Under What Fluid PH and Temperature Combination Does the 316 Stainless Steel Sheath in a Submerged Heater Transition from Passive to Active Corrosion in Sulfuric Acid Solutions

pH-Temperature Domain of Irreversible Breakdown of Passive Film

The corrosion behaviour of 316 stainless steel wrapped electric heating tubes in sulphuric acid (H2SO4) solutions is not a straightforward function of acid concentration or pH alone. Instead, it has a well defined threshold where the protective chromium oxide coating changes from stable passivation to active dissolution with quick and uniform metal loss. This transition is very dependent on the solution temperature and pH (or more accurately the concentration of free sulphuric acid). Below the threshold (generally pH > 1.5 at 25 °C, rising to pH > 2.5 at 80 °C), 316 exhibits passive behaviour with corrosion rates of less than 0.1 mm per year. Above the threshold (more corrosive conditions or higher temperatures) the passive film locally dissolves and the corrosion rate increases to 1-10 mm per year, perforating a 1.5 mm sheath wall within months or weeks. This paper calculates the passive-to-active transition boundaries for 316 sheaths in sulphuric acid service, gives a decision framework for specification of heaters in acidic environments and defines the safe operating windows for reliable long-term performance.

The Electrochemical Mechanism of Passivity Breakdown in Sulphuric Acid

The passive film on 316 stainless steel in sulphuric acid is a chromium-enriched oxide film (mostly Cr2O3 with some oxides of iron and molybdenum). It forms spontaneously at open circuit in solutions with sufficient oxidising power. In dilute sulphuric acid (pH > about 2.0) dissolved oxygen supplies the cathodic reaction (oxygen reduction) which maintains the potential in the passive range. The film is usually 2-5 nm thick and its stability depends on the balance between film dissolution (favoured by low pH and high temperature) and film repassivation (favoured by the presence of dissolved oxygen or other oxidisers). If the pH is lowered below a certain value or the temperature is raised, the dissolving rate of the chromium oxide increases exponentially, since the solubility of Cr 3+ species in acidic solutions increases significantly. If the dissolving rate exceeds the repassivation rate, the layer is locally thinned and bare metal is exposed which dissolves fast (active corrosion). The molybdenum concentration (2.0-2.5 %) is essential for passive stability in reducing acids such as sulphuric for 316. Molybdenum builds up in the passive film and supports the development of a more stable molybdate layer which hinders active dissolution. But even with molybdenum there is a distinct pH-temperature barrier. 316 is a passive corrosion resistant alloy below the threshold. Above the barrier it behaves like ordinary carbon steel.

Quantification of Passive-to-Active Transition Boundaries for 316 in Sulphuric Acid

Passive to active transition of 316 stainless steel in sulphuric acid has been established over the temperature range 25-100 °C by systematic corrosion testing employing weight loss measurements and electrochemical potentiodynamic polarisation. 316 is passive up to ~10 wt% H2SO4 (pH ~0.5) at 25 °C. At 50 °C the passive limit is reduced to 5 wt% H₂SO₄ (pH about 0.8). At 80 °C the limit is reduced to 1 wt% H2SO4 (pH ~1.2). The limit at 100 °C (boiling dilute acid) is around 0.5 wt % H2SO4 (pH about 1.5). Below these concentrations corrosion rates are less than 0.1 mm/year. Corrosion rates are very high above these concentrations, reaching >2 mm/year in 5 wt% H2SO4 at 80 °C and perforating a 1.5 mm wall in <9 months. In high concentration sulphuric acid (over 90 wt%) 316 does indeed become passive again due to the production of a new layer. But such concentrations are rarely found in immersion heating applications. The most hostile situation for 316 is the intermediate concentration range (0.5-15 wt% depending on temperature) when the passive film is unstable. The detailed corrosion rates for 316 sheaths in sulphuric acid are shown in the table below as a function of concentration, temperature and time to perforation.

Concentration H2SO4 (wt%)Estimated pH Solution Temperature (°C) Corrosion Rate (mm/yr)Time to Perforation (1.5mm wall) Recommended for Continuous Service 0.1 1.5 25 0.02 >75 years Yes 0.1 1.5 80 0.05 >30 years Yes 0.1 1.5 100 0.15 10 years Marginal (monitor) 0.5 1.2 25 0.03 >50 years Yes 0.5 1.2 80 0.20 7-8 years Marginal, upgrade to 316L or Alloy 20 0.5 1.2 100 0.80 1.5-2 years Not recommended 1.0 1.0 25 0.05 >30 years Yes 1.0 1.0 60 0.30 5 years Acceptable with inspection
1.0 1.0 80 2.0 9 MonthsNot recommended 2.0 0.8 25 0.15 10 years Acceptable for occasional use
2.0 0.8 50 1.0 1.5 yearsNot recommended 5.0 0.5 25 0.50 3 years marginal (upgrade)
5.0 0.5 50 4.0 4-5 months Not acceptable
10.0 0.2 25 1.5 1 year Not appropriate for 316 unless cold and intermittent
15.0 0.0 25 3.0 6 months Unacceptable
The Extension of the Passive Range by Oxidising Agents

