SS - Aluminum Corrosion, Sail Track

[Triton318]

I have a 1962 Pearson Triton with the original almuminum mast. It has a bronze sail track screwed to it with stainless steel screws. My question is this -- will the corrosion that occurs where the stainless steel screws are in contact with the aluminum mast continue to "spread" over time? Or, does the corrosion just occur in the immediate contact area and then stop?

(I'm trying to decide whether or not to replace this mast before going offshore, and this is just one consideration.)

Thanks.

[skipmac]

The "corrosion" is galvanic reaction between the dissimilar metals, Al and Fe. As long as they are in contact and in a moist, salty environment the reaction and break down of the Al will continue.

The cure is to remove the SS screws, coat with Tef-gel or similar compound that isolates the two metals from direct contact, and replace. Of course with the Al corroded the screws may be really difficult to remove and the hole and threads in the mast may be damaged as well, requiring redrilled larger holes or other cure.

My Pearson also had a steel mast step on the keel resulting in the bottom 2-3 inches of the mast being totally eaten away. Have you checked the base of the mast in the Triton? Does it have the same problem?

[roverhi]

The stainless screws are not the problem, the bronze sail track is. Stainless is fairly close to aluminum on the Galvanic scale, bronze is a long ways away and thus very reactive with the mast.

I've got a slightly newer Pearson (1969) and have removed all the fastenings and hardware on the mast. I've reinstalled it with TefGel for the fasteners and plastic insulation material backing the hardware. Surprizingly, the corrosion was relatively mild even where the bronze goose neck track was against the mast without any insulation.

If you do decide to R&R the mast hardware, an impact screwdriver is a real plus. I've got a 12 volt Hitachi and it's been a god send in getting recalcitrant fasteners loose. Buy lots of straight slot bits as you'll shatter a few.

Another trick is to use battery cables and the boats battery to heat the fasteners. Put an appropriate sized machine screw or bolt in the positive clamp. Attache the negative to the mast as close to the fastener as possible; hold it against the mast right next to the fastener; or, if you are lucky enough to have access to both ends of the fastener, touch or clamp it to the opposite end of the fastener. The dead short created will heat the fastener and primarily the fastener real quick. It's essentially an arc welder so be aware you could weld the fastener in the jumper cable to the head of the fastener in the mast. I've never done it but the fastener to fastener union did get a bit sticky in a couple of instance. I've used this method to get a bunch of fasteners out of the mast and my self steering without a problem. Until I learned this trick, had a number of fasteners I couldn't get out or broke the fastener. Using this technique, I haven't found a fastener I can't remove with the help of my trusty impact screw driver. It may not work first try so have patience. Heat the fastener, spray with penetrating oil, heat again and repeat till the fastener comes loose. May take a couple of overnight soaks in penetrating oil for really corroded fasteners.

FWIW, the threads were all still good in the aluminum on these long corroded in place fasteners.

[Weyalan]

If any of your fastenings are wobbly in the thread, consider drilling and fitting helicoil inserts (make sure you paint them with Duralac, or equivalent). We found that the threads for our goose-neck fitting were a little worn / corroded, so I drilled and tapped and fitted helicoils and they were like brand new again.

[skipmac]

Roverhi I have to disagree that SS and Al are close on galvanic scale and compatible. It will depend to some degree on which alloys of Al and SS. For example 316 SS is actually quite close to silicone bronze on the scale and both are far from most Al alloys.

A fairly comprehensive galvanic scale of various metal alloys.

Galvanic series

I can confirm from personal experience with SS screws in a mast with no bronze parts that were pretty much galvanically welded in place after 3-4 years.