Seawater Pipe Coating Guide in China

Whether you are involved in a new building or conducting a repair docking in China, options for corrosion and erosion control for steel seawater pipelines will be similar. Most shipowners want to maximize the amount of work done in the dockyard because it saves cost. Whether a pipe renewed or repaired in a dockyard will last until the next docking, or will require an expensive intermediate voyage repair, is largely a function of the coating. We will take a look at the pros, cons and price considerations for the different options.

Galvanizing: If the objective is for the pipe to last to the next docking, galvanizing will not be a good option in most situations. Specifically, galvanizing does not perform well in systems with high flow rates and turbulent flow. Pipes with reducers attached to main sea water pumps, in the vicinity of central coolers and freshwater generator piping are examples of pipes that will likely not last if galvanized. That being said, there is a huge difference in performance between a properly galvanized pipe and a pipe that is not properly galvanized. If you are going to use galvanizing, the rules for proper fabrication (all internal welds must be ground smooth) and surface preparation (pickling) still apply, despite many dockyards attempts to just dip anything in molten zinc. Welding on any coated pipe will destroy the coating and this also applies to a galvanized pipe. Galvanizing begins to flake at about 250 degrees which is easily exceeded during welding processes. Beware the can of silver paint. I commonly see galvanized pipes being cut and re-welded insitu at dockyards followed by a spray from the infamous silver paint. In addition to the toxic fumes that result from oxygen cutting or welding on a galvanized pipe, silver paint is not galvanizing and will not protect the pipe. Dockyards frequently promote galvanizing because their profit margin is huge. A dockyard may pay less than USD250 per MT to galvanize a batch of pipes. If this includes proper pickling is often a mystery. The price to the owner is much higher resulting in a big profit for the dockyard.

Vulcanized Rubber Coating: This is a 2-6mm thick sheet of rubber that is cut, fit and glued to the interior of the pipe and then vulcanized in an autoclave. The major issue with vulcanized rubber coating is that you need a skilled technician to do it properly, especially in a pipe with branches, elbows and stub pieces. Most failures in rubber coated pipes are found on the seams. Proper fabrication is very important since any sharp edges on the welding seams will result in pierced coating and a strainer full of rubber or fouled valves. For applications with slurry or abrasives, this is the best coating available. If you intend on using rubber lining thicker than 2mm, you had best fabricate the pipe with a coating allowance or the rubber lined pipe will not fit. Despite the advantages of this coating, most companies in China are not following correct procedures with the result that many rubber lined pipes will not last until the next docking, although if properly applied, this coating may last more than 10 years. Pipes with diameters less than DN200 with branches or elbows may not be possible to rubber line because the technician cannot access the inside of the pipe. Even for large diameter pipes about 2.8 meter is the longest pipe you can rubber line since most companies do not have larger autoclaves.

Fusion Bonded Epoxy (FBE): When this coating is applied, the pipe surface is between 180 and 250 degrees which results in cross-linking of the polymers. Once the coating is cured it will not melt again even if heat is again applied making this a thermoset coating. Welding will destroy this coating just like all coatings. FBE can also be a good long term coating, but you best use an experienced coating inspector with holiday test gear if you intend on using FBE on your pipes because although you can go as thick as 1200microns (1.2mm), typical coating thicknesses range from 250-500microns. If your coating is only 0.25mm thick, it does not take a lot of imperfections in a welding seam to result in a holiday. This coating is typically applied to the interior of pipes by a spray applicator so elbows, branches and longer pipes can create challenges for proper application. It is very unlikely a spray applied FBE coating can be applied to a pipe with more than one elbow and impossible if more than 2 elbows. If you see a long pipe with multiple elbows and branches and you are wondering how they managed to grind all the welding seams and how they managed to get a spray applicator around all the bends, the answer is that they probably did not do something right and there are multiple holidays in the pipe. More expensive than galvanizing, but less expensive than vulcanized rubber lining or PE lining. If the costs for adequate quality control are added, may end up being similar price to rubber lining or PE lining.

Glass Reinforced Epoxy (GRE): While not technically a coating, GRE pipes are sometimes used because if you have the nominal dimensions, you can fit the flanges insitu resulting in a corrosion resistant pipe that can be immediately fit. GRE pipes are also resistant to sulfuric acid and sodium hypochlorite. From a corrosion resistance perspective, the manufacturers often claim a 15 year service life, but in practice that is rarely achieved. The reason is that a GRE pipe is brittle compared to a steel pipe and after 6-8 years or so or any UV light exposure, they can become more brittle. GRE pipes are popular for voyage repairs because even though they are more expensive than coated steel pipes, you can fit a corrosion resistant pipe in a single attendance. The result is often a system made up of mostly coated steel pipes and a few GRE pipes renewed during voyage repairs. If a GRE pipe is disassembled by the dockyard and you think that pipe will be successfully refit, then that is a big gamble. Many times I have seen GRE pipes cracked at the dockyard by steel pipes being forced into position quite a distance from the GRE pipe. GRE pipes do not like tension, torsion, compression or bending and if a pipe cracks, many cracks are not possible to repair. It is not like a coated steel pipe where if you have a leak, a doubler can be easily welded. This makes them less robust than coated steel pipes and many Chief engineers prefer coated steel pipes for this reason. I am sure the GRE manufacturers will disagree with my characterization, but I was previously an Ameron/Bondstrand/Novus installer and I experienced these problems first hand. GRE pipes are typically the most expensive option, not considering CuNi or stainless pipes.

Rotational PE Lining: First a disclaimer that Rotational PE Lining is my core business and I have patents related to this process and built an entire workshop around refurbishing and PE lining pipes in China. Rotational PE lining is the gold standard for internal seawater pipe coating for the following reasons: A typical DN600 PE lined pipe will have a 4-6mm seamless internal PE coating with the flanges machined to 2-3mm. Branches, stub pieces, bends, elbows, all beautifully coated with no seams. We build a plastic pipe inside the steel pipe which acts as the mold during the rotating and heating process. This is achieved by placing a properly grit blasted pipe on a turn-table that turns in 2 axis, placing plastic powder inside the pipe, placing specially designed end caps and then turning the pipe inside an oven until the PE has melted and is evenly distributed across the internal surface. This process was specifically developed to refurbish old pipes that required a coating thickness sufficient to fill in rough surfaces and inclusions found in old pipes that have experienced failed coatings. The result is an even and smooth internal surface. During the coating and melting process, the PE will flow into gaps and fill in inclusions. The presence of the inclusions and rough surface improve the performance of the PE coating by improving the adhesion to the steel substrate. While pipes still require proper fabrication, a 4-6mm coating is more forgiving than a spray applied or FBE type coating meaning it is easier to achieve the proper quality control. Since no seams, rotational PE lining has none of the issues with seams in vulcanized rubber coating. PE lining for new pipes will be similar price to vulcanized rubber lining. Pipe refurbishment using rotational PE lining is the most cost effective and fastest method of renewing pipes because the old pipe is recycled into a new pipe. The life span of a rotationally PE lined refurbished pipe and a new PE lined pipe will be the same 10 plus years, since the life span is a function of the coating. Rotational PE lined pipes offer excellent resistance to most acids and bases, but temperatures are restricted to under 70 degrees since above that temperature the PE will begin to get soft.

David Schaus is the owner and technical director of Irwin Group of companies that includes Irwin Marine Services Ltd (voyage repairs), Irwin Rotational Lining (pipe fabrication and PE lining), OSRO China Ltd (sludge collection, oil spill clean-up, and ballast water handling and treatment) and Legal Beagle Limited (online CPD/RME training accredited by the Hong Kong Law Society).