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Author Topic: Protecting iron and steel hulls against rust  (Read 275 times)

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mattmill30

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Protecting iron and steel hulls against rust
« on: December 27, 2018, 11:52:01 AM »

TL;DR: What is the best practical protection against deterioration of a steel hull from rust or electrolysis? - I already have Zinc anodes

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Apologies if this is in the incorrect forum. I am not a chemist, so my presumption that this is a chemical engineering question may be incorrect.

I own a steel inland waterways vessel (Narrowboat) which is my first live-aboard project.

A common maintenance requirement is "Blacking" beneath the waterline with Bitumen to provide a protective layer against the water to prevent rust. This is meant to be performed every 3 years.

I also already have Zinc anodes.

I have been attempting to research an answer to the question: "What is the best practical protection against deterioration of the hull by rust or electrolysis?"

I have attempted to remind myself of my pre-college Chemistry education by researching on Wikipedia the subjects of chemical bonding and electrolysis, and have lead myself to more questions than answers.

I am aware that Zinc galvanisation is a popular solution to protecting Iron - https://www.youtube.com/watch?v=iRrdAM-3nq0 - though this appears to be at odds with Narrowboat forum recommendations - https://www.canalworld.net/forums/index.php?/topic/81076-red-oxide-paint-vs-hull-blacking/&tab=comments#comment-1710101

So far as I see it, there are three options for protecting a metal object from deterioration:

1) Prevent rusting from occuring by applying a coating of an impervious wear resistant compound, such as Red Oxide
2) Prevent rusting from occuring by coating the steel with a conversion compound which doesn't result in rusting and prevents future rusting reactions.
3) Prevent electrolysis by applying a layer which either acts sacrificially or, ideally, neutralises electrolysis.

The questions I have are:
1 - What is the best wear resistant compound for steel, and what's the reason for that?
2 - What would be the ideal rust-resistant steel compound, which chemicals would be best to achieve that conversion, and again what's the reason for those chemicals achieving that result?
3 - Which chemical is best for preventing electrolysis, how thick of a layer would be necessary, what would be a suitable application compound - e.g. oil-based paint, epoxy or another - and how did you reach that conclusion?

Many thanks
« Last Edit: December 27, 2018, 12:07:09 PM by mattmill30 »
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Enthalpy

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Re: Protecting iron and steel hulls against rust
« Reply #1 on: December 28, 2018, 11:13:19 AM »

Welcome, Mattmill30!

Corrosion is very much experimental knowledge, so don't regret any pre-college knowledge, it wouldn't help you. Things like balanced redox equations bring nearly nothing here.

To protect a boat, you need methods applicable at the proper scale and cost. Chromium or nickel layers would be perfect on small parts in mechanical engineering, but little practical on a boat. I guess the workable methods are already known and you cited some.

About bitumen, it's known meanwhile to be carcinogenic. It's not that dangerous, as my grandfather used it everywhere for decades and he didn't die of it, but it's getting out of fashion now. Not very good for the aquatic life neither, since the lighter components exude from bitumen to form a film at the water's surface.

Zinc (which isn't chromate) is a decent protection on steel but as is, it hampers the adhesion of paint. On street signs it lasts for several decades under occasional rain, so you can hope several years in sweet water. There are other treatments on zinc, applicable on a big scale, that don't use chromates; the colour differs, and they give paint a good grip too.

I'd consider the paints (and primers) used for oceanic boats.
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