Odd corrosion pattern on 531 frame
 

I put these images in the C&V appraisals forum, but I'm not sure it was noticed. I looked at a Trek 720 today and saw some spots of paint damage and minimal rust, along with these worm-like tunneling shapes in the paint. In the photo below you see one in the middle of the photo and another on the right-hand edge of the photo, both on the chain stay. There was a similar corrosion pattern under the bottom bracket.
https://c4.staticflickr.com/6/5770/3...caa164b1_z.jpg

It looks like a thick thread was on the frame when it was painted. How worrisome is this? What would I likely see if I sanded the paint off?

Yes, seen this before. Most often on powder coat jobs, but wet paint too.
After chemical stripping I was amazed the full extent was quite a bit farther that visible from the outside.
Happy I went ahead with refinishing on those bikes, will take a while to become a big problem though, just did not like looking at it.

If you're worried, easiest thing is to sand off the paint in that area, try to minimize the area. Once you confirm that it's purely cosmetic, and nto a frame issue, get some Testor's paint to match the color and touch it back up.

What's the cause of the problem? Looking at it in the pictures leads me to suspect some sort of contamination on the metal when the paint was applied, but I'm unsure.

"RUST SPIDERS"

There was an extensive discussion recently in the Classic Rendezvous group about this subject.

They have nothing to do with powder coating. I've never seen them on any frames from the 1970's, just 1980's and later. Could be due to a change in the paint formulas.

Rust spiders have an organic appearance.

I suspect is there's a possible organic cause for rust spiders, here's why:

There are a lot of microbes that use metals, all kinds of metals! For example, microbes are used to extract copper from low grade ore.

A microbiologist friend and his research team discovered a bacteria that can exist in highly radioactive uranium!

Rust is iron oxide and results when iron and oxygen mix usually from exposure to air or water. A micro-pore in the paint could be enough to get things started.

"Iron Bacteria are small living organisms that naturally occur in soil and water. These nuisance bacteria combine iron and manganese with oxygen to form deposits of “rust”. The most common bacteria known to feed on iron are Thiobacillus ferrooxidans and Leptospirillum ferrooxidans."

Almost all steels contain a small amount manganese as an alloy with iron.

To quote Madonna: "It could happen"

The bottom line is that rust spiders only affect the layer of steel under the paint. Scraping the lifted paint off and using a wire brush to remove the rust followed by some touch-up paint will protect the surface for a while.

verktyg :50:

Chas.

Been reading alotta Analog magazine? Too much Larry Niven? How about helium based life forms vs carbon based? Or intergalactic travel via gravity waves? I'm game ;) I love this stuff!

Stranger than fiction! ;)

verktyg :50:

Chas.

It's rust.
Try to stop it.
After you find a suitable touch up color.

science fiction notwithstanding, it's called filiform corrosion
It's how corrosion progresses under organic coatings and has has nothing to do with microbial-influenced corrosion.
I''m a licensed, professional metallurgist and corrosion engineer.

With powder coat, there is basically just an envelopment of the steel, it does not bond exceptionally well. There are micro bubbles often, those break, bringing an entry point for rust.
Trek's were advertised for a long while as using Imron. My guess the steel was slightly oxidized just prior to paint and the etching primer or seal coat (often with an etching component to help anchor the paint) did not take for some reason. There appears that there may have been an initial nick to the steel, allowing a point of entry to get between the coating and the steel.

My take in general, avoid powdercoat. I have stripped four frames now, all exhibited "spider webbing" of rust below, and dots of rust elsewhere. Perhaps a multistage powder job could help avoid this, but that will by its nature get thicker in appearance than I would want on a lugged steel bike. Maybe on a fillet brazed or tig welded frame.

Fantastic info. Not as sexy a title as rust spiders . . .

Chas, thanks for the info about this being seen in 1980s bikes, which of course fits in this case. Overall it sounds like it's just more cosmetic damage (after repair) to an already cosmetically challenged bike.

I've seen enough Treks from the 80s with disappointing corrosion problems to believe that whatever they were using then (Imron?) didn't work very well sometimes. A lot of paint durability/corrosion problems have to do with prep, not the top coat(s).

Painted aluminum is the worst, and with the environmental/health issues eliminating the use of former chromate primers, it's even tougher.
Very much explains the Trek observations, since 1980 is when chromate disappeared.

The aircraft industry is smartest about defeating it using modern primers.
I once worked on the American Orient Express with it's 70-y-o aluminum coachwork when it was parked at the local train station. and wrote a patch repair procedure.

Picked up this Burley a few years back:
[IMG]https://c7.staticflickr.com/4/3682/1...6a18d276_k.jpgP1010017, on Flickr[/IMG]

I don't believe the bike was ridden much as the bearings and anything oiled/greased was in great shape but it was stored at a summer cottage IIRC. I didn't like the look of it so stripped it down:
[IMG]https://c3.staticflickr.com/8/7359/1...82607243_k.jpgP1010140, on Flickr[/IMG]

Every tube all around the circumference had this condition. Click on the link for more TMI pics.

Thanks for the information and the link.

Is that how rust gets under chrome plating and causes it to lift?

Having worked with all types of metals and metal processing for well over 50 years I hadn't seen "rust spiders" under paint until I started collecting bikes built in the 1980's. None of my bikes from the 1960's or 70's have this kind of corrosion.

verktyg :50:

Chas.

I found one on my bike last week on the left of the top tube, near the cable guide. Didnt see any others in the really quick look i took, though i only really checked around the head tube/top tube/down tube area. I couldn't think of what could've caused it seeing as there wasn't any damage in the area, aside from a tiny bit of rust near the cable guide. Looks like the lines on the Burley.

My '73 Raleigh Competition had rust spiders that didn't show until I stripped the paint. I thought they were cracks at first.

Just a nasty case of "tin worm"

A significant chunk of the bikes I find out here have this spidering under the paint.

I read somewhere that an oxalic acid bath (bare frame, of course) can stop the expansion of these spidering rust patterns without needing to remove paint. This is assuming that there is an entry point that the bath can enter and "chase" the spidering through.

These creatures are seen most often on the top tube, presumably because of sweat dripping.

I see them on 70s bikes. There's a couple small spiders (rust mites??) on my '78 Masi.