Greenhouse Restoration

[BryanW]

There are zinc based protective paints but and it's a 'big but' it is best applied between both metals as the zinc takes the electrolytic damage thus providing a decent degree of protection to the metals. Heres a Google search result, you'll find a fair amount of coatings available for self application.

However, If you seriously want to preserve the framework your only realistic option is to seperate and treat the Steel and Alloy and looking at the rivets they'd need replacing too as their coating has long dissolved. Other than that your whistling in the wind and wasting good after bad etc

[BryanW]

Or get rid of all the steel strengthening bars and replace with new aluminium, eg https://www.aluminiumwarehouse.co.uk/1- ... m-flat-bar

A better more sensible option

[agavedave]

A zinc based coating could be anything,

1). zinc rich paint
2). electroplating
3). galvanised

Galvanising involves cleaning the steel and dipping it into molten zinc. Gives a nice thick durable coating.
https://www.galvanizing.org.uk/galvanizing-process/

Regarding aluminium/steel, the zinc plating on steel fasteners will initially protect the aluminium, once quickly exhausted the aluminium will start pitting around to joint. By that time the fasteners will be well corroded and stuck.


Regards
Dave

[habanerocat]

What exactly is a zinc based coating?

Zinc = Galvanised

Are you talking about the spray on galvanised coatings that come in cans?

I can't seem to get a definite answer as to whether they can be sprayed on aluminium.

[BryanW]

Spray cans are no good for what you're looking to do and you'd need a couple of dozen coats if it was.
Also it would require all surfaces do be very clean and free from any corrosion whatsoever, too costly

[Phil_SK]

Zinc-based solutions are no good: aluminium is more reactive than zinc (and iron) so, if the three are in contact, it's the aluminium that'll corrode preferentially.
I can't see any solution to this problem that involves retaining the steel, as you'll never succeed in either (a) keeping the steel completely away from the air so that it doesn't corrode, as rust is so flaky, or (b) keeping the steel from touching the aluminium, which will result in the aluminium being 'sacrificed'.

[habanerocat]

Or get rid of all the steel strengthening bars and replace with new aluminium, eg https://www.aluminiumwarehouse.co.uk/1- ... m-flat-bar

They are actually angle iron, which is visible if you look closely at two of the four sections (they are reverse views). But I get your point. Unfortunately replacing these would be far to big a job. I'd need a tool room.

[Tony R]

Or get rid of all the steel strengthening bars and replace with new aluminium, eg https://www.aluminiumwarehouse.co.uk/1- ... m-flat-bar

They are actually angle iron, which is visible if you look closely at two of the four sections (they are reverse views). But I get your point. Unfortunately replacing these would be far to big a job. I'd need a tool room.

I see, then buy some angle Al and pay someone to cut to size and drill them. It's the only way you will get a satisfactory finish that will last for life: eg https://www.aluminiumwarehouse.co.uk/al ... ess/1/4-in
Regardless, good luck with an admirable restoration project.

[BryanW]

Zinc-based solutions are no good: aluminium is more reactive than zinc (and iron) so, if the three are in contact, it's the aluminium that'll corrode preferentially.
I can't see any solution to this problem that involves retaining the steel, as you'll never succeed in either (a) keeping the steel completely away from the air so that it doesn't corrode, as rust is so flaky, or (b) keeping the steel from touching the aluminium, which will result in the aluminium being 'sacrificed'.

I always thought zinc was used as the third metal *the sacrificial anode' in such cases but as you say keeping the steel isolated from the aluminium (in this case) is extremely difficult if not impossible. I know some of the zinc coatings/paints are expoxies and do the job well but to protect something that is exposed to the elements 24/7 is the grey area for me. I do know that if salt water is involved then you have a battery.

[Jim_Mercer]

I always thought zinc was used as the third metal *the sacrificial anode' in such cases but as you say keeping the steel isolated from the aluminium (in this case) is extremely difficult if not impossible. I know some of the zinc coatings/paints are expoxies and do the job well but to protect something that is exposed to the elements 24/7 is the grey area for me. I do know that if salt water is involved then you have a battery.

Yes order is Steel - Aluminium - Zinc but it doen't need to be salt water although that works quicker than rain water for galvanic corrosion. What makes things worse for Iron/Steel is a wet/dry cycle so permanently submerged or completely dry is less likely to corrode than areas subject to condensation like most of us get in a greenhouse. Out of interest I have done a bit of googling on currently avsilable paints to see if anything new (ie since I was last involved in things like this) would be suitable but it seems they still require a clean surface for best results so lots of shot blasting or wire brushing required which might be easier than trying to remove the steel angles to either replace by aluminium or insert insulation between steel and aluminium.