On Wed, 28 Mar 2007 13:29:59 +1200, Eric Stevens
wrote:
On Tue, 27 Mar 2007 06:52:12 +1000, Peter Hendra
wrote:
On Tue, 27 Mar 2007 20:51:00 +1200, Eric Stevens
wrote:
On Mon, 26 Mar 2007 17:56:36 GMT, "Gordon Wedman"
wrote:
"Peter Hendra" wrote in message
m...
On Sat, 24 Mar 2007 12:44:13 GMT, Rich Hampel
wrote:
Probability is that the welds are not ground flat and smooth and
crevice corrosion is underway in the 'intersticies' of the weldment
(poor weld technique ... not a full penetration weld).
To be 'corrosion resistant' (chlorides, etc. attack stainless) all the
weld must be flat /smooth by grinding and then polishing/buffing to a
high mechanical mirror polish. Any 'weld-lap' or pin-hole in the weld
will be a source of 'rust' .... gotta be closed and then smoothed. .
This has just made sense as only one of the bolt on "feet" for the
ladder suffers from the problem. I'll try it.
Thanks very much to all who responded.
regards
Peter Hendra
A friend has gone thru this with his stainless. People at his workplace
that know about his sort of thing told him he needed to passivate the welds.
They gave him some solution to paint on the welds. I believe this is
hydrofluoric acid, perhaps a mixture with other acids. Requires very
carefull application. It took away the surface shine. Don't recall the
final result but I can ask him.
It won't be Hydrofluoric Acid, which is one of the nastiest nasty
chemicals on the face of this earth. It's much more likely to be a
50~50 mixture of Hydrochloric and Nitric acids plus, sometimes, a
little Phosphoric acid. You should somewhere find an expert and read
http://www.assda.asn.au/asp/index.asp?pgid=17982
Whatever you do, you don't want a strong brew and you should let time
be your friend.
Eric Stevens
Thanks Eric,
A great site and I love your phrase "let time be your friend" -
poetic. May I have your permission to use it in future - yes, I'll
cite you as the originator? I don't want to get flamed.
I take back most of what I have written. I have just found a little
booklet about stainless steels I bought some years ago from New
Zealand's Department of Scientific & Industrial Research. Here is
their advice:
_____________________________
Pickling and passivation
Acid pickling effectively removes all surface blemishes of low
corrosion resistance, such as iron contamination, oxide scale, and
heat tints. The process involves using an etchant (hydrofluoric acid)
in the presence of an oxidising agent (nitric acid) which inhibits the
etching action. A typical formulation consists of 15% nitric acid and
1-3% hydrofluoric acid, which will remove most scales on immersion of
the stainless steel component for 5-30 min at ambient temperature. As
mentioned previously, passivation treatment involves the use of 15-30%
nitric acid on its own, to restore a passive film which may have been
damaged by heat treatment or contact with harmful chemicals. The
disadvantages of using concentrated chemical etchants, apart from the
handling of hazardous materials, include the risk of excessive
pickling at grain boundaries and hydrogen embrittlement of certain
grades of stainless steels.
Weld dressing and descaling are essential to enhance the corrosion
resistance of welds. If immersion of the welded component in an acid
mixture is not practicable, a weld-descaling paste may be used. This
is an inert base material containing nitric acid and hydrofluoric
acid, or, if a passivation treatment only is required, just nitric
acid. These weld-dressing pastes are applied over the weld in a thin
layer and then left for about 1 hour before the residues are cleaned
off.
All pickling and passivation chemicals applied to welds must be
thoroughly rinsed off with fresh water after the appropriate residence
time for the chemical cleaning is completed.
Electropolishing is another batch process for post-fabrication shop
cleaning and passivation of stainless steel components and smaller
equipment. This technique removes heat tints on welds and surface
impurities such as iron contamination, and it also imparts a highly
corrosion-resistant finish with a pleasant dull-grey appearance to the
treated surface. Electropolishing must be carried out by trained staff
under carefully controlled conditions, because, as with the anodising
process on aluminium, it is possible to produce a defective finish if
the process goes out of control.
______________________________
What I said previously about hydrofluoric acid stands. Its a very nast
material. Treat it with very great respect, even in the diluted form.
See http://ptcl.chem.ox.ac.uk/MSDS/HY/hy...oric_acid.html and
http://adm.monash.edu/ohse/assets/do...acid-draft.pdf
Eric Stevens
Thanks for quote. I knew that stainless reacted with oxygen to form a
non-corrosive surface and that unless you had a flow of water the lack
of dissolved oxygen might result in crevice corrosion but somehow had
never related it to pasivating. I'll now use at least come nitric acid
as an oxidizing agent.
Bruce in Bangkok
(brucepaigeatgmaildotcom)
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