Galvanic corrosion in heat exchangers: tube sheets, water boxes, prevention and repair

Galvanic corrosion in a heat exchanger occurs when two metals of different nobility are electrically connected in the cooling water. The less noble one — usually the carbon-steel tube sheet, channel or water box — corrodes to protect the more noble tubes in copper-nickel, brass, stainless steel or titanium. Area ratio and water conductivity set the severity.

The damage is predictable, which means it can be designed out, controlled or repaired. This guide is written for reliability engineers, exchanger specialists and shutdown planners who need to decide what to inspect, what to specify and what to fix.

Where galvanic corrosion occurs in a heat exchanger

A shell-and-tube exchanger combines several metals by design. Each junction that sees the same electrolyte is a potential cell.

Graphite deserves a note: it sits at the noble end of the galvanic series, so graphite gaskets, packings and graphite-block equipment act as cathodes against steel. For process equipment built around graphite, see our guide to graphite heat exchanger repair.

Tube sheet galvanic corrosion: the mechanism

A rolled or welded tube-to-tube-sheet joint is a permanent metallic contact. The tubes and the tube sheet are therefore always electrically connected; what makes the cell active is the water in the channel, which closes the ionic circuit. The carbon-steel tube sheet becomes the anode and releases metal ions, while oxygen reduction takes place on the noble tube surfaces, which act as the cathode.

The practical consequence is important: you cannot electrically isolate a tube from the tube sheet it is rolled into. Prevention at the tube sheet relies on materials, coatings, cathodic protection and water chemistry — not on isolation.

Area ratio: why a small anode and a large cathode is the worst case

The galvanic current collected by the cathode must be supplied by the anode. When the cathode is large and the anode small, the anodic current density — and so the corrosion rate — is high.

In an exchanger, the cathode is the wetted surface of the tubes, which is very large compared with the steel exposed on the tube sheet face. That is why tube sheets corrode even when the steel looks well protected elsewhere. It is also why coating only the anode is dangerous: every holiday in a coating on the tube sheet becomes a tiny anode facing the entire tube surface, and it perforates faster than bare steel would have corroded.

The same rule works in your favour elsewhere. Stainless bolts in a carbon-steel flange are generally acceptable, because the small cathode is fed by a large anode; carbon-steel bolts in a bronze or stainless component are not.

Electrolyte conductivity and water chemistry

Galvanic corrosion needs an ionic path, so the water sets the reach and intensity of the cell:

• Seawater and brackish water are highly conductive. The galvanic current spreads over a wider area and the whole tube sheet face and water box can be attacked.
• Low-conductivity water (condensate, demineralized water) concentrates the attack close to the junction, where the ionic path is shortest.
• Cooling-tower water sits in between, and its conductivity rises with cycles of concentration.
• Chlorides, dissolved oxygen, temperature and velocity all accelerate the reactions. Deposits and biofouling create local cells of their own.
• Dissolved copper from copper-alloy tubes can plate out on steel surfaces downstream and create new local cathodes.

How to recognise galvanic corrosion at inspection

Inspect before cleaning, then again after. Photographs and a map of the tube sheet, with the affected zones marked, are the baseline for every future comparison.

Typical signs:

• attack concentrated on the less noble metal, strongest next to the junction with the noble one;
• craters around tube holes and thinned ligaments between holes;
• loss of tightness at rolled joints, detected as leaks during a hydrostatic or leak test;
• wastage of the water box near its flange with the tube sheet;
• on cast iron, a surface that looks intact but can be scraped with a knife into soft, dark graphite: graphitic corrosion.

Measure rather than estimate: ultrasonic thickness on water boxes and channels, pit depth on the tube sheet face, and ligament width where the tube sheet is attacked. Galvanic attack is easily confused with other mechanisms. Local, deep attack under deposits or at gasket edges points to crevice corrosion; scattered deep pits on a single metal point to pitting corrosion; smooth, flow-shaped wastage at tube inlets points to erosion-corrosion. The mechanism decides the fix.

Preventing galvanic corrosion in heat exchangers

1. Choose compatible materials. Use the galvanic series in the actual service water to select tube sheets, channels and partition plates close to the tube alloy, or use a tube sheet clad with a compatible alloy on the water side. For new or replacement bundles, this is the most durable measure.

