Corrosion on the mast between Aluminum and Stainless Steel

[Lambretta]

Originally Posted by chala

I get good result from Duralac.

“Duralac is a chromate containing jointing compound designed to inhibit electrolytic decomposition between dissimilar metals – often called galvanic corrosion. It is a single pack, air setting product. Used correctly, Duralac prevents anodic decomposition.”

By the way, a rope wrapped around an aluminium spare, even anodised, can cause corrosion.

+1 for Duralac
Have been using it for 5 years with great results

http://www.llewellyn-ryland.co.uk/downloads/duralac.pdf

[dennismenace111]

Originally Posted by S/V Alchemy

Another vote for Duralac, but really, the choices are Duralac, Tef-gel, Lanacoat, Permatex of the right type or basically anything that will separate the steel from the alu while maintaining the mechanical bond. The other options are to place holes in the mast accessible from behind (i.e. quite low or next to a slot) so you can use a nylon sleeve bushing and a nut (and nylon washers) to again fully isolate the SS fastener from the aluminum mast. Of course, this is rarely possible to achieve.

Lastly, you may want to consider aluminum pop rivets. It is far easier to isolate a SS eyestrap, for instance, from a mast's outside surface than it is to isolate the threads from the tap. You can affix soft shackles to the eyestrap made of covered Spectra or Dyneema and fasten blocks to that. It seems like more fuss and work until you consider the alternative.

You may wish to test your boat for galvanic corrosion and for how the mast "touches" the boat. That could indicate why this is happening beyond the intrinsic issue of different degrees of metallic "nobility".

I would vote against aluminum pop rivets... the likely hood of finding a pop rivet with the same electrolytic properties of the aluminum mast are not likely...

Nylon washers work the best... and in another post, someone mentioned cutting plastic backers from laundry detergent bottles... that would be better than using an old innertube that would hold moisture..

Then the last point, using the proper version of Permatex to isoloate the threads of the screw or bolt.

No matter what you do, anytime you scratch the aluminum oxide surface of the aluminum mast, you are going to eventually get corrosion.

Its an ongoing battle that never ends.

You should see what we had to do on aluminum seaplanes... do you have any idea how many stainless screws and bolts are used? In some cases we even used pure sacrficial aluminum washers along with nylon washers on parts we frequently removed.

Even aluminum rivets in the skin panels would eventually get salt moisture under them and corrode. Regular inspections are de rigueur.

[double u]

Originally Posted by Dan_K

Galvanic corrosion rate is (among other things) proportional to the exposed surface areas of the anode (aluminum in this case) and the cathode (stainless steel). Since painting will have some pinholes, painted aluminum will have these areas of very small exposed area causing the galvanic current to be high in the aluminum and cause a fast aluminum corrosion rate. From the galvanic perspective, painting the stainless and leaving the aluminum exposed would cause the aluminum galvanic corrosion rate to be much less (but isolating stainless from oxygen has it's own problems). Previous comments of galvanically isolating the stainless from the aluminum is good. In naval marine and aerospace uses I have at times specified that the stainless parts be coated with dry film lubricant (like specification MIL-L-46010), but have not seen this coating used in small boats.

I have very much enjoyed this forum, this is my first comment.

...& a very smart one too!

[double u]

btw: even totally electrically insulating the two metals from each others will not cure the problem as long as there can be stagnant water on the aluminium (differential oxygenation will make some places of the Al cathodic, others anodic & presto off the galvanic corrosion kicks

[jonahmulski]

Thank you all for the great information! I really appreciate your thoughts and insights.
I'm going to start going through the mast and boom next week. I like the plastic bottle gasket idea and once I get the screws out (fingers crossed) I'll smear them with tef-gel before putting them back in (it what I have on the boat here in Mexico ).
Cheers!

[jonahmulski]

Oh ya... and I get to sand and paint a lot of corrosion while hanging in my bosins chair. I know... it's hard not to be jealous.

