At What Specific Combination of Hydrochloric Acid Concentration and Sheath Surface Temperature Does a 1.6 Millimeter 316 Stainless Steel Sheath Require Replacement Every 12 Months in Regeneration Tank Heater Service for Ion Exchange Systems?

Hydrochloric acid is the standard regenerant for cation exchange resins . * * For facility engineers responsible for electric immersion heaters in ion exchange water treatment systems . Typical regeneration is done with 5–10% HCl at injection temperatures of 20–40°C, however regeneration tanks are often heated to 50–60°C for efficiency. These heaters are often made with a 1.6 mm 316 stainless steel sheath because of general corrosion resistance in weak HCl. But under certain acid concentration and high sheath surface temperature by watt density, pitting and general corrosion are accelerated to reduce service life to 12 months or less. This article tells you exactly where to change a 1.6 mm 316 sheath on an annual basis in regeneration tank heater service.

Corrosion behaviour of 316 in dilute hydrochloric acid
The passive film of 316 stainless steel is attacked by chloride ions and hydrochloric acid is quite aggressive to 316 stainless steel. Below 3% at temperatures below 40°C, 316 exhibits mild corrosion rates of 0.1-0.3 mm per year. At 5% HCl the rate increases to 0.5-1.0 mm per year at 40°C. Rates of 1.0-2.0 mm/year were seen at 8% HCl. At 10% HCl, rates are above 2-4 mm per year. The minimal structural thickness for a sheath of 1.6 mm is ~0.6 mm. At a corrosion rate of 1.0 mm/year, corrosion penetration of 1.0 mm (from 1.6 mm to 0.6 mm) happens in 12 months. The 12-month replacement threshold is determined when the HCl combined concentration and surface temperature of sheath results in a corrosion rate of 0.8–1.2 mm per year.

12 Month Service Life Critical Thresholds
Immersion testing of 316 samples in hydrochloric acid at temperatures relevant to regeneration tank service indicates that the following combinations of acid concentration and sheath surface temperature will provide a 12-month maximum service life for a 1.6 mm wall.

Hydrochloric Acid Concentration (Weight %) Bulk Solution Temperature Sheath Surface Temperature at 6 W/cm² (12°C rise) Expected Corrosion Rate at Surface Temperature Time to Reduce 1.6 mm to 0.6 mm Service Life to Replacement 3% 40°C 52°C 0.2 – 0.4 mm/year 2.5 – 5.0 years 3 – 6 years 3% 50°C 62°C 0.4 – 0.7 mm/year 1.4 – 2.5 years 1.8 – 3 years 3% 55°C 67°C 0.6 – 1.0 mm/year 1.0 – 1.7 years 1.2 – 2 years 5% 35°C 47°C 0.4 – 0.7 mm/year 1.4 – 2.5 years 1.8 – 3 years 5% 40°C 52°C 0.6 – 1.0 mm/year 1.0 – 1.7 years 1.2 – 2 years 5% 45°C 57°C 0.9 – 1.4 mm/year 0.7 – 1.1 years 0.9 – 1.3 years 5% 50°C 62°C 1.2 – 1.8 mm/year 0.6 – 0.8 years 0.7 – 1.0 years 8% 30°C 42°C 0.6 – 1.0 mm/year 1.0 – 1.7 years 1.2 – 2 years 8% 35°C 47°C 0.9 – 1.4 mm/year 0.7 – 1.1 years 0.9 – 1.3 years 8% 40°C 52°C 1.3 – 2.0 mm/year 0.5 – 0.8 years 0.6 – 1.0 years 10% 25°C 37°C 0.8 – 1.2 mm/year 0.8 – 1.3 years 1.0 – 1.5 years 10% 30°C 42°C 1.1 – 1.7 mm/year 0.6 – 0.9 years 0.7 – 1.1 years 10% 35°C 47°C 1.6 – 2.4 mm/year 0.4 – 0.6 years 0.5 – 0.8 years 
For a 1.6 mm sheath at 6 W/cm2 in an ion exchange regeneration tank with 5% HCl at 45°C bulk temperature the sheath surface will be about 57°C. Corrosion rate is predicted to be 0.9-1.4 mm/year such that the 1.6 mm wall will be reduced to 0.6 mm in 0.7-1.1 years. A 12 month replacement is suitable. Bulk temperature reduction to 40 °C reduces surface temperature to 52 °C, and corrosion rate to 0.6–1.0 mm per year, and increases life to 1.2–2 years.

12-Month Replacement Scheme Safe Operating Envelope
The accompanying table gives the maximum safe bulk hydrochloric acid temperature at various watt densities to provide at least 12 months of service life before wall degradation to 0.6 mm required replacement for a 1.6 mm 316 sheath.

Watt Density (Temperature Rise) Max. Bulk Temp. for 3% HCl Max. Bulk Temp. for 5% HCl Max. Bulk Temp. for 8% HCl Max. Bulk Temp. for 10% HCl
4 W/cm2 (8°C rise) 60°C 52°C 44°C 38°C 6 W/cm2 (12°C rise) 56°C 48°C 40°C 34°C 8 W/cm2 (16°C rise) 52°C 44°C 36°C 30°C 10 W/cm2 (20°C rise) 48°C 40°C 32°C 26°C
For a 5% HCl regeneration tank, at 45°C bulk temperature and 6 W/cm2 (12°C rise, 57°C surface) the safe bulk limit is 48°C. The heater is in the safe zone at 45°C and 12 month service life is possible. With annual replacement the safe limit for 50°C bulk is exceeded and cannot be guaranteed.

Design Modifications to Improve Durability Beyond 12 Months
When the regeneration conditions need a 1.6 mm 316 sheath and the 12 month life is insufficient, 3 modifications can be made to enhance service life. The first is to minimise the watt density using a longer heater. Reducing from 6 W/cm2 to 4 W/cm2 reduces surface temperature rise from 12C to 8C, moving from the 12 month regime to the 1.5-2 year regime for 5% HCl at 45C. The second change is to reduce the acid contact duration by only heating the regeneration tank during the preheating part of the process rather than during the acid injection part of the process. A heater that runs 4 hours every regeneration cycle compared to 12 hours will last 3 times longer in calendar time. The final adjustment is to change to a high silicon stainless steel such as Alloy 20 or a tantalum wrapped sheath. Corrosion rates of the Alloy 20 are < 0.1 mm/year in 5–10% HCl at 60°C thereby providing a service life of > 10 years. For water treatment plants where regeneration takes place on a daily basis, the labour cost of replacing the heater annually usually is greater than the incremental cost of an Alloy 20 heater. When specifying heaters for ion exchange regeneration service, always specify the correct HCl content, bulk temperature and frequency of regeneration. A manufacturer that suggests 316 for 8% HCl at 50°C without stating projected service life isn't giving reasonable advice. The 12 month replacement threshold for a 1.6 mm 316 sheath in hydrochloric acid service is predictable as a function of concentration, bulk temperature, and watt density allowing for precise forecasting of heater life in water treatment operations.