Ask for help on the causes of corrosion of marine seawater filters
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Dear experts and teachers, we have supplied dozens of seawater filters of DN700 and DN800 specifications to the ship. The cylinder body is made of welded carbon steel. The surface treatment is galvanized to 100 microns and then sprayed with seawater paint to 100 microns. The actual construction thickness is greater than this value. The cylinder sizes of the two filters are the same, but the diameters of the inlet and outlet pipes are different, which are DN700 and DN800 respectively. Since the end of last year, we have received complaints from shipping companies. The inner cavities of all DN800 products have been severely corroded, and some filter cylinders have been rusted through. The following is the details of one of the DN800 filters.: November 20: That month we boarded the ship for simple polishing and rust removal, and applied seawater-resistant epoxy paint again. In March and April this year, the ship reported that the situation was even more serious.: This month we received another complaint about DN700 specifications. We only received this one complaint about DN700. It has only been two months since it was installed and put into use. The 12mm thick steel plate cylinder has already rusted through.: Corrosion is concentrated in the welded parts of the filter barrel and the pipe. But we think the rate of corrosion is faster than we realize. The filter valve cover is equipped with a sacrificial anode and a protective cathode. The relevant suppliers believe that it has nothing to do with their anode and cathode. The design flow rate of DN800 filter is 8250m3/h. The DN700 specification filter that suffered rust damage was installed and constructed at the same time using the same process and is currently in good condition. Both ends of the filter are centerline butterfly valves with rubber valve seats, and the back pipe is made of fiberglass. We have supplied many products with the same medium and similar design to other shipping companies, and there has never been such a serious corrosion problem. All kinds of information are quite confusing. Could you please help me analyze the possible reasons? Thanks in advance!
Is the welding joint uneven or insufficient?
I make polyaniline heavy-duty anti-corrosion coatings. It has very good performance in marine anti-corrosion. I have done a lot of marine submersible marks. You can learn more about it.
Galvanized and anti-corrosion filters cannot be welded, as welding will destroy the galvanized and paint protective layers. It is best to use flange connection.
Agree with the above. It may be better to insert the pipe into the cylinder for a short distance at the connection point between the pipe and the cylinder.
The corrosion is concentrated in the welding part of the filter barrel and the nozzle. This is because the anti-corrosion layer was destroyed after welding of the anti-corrosion barrel and nozzle, and the later repair of the anti-corrosion layer did not meet the requirements. What should you pay attention to when doing on-site repairs like this?: One is the surface treatment of metal (according to the requirements of the coating used) ； Second, remove the sharp weld bead (with some rounded corners) and welding spatter and slag on the nozzle. ； Third, the treated parts should be painted with seawater paint and its matching primer, and the thickness should be greater than 250 microns. If conditions permit, some high-performance anti-corrosion materials can be used to prevent corrosion.
Thank you for your careful analysis and guidance. I didn't make it clear, but these filters are welded first, then shot blasted, hot-dip galvanized, and then painted with seawater-resistant paint. I feel like this corrosion rate shouldn’t be considered normal, right? In 2 months, the 12mm thick steel plate was rusted through.
For corrosion caused by the welding joints at the entrance, the overall thickness of your anti-corrosion system should not be thick, especially if no special treatment is done on the welding points, the thickness of the coating at this point should be thinner. After the bottom of the welding part leaks, the cathode peeling area also develops along the welding seam like the cylinder wall. There should be issues with both workmanship and materials.
The seawater corrosion rate can indeed reach such a speed at this welding site, and the weld seam has different potentials.
I think there are two reasons: electrochemical corrosion of the weld, and the other is that the anti-corrosion of the weld is not done well.
There are many specific corrosion factors, mainly from the quality of anti-corrosion paint, coating, welding treatment and other combined reasons. For specific details, you can call Tanggong to consult. The penetration corrosion of chloride ions should be paid great attention to.