2. Isolate where isolation is possible. Dielectric flange kits (isolating gasket, sleeves and washers) can break the metallic path between a water box and dissimilar piping. They do nothing at the rolled tube joint.

3. Coat the cathode, or both — never only the anode. A tube sheet coating is effective when it is part of a system: it covers the tube sheet face and extends over the tube ends, it is holiday-tested and holiday-free, and it is combined with cathodic protection in the water box so that any defect is protected rather than turned into a small anode. Water boxes and channels are coated on all wetted surfaces.

4. Install cathodic protection in the water boxes. Sacrificial anodes bolted in the channel, or an impressed-current system, supply current to the tube sheet and the first portion of the tubes. Anodes are consumed by design and must be inspected and replaced at each outage. With titanium tubes, protection potentials must be kept within the limits recommended by the tube supplier to avoid hydrogen uptake by the titanium.

5. Control the water. Treatment programs for chlorides, oxygen, pH, deposits and biofouling reduce the driving force. Avoid leaving stagnant water in the exchanger during lay-up.

Repairing galvanic damage on tube sheets and water boxes

When the metal loss is non-structural, a cold repair restores the surfaces during a planned outage:

1. Assess ligament widths, gasket seating surfaces, partition grooves and joint tightness. The tube sheet is a pressure-retaining part: its minimum thickness is a design value, and the owner's inspector decides whether the remaining metal is acceptable.
2. Protect the tube bores, then blast the tube sheet and water box to the cleanliness and profile required by the product data sheet, typically near-white metal (SSPC-SP 10 / NACE No. 2, ISO 8501-1 Sa 2½).
3. Test and remove soluble salts (ISO 8502-6 and ISO 8502-9 methods), since seawater and cooling water leave chlorides in the pits.
4. Rebuild craters, ligaments, gasket faces and partition grooves with a metal-filled epoxy, then dress them back to geometry.
5. Coat with an immersion-grade epoxy system extending over the tube ends, measure dry film thickness and holiday-test the whole surface.
6. Reinstall or renew anodes, respect the full cure time from the data sheet, then leak-test before return to service.

Replacement is the right answer when ligaments are too thin, the tube sheet is perforated, joints can no longer be made tight, or cast iron is deeply graphitized. In those cases, replacing the part — with a better-matched alloy — addresses the cause rather than the symptom.

FAQ

Can I just coat the carbon-steel tube sheet? Not on its own. The tube sheet is the anode; any holiday in its coating becomes a small anode facing the large tube surface and corrodes rapidly. Coat the tube ends as well, holiday-test, and add cathodic protection in the water box.

Do sacrificial anodes in the water box protect the whole tube length? No. The current reaches the tube sheet, the water box and the first portion of the tubes near the inlet. Inside the tubes, protection depends on the tube alloy and the water treatment.

Why are titanium tubes in a steel tube sheet such a severe couple? Titanium is very noble and presents a large cathodic surface, so the steel receives a strong galvanic current. Cathodic protection helps the steel, but its potentials must stay within the limits recommended for titanium.

Does galvanic corrosion occur in closed-loop or glycol systems? It can, but conductivity and oxygen are usually lower and inhibitors are present. Attack appears when the inhibitor is depleted or the fluid is contaminated, so fluid analysis is part of prevention.

How do I distinguish galvanic corrosion from erosion at tube inlets? Galvanic attack follows the junction between metals and affects the less noble one. Erosion-corrosion follows the flow: smooth, directional wastage at the tube inlets, often on the tube itself.

In practice

Induscoat inspects and repairs heat exchanger tube sheets, channels and water boxes on site in Quebec and Ontario through its maintenance and rehabilitation service: surface preparation with soluble-salt control, metal-filled epoxy rebuild, immersion-grade coating with holiday testing, and a clear recommendation when replacement is the better option. The heat exchanger repair application page describes the scope. Photos of the tube sheet and water box, the materials of construction and the water analysis are enough to start: request a quote. Technical data sheets for the Induscoat range are published on the brand site, induscoat.com.