[rlcolwell]

Since the aluminum is anodic to the SS, conventional practice is to paint the cathode (the SS) to mitigate galvanic corrosion. Opposite of what might be intuitive, but borne out by convention

[Dsanduril]

A long time ago a rigger pointed me to PVC pipe wrap tape rather than plastic sheets and I've never looked back. Been going strong for decades and I see no corrosion when I pull fittings off the mast (or crossbeam). It's cheap enough, and the advantage over plastic sheets - it's self stick. Put a piece of tape on the mast (or on the back of the fitting), mount your hardware, trim around the edges. Available in different thicknesses, I use the 20 mil. On fasteners I like to use NoAlox or similar anti-seize. Contains zinc which is controversial here on CF but it is my preference.

[GreenWave]

Originally Posted by mike d.

Rap the head of the fasteners with a hammer ! Don't pound them, just rap them, that should loosen the fastenings in the threads. Use a properly sized Phillips head screw driver, take a pair of vise grips, and gently try to rock/turn the screws loose. Do this lightly, and watch the heads as you do this. If you have jarred the screws enough they should back out. If they don't move try it again, just don't bugger the screw head, hitting it or with the screw driver. If you are not in a rush to get it done, try building a dam with good duct tape and polyethylene plastic to make a pocket. Pour in vinegar to cover screws if possible. Let it sit over night. The vinegar will dissolve the corrosion. You may have to do it a few times, then go back to what I said before, and try again. I did my 53 foot mast and boom, this way which were painted 25 yrs. prior, and only had 1 screw that I could not get out, on my spin pole track.

My trick is to soak a cotton ball in vinegar and then apply that to the screw head. Tape over with duct tape. Let sit overnight as you suggested.

[betwys1]

Originally Posted by jonahmulski

Hi,
I'm starting to get corrosion on my aluminum mast and boom around the points where the stainless steel hardware is mounted (see pics). I'm planning on remounting the hardware and would appreciate any input on how to prevent or slow this process from happening again.
Many thanks,
Jonah

Take a look at this table - the bigger the distance between metals the worse!
(If I worked this uneasy method of adding pictures) and keep stainless out of contact with aluminum. If you must screw into aluminum, cadmium plate or galvanized screws minimize corrosion of the aluminum.

[chouliha]

Originally Posted by Dsanduril

A long time ago a rigger pointed me to PVC pipe wrap tape rather than plastic sheets and I've never looked back. Been going strong for decades and I see no corrosion when I pull fittings off the mast (or crossbeam). It's cheap enough, and the advantage over plastic sheets - it's self stick. Put a piece of tape on the mast (or on the back of the fitting), mount your hardware, trim around the edges. Available in different thicknesses, I use the 20 mil. On fasteners I like to use NoAlox or similar anti-seize. Contains zinc which is controversial here on CF but it is my preference.

Been using this for the past 25 years and it works extremely well with great insulating between SS fittings and Aluminum. It’s extremely easy to put on and then cut with a razor blade, punch out holes with the bolt

[GordMay]

Originally Posted by Dsanduril

A long time ago a rigger pointed me to PVC pipe wrap tape rather than plastic sheets ...
... Put a piece of tape on the mast (or on the back of the fitting), mount your hardware, trim around the edges. Available in different thicknesses, I use the 20 mil. On fasteners I like to use NoAlox or similar anti-seize. Contains zinc which is controversial here on CF but it is my preference.

CONDUCTIVE ANTI-OXIDANT JOINT COMPOUNDS:
(As utilized on Copper to Aluminum & Cu to Cu Electrical Joints)
These pastes usually contain conductive (under pressure) Zinc (or Copper) particles in suspension, and shouldn't be confused with insulating dielectric grease.

BURNDY “Penetrox” #CR300
IDEAL “Noalox” #30-030
THOMAS & BETTS “Aluma-Shield”
ITT/BLACKBURN “Contax“ #CTB8
GB ELECTRICAL “Ox-Guard” #OX-100B
ILSCO “DE-OX”
SANCHEM #NO-OX-ID-“A”

For electrical connections, I recommend BURNDY “Penetrox” (copper particles), for 2 prime reasons:
1. Penetrox is non-flammable, whereas others (Ideal “NoAlOx” & ilsco “De-Ox”, etc) will ignite and burn vigorously.
2. Penetrox is a thicker consistency, which will adhere better overhead & at elevated temperatures; whereas others are much thinner, and will drip & often require re-mixing.


For fasteners, in aluminum, I might prefer Ideal "Noalox" (zinc particles).