Steel vs. Aluminum: Corrosion in long-distance touring frames

[Danneaux]

Hi All,

Our Thorns are steel and therefore subject to rust if scratched; rust is oxidation, which is one form of corrosion. We all know the importance of promptly touching-up nicks and scratches to prevent rust taking a foothold in the steel, and this is especially important when a steel frame is powder-coated.  Left alone and unchecked, rust can "tunnel" under the powder-coat and wreak havoc on the steel beneath.

But what about aluminum frames?  I came across an interesting account of what appears to be filiform corrosion setting-into a Koga frame, proving severe enough to cause a warranty replacement. See:
Summary:
http://www.powercycle.net/equipment
Replacement saga:
http://www.powercycle.net/2011/01/15/tumbaco-ecu-bike-work
http://www.powercycle.net/2011/01/18/quito-ecu-more-bike-work
http://www.powercycle.net/2011/01/18/tumbaco-ecu-update
http://www.powercycle.net/2011/02/03/tumbaco-ecu-the-fun-never-ends
http://www.powercycle.net/2011/02/10/tumbaco-ecu-its-complicated
http://www.powercycle.net/2011/02/18/tumbaco-edu-update
http://www.powercycle.net/2011/03/03/tumbaco-edu-back-on-the-road-tomorrow <-- Mention of another tourist with a cracked steel frame
What parts failed on his bike; when and how:
http://www.powercycle.net/bicycle-health

Interesting stuff. 

Best,

Dan.

[il padrone]

My son is riding my old Giant Sedona (1993) frame. It had some early scratches that were touched up (about 95/96), but then it had later scratches in general use. Some I never could work out what caused it, like the 4-5 cm rub on the chainstay near the BB. Anyway these later scratches did not get touched up (got a bit blase).

Many of these have been there for over 10 years now. They have formed a very light corrosion  coat, but have not got any worse at all. The surface corrosion is probably stopping any further rust. CroMoly is marvellous stuff. It generally has about 5% chrome in it. Stainless steel has about 11% chrome content, so I wouldn't worry too much about scratches on CroMoly.

[Danneaux]

Pete,

That Giant Sedona was set-up beautifully for touring and it is nice to know it continues to roll on with your son. D'you know it was through a photo of it I first came to "know of" you online?  The photo I saw of it had your blue Ortliebs with the add-on pockets that later appeared on your Tonka Yellow Thorn.

Very good point about the native rust-resistance of CroMoly steel. My Folder-in-progress is still unpainted and in a garage with 70%+ relative humidity and it doesn't even bear surface rust. Because it is a folder, I can look inside the main frame spar and steerer mast and they, too, are clear of corrosion.

I guess what prompted me to think "corrosion" and put up this post was a Saturday afternoon replacing a friend's top-tube cable guides that had rusted through and had started to get a grip in the top tube itself.  Apparently, things were fine until he mounted the bike on a trainer.  A few sessions on the trainer in the warm room, the rider overheating and working up a good sweat and next thing you know, I'm reminded of Neil Young...Rust Never Sleeps.

Best,

Dan.

[triaesthete]

The beautiful feature on Thorn steel frames are the infilled blow holes (drilled in stays and forks to allow welding gases to escape during manufacture). I have never seen another production frame that has had them closed off and of course this leaves the inside of the tubes completely unprotected from water, road salt and humidity!
Not many other frames actually fail as far as I know but I find it very pleasing that Thorn have done the job properly.
Ian

[Relayer]

The beautiful feature on Thorn steel frames are the infilled blow holes (drilled in stays and forks to allow welding gases to escape during manufacture). I have never seen another production frame that has had them closed off and of course this leaves the inside of the tubes completely unprotected from water, road salt and humidity!
Not many other frames actually fail as far as I know but I find it very pleasing that Thorn have done the job properly.
Ian

I have often wondered about this especially when riding through torrential downpours (happens too often on Scotland), and imagine water ingress via the seat tube or maybe even the headset, it would then be trapped inside without any holes ... thoughts?

P.S. I suppose liberal application of grease is the answer?

[Danneaux]

Ian, Jim,

I recall looking for signs of the vent holes as I uncrated and assembled Sherpa, and concluded the rear triangle's holes are internal to the joints, rather than filled (i.e. seat stays vent into seat tube, chainstays vent into BB).  The fork blades vent on their upper-inside faces, just below the fork crown, where it would be difficult for water to enter unless submerged (see: http://www.thorncycles.co.uk/forums/index.php?topic=3697.msg17393#msg17393 ) Once submerged with the air displaced, it's not coming out of the fork blades unless the bike is laid on its side or inverted. Fenders mask the entry, but it seems unlikely water could get around the corner in normal use even without fenders.

