The electrochemical basis of galvanic corrosion
When two dissimilar metals are connected in an electrolyte, they form a galvanic cell — an electrochemical cell in which the difference in electrode potential between the two metals drives a current. The metal with the lower (more negative) electrode potential becomes the anode: it is oxidised and corrodes. The metal with the higher (more positive, more noble) electrode potential becomes the cathode: it is protected.
The galvanic series ranks metals and alloys in seawater from most active (anodic, corrodes) to most noble (cathodic, protected). Key positions relevant to structural applications:
| Position | Metal / alloy | Behaviour in galvanic couple |
|---|---|---|
| Most active (anodic) | Magnesium | Always corrodes; used as sacrificial anode |
| Zinc | Corrodes when coupled to steel or more noble metals; basis of galvanic steel protection | |
| Aluminium alloys | Active; corrodes when coupled to steel in seawater (despite passive oxide in air) | |
| Carbon steel / iron | Corrodes when coupled to copper, stainless steel, or titanium | |
| Cast iron | Slightly more noble than carbon steel; can act as cathode when coupled to carbon steel | |
| 304 / 316 stainless steel (passive) | Noble when passive; can drive galvanic corrosion of carbon steel at connections | |
| Copper and copper alloys | Noble; drives galvanic attack on steel and aluminium in contact | |
| Titanium | Very noble; can cause severe galvanic corrosion of active metals in contact | |
| Most noble (cathodic) | Platinum, gold | Always protected; never the anode in a galvanic couple |
Factors that control the severity of galvanic corrosion
The potential difference between the two metals is the thermodynamic driving force, but the rate of galvanic corrosion is controlled by several practical factors:
- Electrolyte conductivity — Galvanic corrosion is more severe in seawater (high conductivity) than in freshwater (lower conductivity) or soil (variable conductivity). Offshore and marine environments are therefore the most aggressive for galvanic corrosion.
- Area ratio — The ratio of cathode area to anode area is critical. A small anode connected to a large cathode will corrode at a much higher rate than if the areas were equal, because the cathodic current density (per unit area) remains low while the anodic current density at the small anode is high. A steel bolt in a copper plate will corrode rapidly; a copper bolt in a steel plate corrodes the surrounding steel much more slowly.
- Geometry and distance — Galvanic attack concentrates at the junction between the two metals. The severity diminishes with distance from the junction, particularly in lower-conductivity electrolytes.
- Temperature — Higher temperatures generally increase corrosion rates by increasing electrolyte conductivity and reaction kinetics.
Common galvanic corrosion scenarios in industrial structures
- Steel-aluminium connections — Carbon steel fasteners in aluminium structural members (or vice versa) in marine or offshore environments. Aluminium is the anode; severe pitting of the aluminium occurs around the steel fastener.
- Steel-stainless steel connections — Stainless steel handrails, gratings, or fittings connected to structural carbon steel. The carbon steel corrodes at the interface — a common failure in coastal construction.
- Steel-copper grounding connections — Copper earthing conductors attached to steel structural members drive severe galvanic attack of the steel at the connection point.
- Steel in contact with copper plumbing — Galvanic couples in building services where steel pipework connects to copper; corrosion of the steel at the transition.
- Offshore jacket cathodic protection system connections — Sacrificial aluminium anodes connected to carbon steel jacket; the aluminium corrodes preferentially, protecting the steel.
Zinc-rich primers: using galvanic principles to protect steel
The same galvanic principle that causes damage when steel is connected to a more noble metal is deliberately used to protect steel when it is connected to a more active metal — specifically zinc. Zinc-rich primers contain a very high concentration of zinc powder in the dry film (typically 65–95% by weight, or 77–85% by volume for effective galvanic protection) dispersed in an organic or inorganic binder.
The protection mechanism: when the applied coating is breached — by mechanical damage, corrosion at a defect, or edge failure — the zinc particles in the primer are exposed to the environment and form galvanic couples with the surrounding steel. Zinc is the anode; steel is the cathode. Zinc corrodes preferentially, consuming itself to protect the steel at and around the breach. This cathodic (galvanic) protection is the defining property of zinc-rich primers and distinguishes them from barrier-only epoxy systems.
This is why zinc-rich primers are specified for the most demanding corrosion protection applications: offshore structures, major bridges, wind turbine towers, and industrial plant in C4–C5 environments. A zinc-rich primer system, even after being mechanically damaged, continues to protect the exposed steel rather than simply allowing corrosion to initiate immediately at the defect.
