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Posted by daestrom on September 2, 2007, 5:32 pm
 


I was only talking about carbonates and their affect on Al corrosion.  The
link I found pointed out that dissolved halides were much worse at removing
the protective oxide layer than dissolved carbonates.

Carbonates in a boiler are bad news for several reasons and you don't need
900F to suffer.  When you heat water the solubility for CaCO3, Mg2CO3 and
others drops off and they come out of solution.  This is what forms scale on
heat transfer surfaces (even in home water heaters, and LP boilers).  The
really bad part is that once the CaCO3 plates out, it will not re-dissolve
into the boiler water when you shutdown the boiler.  You can't get the scale
back off with out 'mechanical' cleaning or 'chemical' cleaning.  One way to
combat this in the old days was to add tri- and di-sodium phosphates.  The
Na would combine with the CO3 to form a scale that was easier to remove.
The first trade name of such boiler-chemistry control was 'CalGon' (same as
the retail soap) which stood for 'calcium-gone'.  But the large amount of
phosphates wasn't so good for the environment so phosphates have been
regulated.

In steam plants it is now common to use 'polishers' (ion-exchange resin
beds) to continuously demineralize the condensate/feed-water.  Keeping total
conductivity down well below 0.1 micromhos/cm is quite common.  This slows
the buildup of TDS in the boiler water.

Dissolved oxygen in boiler is also bad for Cl-stress corrosion.  But modern
low-carbon steels have a problem with too-low an O2 content as well.  Yes,
O2 levels are controlled with several techniques (deareating feed tanks,
contact feed-water heaters and such).  We actually have to keep O2 levels
between 10 and 50 ppb for best results.

daestrom


Posted by Neon John on August 25, 2007, 3:49 pm
 
On 24 Aug 2007 13:46:41 -0400, nicksanspam@ece.villanova.edu wrote:


Radiator performance is readily available from the manufacturers, if not in their
catalogs then from an inquiry.  Have you looked at, say, Modine's website.


Coolant anti-corrosion/passivation chemical packages are available separate from
antifreeze.  These are used to maintain the anti-corrosion protection in large
volume
coolant systems where one wouldn't change many gallons of antifreeze just
because the
protection package is depleted.  Stationary diesels, locomotive engines, things
like
that.

Additionally, the aluminum can be completely protected from corrosion by fresh
water
using cathodic protection.  There are many thousands of RV water heaters out
there
with aluminum tanks that handle all sorts of tap water without corrosion.  The
reason
is the magnesium anode screwed into the tank.  I would think that an RV
replacement
anode could easily be fitted to an aluminum radiator.

John

--
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com  <-- best little blog on the net!
Tellico Plains, Occupied TN
I'm going crazy. Wanna come along?


Posted by Robert Scott on August 28, 2007, 2:26 pm
 

Aren't there also some active systems that do the same thing using electric
power instead of relying on cathodic action to provide the low voltage
differential?  Of course they would stop working when the power went out, but
hopefully the amount of damage caused during the short power failure would not
amount to much.


Robert Scott
Ypsilanti, Michigan

Posted by daestrom on August 31, 2007, 3:06 pm
 

Yes.  Active service naval ships use cathodic protection with zinc anodes
that have to be replaced every year or two.  But *inactive* ships in the
'mothball' fleet are protected by suspending separate electrodes in the
water around the ship.  A small voltage impressed on these (only need about
1-2 V) will protect the hull from corrosion.  It's much simpler to replace
the suspended electrodes (a single sailor can pull one up and replace it)
than to put the whole ship in drydock.

You can also find these 'active systems' in marinas for private yachts as
well.  Same idea.  If the power goes out for a couple hours, you don't see
any real damage.  After all, the hull is designed to stand up to seawater
corrosion for many years, the 'active systems' just extend that further.

daestrom


Posted by dances_with_barkadas on September 2, 2007, 11:24 am
 
I'm in active service in the Dept of the Navy as a shipriding Marine
Engineer.  I live in a world where we pay a lot of attention to the
things that really could end up being showstoppers or inspection-
passing obstacles; and usually don't have any time left for the
theoretical niceties.  I'm not qualified to discuss the theoretical
utility of active cathodic-protection systems;  I can report that I've
never seen a Chief Engineer direct an underling to pay attention to
the cathodic protection system.

Magnetic Silencing seems to fall into the same category.  I guess that
will change the first time the Iranians use a torpedo against us.


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