Avoiding Rust: Science of Corrosion on Ships

Every sailor eventually asks, “How do I stop corrosion?” The sea constantly eats away at your metal hull. And corrosion seems to have a mind of its own, destroying one segment while completely ignoring other parts of the hull. Why is this? I have no easy answers. No simple fix for corrosion, because it isn’t a simple thing. If you want to fight corrosion and preserve your ship, we need some time to understand the subtle chemical processes that drive corrosion, and how to stop that chemistry.

Introduction

I remember sitting at dinner one night with some other naval architects, talking shop and relaxing a little. When one of the older senior engineers stood up and announced boldly, “A toast. To corrosion! The best job security we can get.” Corrosion kills most ships. Rust slowly eats away at the hull until your shinny new ship looks like a piece of brown grizzled Swiss cheese. Ew!

Considering the deleterious effects of corrosion, every sailor eventually asks, “How do I stop this horrible thing?” No easy answers. No simple fix for corrosion, because it isn’t a simple thing. If you want to fight corrosion and preserve your ship, we need some time to understand the subtle chemical processes that drive corrosion, and how to stop that chemistry.

Science of Corrosion

Corrosion is an electrical and chemical process. All corrosion involves the movement of electrons from one atom to another. From chemistry, we divide corrosion into two critical steps, tracking the movement of electrons:

Reduction
Oxidation

In the reduction reaction, oxygen atoms are drifting around looking for electrons. Natural oxygen in the air is mostly stable and paired up. But if we add some water into the mix, the gaseous oxygen can free up. This process requires that we add in some electrons. Oxygen really wants those extra electrons and will happily steal them from any nearby atoms.

Along comes a helpless atom of iron, found in the steel hull of a ship. (It can be other metals as well, but we’ll stick with iron for now.) Metals easily give up their electrons. So, the conniving oxygen atom steals two electrons from the iron atom. This is the oxidation reaction. But what about the poor iron atom?

Now comes the second part of the process, where we balance out the equation. The iron atom tries to fill out its valence shell and replace the two missing electrons. It runs into that hydroxide molecule (OH^–) left over from the oxidation reaction. Wouldn’t you know, they match up perfectly to balance out their electrons. So the iron and hydroxide get together and form iron oxide. More commonly known as rust.

All the atoms are happy. All the electrons balanced out. Only problem, the humans don’t like the rust. It’s an inferior compound compared to the iron.

The really devious thing to understand about corrosion is the electrons. Electrons were the main thing transferred between the atoms in this chemical reaction. But electrons travel easily through metals. They are happy to move far away before meeting up with the matching atoms. (Far in terms of atomic distances.) The site where the iron gives up electrons and the site where the hydroxide steals them may be in two different locations. This separation leads to some odd behavior with rust. So, how do we stop the rust?

Stopping Corrosion – Paint

If we want to prevent corrosion, either remove the ingredients or stop some part of that electron exchange. For corrosion to happen, we need three ingredients:

Metal willing to give up electrons
Oxygen that wants to take electrons
Water to balance out the chemical process

Another key element to notice is the concentration of these ingredients. For example, imagine a sealed tank where we remove all the oxygen. Pump in pure nitrogen to remove the oxygen. In reality, we still have a few leftover oxygen atoms. But nobody cares about a few atoms. That won’t do much damage. For significant corrosion, we need a high concentration of the ingredients.

Looking at the exterior of a steel ship, this is also why we see more corrosion right at the waterline. Wave action constantly mixes in a fresh supply of oxygen. Go deeper, and corrosion slows down due to less available oxygen.

Removing the oxygen is one strategy to completely stop corrosion. But it’s not very practical. Our atmosphere is full of oxygen and water vapor. Especially for a boat sitting on the water. Any metal exposed to the atmosphere experiences corrosion. If we can’t eliminate the oxygen, we need to separate it from the metal. For this separation, we use a highly technical combination of advanced chemical compounds: paint.

On commercial ships, we use paint to prevent corrosion. That’s the primary purpose. The paint does not stop the flow of electrons. Electrons can still flow through the paint and through the metal. Instead, the paint separates the water and the metal, preventing the reduction reaction. Without that reduction reaction to balance the equation, the oxygen atoms quickly run out of iron atoms willing to donate their electrons. No reduction reaction, no corrosion.

This is not the same paint you use on your house. Ship paint is typically based on epoxy or polyurethane. It’s more like an advanced glue. We form a solid layer of plastic over the steel, with a little pigment added in. Despite the advanced nature, paint is much more fragile than steel. And when paint fails, we get pitting corrosion.

