A few months back, I sat across from a lead engineer at a marine equipment firm, and he asked me a question that stopped me mid-sip of my coffee: “Can bonded seals even hold up under 500 meters of saltwater pressure?” It wasn’t a gotcha question—his team had burned through three off-the-shelf rubber gaskets in a subsea valve prototype, and his whole project was on pause. As someone who’s spent 12 years in the bonded seal supply game, I didn’t just throw out a yes or no. I walked him through the messy, real-world line between “bonded seals can work” and “they work for your underwater application.” That conversation stuck with me, because it’s the exact confusion most people have when they first start looking at sealing solutions for water below the surface. Bonded Seal
First, let’s start with what a bonded seal actually is, for anyone new here. Unlike standard O-rings, which are just a single elastomer ring, bonded seals are made by vulcanizing (fusing under heat and pressure) a flexible elastomer layer to a rigid metal core. The metal core is usually stainless steel, carbon steel, or bronze, depending on the environment, and the elastomer is your standard lineup of nitrile (NBR), Viton (FKM), EPDM, or HNBR. The metal part is what sets bonded seals apart: it lets them seat into a precision groove to create a metal-to-metal seal first, then the soft elastomer compresses slightly to stop fluid from getting through. That dual action is why they’re used in everything from hydraulic pumps to aerospace actuators, but it’s also why the underwater question isn’t a simple one.
The short answer? Yes—bonded seals absolutely can work in underwater applications. But the details matter more here than in almost any other industry. Let’s break down why that is, and the caveats that make it or break it.
First, let’s talk about why bonded seals are often a better fit underwater than their competitors. Saltwater is corrosive, and subsea pressure can crush weaker seals if they don’t have a solid structure to hold their shape. A standard O-ring, for example, is just a rubber ring: at depth, high pressure can cause it to extrude into tiny gaps between mating parts, leading to leaks or total failure. Bonded seals’ metal core acts as a backbone, so extrusion is far less likely. The metal core also takes most of the load when you install the seal into a groove, so the elastomer only compresses enough to create a seal, not so much that it’s crushed under the weight of surrounding water over time.
Corrosion is another big factor. If you pick the right metal core and elastomer combo, bonded seals can stand up to saltwater far better than many elastomer-only seals. For example, 316 stainless steel is the go-to metal for marine bonded seals—its chromium content creates a protective layer that resists saltwater corrosion far longer than common carbon steel. Pair that with an elastomer like HNBR, which is resistant to both saltwater and moderate oil, and you’ve got a seal that can handle constant immersion without breaking down. I’ve supplied these exact seals to a company that makes underwater ROV manipulator arms, and they’ve reported zero seal failures in 18 months of testing at 1,000 meters, which is way better than the O-rings they were using before.
But here’s the catch: not all bonded seals are made equal for underwater use. The biggest mistake I see engineers make is picking a standard bonded seal off the shelf and assuming it works for deep water. Let’s talk about the factors that determine if a bonded seal is right for your underwater application.
First, pressure rating. Subsea applications range from shallow coastal water (a few meters deep) to full ocean depth (over 10,000 meters), and each requires a different seal specification. Most standard off-the-shelf bonded seals are rated for pressures up to 100 or 200 bar, which is fine for shallow work like dockside pumps or small underwater cameras, but not for deepwater projects. For high-pressure applications, you need custom-tailored bonded seals with thicker metal cores and elastomers that are formulated to resist compression set at high pressure. Compression set is when an elastomer stays deformed after being compressed, which would leave gaps for water to seep through. HNBR and Viton, for example, have far better compression set resistance than NBR at pressures over 300 bar, so they’re non-negotiable for deep subsea work.
Next, elastomer compatibility. Saltwater isn’t just water—it’s a mix of salts, minerals, and sometimes chemicals like antifouling agents that can break down elastomers over time. EPDM is often used for general water sealing because it stands up to pure water, but it’s not great at resisting saltwater or petroleum products. If your underwater application involves hydraulic fluid (which most subsea valves or actuators do), you need an elastomer that works with both saltwater and whatever fluid you’re using. HNBR is perfect here—it’s resistant to saltwater, hydraulic oil, and moderate chemicals, which makes it the most popular choice for marine bonded seals these days. Viton is another option if you’re dealing with high temperatures along with high pressure, like in subsea equipment that runs during hot coastal seasons.
Then there’s the metal core material. Like I mentioned earlier, 316 stainless is standard for marine use, but there are cases where you need something even more corrosion-resistant. For example, if you’re working in areas with high levels of sulfur or other corrosive elements (like near volcanic seeps or offshore drilling sites), you might need a duplex stainless steel core, which is stronger and more corrosion-resistant than 316. I once had a client working on offshore oil pipelines that needed seals for wellheads, and standard 316 steel corroded after just six months in their specific operating environment. Swapping in a 2205 duplex stainless steel core fixed the issue—no more corrosion, no more leaks. The metal core also needs to be coated if you’re dealing with extremely corrosive water; sometimes we use a PTFE coating on the core to add an extra layer of protection, though that’s only necessary for the harshest environments.