The above passive-active limits are applicable to aerated solutions (i.e. oxygen dissolved in the solution). Under deaerated or reducing circumstances (e.g. sulphuric acid free of oxygen and with hydrogen from corrosion present) the passive range narrows considerably due to limitation of the cathodic reaction (oxygen reduction). 316 can be actively corroded within hours instead of months at 80 °C in 0.5 wt% H2SO4 with deaeration. However, the addition of oxidising chemicals, such as nitric acid, ferric (Fe3 +) or cupric (Cu2 +) ions, expands the passive range to higher concentrations and higher temperature. In sulphuric acid containing 1000 ppm Fe3+ 316 stays passive up to 10 wt% at 80 °C-a tenfold improvement . In heater applications where the process solution incorporates oxidising species as an inherent component (e.g., pickling baths containing ferric sulphate, or waste acid streams from metal finishing), 316 may be suitable even at conditions that would lead to active corrosion in pure acid. The problem with depending on the existence of oxidisers is that process disturbances might lower their concentration and induce abrupt acceleration of corrosion. The passive-active boundaries in the preceding table should be unambiguously satisfied under worst-case (minimum oxidising potential) conditions for engineers to specify 316 for sulphuric acid service.

Actual detection of active corrosion in running heaters

Three field indicators show that active corrosion has started for 316 sheaths working in sulphuric acid solutions close to the passive-active boundary. The first is a colour change of the solution: vigorous dissolution of 316 creates green Fe 2+ and Ni 2+ ions, converting the normally colourless or slightly yellow acid to a pale green or blue-green tint. The second is the evolution of hydrogen gas. The active corrosion of stainless steel in acid is accompanied by the production of hydrogen bubbles on the sheath surface, which can be observed as a stream of fine bubbles rising from the heater. In passive state, there is no hydrogen evolution, because the cathodic process is oxygen reduction. The third sign is a quick decline in heater sheath temperature for the same power input. The electrical resistance of the internal heating wire remains the same. The sheath wall is becoming thinner and has less thermal resistance. If the power density is constant the outside surface temperature will be reduced. The considerable wall thinning is evidenced by a measured surface temperature reduction of 5-10 °C during 1-2 months. If the heater can be removed for inspection, wall loss can be followed over time with an ultrasonic thickness gauge for quantitative monitoring. Begin preparing for replacement if the corrosion rate exceeds 0.3 mm per year.

Alternative Alloys When 316 Goes Beyond the Passive Limit

For the sheathed heater, if the necessary sulphuric acid concentration and temperature are outside the passive range for 316, various more resistant alloys are available. Alloy 20 (Carpenter 20) comprises 32-38 % Ni, 19-21 % Cr and 2-3 % Mo plus copper and provides excellent resistance to dilute sulphuric acid up to 10-15 wt% at boiling. Similar performance with a little less nickel is Incoloy 825 (Alloy 825). Alloy C-276 (Hastelloy C-276) or Alloy B-3 (Hastelloy B-3) are required for larger concentrations (15-50 wt% at moderate temperatures), but are considerably more expensive. For very dilute acid (less than 1 wt%, pH>1.0) up to 80 °C adequate life (5-10 years) is provided by 316L with low carbon and proper surface polish. In any application where the temperature and concentration combination is close to the active region, the corrosion risk is eliminated completely by selecting a heater with a replaceable sheath or lined vessel with external heating, eliminating the heater from direct contact with the acid.

Conclusion: Safe Operating Limits for 316 in Sulphuric Acid Services

Passive corrosion rates of 316 stainless steel encased immersion heaters are 0.02-0.2 mm/year, but active rapid dissolution rates are 1-10 mm/year in sulphuric acid solutions. The pH-temperature threshold for transition from passive corrosion to active fast dissolution is largely dependent on acid concentration. 316 can tolerate up to 10 wt% H2SO4 at ambient temperature. At 80 C the acceptable limit is reduced to 0.5-1 wt%. Even at 0.5 wt% the corrosion rates are minor at 100 °C. When engineers specify 316 sheathed heaters for acidic operation, they must determine the actual process acid concentration and maximum operating temperature, then compare those values to the quantitative boundaries of the table above. If the combination falls within the marginal or unacceptable range then either the heater must be derated to a lower temperature (by reducing power density), the standard must be upgraded to a more corrosion resistant alloy, or the heating technique must be changed to indirect or external heating. By tying the passive-active transition to quantifiable chemical and thermal characteristics, the methodology proposed here enables purchasers to anticipate the service life of 316 sheaths in sulphuric acid settings and avoid the rapid aggressive corrosion that happens after the passive film fails.