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Aluminum horse trailer corrosion protection
Q. Have a aluminum horse trailer with floor stains that horse urine has discolored & believe has eaten a small hole thru metal, also the painted exterior has peeled in several spots. Would like to repaint in needed areas & protect floor for the future. HELP. Thanks.
John Leebuyer - Pasadena, Texas
September 1, 2008
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Q. Thank you in advance for your advice. I want to protect the floor of my all aluminum horse trailer. The floor is subject to urine, and friction from shod horses. The usual method is to cover the floor bids three-quarter inch rubber mats. That the urine runs down and pools underneath the mat causing corrosion. My horse urinates every single time he steps into the trailer. The mats are heavy and awkward to clean and get dry on both sides and rinsing the trailer after each use. Is there a sealant that will flex under the weight of the horses without cracking? I am considering removing the mats and replacing them with chipped rubber pebbles. This can be shoveled out and rinsed when soiled. But I still need to protect the aluminum underneath. My other option would be to spray in a hard surface such as a rhino liner. Do you have any suggestions?
Jean LukensEquine - Leverett, Massachusetts USA
May 19, 2014
A. Hello Jean. From the way you have described it, this sounds to me like an ideal application for a rhino liner type coating.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
May 2014
A. Black Rubber barn matting is what is causing the corrosion on the aluminum trailer floors. I have seen this many times.
What is happening is Galvanic corrosion cause by dissimilar metals
Graphite (LEAD) is added to the rubber to make it black. Black rubber will eat up aluminum in short order unless it is isolated. With multiple coats of epoxy primer.
I have seen this happen many times on commercial aluminum boats with black D-rubber bolted to exterior of the vessels.
The aluminum behind the Black D-Rubber will eat out in no time. We use GREY D-Rubber and there is no graphite to eat the aluminum.
Daigle Welding & Marine LTD - Campbell River, BC, Canada
June 24, 2016
Q. Hi Steve, I don't know if the dissimilar metals is a factor on our horse trailer. We have an aluminum deck that has been coated with a rubberized bedliner like Rhino. On top of that was laid the thick black rubber mats. The trailer is about 14 years old and after acquiring, I removed the thick rubber mats to find the Rhino lining had separated from the aluminum. I peeled it up to find corrosion on the aluminum in some areas and spotless aluminum in others.
Since the black rubber mats were not in contact with the aluminum and galvanic reaction requires an electrical path which was not provided due to the Rhino, I suspect that the oxidation is a result of two things; urine leaking through and propagating along the interface between the Rhino and the aluminum and from water leaking up from the bottom through the joints and rivets.
I believe the Rhino linings may be helpful but I am inclined to remove the oxidation then apply some other type of coating such as epoxy. Since the rubber mats will protect the coating from abrasion and the epoxy is providing electronic isolation (relatively speaking), I think that things will be better.
Thoughts?
erex engineering - Camden Point
February 18, 2017
A. Hello Ken
Very interesting question regarding Rhino Bed liner as a insulator between the rubber and the aluminum floor and why you still have corrosion.
I have been spraying Polyurethane insulation/floatation in my Eaglecraft aluminum boats for 32 years. About 25 years ago I discovered that the polyurethane was having a chemical reaction under the waterline where it is exposed to moisture with the aluminum; 1/4" aluminum bottom plate was being being eaten away and in some spots right through.
At the time I knew a corrosion expert with the Canadian Navy.
We discussed this in great length and after research it was concluded that if I were to continue to apply polyurethane in the boats I would need to insulate the bare aluminum from the hull.
We have since been using a thick BARRIER coat of Epoxy paint to the aluminum prior to spraying the Polyurethane.
Since we started doing this we have not had any more problems with corrosion.
I googled Rhino Liner and found that it is a Polyurethane product.
Daigle Welding & Marine, EagleCraft Boats - Campbell River BC Canada
A. Just clarifying a statement above re graphite. Graphite, although the material used in pencil leads, is not actually lead (the metal element Pb); graphite is a form of carbon and is more akin to coal.
A cousin to graphite, carbon black, is the usual material added to rubber to make it black. It is not lead either.
As far as I know, the difference in the anodic index between carbon and aluminum is not high enough to be a problem (and I question whether there's enough carbon of the right type in rubber to be an issue anyhow …)
I wonder whether the issue with oxidation of aluminum against rubber has more to do with the water trapped by the rubber against the aluminum, than with any reaction between the two materials?
- Nanaimo, BC, Canada
May 20, 2018
Hi S., thanks for the input. Yes, corrosion can be a complex phenomena where different things happen not only because of trapped water but also because of anaerobic conditions, such that Steve's observation about corrosion under black rubber specifically might be accurate despite aluminum and carbon being 'close' on the "seawater series".
Although you understand it, something that further complicates things for other readers is that people tend to think of aluminum as corrosion resistant because of the everyday performance they see from it. But actually, aluminum is a very active metal which happens to benefit from forming a corrosion resistant oxide surface under everyday conditions. When those conditions do not exist it might corrode rapidly.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
May 2018
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