Sorry for being slow to continue this thread. It took me a few days to put some more information together.



Here is another reference, agreeing that chemical corrosion propagates from ferrous grain boundaries. This, from a document on the use of etching solutions to prepare samples for metallurgical testing.

National Board of Boiler Inspectors - Grain Boundaries

ETCHING:
In order to observe the microstructure, a piece of the metal is smoothly polished to a plane and mirror-like finish. The prepared surface is chemically attacked with dilute acid for a short period, a process called "etching." The grain-boundary atoms are more easily and rapidly dissolved or "corroded" than the atoms within the grains. A small groove is left at the grain boundaries. Since a groove will not reflect light as do the flat, polished grains; the grain boundaries appear as black lines, and the structural details are visible.



So to review; in finishing of damascus steel, the goal is to create a bold display of the damascus pattern. Knifemakers use destructive electro chemical corrosion to create deep etching and topography between the layers. Damascus gun barrel finishing, typically uses weaker etchant and acid solutions to create only a small amount of topography and gently develop the light vs. dark coloring to display the damascus pattern. BOTH, finishing processes, utilize electro chemical corrosion reactions to achieve the finish on the surface of the material.

Steel is the ferrous material in damascus that is very reactive to the etchant solution. As corrosion propagates at the grain boundaries, more active corrosion is facilitated by a ferrous material with a small grain structure. The carbon in steel also tends to precipitate into the grain boundaries, as cementite. The particles of cementite in the grain boundaries, provide more anodic connections for the electro chemical corrosion reaction. Steel being very reactive to corrosion, will create the colored (brown/black) layers in the damascus pattern.

The other layers in the damaacus pattern need to be of a light color. There are two ways to accomplish this. Simply use a ferrous material that has a larger and less dense grain structure. Or, use a ferrous material that has alloys that resist corrosion. As noted in the earlier comment about the steels that knifemakers use for damascus steel, 15N20 steel, with a corrosion resistant 2 percent nickel content is laminated with the much more reactive 1084 steel.

Damascus steel gun barrels have wrought iron, as one of the ferrous materials used. Wrought iron lacks the alloys necessary to create fine grain structure and it also lacks enough carbon to precipitate out of the ferrous grains to create a high number of anodic connections to facilitate electro chemical corrosion. But more importantly it contains silica, which acts as a resist to the etchant solution, inhibiting the effects of electro chemical corrosion.

Artmetal.com - Wrought Iron

The slag in wrought iron also provides natural corrosion resistance. Let's face it, nearly all ferrous metals rust, but wrought iron does a better job at handling it. As corrosion progresses, the fibers tend to disperse the rust into an even film, which gives the metal a natural brownish appearance. This film repels the scattering spotty corrosive attack t that other metals endure.
Because of its corrosion resistance, wrought was the metal of choice in earlier years for marine use, bridges, and girders. In fact, in extremely corrosive areas, an architect may still specify the metal.

Real Wrought Iron Company

The important corrosion properties of wrought iron are therefore due to its impurities in the form of slag. It follows that iron and steel without the slag will not exhibit the same corrosion resistance, and this is what is found in practise. Furthermore the slag has additional advantageous in relation to traditional forging techniques, especially fire welding.



So finally…

My THEORY is that the wrought iron layers of gun barrel damascus are always white. Because wrought iron is resistant to the electro chemical corrosion process. The steel layers are always the layers that bear the color (brown/black). Because the grain structure and carbon content of steel promote the electro chemical corrosion reaction.

I would really like to hear from the guys who finish damascus gun barrels. Which material do you perceive etches light and dark? Do you do your finish work with a plan to turn either the wrought iron or the steel dark? Are you able to reverse the colors on the barrel by changing your process?

Steve Culver
Steve Culver Knives