What kind of corrosion is this?

[Popcorn]

Originally Posted by gray_sailor

Hi!


This is a picture of the main boom on catamaran in Turkey.
We have a discussion about the type of corrosion and its nature.
Depending on the answer there are various reasons and remediation action for this corrosion.


Is it Pitting or Electrolytic?
What do you think?




Hi Gray Sailor:

Suggestion:
Determine if the pitting is coming from the inside. Take a worse case test area, and using a high speed drill, and small drill bit, and test if it goes through showing a powdery gray dust all the way. If so, than your boom is slowly going to fail from weakening metal corrosion, especially since it is showing everywhere along the boom.
If not, just take out the corrosion by lightly touching the aluminum with the drill bit. This may make the boom look additionally pitted, but should stop the corrosion.
To prevent further corrosion of each area drilled, use a Cold Galvanizing spray (found at Ace or The Home Depot, or can of such. Block off spraying the area, but if you fill the indent, be sure compatibility of paint is good. Your boom may look like a spotted spear, but you can eliminate the spreading of, or life of your boom. Later you might prime and paint the boom as to the color you wish and eliminate the unsightly pattern problem you see.
If you find an Aluminum paint spray that would work, other than Cold Galvanizing spray paint, Please let me know.
Another thought is that I think the boom aluminum was not made properly and would not pit from others grinding near by. If that were so, than you would see stain (usually brown) on deck, plastic dodger parts and sail materials and not just the boom, unless the boom was stored in a shop with foreign metal cuttings about somewhere else.
Popcorn
PNW


Is anyone with appropriate competence able give us an opinion?
Thank you very much!



P.S. I vote for pitting

Hi Gray Sailor:

Suggestion:
Determine if the pitting is coming from the inside. Take a worse case test area, and using a high speed drill, and small drill bit, and test if it goes through showing a powdery gray dust all the way. If so, than your boom is slowly going to fail from weakening metal corrosion, especially since it is showing everywhere along the boom.
If not, just take out the corrosion by lightly touching the aluminum with the drill bit. This may make the boom look additionally pitted, but should stop the corrosion.
To prevent further corrosion of each area drilled, use a Cold Galvanizing spray (found at Ace or The Home Depot, or can of such. Block off spraying the area, but if you fill the indent, be sure compatibility of paint is good. Your boom may look like a spotted spear, but you can eliminate the spreading of, or life of your boom. Later you might prime and paint the boom as to the color you wish and eliminate the unsightly pattern problem you see.
If you find an Aluminum paint spray that would work, other than Cold Galvanizing spray paint, Please let me know.
Another thought is that I think the boom aluminum was not made properly and would not pit from others grinding near by. If that were so, than you would see stain (usually brown) on deck, plastic dodger parts and sail materials and not just the boom, unless the boom was stored in a shop with foreign metal cuttings about somewhere else.
Popcorn
PNW

[BruceS]

I had a farmer friend hoist his unused alum dinghy up above some machinery in his implement shed to get it out of the weather.
When he got it down it was peppered with holes caused by bird crap.
Swallows, starlings & spoggies (Sparrow).

[Seymore]

It's pitting.

But pitting is best treated as electrolytic by corrosion engineers. Thus.....

--------------

Does the pitting appear on the main mast, and mizzen mast and boom? On the photo, the pits look to be on the top half of the boom and the bottom looks to be clear. If so I'd vote for grinding dust, welding dust or something similar causing the pits.

The pits look to be active corrosion. Using a pin, dig into the bigger holes looking for tiny particles of dust or metal. The tiny particle is the cathode and the aluminum is the anode. The pits are a tiny corrosion cell--the anode (aluminum) corrodes to protect the cathode (particle). Examine the particles carefully, like under a microscope, and try to decide what they are.

If the mast and mizzen are clear, there is no grinding dust, and the pits are present on the bottom of the boom, I'd vote for manufacturing defect. 8 ))

Corrosion cells need water to work. If you decide to paint the boom, heat it first to drive out the water.

If it was my boom, I'd consider putting it back in service as it is, for a while. The pits don't look to be structural damage.

[team karst]

One thing not mentioned is the aluminum alloy that was used. If it was 6000 series like 6061, then bad, with its content of copper in the alloy mix. 5000 has nil copper. But, you will need a portable XRF gun to analyze it. Fast and non-destructive, but you need to get one. Try a high end scrap metal yard, they have them. An employee will tag along, since they are around $20k to buy.

