How to Repair Corrosion on Aluminum Convectors

grassgd

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).

Some of this information can be found spread through the various forum topics but I hope this can be a single place to find information on this specific issue without having to read all the threads. If anyone else has specific information on this topic, chime in too.

KK1L

What I find most interesting about the detailed photos is how the corrosion is all on the exterior of the aluminum tubing. I would bet this has been discussed already somewhere. Why is this?

It is a night and day difference on the interior compared to the area trapped under the rubber hose. So which of these is the reason?
1) A reaction between the rubber, the aluminum, and the transfer fluid?
2) The trapped transfer fluid loses its anti-corrosion properties (changes alkalinity) over time and corrosion occurs at an accelerated rate? There is no exchange of fresh fluid in this area. It is stagnant. In the rest of the system there is a large volume of fluid which exchanges at least periodically.
3) Something else??

Is there some industry analysis of this effect? I would be surprised if there was not.

EDIT: It did not take too much searching to find this.
https://www.boydcorp.com/blog/avoiding-galvanic-corrosion.html

"In general, corrosion can be reduced through pH control and corrosion inhibitor use. The inhibitors attach to metal surfaces to passivate them and prevent corrosion. It is also important to maintain a stable water flow to avoid stagnant zones inside the cooling system, which can cause corrosion."

This sounds like it supports protecting the surface under the hose connection from exposure to transfer fluid, and changing the fluid occasionally.

grassgd

KK1L said:

What I find most interesting about the detailed photos is how the corrosion is all on the exterior of the aluminum tubing. I would bet this has been discussed already somewhere. Why is this?

It is a night and day difference on the interior compared to the area trapped under the rubber hose. So which of these is the reason?
1) A reaction between the rubber, the aluminum, and the transfer fluid?
2) The trapped transfer fluid loses its anti-corrosion properties (changes alkalinity) over time and corrosion occurs at an accelerated rate? There is no exchange of fresh fluid in this area. It is stagnant. In the rest of the system there is a large volume of fluid which exchanges at least periodically.
3) Something else??

Is there some industry analysis of this effect? I would be surprised if there was not.

EDIT: It did not take too much searching to find this.
https://www.boydcorp.com/blog/avoiding-galvanic-corrosion.html

"In general, corrosion can be reduced through pH control and corrosion inhibitor use. The inhibitors attach to metal surfaces to passivate them and prevent corrosion. It is also important to maintain a stable water flow to avoid stagnant zones inside the cooling system, which can cause corrosion."

This sounds like it supports protecting the surface under the hose connection from exposure to transfer fluid, and changing the fluid occasionally.

Why is the corrosion only on the od of the aluminum under the EPDM hoses?

I haven't seen any definitive reason other than educated guesses.  My guess, it is in an area deprived of a continued supply of corrosion inhibitor in the glycol since it is a stagnant crevice that glycol seeps into but once the corrosion inhibitor is consumed, it is unprotected to corrosion.  This same method would suggest the glycol in the craves would become oxygen deprived also.  I read an article that states this area deprived of oxygen next to the oxygen rich glycol can lead to the pitting potential like we are seeing.

So which one is the reason?

I think it is your #2 which is what I described above also, but just a guess but apparently Alde has not given one.

That is a very good article you found and referenced, and your conclusion about coating the ends and glycol changes seems to be the best approach at this time in my mind. 

The glycol change every two year is the recommended practice along with the use of Rhomar glycol but recent information has shown a trailer with the Rhomar glycol change 2 years ago plus all new convectors and connectors had been found with corrosion on the inside of the aluminum pieces in the bottom of the glycol tank plus some pin holes in the new convectors. 

As soon as you think you understand what's going on, something else pops up.