Galvanic corrosion
in flanged joints —
why mixed metals cause
leak and failure risk
Scope note: This article is a material-selection reference. Confirm the exact medium, pressure, temperature, flange condition and current material datasheet before specifying the gasket.
The electrochemical mechanism — what actually happens
When two metals with different electrochemical potentials are in contact in the presence of an electrolyte — any conducting liquid, including moisture, condensate, soil water or seawater — they form a galvanic cell. The electrochemical circuit is completed through metallic contact and the electrolyte. The less noble metal acts as the anode and corrodes, releasing metal ions into the electrolyte. The more noble metal acts as the cathode and is protected from corrosion.
This is not a uniform process across the joint. The corrosion concentrates at the anode — the less noble metal — while the cathode may appear unaffected. In a flanged joint, the anodic metal progressively loses material at the interface, which may alter the flange face geometry, widen the effective gap, and degrade the sealing surface over time.
Schematic illustration only. Actual current path in a flanged joint involves both the flange faces and any metallic fasteners bridging the joint.
Why the fasteners matter as much as the flanges
A common error when considering galvanic corrosion in flanged joints is to focus only on the two flange faces. The bolts, nuts and washers are also metallic components that bridge the joint — and if they are a different material from either flange, they introduce additional galvanic pairs.
Stainless steel bolts on a carbon steel flange, for example, may protect the bolt threads from corrosion while accelerating attack on the carbon steel flange at the bolt hole contact area. The relative surface areas matter: a large cathode connected to a small anode accelerates the anodic attack more than a small cathode connected to a large anode. A small stainless steel fastener in a large carbon steel flange is less damaging than a large stainless steel flange connected to a small carbon steel fitting.
Factors that increase galvanic corrosion rate
Where galvanic attack appears in a flanged joint
The location of galvanic attack depends on which metal is the anode and which electrolytic path is available:
- Flange face surface: material loss at the sealing contact area, which can alter face geometry, increase effective roughness or create pitting that prevents full gasket contact. A face that has been galvanically attacked may appear superficially intact but no longer provides the flat, finished surface needed for reliable sealing.
- Bolt holes and fastener contact areas: corrosion at the bolt-flange interface, particularly where different metals are in contact through the fastener. Seizure of fasteners in corroded bolt holes is a common consequence, making disassembly difficult and potentially requiring cutting or drilling out of bolts.
- Flange body wall thickness: in severe or long-duration cases, galvanic attack may reduce the wall thickness of the anodic flange, affecting structural integrity rather than just the sealing face.
Risk by environment and metal combination
| Environment | Metal combination example | Indicative relative risk | Notes |
|---|---|---|---|
| Seawater / marine | Carbon steel + copper alloy | HIGH | Highly conducting electrolyte, large potential difference |
| Seawater / marine | Carbon steel + stainless steel | HIGH | Carbon steel is strongly anodic — isolate or coat |
| Buried pipework | Carbon steel + copper earth connection | HIGH | Soil electrolyte, long current paths possible |
| Industrial outdoor | Galvanised steel + copper | MODERATE–HIGH | Zinc coating sacrificed, then steel attacked |
| Controlled indoor | Carbon steel + stainless steel, dry | LOW–MODERATE | Risk is typically lower while dry — rises with any moisture ingress or condensate. |
| Any environment | Carbon steel + low-alloy steel | LOW | Small potential difference — galvanic contribution typically minor |
Risk assessments above are indicative only. Actual corrosion rate depends on the specific alloys, electrolyte chemistry, temperature, geometry and exposure conditions. Consult a corrosion engineer for critical applications.
Isolation kits — what they do and what they do not
A flange isolation kit is designed to electrically isolate a flanged joint by interrupting the metallic current path between the two flanges. A full flange isolation kit typically includes:
- A dielectric gasket — electrically non-conducting, isolating the two flange faces from direct metal-to-metal contact
- Isolation sleeves — non-conducting sleeves fitted over each bolt, isolating the bolt shank from the flange bore
- Isolation washers — non-conducting washers under the bolt head and nut, isolating the fastener from the flange face at each end
What isolation kits can do
Break the metallic current path at the flanged joint, interrupting the galvanic circuit between the two piping systems at that point. This is particularly relevant in cathodic protection systems — isolation flanges prevent interference between protected and unprotected sections of pipework.
When correctly specified and installed, a full isolation kit can significantly reduce galvanic current flow through the joint. They are a standard component in many gas, water and oil pipeline systems at material transitions.
What isolation kits cannot do
In many flanged joint arrangements, a dielectric gasket alone — without isolation sleeves and washers — does not provide full electrical isolation. If the bolts remain in metallic contact with both flanges, a current path remains through the fasteners, and the galvanic circuit is not broken.