I don't recall seeing vent holes in the head tube, and suspect they might be covered by the headset cup collars. I have to admit I was focused on getting the bike together at the time. 

I have heard some Nomad and Raven owners describe a blocking plate in the lower end of the seat tube, above the BB.

I agree, Thorn's frame sealing is a very clever, wonderful, largely unappreciated feature. I always had to plug the holes on my other bikes with kneaded beeswax, so this was a nice bonus.

The Shimano external bottom bracket has a nice sleeved tube connecting the bearing units, and this should help prevent water entry to the frame from the sides.

I do believe leaving the bike upright when parked does wonders to prevent the intrusion of water via the bearings. Before I had my Click-Stand, I always covered the saddle with a dry sack that extended low enough to buckle around the underside of the top tube-seat tube junction; this prevented water entry around the seatpost, even in Belgium's torrential rains.  The crankarm spider shielded the BB to a degree, and grease in the headset helped there, but upright is definitely better to prevent rainwater entry.

I'll soon be removing Sherpa's seatpost so I can fit my custom-made spoke carrier to the inside of the post and I'll take another look for vent holes then.  Meanwhile, I have generously greased my seatpost with Phil Wood grease, and the seatpost-mounted alarm also serves as a shield against the direct entry of water into the seatpost top (it clears the seattube top by a couple millimeters). In the past, I've had great luck fitting a nitrile o-ring to the seatpost, pressing it down against the top of the seatpost collar with just a smidgen of grease between it and the seat tube collar/seat tube top.  It is unobtrusive, nearly invisible and amazingly effective. It also works a treat for helping to "seal" quill handlebar stems and prevent direct entry of water.

Best,

Dan.

[il padrone]

The beautiful feature on Thorn steel frames are the infilled blow holes (drilled in stays and forks to allow welding gases to escape during manufacture). I have never seen another production frame that has had them closed off and of course this leaves the inside of the tubes completely unprotected from water, road salt and humidity!

Sorry, the pedant in me is pressing to get out. You do of course mean "leaving the inside of the tubes completely protected from water, road salt and humidity"

 

[macspud]

Sorry, the pedant in me is pressing to get out. You do of course mean "leaving the inside of the tubes completely protected from water, road salt and humidity"

I read it as "I have never seen another production frame that has had them closed off and of course this leaves the inside of the tubes completely unprotected from water, road salt and humidity!" meaning that all the other production frames that he's seen have been unprotected and open to the vagaries of the conditions.

Though maybe it would have read better as "I have never seen another production frame that has had them closed off and of course this leaves the inside of their tubes completely unprotected from water, road salt and humidity!"

Oh the wonders of the English language

[expr]

On the custom frames made in somerset by Ks at the time he didnt use vent holes, the last stage of the brazing around the joint was the vent and on final cover up this produced the full seal. the only exposed steel to corrosion is the seattube and headtube, this can be wax oiled or similar.

[farmer palmer]

I've always been concerned about rust on my steel frames and over the years i've just spayed some WD40/GT85 etc into the vent holes and down the seast tube etc every now and then. The bikes are now stored in my shed where the central heating boiler is so they're always nice and dry rather than dripping in condensation when the weather changes in a cold and clammy shed.

It's the wet salty spray on our Scottish roads I was always concerned about and when I used to ride a lot in the winter i'd liberally coat the bike with corrosion inhibitor to keep it in tip top condition or everything would start and seize up.

I bought my first aluminium framed MTB a couple of years ago and treated it the same as my steel framed bikes and so far there's no corrosion even though I regularly ride the bike through stream crossings and along the beach......Not good when the tides coming in and the bikes partially submerged in our salty and very cold North Sea........!

FP.

[triaesthete]

Il padrone, read it again.  'tis two sentences. The second applies to frames wot ain't Thorns. +1 Macspud.

Relayer , barring full submersion I think the Thorn system is "more adequate" than everyone elses.

Dan, I think the plastic sleeve on Shimano bottom brackets is primarily to protect the inside faces of the bearings from water rust and grit inside the frame.

Personally I run a Thomson seatpin with rock hard anodising that is unlikely to corrode into a steel frame. I like to install it dry for better clamping as I've had self lowering saddles in the past. To stop corrosion in the tubes I use Scottoiler FS365. I also spray this around the seatpin area after the pin is installed, then seal the joint with insulation tape so water does not run down into the frame.

Years ago a friend had the seat tube on an old steel frame snap just above the bottom bracket. To get home he found someone else with an overlong seatpin, cut the surplus length off and pushed it right down the seat tube to bridge the breakage internally!

Ian