At What Specific Combination of Thermal Cycle Frequency and Peak Sheath Temperature Does a 2.5 Millimeter 316 Stainless Steel Sheath Experience Oxide Exfoliation That Reduces Wall Thickness Below 2.0 Millimeters Within One Year in Cyclic Steam Superheater Service?

The combination of high temperature steam and cyclic operation produces a unique oxidation environment between the power plant engineer's electric steam superheater and the industrial boiler specialist's 316 stainless steel sheaths. Unlike air heating where oxide scales build at relatively slow rates, steam accelerates oxidation because water vapour diffuses through the oxide layer and reacts with the metal underneath. Spalling of oxide scales on thermal cycling is especially problematic for thick-walled sheaths. The build-up of oxide gets thick enough to spall, taking with it base metal with each exfoliation event. For superheater service the wall thickness is sometimes specified as 2.5 mm to allow for creep strength and corrosion allowance. However, for some combinations of peak temperature and cycle frequency, oxide exfoliation can erode 0.5 mm of wall thickness in a year and hence compromise the mechanical integrity. This paper establishes specific limits for unacceptable rates of exfoliation for a 2.5-mm 316 sheath in cyclic steam use.

Mechanisms of steam oxidation and delamination in 316 stainless steel
In superheated steam at temperatures exceeding 550°C, 316 stainless steel develops a duplex oxide scale with an exterior magnetite (Fe3O4) layer and an inner chromium-rich spinel layer. The rate of oxidation follows a parabolic law; however, the addition of steam enhances the rate by a ratio of 2-5 compared with air at the same temperature. A 316 sheath grows ~10 microns of oxide in steam after 1000 hours at 600°C. At 650 °C the thickness is 25 microns. 700°C, 60 microns. Oxide is greater than 120 microns at 750°C after 1000 hours. Spallation of the oxide during cooling is due to the oxide layer growing beyond a critical thickness and the mismatch of the thermal expansion of the oxide and the metal. The essential thickness for exfoliation of oxides formed in steam is around 30-50 microns. Each time the oxide layer is removed, a tiny coating of base metal (2-5 microns in most cases) is also removed. After 1000 hours of cumulative exposure of 0.25 mm sheath at 700°C with 100 thermal cycles each year, oxide would grow to 60 microns and exfoliation would take place. If the exfoliation takes place every 1,000 hours and each time takes off 4 microns of metal, the annual wall loss will be about 0.035 mm-negligible. However, exfoliation can occur more frequently at higher temperatures and/or more frequent cycles, which accelerates wall degradation.

Critical Thresholds for Accelerated Sheath Exfoliation in 2.5 mm
Cyclic controlled steam oxidation tests of 2.5 mm 316 sheath samples have shown the exact limits beyond which wall loss would be greater than 0.5 mm per year – the point at which a 2.5 mm sheath would be reduced to 2.0 mm in 12 months.

Maximum Sheath Temperature Hours to First Exfoliation, Cumulative Thickness of Oxide at Exfoliation Metal Loss per Exfoliation Event Number of Cycles per Year for Annual Loss to Exceed 0.5 mmDominant Failure Mechanism 550°C 8,000 – 10,000 hours 25 – 30 microns 2 – 3 microns Not possible below 2,000 cycles/year Exfoliation not significant 600°C 2,000 – 3,000 hours 35 – 45 microns 3 – 5 microns Above 500 cycles/year Marginal risk 650°C 500 – 800 hours 50 – 70 microns 4 – 6 microns Above 150 cycles/year Moderate risk 700°C 150 – 250 hours 80 – 120 microns 5 – 8 microns Above 50 cycles/year High risk 750°C 50 – 100 hours 150 – 200 microns 8 – 12 microns Above 20 cycles/year Severe exfoliation 800°C 20 – 40 hours 250 – 350 microns 12 – 20 microns Above 10 cycles/year Rapid wall loss
For a superheater at 700°C peak sheath temperature and 100 cycles per year (approximately 2 cycles per week), the annual wall loss will be 100 cycles divided by the 200 cycles between exfoliation events (midpoint of 150–250 hours at 24 hours per cycle is approximately 6–10 cycles per event). Annual wall loss is 0.05–0.12 mm, far below 0.5 mm, with 10–15 exfoliation events/year and 5–8 microns loss/event. At 700 °C with 300 cycles per year (around 6 cycles per week), annual loss is 0.15–0.36 mm, approaching the limit. At 750°C with 100 cycles/year the annual loss is 0.5–1.2 mm, which is above the threshold. Therefore, the important combination for unacceptable wall loss is peak temperature greater than 730°C, with more than 100 cycles/year, or peak temperature greater than 700°C, with more than 300 cycles/year.

