Industry searching for better ways to reduce ballast tank corrosion
Improving the protection of ballast tanks from corrosion is an issue of increasing importance to regulators, shipyards and ship owners.
In January 2006, Intertanko and the International Association of Classification Societies (IACS) announced new revisions would be made to the IACS standards for corrosion prevention coatings in seawater ballast tanks.
The purpose of the new guidelines is to “assist IACS inspectors in making a better assessment of the condition of ships’ coatings,” according to Intertanko. “The guidelines would also set minimum standards for coating maintenance and for proper coating repairs.”
For newbuilds, recommended design thickness and composition of steel used for new ballast tanks are being reviewed as the new rules are being formulated. Failure of ballast-tank coatings and subsequent corrosion damage throughout the life of a ship are realities that must always be considered.
The new IACS rules will “reflect the different rates at which different areas of the ship corrode in service,” said Dr. Kirsi Tikka, vice president of special projects at the American Bureau of Shipping.
The problem of protecting ballast tanks is compounded by the extent of surface that must be coated and by the difficulty of reaching the areas that need protection.
“The ballast tanks represent more than 50 percent of the entire coated area per vessel and, as large parts of the ballast-tank areas are inaccessible during service, the quality of surface preparation and application is of crucial importance,” according to a Hempel (USA) Inc. technical report.
A report by Ameron International Corp. also noted new challenges: “The double hull initiative is creating millions of square meters of ballasted spaces in tankers and other types of vessels that previously used that space for the carriage of cargoes. In the case of crude oil carriers, the cargo itself had anticorrosive properties and eliminated the need for coatings in what used to be referred to as the wing tanks of a single hull vessel.”
In a report issued by Det Norske Veritas about rules for protective coatings, the authors Erik Askheim, Olav Nakken and Bjorn K. Haugland observed that corrosion of ballast tanks is often the reason for scrapping of ships.
The proposed IACS rules are largely the result of problems experienced with coatings in the ballast tanks of tankers and bulk carriers. But the industry is also addressing ballast-tank issues for other types of vessels. New international rules for ballast water exchanges at sea have drawn attention to the long-term structural integrity of ballast tanks. In March, the IMO Sub-Committee on Ship Design and Equipment moved to require a new performance standard for all dedicated seawater ballast tanks constructed of steel on new ships of 500 gross tonnage or larger.
One of the driving forces behind current developments for better ballast-tank corrosion protection is the U.S. Navy’s Naval Sea Systems Command’s Materials Engineering group. It has selected five coating systems for corrosion protection of seawater ballast tanks and other critical shipboard applications. Sherwin-Williams Dura-Plate UHS, Sigma Coatings Edgeguard, Sigma Coatings Sigmaguard BT (control), Jotun 591, and International Intergard 180 all meet the Navy’s requirements for new low-VOC/HAP content for the new solventless edgewrap coating systems.
One of the Navy’s major goals is the development of coating systems with a working life of 20 years instead of the now common five to seven years. The Navy anticipates savings of millions of dollars during the next 20 years by using these new coating systems to reduce the costs — labor, materials, hazardous waste disposal — associated with the five- to seven-year cycles for ballast-tank painting.
Eric Bosanac, director of marine and offshore programs at The Sherwin-Williams Co., noted, “Initial results for what the Navy has done have been very positive and resulted in a drastic decrease in maintenance and recoat work they had to do.”
For commercial vessels, achieving 20-year cycles for the ballast-tank coatings may be more difficult because of varying quality standards during maintenance work at the world’s shipyards.
“A good coating applied on a well prepared surface at the newbuilding stage is the most cost effective means of avoiding corrosion,” noted the Det Norske Veritas report. However, it pointed out that “Coatings applied by crew or in repair yards on ships in operation will often have a rather short useful life … because the surface preparation and strict control of temperature and humidity conditions necessary for a good result are not obtained.”
Paint manufacturers understand the coating application challenges facing shipyard workers performing repair work. “Ballast tanks are complicated structures with many angles and stiffeners creating the complicated geometry that achieves structural integrity but can be hard to work in,” said Bosanac. “Every surface must be properly prepared and paint coats applied without any slights or misses for top quality.”
Standard procedures require the painter to mix the two-part paint product, then enter the ballast tank frequently by a ladder to apply the repair coating.
One solution from Sherwin-Williams is a two-component cartridge packaging and dispensing system designed to reduce mixing preparation time to 10 seconds and with a guaranteed selectable mixing ratio for the application. Bosanac explained that waste is reduced since only the amount of coating immediately needed is mixed and the partially used cartridges can be capped for later use. Other variations of the new system allow shipyard workers to use a small pump unit for larger areas or a pneumatic gun that atomizes the paint product.
Ameron’s high-solids, low-VOC Amercoat 240 Universal Epoxy uses “well proven surface tolerant technology” and provides the benefit “of being able to be applied during a wide variety of ambient climate conditions,” according to an Ameron report.
To minimize shipyard time, the report noted some vessel operators have tank coating work performed at sea while the ships are in service. However, the report cautioned, “Many high performance tank linings, even though they may exhibit some surface tolerance qualities, may not have the rheology (fluid-flow characteristics that greatly affect application characteristics) for ease of use under this most adverse of conditions.”
Paint manufacturers are continually developing better coating products to protect seawater ballast tanks, while exceeding environmental standards. Ballast-tank coating repairs after a ship has been in service are not unusual, but should not be the result of false economy measures during the ship’s initial construction. As the Det Norske Veritas report stated: “The application of high quality coating in seawater ballast tanks at the newbuilding stage is cost effective compared with upgrading by means of steel renewals later on.”