Isolation kits do not address galvanic attack that is already established, reverse material loss that has occurred, or prevent corrosion from other causes such as crevice corrosion or general atmospheric attack. They also require correct installation — a damaged or incorrectly fitted sleeve allows the bolt to contact the flange bore, reinstating the current path.
The gasket material in an isolation kit is selected for electrical resistance, not just sealing. A standard compressed fibre gasket — even if non-conducting — is not the same as a purpose-designed dielectric gasket for a flange isolation application. Purpose-designed isolation gaskets are typically made from materials such as glass-reinforced epoxy (GRE), phenolic laminates or PTFE-based compounds, chosen for their combination of sealing performance and high electrical resistance. Confirming that the gasket meets the electrical isolation specification for the application — not just the pressure and temperature rating — is part of correct isolation kit selection.
What to look for — signs of galvanic attack at a flanged joint
- Preferential corrosion of one flange face only: if one flange shows significantly more corrosion than the mating flange, galvanic action may be a contributing factor alongside general corrosion. The more corroded face is typically the anode.
- Pitting at the sealing contact area: galvanic attack can produce pitting corrosion on the flange face in the area of electrolyte contact. Pitting within the gasket contact zone alters the effective sealing surface and may prevent full gasket conformance.
- Seized or corroded fasteners: bolts that are difficult or impossible to remove, with significant corrosion at the nut-to-flange interface, may indicate galvanic attack at the fastener contacts. The bolt material and flange material should be checked for compatibility.
- White or orange deposits at the joint: corrosion product deposits at the flange face area — white aluminium oxide, orange iron oxide — visible from the outside of the joint may indicate active galvanic corrosion at the interface.
- Repeated slow leaks after gasket replacement: if new gaskets repeatedly fail to hold at a joint where face condition appears acceptable, progressive face damage from galvanic attack reducing the effective sealing geometry may be a factor.
Galvanic corrosion is slow — it is easy to miss until significant damage has occurred. A joint that appears sealed and structurally sound may have undergone significant material loss at the flange face or fastener contacts over months or years. Periodic inspection of flanged joints at material transitions — particularly in outdoor, marine, buried or damp environments — should include assessment of face condition and fastener integrity, not just leak checking.
Galvanic corrosion in a flanged joint is an electrochemical problem, not a mechanical one — and it requires an electrochemical solution.
Different metals in electrical contact with moisture present will produce a galvanic cell. The less noble metal corrodes. In a flanged joint, this can affect the sealing face, the fasteners, or both. The rate depends on the potential difference between the metals, the electrolyte, the area ratio and the environment. Isolation kits — correctly specified with a dielectric gasket, isolation sleeves and washers — can interrupt the galvanic circuit at the joint. They do not reverse existing damage and require correct installation to be effective. For critical applications, material selection at the design stage — choosing compatible metals or applying appropriate coatings — is the more reliable long-term solution.
Practical FAQ
What causes galvanic corrosion in flanged joints?
Galvanic corrosion in flanged joints occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte — typically moisture, condensate, soil water, seawater or a process fluid. The two metals form an electrochemical cell. The electrochemical circuit is completed through metallic contact and the electrolyte: the less noble metal (the anode) corrodes preferentially, while the more noble metal (the cathode) is protected. In a flanged joint, this can affect the flange faces, the fasteners, or the flange body itself. The rate and severity depend on the electrochemical potential difference between the metals, the conductivity of the electrolyte, the relative surface areas of anode and cathode, and the temperature of the environment.
Does a dielectric gasket stop galvanic corrosion?
A dielectric gasket can electrically isolate the two flange faces, breaking the direct metal-to-metal contact path. This may reduce or interrupt the galvanic circuit at the flange face interface. However, electrical isolation of the flange faces alone is not always sufficient to prevent galvanic corrosion risk in a flanged joint — if the fasteners remain in metallic contact with both flanges, a current path may remain through the bolts and nuts. Full electrical isolation of a flanged joint usually requires an isolation kit that includes a dielectric gasket, isolation sleeves for the bolts, and isolation washers under the nuts. Even then, the effectiveness of the isolation depends on the quality of installation and whether other current paths exist in the system.
Which metals are most at risk from galvanic corrosion in flanged joints?
The risk depends on the electrochemical potential difference between the metals in contact. Metals far apart in the galvanic series — such as zinc or aluminium paired with stainless steel or copper — have a large potential difference and are generally more susceptible to galvanic attack. Metals close together in the galvanic series — such as carbon steel and low-alloy steel — have a smaller potential difference and are generally at lower risk of rapid galvanic corrosion under comparable conditions. In practice, common higher-risk combinations in industrial piping include carbon steel flanges connected to copper alloy or stainless steel components, and galvanised steel connected to copper. The specific electrolyte, temperature and geometry of the joint affect the actual corrosion rate.