Pete,
A humid environment as you describe is not a "dry" environment. I was thinking more of a desert or manufacturing facility where condensation didnt occur. Even if it did and it evaporated routinely my experience has been that galvanic corrosion doesnt occur. For a galvanic cell to exist one needs an electrolyte. This has to be a fluid in whch electrons will move. Condensing water doesnt fit into that category until something dissolves in it such as gases from the environment or salts from the surface of the pipe or bugs grow and their excretia creates an acid environment. Hence marine environments, where a salt layer may rest on the pipe, represents a greater challenge than a dry factory environment.
I have come across stainless steel/aluminium interfaces inAvtur and Avgas aviation fuels service that corroded. The cause was the bugs that lived in the minute amounts of water in the fuel could chew out the aluminium. By chew out I mean that they cause a galvanic cell by changing the pH of the local environment.
Geoff
--- Pete Burns <peteburn@optonline.net> wrote:
thanks
Pete
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
<a href="http://www.ionikconsulting.com">http://www.ionikconsulting.com</a>
Got a question? Why not ask the panel for free at
<a href="http://www.ionikconsulting.com/04_panel.htm">http://www.ionikconsulting.com/04_panel.htm</a>
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: <a href="/group/PipingDesign/post?postID=IMRzm9I-MCIBGKq88zO2tDrqPegSu4qE4PF_KAy1vmX1p8zxWHYYiNFj0RFvAg_h_L-NAtI2XD8qE1EN4_L1VHLp">PipingDesign@yahoogroups.com</a>
> 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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