- Please comment on safty of stainless steel
by chuck/pierre.mit.edu (Chuck Parsons) (29 Jan 92)
by chuck/pierre.mit.edu (Chuck Parsons)
Date: 29 Jan 92
In article <1992Jan24.185119.15069-at-cbnewsh.cb.att.com>, shine-at-cbnewsh.cb.att.com (stephen.c. writes...
>In article jto-at-litwin.com (John O'Beck) writes:
>>>In article unllab-at-violet.berkeley.edu (LBL lighting laboratory) writes:
>>>>I am thinking of using an immersion heater that is sealed inside a stainless
>>>>steel rod in a freshwater tank...
>>As far as I know, it is the marine environment that harms the stainless
>>from chloride stress failure. The stainless does not leach enough of
>>anything into the tank to cause harm. Titanium is used for salt-water
>>because it works - stainless does not. hhhhat is one reason that chillers
>>Stainless is fine for fresh water.
>I recall a metallurgist-type who posted a table of the different grades
>of stainless steel and in what environments they could safely be used.
>The posting indicated that not all stainless steel grades are safe for
>freshwater. I don't remember if any qualified for use in a marine
I looked into this question in some detail. I did post some
results but I didn't save them.
The fundamental problem is that stainless steel is so generic a term
as to be useless. There are litlerially hundreds of alloys that contain
very different compositions.
However if you call up your local metal dealer chances are good
that they will only have two types of stainless steels. Both
are 3XX series which (in laymans terms ) means that they are mostly iron,
with just enough chromium and nickel added to make them corrosion
resistant. I'm sorry that I don't remember the number of the cheaper
one (30 something) but the better one is Alloy 316. The cheaper stainless is
totaly unsuitable for saltwater, and you will see rust if you
leave it saltwater for very long.
The cheap (hence common) grades of stainless fair very poorly in
saltwater. They are primarly iron with smaller amounts of chromium
and nickel (5-25% or so). I don't believe that any of these is highly
toxic to fish but I don't know for sure. It is common to find that
all kinds of other things are added to the alloy in <=1-2% quantities for
magical metalurgical reasons. Tungsten, Copper, Molybedeum, Titanium,
Cobalt are a few of the more common things.
316 is reasonably good for non critical applications where it remains
submerged. _If_ I remeber right, it has no copper. It is often used in
saltwater pumps/powerheads for the shaft. It corrodes faster if it goes
through constant wet dry cycles than if it is always submerged. It is not
suitable for chillers, because the they have thin walls (for high heat
conductivity) and usually have freon gas at high pressure on one side. The
stress corrosion cracking, refered to in the other post will soon (<1year)
cause a crack and the freon will bubble into your tank, stress corrosion
cracking is caused by Cl ions hence the reason saltwater is so bad. Many
stainless steels stand up better in concentrated sulfuric acid than seawater.
If you leave 316 in saltwater for long periods (years), it will show
signs of corrosion, like pitting. Thus you can be sure that iron, nickel
and chromium are getting into the tank.
Titanium (often alloyed with small amounts aluminum and copper to
increase strength) can sit in seawater for years and show no signs
Many grades of stainless, (like the ones they make replacement
hip joints out of). Can perform even better than titanium. The
trouble is that they are just as expensive (often much more) than
titanium. With minimum orders, etc it is hard to get titanium
for a chiller for less than you can buy a chiller for. I bought
some, but $250 bucks was the smallest quantity I could buy
(for $300 I got 4 times as much though). Because it comes in 20ft
lengths it had to be shipped by truck, which cost almost $100.
Don't use the heater unless you can find out what alloy it
is. $12 will get you a new vis-therm mailorder.