At What Critical Threshold of Sulfuric Acid Concentration and Temperature Does a 1.6 Millimeter 316 Stainless Steel Sheath Become Unsafe for Continuous Immersion Heating Above 15 Watts Per Square Centimeter?

Chemical process engineers designing electric immersion heaters for sulphuric acid service in metal pickling lines, battery manufacturing and chemical synthesis reactors need to know the concentration-temperature limits at which passive film breakdown occurs precisely, if they are to select 316 stainless steel as the sheath material. Of the mineral acids only sulphuric acid is shown to be maximally corrosive to 316 stainless steel at intermediate concentrations (40–60%) and decreases at lower and higher concentrations. A typical standard of 1.6 mm wall thickness provides a compromise between the practical heat transmission and the mechanical robustness. But operating above the crucial mix of concentration, temperature, and watt density can lead to rapid active corrosion, reducing sheath life from years to weeks. In this paper, the exact limits of the 1.6 mm 316 sheath, which is not safe for continuous immersion heating at power densities more than 15 W/cm2 .

The Change from Passivity to Activity in 316 Exposed to Sulphuric Acid 
The corrosion resistance of 316 stainless steel in sulphuric acid is dependent on the stability of the passive chromium oxide coating. The passive film is stable in dilute sulphuric acid at concentrations below 15% up to 60 °C, and corrosion rates are less than 0.1 mm per year. At increasing concentration to 40-60% the reducing tendency of the acid dominates the passive film and there is a shift to active corrosion with rates of 5-20 mm per year. At concentrations above 70%, the sulphuric acid is again oxidising, repassivating the 316 surface and decreasing the corrosion rates to below 0.5 mm/year. The sheath surface temperature is typically 20–50°C above the bulk acid temperature due to the heat flux for an immersion heater. The sheath surface temperature of a 15 W/cm^2 heater in 50°C bulk acid can be 80–90°C. This results in a higher surface temperature and a shift of the passive-to-active transition to lower concentrations. A 30% bulk acid concentration that is safe at 50C can be aggressive at the sheath surface if the local temperature surpasses 70C. Hence, the safe working window for a 1.6 mm 316 sheath is determined by the highest surface temperature, not the bulk acid temperature.

Critical Thresholds for a 1.6 millimetre wall at 15 Watts per square centimetre
Experimental corrosion testing on 316 samples with a surface polish typical of swaged heater sheaths (0.5 to 0.8 microns Ra) yields the following essential thresholds for active corrosion start. The critical concentration of sulphuric acid for the breakdown of passive film is around 55% at the surface temperature of 60 °C. The critical concentration is reduced to 48% at 70°C surface temperature. At 80 °C, the threshold decreases to 42 %. At 90°C it is 38%. At 100°C it reduces to 32%. For a 1.6 mm wall sheath at 15 W/cm 2, the temperature rise from bulk acid to sheath surface is about 25-30 °C in naturally circulated acid. The bulk acid is at 50 °C and the sheath surface at 75-80 °C. Therefore the critical concentration is around 40%. If the bulk acid concentration is above 40% at this temperature, active corrosion will start within days. Once active corrosion has started the corrosion rate is usually 5-15 mm/year therefore a 1.6mm sheath will perforate in 1-4 months. Below the critical concentration, at the same surface temperature, the corrosion rate is below 0.2 mm/year, allowing a service life of 5-8 years. It's a sudden shift, not a gradual one. A change of 2-3% in concentration or 5°C in temperature can imply the difference between years of operation and catastrophic failure within weeks.

1.6 mm 316 Sheath in Sulphuric Acid - Safe Operating Envelope
The following table describes the safe operating envelope for a 1.6 mm 316 stainless steel sheath at a watt density of 15 W/cm2 . Values represent combinations of bulk acid concentration and temperature for which the temperature of the sheath surface is below the passive-to-active transition temperature.

