Why Monel Is So Hard to Replace: The Metal That Beats Steel at Sea

Added: 2026-05-30

[00:00:00]Picture a valve at the bottom of a desalination plant. Sea water has been moving through it under pressure for 15 years without stopping. The water is salty, slightly acidic, and constantly forcing its way through every microscopic gap in the metal. Steel would have failed years ago. Stainless steel would be pitting.

[00:00:29]The valve is made of Monell and it is still performing exactly as the engineers intended. We covered Monell before, a nickel copper alloy born from a Canadian mine in 1906, trusted in places where failure is not allowed. But there is a follow-up question. If Monell is this good, why not use it everywhere?

[00:00:49]The real reason Monell is hard to replace, is not about price. It is about what happens to everything else in the specific environments where Monell is asked to work. Let's get into it right here on Small Things.

[00:01:10]The problem only Monell solves.

[00:01:14]At its core, Monell does not solve the problem of needing a strong metal. It solves the problem of needing a metal that stays strong in the places that destroy everything else. There is a difference between corrosion resistance in a kitchen sink and corrosion resistance under sustained chemical attack from salt water and brine over decades of continuous use. Every obvious substitute fails at least one critical test. Standard steel rusts. Stainless steel can crack silently under the combined pressure of mechanical load and chloride ions in flowing seawater. The component looks completely fine until it does not. Manel does not crack in flowing seawater. It corrods at roughly 0.0025 mm per year. That is so close to nothing that engineers treat it as permanent.

[00:02:05]When stainless steel seemed like the answer, it did not start as a niche material with no competition. When Monell arrived in the early 20th century, it was genuinely revolutionary.

[00:02:17]Steel corroded in seawater. Aluminium was too soft for high-pressure systems.

[00:02:23]Monell could be used in ships, in chemical plants, in the early aviation industry, and in household sinks, and it held up in all of them. For decades, it had almost no credible rival in harsh environments. Then stainless steel arrived, improved steadily through the middle of the century, and offered something Monell could not, a lower price. By the 1950s, engineers began specifying stainless steel for applications that had previously used Manel. The switch made economic sense in many environments. In a food processing facility, in an architectural setting, in a piece of kitchen equipment, stainless steel performed well enough and cost significantly less. Manel began to look like an expensive solution to a problem that stainless steel had already solved, except it had not solved it everywhere. In seawater, the chloride ions that make salt corrosive do something specific to stainless steel that they do not do to monell. Stainless steel protects itself through a thin passive oxide layer on its surface. In most environments, that layer is stable and effective. But in high chloride conditions, particularly under mechanical stress, chloride ions penetrate and break down that layer. The metal beneath corrods. The process is invisible until the structure develops a crack. Engineers call it stress corrosion cracking. It does not announce itself. A component can look perfect from the outside while failing from within. Monell resists this mechanism because of what nickel and copper do together at the surface. Unlike the passive film of stainless steel, Monel's surface protection does not depend on a fragile oxide layer that chloride ions can penetrate. The resistance is structural, built into the alloy itself, not sitting on top of it as a coating.

[00:04:14]That distinction turns out to be the reason Monell is still specified for marine valves, propeller shafts, and seawater pumps more than a century after it was developed in applications where stainless steel has been tried and found to fall short. The metal that was supposed to be replaced never quite was because the problem it was solving turned out to be harder than expected.

[00:04:38]What seawater actually does. So what is actually happening when seawater meets metal over a long period of time? Think of it less like water and more like a slow chemical argument that the metal is always losing. Seawater is not just salt and water. It is an electrolyte, a conductor of electrical current, which means it creates the conditions for electrochemical reactions between any metal it contacts and the oxygen dissolved within it. Add chloride ions, which are the chemically active component of salt, and you have a medium specifically hostile to the passive protective films that most corrosion resistant metals rely on. At normal temperatures and pressures, stainless steel handles this well. But at higher flow velocities, at raised temperatures, under mechanical stress, or when the seawater is carrying dissolved gases like hydrogen sulfide found in offshore oil environments, the conditions tip, the passive film fails, localized pitting begins. In some cases, the pitting progresses into cracking that spreads through the metal's grain structure. The component has not been hit, scratched, or overloaded. It has simply been corroded from within by a process that was invisible while it was happening. Monel works differently. Its nickel copper structure forms a surface that is resistant to chloride attack without needing to maintain a delicate passive film. It performs particularly well in flowing seawater where its corrosion rate drops to almost nothing.