Groove design is another factor that’s often overlooked, and it’s make-or-break for underwater bonded seals. Even the best seal in the world won’t work if the groove it’s seated in is poorly designed. Underwater, pressure can act on the seal from all sides (not just one like in hydraulic applications on land), so the groove needs to be precision-machined to account for that. Too much gap in the groove, and the elastomer can extrude into it under high water pressure, leading to failure. Too little, and the elastomer is overcompressed, leading to compression set and leaks after a short time. For subsea applications, we always work with engineers to design custom grooves that account for operating depth, pressure, and the specific seal we’re providing. For example, for seals rated for over 500 meters, we add anti-extrusion rings made of PTFE to the groove, which prevent extrusion even at the highest pressures.
Now, let’s talk about real-world examples, because theory is great, but actual performance is what matters. Last year, we supplied a batch of custom bonded seals to a company that makes underwater sensor buoys. These buoys are deployed in the Atlantic, at depths up to 800 meters, and they need to hold a seal that keeps electronics dry for up to five years without maintenance. We used 316 stainless steel cores, HNBR elastomers, and precision-machined grooves with anti-extrusion rings. A year and a half later, the client reached out to say that every single seal was still holding—no leaks, no corrosion, no signs of wear. That’s not a fluke; it’s the result of matching the right seal specs to the application.
On the flip side, I’ve seen what happens when you cut corners. A startup that was making small underwater drones tried to use standard off-the-shelf bonded seals for their prototype, rated for 100 bar, and deployed them at 300 meters. After a week, two of the five drones had leaked, and the electronics were ruined. The issue? The standard seal’s metal core was too thin, so it deformed under the pressure at 300 meters, and the NBR elastomer wasn’t resistant enough to the saltwater’s high mineral content. We redesigned their seals with thicker 316 cores and HNBR, and their full production run of 50 drones had zero seal issues. That’s why custom isn’t always more expensive—sometimes it’s the only way to avoid costly failures.
Another common question I get: do bonded seals work in dynamic underwater applications, like rotating shafts or moving rams? The short answer is yes, but with the right elastomer. For dynamic uses, you need an elastomer that has good wear resistance, because the seal is moving against a mating part. HNBR again is a good pick here, but sometimes we add a thin layer of PTFE to the elastomer surface for extreme dynamic applications, which reduces friction and wear. I’ve supplied seals for subsea pump shafts that have been rotating at 1,500 RPM for over two years at 1,200 meters, and they’re still going strong.
There are, of course, limitations. Bonded seals aren’t the right choice for every underwater application. If you’re working at full ocean depth (over 10,000 meters), even the strongest metal cores can start to deform, and you might need a different sealing solution, like a metal-to-metal seal or a custom elastomer O-ring with back-up rings. If your application involves extreme temperatures over 150°C or below -40°C, standard bonded seals might not hold up, though we can work with specialty elastomers to extend that range. And if you need a seal that’s fully transparent (like for an underwater camera lens), bonded seals probably aren’t the way to go—though there are other options, like liquid gaskets, that are better there.
At the end of the day, when an engineer comes to me asking if bonded seals can work for their underwater project, my answer is always: “It depends on your specs.” But what I’ve learned over the years is that most applications can work with bonded seals, as long as you don’t just grab a cheap off-the-shelf part. The key is to partner with a supplier who understands the unique demands of underwater environments, who can help you pick the right metal core, elastomer, and groove design for your specific depth, pressure, and operating conditions.
If you’re working on an underwater project—whether it’s a small sensor buoy, an ROV, a subsea valve, or something else entirely—and you’re tired of dealing with seal failures in saltwater, let’s talk. I’ve spent thousands of hours working with marine engineers to solve sealing problems, and I can help you figure out if a bonded seal is the right fit for your application, or point you to a solution that is. Don’t let bad seals derail your project—reach out to discuss your needs today.
TCN TCV Seal References:
- Standard Specifications for Bonded Seals for Marine and Subsea Applications, International Organization for Standardization (ISO) 6194, 2021.
- Corrosion Resistance of Stainless Steels in Seawater, NACE International Publication, 2019.
- Elastomer Compatibility with Seawater and Hydraulic Fluids, Society of Tribologists and Lubrication Engineers (STLE), 2020.
- Pressure Extrusion Resistance of Sealing Materials for Deep Subsea Use, Offshore Technology Conference (OTC) Paper 30217, 2020.
DEDE SEAL Co., Ltd.
DEDE Seal Co., Ltd. is one of the most experienced bonded seal manufacturers and suppliers in China for over 35 years, featured by quality products and good price. Please feel free to buy bulk customized bonded seal made in China here from our factory. Welcome to contact us for pricelist.
Address: Hunan Road, Pudong New Area, Shanghai, China
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