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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