Steel Bulkhead Repair: Corrosion Zones, Coatings and Patching

Steel sheet piling is strong, stiff and available in long lengths, which is why it appears on everything from commercial wharves to residential canals. Its weakness is corrosion, and that corrosion is not uniform. A wall can look sound at the top while losing steel quickly a few feet below the waterline. Good repair decisions start with knowing where to look and what each repair can realistically do.

For background on the material, see steel sheet piling. This guide focuses on deterioration and repair.

Where steel walls corrode

The Department of Defense waterfront maintenance manual (UFM 4-150-07) states that corrosion is the major cause of deterioration of steel waterfront structures, and that its severity varies with exposure zone. It also notes that corrosion rates in seawater are much higher than in fresh water.

The UFM states that carbon steel corrodes in all zones, but the most severe corrosion occurs in the splash zone and just below mean low water. Its underwater inspection checklist starts at the splash and tidal zones and at about 24 inches below mean low water, "where most mechanical and corrosion damage is found."

Accelerated low water corrosion

A European Commission research report on steel piling describes accelerated low water corrosion (ALWC) as a microbially influenced phenomenon at the low water level that causes premature perforation of some steel sheet pile walls. The report noted that the mechanisms were not fully understood. Speaking to the trade journal Materials Performance, a Naval Research Laboratory scientist described ALWC as reported worldwide on unprotected steel piling in tidal seawater and brackish water, and said several research groups attribute it to a combination of sulfate-reducing bacteria and other organisms in fouling layers.

Freshwater is not immune

In the Duluth-Superior Harbor on Lake Superior, steel sheet piling expected to last 50 years showed perforations in under 10 years, according to the same 2006 Materials Performance article. Wisconsin Sea Grant reports that pitting was concentrated in the first 4 to 6 feet below the waterline, tapering off around 10 feet. Researchers have investigated microbial involvement but describe the evidence as not conclusive.

Assessing a corroded wall

1. Clean before judging. The UFM calls for removing marine growth from a sample area before inspecting for rust, scale and holes.
2. Sound the steel. A hammer can reveal scaled or hollow areas.
3. Measure thickness. Calipers can measure webs and flanges. Ultrasonic gauges give more complete readings, though the UFM cautions that pitting can produce erroneous results.
4. Check existing cathodic protection. The UFM lists acceptable potentials of about -0.80 to -0.90 volts against a silver/silver chloride reference cell, and notes anodes should be replaced when more than 80 percent consumed.
5. Look behind the wall. Holes in the sheeting can show up as lost backfill and ground subsidence.
6. Check alignment and the toe. Deflection can signal tieback failure or overload, and loss of soil in front of the wall can let the toe kick out.

Much of this happens underwater. The UFM expects an engineer to brief the diver and evaluate the findings.

Protecting steel that is still sound

Coatings

The UFM groups maintenance coatings by zone: underwater-curing epoxies for submerged steel, heavy-build systems for the intertidal and splash zone, and conventional systems above the splash zone. Preparation matters as much as the product. It lists removal of growth, unsound coating, rust and surface chlorides, followed by abrasive blasting, and recommends consulting a coating specialist.

Cathodic protection

Cathodic protection makes the steel the cathode in an electrochemical circuit. Galvanic systems use sacrificial zinc, magnesium or aluminum anodes, while impressed current systems use a rectifier and longer-lived anodes. The UFM is clear about the limits: cathodic protection is restricted to submerged or buried structures, and it "cannot restore the material already lost by corrosion." Both types need periodic monitoring.

For ALWC specifically, the European Commission study evaluated low-alloy steels, periodic cleaning, cathodic protection and organic coatings. It found that cathodic protection and organic coatings, alone or combined, gave a high level of protection and recommended them.

Restoring steel that has been lost

The UFM notes that patching "will not prevent further corrosion" and that its success depends on the surrounding steel being sound. It warns that continued deterioration of a weak structure, particularly near tiebacks, could lead to rapid failure and poor use of repair funds.

For serious deterioration, it describes driving new steel sheet piling in front of the existing wall, installing grouted tie rods through both walls or adding deadmen, and filling the space between with concrete, while maintaining electrical isolation between old and new steel. When reinforcing tiebacks, it calls for enough clearance to electrically isolate new tie rods from the piling, plus packaged anodes on the rods. See tieback and anchor repair.

Mixing metals

The UFM records failures of sheet pile bulkheads with interior wales caused by galvanic incompatibility between metals. Any repair that adds new plates, bolts, rods or anodes should be reviewed for material compatibility.

Next steps

Record what you can see above water, including rust bands, holes, coating failure and settlement behind the wall, and use the seawall health check to organize it. Our bulkhead inspection guide explains what a professional inspection typically covers, and bulkhead repair vs. replacement covers how owners weigh the options.