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Corrosion basics for kiteboarders

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plummet
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Corrosion basics for kiteboarders

Postby plummet » Sat Jun 22, 2019 12:02 pm

Hey bros,
Here's a quick run down on how corrosion works and how you can apply that knowledge to kiteboarding.

To transform metals from their natural state (eg iron ore)we expend large amounts of energy to form them into the metals that we know today. Once we have transformed the metals into the composition that we want they are known to be in a meta-stable state. The metal over time will shed energy and return to its original state. This is what we know as corrosion

All metals with the exception of gold have an oxide layer. That layer could be an active layer that continues to corrode. Eg iron oxide, rust.
Or a passive oxide layer could be formed that protects the underlying material. EG the chromium oxide layer found on stainless steels.

Corrosion is an electrochemical process.
Corrosion acts in the same way a battery works.

You need an anode, Cathode, an electrolyte solution, water and oxygen and temperature. If you can remove or reduce any of these factors you can reduce/slow the corrosion process!
Galvanic_electrolytic.jpg
The difference between a battery and galvanic corrosion cell is that the anode and cathode are constantly touching each over. This allows the corrosion to propagate unchecked.
20.8.1A.jpg
All metals have voltage potential. When the voltage potential varies between the 2 metals we have a possibility of creating a galvanic cell.
The metal with the highest voltage becomes the cathode and the metal with the lowest voltage becomes the cathode. For galvanic corrosion to take place is standard services (eg in your house) you need a voltage potential difference of 0.5V.


However kitesurfing in the sea is not a standard service, It is considered a severe service. In a server service, the recommendation is to have no more than 0.15V potential difference between to two metals.
What is a server service? Service that increases the likelihood of corrosion. Increased temperature increases corrosion protential, increased moisture and increased reactivity in the elecrolyte solution eg salt water compared to fresh water


How do we know what voltage potential our metals have? Are they compatible? and will they be susceptible to corrosion.

We look at something called an anodic index or galvanic chart,

How do we know what voltage potential our metals have? Are they compatible? and will they be susceptible to corrosion.

We look at something called an anodic index or galvanic chart,
001-Galvanic_Series_of_Common_Metals_Scale_of_Nobility_Cathodic_Anodic_Reactance_of_Metals_Corrosion_of_Metals.png
If we take 2 common material used in kiteboarding. Aluminum and stainless. How far apart are these on the anodic index?
Much more than 0.15V.There will be galvanic corrosion in a server environment between aluminum and stainless steel.
Which material will corrode? the alloy or the stainless? The cathode will not corrode and the anode will. In this instance, the alloy is the anode and the stainless is the cathode.

How do we slow galvanic corrosion?

We can insulate the 2 dissimalar materials. If the 2 different metals are not touching then we have broken the battery connecttion much like turning off the switch. EG use a plastic washer.
Use materials within 0.15V potential differene.EG stainless with stainless.
Control the moisture between the 2 materials. Using a sealant or lubricant can keep moisture out off fasteners and prevent oxygen, moisture and elecrolyte solution.
Place a protective coating over the anode and cathode.
ways-to-prevent-galvanic-corrosion.png
So.... You say I don't have alloy to stainless, I only have stainless to stainless bolts. I wont get corrosion because we have the same voltage potential right?

Wrong You still have a chance for corrosion. This is where pitting and crevice corrosion come to play.

As mentioned above stainless steels have a protective oxide layer that prevents corrosion. This is because chromium has been added to the composition of the steel and creates the chromium oxide passive layer.



If the passive oxide layer breaks down and you have the reactive environment like sea water and warm temperatures then corrosion can take place. Interestingly Stainless steels have 2 voltage potentials. A voltage potential with a passive oxide layer and a lower voltage potential when the passive oxide layer breaks down. This means that a galvanic cell can be created on the surface of the stainless and a pit can start to be created.
corrosionpit.jpg
corrosionpit.jpg (38.5 KiB) Viewed 727 times


As the pit depth increases the corrosion products on the deprives the pit of oxygen removing the materials ability to regenerate its passive oxide layer. The pit becomes increasingly more anodic and reactive increasing the corrosion rate. Pitting corrosion is 10 x faster than general corrosion.
6C2W1aZ5vrzd5iFDbZXhwK9jnD2LSzkYRo3psLiJWZU17qgSANusnmk.jpg
6C2W1aZ5vrzd5iFDbZXhwK9jnD2LSzkYRo3psLiJWZU17qgSANusnmk.jpg (27.26 KiB) Viewed 727 times
As you increase the temperature likelyhood of pitting corrosion increases, as you increase the reactivity of the electrolyte solution you increase the rate of pitting corrosion.

How do you prevent pitting corrosion?
Select appropriate materials that will work in the service you use.
Wash down after use in fresh water, This removes the electrolyte solution
Keep gear stored dry,


Not all stainless steels are created equal. 304 and 316 are common grades used in kiteboarding.
untitled.png
untitled.png (12.43 KiB) Viewed 727 times
As you can see in this critical pitting temperature chart,

304 SS has a low temperature of when pitting corrosion takes place.

316 includes 2.0-2.5% molybdenum. This increases corrosion resistance. When selecting stainless parts for kiteboarding. Try to select 316 parts where possible.


