Faced with a load of corrosion or rust stains, it’s such an appealing thought that they could magically be made to vanish, lifted away by the application of a bit of acid.

No frenetic sanding, scrubbing or scraping. Just soak it, have a cup of tea and come back to find grubby bits gleaming, and seized parts gliding effortlessly.

The reality is never quite so simple and on a boat there are only a few situations where acid really helps.

In fact, there are a lot of situations where it can make things a lot worse.

Knowing where it can be applied requires an appreciation of the chemistry and metallurgy involved. And thought also has to go into how to dispose of it safely.

So firstly a bit of chemistry: Acidity is measured on the pH scale. pH stands for Potential Hydrogen because acids contain highly reactive hydrogen ions.

These attack rust molecules and turn them into something that can dissolve in water and be washed away.

The more hydrogen ions there are in a liquid, the lower the pH number is, down to 1, and the more aggressive the acid will be.

A neutral liquid will have a pH of 7 and an alkali which absorbs hydrogen ions, rather than supplies them, will have a pH up to 14 depending on how aggressive an alkali it is.

As a rough rule, acids are good at dissolving calcium build-ups, and alkalis are good for organic build-ups.

But don’t be complacent in thinking only acids are the dangerous stuff.

Alkalis can also be highly damaging to your skin, the environment and your boat.

Sodium hydroxide, for instance, is called caustic soda for a reason, so make sure you wear the appropriate PPE gear for the type of acid you are using.

To have a less abrasive acid, you can either choose a weaker acid which by its chemistry has fewer hydrogen atoms available in its molecules, or you can dilute a stronger acid with water which will reduce the concentration of hydrogen ions present and so reduce the pH, and make it less abrasive.

So in which situations can acid be useful on a boat?

Cleaning with acid: the toilet

Urine mixing with salt water reacts to form a calcium carbonate. That builds up inside your pipes and holding tank. It’s a hard, nasty, chalky substance.

Over time it restricts the flow like a sclerotic artery until it gets completely blocked, invariably at the most inopportune moments.

Everyone’s least favourite job is unblocking a toilet that’s jammed mid-flush.

Giving the pipes a healthy flush out after each use means there’s no reactive solution sitting in them, and that’s a good way to slow down the build-up.

Seven pumps on a Jabsco flush handle clears 1m of pipe according to the manual, so measure the length of pipe in your system to the thru-hull and advise the crew on how many pumps to do after clearing the bowl.

Occasionally it’s good to soak the pipework with an acid to dissolve the calcium carbonate that has built up in there.

Ideally use a weak acid that isn’t harmful to the sea, because the only way to get it out is by flushing it into the ocean.

A weaker acid needs a longer time to react so time the acid treatment for when the heads won’t be in use for a day or two.

We’ve used citric acid, a powder you can buy from chemists and add water to form an acid of pH between 3-6, depending on how dilute you make it.

It’s food-grade safe, and it’s found in lemons. We’ve also used white (not malt) vinegar, which is a little stronger.

Before starting we rinse the pipes out with fresh water and then add 4lt of a water/acid mix and pump it into the pipes and let it sit for as long as we can.

The first time we did it on our used boat, we left it for a week and the dissolved carbonate inside the pipes was like the consistency of a Slushy ice drink.

Cleaning with acid: heat exchangers

Engine heat exchangers have seawater flowing through them.

With that comes the opportunity for sea life to deposit calcium on them and clog up the narrow pipework.

It’s like a small reef being created inside your engine.

The cores are made of copper which is resistant to reacting with hydrogen ions, so you can really go for it when choosing an acid strength.

In the lab, muriatic acid is known as hydrochloric acid. It’s about as strong as it gets with a pH of between 1-2.

When you buy it, the bottle label will tell you how dilute it is. 8% is not untypical so the pH you’ll be using will be a lot weaker than 1 or 2, but even so, it’s still a good idea to dilute it further for your first attempt on a job like this to get the measure of its effectiveness.

Balance contact time against concentration. Using it undiluted requires more care and monitoring, but it’s really satisfying to watch hard caked-on deposits that are impossible to scrape off and were starving your engine of cold water, just disintegrating before your eyes and the clean smoothness of the copper returning.

Take time to think about PPE to protect your skin and eyes with stronger acids. You might find holes developing in clothes that come into contact with it too.

