Very difficult situation as the oxide is much more inert and non-reactive than the base metal and of course your stanchions are mounted to a deck
substrate which also is likely to be much less inert. Acid strong enough to dissolve the oxide will corrode everything else and is hazardous.
I suspect physical force, torque is a potential.
The two aluminum
surfaces are likely to gaul together if you can break the bond, which bonding has caused by having formed oxide bridging, basically like a chemical weldment.
A stainless steel
bolt and aluminum
could cause galvanic corrosion
Snipet of guidance:
It is no doubt that stainless steel
screws are the best when it comes to holding together materials such as aluminum sheets
so that when faced with a variety of elements they do not break open. However, the dissimilar nature of these two metals puts them at risk of getting corroded resulting in their destruction altogether. Steel
screws can be used with aluminum but some precautions must be taken if they are to work
Tips to consider when using stainless screws on aluminum
Do not expose to water
is an electrolyte that can easily corrode either the aluminum or stainless steel material when they come into contact with each other. Therefore, it is important to always separate the two metals as soon as possible especially if the environment
has salty water or anything close to it.
To reduce chances of the metals getting corroded reduce the contact the two metals have by creating a non-reactive barrier. The best barrier material is plastic washers which ensure that the metals are sealed at their point of contact. Also, to reduce their contact with water, connecting points can be protected using tape or even paint
depending on your preference.
Check the surface area of metals
Always ensure that the aluminum part of the material is larger than the stainless steel screws to avoid it being eaten away. Aluminum which has an anodic nature will quickly disintegrate when it comes into contact with salty water, and will do the same when its surface area is less than that of stainless steel.
On this subject purely discouragement from other forum threads:
"Well, if they are Aluminium stanchions in aluminium bases bolted to the toerail with stainless screws and bolted through the deck
, just like my Moody 33, then it's practically impossible to remove the stanchion without destroying the base. New bases are supplied with a plastic liner between the post and the base. The reason they corrode solid is aluminium post in aluminium base with stainless bolts & split pins continually washed with salt water
in a big way. The only way to remove the post is to destroy the base(probably damaging the post in the process) using a combination of drilling/hammering/disc cutter
& any other method you can think of. Liquids and heat I found were not any good. The other problem is access to the nuts inside under the deck, as soon as you turn them the bolt turns as well, sometimes you are lucky and you can get them off. I found I had to drill out the countersunk screws which hold the bases to the toerail.
If you are lucky you'll get the stanchion complete with base off in one piece, if not they will both be damaged beyond repair
I found it took anything from ½ hr to 4 hours for each stanchion. It took two winters to complete the job.
Bases and stanchions still available from chandlers & internet
& ebay. Not cheap
if bought from chandlers. Negotiate to get a good deal. Use dichromate paste when reassembling to prevent corrosion between stainless & aluminium."
You are wasting time even trying wonder sprays. The products of corrosion on ally are more bulky than the ally metal so as a pipe in socket joint corrodes it gets ever tighter.
Most effective way is to remove stanchion and base, put the stanchion in the vice and leave the blowlamp playing on the base itself for 10 mins or so. Then use a stilson.
The less frustrating way is to junk the lot and replace with new.
Am about to try the same task today, for the third time on my boat
. The blowlamp has finally worked before but this time...... we shall see