: CORROSION REMEDIATION
The mysterious orange slime heading to eat your port By Daniel Leonard, P.Eng, Vice President, Westmar Advisors Inc.
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peak 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 38 — PACIFIC PORTS — September 2021
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.