Paint Failure – Pitting Corrosion

Plenty of mishaps create small cracks and holes in that pristine coat of paint. Maybe a pebble hit the hull and chipped off a small piece of paint. The metal expands and contracts with the heat of the day, which eventually generates small cracks in the surface coating. These small chinks in the armor allow corrosion to get in. Those chinks show up as pitting corrosion.

Figure 3‑1: Pitting Corrosion Example [1]

Pitting corrosion creates tiny little pits that eat deep into the surface of the metal. And the pits form much faster, because we don’t have equal areas. The reduction reaction only happens at the crack in the paint. Those metal atoms donate their electrons. But corrosion gets driven by the contact area to transfer electrons from the metal to nearby hydroxide molecules. The electrons can travel through the metal to a large region around the pit. There is much more surface area to accept electrons compared to the small pit that donates them. (Figure 3‑2) This leads to much faster corrosion.

Figure 3‑2: Pitting Corrosion Diagram [2]

Fighting Corrosion – Practical Solutions

Paint doesn’t last forever. Eventually it cracks, letting in corrosion. What’s the solution? MORE PAINT! Ships constantly renew their paint. Touchups and even complete replacements. Paint is a constant battle to stop corrosion.

Despite this effort, we can’t constantly renew every surface on the ship. Some corrosion becomes inevitable. And acceptable. Yes. You heard me. The engineers expect limited corrosion. We intentionally make the metal thicker than it needs to be. We call this the corrosion margin. Extra metal so you can afford to lose a little to corrosion.

Problem though, corrosion isn’t equal and fair. It concentrates in specific locations and unevenly attacks small spots on the ship structure. And we can’t add extra metal to just the problem locations. So corrosion margin offers limited benefits. Even with a corrosion margin, a middle aged ship still needs to replace some of its steel. Areas where corrosion ate unevenly fast. Although this sounds bad, it’s a lot better to replace some of the steel compared to the entire ship.

Sacrificial Anodes

Corrosion margin only offered an equitable blanket of protection. Sacrificial anodes protect with more surgical precision. When we talk about corrosion, we normally mean iron in the steel hull. But remember that corrosion is about transferring electrons, regardless of the metal that donates those electrons. So, get a different metal. Some metals give up their electrons easier than iron. Zinc and aluminum are the two preferred alternatives. We bolt a lump of zinc to the hull in critical locations. This lump is called a sacrificial anode, and it does exactly what is says. The zinc donates the electrons, instead of the iron. This slowly eats away at the anode but helps preserve the steel. Sacrificial anodes offer two advantages:

The anode is less critical. We literally bolt it to the outside of the hull. It has nothing to do with hull strength or holding a watertight boundary.
The anode is cheaper than steel. We can afford to replace the anode every five years or so.

Cheap becomes a relative term on commercial ships. A large commercial freighter will probably install 20-50 anodes along the hull, with clusters near the rudder and propeller. That adds up to several hundred pounds of anodes. And thousands of dollars, every five years.

We typically replace the anodes at the major maintenance periods that occur every five years. This schedules comes from an overabundance of caution. The zinc anodes rarely dissolve in five years. But we don’t monitor them daily, and we don’t know exactly how long they last. It depends on the operating conditions for the ship. That’s the downside of sacrificial anodes. They don’t completely stop corrosion, and it’s a guess when to replace them. But they are cheap, which is the major benefit.

Impressed Current Systems

What if we could monitor the hull corrosion? Even better, what if we had a shield to completely stop corrosion along the hull surface? Impressed current systems offer that solution. Remember that corrosion starts because oxygen atoms want to steal electrons. Instead of taking those electrons from poor unsuspecting iron atoms, what if we just provide a ready supply of fun-loving electrons?

Impressed current systems generate a mild electrical charge on the outside of the hull surface. That electrical charge pumps free electrons along the hull surface. Then the hydroxide molecules take those free electrons instead of stealing from the iron atoms. This requires active power input 24/7. And the electrons don’t just hang out on the hull surface. To make the circuit work, electrons flow out from a source point (the cathode), travel along the hull surface, and go into a receiving point (the anode). Large ships may have multiple cathodes and anodes. And we need sensors along the hull to monitor the electricity. So impressed current systems get complicated, but the benefit is a pretty effective shield against corrosion.

The problem with impressed current systems is the people (and nearby sea life). We could create an impressed current system that utterly floods the hull with spare electrons. Thousands of volts of power, creating an unstoppable shield. Unfortunately, that much power requires a huge supply of electricity, at a fairly large cost. Even more problematic, that much power electrocutes any humans and sea life in the region. Remember that impressed current electrifies the hull, which is connected to every other metal surface on the ship, including all the interior structure. So we need to be extremely careful with the electricity supply in an impressed current system.

