How to Repair Corrosion on Aluminum Convectors

Many of us have made repairs to the aluminum convector in the Alde heating system. Since this is an ongoing issue facing many owners, I thought I would summarize what I “think” I know about the available methods to deal with the issue other than just buying new components. Hopefully this will help others starting out with a concise description of the options available to them.

Please if anything I write is wrong, don’t hesitate to call me out.  Everything I am writing is true to the best of my current knowledge but if I am wrong on anything, I would like to know.

To mitigate this type of corrosion beneath the EPDM rubber connections, providing a barrier seems to be the best practice so far.  Some have applied this barrier to the cleaned up corroded ends if the end was considered serviceable or where the end was not considered serviceable, a few fins are removed exposing un-corroded aluminum and the corroded end cut off. 

With a fresh un-corroded end, the convector could be reinstalled with or without a protective barrier. 

The two types of barriers I’m familiar with are heat shrink sleeves which apparently have some sort of adhesive underneath or a type of coating that bonds to the aluminum.

 In either case, the aluminum should be properly cleaned to remove any residual corrosion or in the case of a cut back end to be coated (not a heat shrink sleeve), cleaned to prepare the aluminum for proper bonding of the coating.  This can best be performed using sandblasting (or glass beading) or with chemicals like phosphoric acid.  Phosphoric acid is used to etch new metals prior to the application of paint primers and in other cases to remove corrosion from metals prior to painting.  It can be found in many familiar products like Naval Jelly, etc.  It is nothing like other acids that will attack the base metal and destroy the part while removing corrosion too.  There may be other chemical methods that I am not aware of also.

 Using an abrasive to clean a corroded convector end that is considered serviceable likely would not get corrosion in microscopic pits which may or may not cause a problem depending on what it is covered with.  There are many products used in auto restoration for encapsulating corrosion, usually rust on steel since most autos are made with steel but I believe removing all the corrosion is a best practice. 

Whatever method is used, if applying a bonded type of coating, the aluminum should be coated as soon as the cleaning process is complete before the aluminum surface oxidizes which it does within an hour or so if I remember correctly.   There are primers specifically made to pre-etch aluminum before applying a primer coating that deals with this oxidized surface which would otherwise negatively affect the coating bond.  This is the normal practice in the marine industry working on aluminum yachts where a large area is prepared before applying a 2 part epoxy primer.  Since the time between preparing the large surface is too long before coating, the pre-etch coating is applied first to deal with the aluminum oxidation then the two part epoxy primer is applied.  Typically 2 coats of epoxy primer are used to achieve a 15 mil coating epoxy primer dry film thickness to provide a barrier coating.

I used a 2 part epoxy primer an applied two coats soon after glass beading the area to be coated. 

I saw where others used a 2 part Defcon Product or maybe it was a 2 part JB Weld product, not sure.  These other products likely will work just a good but application would be slightly more difficult since Defcon is thicker, like a putty and JB Weld is somewhere in between from my experience with them.  The least expensive I believe would be the JB Weld then the Devcon followed by the epoxy primer.  Note single part epoxy is not the same as a 2 part epoxy.  For a small area like dealing with convector ends, if you wanted to use a 2 part epoxy, there are special aerosol cans that mix the 2 part epoxy when sprayed which can be purchased from The Eastwood and other companies catering to the auto restoration market.  Also POR is another product which is a single part paint used to encapsulate corrosion used widely on auto restoration.  Rust-O-lum makes a self etching primer in aerosol cans I have used extensively on steel and aluminum that may perform well also.  Whatever product used remember, preparation is the key to success because the bond in most impacted by the quality of the preparation.

The aluminum connectors and the check valve are usually just as corroded as the convectors (check valve shown below):

There are two solutions:

1.       Buy the new plastic connectors but if any of the EPDM rubber hose ends are damaged from the corrosion and bulging, new hoses would need to be purchased also since cutting off the damaged end may cause the connector to be too short

2.       Use marine stainless steel 7/8” railing to cut new connectors to any length needed to account for the reduced hose length after cutting off the damage ends.

In fact, I managed to salvage the plastic check valve from the corroded one shown above and installed it in a piece of the same stainless steel used for the connectors.

The only issue is crimping the downstream end slightly to keep the plastic valve from being pushed out of the tube or making a circular indentation like in the original check valve.  I actually did the later using the tubing cut off tool by replacing the cutter with a steel wheel (required some lathe work – not for the average person).