Maximum Safe Bulk Temperature for 1.6 mm Sheath at 15 W/cm2 Bulk Concentration of Sulphuric AcidSheath surface temperature at max safe bulkEstimated Corrosion Rate Estimated Life at 1.6 mm 0 – 10% 95°C 120 – 125°C 0.08 – 0.15 mm/year 8 – 12 years 10 – 20% 90°C 115 – 120°C 0.10 – 0.20 mm/year 6 – 10 years 20 – 30% 80°C 105 – 110°C 0.15 – 0.30 mm/year 4 – 8 years 30 – 35% 65°C 90 – 95°C 0.20 – 0.40 mm/year 3 – 5 years50°C 75 – 80°C 0.30 – 0.60 mm/year 2 – 4 years 40 – 45%Unsafe at any temperature More than 65°C Active corrosion > 5 mm/year Weeks to months
45 – 60% Not safe at any temperature Any Active corrosion > 10 mm/yearDays to weeks 
60 – 70% 50°C 75 – 80°C 0.50 – 1.00 mm/year 1 – 2 years (transitional)80°C 105 – 110°C 0.20 – 0.40 mm/year 3 – 5 years (re-passivated)
80 – 93% 100°C 125 – 130°C 0.10 – 0.20 mm/year 6 – 10 years
Over 93% 120°C 145 – 150°C 0.05 – 0.10 mm/year 10 – 15 years
For bulk acid concentrations from 40 to 60% no combination of bulk temperature and watt density will give a safe 1.6 mm 316 sheath for continuous service. The passive film is unstable at any temperature higher than 30°C, and the high temperature on the sheath surface caused by the heat flow insures active corrosion. At this concentration range, engineers need to specify a more corrosion resistant alloy such as Alloy 20 (Carpenter 20) or Alloy 825, or employ a non-metallic heating technology such as a graphite or silicon carbide immersion heater. For example 38% sulphuric acid at 55°C bulk temperature, at the edge of the safe envelope, a slight process upset can push the sheath into the active region. Engineers must apply a safety margin of at least 5°C below the maximum safe temperature as specified in the table.

Design Modifications to Expand Safe Operating Envelope
When process conditions approach the critical thresholds for a 1.6 mm 316 sheath, three design adjustments can expand the safe operating environment without changing the sheath material. The first is to minimise the watt density. If we work at 8 W/cm² instead of 15 W/cm² the sheath surface temperature drops by about 15–20°C. A heater is dangerous at 15 W/cm 2 in 38% sulphuric acid at 60°C (surface temperature 85–90°C) and safe at 8 W/cm 2 (surface temperature 70–75°C). The trade off is larger heater length or diameter . A second adjustment is to increase the flow of acid across the sheath surface. Forced circulation by a pump or an agitator raises the heat transfer coefficient from 200 – 500 W/m 2 K in natural convection to 1000 – 2000 W/m 2 K in turbulent flow, lowering the surface temperature rise by 40 – 60 %. Thus a heater which produces a 30 deg C rise in stagnant acid may produce just a 12 deg C rise in well-circulated acid, and maintain the sheath surface below the critical level. The third change is the addition of an oxidising agent to the acid. The transition from passive to active states can be shifted to higher temperatures and concentrations by maintaining the oxidising potential of the solution by a tiny concentration (0.1-0.5%) of the nitric acid or hydrogen peroxide. But this is not always practicable due to process constraints. In critical installations where unforeseen heater failure might cause considerable production losses, engineers should install a corrosion monitoring probe – a tiny 316 coupon electronically attached to the heater sheath. A sudden increase in galvanic current between the probe and a reference electrode indicates early breakdown of the passive film and allows the operator to reduce power or change acid content to avoid sheath damage. When specifying heaters for sulphuric acid service, always mention to the supplier the bulk concentration and the expected bulk temperature range and request a calculated sheath surface temperature for the suggested watt density and wall thickness. If the manufacturer can't do this calculation, don't trust them with the application. The difference between safe operation and catastrophic failure in sulphuric acid is sometimes less than 10°C of sheath surface temperature and accurate thermal analysis is important for dependable heater specification.