Stainless screw in Aluminium

[Dino]

So a friend has a mobo with a very old set of wiper motors. There is a large stainless countersunk screw holding it together but it appears that the screw has reacted with the aluminium body and is seized. I have tried WD40, etc but no joy. The flat head countersunk screw has a damaged slot so I will probably have to grind a deeper slot.
Does anyone have any tricks for resolving this issue?

 

[vyv_cox]

The usual cause of this common problem is that insoluble seawater salts, mostly carbonates, accumulate in the crevices between the two metals. Hydrochloric acid will dissolve these very quickly. Apply the acid drop by drop as it may attack the aluminium, although fairly slowly. Once free wash liberally in fresh water

 

[geem]

The usual cause of this common problem is that insoluble seawater salts, mostly carbonates, accumulate in the crevices between the two metals. Hydrochloric acid will dissolve these very quickly. Apply the acid drop by drop as it may attack the aluminium, although fairly slowly. Once free wash liberally in fresh water

Yesterday I had a problem with one of my mainsail cars. A car failed a few months ago so I replaced it with a spare I had. The one that failed had a s/s bolt going into the batten box. The spare car I fitted had an aluminium bolt. Yesterday, the car with the aluminium bolt failed. The bolt had seized to the brass nut inside the car. No way could I remove it so I cut of the aluminium bolt head and drilled out most of the bolt. I then managed to remove the brass nut with the remains of the aluminium bolt inside. Dropping it into hydrochloride acid dissolved the aluminium really quickly. It left me a perfectly clean nut. The whole thing took about 15mins. I was using 33% pure hydrochloric acid.

 

[KompetentKrew]

I have tried WD40, etc but no joy. ...
Does anyone have any tricks for resolving this issue?

Did you try kettles and kettles of boiling water?

 

[vyv_cox]

Yesterday I had a problem with one of my mainsail cars. A car failed a few months ago so I replaced it with a spare I had. The one that failed had a s/s bolt going into the batten box. The spare car I fitted had an aluminium bolt. Yesterday, the car with the aluminium bolt failed. The bolt had seized to the brass nut inside the car. No way could I remove it so I cut of the aluminium bolt head and drilled out most of the bolt. I then managed to remove the brass nut with the remains of the aluminium bolt inside. Dropping it into hydrochloride acid dissolved the aluminium really quickly. It left me a perfectly clean nut. The whole thing took about 15mins. I was using 33% pure hydrochloric acid.

Concentrated HCl is far more aggressive than the usual stuff sold all over Europe for domestic use, about 6% w/w.

Pure aluminium reacts quite rapidly but some of the alloys are far more resistant. My test on a VHF antenna in a masthead bracket, probably a 5xxx alloy took the corrosion product out perfectly with no visible attack on either metal.