Flanges can go through two types of corrosion in their lifetime: Crevice and Pitting. Pitting Corrosion occurs on the face of the flange and often appears in groups or clusters. This kind of corrosion causes small pits or cavities to form on the surface of the material. Pitting corrosion is best prevented by proper alloy selection. A very high rate of Crevice Corrosion occurs when there is a build-up of concentrated substance between two adjoining flanges. It is the type of corrosion that can be very damaging as it is not easy to inspect the areas where it is occurring.
The method of monitoring flange damage is to disassemble the joint and visual inspection of the surfaces. It is not ideal due to the downtime and cost but is necessary. There has been some development using some non-destructive techniques to inspect the amount of corrosion to the flange and fitting assembly.
Methods of prevention
Maintenance of paints is the most common method to prevent corrosion. The paints are based on epoxy or urethane. Most paints are likely to bond directly to the substrate as a hard coating. While the use of maintenance paints, there must be an ideal amount of application. If the paint is too thin, the area tends to be ineffective, and if the paint is too thick, it will cause seizing to the fastenings. There are different flange sizes, shapes, and angles – you may not be able to coat the whole area. After the inspection of the flange, another layer of coating needs application. A benefit to using maintenance paints is that it does not require specialist equipment or hot work.
Mechanical solutions are usually rigid clamps, or coverings encapsulated in the flanges. These clamps and covers are made from plastic or stainless steel. Another solution is the use of semi-solid tapes. They can be composed of viscoelastic polymers, wax, petroleum embedded into the fabric for wrapping. These are used specifically for their water-repellent nature of semi-solid polymers.
All preventive methods come with pros and cons. There is no solution out there that may entirely prevent corrosion. Regular inspections and maintenance can come out to help prevent rapid corrosion.
Four ways to repair corroded flanges
Add polymer composite to rebuilt the flange face.
Add material to seal face or ring grove and then machine within flange tolerance.
Machine the ring grove or sealing face within the flange tolerance.
Removing the damaged flange and weld on a new one.
Three factors to influence corrosion resistance during operating conditions
Corrosive agent concentration.
Purity of corrosive agent.
The temperature of corrosive agent (higher temperature means more rapid corrosion).
A few flange materials are subject to stress corrosion cracking. The fact must be considered while the selection of gasket type and materials. When you select a proper gasket, it can be helpful to prevent flange corrosion from aggressive chemicals.
Repairing Corroded Flanges
When a flange face is damaged, it is no longer sealable by a gasket and must be either repaired or replaced. In general, there are four basic kinds of repairs to consider:
Removal of the corroded flange and welding a new one.
On-site machining of the sealing face or ring groove within the flange tolerance.
Adding materials and site machining of the sealing face or ring groove.
Using polymer composite repair materials to rebuild the face of the flange.
Portable on-site machines that can help.
Flange Face Repair
A flange facing machine is the ideal product to machine the flange face sealing surface. It helps to get the job done accurately and quickly. There is a wide range of options available for you. It includes models that can be mounted externally or internally. Smaller machines help face diameters as small as 2” and the largest capable of facing up to 120” diameter.
Additionally to machine a conventional flat sealing face, machines with various attachments or conversion kits for machining RTJ grooves and compact flange joints.
When it becomes impossible to repair the flange, the remaining option is to remove the flange entirely. You can carry it out by using a split frame/clamshell pipe cutter. As per the suggestion, the machine comprises two sections fixed and clamped together around the pipe diameter.
You can prevent flange corrosion with many methods, as different types of flange require different ways to deal. When you deal with chemical oxidizers/HF acid, it is essential to select a sturdy gasket that will prevent corrosion.
As a preventive solution on plant equipment, prolong its life and avoid costly shutdowns, it is best to use recommended torque values and proper bolting techniques when you install a gasket. Regular inspections can prevent rapid corrosion from happening.
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