by: john walters, harrison lam engineering 45, materials...

By: John Walters, Harrison Lam Engineering 45, Materials Science December 9, 2013

SRJC

Through questionable means we obtained several rivets believed to have originally been part of the Golden Gate Bridge between January 5, 1933-May 28, 1937 all the way till the 1970’s.

http://goldengatebridge.org/research/facts.php#CorrodedRivets

In order to verify its authenticity we decided to examine the microstructure of the rivet in question.

Unfortunately due to the high degree of oxidation the determination of its microstructure from its surface was somewhat improbable.

Therefore we resorted to more intricate methods.

Hacksaws and table clamps!

To examine the microstructure we first sawed off the end of the rivet and deburred the edges.

Then we put it into a mold, filled it with a plastic mold powder and heat treated it until the powder solidified.

Next we sanded it with 100, 320, 400, and 1000 grit sandpaper rotating it 90 degrees for every 15 passes.

Polished it with a 10 micron solution on a buffering wheel, then etched it with Ferric Chloride.

25x magnification 100x magnification

250x magnification 525x magnification 950x magnification

Can you find the carbon? Neither could we. Dots are particulates of plastic molding, not graphite flakes.

25x magnification 100x magnification

250x magnification 525x magnification 950x magnification

Can you find the carbon? Neither could we. Dots are particulates of plastic molding, not graphite flakes. So what is it exactly?

Our sample: 100x Bainite ATSM-A36 2000x

Fine ferrite grains of aluminum deoxidized low carbon steel 50x

Dorman, Long & Co. Silicon Steel, 100X Hot Rolled

Our sample: 100x Bainite ATSM-A36 2000x

Fine ferrite grains of aluminum deoxidized low carbon steel 50x

Dorman, Long & Co. Silicon Steel, 100X Hot Rolled

We couldn’t tell

Close enough

Through careful analysis of the specimen (and because Younes said so) we concluded that our sample had very low carbon content, it’s essentially just a cheap hunk of iron.

We were unable to find the grade of steel they used in the initial 1933-1937 construction, so we can’t ‘prove’ our rivet was in the Golden Gate Bridge, but come on, it’s red, rusted, and cheap to make, what more proof do you need? Also the original rivets had diameters of 1” shaft and 1.5” at the head, ours measured 1” on the shaft and about 1.5” on the head. http://en.wikipedia.org/wiki/File:Golden_Gate_Bri

dge_Rivet_1937.jpg

Due to friction the fragment I sawed off ended up too hot to handle, I got a blister The first mold wasn’t cooled slowly enough, the plastic was brittle and contaminated the sample. The second molding wasn’t heated completely, the plastic smudged and also contaminated the sample, but marginally less so.

We used the right polishing solution on the wrong polishing wheel

The sample wouldn’t etch properly, we had to re-etch it several times (Rather Younes did-Thank you Younes!)

The listed magnification of the microscope was not to be trusted, we had to do it manually

Here’s how they used rivets in the old days, it explains why our specimen’s grain structure was so warped!

(As if you didn’t notice)

http://www.youtube.com/watch?v=CVjS1DsqYvo

In the 1970’s due to corrosion the golden gate rivets were replaced with ASTM A-325 high-strength bolts and ASTM A-325 galvanized high-strength bolts. The last rivet to be put in the Golden Gate Bridge during its construction was made of gold.

http://www.archinomy.com/case-studies/1908/golden-gate-bridge-san-francisco