Flange face corrosion repair: rebuilding a sealing surface without machining or welding

Why a new gasket does not fix it

A flanged joint seals because the gasket is compressed between two flat, continuous surfaces. Take away the flat surface and no gasket, no torque and no sealant will hold it. When a joint leaks after a gasket change and a torque check, the sealing faces are the first thing to look at, and they are often the last thing that gets looked at, because it means breaking the joint again.

Flange face damage has three common origins. Crevice corrosion under the gasket, where a stagnant film of process fluid sits in the narrow gap and attacks the metal — the mechanism described in the crevice corrosion guide. Erosion at the bore edge, where a turbulent or solids-laden stream washes out the inner part of the face. And pitting across the face from a corrosive process stream that finds its way past a leaking gasket, so the leak causes the damage that causes the next leak.

In all three cases, the problem is a face that is no longer flat and continuous. Adding more bolt load simply crushes the gasket further into the pits without sealing them.

Inspecting a flange face

The inspection is simple and belongs in every gasket change. With the joint open and the old gasket removed, the face is cleaned and examined against three questions.

Is the damage within the gasket seating area? Pitting outside the gasket footprint — on the outer part of a raised face, or on the flat ring around it — is cosmetic. Pitting under the gasket is a leak path. A line of pits that follows the inner edge of the gasket is the classic crevice corrosion signature.

How deep is it? Measured with a depth gauge or a straightedge and feeler gauge across the face. Depth determines the repair option. Shallow pitting of a few tenths of a millimetre may be within the tolerance of a soft sheet gasket. Pits of 1 to 3 mm are the normal territory for an in-place rebuild. Damage that has removed a substantial part of the raised face, or reached the hub, is a machining or replacement question.

Is the face still flat and square? A face that has been overheated, over-torqued or subjected to piping loads can be warped or dished even without corrosion. A straightedge in two directions shows it. Warpage is a machining problem, not a filling problem.

The face finish matters too. ASME B16.5 specifies a serrated finish of 3.2 to 6.3 µm Ra for standard raised faces, which is what a spiral-wound or sheet gasket is designed to bite into. A face polished smooth by erosion, or one that has been ground by hand in the field, may be flat and still not seal a spiral-wound gasket.

The three repair options

Replacement

Cutting out the flange and welding in a new one is the definitive repair. It is also the most expensive, because it involves hot work on live piping, a welding procedure, a hydrotest and usually a longer isolation than the maintenance window allows. On a stub-end and lap-joint arrangement the stub end can be replaced without touching the pipe, which changes the calculation.

In-situ machining

A portable flange facing machine mounts on the flange bore or the outside diameter and cuts a new face in place. It restores flatness, removes pitting to the depth of the cut, and can produce the serrated finish the gasket needs. It is the right answer for a warped face, for deep damage that has to be removed rather than filled, and for ring-type-joint grooves, which have to be machined to gauge.

Its limits are the material removed and the access needed. Each cut reduces the raised face height, and B16.5 raised-face flanges do not have much to give. A flange that has already been machined once may not survive a second pass. The machine needs clearance around the flange and a rigid mount, which is not always available in a congested pipe rack.

In-place rebuild with metal-filled epoxy

For pitting and erosion within the gasket seating area, on a face that is still flat and square, the fastest repair is to fill the damage with a metal-filled epoxy paste and re-form the face flat. The repair is done cold, with no welding permit, no grinding sparks and no machine to mobilize. It is the flange-face application of the cold bonding and metal repair family of products, and the same rules on surface preparation and product selection apply.

How an epoxy flange face rebuild is done

The procedure has five steps, and the quality of the result is decided in the first two.

Surface preparation. The damaged face is abrasive blasted or, where blasting is impossible, mechanically abraded to bright metal with a coarse profile. Pits are opened out so the paste can reach the bottom; a pit with a narrow mouth and a wide base traps air and fails. The face is then degreased with a solvent that leaves no residue. Any trace of process fluid, oil or old gasket material in the pits is a future bond failure.

Product selection. Metal-filled epoxy pastes are formulated for exactly this use: high compressive strength, machinable after cure, and resistant to most process fluids. The product's service temperature and chemical resistance are checked against the line conditions from the data sheet — typical metal-filled epoxies are rated for continuous dry service somewhat above 100 °C and lower in wet or immersed conditions, so a hot line needs a high-temperature grade. Bolt load is also a consideration: the cured paste must carry the gasket seating stress without creep.

Application. The paste is pressed firmly into the pits and over the face, working it into the profile with a spatula to exclude air, then built slightly proud of the original surface.

Forming the face. A flat plate — a machined blank, a piece of thick glass or a plate faced with a release film — is coated with release agent and pressed onto the wet paste, aligned to the flange, and held until the paste cures. When the plate is removed, the face is flat, at the original height and with the surface texture of the release film. For flanges that need a serrated finish, the cured face can be lightly machined or a serrated forming plate used.

Cure and inspection. The paste cures to a machinable, load-bearing state in a few hours at room temperature, longer when cold, per the data sheet. The face is then checked with a straightedge for flatness and the joint made up with a new gasket to the correct torque.

An experienced two-person crew handles a typical pipe flange rebuild in a few hours of work time plus the cure, which is why the technique suits a short isolation or a shutdown with a long list of joints to address.

Limits of the in-place rebuild

Epoxy is not the answer for every flange. It should not be used where the face is warped or dished — filling a bent face produces a flat epoxy surface on a bent flange, and the joint will still leak under piping loads. Ring-type joints, whose grooves are gauged and carry very high seating stress, are machined. Very high-temperature lines, and lines where the process fluid attacks the epoxy chemistry, are outside the product window. And a face where the damage extends past the gasket area into the hub is structural, not a sealing problem, and belongs in the pressure piping repair process under API 570 rather than in a filling operation.

Within those limits, an epoxy rebuild is a recognized repair, and ASME PCC-2 devotes an article to flange repair and conversion that sets out the engineering assessment expected before any flange repair method is chosen. The same document's approach to non-metallic repairs is covered in the ASME PCC-2 guide.

In practice

Induscoat rebuilds corroded and eroded flange faces in place for plants in Quebec and Ontario as part of its maintenance and rehabilitation service — surface preparation, metal-filled epoxy application, face forming and post-cure inspection — and advises on when machining or replacement is the better choice. A photo of the open face and the line conditions (fluid, temperature, pressure class) are enough to assess a joint: request a quote. Technical data sheets for the Induscoat range are published on the brand site, induscoat.com.