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Keeping it steel

Carbon steel may be strong, but it is no match for the corrosive effects of seawater. However, cathodic protection helps preserve the integrity of offshore steel structures.

Carbon steel — durable, flexible, and relatively inexpensive — is the material of choice for offshore engineers. But it has one significant disadvantage — rust. 

“We want to build things out of carbon steel, and when you put carbon steel in seawater, it corrodes. It disintegrates and dissolves into the ocean,” Matthew Taylor, head of science and technology at Deepwater Corrosion Services, says.

Engineering metals such as steel are not stable on Earth in the long term, he explains.

“We live in an environment of water, chlorides, and other salts, and that’s just not compatible with metal,” he says. “We have a metals-based society that relies on machines.”

The most widely applied technology to preserve those machines is cathodic protection, which comes in two forms — galvanic and impressed current. The galvanic method uses sacrificial metal, or an anode, so “we can define the location of the corrosion”, Taylor says.

Demyst_Typical JOB DONE: An aluminum sacrificial anode affixed to subsea drilling equipment exhibits the pitting and corrosion indicative of proper cathodic protection.
An anode is where the corrosion occurs, while a cathode is where the cathodic reaction — that is, oxygen consumption or hydrogen production — occurs.

Connecting an anode to a steel structure in seawater ensures “all the corrosion can occur on the anode and the rest of the structure is the cathode and does not corrode”, Taylor says.

With impressed current, a method often used for ships and other floating steel structures, the protected metal connects electrically to a stable anode through a power supply.

One misconception is that all metal surfaces need to be within line of sight of an anode to be protected — not true, Taylor insists.

Another common but mistaken belief is that more anodes means more protection. In fact, anodes closely spaced tend to be less efficient due to mutual interference of added current trying to flow through the seawater, Taylor says.

Demyst_Sacrificial PROTECTION: Lustrous aluminum platform anodes before installation and exposure to corrosive seawater.

“Seawater can only conduct so much current.,” he explains. “They’re just fighting with each other to get through one door.”

Another misunderstanding involves protection confirmation. Some clients request proximity measurements, without actually touching the anode or structure.

“That’s physically impossible. You must touch the unit to do proximity measurements,” Taylor says.

Cathodic protection has evolved over the years, a fact that eludes some project planners.

“We get a lot of granddad engineering: ‘It was good enough for my granddad. We’ve been doing this since the 1960s, and nothing has changed.’ They’ve been taking old cathodic protection designs and pasting them onto new equipment,” he says.

Thinking long-term

The industry is using more corrosion-resistant metals, such as superduplex and other alloys. These metals do not require cathodic protection, but unless protection plans account for that, the corrosion-resistant alloys will draw the current nonetheless, causing the overall protection plan to underperform. Taylor refers to this as a “phantom cathode”.

“Think of the anode as a fuel and as you lose mass on the anode, you run out of anode and it can no longer protect the structure,” Taylor says. “The phantom cathode makes your ‘fuel’ burn much faster.”

For metal to undergo electrochemical corrosion in typical seawater environments, there are four basic requirements.

First, there must be an electrolyte that conducts the current from the anode to the cathode. Seawater is that electrolyte. The higher the salt content of the water, the more conductive it becomes.

DCS IMPRESSED: RetroClamps with ROV-compatible handles sit atop the pod, ready for electrical connection to the structure. Once the clamps are connected, continuity is created and the structure is protected.

There must be an anode material, which is the metal being attacked, releasing electrons and cations (positively charged ions).

Third, there must be an electric path that transports the electrons from the anode to the cathode.

Fourth, reactions on the cathode demand the electrons from the anodes, completing the circuit, functioning like a battery.

“All electrochemical corrosion control is cathode controlled. If you use a big cathode, the anode will corrode faster. A small cathode results in a slower corrosion rate,” Taylor says.

Temperature affects corrosion rates, and water velocity increases corrosion. Rates of corrosion in seawater are predictable except in situations such as the Gulf of Mexico’s loop current.

“Long term, the corrosion rate of carbon steel in seawater is 0.1 millimetres per year,” he says.

Some companies opt to use more steel initially on their structures to allow for corrosion, but correctly planned and executed cathodic protection from the outset can minimise the amount of steel required, Taylor says.

Local anodes are attached to the surface of a structure, often during the construction phase.

Semi-remote anodes are placed a distance from the structure and connected by cable. That technology was invented in the early 1990s for use on pipelines in the Gulf of Mexico, as the heat from the pipelines was reducing the anodes’ effectiveness.

Demyst_Platform EXPOSURE: Underside of an offshore platform exhibiting typical carbon steel corrosion.

Semi-remote anodes can be put into service nearby when local anodes are near the end of their functional lives. Modern retrofit jobs can be done in a day or two, Taylor says, compared with more than a month for cutting off old local anodes and welding on new ones in the same spot, as was done in the past.

The retrolink is a hanging anode string that Taylor calls a good short-term solution for cathodic protection of small structures.

Suitable for water depths up to 100 feet, Retrolinks are economical and provide five to 10 years of protection, he says.

“There’s been no new innovation in the science of cathodic protection in a long time. It’s all in the art now,” Taylor notes.

Innovation in cathodic protection, he says, will come in the form of ways to reduce installation costs, reduce risk to people, reduce the number of objects that are on the seabed for protection, improved sensing technologies and improved prediction modelling.

The industry is increasingly concerned about extending the design life of facilities, so operators are beginning to think about cathodic protection much earlier in the design phase.

“The industry is getting a lot more far-sighted than it used to be,” Taylor says.

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