Rate of rust

[jenevers]

Is there a generally accepted, rule of thumb, rate of how much steel rusts annually (eg 0.1mm/year)?

[Tonka]

Mild steel rusts at 7 thou of an inch when untreated in salt water.

[Alan de Enfield]

1 minute ago, Tonka said:

Mild steel rusts at 7 thou of an inch when untreated in salt water.

Per day ?

[jenevers]

7 minutes ago, Alan de Enfield said:

Per day ?

Hope not?

[Tonka]

per year. I was told this by a metallurgist. Not sure what the value of sea water was. So a boat is painted which slows this down

[Momac]

Paragraph 6.2 of the following suggest some figures , the splash zone being the worst area .  Fresh water corrosion rate about half that in salt water.

https://www.sciencedirect.com/topics/engineering/submerged-zone

 

[Jen-in-Wellies]

However, in a pit the corrosion rate at the bottom is higher than usual, which is why boat surveyors don't like to see pitting in the hull.

Jen

[Captain Pegg]

The actual answer to the question isn’t very helpful since in normal atmospheric conditions or under fresh water the rate of corrosion of mild steel due to rusting is theoretically so slow that it would never be a problem within the life of any canal boat.

 

However that’s not what causes problems for canal boats. We are worried about electrolytic or chemical corrosion or erosion and corrosion in combination and in those respects the question is essentially like asking the length of a piece of string.

 

What’s the purpose of asking?

 

JP

 

 

[Balliol]

........

 and corrosion rates are higher in flowing water, and anyway most inland boats corrode away from the inside.

[Captain Pegg]

11 minutes ago, Balliol said:

........

 and corrosion rates are higher in flowing water, and anyway most inland boats corrode away from the inside.

The former will be a product of the mechanical action of the water eroding the rust layer. Pure steel has an unstable surface and hence rusts quickly initially. Once that happens the rust forms a partial barrier to further oxidisation which if left undisturbed is fairly effective. However rust is fairly easily dislodged so mechanical action such as flowing water or tidal effects will disturb it and expose fresh steel that will quickly rust again. Hence a cycle resulting in more aggressive corrosion can be created. Wetting and drying through rainfall isn’t really a problem but put a constant large drip onto the same spot on an untreated steel plate and you will see the effect.

 

The second point is definitely true. I was comparing notes a couple of weeks ago with one of my fellow moorers regarding the amount of rust we have each removed from the top surface of the original baseplates on our 45 and 51 year old boats.

 

JP

 

 

[jenevers]

44 minutes ago, Balliol said:

anyway most inland boats corrode away from the inside.

Why’s that?

[DandV]

My experience with buried 8mm and 10mm thick  underground mild steel tanks coated in blacking, Shell flintcote, was that their life was 15years plus ot minus 14 years.

The shortest life was 10months when the entire 20mm overhang  shell plate extension over the end plates was missing for 1/4 of the tank circumference both ends of the tank. Other tanks removed after 20 plus years underground were as new. In marine structures the high rate corrosion  zone is the spray zone,  and in all steel structures stray electrical currents either from afar or local electrolytic corrosion.

An investment  in the supply, installation/ application and importantly maintenance of  quality galvanic isolation, cathodic protection and coating system  will considerably enhance the life of steel.

 

[WotEver]

35 minutes ago, jenevers said:

Why’s that?

Cos it’s damp and warm and generally ignored. 

[David Mack]

8 minutes ago, WotEver said:

Cos it’s damp and warm and generally ignored. 

 

And after the boat is fitted out it never again gets painted.

[Laurie Booth]

4 hours ago, Tonka said:

Mild steel rusts at 7 thou of an inch when untreated in salt water.

Per hour, wow

[MtB]

On 16/03/2020 at 18:27, jenevers said:

Is there a generally accepted, rule of thumb, rate of how much steel rusts annually (eg 0.1mm/year)?

No.

 

hope that helps....

 

 

[blackrose]

On 16/03/2020 at 20:31, Captain Pegg said:

The actual answer to the question isn’t very helpful since in normal atmospheric conditions or under fresh water the rate of corrosion of mild steel due to rusting is theoretically so slow that it would never be a problem within the life of any canal boat.

 

 

 

 

Not sure about that as a statement? Whose theory is that?

 

It might depend on how far under the water we are talking? Unpainted mild steel will certainly corrode around the waterline and just under the waterline at a rate fast enough to be of concern, which is why we stick anodes on and repaint them regularly. That's the difference between theory and reality I suppose

Edited March 18, 2020 by blackrose

[Balliol]

Simple oxidation corrosion, as opposed to galvanic or electrolytic corrosion, requires oxygen. There is less available where steel is immersed in water, excepting that weed growth will hold some oxygen. On the interior surfaces general dampness, condensation etc. will be fed by a more generous supply of oxygen, hence oxidation.

 

The other point of course is that people do generally paint their hulls externally, but not internally. How many very smart boats have rusty gas lockers? Many shells are painted with a quick flash of cheap red oxide primer only, then fitted out, and that is how they remain for decades.

