The Ocean Eats Steel Faster in Some Harbors Than Others
By Ed Brannigan, Marine contractor. Reviewed by Amanda McCallister, editor.
Cathodic Protection & Anodes | Offshore & Subsea Diving April 2011, Port of Gulfport, Mississippi. I was leaning over a bucket of green water while a crew yanked a rusted Z-pile out of the slip. It had been driven nine years earlier as part of a forty-pile berth. Half the bottom flange was gone. The rest came up looking like coral. Terminal manager walked over, said 'That's a fifty-year pile, Ed.' I said, 'Not in this harbor, it isn't.' That night I went back to the shop and looked at our corrosion maps. The same spec steel in the Gulfport slip was losing an eighth-inch a year. In a cold, clean harbor like Eastport, Maine, it barely loses half that. The difference isn't the sea. It's what's in it, and what's working on it. Over 30 years I've pulled enough failed piles to know how to read a harbor. Here's what tells you whether your steel will last a construction cycle or a career.
The Water Is Not Seawater—It's a Cocktail
Start with chlorides. Saltwater is saltwater, but the concentration changes with the harbor. A creek with heavy rain runoff might be almost brackish, and that's actually richer in oxygen. Oxygen is the oxidant that drives the attack. But the real killers are the extras. I've seen harbors where an upstream chemical plant or a tannic swamp turns the water acidic enough to eat the coating off in a season. If your slip has black, smelly water, you're likely looking at a low-oxygen environment with sulfides. That's not a slower attack—it's a different animal. Get a baseline water chemistry test before you spec your steel.
Galvanic Pinball
The single easiest way to kill steel in a harbor is to tie it to a bigger piece of metal through the water. If your dock shoes have copper straps, or a boat in the slip is dumping copper bottom paint into the water, you've set up a battery. The steel is the anode, and it donates metal to the copper. Same thing goes for stray DC current from shore power or a badly grounded welder. Six hours a day of that and you'll lose an anode's worth of pile a year. I once inspected a marina where every steel pile at the electrical pedestal was pitted like a golf ball. The ones at the end of the dock were fine. It wasn't the salt—it was the current.
Warm Water and Current
Temperature runs the whole show. Every 10 degrees C of water temperature roughly doubles the corrosion rate. That's why a pile in Mobile Bay is shot in 15 years, while the same pile in Nova Scotia can go 40. Current also matters. Fast-moving water scrubs the rust film off the steel. Rust is not the enemy—it's actually a semi-protective layer. Scour removes it, exposing fresh metal. Harbors with strong tidal rip or propeller wash from tugs cut steel the way a knife cuts cheese. If your berth is at a bend in the channel, plan on replacing your piles sooner.
Microbes Under the Mud
The worst corrosion I've seen isn't in the salt water. It's at the mudline, where the pile disappears into the bottom. There, sulfate-reducing bacteria live in the oxygen-starved sediment. They create hydrogen sulfide, which reacts with the steel and forms an acid environment. This is microbially induced corrosion, and it doesn't look like rust. It looks like a black scab with blisters underneath. When you break open the blister, the steel is pitted and brittle. I've seen piles fail below the mudline after a decade. You can't see it from the top, so you have to inspect with a sonar or pull a test pile. That's the only way to know.
The Ten-Year Checkup
So what do you do with a hostile harbor? First, get a water and sediment analysis. If it's acidic or high in sulfides, you need a thicker pile jacket or a sacrificial anode system. Second, never combine copper and steel without an anode. Third, coat only the above-mud section. Below the mud, the coating gets scraped off during driving, so it's better to rely on anodes and thickness. Fourth, set a scheduled inspection. Every five years is a good interval; if your harbor is aggressive, do three. We now use ultrasonic thickness gauges on test piles. It costs a day of boat time. Replacing a pile costs $10,000 and two weeks of downtime. You do the math.
Frequently Asked Questions
What's the best metal to use if my harbor is aggressive? For piling, stick with ASTM A690 or A572. Don't expect stainless steel to save you—it can suffer pitting in this saltwater too. The protection is the coating and the anodes, not the alloy. Can I just paint steel with any marine paint and be done? No. The only coating that works in the splash zone is a full epoxy system with a corrosion inhibitor. You need to do an abrasive blast, apply a zinc-rich primer, and then a high-build urethane. And even then, you need anodes at the underwater section. There's no one-coat miracle. How do I know if my harbor is killer before I build? Ask the local engineers at the Army Corps of Engineers if they have corrosion data for the port. Or get a simple water test done by a lab. They'll check pH, chloride, sulfides, and dissolved oxygen. That gives you a baseline. But the best indicator is your neighbor's pile. If the dock down the street has rust blisters at the waterline after five years, treat yours accordingly.