A PCB plating line replaced a PTFE immersion heater after only eight months of service. The sheath surface showed a pattern of deep, localized pits concentrated on the side facing the rectifier bus bars. The PTFE was intact elsewhere. The pits had penetrated the sheath and caused a leak. The plant's rectifiers were not isolated from the tank, and stray current was flowing through the plating bath to the grounded heater.
Stray current is an electrical current that flows through unintended paths in a plating system. It originates from rectifiers, bus bars, and electrical connections that are not properly isolated. The current enters the plating bath and finds a path to ground through the tank structure, the heater, or the plumbing. Where the current leaves the metal surface, it causes corrosion.
How Stray Current Causes Pitting
Stray current corrosion occurs at the point where the current leaves the metal surface and enters the electrolyte. The metal at that point becomes anodic and dissolves. The corrosion is localized and can penetrate the metal rapidly.
The amount of metal lost is proportional to the current. A stray current of just 1 ampere can dissolve approximately 1 kilogram of steel in a year. A current of 10 amperes can dissolve 10 kilograms-enough to destroy a heater sheath in months.
| Stray Current | Metal Loss Per Year | Penetration Rate on Steel | Time to Perforation (2 mm Sheath) |
|---|---|---|---|
| 0.1 A | 0.1 kg | 0.013 mm/year | >150 years |
| 1 A | 1 kg | 0.13 mm/year | 15 years |
| 5 A | 5 kg | 0.65 mm/year | 3.1 years |
| 10 A | 10 kg | 1.3 mm/year | 1.5 years |
| 50 A | 50 kg | 6.5 mm/year | 3.7 months |
Stray current and its effect on steel corrosion in plating baths
Why the Side Facing the Rectifier Is Most Affected
The stray current flows from the rectifier bus bars through the plating bath to ground. The current density is highest near the source-the bus bars-and decreases with distance. The side of the heater that faces the rectifier receives the highest current density. The pits form on that side.
The current also follows the path of least resistance. If the heater is grounded and the tank is not, the current may concentrate on the heater. The heater becomes the preferred path to ground, and the pitting is concentrated there.
Why PTFE-Coated Heaters Are Still Vulnerable
The PTFE coating on a heater is an electrical insulator. It should block stray current from reaching the metal sheath. But the PTFE is thin, and it may have defects-pinholes, scratches, or thin spots-that allow current to pass.
Once current reaches the metal sheath through a coating defect, it creates a local corrosion cell. The metal at the defect becomes anodic and dissolves. The pit deepens and may penetrate the sheath.
The stray current also accelerates PTFE degradation. The current generates heat at the defect, and the heat degrades the PTFE around the pit. The degraded PTFE is more permeable, allowing more current to pass.
Preventing Stray Current Corrosion
The most effective prevention is to eliminate the stray current. This means isolating the rectifiers, bus bars, and electrical connections from the tank and the plumbing. The rectifier output should be connected only to the anodes and cathodes, with no other paths to ground.
Grounding the tank and the heater to a common ground can reduce the potential difference that drives stray current. The grounding should be done at a single point to avoid ground loops.
Using an isolated heater-one that is not electrically connected to the tank-can prevent stray current from flowing through the heater. The heater is powered by an isolation transformer or a grounded supply that is separate from the plating circuit.
| Prevention Measure | Effect on Stray Current | Implementation | Cost |
|---|---|---|---|
| Rectifier isolation | Eliminates source | Electrical work | Moderate |
| Single-point grounding | Reduces potential difference | Electrical work | Low |
| Isolated heater supply | Blocks current path | Transformer or separate supply | Higher |
| Coating integrity | Blocks current at defects | Quality control | Moderate |
Prevention measures for stray current corrosion in plating lines
When Stray Current Indicates a Design Problem
A heater that suffers stray current pitting despite rectifier isolation and proper grounding indicates that the tank layout or the electrical system is creating an unavoidable potential difference. A custom PTFE immersion heater configuration with an isolated supply, a thick coating, and a defect-free surface addresses the root cause of stray current corrosion. The specification process requires the rectifier layout, the grounding system, the bath chemistry, and the required wattage. For facilities with stray current problems, this application-specific approach reduces pitting and extends heater service life.