 

Anybody versed in restoring old boats such as Large Woolwiches will know that most of the classic failure points are the result of internal corrosion, not external corrosion or abrasion.

 

 

[Captain Pegg]

26 minutes ago, blackrose said:

 

Not sure about that as a statement? Whose theory is that?

 

It might depend on how far under the water we are talking? Unpainted mild steel will certainly corrode around the waterline and just under the waterline at a rate fast enough to be of concern, which is why we stick anodes on and repaint them regularly. That's the difference between theory and reality I suppose

 

15 minutes ago, Balliol said:

Simple oxidation corrosion, as opposed to galvanic or electrolytic corrosion, requires oxygen. There is less available where steel is immersed in water, excepting that weed growth will hold some oxygen. On the interior surfaces general dampness, condensation etc. will be fed by a more generous supply of oxygen, hence oxidation.

 

The other point of course is that people do generally paint their hulls externally, but not internally. How many very smart boats have rusty gas lockers? Many shells are painted with a quick flash of cheap red oxide primer only, then fitted out, and that is how they remain for decades.

 

Anybody versed in restoring old boats such as Large Woolwiches will know that most of the classic failure points are the result of internal corrosion, not external corrosion or abrasion.

 

 

Thank you @Balliol for answering the question.

 

The whole point of my response to the OP was that while there is a technically correct answer to the question posed the reality is that it’s pretty much meaningless.

 

Without catalysts or erosion to accelerate corrosion the normal rusting process of steel is very slow. I know this from a quarter of a century of inspecting and assuring untreated steel products in a safety critical environment. In water there are many more issues to complicate the process so the divergence between theory and practice is likely to be greater despite the lack of oxygen. Plus a canal boat hull is subject to quite a lot of mechanical action at and below the water line given the frequency with with it comes into contact with the bank or the bottom.

 

Water that gets onto the top surface of the baseplate is very likely to be contaminated with chemicals that act as catalysts. In the case of working boats the obvious one is coal dust but even on a modern boat leaking waste water with detergents or cleaning product residue can be damaging.

 

JP

[Scholar Gypsy]

When I looked at a particularly inaccessible part of the bilges a) I wished I hadn't b) I weighed the rust from a given area and converted that into mm. 

 

If I got my sums right, 1.2grams of rust per square cm worked out at 1mm of steel. Not bad for 40 years....

 

 

[blackrose]

On 16/03/2020 at 20:57, Balliol said:

........

 and corrosion rates are higher in flowing water, and anyway most inland boats corrode away from the inside.

Because most are wet inside. If you make sure they stay dry inside they don't corrode. Simples.

[blackrose]

58 minutes ago, Balliol said:

Simple oxidation corrosion, as opposed to galvanic or electrolytic corrosion, requires oxygen. There is less available where steel is immersed in water, excepting that weed growth will hold some oxygen. 

 

 

 

Yes but this seems to ignore the fact that the hull of any steel boat that's immersed in water will at some point intersect with the waterline and at that point (and just below) there is plently of oxygen for corrosion to occur at a faster rate.

24 minutes ago, Captain Pegg said:

 

Thank you @Balliol for answering the question.

 

The whole point of my response to the OP was that while there is a technically correct answer to the question posed the reality is that it’s pretty much meaningless.

 

Without catalysts or erosion to accelerate corrosion the normal rusting process of steel is very slow. I know this from a quarter of a century of inspecting and assuring untreated steel products in a safety critical environment. In water there are many more issues to complicate the process so the divergence between theory and practice is likely to be greater despite the lack of oxygen. 

 

But as I've said there's always oxygen and in practice steel hulls do corrode rather quicker than your posts seem to suggest.

 

Anyway, I won't bother to argue with you. Despite what you've seen in your quarter of a century of inspecting and assuring untreated steel products in a safety critical environment, we all know how many unmaintained rusty hulls sitting in the water that we've seen too.

Edited March 18, 2020 by blackrose

[Captain Pegg]

9 minutes ago, blackrose said:

 

Yes but this seems to ignore the fact that the hull of any steel boat that's immersed in water will at some point intersect with the waterline and at that point (and just below) there is plently of oxygen for corrosion to occur at a faster rate.

 

But as I've said there's always oxygen and in practice steel hulls do corrode rather quicker than your posts seem to suggest.

 

Anyway, I won't bother to argue with you. Despite what you've seen in your quarter of a century of inspecting and assuring untreated steel products in a safety critical environment, we all know how many unmaintained rusty hulls sitting in the water that we've seen too.

Of course it ignores the fact. Why do you think I said at the outset that a direct answer to the question was meaningless? That answer can only be given when considering the base case of pure rusting without the compounding factors as I believe I have explained more than once now.

 

There is always going to be some oxygen present in the water around a boat but that isn’t the critical thing. It’s the catalysts and other actors that are present in water that are key to the rate of corrosion rather than the amount of oxygen. Without them there wouldn’t be a problem, that’s evidenced by what happens above the water line where there is more oxygen and therefore in theory there should be more corrosion, but that’s not generally what happens.

 

JP