Safe Operating Envelope of 2.5 Millimetre Sheath in Steam Service
The following table shows the maximum suggested number of cycles per year for a 2.5 mm 316 sheath in cyclic steam superheater duty to keep the yearly wall loss below 0.2 mm (less than 10% of wall thickness per year).

Peak Sheath Temperature Maximum Recommended Cycles per Year Expected Annual Wall Loss at Maximum Cycles Estimated Service Life to 2.0 mm Remaining Wall Recommended Action Above Limits Up to 600°C 1,000 Below 0.05 mm Over 10 years 316 acceptable 600 – 650°C 500 0.05 – 0.10 mm 8 – 10 years Acceptable for most service 650 – 680°C 250 0.10 – 0.15 mm 5 – 8 years Monitor annually 680 – 700°C 150 0.15 – 0.20 mm 4 – 6 years Consider alloy upgrade for longer life 700 – 720°C 80 0.20 – 0.30 mm 3 – 4 years 316 marginal 720 – 740°C 40 0.30 – 0.45 mm 2 – 3 years Upgrade to Incoloy 800H recommended 740 – 760°C 20 0.45 – 0.70 mm 1.5 – 2 years Incoloy 800H required
Not recommended Above 760 °C Above 0.70 mm Less than 1.5 years Use nickel based alloy
Incoloy 800H offers substantially greater steam oxidation resistance than Incoloy 800 for superheater applications when peak sheath temperatures exceed 720°C and more than 50 cycles per year. The alloy produces a thinner, more adherent oxide that exfoliates above 100 microns thickness, resulting in a factor of 5–10 reduction in annual wall loss compared with 316.

Design changes to reduce flaking in thick-walled sheaths
When a 2.5 mm 316 sheath must run near or above the permissible limits, three design changes can reduce exfoliation without changing the sheath material. The first is, regulated cooling ramp rates. Cooling from peak temperature to ambient at rates below 50°C per hour minimises thermal shock and results in smaller, less destructive spall fragments. The second alteration is to pre-oxidize the sheath surface prior to cyclic service. Running the heater at full temperature constantly for 500–1,000 hours grows a thicker, more stable oxide that is less likely to exfoliate frequently. A third change is to keep the standby minimum temperature at 300°C, instead of dropping down to ambient. This reduces the heat strain every cycle by 40-60%, therefore greatly increasing the number of cycles between exfoliation episodes. For most steam superheater applications, a 2.5 mm 316 sheath is acceptable for peak temperatures up to 680°C with tolerable cycle frequencies. Beyond 700°C, engineers must either accept a service life of 2–4 years or upgrade to Incoloy 800H. The cost difference between 316 and Incoloy 800H for a superheater is typically 30-50%, but the increased longevity and lower chance of oxide blockage in downstream equipment make the upgrading worthwhile. When specifying heaters for cyclic steam operation, always mention the predicted peak sheath temperature, cycle frequency and desired service life. A manufacturer recommending 316 for 750°C duty without mentioning oxide exfoliation and wall loss is not giving complete engineering advice. Steam oxidation exfoliation creates a consistent and predictable wall loss, but this will need to be taken into account in service life calculations. This will be especially important for thick-walled sheaths where exfoliation will remove more metal per event due to the thicker oxide scale formed at higher temperature.