[00:06:08]The one condition where it is less effective is stagnant saltwater where biological fouling can change the local chemistry near the surface. But in the moving pressurized chemically active seawater of real marine systems, it remains one of the most durable materials available. And there is one more thing seawater does that most people do not think about. It connects metals. Any two dissimilar metals in contact in a conductive liquid will drive an electrochemical reaction between them where the less noble metal corrods to protect the more noble one.

[00:06:43]This galvanic effect means that choosing the right material is not just about how it performs on its own but how it behaves beside every other material in the system.

[00:06:54]Any tough alloy will do. The assumption many engineers carry when they first encounter Manel's price is that some newer, cheaper alloy must by now have made it redundant. That assumption underestimates how specific the problem is. Several alternatives have been developed and are used successfully in certain marine environments. Super duplex stainless steels offer excellent resistance to stress corrosion cracking and are strong enough for structural marine applications. An alloy called AL6XN was developed specifically to combat seawater corrosion and performs well in desalination and power plant contexts.

[00:07:33]These are not inferior materials. They are good alternatives in the right context. But super duplex steels are more difficult to weld than Monell. And in certain acidic environments, particularly those involving reducing acids rather than oxidizing ones, they do not match its resistance.

[00:07:51]AL6XN carries its own cost premium and supply chain constraints that make it less available in parts of the world where Manel is already established and stocked. Manel's position is not based on being irreplaceable everywhere. It is based on being reliably excellent in a specific difficult category where the cost of failure far exceeds the cost of the material. A valve deep inside an offshore platform is expected to function for decades without maintenance windows. In critical systems, the engineer does not ask what the cheapest material that might work is. They ask what the most reliable material with a proven track record is. Specifying a cheaper alternative that lasts 8 years instead of 20 is not saving money. It is deferring a much larger cost.

[00:08:41]Now, the sea still needs it.

[00:08:44]Today you will find Manel in the systems that connect modern industrial life to seawater, deep pressure and corrosive chemistry.

[00:08:53]Desalination plants supplying drinking water to millions across the Middle East and North Africa use monol in high-pressure intake systems where concentrated brine at elevated temperatures would destroy standard stainless steel within years. The plants run continuously for decades. Every hour of unplanned downtime costs more than any material savings a cheaper valve could offer. Offshore oil and gas platforms use Monell in downhole tools, valve seats, and pump components exposed to sour gas environments where hydrogen sulfide and seawater combine into one of the most corrosive mixtures industrial engineering encounters. These are not environments where anything can be easily swapped out mid operation. The cost of a failed component a kilometer below the seabed is not the cost of the part. It is the cost of the shutdown, the recovery, and the environmental liability. In recreational boating, monell rivets are still used on certain aluminium framed boats precisely because stainless steel rivets in contact with aluminium in seawater would drive galvanic corrosion through the frame. In chemical processing, Monell handles hydrofluoric acid in conditions where most other metals would be consumed within months. There is no obvious substitute that does this reliably at comparable cost over a comparable service life. The forgotten metal turns out to still be the only answer in more places than its price would suggest.

[00:10:26]Strong enough is not always enough.

[00:10:29]So why is Monell so hard to replace?

[00:10:31]Because the environments where it works are the ones where almost everything else eventually stops working. In ordinary conditions, steel is enough. A kitchen, a warehouse, a car chassis, strong enough is enough. But in seawater under pressure, in chemical systems handling acids and brine, a material that performs well for 5 years then requires replacement is not a solution.

[00:10:55]It is a delay. Manel is not valuable because it is the strongest metal in the world. It is valuable because it solves a very specific problem better than cheaper materials can. Some materials are saved for the few places where failure would cost far more than the metal itself. The quiet valve at the bottom of the desalination plant is still there, still working, still unreplaced.

[00:11:19]If this changed the way you think about the materials holding together the systems you depend on every day, let us know in the comments what object or material you want us to look at next. We might make it our next episode. Thanks for watching. History often hides in the smallest details.