CORROSION REMEDIATION

:CORROSION REMEDIATION The mysterious orange slime heading to eat your port

By Daniel Leonard, P.Eng, Vice President, Westmar Advisors Inc.

Speak to the Director of the Port of Alaska, Stephen Ribuffo, and he will tell you all about the horrors of a mysterious orange slime that is slowly and methodically eating his port. “Not to put too fine a point on it, you’ve got a lot of ugly in your future — costly and ugly.”

The technical term for the reaction that causes the slime is accelerated low water corrosion (ALWC), which is a form of microbial induced corrosion (MIC). In simple terms, ALWC is caused by a form of bacteria that is thought to be brought from port to port in vessel ballast water and attaches to steel in a narrow band at the lowest level of the tidal range.

The bright orange slime that appears on the surface of the steel is the result of the bacteria producing sulphuric acid that will eat through the steel at a rate up to 10 times faster than saltwater, or two mm/year (5/64 inch/year). When many of the steel sheet piles and steel pipes found in a port have a thickness of only 12.5 mm (1/2 inch), bacteria that can eat through those elements in less than 10 years is a serious threat.

The orange slime, or bloom, covers a black corrosion product that, when removed, reveals shiny pitted steel. The fact that it is possible to see shiny carbon steel in saltwater shows how aggressive the corrosion is.

Although the phenomenon has been identified in numerous ports around the world, from Scotland to Australia, and from Canada’s Arctic to the southeast U.S. in North America, the reason for why it is in one port instead of another remains a mystery. While many scientists are reasonably certain that it is transferred from one port to another by vessels, other scientists believe that it is In simple terms, ALWC is caused by a form of bacteria that is thought to be brought from port to port in vessel ballast water...

also found naturally in waterways that have the right combination of nutrients.

In addition to Alaska, ALWC has now been confirmed at a port in northern B.C. and there are concerns that it could soon be found in other ports on the West Coast.

Dealing with invasive species is not a new problem for international and domestic shipping. The International Maritime Organization’s International Convention for the Control and Management of Ships’ Ballast Water and Sediments was adopted in 2004 and requires vessels to exchange ballast water in open ocean before entering a new country that has ratified the convention. As well, there have been additional rules in the Great Lakes for several years to attempt to prevent the spread of mussels. But under the existing regulations outside of the Great Lakes, smaller vessels transiting within Canada have been exempted from any ballast water regulations. Now, Transport Canada is proposing new regulations that, by 2024, would require vessels over 50m in length to meet the requirements of the international convention and vessels under 50m would be required to make the best efforts to meet the convention.

But what can be done if ALWC already has a foothold in your port?

The Port of Alaska is currently spending about USD two million per year installing bolted steel jackets over steel pipe piles with severe corrosion. Each jacket costs about USD 32,000 and is intended to restore vertical load carrying capacity. There are mixed opinions on whether the jackets will perform as well as the original piles in a large earthquake. The remediation costs for sheet pile bulkheads are higher than the steel jackets for pile structures, requiring more expensive and complex solutions.

There are other potential rehabilitation solutions depending on the amount of corrosion that has occurred: running a substantial electric current through the steel structures to make them the cathode of an electrochemical cell; applying a non-corrosive protecting coating such as epoxy or cementitious mortar; installing a non-structural, non-corrosive barrier between the steel structure and the water such as PVC sheet piles; or welding plates over top of the corroded zones.

Cleaning away the ALWC requires that workers have extensive protection measures to avoid ingesting bacteria and becoming infected with a blood disorder called leptospirosis.

A significant challenge to implementing a mitigation scheme is the location of the problem — the very lowest tidal elevation. There are only a few hours every year where the structure where ALWC is attacking is above the water level. Various movable and modular cofferdam systems have been developed to allow workers to apply coatings underwater and, in some cases, divers install the repairs. These measures can add significant cost to any mitigation scheme.

Regardless of the mitigation option, owners and their asset management contractors require education about what to look for to correctly identify if ALWC is present. Early identification provides time to employ a lower cost mitigation measure. In some cases, ALWC does not present as shown in a textbook and so samples of the corrosion or steel coupons are taken and sent to a lab for verification.

The worst potential outcome is complete replacement, like was required for a dock on the North coast of Quebec. Several steel sheet pile cellular structures built in 1971 were found to have ALWC in 2003 and no feasible rehabilitation options were available due to the extent of the corrosion. New, concrete caissons were installed offshore of the steel structures in 2007 so that the steel structures could be left to fully corrode safely.

For new structures, several precautions can be incorporated into design. Uncoated steel structures can be avoided altogether or pre-wired so that an induced cathodic protection system can be added easily later. But the yearly costs of electricity and maintaining the system can be high. If steel is being used, robust coating systems can be applied to the exposed surfaces of the steel. Several marine structures in B.C. for instance, including Vancouver Convention Centre West, have a polyethylene coating on the steel pipe piles that was applied at a factory and is what is often used for protecting buried oil and gas pipelines. The polyethylene coating is so robust that marine contractors have had to use woodworking routers in the field to remove the coating so pipe sections can be welded together. There is also a new grade of steel called AMLoCor that has been developed to lower the rate of corrosion from saltwater and ALWC.

Photo sourced from Port Technology International A significant challenge to implementing a mitigation scheme is the location of the problem — the very lowest tidal elevation.

There are currently no regulatory requirements to plan for a future with ALWC; it is up to each owner and each design engineer to decide whether precautions or preventive measures are appropriate. With all the changes currently being considered in new regulations to deal with climate change and earthquakes, such as sea level rise, storm surges, changing weather intensity, and increased maximum credible earthquake, something like ALWC can easily be overlooked. Based on the recent experiences at the Port of Alaska, the business case justifying making an investment in prevention at the design level is clear.

Daniel Leonard is an experienced senior engineer, project manager, and executive with a demonstrated history of working across several sectors in the consulting industry with a primary expertise in port planning and marine structures. He can be contacted at 1 604 770 4787 / dleonard@westmaradvisors.com.

Photo courtesy Daniel Leonard, Westmar Advisors

Accelerated low water corrosion (ALWC) caused by a form of bacteria can corrode steel at a rate up to 10 times faster than saltwater. Left unchecked, ALWC can become a costly challenge for ports to replace their infrastructure.