From: email@example.com (Bob Wilson)
Subject: Re: Side effects of potting compound !
Date: Fri, 08 Nov 2002 04:55:47 -0000
Organization: Your Organization
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In article ,
>In article ,
>a (Bob Wilson) wrote:
>>This was also my own experience. In theory, none should appear at all
>>a) copper is unaffected by common acids, and b) there should be no water
>>present to ionize the acetic acid produced. But of course, nothing is
>>>Allowing it to cure in free air produced *no* corrosion whatsoever so
>>>matter what else was done.
>>Exactly. Unfortunately, too many people forgot their Grade 9 science, and
>>got scared when they heard that normal RTV generates acetic **ACID**
>Your error is basing your understanding of corrosion on that 9th grade
>science. Just because copper does not instantly corrode when dunked in
>acid, does not mean that acetic acid will not contribute to long term
>corrosion - it will. And you cannot avoid the persence of water, it is
>in a layer many molecules thick on all surfaces exposed to the atmesphere.
>Try reading the ASM Metals Handbook Volume 13, Corrosion, if you want to
>understand how electrolyte aided crevice corrosion occurs, and why the
>electrolyte in itself does not need to be chemically corrosive to cause
>corrosion. Any other modern book on corrosion will explain this also.
>But like I said before, this is a long term effect which occurs over a
>of years, depending also on the inevitable porosity in the gold plating on
>the trimpot wiper, and since long term reliability is not an issue here ...
So how are you going to get RTV to continually produce acetic acid for
years? After a few days, it is fully polymerized and no more acetic acid is
produced. RTV is a very permeable material (the most permeable of all common
polymers), and whatever acetic acid vapours are trapped in cavities, will
diffuse out long before "years".
This has little to do with crevice corrosion, which is corrosion
(particularly of ferrous metals) which is dramatically accelerated in the
near absence of oxygen. We were talking about simple acetic acid attacking
copper here, which cannot occur unless there is an oxidizing environment.