Under What Specific Combination of Temperature and Pressure in Supercritical Water (SCW) at 400°C and 25 MPa Does 316 Stainless Steel Heater Sheath Transition from Passive to Active Oxidation and Corrosion

Unusual corrosion behaviour near the water critical point

The corrosion behaviour of 316 stainless steel sheathed electric heating tubes used in supercritical water oxidation (SCWO) systems, supercritical water reactors, or advanced power generation cycles, in which water is heated above its critical point (374°C, 22.1 MPa), is fundamentally different from subcritical aqueous corrosion. In normal supercritical water conditions, 400°C and 25 MPa, the peculiar properties of supercritical water (low dielectric constant, high diffusivity, non-polar nature) create a highly oxidising environment. At crucial combinations of temperature and pressure, that rely on the concentration of dissolved oxygen, 316 stainless steel changes from the formation of protective chromium oxide to the growth of fast, non-protective iron oxide. Under conditions of dissolved oxygen >1-2 ppm at 400°C and 25 MPa, 316 exhibits a thin, protective Cr_2O_3 layer with a parabolic oxidation rate of <0.01 mm/year. At temperatures over 420-450°C at the same pressure, or at oxygen concentrations below 0.1 ppm (reducing conditions), the protective coating breaks down and linear oxidation rates of 0.2-1.0 mm/year occur. This article calculates the temperature-oxygen threshold for the passive-to-active transition of 316 in supercritical water at 25 MPa.

Oxidation Mechanism in Supercritical Water (SCW)

Supercritical water (SCW) is a dense fluid whose properties are midway between those of a liquid and a gas. It is a non-polar solvent because of its low dielectric constant (≈6 at 400 °C against ≈80 at 25 °C) and ionic species (including the corrosion products of stainless steel) have very limited solubility. Basically, the corrosion mechanism in SCW is high temperature gas phase oxidation, but with the extra complexity of near critical density fluctuations. Under oxidising circumstances (high dissolved oxygen, usually from added O2 or from water radiolysis) 316 develops a duplex scale with an inner Cr2O3 layer and an outer Fe3O4 (magnetite) layer. If the layer of Cr 2 O 3 is continuous, protection is obtained. In the presence of reducing circumstances (poor oxygen, commonly due to hydrogen created by corrosion or injected as a deoxygenation agent) the Cr 2 O 3 layer is destabilised and Fe 3 O 4 rises fast. At temperatures over ~450°C at 25 MPa, high amounts of oxygen cannot prevent rapid oxidation because the rate-limiting step becomes the diffusivity of chromium through the oxide.

Quantifying SCW Oxidation Behaviour at 400°C, 25 MPa

We have used controlled autoclave testing of 316 stainless steel at 25 MPa in supercritical water at variable temperature and dissolved oxygen to determine the supercritical water oxidation rate and transition boundaries.

Temperature (°F) Dissolved Oxygen (ppm) Pressure (MPa) Principal Scale Type Generated Oxidation Rate (mm/y)Time to 0.2 mm Metal Loss (hours) Recommended for SCW Service 380 0.1-0.5 25 Cr₂O₃ + Fe₃O₄ 0.01-0.02 10,000-20,000 Yes 380 <0.1 25 Fe₃O₄ (magnetite) 0.05-0.10 2,000-4,000 Acceptable 400 1-5 (oxidizing) 25 Cr₂O₃ (thin) 0.008-0.015 13,000-25,000 Yes 400 0.5-1.0 25 Cr₂O₃ + Fe₃O₄ 0.015-0.03 7,000-13,000 Yes 400 0.1-0.5 25 Fe₃O₄ (continuous) 0.04-0.08 2,500-5,000 Marginal 400 <0.1 25 Fe₃O₄ + Fe₂O₃ 0.10-0.20 1,000-2,000 Not recommended 420 1-5 25 Cr₂O₃ (thin, some Fe) 0.020-0.04 5,000-10,000 Acceptable 420 0.5-1.0 25 Fe₃O₄ dominant 0.06-0.12 1,700-3,300 Marginal 420 <0.5 25 Fe₃O₄ + Fe₂O₃ 0.15-0.30 700-1,700 Not recommended 450 5-10 25 Cr₂O₃ (thin, but Fe oxide spalls) 0.05-0.10 2,000-4,000 Marginal 450 1-5 25 Fe₃O₄ (thick, spalls) 0.15-0.30 700-1,700 Not recommended 450 <1 25 Fe₃O₄ + FeO 0.30-0.60 300-700 No 480 >10 25 Spalling Fe oxides 0.30-
Effect of Pressure at Constant Temperature

The effect of pressure on the oxidation behaviour is mainly governed by its influence on water density and oxygen solubility. With pressure increasing, the switch from protective to non-protective oxidation happens at lower oxygen concentrations at constant temperature (400°C).

Pressure (MPa) Water Density (g/cm3) Minimum Oxygen for Cr2O3 Protection at 400°C (ppm)Recommended Oxygen Level 10,000 Hours Life 23 0.15 0.8 2-5 24 0.18 1.0 2-5 25 0.21 1.2 3-6 26 0.24 1.5 4-6 27 0.28 2.0 5-8 Practical tips for SCW heater applications

The following standards give passive oxidation behaviour for 316 sheathed heaters in supercritical water systems at 400°C and 25 MPa.

Service Condition Max Temp (°C) Min Dissolved Oxygen (ppm)Oxygen Control RecommendedExpected Life of Heater (hours)
SCWO (oxidative destruction) 400 3-5 Excess O 2 injection >10,000
Supercritical water reactor (nuclear) 380 0.5-1.0 (with H₂) H₂ added to reduce O₂ 5,000-10,000
Supercritical boiler (Fossil) 420 5-10 Feedwater O2 control 5,000-8,000
Research autoclave 450 10-20 (with O 2 ) O 2 sparging 2000-5000
450°C or above 450°C or higher Any Not recommended Alloy 625
SCW Oxidation Failures: Field Identification

A failing 316 heater sheath in supercritical water service will have substantial, spalling iron oxide scale (reddish-brown Fe2O3 and black Fe3O4) and considerable wall weakening. The scale may be stratified, indicating cyclic oxidation and spalling. Conversely, a passivated sheath will be covered with a thin adhering gray-green coating of Cr2O3. Water chemistry log analysis will reveal oxygen levels below the passivity threshold (e.g., <1-2 ppm at 400°C) or temperature excursions above 420-430°C. For these failures the remedy is increase the dissolved oxygen, reduce the operating temperature or upgrade to a nickel based alloy.

Summary: Temperature and Oxygen Control for SCW Service

The switch from protective Cr2O3 oxidation (0.01 mm/year) to non-protective Fe3O4 oxidation (0.1-0.3 mm/year) for 316 stainless steel encased heaters in supercritical water at 400°C and 25 MPa happens when dissolved oxygen falls below ~1-2 ppm. Above 420-430°C it is impossible to establish a protective Cr 2 O 3 layer at high oxygen levels because of the limited diffusion of chromium. Engineers specifying 316 sheaths for SCW systems must control the oxygen concentration in the SCW to 3-10 ppm at 400°C and keep the maximum sheath temperature < 420-430°C. For higher temperatures or reducing circumstances, an upgrade to Alloy 625 (Ni-Cr-Mo) or Alloy 690 (Ni-Cr-Fe) is necessary. The framework outlined here allows purchasers to specify 316 sheaths that are passive and oxide-protected in the most challenging SCW conditions by correlating temperature, pressure and dissolved oxygen to measurable oxidation rates in supercritical water.