Corrosion on the mast between Aluminum and Stainless Steel

[Suijin]

The 30 year old fasteners came out of my mast with an impact driver without much of a problem. Retapped the holes to core out corrosion. Used a stainless wire wheel to take the spot corrosion down to new metal, etched, primed, painted with two-part, and gooped the hell out of any stainless that went back on with Tef-gel.For flat plates against the mast, like line guides into the mast, I cut vinyl sheet templated from the fitting and slid them underneath.

All that said and done, corrosion comes back quick if you don't 1) get down to bare metal and 2) over apply the Tef-gel.

Tef-gel is really nothing more than PTFE and it works as a barrier. You have to use it liberally and make sure every surface is coated for it to be effective.

[Tetepare]

Originally Posted by topmast

Has anyone tried painting with Cetol? Blasphemy? Isolate metals as best you can and make it look like wood..No aluminium on my boat but lots of galvi. Worked.

Sounds silly but u might b on to something. Cetol on wood might last two years. Cetol on fiberglass stays forever. On aluminum? Hmmmm

[S/V Alchemy]

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.

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".

[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.

[Cadence]

Originally Posted by DeepFrz

Painting aluminum is not quite as easy as SailingFan suggests. Don't forget the etch coat.

You always seem to be a wealth of good information and research.

[DeepFrz]

I wish that were so. But thanks for the compliment.

[JPA Cate]

Another product we've found very good for the threads of the s/s screws is silver anti-seize, although, we have started using plain anhydrous lanolin. Underneath the larger plates, we use automobile inner tube material, using the object for a template as suggested above, have also used pvc electrical tape, and I'm told those skinny silicone cutting board things, work well, too.

Another way to "finish" a stripped mast is to first, sand it's length longitudinally, base to top, till it's all done and even, then coat it with clear polyurethane. Your mast will stand out among all the others, as the shiniest, particularly in low sun light angle conditions.

Ann

[svrodeorm]

Why this Hassle and cost of painting alu spars.?

I've got a painted spar (American built boat) and honestly I can't see any sort of benefit unless you are masochist who happens to love the looks of it. In Europe most spars are anodized alu which is far better IMHO, and totally maintenance free. (Obviously the issues of SS fasteners on alu is always there, lanolin or any other animal grease takes care of that, but much cheaper than Teflon based brand name stuff.

When time comes to strip off whatever paint there is left on my mast, I will simply leave it bare. Marine aluminium alloys oxidize, giving it a thin protective layer of "alumine". . Guess for some this grey appearance could be disturbing. Look at a French built aluminium boat with unpainted top sides, that will give you an idea.

A natural pleasant look once you get used to it ;-) no need to keep anything on a boat shny and glossy,

[Elie]

Anti seize product sold in electrical departments, mostly used to prevent corrosion between aluminum and copper electrical wires. Be generous with the stuff. If you can raplace the screws by stainless steel rivets, so much the better: Rivets can be drilled to remove them, if needed.

[peter57]

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

The plates Sikaflexed or other similar barriers like cetel plastic shaped keeps that problem away. The main problem as mentioned is the fastenings if you can find Monel metal fastenings and use Duralac or Tef-Gel, this will slow the problem right down with carefully application, plus you can reuse the screws again and the tapped threads in the Aluminium are usually good to go again and do not need oversizing the fasteners because the aluminium normally needs re-tapping when stainless has been used.

Monel pop rivets can still be obtained but finding bolts will be difficult. If you are in the States you have a better chance of finding them.

Cheers, Peter.

[Don C L]

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.

Welcome here! Tell us more about naval marine and aerospace applications we can try out here too!

[Don C L]

Originally Posted by svrodeorm

Why this Hassle and cost of painting alu spars.?

I've got a painted spar (American built boat) and honestly I can't see any sort of benefit unless you are masochist who happens to love the looks of it. In Europe most spars are anodized alu which is far better IMHO, and totally maintenance free. (Obviously the issues of SS fasteners on alu is always there, lanolin or any other animal grease takes care of that, but much cheaper than Teflon based brand name stuff.

When time comes to strip off whatever paint there is left on my mast, I will simply leave it bare. Marine aluminium alloys oxidize, giving it a thin protective layer of "alumine". . Guess for some this grey appearance could be disturbing. Look at a French built aluminium boat with unpainted top sides, that will give you an idea.

A natural pleasant look once you get used to it ;-) no need to keep anything on a boat shny and glossy,

I agree. My old boat has a bare mast, the original, 54 years old. Not the prettiest but better than seeing paint chip off IMO. On my previous boat I did paint the mast, and it did look beautiful for quite a while. But now, the lower the maintenance the more beautiful something is...

[knockabout]

duralac paste is designed for assembling dissimilar metals / lanolin grease is good as an alternative for field maintenance and as a regular check wipe with an olive oil soaked cloth / we are using marine 66 spray at the moment and coated mast and fittings and pulled a cloth soaked with marine 66 with a retrieval rope tied to the bottom of it up the mast track and worked it up and down a few times spraying the track above the cloth to add more lube treatment / we have not been out of the mud pen to test whether it's cruising boat proof. looking good so far though / marine 66 is made by CRC and is some sort of new synthetic inhibitor designed for salt or fresh / is also a food grade spray / works well on the bar-b-que / careful spraying on the stainless kettle makes the dandelion tea taste strange / haven't tried it cooking eggs yet

[SailingFan]

Originally Posted by Ann T. Cate

Another product we've found very good for the threads of the s/s screws is silver anti-seize, although, we have started using plain anhydrous lanolin. Underneath the larger plates, we use automobile inner tube material, using the object for a template as suggested above, have also used pvc electrical tape, and I'm told those skinny silicone cutting board things, work well, too.

Another way to "finish" a stripped mast is to first, sand it's length longitudinally, base to top, till it's all done and even, then coat it with clear polyurethane. Your mast will stand out among all the others, as the shiniest, particularly in low sun light angle conditions.

Ann

Yupp....

[Don C L]

Originally Posted by Ann T. Cate

Underneath the larger plates, we use automobile inner tube material, using the object for a template as suggested above, have also used pvc electrical tape, and I'm told those skinny silicone cutting board things, work well, too.
Ann

Oh yeah, I forgot, I put a washer I cut out of a thick laundry detergent bottle between the base of my mast and the stainless mast plate. Seemed like pretty tough stuff, we'll see how it holds up to the pressure. The price was pretty good.