And also consider how you’re going to dispose of it before you start working with it.

Reacting it with an alkali will create a safer fluid to discard but make sure the residues that settle in the bottom of your bucket created in the cleaning process are disposed of properly.

Cleaning copper with acid can create a green sludge containing copper, which is highly toxic to marine life.

Aluminium alternatives

The heater core is housed in an aluminium casing. Don’t be tempted to clean this with acid.

The aluminium becomes porous and pitted and the smooth mating surfaces where the gaskets have to seal can get so roughed up that the part becomes a write off.

Avoid allowing aluminium to come into contact with acids.

Aluminium reacts with air to form a protective film so in theory it doesn’t rust, but corrosion from steel bolts and brackets do stick to it, and it can be corroded by galvanic currents when in contact with dissimilar metals.

Instead, I used an air compressor and adapted air gun; I built an improvised sand blaster and used that to clean up the outside of a pitted saildrive leg.

The sand needed to be very dry, and it was only useful for very small areas.

It did work well on the recessed pitted metal where even a wire brush couldn’t reach and I was able to prepare the surface for painting with an etch primer.

I’ve also seen corrosion on aluminium being cleaned up using electrolysis with a car battery, the right electrolyte and a sacrificial electrode.

Cleaning with acid: gelcoat

Rust stains in white gelcoat will withstand any amount of scrubbing.

Iron filings that fall on the surface will bed in and create yellow spot stains.

Stainless fittings can leave rust-flow stains where they join the deck and topsides.

There are several acid wash products available on the market to address precisely this.

You can also make up your own, with detergent and oxalic acid.

Scrub it on with a brush, leave it for a while and then rinse it off. How long you need to leave it on depends on how deep the stain is, and how strong the acid is.

Start off weak and quick, and repeat again and again in small increments of strength or time.

Being too aggressive isn’t good for the gelcoat and can lead to spider cracking in extreme cases.

Don’t underestimate the rinsing needed.

If you don’t get all the acid off completely it will make a much bigger brown stain downstream of where you were cleaning.

Rust removal from steel

We had our chain re-galvanised by a professional company, and to prepare the rusty chain for galvanising, they did what they called a ‘chemical cleaning’, which essentially involved soaking it in industrial-strength acid for a few days until it came out shiny.

But the strength and volume of acid needed for this means this isn’t the sort of process that you can replicate.

Personally, I’ve not had much success with cleaning rusty steel with acid.

As an experiment, I tried to clean up a set of ring spanners that had repeatedly been used for work in the saltwater.

With hindsight, a wiser move would have been a thorough freshwater rinse after each use.

While the muriatic acid managed to remove some of the rust, it didn’t clean it back to bare metal, so the spanners still would have needed polishing with a wire wheel before re-chroming or etching.

It’s important to thoroughly rinse the acid off the metal after treatment and protect it with oil or paint without delay, or the rust will grow back very quickly.

Cleaning with acid: watermaker membranes

Reverse osmosis watermaker membranes can be thought of as a very fine filter.

If they aren’t used and left to sit in sea water the pores can get clogged with carbonate build-up.

Flushing them through with food-grade acid, like citric acid, can bring them back to life.

By circulating it through the machine for an hour the mechanical abrasion of the flowing acid also helps with the cleaning.

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It won’t be as good as new but can restore enough functionality to keep the machine working.

Watermaker manufacturers sell cleaning products to restore membranes, and these are sometimes little more than repackaged citric acid, so a DIY approach can save some money.

The pores can also be blocked by algae, and flushing them with a warm alkali can dislodge and dissolve that bringing more performance back to the membrane.

Save both solutions and carefully mix them to neutralise each one before disposal.

After cleaning with these chemicals it’s a good idea to thoroughly rinse out the system by running it for a while before collecting any of the output freshwater.

Dropping acid

While it may sound like a magic elixir that will save you elbow grease, there are only a few very specific applications where acid is the best approach.

And even in those cases, using acid can very often be more hassle than a few rounds with a wire brush, especially when you consider the PPE and clean-up effort required.

Using it judiciously and repeatedly with small increments in either concentration or contact time is the safest way to get results without damaging the part you’re trying to save.

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Enjoy reading Cleaning with acid and its uses onboard?

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