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Hi Gary, I think you have put your finger on it. That erosion of the exterior of the WOB is something I have never seen and your reasoning that the least noble metal eroded sounds right. Brass is an alloy of copper and zinc. However bronze alloys come in a variety of compositions, I believe some with zinc and some not.(but I am no expert on alloys) I always thought the WOB was a bronze, if so it would seem likely that Kents did indeed contain zinc. Whatever I am sure you have picked the cause. Once again we see good reason to stay with the Amel original. If the bronze WOB had been replaced with a stainless what may have happened?
SM 299 Ocean Pearl
On 31 March 2020 at 06:33 "Gary Silver via Groups.Io" <garysilver@...> wrote:
On Sun, Mar 29, 2020 at 11:55 AM, karkauai wrote:
bottom line for me is that while hull potential is a good measure of overall bonding efficacy, we must REGULARLY measure continuity at ALL metals exposed to sea water to confirm that they are protected. It goes on my quarterly maintenance list today.
Hi Kent: What a great find and a great discussion along with Joel's post. Sooooo valuable. Thanks for sharing. Glad you got that WOB (wearing out bearing) out. It is still a bit of a mystery to me as to why only that corroded. The color (salmon like) looks like de-zincification. I am only speculating but, since brass is copper and zinc, the zinc in the brass was the least noble element in that vicinity (i.e.the prop, c-drive shaft etc.) Everything must have all been more noble metals, so with no continuity of the C-drive to the normal sacrificial anodes (on the rudder), the WOB became the sacrificial anode for your C-drive. I guess it only corroded where it was in contact with sea water and hence the lack of corrosion "inside" the lip seals etc.
Again, thanks for you invaluable reminder about preventative diagnostics on our bonding system.
Gary S. Silver
s/v Liahona (Puerto Del Rey Marina, in-accesible due to Puerto Rico's COVID-19 quarantine)
SM 2000 # 335