximation, but we can make good use of it here. Given this rule, we can say that whatever is happening is happening at a rate proportional to 2 raised to the power of the change in temperature divided by 10. In the regime we are looking at, it means that the rate of corrosion increases by a factor of 1000 for every rise of 200 degrees F.
    • On a very small scale, the corrosion is proportional to the temperature rise of the propellant gases. On a large scale, it is proportional to the temperature rise of the barrel.
    • Shear stress is any combination of pure tensile and compressive stresses. Pure uncombined tensile and compressive stresses do not exist in nature (except at the atomic level). Everything that bends and flows does so under the influence of shear.

    Okay, understanding that we can proceed with some general facts regarding the processes that affect bore erosion.

    Bore erosion is a combination of very small scale chemical processes and larger scale mechanical processes. Metals consist of small highly ordered crystal grains mechanically cemented together. The crystals are capable of flow under shear stress, the cement boundaries are relatively brittle and are broken under shearing stress.

    The condition in the bore during ignition is hellish. Gases and particles are churning around at rates of thousands of meters per second, and temperature is rising to several thousand degrees, expanding the gases and increasing pressure "explosively".

    At a microscopic scale, the surface of the bore is a forest of burrs and deep chasms. the tiny tips of these burrs are literally ignited in the hot, oxygen rich, burning gas atmosphere, and new burrs are being raised by abrasion from solid particles of unburned and partially burned propellant and lubricant. The remaining exposed metal is partially oxidized, and the oxides formed are highly resistant to further corrosion (Gee, sounds just like rust bluing, doesn't it?).

    Unfortunately the protection of the oxide layer is short lived, since the next blast of hot gas and particles tears most of it away.

    At the same time that all of this is going on, the barrel metal is heating and expanding, causing failure of the cement between the metal grains, allowing hot gases to attack the exposed faces.

    Interestingly, this all happens at very nearly the same rate for pure iron and high alloy stainless.

    That should be a big enough wad of chaw for folks to get their jaws around at this point.

    NEXT: Inhibiting corrosion

    I would be very interested in hearing bore erosion stories from forum members in this thread.
     
    Last edited: May 13, 2015