Propeller Nut - What happened to this Nut

The picture of the Nut is M30 locking nut which is one of the nut shown in drawings. The corrossion has happened between the outer thread and outer side of the bolt

 

Stray current is understood, but I want to understand between which two materials happened.

 

That depends on the potential (voltage) difference. If it was a passive galvanic cell, you can look up the values on a chart. Were there no electrical connections at all, not even batteries for the engine?

 

@alby joy - Looks from your profile that you are in India. I don't know what organizations you have there, here in the USA we have ABYC that has resources that you can tap into. I'm attaching below some of the ABYC resources, you may have something similar in India, I don't know.

But I would suggest that you find a professional to help you with your problem. From here, it just isn't possible to analyze what you have going on other than to throw out some WAGS (wild a$$ guesses). Without the parts and the boat to examine in person, it's really just a lot of guess work as to exactly what is going on. Just my 2 cents worth.

dj

---
ABYC consumer page. ABYC - American Boat Yacht Council - American Boat and Yacht Council https://abycinc.org/mpage/recreationalboaters


The “Find a Certified Technician” link can be used to search for a marine corrosion certified technician. https://abycinc.org/mpage/findatech


Additionally ABYC has several online courses that target corrosion including a Basic Marine Electrical & Corrosion Mitigation.

The complete catalog can be searched here.

ABYC Online Learning: Home https://abyc.elevate.commpartners.com/

 

The boat is not commissioned. Even we have not connected the battery to engine

 

I understood that there are many certifications & online course which can enhance the skills or the capabilities. From the members of this group- I understood a lot of experts are available here to help. Experience is a costly gift that can't be earned by just certifications. In India, the marine market is not so big when it is compared with U.S. This has also impacted the availability of certain marine grade materials.
Additionally, we have classification societies like Indian register of shipping with its own set of rules and regulations. We are following that but the material grade -suppliers provide this kind of non-marine grade results in these damages.

 

If this is the same boat, this is sounding a lot like marina induced stay current. Maybe there is a large metal object on the bottom under one side of the boat contributing to the cell, if that's a thing.

Walking a marina in Scotland recently, I saw an old shopping cart sitting in the water just off the transom of a boat and wondered if that could become a potential element in galvanic corrosion with the vessel only half a meter away.

-Will (Dragonfly)

 

It would, particularly if is chromed.

 

To get to the root of your problem, you need someone to be able to look at your specific boat, in it's specific location. There are indeed a lot of experts here, but no expert can tell you the whole story without being able to do some measurements and testing where your boat is.

dj

 

If you keep the cap over the nut, dispace the water with MARINE grease, not soap based greases and probably smear some silicone gasket maker over the cap to prop hub.

Other than this, how about using a duplex SS nut, not 316.
Have no idea if you could buy this yourself though.
Stainless Steels - Duplex Stainless Steels Properties, Fabrication and Applications https://www.azom.com/article.aspx?ArticleID=2870#:~:text=Duplex%20stainless%20steel%20are%20extremely%20corrosion%20resistant.,even%20more%20resistant%20to%20corrosion.
Corrosion Resistance of Duplex stainless steel
Duplex stainless steel are extremely corrosion resistant. They have high resistance to intergranular corrosion. Even in chloride and sulphide environments, Duplex stainless steel exhibit very high resistance to stress corrosion cracking.

The super duplex grades are even more resistant to corrosion.

Or discard the cap, and let the nut be exposed, that should give it enough oxygen to help it not corrode. And you could uses an RTV silicon gasket maker on the shaft threads, and nut to seal out water too.