Molasses and Gray Cast Iron - WARNING

[Autofil]

Equipped with my big IBC container (260 gallon) converted for molasses rust removal, I have done several rust strip batches of various rusted truck parts. With a 1:10 molasses/water mixture, soak time has been 2-3 weeks heated to 100 degrees F. 

 

I have noticed that regular cast iron get very clean, but may loose the smooth surface finish. The same goes for regular steel. Machined surfaces maintain a smooth appearance. 

 

The big mistake and a even bigger jump in my learning curve came today; picking up the last batch consisting of rare/hard to find parts for my 1935 Studebaker truck build. The clutch housing and the water pump housing was obviously casted in the popular gray cast iron. Despite being popular among automotive design engineers, the material do not react very well with molasses.... Big mistake! 

 

Graphitic Corrosion in Grey Cast Iron is the terminology.

 

Approximately a 1/32" thickness of the normally solid material hits a weak spot; getting soft and very much like the graphite of your pencil. You can scrape the surface of the casting, and it will leave a new harder surface beneath. All details like machined surfaces gets wobbly or flaking, and once cleaned up, the details are gone (at least the tight tolerances). Fine threads are gone too. 

 

I will be able to save the clutch housing, even if it means to re-surface the mating surfaces. The water pump....not really sure, I may need to re-manufacture some details as a minimum.... 

 

A tough lesson learned..... Mechanical rust removal, OK. Chemical rust removal, a completely different ballgame.

Edited January 3, 2018 by Autofil (see edit history)

[jpage]

I had that happen to a Model A tranny case using the ZEP industrial rust remover to clean it. I don't remember the exact material but it just turn the casting so soft you could scrape off the iron with a fingernail!

[Spinneyhill]

I have used electrolytic rust removal on castings. That worked, but revealed a deep crack that made the part unusable. I suppose it is important to make sure the electrolyte is slightly alkaline.

Edited January 4, 2018 by Spinneyhill (see edit history)

[Autofil]

1 hour ago, jpage said:

I had that happen to a Model A tranny case using the ZEP industrial rust remover to clean it. I don't remember the exact material but it just turn the casting so soft you could scrape off the iron with a fingernail!

That is how soft it gets. The thing is that for molasses, when the rust is gone, the active part of the molasses will continue processing the iron. For regular iron parts, the material loss is fairly limited. For gray cast iron, the molasses will remove the iron structure in the surface of the parts, leaving only the non-corrosive elements of the cast iron, which are soft and brittle. I will assume the same process was initiated with the ZEP industrial rust remover.

 

I lesson learned indeed.

[Autofil]

1 hour ago, Spinneyhill said:

I have used electrolytic rust removal on castings. That worked, but revealed a deep crack that made the part unusable. I suppose it is important to make sure the electrolyte is slightly alkaline.

I think electrolytic is better in that sense. It will only remove rust, not the conductive material. An balance the solution pH level yes.