At What Specific Combination of Sulfuric Acid Concentration and Temperature Does a 2.0 Millimeter 316 Stainless Steel Sheath Require Derating Below 5 Watts Per Square Centimeter to Prevent Active Corrosion 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, running at normal watt densities of 8-12 W/cm2 causes the sheath surface temperature to be 20-40oC higher than the bulk bath temperature, which can lead to the sheath entering the active corrosion regime with corrosion rates of greater than 5 mm per year. In this paper, we identify the applicable temperature-concentration combinations for which a 2.0 mm 316 sheath needs to be derated below 5 W/cm2 to stay inside the passive corrosion regime.

316 Stainless Steel in Sulphuric Acid: The Transition from Passive to Active
Article 30 states that in sulphuric acid the passive-active transition of 316 stainless steel is largely dependent on temperature and concentration . Corrosion rates are < 0.2 mm yr−1 in the passive regime. 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 in the transition zone where the bulk conditions are passive, but the increased sheath surface temperature, due to heat flux, pushes the sheath into the active regime. The temperature increase from the bulk to the sheath surface is around 25–30°C for a sheath of 2.0 mm at 10 W/cm². A bulk bath at 40 °C with 15 % acid (passive) results in a sheath surface temperature of 65–70 °C, which exceeds the 45 °C transition, so active corrosion occurs at the sheath surface despite the fact that the bulk bath is passive.

Critical Derating Thresholds 2.0 Millimetre Sheath
Derating needs from corrosion testing of 316 samples, at temperatures and concentrations pertinent to pickling baths, are as follows. Sheath with 2.0 mm wall. Values are the maximum watt density that maintains the sheath surface temperature at least 5°C below the passive-active transition temperature for the given 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)Max. Allowable Watt Density for 2.0 mm. Sheath Recommended Action 10% 30°C 55°C 50°C 12 W/cm² Safe at standard watt density 
10% 40°C 55°C 50°C 6 W/cm² Derate 10% 45°C 55°C 50°C 3 W/cm2 Marginal; Consider alloy upgrading 10% Above 48°C 55°C 50°C Not possibleTitanium or Alloy 20
15% 25oC 45oC 40oC 10 W/cm2 Safe in moderate watt density
15% 30°C 45°C 40°C 6 W/cm² Derating necessary
15% 35°C 45°C 40°C 2 W/cm2 Not practical change alloy
15% Above 38°C 45°C 40°C Not possible 316 not practical
20% 20°C 38°C 33°C 8 W/cm² Safe at low watt density
20% 25C 38C 33C 4 W/cm2 Major derating 20% Above 28C 38C 33C Not possible 316 not suited
25% <25°C 32°C 27°C <3 W/cm² 316 not recommended
25% > 25°C 32°C 27°C Not possible Use Alloy 20 or Titanium
For a typical pickling bath at 15% sulphuric acid and 35°C bulk temperature, the 2.0 mm 316 sheath must be at or below 2 W/cm2 to prevent active corrosion. At this watt density the heater would be an impractically lengthy heater for average power requirements of 10-20 kW. So 316 is not the right fit for this service. Engineers should consider using Alloy 20 (Carpenter 20) or Alloy 825, both of which have passive-active transition temperatures 30–50°C greater than 316 in sulphuric acid.

Design Changes to Prevent Active Corrosion Without Derating
If the pickling bath conditions are close to the transition boundary and a 2.0 mm 316 sheath is already in place or is necessary, three design adjustments can avoid active corrosion without dropping the watt density below practical limits. The first is to improve acid circulation across the heating surface. The turbulent flow with a velocity of 1–2 m/s decreases the temperature rise of the sheath surface from 25–30 to 10–15 °C, which can prevent the sheath from reaching the transition point. The second alteration is to add an oxidising agent in the bath. Small additions of ferric ions or of hydrogen peroxide can shift the passive-active transition to higher temperatures by 10-20 degrees C, returning to passive behaviour. The third change is the dual-watt-density heater design that has lower power density in the lower part of the bath with the greatest temperatures due to stratification . For most pickling bath applications the engineering solution is to upgrade from 316 to a more resistant alloy. Typically 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 improves from months to 5-10 years. When specifying heaters for pickling baths the engineers should supply the supplier with the complete composition of the bath, the temperature range and the degree of turbulence anticipated. 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. A 2.0-mm sheath that is losing 5 mm/yr in active corrosion will perforate in less than 5 months. Derating <5 W/cm 2 is seldom feasible for commercial pickling baths, therefore alloy upgrade remains the only reliable remedy for most applications.