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Rust - how much of my hull has disappeared?

Scholar Gypsy
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Scholar Gypsy Veteran

Scholar Gypsy

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Posted (edited)

I thought the following calculation (assuming it is correct - please let me know if you spot any errors) might be of wider interest.

 

When I was replacing the rotten floorboards under the water pump at the weekend (photos here), I was rootling around in the bilges (not exposed since that bit of the boat was built in 1982) and picked up some substantial sheets of rust, which came away in my hand. This was a very localised problem - nothing similar on the other side of the boat. The rubbish bag seemed rather heavy as I was walking to the rubbish tip...

 

Anyway, I took a small sample home.

 

It measured approx 18 x 14 cm

 

After drying overnight in a jam jar on a radiator, it weighed 11 oz on the kitchen scales (My wife is a research scientist but for some reason did not want to take the sample into her lab). As 1 oz = 28.3 grams (I will round all workings quite heavily, all data was sourced from Wikipedia) this is about 312 grams.

 

So the mass of rust per cm2 is about 1.2g = 312/(18x14)

 

Now rust is Fe2O3 and the molecular weights of Iron and Oxygen are 56 and 16, so iron makes up 70% of the mass of the rust = 112/(112+48)

 

So the mass of iron in the rust per sq cm is 70% x 1.2 = 0.8 g

 

The density of iron is about 8 g/cm3

 

So therefore about 0.1cm = 1mm of steel has turned into rust. (0.8/8)

 

The bottom plate was originally 1/4 inch, or 6mm plus, so I don't need to panic.

 

This also chimes with the 35 year survey done 12 months ago, which said:

 

"The base plate was originally fabricated using 6.0mm plate

(nominal).
Very few ultrasonic measurements could be taken and
were restricted to the outer plate edges due to the close
proximity of the dock floor.
Ultrasonic measurements show the base plate to be
between 6.0mm to 5.4mm.
Pitting is present at up to 0.8mm."

 

 

PS It was interesting to note that the pitting on the uxter plate was where a bucket has lived for the last 20 years. No longer!

Edited by Scholar Gypsy
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Scholar Gypsy Veteran

Scholar Gypsy

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Drying the rust sheet on a radiator overnight wouldn't have removed all the water within it, so the extent of corrosion is probably less than you calculated.

 

Thanks - I should have said that the weight reduced by about 10% over 24 hours. I will try again in a few days...

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Scholar Gypsy Veteran

Scholar Gypsy

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Are there any holes in your Iron?

 

Have you hit it with a hammer?

 

 

Doinggg is good. Dub is not...

 

No holes yet - though I have left the weedhatch off for a couple of weeks so the bilge paint can dry properly.

 

No pitting on the inside of the hull, that I could see (apart from the stuff on the uxter plate, mentioned above, due to a damp bucket being in contact). Does your hammer test work only when the boat is out of the water?

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Neil Smith Experienced

Neil Smith

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I read that you can also calculate metal loss by measuring the thickness of the piece of rust with a micrometer or similar, can't remember the calcs but something like 4 mm of rust equates to 1 mm of steel, don't hold me to this figure though as was a long time ago.

 

Neil

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Alan de Enfield Veteran II

Alan de Enfield

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So the mass of rust per cm2 is about 1.2g = 312/(18x14)

 

I am thinking there may be an error in the calculations as there does not seem to be any allowance of rust 'thickness'

 

Surely a piece of rust 18 x 14 that is 1.0mm thick will have a different weight to a piece 18 x 14 x 0.5mm thick.

 

Your conclusions are almost correct so maybe you have allowed for thickness somewhere in the calculations and I cannot see it.

 

The 'official' figures I have found on steelmakers websites vary depending on steel type but are generally in the "rust will be 7 or 8 times thicker than the steel loss"

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Scholar Gypsy Veteran

Scholar Gypsy

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So the mass of rust per cm2 is about 1.2g = 312/(18x14)

 

I am thinking there may be an error in the calculations as there does not seem to be any allowance of rust 'thickness'

 

Surely a piece of rust 18 x 14 that is 1.0mm thick will have a different weight to a piece 18 x 14 x 0.5mm thick.

 

Your conclusions are almost correct so maybe you have allowed for thickness somewhere in the calculations and I cannot see it.

 

The 'official' figures I have found on steelmakers websites vary depending on steel type but are generally in the "rust will be 7 or 8 times thicker than the steel loss"

 

As David Mack says in his post, I didn't attempt to measure the thickness of the rust.

 

I measured the mass of rust per unit area, and then converted that to mass of iron per unit area, and then converted (ignoring the carbon content in the steel) that to a thickness of solid steel. I am implicitly assuming that the flaky stuff was fully oxidised.

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Aunty_Rinum Contributor

Aunty_Rinum

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Can you provide a link or source for that figure ?

 

Most of the steel companies claim 7 or 8.

I read this recently; it was based on a report by the Canadian Government and I can't find a link now. I have to admit I was surprised and am happy to be corrected if it's wrong.
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Alan de Enfield Veteran II

Alan de Enfield

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I read this recently; it was based on a report by the Canadian Government and I can't find a link now. I have to admit I was surprised and am happy to be corrected if it's wrong.

 

No personal knowledge, just a google search :

 

http://www.steelconstruction.info/Corrosion_of_structural_steel

 

and an old thread on the same subject

 

http://www.canalworld.net/forums/index.php?showtopic=68459

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