How Does the Presence of a Rubber Lining on the Tank Wall Change the Galvanic Corrosion Risk for an Unbonded Titanium Heater in the Same Electrolyte?

And for a process engineer putting a titanium immersion heater in a steel tank with rubber lining, the risk of galvanic corrosion is very different than if the tank were unlined steel. The titanium heater is galvanically linked to a huge carbon steel surface area in an unlined tank of steel. The steel (anode) is corroded preferentially protecting the titanium (cathode). The wall of the steel tank corrodes but the titanium heater does not get corroded. The insulator rubber in a rubber-lined tank electrically insulated the steel from the electrolyte. There is now no big cathodic area for the titanium heater to couple with. However, if there is any minor defect in the rubber lining (a pinhole, scratch or a damaged area), a very damaging galvanic couple is created, in which a very small amount of exposed steel (anode) is coupled to a very large area of titanium (cathode). The galvanic current concentrates on the small steel fault and causes quick deep pitting of the steel tank wall under the rubber triggering lining detachment and tank failure. The tank fails but the titanium heater itself is unharmed.

The Mechanism of Galvanic Corrosion in Rubber-Lined Tanks
Rubber Lining . The rubber lining of a rubber lined tank electrically isolates the steel substrate from the electrolyte . The titanium heater is electrically insulated from the steel when it is immersed. No galvanic current will pass and the titanium and the steel (covered by the undamaged rubber) will not corrode. But if the rubber liner is defective (pinhole, cut or damaged area) the steel exposed acts as a tiny anode. The huge titanium heater (and any other wetted metal surfaces) is a giant cathode . The ratio of cathodic area to anodic area might be greater than 1,000:1. The galvanic current is focused on the small steel flaw, resulting in a current density of 1 to 10 mA/cm2 at the fault. The high current density quickly dissolves the steel, creating a deep trench that undercuts the rubber lining. The rubber loses its grip, maybe coming off and exposing more steel and therefore increasing failure. As the titanium heater itself is the cathode , it is protected and is not damaged .

Quantifying the galvanic current density at defects in rubber linings
Titanium Heater Surface Area (cm2) Rubber Defect Area (cm2) Cathode/Anode Area Ratio Galvanic Current (μA) Current Density at Defect (mA/cm2) Time to Perforate 6 mm Steel Tank Wall (years) 1,000 10 (large defect) 100:1 1,000 0.1 5-10 1,000 1 (pinhole) 1,000:1 1,000 1.0 0.5-1 1,000 0.1 (small scratch) 10,000:1 1,000 10 0.05-0.1 (weeks to months) 2,000 0.1 20,000:1 2,000 20 0.03-0.05 (months) 5,000 0.1 50,000:1 5,000 50 < 1 month 10,000 0.1 100,000:1 10,000 100 < 2 weeks
A Scenario on the Installation of Titanium Heaters in Rubber-Lined Tanks: A Guide
Condition of Tank Lining Heater Electrical BondingGalvanic Risk Recommended Action
Rubber is OK, no flaws.Heater separated (not bonded)Very low (no circuit) OK. Annual lining of monitor.
No flaws, rubber intactHeater linked to steel (earth)Low (no touch to steel of electrolyte)Optional but recommended.
small pinholes (< 1 mm2) in rubberHeater isolated High (localized pitting of steel)Fix pinholes. The heater is isolated by nylon bushings.
Pinprick holes in rubberSteel bonded heaterLow (titanium and steel at equal potential)Bonding is equalizing potential and removing galvanic couple.
Rubber big damaged areaHeater isolated Very high (huge steel exposed area)Eliminate heater or fix lining immediately.
Age of rubber lining > 10 years (unknown faults)Heater isolated Unknown (perhaps large)Do a spark test on the lining. Heater insulation.
Any tank with rubber lining and cathodic protectionHeater isolated Low (CP protects steel) OK. Check that the CP system is working.
Engineering Solutions to Mitigate Galvanic Risk
Three options are available to prevent the danger of galvanic corrosion of lining defects when a titanium heater is to be installed in a rubber-lined tank. In the first option the titanium heater is electrically connected to the steel tank wall (for example, by a copper wire attached to an uncoated fitting). If the titanium and steel are at the same potential then the galvanic current is zero even if the rubber lining has faults. The steel is protected by the huge cathodic area of the titanium (cathodic protection). The second method is to attach a sacrificial anode (zinc or aluminum) to the titanium heater. At any lining flaw the anode corrodes, not the steel. The third method is to spark test the rubber lining before installing the heater, to find and rectify all faults. Pinholes are detected by a high-voltage spark tester (5-15 kV) that closes a circuit to the steel substrate. If a fault is identified it is corrected by a rubber patching substance.

Conclusion: Heater-Insulated Rubber-Lined Tanks Create High Galvanic Risk at liner defects
The addition of a rubber lining in a tank wall completely alters the galvanic corrosion risk for an unbonded titanium heater. The big steel area is the sacrificial anode to protect the titanium heater in an unlined steel tank. The steel is electrically insulated from the electrolyte in a rubber-lined tank. When the rubber liner is defective (pinhole, abrasion or damage) an extremely harmful galvanic couple is formed: a very small steel anode paired to a very huge titanium cathode. The galvanic current density at the flaw may approach 1-100 mA/cm², perforating a 6 mm steel tank wall in weeks to months. The titanium heater itself is okay, however the tank breaks severely. To avoid this mode of failure titanium heaters in rubber-lined tanks should be electrically linked to the steel tank wall to equalize potentials and eliminate the galvanic couple. For example, ordering a titanium heater for a rubber-lined tank, asking electrical bonding hardware and spark testing of the lining helps ensure that galvanic corrosion at lining faults doesn't cause premature tank failure.