Pete,
There are a number of references regarding the level of humidity above which aqueous corrosion reactions can occur. Typically however, at humidities about about 70%-80% RH you can expect to find some condensation on the surfaces, and the possibility of galvanic corrosion.
This can be seen when you have flash rusting (or gingering) of freshly blasted steel. (Incidentally - steel self corrodes because of the microstructure of the steel having slightly different potentials).
Hope this helps some.
Regards
Andrew Lindsay
Senior Corrosion Engineer
IONIK CONSULTING
Quoting Pete Burns <peteburn@optonline.net>:
> Mr Lindsay,
> I have been following the galvanic corrosion thread here, I have a basic
> question for you (or anyone). I know the action is like a cell and requires a
> cicuit path of conductivity, a medium like water. I know underground systems
> provide this.
> Another regular here Geoff I think said in a dry environment Galvanic
> corrosion is not a big concern. BUT what about a humid environment like a
> steam plant - very high humidity - wouldn't a copper - steel combination be
> at risk there ?
>
> thanks
> Pete
>
> ----- Original Message -----
> From: Andrew Lindsay
> To: PipingDesign@yahoogroups.com ; Ahmed Vawda
> Cc: 'PipingDesign@yahoogroups.com'
> Sent: Monday, June 09, 2003 4:26 AM
> Subject: RE: [PipingDesign] Galvanic corrosion
>
>
> Ahmed / Jagdeep,
>
> Ahmed, you and many others in this forum are correct regarding the effects
> of
> differing potentials on galvanic corrosion. However, what you also need to
>
> look at when considering passive metals, such as stainless steels, is their
>
> potential when the passivation layer is lost.
>
> Passivity may be lost for a number of reasons, but for a stainless steel,
> the
> main reason for loss of passive layer is the loss of oxygen from the
> system.
> Under conditions where oxygen is removed from a system, it is possible for
> the
> stainless steel potential to reduce (become more negative) and approach the
>
> same solution potential as normal carbon steel.
>
> If you could give me some more information I could probably suggest to you
>
> what may be the root cause and possible material replacements that would
> work
> in your system. There are a number of process and environment variables
> that
> have not been given in the previous discussions and these would need to be
>
> addressed successfully prior to making any further comment.
>
> Best regards
>
> Andrew Lindsay
> Senior Corrosion Engineer
> IONIK CONSULTING
> http://www.ionikconsulting.com
>
> Got a question? Why not ask the panel for free at
> http://www.ionikconsulting.com/04_panel.htm
>
> Quoting Ahmed Vawda <avawda@aksugar.co.ae>:
>
> > Jagdeep
> > Galvanic corrosion is the corrosion damage caused when two dissimilar
> > materials are in contact. It occurs when two (or more) dissimilar
> metals
> > are
> > brought into electrical contact under water.
> > Metals have different electrical properties. Metals that are "more
> noble"
> > will corrode quicker than the "less noble" metal. When a galvanic
> couple
> > forms, one of the metals in the couple becomes the anode and corrodes
> > faster
> > than it would all by itself, while the other becomes the cathode and
> > corrodes slower than it would alone. In your case the noble to less
> noble
> > metals are in the ffg order : copper, brass and then SS. You haven't
> > specified which metal is corroding faster, but according to the EMF
> table,
> > the SS would be the main victim of this coupling
> >
> >
> > Ahmed Vawda
> > Process Engineer
> > Al Khaleej Sugar - Dubai
> >
> > -----Original Message-----
> > From: punnu_h2003 [mailto:jsingh11@rediffmail.com]
> > Sent: Monday, June 02, 2003 8:11 AM
> > To: PipingDesign@yahoogroups.com
> > Subject: [PipingDesign] Galvanic corrosion
> >
> > Thanks for some stuff on the subject. But I am specifically looking
> > for the possible reason of corrosion when SS tube and a Copper tube
> > have a Brass coupling to join them. What could have triggered the
> > Galvanic Corrosion.
> >
> >
> > -----Original question-------
> > My question relates to the Compatibility of distinct materials when
> > used in assembly. To be specific, there is a case where a SS tube is
> > joined to a Copper tube wit a brass coupling. At the joint, corrosion
> > has set in. Why and what is the cause? What metal properties of these
> > materials are conflicting causing corrosion.? What is actually a
> > Galvanic corrosion?Thanks. I hope to get good and informative
> > response.
> >
> > Jagdeep
> >
> >
> >
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> >
> >
> >
> >
>
>
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