Joshua Culp
10-14-2010, 6:46 AM
A few months ago someone posted that their cast iron table top would immediately re-rust. They tried every method and chemical that readers could suggest and nothing worked.
I read the following post on Smokstak.com, an antique engine website, and thought it may be useful to some folks here as well. The poster's username on Smokstak is speleausmining.
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The other distinct possibility if break cleaner or de-greaser does not get it resolves is chemical and has to do with the fuel they used for smelting the iron and to a lesser extent the ore they smelted. Alot of Iron was smelted with coal which depending on grade can be from low to high sulphur content. For iron smelting they tended to use antracite or other high grade coals because of this propblem. Basically if there was a low grade coal in the smelt (Low grade coal is often high in sulphur) your resulting product will be a high sulphur content cast iron. Sulphur and iron want to combine to form Iron sulphides (FeSO2). Sulphur and iron will react with even minor ammouts of moisture in the air to cause oxide coating to form fast, often almost before your eyes. Sulphur is tricky to get out of the iron. Heating in a reducing enviroment (oxygen poor flame) can sometimes drive it out. I have run into this problem before on artifacts recovered at old smelter sites (incomplete castings etc). The real drag is unless you can resolve it even sealing and painting rarely results in halting the deterioration and may actually accelerate it. I had a partial leaver casting from an early iron foundary literally disintegrate to dust over a period of months from this. It was fine when first uncovered from a clay soil (the clay soil kept the artifact sealed in a Oxygen poor enviroment knee-caping the oxidation process), in fact it looked like it could have been cast last year when recovered but on exposure to air one area startend to oxidise dramatically and disinegrate. Its a similar chemical action that causes suphric Acid to be so destructive to iron it is spilled on. Iron will naturally rust forming iron oxide (FeO2) but presence of salts or suphur will cause it to form less stable and mor complex compounds. Its the same process that causes the interior of the combustion chamber and exhaust parts of an engine to deteriorate at a greater rate then elsewhere in the engine. Again sulphur fromt he fuel burned is the problematic agent.
---------- Post added 10-14-2010 at 12:04 AM ---------- Previous post was 10-13-2010 at 11:59 PM ----------
If it is a sulphur, salt, etc problem the cast iron in that area will while visually appearing fine be noticably softer then elsewhere in the casting. Often soft enough that a rasor blade can carve out deep gouges almost effortlessly in that area.
I read the following post on Smokstak.com, an antique engine website, and thought it may be useful to some folks here as well. The poster's username on Smokstak is speleausmining.
------------------------
The other distinct possibility if break cleaner or de-greaser does not get it resolves is chemical and has to do with the fuel they used for smelting the iron and to a lesser extent the ore they smelted. Alot of Iron was smelted with coal which depending on grade can be from low to high sulphur content. For iron smelting they tended to use antracite or other high grade coals because of this propblem. Basically if there was a low grade coal in the smelt (Low grade coal is often high in sulphur) your resulting product will be a high sulphur content cast iron. Sulphur and iron want to combine to form Iron sulphides (FeSO2). Sulphur and iron will react with even minor ammouts of moisture in the air to cause oxide coating to form fast, often almost before your eyes. Sulphur is tricky to get out of the iron. Heating in a reducing enviroment (oxygen poor flame) can sometimes drive it out. I have run into this problem before on artifacts recovered at old smelter sites (incomplete castings etc). The real drag is unless you can resolve it even sealing and painting rarely results in halting the deterioration and may actually accelerate it. I had a partial leaver casting from an early iron foundary literally disintegrate to dust over a period of months from this. It was fine when first uncovered from a clay soil (the clay soil kept the artifact sealed in a Oxygen poor enviroment knee-caping the oxidation process), in fact it looked like it could have been cast last year when recovered but on exposure to air one area startend to oxidise dramatically and disinegrate. Its a similar chemical action that causes suphric Acid to be so destructive to iron it is spilled on. Iron will naturally rust forming iron oxide (FeO2) but presence of salts or suphur will cause it to form less stable and mor complex compounds. Its the same process that causes the interior of the combustion chamber and exhaust parts of an engine to deteriorate at a greater rate then elsewhere in the engine. Again sulphur fromt he fuel burned is the problematic agent.
---------- Post added 10-14-2010 at 12:04 AM ---------- Previous post was 10-13-2010 at 11:59 PM ----------
If it is a sulphur, salt, etc problem the cast iron in that area will while visually appearing fine be noticably softer then elsewhere in the casting. Often soft enough that a rasor blade can carve out deep gouges almost effortlessly in that area.