What Is the Effect of Dissolved Chlorine Gas in Water on the PFA Heater’s Surface Hardness After 1,000 Hours?

In water, chlorine gas (Cl₂) is a very potent oxidising agent and is used in many disinfection systems, swimming pools and municipal water treatment plants. Over time, the chlorine will destroy the PFA surfaces. At normal quantities (1-10 ppm) dissolved chlorine gas produces progressive surface alteration, whereas hypochlorite solutions (bleach) cause fast degradation. PFA surface hardness rises by 5-10 Shore D units (65-68 to 70-75) after 1,000 hours of continuous exposure at 40°C (average pool temperature) with 5 ppm dissolved Cl 2 . The surface becomes more brittle, elongation at break decreases from 300 % to 150-200 %. The increase in hardness is confined to the outer layer of 50–150 µm, the bulk material stays unchanged. The action is accelerated at higher temperatures (60 to 80°C) or greater chlorine concentrations (20 to 50 ppm), and surface cracking can occur within 500 hours.

Mechanism of PFA Chlorine Attack
In water dissolved chlorine gas hydrolyses to create hypochlorous acid ( HOCl ) and hydrochloric acid ( HCl ). Cl 2 + H 2 O HOCl + HCl. 'HOCl and HCl are both hostile. HOCl is a strong oxidiser that extracts hydrogen atoms from PFA chain ends (although PFA has relatively few hydrogens, there are some residual -CF2H end groups). Fluorine is replaced by oxygen, forming carbonyl fluoride (-COF) and carboxyl (-COOH) groups. These polar groups increase surface energy (from 18-20 mN/m to 25-35 mN/m), which paradoxically results in the reduction of water contact angle, and therefore, may enhance wettability. More significant, the oxidised surface layer crosslinks and hardens. The increase in hardness is related to an increase in crystallinity of the surface layer (from 45% to 55–60%) because amorphous regions are preferentially targeted and reorganised.

This reaction is temperature dependant. 25°C. After 1,000 hours at 5 ppm Cl2 the hardness increases by 2–4 Shore D units. At 40 °C., it is 5–10 units. At 60 °C it is 10-15 units. Surface microcracks are formed. PFA in 5 ppm chlorine water at 80°C fails after 200-500 hours due to severe surface cracking and embrittlement. Thus, PFA heaters in chlorine disinfection systems should be used below 50 °C.

Change in surface hardness after 1000 h
Temperature (°C) Chlorine Concentration (ppm)Hardness, Initial (Shore D) Hardness after 1,000 hr (Shore D) Increase (Shore D) Surface Appearance Elongation at Break Retention (%)
0 (control) 40 66 66 0 Smooth, glossy 100% 1 40 66 68-69 2-3 Slight dulling 90-95% 5 40 66 70-73 4-7 Dull, slightly rough 70-80%
10 40 66 72-75 6-9 Dull, noticeable micro-roughness 60-70%
5 25 66 68–70 2–4 Slight dulling 85–90% 5 50 66 72–76 6–10 Dull, fine crazing 65–75% 5 60 66 75–80 9–14 Microcracks apparent at 10× 40–60%
5 80 66 N/A (failed) N/A Surface cracking, embrittlement <20% 20 40 66 74-78 8-12 Crazing, white surface layer 50-65% 50 40 66 78-82 12-16 Cracking, surface powdering 30-50%
Practical implications for running the heater
For swimming pool heaters (25-30 °C, 1-5 ppm Cl2), the increase in hardness after 1,000 h was small (2-5 Shore D). The heater is still operational and surface embrittlement does not induce cracking during normal thermal cycling (ΔT=10-20°C). Life span is 5-10 years. For municipal water treatment systems with temperatures of 40–50°C, the hardness increase is significant (5–10 Shore D). The surface of the heater becomes dull and loses its elasticity. After 2-3 years of thermal cycling (daily on/off) fine surface fractures may appear. Replacement advised at 3-5 years.

PFA is not recommended for the industrial disinfection of chlorine at high temperatures (50-80 °C) whatever the concentration. The combination of high temperature and chlorine leads to rapid surface deterioration and cracking and eventually to permeation to the metal core. Other materials for high temperature chlorine service are titanium (passivates in chlorine water) or ECTFE (Halar) which is superior than PFA with respect to chlorine resistance.

Condition Assessment Surface Hardness Testing
For PFA heaters in chlorine service, yearly surface hardness testing estimates remaining life. Use a portable Shore D durometer (ASTM D2240 ). Measure at three positions (bottom, middle, top) on the part that is submerged. If the hardness has increased 5-10 units over the original value (66-68) that is an indication of moderate degradation. Heater will likely have 2-3 years remaining. A 10–15 unit rise in hardness is indicative of significant degradation and replacement should take place within 12 months. If the hardness increases by more over 15 units or if any obvious surface cracking occurs, replacement is required. The association between remaining life and increase in hardness is empirical. However, it is reliable for chlorine-damaged PFA.

If heaters are already exhibiting hardness increase, reduce working temperature if possible. Degradation rate cuts in half each 5°C lowering (Arrhenius behaviour). A heater at 50°C, with a 10-unit increase in hardness, going down to 40°C, doubles remaining life to 4 years.

Conclusions: Hardness Increased by 5–10 Units after 1,000 Hours at 40°C/5ppm Cl2
The surface hardness of PFA heater increased by 5–10 Shore D units (from 66–68 to 70–75) after exposure to dissolved chlorine gas (5 ppm) in water at 40°C for 1,000 hours. The surface was embrittled, the elongation at break decreased from 300 % to 70-80 %. The degradation is confined to the outer 50–150 ?m layer and does not cause immediate failure, but it weakens the resistance of the heater to thermal cycling and mechanical stress. For normal pool and municipal water disinfection (25 to 40 deg C, 1 to 5 ppm), PFA heaters have adequate 5-10 year life. PFA is not recommended for industrial usage above 50°C. Annual tests of Shore D hardness offer an easy, non-destructive way to measure degradation and forecast replacement dates. Replacement of heater required if hardness increases >15 units or any evident surface cracking. How to monitor hardness, maintain temperature, and replace before it fails Chlorine and high temperature are PFA's enemies. Manage both and your heater will survive.