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May 20 - May 26, 2013

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



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Selected Answers

From Simon Latty of MCT Group on July 8, 2013:
There is a very good argument for coating flange faces, new and old. Most joint faces are often subjected to various types of corrosion, which can go unnoticed until it is too late, and the flange needs to be replaced, which would involve hot work and PWHT, neither of which is favorable, especially on a live plant. Polymeric coatings have been used extensively for refurbishment of deteriorated flange faces and for protection of new flange faces prior to going into service. It is even possible to replicate the serrations in an appropriate coating, and this type of treatment can eliminate further corrosion, which means that in many cases a coated/refurbished flange face can be considered better than a new one. As for pressure retention, if carried out correctly, this type of flange face coating can withstand pressure in excess of 150 bar.

From corey harink of Desert Blasting & Painting Ltd. on May 29, 2013:
I coated over 14,000 raised face flanges last year along with internals of spool pipe. We coat the internals for protection against the fluid flowing through the pipe. Without coating the flange faces, there would be no point in coating the internal spool at all. We have been coating this many flanges a year for more then 15 years and we never have issues with it. There does have to be some consideration for changing the gasket type to accommadate for the coated surface... linebacker gaskets are the norm here for that purpose. Spiral wound gaskets work; however, the metal rings in them cut the coating and if left bolted together, there are normally no issues. However, if taken apart, normally the coating on the flange face has damage, and re-use with out remedial work is not common. So for corrosion protection of internally lined pipe with raised face flanges, YES you must definitely coat the faces.

From Atanas Cholakov of ACT on May 25, 2013:
Normally, the raised surface of a flange is never blasted or painted. (Those surfaces are sometimes referred to as machined, and this is where the gasket sits.)  The rest of the flange surface, though, is blasted, and a coating is applied. If the question originates from a project, the best answer can be found in the coating spec. Information should be provided, or else prepare an RFI and seek clarification.

From Robert Mexter of BC Hydro on May 23, 2013:
I have to assume you are referring to a flanged or bolted sections of some type of structural member? If so, I would agree that the section (flange) should be coated. In most cases we use an inorganic/organic zinc rich primer on all structural flanges. Overall, this has worked well, and we have never had any type of corrosion issues with our flanged areas.

From Brian Chapman of Cadillac Fabrication on May 23, 2013:
I submitted this question with respect to flanges on large industrial duct work....not piping and vessels. My original question was how to deal with plate and angle flanges on ducts.

From Mario Serra of Saras on May 22, 2013:
Which kind of corrosion? Internal or external? Normally, the answer is yes on both surfaces when required, but not in the seat gasket surface because a proper roughness and hardness is necessary to guarantee the tightness. At the same time, corrosion in the flat surface beside the gasket is found  to require maintenance.

From William slama of International Paint/Ceilcote Products on May 20, 2013:
This reply is with respect to vessels (or pipes) containing a liquid that is corrosive to the flange material. With typical carbon steel vessel construction, almost all aqueous solutions will eventually (or much sooner) corrode the steel surfaces. That, of course, is why the vessel is coated (lined). It is imperative that the gasketed surface of any flange seal inside of the bolt ring; otherwise it would leak through the bolt holes. However, depending on the flange and gasket design, the seal may occur a little out onto the flange face. Even if the nozzle surface coming up to the flange is coated, there will likely be a line of attack available where the edge of the flange is exposed to the corrosive. So the only reliable procedure is to coat/line the surface of the flange. In doing that, the resulting surface of the lining on the flange face must be smooth and flat enough to accomplish a gasketed seal when bolted up to a similar flange of flat cover. For thick linings a special "pressing" technique is used to produce the flat smooth surface required. The simple rule is do not let the corrosive touch the steel.

From Bonny Njimogu of Construction Specialist Services Ltd. on May 20, 2013:
Surfaces of flanges should be greased rather than coated to enable easy gasket removal.Coated surfaces develop rust fast especially after coating failure.

From William Gusnard of Southern Company Services on May 20, 2013:
My answer to this question is no. I have been in piping design a very long time and one of the problems with coating flanges is that the plus/minus thickness allowed on coatings creates a problem with the flange gasket in that it will not seal as well with a coating material applied to the flange face. I recently worked om a project where we calculated these numbers and in order to get a good seal, we needed to go with a thicker than normal gasket, which then created stress problems on the flange. My coating spec has details in it to show how to properly coat a flange.

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