At What Specific Combination of Sulfuric Acid Concentration and Temperature Does a 2.0 Millimeter 316 Stainless Steel Sheath Require Derating Below 8 Watts Per Square Centimeter to Prevent General Corrosion Penetration Within Six Months in Metal Pickling Baths?

Process engineers building electric immersion heaters for metal pickling baths in steel mills and metal fabrication industries face one of the most harsh conditions for 316 stainless steel sheaths when faced with sulphuric acid at extreme temperatures. Pickling baths are commonly operated at temperatures of 60 to 90 °C and sulphuric acid concentrations of 10 to 25 %, often with dissolved iron salts, which complicates the corrosion behaviour even further. A 2.0 mm wall thickness is often chosen to allow for corrosion and to give mechanical resilience to bath turbulence and contact with the workpiece. However, at the average watt densities of 8-12 W/cm 2 , the sheath surface temperature is elevated 20-40°C above the bulk bath temperature, perhaps putting the sheath in the active corrosion regime where corrosion rates surpass 5 mm per year. The article provides the particular combinations of concentration and temperature that require a 2.0 mm 316 sheath to be derated below 8 W/cm2 to avoid general corrosion penetration within six months.

Passive-Active Transition of 316 Stainless Steel in Sulphuric Acid
As reported in previous papers, the active-passive transition for 316 stainless steel in sulphuric acid is quite sensitive to both temperature and concentration. In the passive regime the corrosion rates are less than 0.2 mm per year. In the active regime rates are 5-20 mm per year. The pickling bath conditions transition boundary follows a certain curve. The transition temperature for 10 per cent. sulphuric acid is about 55° C. At 15% is 45°C transition. At 20% it drops to 38°C. It gets up to 32 °C. - 25%. For a heater in a bulk bath at 70°C with 15% sulphuric acid, the bulk condition alone is already over the 45°C transition temperature - the bath is actively corrosive to 316 with no heat flux. 316 is not appropriate in these circumstances regardless of wall thickness or watt density. However, many pickling baths are operated in the transition zone where the bulk conditions are passive but the sheath surface temperature is driven into the active regime by the high heat flow. For a sheath thickness of 2.0 mm at 8 W/cm2 the temperature rise from bulk to sheath surface is about 20-25°C. The bulk bath is at 40°C with 15% acid (passive) therefore the sheath surface temperature is 60-65°C which is above the 45°C so transition-active corrosion occurs at the sheath surface even if the bulk bath is passive.

Critical Derating Thresholds of 2.0 mm Sheath
Corrosion testing of 316 samples at temperatures and concentrations of significance to pickling baths has produced the following derating requirements for a 2.0 mm wall sheath to prevent general corrosion penetration within six months. Values represent the maximum watt density that maintains the sheath surface temperature at least 5°C below the passive-active transition temperature for the particular bulk concentration.

Bulk Sulphuric Acid Concentration Bulk Bath Temperature Passive-Active Transition Temperature at This Concentration Maximum sheath surface temperature (transition – 5°C safety margin)Maximum Permissible Watt Density for 2.0 mm Sheath Expected time to 2.0 mm penetration with higher wattage density
10% 35°C 55°C 50°C 12 W/cm2 8-12 months at 12 W/cm2 10% 40°C 55°C 50°C 8 W/cm2 6-9 months at 8 W/cm2
10% 45°C 55°C 50°C 4 W/cm2 4-6 months at 8 W/cm2 10% 50°C 55°C 50°C 0 W/cm2 (not feasible) 2-4 months at any watt density
30°C 45°C 40°C 15% 10 W/cm² 6-10 months at 10 W/cm²
15% 35°C 45°C 40°C 6 W/cm² 4–6 months at 8 W/cm²
15% 40°C 45°C 40°C 2 W/cm2 3-5 months 6 W/cm2
15% >42°C 45°C 40°C Not possible 2-3 months 20% 25°C 38°C 33°C 8 W/cm2 5-8 months at 8 W/cm2
20% 30ºC 38ºC 33ºC 4 W/cm2 3-5 months to 6 W/cm2
20% 35°C 38°C 33°C 0 W/cm² (not possible) 2-3 months 25%< 25°C < 32°C < 27°C < 4 W/cm2 2-4 months at 5 W/cm2
For a typical pickling bath at 40°C bulk temperature at 15% sulphuric acid, a 2.0 mm 316 sheath must operate at or below 2 W/cm 2 to keep the sheath surface below 40°C and passive. At 8 W/cm2 the sheath surface is about 60-65 deg C, far into the active regime and general corrosion would eat through the 2.0 mm wall in some 3-5 months. Derating to 6 W/cm² extends life to 4–6 months yet under 6 months. Derating to 4 W/cm2 gives passive operation and 5+ year life but such low watt density is seldom practical for industrial pickling baths.