Surface preparation requirements for zinc-rich primer systems
The galvanic protection mechanism of zinc-rich primers depends on intimate electrical contact between the zinc particles in the primer and the steel surface. Any contamination that interrupts this contact — residual rust, millscale, oil, or soluble salt deposits — reduces the galvanic efficiency and the effective cathodic protection the primer provides. This is why zinc-rich primers have among the most stringent surface preparation requirements of any industrial coating system:
Organic zinc-rich primers (epoxy binder)
- Minimum surface preparation: SSPC-SP10 / Sa 2½ (near-white metal blast)
- Anchor profile: typically 40–75 µm Rz per TDS
- The epoxy binder forms a mechanical bond to the steel surface; the galvanic mechanism requires zinc-to-steel electrical contact through the primer matrix
- Applying organic zinc-rich primer over inadequate preparation significantly reduces its effective protection period
Inorganic zinc silicate primers
- Minimum surface preparation: SSPC-SP10 / Sa 2½ to SSPC-SP5 / Sa 3 — most manufacturers specify SP-10 as the minimum; SP-5 is preferred and often required by project specifications in C5 environments
- Anchor profile: 40–75 µm Rz, achieved with angular abrasive to maximise mechanical bonding
- Inorganic zinc silicate undergoes a chemical reaction (silicate hydrolysis and condensation) with the steel surface to form a direct bond. Contamination between the silicate binder and steel prevents the bond chemistry from proceeding correctly.
- Inorganic zinc silicate is the most demanding primer for surface preparation. Applying it over inadequate preparation produces a primer that will disbond under the thermal and mechanical stresses of service — providing neither barrier nor galvanic protection.
Soluble salt limits for zinc-rich systems
Soluble salt contamination under a zinc-rich primer drives osmotic blistering regardless of the galvanic protection the zinc provides. For C4–C5 atmospheric applications: ≤30–50 µg/cm² NaCl equivalent. For immersion and offshore applications: ≤20–30 µg/cm². Verify with the Bresle patch method (ISO 8502-6/9) before primer application.
Preventing galvanic corrosion at dissimilar metal connections
Where dissimilar metals must be connected in corrosive environments, the primary mitigation measures are:
- Electrical isolation — Insulating sleeves, gaskets, and washers at the connection prevent the galvanic cell from forming. This is the most effective measure but requires design-stage implementation and ongoing maintenance of the isolation components.
- Coating the anodic metal — Applying a high-performance coating to the less noble metal (the one that would corrode) interrupts the galvanic circuit. The coating must be applied to properly prepared steel (SP-10 minimum) and maintained intact — any holiday or damage re-establishes the galvanic couple at that point.
- Coating both metals — Coating both the anodic and cathodic metals reduces the cathode area available to drive the galvanic reaction and minimises the galvanic current.
- Sacrificial anodes — Attaching a sacrificial anode of a more active metal (aluminium or zinc anode on an offshore structure) provides cathodic protection of the steel without requiring an external power supply.
- Material selection — Where possible, selecting metals close together in the galvanic series for joined components minimises the galvanic driving force.
Key takeaways
- Galvanic corrosion occurs when dissimilar metals are electrically coupled in an electrolyte. The less noble metal (anode) corrodes; the more noble metal (cathode) is protected. Severity is controlled by the potential difference, electrolyte conductivity, and the area ratio of cathode to anode.
- Zinc-rich primers exploit the galvanic principle deliberately — zinc (more active than steel) corrodes sacrificially to protect steel at coating defects. This cathodic protection mechanism is what distinguishes zinc-rich primers from barrier-only epoxy systems.
- Organic zinc-rich primers require SSPC-SP10 / Sa 2½ minimum. Inorganic zinc silicate primers require SP-10 to SP-5 minimum. These requirements are not conservative specification padding — they are mechanistically necessary for the galvanic protection mechanism to function.
- Soluble salt contamination under zinc-rich primers causes osmotic blistering even when the zinc’s galvanic function is intact. Always test and control salt levels before zinc-rich primer application.
- At dissimilar metal connections in corrosive environments, the primary mitigation options are electrical isolation, coating of the anodic metal to SP-10 minimum, and sacrificial anode attachment.