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"Will heptahydrate or sulphate result in Fe rusting??!"

April 11, 2016

Q. I was wondering if the presence of heptahydrate (MgSO4) OR sulphate (O4S-2) will cause an Fe product to rust? If so, I need the chemical equations of these... :)

Ryan Packer
- Hervey Bay, QLD, Australia

April 2016

A. Oh the strange strange things we wonder when we set to wondering, Ryan :-)

The pH of MgSO4 is probably 5.6 - 6.5, and it's a mild neutral salt. I would say, if pure, it's non-corrosive to steel in normal concentrations, that is, I'd guess that steel will rust in plain water at about the same speed that it will rust in water with MgSO4 in it, but I don't have a reference to prove or disprove my guess. I would not write:

MgSO4 + Fe => FeSO4 + Mg

Sulphate, SO4-2, is the anion in MgSO4 and, in this context therefore I would say it is not corrosive in normal concentrations.

Are you sure that you thoroughly understand the question you asked? As I often say here, it's fine to ask for help determining the answer to a question you clearly understand; but if you don't even understand the question, please let your teacher know -- just posting it probably won't help you make sense of it. Good luck, and I hope this discussion of heptahydrate (magnesium sulphate-wise) was helpful :-)

Otherwise, again, please tell your teacher that you don't understand the question.


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Ted Mooney, P.E. RET
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