Turbo Fin Mast Corrosion Issue

So, never mind that these fin masts are bent. A submerged rock hit my mirage drive fins and bent them
I dropped the fins and removed the masts to straighten them.
These were new replacements that were installed 6 months ago.
I had a fin mast failure last October and replaced both with two new ones. One was purchased in October and the other one was from a Hobie Turbo Mirage drive repair kit, approx 2 years old

I was very surprised to find so much corrosion on the fin masts in 6 months. When installing this last set, I applied a coating of Superlube grease (synthetic) hoping that it will help prevent corrosion.
I mostly kayak in saltwater and thoroughly rinse off everything after every trip.





If you look at the last picture ^^^^ you will notice that there are round surface corrosion areas, along with the rusty crack that is developing


The reason for my concern is that I am convinced that the first mast failure was caused from deep corrosion after 3 years of usage as seen in the pictures below




This is the other mast that didn't fail that is 3 years old, and that I replaced with the new ones 6 months ago


So, my question is: Is there a reason that the 17-4 stainless fin masts are corroding in such a short amount of time?

Are masts made from 17-4 srainless? I have seen many of these masts pit like this and then brake. Always in the same area as in your picture. I wonder if it has something to do with the 2 different metals. The ss masts get screwed into brass inserts in the sprockets.
One thing I have been doing to help this is to spray wd40 into the little hole in the end of the turbo fin after each use to flush out the saltwater. The saltwater left between the mast and fin takes a long time to dry out and continues to corrode. Displacing it with WD40 seems to stop this.

Sounds like micro cracks may appear from fatigue or repeated bending. That may allow contaminants to collect and weep rust from the cracks? I am told that brass would be sacrificial in the relationship, so not likely caused by the brass part.

Can you send us one of these masts for inspection?

_________________
Matt Miller
Former - Director of Parts and Accessory Sales
Warranty and Technical Support
Hobie Cat USA
(Retired 11/7/2022)

Most stainless steels have some iron/steel in them. Specific marine stainless products are typically dipped in strong acid to remove any iron from the surface, this is apparent by the dull grey appearance of these typically very pricey products ( same or similar steel,, just treated with acid to remove the surface iron, and they get double the price (lol)). Any of the heat treatable 400 series stainless steels need the iron content to be able to be hardened. The Iron below the surface making up the internal grain of the steel is safe from corrosion.
So in otherwords if you buy stainless bolts at home depot and use them in a salt water enviornment you will likely get a little surface rust, (only on the surface, inside is still sound).
Salt water is very corrosive and ten times more corrosive if allowed to wick into narrow areas where it can't dry out, or be flushed with fresh water. In the example of your mirage shafts if you pack them with grease so the salt water can't wick in in the first place and do it's dastardly deed of eating the iron. Then periodically removing and cleaning the fins and re-apply the grease the rods should live a happy life (well happy until you hit an oyster bed and wreck them anyway). That's kinda why we always pack the prop shafts with grease on our motors and remove the props once in a while, because the salt wicks in and corrodes the prop shaft if the salt is allowed to get in there in the first place. If the salt water can't wick in there in the first place, you shouldn't get any corrosion except minor surface rust. I can see the deep pits from long term salt exposure in your pics, you can rinse the mirage drive off till the cows come home, and it won't remove the wicked in salt water, just sayin.
FE