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December 11 - December 17, 2016

Should the surfaces of flanges be coated to prevent corrosion? Why or why not?


Selected Answers

From mauricio gomez of jifco on January 3, 2017:
In my own experience working on fabricated piping for different water districts ( East Bay Municipal Water District, Metropolitan Water District of Southern California, Eastern Municipal Water District),  every one has its own requirements for faces of the flange serrations, and some require coating, depending on what kind of service the piping is going to experience, such as sewer water, drinking water, hot air, or injection of chemicals, etc.  Determining when to coat is  the job of the project engineer or the person who writes the job specification!!!

From William Gusnard of Southern Company on December 27, 2016:
No, the average coating thickness is 3-5 mils. A 1/8-inch-thick gasket does not have enough compressibility to compensate for a coating on both flanges. This will leave gaps in the gasket which are now leak spots. I started life as a piping design engineer and am now a coating specialist who also handles the piping specs as well.

From Mohsen Saeedikhani of NUS on December 20, 2016:
Consider the flange face is coated by a good paint, for instance,  zinc-rich . Does the paint survive under high internal pressures such as  200 bar (2900 psi)?

From bart de cremer of acotec nv on December 20, 2016:
Of course, flanges should be coated wit a solvent-free epoxy to prevent corrosion and rust creep.

From Sal Bustos of MWD on December 19, 2016:
The answer is Yes and No. As a coater/applicator, and now manager, I've seen both coated and uncoated flanges on water-deliver systems with raw and treated water. Most notable were on the older systems where flanges were coated with a red lead primer. With that said, I'd say coat the exposed portion of the flange with an appropriate coating for the service condition and leave the gasket sealing area bare or coat lightly with a zinc coating. The thought here is, generally, a gasket grease is applied and the bolt-down pressure will seal as well as any coating applied, thus keeping oxygen out and corrosion to a minimum.

From Paiboonsak Saengsomboon of JT Marketing Co., Ltd. on December 15, 2016:
It is necessary to coat the flange's face  to prevent the crevice corrosion.

From Warren Brand of Chicago Coatings Group on December 13, 2016:
Having worked on 100s, perhaps 1000s of projects involving flange and manhole faces, we tend to coat the faces for the same reasons William mentioned. We tend to apply the coating system a bit thicker on the flange face in case the owner needs to lightly sand the coating system to make it smooth and/or perfectly flush. Of course, the edge of the flange face is critical and we usually stripe coat that right angle, before coating the face.

From Per Gabrielsson of Free Lance Consultations and Inspections on December 13, 2016:
If the flange face is coated with multiple layers, tightening problems may occur and the coatings may split. A single coat, for instance, of 1-component, high-zinc-content polyurethane would be a reasonably good solution (does not require a blasted, rugged profile as inorganic zinc does).

From William Slama of International Paint/Ceilcote Products on December 12, 2016:
We have encountered this many times in the past where flanges get severe corrosive attack. For some reason, some plant people or applicators do not want to coat (or line) flange faces. To me, that is unbelievable. One must look only at the effectiveness and actual expected sealing area that the gasket accomplishes. At the very least, there will be a line at the edge of the flange that is exposed to the chemical solution. More likely, the actual "sealing" line will be a bit out onto the flange. Given that the flange face can easily be coated or lined, I don't see any reason not to line them with the same system used in the vessel.

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Tagged categories: Corrosion; Corrosion control coatings; Corrosion protection


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