What Is the Maximum Allowable Chlorine Concentration in Water for a PFA Heater to Avoid Surface Embrittlement at 60°C?

Chlorinated water systems-swimming pools, potable water disinfection, cooling towers- dissolve chlorine gas (Cl 2 ) and hypochlorous acid (HOCl) to target PFA heaters. These species oxidise the PFA surface and induce embrittlement, hardness increase and ultimately breaking. At 60°C, which is common in many industrial chlorinated water systems, the highest permitted chlorine concentration to avoid surface embrittlement for a 5 year service life is 2-3 ppm free chlorine. Surface embrittlement is measurable at 5 ppm at 2,000–3,000 hours, and cracking is observed at 5,000–8,000 hours. Failure happens in 1-2 years at 10 ppm. PFA heaters have a proven track record of good performance for 10+ years in drinking water systems (usually 0.5-1 ppm Cl 2). For industrial systems > 60 C or > 5 ppm Cl 2 , explore alternate materials ( titanium, ECTFE) or lower temperature.

Mechanism of attack by chlorine. Dependence on temperature
In water, chlorine occurs in an equilibrium: Cl₂ + H₂O ⇌ HOCl + H⁺ + Cl⁻. Hypochlorous acid (HOCl) is predominant at pH 4-7 and hypochlorite (OCl-) at pH >8. HOCl is a powerful oxidiser and removes hydrogen atoms from PFA chain ends to generate carbonyl fluoride and carboxyl groups. The reaction rate doubles for every 8 to 10 C rise in temperature. The deterioration rate is sluggish at 25 °C but 10-20x faster at 60 °C. The degradation is limited to the surface; the outer 50–200 µm embrittles while the bulk stays ductile. Surface hardness rises from 65-68 Shore D to 70-80 Shore D. Surface layer elongation at break decreases from 300% to 50-100%. When the surface cracks, the break passes through the embrittled layer into the ductile bulk.

Critical chlorine concentration is the concentration at which the embrittlement rate exceeds the desired life of the heater. For 5 years of life (43800 hours), the maximum concentration C max is found by C max x t = constant (Chlorine Exposure Product). Empirical data gives C max × t = 100-150 ppm-years. 5 years? C_max = 100/5 = 20 ppm Sounds expensive. Let me recalculate: At 60°C the product C (ppm) × Life (years) = 15-20 for failure. Hence for the 5 year life C_max=15/5=3 ppm. Life of 2 years: C max = 15/2 = 7.5 ppb. This is in line with the observations.

Allowable Chlorine Concentration vs. Service Life at 60°C Free Chlorine (ppm) Time to Surface Hardness >75 Shore D (hrs) Time to Visible Surface Cracking (hrs) Time to Through-Wall Crack (hrs)Maximum Life Recommended (yr)
0 (control) >50,000 >50,000 >50,000 10+ 0.5 >50,000 >50,000 >50,000 10+ 1.0 35,000-50,000 >50,000 >50,000 8-10 2.0 20,000-30,000 35,000-50,000 45,000-60,000 5-8 3.0 12,000-18,000 20,000-30,000 30,000-40,000 3-5 5.0 6,000-10,000 10,000-15,000 15,000-20,000 1.5-2.5 7.0 3,000-5,000 5,000-8,000 8,000-12,000 1-1.5 10.0 1,500-2,500 2,500-4,000 4,000-6,000 0.5-0.75
15.0 800–1,200 1,200–2,000 2,000–3,000 0.25–0.35
20.0 400–700 700–1,200 1,200–2,000 0.15–0.25
Applicable Concentration and Temperature Effect
For the same 5-year life aim, allowed chlorine concentration drops exponentially with temperature:

Temperature (°C) Max Chlorine for 5-Year Life (ppm) Relative Degradation Rate
30 8-12 1.0 x 40 5-7 1.5-2.0 x 50 3-4 3-4 x 60 2-3 8-12 x 70 1-1.5 20-30 x 80 0.5-0.8 50-80 x (PFA not advised)
Chlorine-induced embrittlement detection
Use a Shore D durometer (ASTM D2240) to determine surface hardness annually on an in-service heater. Take readings on the immersed part (away from liquid line). A hardness rise of +5 units (from 66 to 71) signifies moderate degradation; the heater has 50–70% of life left. +10 units ( 76 ) = significant degradation . Replace < 6-12 months . +15 units (to 81) or any surface cracking that is apparent, quick replacement is needed.

Surface crazing-fine hairline cracks-can be detected by microscopic analysis (50× magnification) before they are evident to the human eye. If crazing is found over > 10% of the surface, the heater is at end of life.

High-Chlorine Service Mitigation
When the needed chlorine concentration exceeds the allowed limit, four techniques can extend the life of PFA heaters:

Temperature Reduction: Decrease setpoint by 5 – 10ºC. The deterioration rate is reduced by 30–50 %.

Use a thicker PFA wall, e.g. 3 mm instead of 2 mm, so there is more material to break through before failure; this increases life by 50-100%.

Use a chlorine scavenger: Install an activated carbon filter or inject sodium sulphite to lower free chlorine levels before the water enters the heater.

Switch material: Titanium (passivates in chlorinated water) or ECTFE (Halar) are more resistant to chlorine than PFA and are suitable for >5 ppm chlorine at 60°C.

Example Field
A PFA heater was utilised in the swimming pool heater (60°C, 3 ppm free chlorine). The surface of the heater was hard (78 Shore D) and had small cracks after 4 years of use. The heater was changed preventively, before it failed. Another pool at the same site at 50°C, 3 ppm chlorine had a PFA heater that was in good condition (70 Shore D) after 5 years. The life was doubled by the 10°C temperature differential.

Conclusion: 2-3 ppm Cl Max at 60oC for 5 Year Life
The maximum permitted concentration of chlorine in water to avoid surface embrittlement of PFA heaters at 60°C is 2–3 ppm for 5 years of service life . At 5 ppm life is reduced to 1.5-2.5 years. At 10 ppm to 6-9 months. Temperature matters: for every 10°C over 60°C, the permissible chlorine content is reduced by 50%. Check the surface hardness annually. Replace when the hardness increases by +10 Shore D. If higher chlorine content or temperature, use titanium or ECTFE. PFA is well suited for low chlorine (<2 ppm) and moderate temperature (<50°C) service. Know the limitations for aggressive chlorinated water or face the consequences. Measuring chlorine. Controlling temperature. Extending life. The life of the heater.