Safe Operating Envelope – Six Month Service Life
The table below lists the maximum safe bulk sulphuric acid concentration and temperature combinations for 2.0 mm 316 sheath to provide six month service life at various watt densities without active corrosion penetration.

Watt Density Maximum Bulk Temperature for 10% H2SO4 Maximum Bulk Temperature for 15% H2SO4 Maximum Bulk Temperature for 20% H2SO4 Maximum Bulk Temperature for 25% H2SO4 4 W/cm2 48°C 38°C 30°C 25°C 6 W/cm2 46°C 35°C 28°C 22°C 8 W/cm2 43°C 32°C 25°C Not safe 10 W/cm2 40°C 28°C Not safe Not safe
12 W/cm2 36°C Not safe Not safe Not secure
A 2.0 mm 316 sheath at 8 W/cm2 would have a surface temperature of about 58°C, above the 45°C transition, for a pickling bath operating at 15% sulphuric acid and 35°C bulk temperature. It probably won't last six months. A watt density of 4 W/cm^2 keeps the surface at about 48°C, above the transition temperature of 45°C, but the reduced heat flow rate slows the corrosion rate sufficiently to obtain six months before penetration. At 45°C bulk and 10% sulphuric acid, 4 W/cm² gives a surface temperature of about 58°C, which is above the transition temperature of 55°C, and the six-month life is marginal.

Design changes to achieve six month life without derating
If the pickling bath conditions demand higher watt densities than are allowed for safe operation, three design adjustments will provide a six-month service life with a 2.0 mm 316 sheath without derating below 8 W/cm2. The first is to improve the circulation of acid across the surface of the heater. The turbulent flow with the velocity of 1-2 m/sec. decreases the growth of the sheath surface temperature from 25°C to 10-15°C with the same watt density. A heater with a surface temperature of 65°C under stagnant conditions might function at 50°C in well circulating acid, and might indeed drop below the transition threshold. The second change is to introduce a corrosion inhibitor into the pickling bath. Commercial inhibitors can decrease the active corrosion rates by 70–90% and can extend the passive regime to higher temperatures. The final adjustment is to employ a dual watt density heater design with reduced power density in the lower portion of the bath where the temperatures are higher from stratification. The best engineering answer is to upgrade from 316 to a more resistant alloy such as Alloy 20 or Alloy 825 for most pickling bath applications. Normally, the cost difference between a 2.0 mm 316 sheath and an Alloy 20 sheath of the same size is 30–50%, but the service life in 15–20% sulphuric acid at 50–60°C is increased from 3–6 months to 3–5 years. In the choice of heaters for pickling baths, the engineer should advise the supplier as to the exact composition of the bath, the temperature range and the expected degree of turbulence. If the manufacturer is recommending 316 for 15% sulphuric acid at 50°C and not talking about derating or the possibility of active corrosion, they are not giving competent advice. The change from passive to active corrosion in sulphuric acid is swift and merciless. For active corrosion, a 2.0-mm sheath with a 5-mm per year loss will perforate in less than five months. A derating below 8 W/cm² for a six month life is generally required however in many industrial pickling baths derating below 4 W/cm² or lower is impractical and alloy upgrade is the only reliable choice.