How to Protect Flanges from External Corrosion

Exposure to offshore salt spray, general weathering and chemical contact can all impact flanged connections. When bolts and flanges are made of dissimilar metals, corrosion can develop and spread even faster.

Belzona 3412, a liquid-applied, moisture-curing membrane system, can protect flanges from corrosion for the long term. Unlike conventional paints, it offers excellent edge retention and film build around complex geometries. Bolts can be easily accessed by cutting and peeling the membrane (when Belzona 8411 release agent/corrosion inhibitor is also applied as part of the system). The system can then be resealed again to protect flanges from corrosion.

The system can be brush- or spray-applied. Let’s look at the spray application.

Supplies & Tools Needed for a Flange Corrosion Protection Application:


  • Mix Belzona 3412 (or Belzona 3411)

Mix full units of the base and solidifer thoroughly in accordance with the relevant IFU in a mixing bowl with a spatula provided. Belzona 3412 is a two-coat system, available in grey and orange. (Belzona 3411 is available in grey and beige)

  • Fill cartridge

Immediately transfer mixed product into the cartridge. Belzona 3412 and Belzona 3411 are moisture-cured and will not cure until being sprayed onto the flange. This convenient feature enables several cartridges to be filled and used over several days on larger jobs.

When ready to apply, cut the end of the cartridge thread horizontally. Re-tighten the cartridge support tube until it closes.


  • Adjust air flow

Tighten the Jet control handwheel clockwise to obtain a gradual closing of the air passage and therefore a reduction in atomisation (more orange peel effect). Loosen the Jet control handwheel anti-clockwise to obtain a gradual opening of the air passage and therefore an increase in atomisation.

  • Adjust product feed

Tighten material control handwheel clockwise to obtain a gradual reduction in product flow. Loosen anti-clockwise to increase flow. Combined adjustment of the handwheels enables the user to obtain varying degrees of smoothness in the finish. The distance from which the material is sprayed determines the variation in characteristics of the spray itself.

  • Pull trigger to spray

The gun trigger activates with a 2-stroke principle. The first activation delivers air via the jet adjustment handwheel which must be open, while the second stroke also activates material delivery.

  • Connect the spray gun to air system

Connect the gun to a compressed air system featuring the following characteristics: maximum air humidity: 5%, air temperature range from -10°C to +70°C, system flow rate of at least 500 l/min, system equipped with lubrication.

  • Surface preparation, application of the bridging tape, Belzona 8411 and bolt caps

To protect flanges from corrosion, manual surface preparation is sufficient to achieve optimum adhesion between the membrane and the substrate. If the flange is painted, use abrasive paper to roughen the termination points. If the flange is corroded – a wire brush. Apply bridging tape over the flange. Protect prepared termination points with masking tape. Spray or brush-apply release agent/corrosion inhibitor, Belzona 8411, and fit the bolt caps.

  • Application of Belzona 3412 (or Belzona 3411)

Remove masking tape from the termination points. Apply masking tape or release film at the edges for a neat finish. Spray apply first coat of the Belzona membrane. Add the reinforcement tape, Belzona 9311, at the termination points and the centre of the flange. Apply the second coat and remove masking tape.

  • Completed application and inspection

Belzona’s corrosion protection solution has been tested for 6000 hours in a salt spray cabinet. It is also resistant to UV and general weathering. Protection will last until the next bolt inspection when the system can be simply cut, peeled and resealed to reinstate protection.

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