Summary for bi-metallic corrosion Discussion
Bi-metallic corrosion of electrogalvanized espagnolette bolt is discussed in terms of whether stainless screws could have caused the damage. Alex Sturrock asks if the stainless screws acted as cathodes, the cam section as an anode, and the bolt rod as an electrolyte, and whether limewash or plaster with higher pH could have accelerated attack. The reply says a photo would help and notes the cam section’s exact metal is unclear, but if the zinc electrogalvanizing is anodic to the cam, it should galvanically protect the cam rather than corrode it. The stainless connection would more likely speed corrosion of the zinc plating, not the cam itself. The answer also corrects the terminology: the electrolyte is the wet wood or other liquid medium, not the bolt. Overall, the thread suggests the observed corrosion is more likely related to zinc sacrifice and local environmental conditions than to stainless screws directly attacking the cam.
Q. I can send photographs, if required, of an electrogalvanised steel espagnolette bolt used in window manufacture. The middle (cam) section is made from a different alloy. I was instructed to use stainless steel screws to secure it to the underside of the window sash as the timber species would corrode an anodised screw. The cam section is inserted into a mortice and obviously is subjected to different environmental factors than the rest of the component. It would seem that the stainless steel screws have acted as cathodes, the cam section as an anode and the rod as an electrolyte. Is this correct? If not what has happened? Is it possible that limewash/plaster could have been an accelerator as it has a much higher pH than rainwater? I’m quite baffled. Any help would be greatly appreciated.
Alex Sturrock
– Dundee, England
May 10, 2012
! Yes a photo should help. And I’m not quite following what this cam section is, or what metal it is made of, if known.
Still, if the zinc plating (electrogalvanizing) is anodic to the cam section, which I would expect, it should galvanically protect the cam, such that the stainless steel connection would only accelerate the corrosion of the zinc plating rather than the cam. Thanks.
On a semantics matter, an electrolyte is a liquid that is capable of transmitting ions (metal salts). The wet wood would be the electrolyte rather than the bolt.
Regards,
Ted Mooney, P.E.
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May 10, 2012