Old 13th January 2007, 11:17 PM   #1
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Default Rusty stain around screws.

Somebody asked the team about rusty stains around screws on an Aero 117. I had the same problem on a Carve 123.
This stain is easily removed with clear vinegar.

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Old 15th January 2007, 07:30 PM   #2
Ola_H
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Default RE: Rusty stain around screws.

Nice tip.

Cheers,
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Old 15th January 2007, 11:55 PM   #3
Jean-Marc
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Default RE: Rusty stain around screws.

Guest,

Thanks for the chemistry tip. This reminded me of yellowing hull and how to make them white again.

Acetic acid in commonly found in vinegar. You can try citric acid as well which is commonly found in lemon juice (less stinky than vinegar).

Anything which is weakly acidic will works to remove singly positive charged salt (Na+, K+, etc...) and doubly positive charged salt deposits (Ca++, Mg++, Cu++, Fe++, etc...) commonly found dissolved in fresh and salt water. Some freshwater or saltwater windsurfing areas may contain higher or lower concentration of each of these mineral salts, hence producing different degree of hull's "yellowing".
The mechanism of "yellowing" or "rusty stain" removal is by chemical competition with H+ protons found in the acid. The higher concentration of the acid is , the higher number of H+ proton is found and the quicker the competition reaction goes.

It looks like the plastic surface of the hulls made by the Cobra factory is negatively charged to allow the binding of salt deposits by coulombic interactions. The cure would be to coat the hull surface with a "paint" containing a chemically inert agent. I guess a metallic ion such as Aluminum or Zinc could be used to render the plastic surface non-reactive chemically and corrosion-free. Unfortunately, I'm not educated in plastic engineering, so I can't elaborate further. The plastic bottle industry or the food plastic packaging industry should know the solution to this problem.

The same principles described as above do apply to cure monofilm sails which have turned white/yellow with time.

Cheers !

Jean-Marc
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Old 16th January 2007, 01:21 AM   #4
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Default RE: Rusty stain around screws.

This is poster 1 again.

The stain in this case is Iron Oxide (rust) from the screws. I first thought that the screws were a low quality stainless but a very strong magnet would not attract them. I think that the rusting was the result of crevice corrosion.
This is where part of a stainless object which is shielded from oxygen develops a different voltage from the part which is exposed when immersed in an electrolyte (in this case salt water). Biological activity or and corrosion could have lowered the oxygen in the screw hole.
When I got this rust stain developed, the board had been left in a bag and there was evidence of rusting at the edge of the hole but nowhere else except of course from the anti twist plate which corroded badly. This may have made a further contribution to the galvanic cell which caused the rusting at the air boundary.

I was able to remove the stain quickly using a brush and vinegar.
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Old 16th January 2007, 11:39 AM   #5
Jean-Marc
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Default RE: Rusty stain around screws.

Quote:
Guest
This is poster 1 again.

there was evidence of rusting at the edge of the hole but nowhere else except of course from the anti twist plate which corroded badly.
Looks like to me the metallic anti-twist plate is the culprit of first corrosion which secondly spreads to the screws threads, and not the reverse, what do you think ?

Cheers !

JM
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Old 17th January 2007, 01:36 AM   #6
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Default RE: Rusty stain around screws.

The rust on the screw was at the outer edge of the hole. The screw was perfectly clean where it went through the strap. I used a 10X eyepiece to have a good look at it. I concluded at the time that the rust at that point did not come from the anti twist plate. The rust on the white deck pad was greatest near the hole and became less with increasing distance from it.

I read a text on marine metal corrosion about 30 years ago. I have forgotten most of the details. It went into great detail on the causes of corrosion in stainless steel. It is most inert when exposed to oxygen.

I have given my observations. Reading the post about the Aero it appears that the problem continues.

I fitted Da Kine straps and cleaned the foam with vinegar. Both worked well.

I am now back to where I started.
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