Lastly what is crevice corrosion? Crevice corrosion works in the same manner as pitting corrosion. However a the crevice deprives the material from oxygen and retains moisture and electrolyte solutions becoming increasingly more reactive far more quickly than pitting corrosion. Pitting corrosion is up to 100 times faster than general corrosion and starts at a lot lower temperature than pitting corrosion.
fetch.png
fetch.png (11.72 KiB) Viewed 727 times
How do we prevent pitting corrosion.

Also the same techniques as pitting corrosion.
We can also add drain holes so crevices can be flushed and washed out,
Sealant to prevent ingress into the crevice
On a design standpoint try to remove as many crevices from the design as possible.


So there you have it. A quick guide to corrosion. Does your brain hurt?

Here's a sumary, Corrosion works like a battery. If you can remove or reduce the factors that cause corrosion then you can slow the corrosion process.
Let me know if you have any questions.
Last edited by plummet on Sat Jun 22, 2019 9:50 pm, edited 1 time in total.
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Bille
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Re: Corrosion basics for kiteboarders

Postby Bille » Sat Jun 22, 2019 3:47 pm

GREAT post "plummet" !! :thumb:

So it's Not a great idea, to place those SS -- T-nuts, for the foot-straps , directly
into carbon ?

Bille

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Re: Corrosion basics for kiteboarders

Postby wrogu » Sat Jun 22, 2019 4:27 pm

How do we prevent pitting corrosion.
Well the best way is to use material which is not susceptible to pitting/corrosion in given conditions.
316 ss is not considered to be resistant to corrosion in seawater. Alloys with higher PREN than 316 are also not considered immune.
For 20C water 3% NaCl, pren 40 and higher is considered immune. And a design flaw can screw you.

Other good points:
Wash in freash water - as water evaporates chloride concentration increases - bad case for any form of corrosion including pitting.

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Re: Corrosion basics for kiteboarders

Postby wrogu » Sat Jun 22, 2019 4:30 pm

Bille wrote:
Sat Jun 22, 2019 3:47 pm
GREAT post "plummet" !! :thumb:

So it's Not a great idea, to place those SS -- T-nuts, for the foot-straps , directly
into carbon ?

Bille
There should be no issue as there should be no electrolyte between carbon and tnut. Plus rest of the carbon is insulated by the resin. It is a composite after all.

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Re: Corrosion basics for kiteboarders

Postby RustyChain » 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".

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Re: Corrosion basics for kiteboarders

Postby plummet » Sat Jun 22, 2019 8:01 pm

wrogu wrote:
Sat Jun 22, 2019 4:27 pm
How do we prevent pitting corrosion.
Well the best way is to use material which is not susceptible to pitting/corrosion in given conditions.
316 ss is not considered to be resistant to corrosion in seawater. Alloys with higher PREN than 316 are also not considered immune.
For 20C water 3% NaCl, pren 40 and higher is considered immune. And a design flaw can screw you.

Other good points:
Wash in freash water - as water evaporates chloride concentration increases - bad case for any form of corrosion including pitting.
All Good points.

Regarding 316 in marine environments. In reality, we do get a reasonable service life out of 316. Guys in hotter climates will have more corrosion that guys in colder climates. Rinsing/dryinh and sealing up crevices with sealant or lubricant will help extend the life of 316 dramatically. For kiting crevice corrosion of fasteners is going to get you well before pitting corrosion becomes and issue.

Ever try to purchase any fasteners of a higher grade to 316? I becomes really expensive and difficult to procure. A simple process of washing/drying/sealing is the answer.

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Re: Corrosion basics for kiteboarders

Postby plummet » Sat Jun 22, 2019 8:10 pm

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".
Hahahaha,,, i'm loving the irony of your name "Rusty Chain" discussing corrosion...

Yep. But knowledge is power and many people think stainless will never corrode when it can. Consider yourself now informed. You can now apply the knowledge learned here too yourself and simply rename yourself "chain"

There is another form of corrosion protection you can invoke to stop your chain from rusting. You could apply some cathodic protection in the way of a sacrificial layer of anodic material. In the case of steel chain the sacrificial anodic material is Zinc. Hot dip zinc chain you will be protected for a number of years.

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Re: Corrosion basics for kiteboarders

Postby jaros » Sat Jun 22, 2019 8:30 pm

So, would a zinc anode (like on a motor boat) do the job on alu hydrofoil? If so, why no one is using it?

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Re: Corrosion basics for kiteboarders

Postby AndersP » Sat Jun 22, 2019 9:04 pm

Alpine use zinc anodes on their fuselages since last year.

plummet
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Re: Corrosion basics for kiteboarders

Postby plummet » Sat Jun 22, 2019 9:53 pm

jaros wrote:
Sat Jun 22, 2019 8:30 pm
So, would a zinc anode (like on a motor boat) do the job on alu hydrofoil? If so, why no one is using it?
I don't see a way to get the zinc anode into the crevice of the thread which is where the galvanic corrosion will occur for the stainless fastener/aluminium fuse issue.


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