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Corrosion basics for kiteboarders

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wrogu
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Re: Corrosion basics for kiteboarders

Postby wrogu » Sat Jun 22, 2019 10:01 pm

Al-In-Zn anode would be better or even risking a magnesium one -for an 'Al' mast that is. For uniform corrosion. 'CP' for pitting (negative polarization) is also benefitial. Crevices are indeed an issue. I also do not see a threat from OH- ions generation as foils moves (Al is prone to alkaline corrosion).

As I am a Baltic kiter - I am yet to find crevice and pitting issues in my gear.
I also doubt that you would find 'real issues' with 316 steel with your 316 bolts in any environemnt. Actually when it comes to corrosion engineering the most important issue is: life expectancy vs. cost of corrosion protection. In most cases even if you would find crevice/pitting issues the best way would be to have a second set of bolts rather than going for a higher grade alloy.


Btw. One corrosion issue you might have missed - and it could be actually prety dangerous is intergranullar corrosion - a bad weld in kiting hook could be very dangerous if it is left in corrosive environment.

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Re: Corrosion basics for kiteboarders

Postby apollo4000 » Sun Jun 23, 2019 10:45 am

RustyChain wrote:
Sat Jun 22, 2019 4:42 pm
That's a long winded way to say "Rinse your shit", also "Bolts may seize over time, yo".
Nice and simple. I’m with you.

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Re: Corrosion basics for kiteboarders

Postby JakeFarley » Sun Jun 23, 2019 12:32 pm

For any threaded bolts, screws, a simple solution is to use tef gel or Teflon tape used for plumbing pipe thread sealing. It insulates dissimilar metals (but not entirely) to retard the corrosion process. I've used Teflon tape so that the insert bolts, screws don't seize up due to corrosion. Even if you rinse with fresh water after every use, there is still some salt water that you can't rinse away unless you completely dismantle every time. I periodically (6 mos.) remove the insert screws and reapply new teflon tape.

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Re: Corrosion basics for kiteboarders

Postby wrogu » Sun Jun 23, 2019 4:29 pm

But in kiting gear you rarely see galvanic corrosion as you rarely connect metals.

Thumbs up rule is to use small cathode (more noble metal) as a fastener/bolt and large anode.

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Re: Corrosion basics for kiteboarders

Postby plummet » Sun Jun 23, 2019 8:22 pm

Wrogu, galvanic corrosion is an issue for individuals using aluminum hydrofoil masts with stainless fasteners.

Fair call re intergranular corrosion. I'll upload info tonight.

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Re: Corrosion basics for kiteboarders

Postby wrogu » Mon Jun 24, 2019 5:52 am

plummet wrote:
Sun Jun 23, 2019 8:22 pm
Wrogu, galvanic corrosion is an issue for individuals using aluminum hydrofoil masts with stainless fasteners.

Fair call re intergranular corrosion. I'll upload info tonight.
Good point - I do not foil myself ;) Anyway using SS bolts is still better than other way round.
An Al-In-Zn anode could help. Given the time it spends in water and its surface and paint - it could be quite tiny and easily retrofittable.
I could even model potential and current distribution if I had CAD geometry .. :wink:

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Re: Corrosion basics for kiteboarders

Postby Onda » Mon Jun 24, 2019 7:53 am

Very interesting thread, guys!
I have seen very clear corrosion of my Onda fuselage (Al) even after the first short use in North Sea and I was shocked, to be honest!
The corrosion (white spots of Al oxides) appeared at and near the contact points between the mast (C) and the fuselage (Al). To complicate things, all is screwed together with 316 screws.
Looking at the potentials table it becomes obvious that any contact to Carbon is a killer for almost every metal, particularly Al (if not VERY propperly anodized - which is an art for complex shapes).

I thoroughly rubbed the whole fuselage with Li-grease and left it covered with grease for several days.
That helped a lot, no more corrosion from then.

I dismantle and sweet-water-flush and dry my foil after each use!

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Re: Corrosion basics for kiteboarders

Postby plummet » Mon Jun 24, 2019 7:56 am

I'm interested to see how you could get an anode into a place close enough to become sacrificial considering there are typically tight fits and countersunk fasters without washers etch. But its an interesting thought that might become a sellable item if an easily retrofittable solution was developed! Or something that could be sold upfront to the foil manufacturing companies!

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Re: Corrosion basics for kiteboarders

Postby wrogu » Mon Jun 24, 2019 8:06 am

Onda wrote:
Mon Jun 24, 2019 7:53 am
Very interesting thread, guys!
I have seen very clear corrosion of my Onda fuselage (Al) even after the first short use in North Sea and I was shocked, to be honest!
The corrosion (white spots of Al oxides) appeared at and near the contact points between the mast (C) and the fuselage (Al). To complicate things, all is screwed together with 316 screws.
Looking at the potentials table it becomes obvious that any contact to Carbon is a killer for almost every metal, particularly Al (if not VERY propperly anodized - which is an art for complex shapes).
Now this is interesting, did not know there are sold masts and 'wings' made of different materials. I guess the mast is made of Al to reduce price.
plummet wrote:
Mon Jun 24, 2019 7:56 am
I'm interested to see how you could get an anode into a place close enough to become sacrificial considering there are typically tight fits and countersunk fasters without washers etch. But its an interesting thought that might become a sellable item if an easily retrofittable solution was developed! Or something that could be sold upfront to the foil manufacturing companies!
This is true, if water gets into crevice it is an issue from CP point of view (given anode is placed somewhere on the mast). First of all I would avoid using Al masts with carbon wings. All Al wing with a SS bolt would be less of an issue.

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Re: Corrosion basics for kiteboarders

Postby plummet » Mon Jun 24, 2019 8:18 am

Ok, what is Intergranular corrosion?

Intergranular corrosion is the preferential attack of the grain boundaries within the metal crystal structure. Why does it happen and how can we guard against it?

When stainless steels are heated for between 500 and 800 deg C for a period of time they can become sensitized. Carbon can start to precipitate and joins with chromium to cause chromium carbide precipitation. The net effect once the steel has cooled down is chromium depleted zones in the grain boundaries. This means at those zones the material has lower corrosion resistance. In the right enviroment corrosion can start and propogate through the grain boundries of the steel.
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This typically takes place when welding in the heat affected zones around the weld if the weld has been keep hot for too long.

How do we prevent intergranular corrosion?

By far the easiest method is to control the chemistry of the steel you are welding.
Standard 316 will take approximately one minute of heating into the critical zone to be susceptible to chromium carbide precipitation.
However if you specify 316L low carbon content stainless then it will take 10 minutes approximately to be susceptible chromium carbide precipitation. Since most welded stainless widgets are small and will not be welded for more than 1 or 2 mins selecting 316L is the easiest method to guard against intergranular corrosion.


However, as a consumer you do not control the material selection process.
What else can you do? Heat treatment of parts to redissolve the chromium is an option. But this is not cost effective for a one-off part,

You can follow your washdown/dry proceedure for these parts and do periodic inspection and replacement. If a part shows corrosion on or by a weld replace the part asap.
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