Molasses has etched cast iron?!

The below is bugging me and I really want to understand why this (albeit not a big deal) happened.

I wanted to clean up my Tom Senior main vertical slideway. They famously hand a hand frosted finish. I wanted to maintain it as best I could.

A very experienced engineer relative said that a molasses solution soaked roll, draped over the area, will eventually let me wipe off the inert iron oxide.

I uncovered it 24 hours later for a look, and was disappointed to see that the whole surface was dulled.
After trying water, kerosene, WD40 to no avail, I deduced the iron’s been etched slightly, thus impeding the light reflection.
Despite a good polish, with Autosol, it’s not much better.
...I’m annoyed the surface has lost its shine.


Interesting the spots I missed with the tissue, seem unaffected. This supports my etching theory.
Please see the before and after photos.

I also submerged the front bearing plate in molasses for the same amount of time.By contrast the steel bearing plate came out very well....steel isn’t susceptible to molasses etching?


furthermore, I just read on a forum that molasses can attack grey cast iron?
Since I understand molasses works by chelation (A level chemistry to the rescue!!),’this doesn’t explain the acidic behaviour and I originally avoided acids for this reason.

many thanks for reading this far 😉

I've de-rusted quite a lot of steel components by immersion in molasses, with complete success, producing a bright finish, after the residual crud has been easily rubbed off. However, on some high-carbon steel items (gravers), the resulting finish has been dull grey, rather than the expected bright finish. I'm aware of warnings that air must be excluded from the surface to be treated, otherwise more corrosion occurs. I suppose it's possible that the soaked-tissue method allows oxygen to get to the surface, so perhaps the problem is further corrosion (oxidization), rather than etching (just material removal). However, this doesn't explain the poor finish achieved by immersing high-carbon steel.

I can only assume that the molasses sequesters some iron from the surface, leaving excess carbon, carbide, whatever, on/in the surface, dulling it. Or, perhaps, the grey represents black oxide, from rust conversion, in the 'pores' of the iron, which is consequently difficult to remove. Any chemists / metallurgists out there who know?

Have you tried to recover the cast-iron lustre by wire-brushing or using fine wire wool?

We discussed chelation in a previous thread, and I subsequently tried to find out more,only to discover that it's rather mysterious...

Many thanks for your reply Kiwi.

Sorry I wasn't clear, photo below is the "front bearing plate" I referred to. I assume this is made of steel? This *was* immersed in a bucket of molasses ie no oxygen contact. As you mentioned with your successes, this piece clearly worked very well in under 24 hrs of immersion. Or were your referring to your gravers?

Anyway, the bottom left shows the dulling/patchiness I described in my original post. Yes of course it's not a big deal, I'm just a perfectionist who wants to learn a much as I can along the way.

Aye, chelation is like magic pixie chemistry if you ask me...

For the benefit of others, I asked an academic engineer friend. Although he's avoided the molasses method at home himself, he responded with:

Grey cast iron is far more easy to etch than steel - a fact exploited during the production of metallography specimens in a foundry. I assume this is due to the rough/open surface compared with steel - all those open grain boundaries to attack. Molasses (I think) contains phosphoric acid which makes me think your casting has been etched. Perhaps more is going on with the high level of silicon and phosphorous likely to be in the cast iron??

Edited By choochoo_baloo on 28/11/2019 15:42:11

Forgot to ask. For rectifying - ideally restoring to the original finish - what method & abrasives should I use (I'm a novice)?

I was told: I suspect you will have to resort to using 1200 grit carborundum paper with WD40 to remove the etched layer. Wrap it around a parallel.

Don't restore the original finish any more. You are looking at a cosmetic effect. Metal removal will turn the cosmetic effect into 'wear' that will need work to correct.

+1

Function trumps appearance. The molasses has probably removed an unmeasurably thin layer.

You will lose accuracy and the scraper marks if you use abrasives.

Neil

I believe what happens with molasses is that the sugar ferments into alcohol which in the high air areas oxidises in to acid and that then etches the metal. Total immersion excludes more air.

What exactly is Molasses? It's produced whilst refining sugar, and - being a natural product - its contents vary wildly depending on a multitude of factors, including where in the world it was produced and how it's subsequently processed. Being sticky and slightly acid is promising in a rust remover but it's not a reliably pure chemical intended for that purpose. No-one knows how a particular sample might react with an expensive machine tool!

Is it wise to trust that "very experienced engineer relative"? In my family "very experienced" is a euphemism for "out-of-date"! Whilst other people's folk remedies are suspect, you can always trust mine. What's needed for this job is fresh Maiden's Water. I make my own...

Dave

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Ok thanks chaps. You've confirmed my initial thought - abrasive would be a mistake. I shall let sleeping dogs lie.

On the up side, I assume the minuscule amount of etching that's occurred will hold slideway oil better than being really shiny?

Choochoo_baloo, I've PM'd you.

Perhaps the process is that the sugar in the molasses oxidises to alcohol, and then further oxidises fully to acid? Possibly Acetic, which slowly dissolves the Ferrites in the cast iron to etch the surface.

Cannot think that an aldehyde or ketone would etch.

Is there a Chemist in the house?

Howard