From: REMOVE_THISdwilkins@means.net (Don Wilkins)
Newsgroups: sci.chem
Subject: Re: De-scaling method?
Date: Tue, 27 Jul 1999 01:25:41 GMT

On Sun, 25 Jul 1999 10:26:40 +0100, Andrew Holmes
<andrew@biologist.demon.co.uk> wrote:

>In article <37955577.45A4@chesapeake.net>, dgf@chesapeake.net writes
>>I own a boat with inboard engines that are fresh and raw water cooled.
>>The block is freshwater cooled, the exhaust manifolds are raw water
>>cooled.  Problem is the water passages in the exhaust elbows are badly
>>caked with scale.  There is still plenty of metal left on the inner
>>walls, so I'd like to salvage these parts if possible.  Can anyone give
>>me a suggestion as to how I can dissolve or break down the scale without
>>damaging the elbow itself (steel)?
>>
>>-Dave
>
>In a previous employ at a plant which produced calcium sulphate, when we
>wanted to determine the CaSO4 content of a product sample we would
>dissolve it using ammonium carbonate. I'd suggest that this might
>dissolve the scale, although it might take some time.

Well sort of.

What actually takes place is that CaSO4 is more soluble than CaCO3.
The end result is that the insoluble calcium sulfate is converted to
insoluble calcium carbonate. The good thing about this for the guy
with the problem is that the calcium carbonate is going to end up as a
finely divided powder instead of a hard pack scale.

If the parts can be placed in a container with a carbonate and heated
near the boiling point then it might be fast enough to solve the
problem.

This process is used in analytical chemistry to  e.g. "dissolve"
BaSO4. One can use Na2CO3 which may be easier for the non-chemist to
obtain (washing soda).

It might be worth trying a dilute solution of muriatic acid (local
hardware store). If the scale contains a lot of carbonate this may
remove the scale. This is not the solution to dump the parts in and
come back next week to see how the scale is dissolving. Too strong,
too hot, and too long and the steel parts may disappear.