[double u]

please:
pitting can be caused by electrolytic & galvanic corrosion....
of course it is "pitting": everybody can see "pits".
& if it really is a copper-containing alloy...you have your galvanic element right there...nearly as bad in seawater as brass...
to further educate yourself, gentlemen:
https://www.amazon.com/-/de/dp/15740...7832069&sr=8-1

[Cheechako]

That is just salt corroding aluminum. It's very very common. SO I don't know what you should call it, chemical corrosion?
It's often prevalent anywhere on a mast used offshore.
Salt water splashes on the metal, it dries leaving a grain of salt on the surface. (that's how much table salt is made, salt water dried out leaving.... wait for it! SALT!)

The salt attacks aluminum unless heavily anodized..... and eventually that also.
Marine aluminum alloys are not impervious to corrosion any more than "stainless" steel is impervious to stain or corrosion. Semantics.

The preventive cure is to fresh water rinse all over every time you use the boat.

I inspected an aluminum powerboat once that was not very old but used in salt water. The hull was made of Marine Aluminum Alloy. The trailer had those bunks covered with green carpet. The boat had sat on the bunks over the winter. There were 4 stripes on the hull bottom full of deep pits from the salt water sitting trapped in the carpet.

"Saltwater and Metal
The combination of moisture, oxygen and salt, especially sodium chloride, damages metal worse than rust does. This combination corrodes, or eats away at, the metal, weakening it and causing it to fall apart. Saltwater corrodes metal five times faster than fresh water does and the salty, humid ocean air causes metal to corrode 10 times faster than air with normal humidity. Bacteria in ocean water also consumes iron and their excretions turn to rust."

"Electrochemical Corrosion
One form of corrosion that occurs when metal and saltwater get together is called electrochemical corrosion. Metal ions dissolve in water and saltwater conducts electricity and contains ions, which attract ions from other compounds. During electrochemical corrosion, electrons from other compounds are attracted to the metallic ions. Saltwater attacks the metal and corrosion occurs."

[double u]

we had a whole boat made of aluminium (10 years & 2 rtw), & nowhere did we see pitting like that. Look at any unpainted Ovni - none of them will have such pits.
OTOH I recall seeing a boat on the hard in Raiatea in 90, & the underwater surface looked like a hedgehog from 10m away. On closer inspection the owner had drilled hundreds of holes & put toothpicks through them for the welder not to miss one. There had been extensive pitting under water from the wrong type of antifouling & he drilled all pits out & had them welded shut.

[Cadence]

If it is the result of iron filings, as seems to be the consensus of opinion, I wonder if wiping it down with Ospho might arrest the progress?

[CharlieJ]

I inspected an aluminum powerboat once that was not very old but used in salt water. The hull was made of Marine Aluminum Alloy. The trailer had those bunks covered with green carpet. The boat had sat on the bunks over the winter. There were 4 stripes on the hull bottom full of deep pits from the salt water sitting trapped in the carpet.

This is a textbook example of poultice corrosion.

[CaptainMarkS]

Lightning strike? Manufacturing defect? Interesting it's just the boom.

[timb7734]

To me this appears as acid pitting, I have much experience with corrosion in stainless steel and this looks exactly like that. Something acidic or chlorinated had to sit on the metal in the absence of fresh air/oxygen.

[Sailing August]

That looks like carbon steel pitting. Possibly picked up when someone was grinding steel in the vicinity of your vessel while in a boat yard. This can happen with both SS and Alu.
Use some "Naval Jelly" to passivate the rust spots and brush it in the spots with an aluminum or SS tig brush. Don's use a carbon steel brush.

[Grigory]

Aluminium ("silver") wheel paint for cars maybe good. It is durable, salt-resistant, and looks good. A possible reason is also that boom was manufactured from Cu-contained alloy, like duralumin.

[Kolchac]

This mast is galvanized (protected). I think that somebody damaged this layer mechanically by something like sandblaster or dropped it on gravel. So damaged spots rusted. Anyway, looks bad.

[betwys1]

[QUOTE=gray_sailor;3454902]Hi!


This is a picture of the main boom on catamaran in Turkey.
We have a discussion about the type of corrosion and its nature.
Depending on the answer there are various reasons and remediation action for this corrosion.
Is it Pitting or Electrolytic?
What do you think?
/Quote


Alumatron wiped on after thorough clean, degrease - Dawn etc. is recommended on high test alloy