At What Specific Combination of Brine Concentration and Sheath Surface Temperature Does a 1.2 Millimeter 316 Stainless Steel Sheath Require Replacement Every Three Months in Marine Refrigeration Heater Service?

One of the most challenging corrosion conditions for 316 stainless steel sheaths is the combination of saline filled condensate and cyclic operation for marine refrigeration engineers in charge of defrost heaters in freezer rooms, cold storage containers and seafood processing facilities. Marine refrigeration defrost heaters operate in an environment where chloride levels from saltwater spray, brine leaks and condensate evaporation can be as high as 50,000-150,000 ppm, well above normal industrial standards. A common wall thickness for these heaters is 1.2 mm due to space constraints and thermal response requirements. However, for some pairs of brine concentration and sheath surface temperature, pitting penetration can take place in as little as three months, requiring frequent and costly replacement. This paper describes the exact limits at which a 1.2 mm thick 316 sheath exceeds the three month service life limit in marine refrigeration heater service.

Pitting Corrosion of 316 in High Concentration Brine at Defrost Temperatures
How do marine refrigeration defrost heaters work? When energised, defrost heaters melt ice build up on evaporator coils. The temperature of the sheath surface generally increases to 150-250°C during defrost cycles when in the brine environment of chloride levels of 30,000 ppm (seawater) to greater than 150,000 ppm (evaporated concentrated brine). At such high chloride concentrations, the critical pitting temperature of 316 is significantly lowered. In saltwater (30,000 ppm chloride) the critical pitting temperature is ~25°C. 50,000 ppm is down to 15°C. At 100,000 ppm the critical temperature drops below 0°C thus pitting happens at any temperature above freezing. For a defrost heater the sheath surface temperature in service is 150–250°C, well over the critical pitting temperature at any concentration. Pitting starts within hours of the first exposure. Once started, pit propagation rates in warm, high-chloride brine are quite fast, averaging 2–5 mm/month at 50°C and 5–10 mm/month at 80°C. In this setting a 1.2 mm sheath penetrates 1-3 weeks after pit commencement. The three month service life is only accomplished when the heater is used occasionally with long off-cycles, which allows the brine to dry up and reduces the duration in corrosive conditions.

Three Month Life Critical Brine Temperature Combinations
Field failure data for maritime refrigeration systems and accelerated laboratory testing of 1.2 mm 316 sheath samples in concentrated brine reveal the following combinations of brine concentration and sheath surface temperature which result in a maximum service life of three months.

Brine Chloride Concentration (ppm) Critical Pitting Temperature This Concentration Sheath Surface Temperature During Defrost Pit Propagation Rate (mm/month) Sheath Temperature Time to 1.2 mm Penetration After InitiationTotal Service Life (Start-up + Propagation) 30,000-40,000 (seawater)40,000 - 60,000 10 - 20°C 150 - 200°C 4 - 7 mm/month 0.2 - 0.3 months 0.3 - 0.5 months 60,000 - 80,000 5 - 10°C 150 - 200°C 5 - 8 mm/month 0.15 - 0.25 months 0.25 - 0.4 months 80,000 - 100,000 0 - 5°C 150 - 200°C 6 - 10 mm/month 0.12 - 0.2 months 0.2 - 0.3 months 20 - 25°C 150 - 200°C 3 - 5 mm/month 0.3 - 0.4 months 0.5 - 1 month
Over 100,000Below 0 °C 150 – 200 °C 8 – 15 mm/month 0.08 – 0.15 months 0.1 – 0.2 month
Theoretical perforation times for a 1.2 mm 316 sheath at average marine refrigeration brine concentrations (30,000–50,000 ppm) are on the order of 2–4 weeks. The three-month service life, in fact, is possible only if the heater is used infrequently, say, 4 to 6 defrost cycles per day, each lasting 10 to 20 minutes. Actual corrosion period is only 60-120 hours, but overall exposure at increased temperature is 1-2 hours per day, giving calendar life of 2-4 months. The three month replacement criterion is met when the total time of corrosion exposure is roughly 100–200 hours.

Safe Operating Envelope Three Month Service Life
The table below specifies the maximum safe cumulative defrost hours per week for a 1.2 mm 316 sheath in marine refrigeration service to enable a three month replacement interval. Values are typical brine concentrations in different marine settings.

Marine Environment Typical Brine Chloride Concentration (ppm) Maximum Safe Defrost Hours per Week for 3-Month LifeTypical Defrost Cycles Per Day (15 Minutes/Cycle)Recommended Replacement Interval for Normal Operation
Well drained coils, Open Ocean30,000 – 35,000 10 – 15 hours 6 – 8 cycles (1.5 – 2 hours) 2 – 3 months
Coastal harbour Moderate evaporation 35,000 – 50,000 5 – 10 hr 6 – 8 cycles (1.5 – 2 hr) 1 – 2 mo
Enclosed bilge or drip pan 50,000 – 80,000 3 – 5 hours 6 – 8 cycles (1.5 – 2 hours) 3 – 6 weeks 
Drainage, weak, evaporated brine 80,000 – 120,0001 – 3 hours 6 – 8 cycles (1.5 – 2 hours) 2 – 4 weeks
Brine concentrated no run-offOver 120,000Less than 1 hour 6-8 cycles (1.5-2 hours) 1-2 weeks
A typical maritime refrigeration system running 8 defrost cycles per day at 15 minutes per cycle (2 hours of thaw per day, 14 hours per week) will surpass the 10-15 hour weekly limit with a 1.2 mm 316 sheath under open ocean conditions (30,000-35,000 ppm). Field experience indicates that the service life should be 2-3 months. At increasing brine concentrations, life is measured in weeks.

Design Changes That Will Last More Than 3 Months
When a 1.2 mm 316 sheath must be utilised in marine refrigeration service and three month replacement is unacceptable, four design adjustments can improve service life. The best and first is to increase the coil drainage after thaw. This eliminates the standing brine and lowers the time of contact of the sheath with the corrosive electrolyte. Sloped mounting and thoroughly draining drip pans can increase life 2–3 times. The second change is to decrease the frequency and duration of defrosting. Demand defrost (when ice is detected) instead of timed defrost can reduce defrost hours by 50-70%. The next alteration is to coat the sheath with a protective coating. Electroless nickel or fluoropolymer coatings can offer a barrier between the 316 and the brine and can increase life by a factor of 3 to 5 times. However, coatings must survive thaw temperatures of 200°C without degradation. The fourth alteration is to use a more pit-resistant alloy. Titanium (Grade 2 or Grade 7) is virtually pitting immune in marine refrigeration service with a critical pitting temperature of above 100°C even in 100,000 ppm chloride. A titanium sheath (0.8 mm wall thickness) will survive 10-20 times longer than a 1.2 mm 316 sheath. In marine refrigeration systems where the heater must be replaced by defrosting and gaining access to the coil, the labour cost of frequent replacement is usually more than the cost difference between 316 and titanium. Engineers specifying defrost heaters for new marine installations should default to titanium for any application where brine exposure is unavoidable. For current 316 heaters, tight drainage and decreased defrost timing are the most feasible means to extend life beyond three months. The three-month replacement threshold is not a set value but a predictable consequence of brine content and accumulated defrost hours. By evaluating both values, engineers may predict replacement timeframes and schedule maintenance, preventing unexpected defrost failures leading to coil freezing and refrigeration downtime.