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April 29 - May 3, 2019

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

Selected Answers

From Ron Berry of NZ Corrosion Services Ltd on May 23, 2019:
Always prepare and paint up to the record face with zinc and an HBE. Record faces should not be blasted as this can have an effect on the sealing, depending on the pressure rating of the flange.

From Simon Daly of Hempel A/S on May 10, 2019:
The need for flange face protection, of course, depends upon many factors. However, protection of the sealing face may be required depending upon the corrosivity of the fluid, gasketing arrangement etc. Even precision-machines faces may suffer corrosion in certain circumstances. The use of polymer materials across the sealing area is now a common method of protecting flange faces but is still not widely adopted due to the extra time and effort required. This can be used successfully, even in high pressure systems. Application requires specific care and attention to allow correct sealing of the gasket but once completed will offer long-standing protection, even in highly corrosive situations. Additionally, the extension of internal coatings into either a rebate groove on the inner flange edge or across the flange face provides a more effective way of terminating the coating and isolating it from the effects of fluid flow.

From subodh kulkarni of larsen and toubro on May 7, 2019:
Surface of flanges are required to be coated to prevent corrosion. Flange faces including bolt holes are to be considered as external surfaces as they are exposed to the environment and hence to be coated to prevent corrosion. Gasket faces need not be coated as they are precision-machined and are not exposed to the environment.

From Marco Fabio Ramenzoni of ZINGA METALL BRASIL on May 3, 2019:
Absolutely! I agree with Mark Maresko. Yet, my preference, always when possible, is for active cathodic protection applied before assembly and easy to repair and reload the active content of the coating. For a machined face, a possibility is to use a soft material layer with the same active anode content.

From Mark Maresko of NA on May 1, 2019:
Certainly, all metal that is vulnerable to corrosion should have some kind of corrosion control measures such as protective coatings, corrosion allowance, cathodic protection, or chemical control. Applicable codes such as ASME and the requirements of the gasket manufacturer should be satisfied. In my experience the machined flange face is not coated (except for temporary protection provided by a removable rust preventative) because the gasket requires very specific tolerances to be effective. Coating over a machined face negates the precise tolerances achieve through machined metal surfaces. In addition, the fluid (water) is often not corrosive to steel due to chemical control. Even in the event of potential corrosion, the rate may be acceptable due to the thickness of the metal. Keep in mind that my advise is based on my perspective, mostly in the power market and process piping. The right answer requires consulting the process engineer, corrosion engineer, gasket manufacturer, and coating manufacturer, considering the specifics of your particular project.

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Tagged categories: Corrosion protection; Quality Control

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