On Tue, 27 Mar 2007 17:35:18 GMT, Don W
wrote:



Bruce wrote:

The acid treatment was "passivating".

My experience is that unless the stainless is polished to a mirror
bright finish you will get staining. All the bits have to be polished;
Assuming that it is a bolt on fitting the bolts and washers that are
exposed to sea water need to be polished.

I've been making things out of stainless and putting them on boats for
quite a number of years and I have found that if the part has all the
welds ground smooth (so the welding ripples don't show), no pin holes
or voids in the weld and then polished bright I seldom get staining.
If I don't do this I always get staining.

snip...
Bruce in Bangkok
(brucepaigeatgmaildotcom)


Hi Bruce,

I've just recently started building stainless
parts for my two sailboats. I'm using 316 SS and
316L SS exclusively and passivating with Citrisurf
which is a strong citric acid. It can also be
used for electropolishing, and I intend to try
using it for that in the future.

Google "Citrisurf". It is supposed to be better
and more environmentally friendly than nitric acid
paste. Several of the welders that post to
sci.engr.joining.welding have used it with good
results. I've only started using it about six
months ago, so the stuff I've built has not been
out in the salt spray long enough to tell how good
the passivation was.

Don W.

Don,


I've tried things ranging from purpose made passivating stuff to
vinegar. At the moment I am using a tile cleaner sold, here in
Thailand in every supermarket, that is 20% hydrochloric acid.

One comment I probably should make is that all the passivation
chemicals work by dissolving (eating) some material from the surface
of the piece being worked on. Of course this is how it works, removing
all the oxides and other rubbish from the just welded surfaces. BUT,
it also etches the nice shiny surface of the expensive piece of
stainless you just welded.

My experience is that if you don't polish the etched surface back to a
mirror finish you still get staining.

In another message I mentioned some dinghy lifting brackets I made
that stained. They had been left in acid overnight and were perfectly
clean but with a matt finish when I installed them -- thought the matt
finish would hold the paint better -- wrong.



















Bruce in Bangkok
(brucepaigeatgmaildotcom)

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