Can you paint, grease or seal the flange joints of a flameproof enclosure?
Abstract
Grease, yes: a thin film of non-setting corrosion-inhibiting grease on clean joint faces is allowed. Paint, sealant or added gaskets on flamepath faces, no. Grease-bearing tape outside a flanged joint is allowed only within strict code limits (one layer, never in IIC).
You may grease the joint faces, but you must not paint, glue or gasket them. A thin film of a non-setting, corrosion-inhibiting grease on clean flamepath faces is the normal way to protect an Ex d joint. Paint, powder coating, silicone and other hardening sealants, and gaskets that are not part of the certified design are not allowed on the faces or in the gap. Grease-bearing tape wrapped around the outside of a flanged joint is a special case that installation codes allow only within tight limits.
Short answer
- Grease: allowed. IEC 60079-1 permits corrosion-inhibiting grease that does not harden with age, contains no evaporating solvent and does not corrode the joint. Use the type the manufacturer names.
- Paint and powder coating on the joint faces: not permitted by IEC 60079-1. Other coatings are possible only where the manufacturer has shown that they do not affect the flameproof properties, which is a design matter, not a site choice.
- Sealants, thread-locking compounds, silicone: not on flamepaths. A draft IECEx decision sheet (2016) explains that a joint with a hardening compound can produce hot particles in an internal explosion once the joint has been disturbed.
- Gaskets and O-rings: only where the certified design includes them. IEC 60079-1 treats them as a supplement that must not change the gap or interrupt the flamepath width.
- Tape around the outside of a flanged joint: at most a single layer of non-hardening grease-bearing tape, generally for IIA atmospheres only, never IIC. Follow the code and the instructions that apply to you.
Why the joint faces must stay bare
A flamepath cools the hot gases of an internal explosion as they squeeze through a narrow, long gap. Its gap and width were tested and certified with clean metal faces. Anything that changes the faces changes the joint:
- Paint and hard coatings fill or bridge the gap unevenly, can stop the cover closing fully, and can hold the joint open when they flake.
- Hardening sealants and thread locks set as a solid layer. Once the joint has been opened or disturbed, the broken layer can produce hot particles that are ejected through the gap during an internal explosion.
- A gasket added between flange faces increases the gap by its own thickness and can stop the metal faces meeting at all.
That is why IEC 60079-1 permits only grease that stays soft and plating up to 0.008 mm without further assessment, and why IEC 60079-17 asks inspectors to check that flange faces are clean and undamaged and that gaskets, if fitted by design, are in good condition.
How to protect flamepaths against corrosion
- Clean the faces with a non-metallic scraper and a non-corrosive cleaning fluid. Do not use metal tools or abrasives on the faces.
- Inspect for pitting and scratches. Damage across the width of the joint is a repair matter under IEC 60079-19, not something to fill with sealant.
- Grease lightly with the grease specified by the manufacturer. IEC 60079-17 asks for blind bolt holes to be kept clear of grease, so that the bolts can seat fully.
- Refit the cover with every bolt, of the specified type, tightened to the specified torque.
- Shorten the inspection interval in salt-laden or wet locations rather than adding layers of protection that are not certified.
| What | On flamepath faces | Notes |
|---|---|---|
| Non-setting corrosion-inhibiting grease | Allowed | Type named by the manufacturer; thin film |
| Paint, powder coat | Not allowed | IEC 60079-1 |
| Silicone, hardening sealant, thread lock | Not allowed | Draft IECEx guidance: hardening material only on threaded joints not meant to be serviced, tested and stated on the certificate |
| Added gasket or O-ring | Not allowed | Only gaskets that are part of the certified design |
| Grease-bearing tape outside a flanged joint | Restricted | One layer; generally IIA only; never IIC |
Tape: why the limits are so tight
Wrapping non-hardening grease-bearing tape around a flanged joint is an old weather-proofing method. Tests by the former UK Safety in Mines Research Establishment, as reported by an engineering training institute, found that one layer had little effect on the safe gap with a IIA gas, but reduced it by about 50 % for IIB and about 80 % for IIC. Older British practice therefore allowed tape on IIB equipment only after consulting the manufacturer. The tape changes how the hot gas leaves the joint, and the user cannot compensate because the gap is fixed by the certified design. Repair guidance under IEC 60079-19 also refers external tape back to the installation code. Tape is therefore never a substitute for grease, and it is never used in IIC areas.
Common mistakes
- Painting a whole enclosure, cover joints and nameplate included, during a site repaint. Mask the joints, breathers and labels.
- Running a bead of silicone around the cover to stop water ingress.
- Fitting a gasket from a general-purpose enclosure because the original "had none".
- Using two or three layers of tape, or using tape on IIB or IIC equipment without checking the code and the instructions.
Related on excodex
- Flameproof enclosures (Ex d): how flamepaths stop an internal explosion
- Common Ex inspection findings and how to fix them
- Inspection of Ex installations under IEC 60079-17: initial, periodic, visual, close and detailed
- How are flamepath gaps on an Ex d enclosure checked during inspection?
- How close can a wall or pipe be to the flange of an Ex d enclosure?
Standards and further reading
- IEC 60079-1:2014 (edition 7), flameproof enclosures "d": corrosion protection of joint surfaces (no paint or powder coat, non-hardening grease, plating) and gaskets as a supplement; publicly available as incorporated by reference: law.resource.org. A 2026 edition has since been published.
- IEC 60079-14, installation: protection of flameproof joints (tape limits) and the grease rule for parallel-threaded entry devices; the Polish standards body's interpretation of the grease rule: pkn.pl
- IEC 60079-17, inspection and maintenance (current edition 2023); the 2007 edition, adopted in India as IS/IEC 60079-17, contains the cleaning and greasing rules paraphrased above: law.resource.org
- IEC 60079-19, repair (current edition 2025); 2006 edition as IS/IEC 60079-19: law.resource.org
- IECEx ExTAG draft decision sheet on non-metallic seals of threaded joints (hardening materials, testing): iecex.com
- Engineering Institute of Technology, Ex d weatherproofing alert (tape tests by the UK Safety in Mines Research Establishment): eit.edu.au
This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.