Figure 6‑1: Animation of Impressed Current System

We limit current flow through the human body by limiting the voltage of the impressed current system. The average human can survive around 100V of voltage. That level will hurt, but a few seconds of exposure probably won’t do permanent damage. [4] We keep an impressed current system far, far, far below that. The rules for international shipping (IMO) limit an impressed current system to a maximum of 1.20 volts. [5] That’s only 1.2% of the danger level.

Yes, we want a low voltage, but that extremely low value isn’t for your benefit. It’s to save the paint. Safety requires us to keep impressed current running weak, which means this shield is not perfect. Corrosion can still happen. We use impressed current systems in combination with other corrosion protection like paint and sacrificial anodes. It’s not perfect, but definitely preferrable to damaging the ship or the humans.

So why use impressed current if they fail as a perfect solution? Cost. Impressed current slows down corrosion, which means that the paint and anodes last longer. Especially the paint, which is very expensive. The other benefit is monitoring. Conventional corrosion protection is a passive effective, with no clue when you need to replace things. That prompts an over-abundance of caution, replacing items well before needed. But with impressed current, we get feedback and monitoring. The system doesn’t start at its maximum voltage. It starts lower and slowly increases its voltage to maintain the protection. That gives you a way to monitor the hull. You know that if the impressed current system is pumping at full bast, then it’s time to renew the hull paint and replace the sacrificial anodes. Impressed current removes a lot of the guesswork from corrosion protection.

Conclusion

Corrosion can be subtle and sinister. It doesn’t give up easily. The chemical processes work at the atomic scale, finding any little crack to get through. But once we understand that chemistry, we know how to fight it. Interrupt any link in the chain of chemical processes that drive corrosion, and you stop the whole thing.

Unfortunately, corrosion will always be an enemy at the gates, requiring constant vigilance. But thankfully, once we spot it, the enemy is easily defeated with a host of tools at our disposal. We can’t ever win the fight against corrosion, but we have the tools to win battles. Slow down corrosion and draw out the war to last decades. That is how we ensure a long ship life.

References

[1]	C. Delgado, “Corrosión por picadura en aluminio – 01,” Wikimedia Commons, 29 April 2013. [Online]. Available: https://commons.wikimedia.org/wiki/File:Corrosi%C3%B3n_por_picadura_en_aluminio_-_01.jpg. [Accessed 2024 01 Sep].
[2]	D&D Coatings, “What is Pitting Corrosion,” D&D Coatings, 26 May 2019. [Online]. Available: https://www.ddcoatings.co.uk/2276/what-is-pitting-corrosion. [Accessed 01 Sep 2024].
[3]	J. Llamas, “ICCP Hull Corrosion Protection Systems,” YouTube, 10 Mar 2015. [Online]. Available: https://www.youtube.com/watch?v=d_fiXblgZdE. [Accessed 07 Jan 2026].
[4]	S. G. Burns, “Safe Levels of Current in the Human Body,” University of Minnesota Duluth, [Online]. Available: https://www.d.umn.edu/~sburns/EE2212/L-Safe-Levels-of-Current-in-the-Human-Body.pdf. [Accessed 3 Sep 2024].
[5]	“Chapter 15 – Corrosion Protection, Section 4 – Cathodic Protection,” Classifications Register Rules and Regulations, 2024. [Online]. Available: https://www.imorules.com/LRMAT_CH15_4.html. [Accessed 03 Sep 2024].
[6]	Cathwell, “Cathodic Protection Explained,” Cathwell, [Online]. Available: https://cathwell.com/cathodic-protection-explained/. [Accessed 29 Jan 2026].
[7]	Astro Pak, “Astro Pak: Pitting of Stainless Steel – Narrated,” YouTube, 26 Dec 2017. [Online]. Available: https://www.youtube.com/watch?v=Zbw_qMafWiA. [Accessed 26 Feb 2026].
[8]	The Millennial Seafarers, “Maintenance on Board: Chipping,” YouTube, 6 Feb 2021. [Online]. Available: https://www.youtube.com/watch?v=-MziSsDfSSg. [Accessed 26 Feb 2026].
[9]	PressTube, “Breaking Down OLD Zinc Anodes into NEW \| INSANE CAST,” YouTube, 15 Jun 2023. [Online]. Available: https://www.youtube.com/watch?v=cDJL5jNuSE4. [Accessed 26 Feb 2026].