Under What Specific Combination of Feed Water Dissolved Oxygen Concentration and pH Does 316 Stainless Steel Heater Sheath Suffer Flow-Accelerated Corrosion (FAC) at 200°C in High-Velocity Boiler Feedwater

Dissolution of the Passive Film in High Purity High Temperature Water

The high velocity boiler feedwater systems operating at 200°C (typical for high-pressure boiler economisers or feedwater heaters) can be a problem for 316 stainless steel sheathed electric heating tubes due to flow-accelerated corrosion (FAC), which is thought to be immune to this phenomenon (which occurs mainly in carbon and low-alloy steels). The FAC of 316 is different from the traditional FAC of magnetite (Fe3O4) layers on carbon steel where it is a direct dissolution of passive film by the combined impacts of high flow velocity, high temperature and unique water chemistry. The passive film on 316 dissolves at 200°C, when the dissolved oxygen is less than 5 ppb (deaerated) and the pH is greater than 9.5 or less than 6.5. Under such conditions wall thinning rates of 0.1-0.5 mm/year have been seen in high velocity zones (>5 m/s). This paper assesses the dissolved oxygen and pH thresholds for FAC initiation on 316 sheaths at 200°C.

Mechanism of high temperature solubility of passive films

In high-temperature water (200°C) the passive film on 316 is a bilayer of chromium oxide (Cr₂O₃) and iron chromium spinel (FeCr₂O₄). These oxides are very sensitive on pH and dissolved oxygen for their solubility. When deaerated (DO <5 ppb) the corrosion potential is in the active or passive-transition area where the passive film is less stable . At high pH (> 9.5) chromium oxide is dissolved as chromite (CrO 2-) or chromate (CrO 4 2-). At low pH (<6.0) the film dissolves into Cr3+ ions. High flow velocity (>3-5 m/s) removes the dissolved species constantly and prevents the repassivation and maintains a high dissolving rate. The activation energy for FAC of 316 is about 40-60kJ/mol, i.e. the corrosion rate doubles every 15-25°C above 150°C.

Quantified FAC Thresholds of 316 at 200°C in High-Velocity Water

Below are shown corrosion rates and wall thinning behaviour for 316 stainless steel in a controlled loop test at 200°C with variations in dissolved oxygen, pH and flow velocity.

Dissolved Oxygen (ppb) pH at 200°C Flow Velocity (m/s) Dominant Oxide Solubility FAC Rate (mm/yr)Time to 0.5 mm Wall Loss (hours) Recommended for 200°C Service <5 (deaerated) 6.5-7.5 Any Minimal <0.02 >25,000 Yes <5 7.5-8.5 <3 Minimal <0.02 >25,000 Yes <5 7.5-8.5 3-5 Slight 0.02-0.05 10,000-25,000 Acceptable <5 8.5-9.5 3-5 Cr(OH)3 solubility 0.05-0.15 3,500-10,000 Not recommended <5 9.5-10.5 >3 Chromite dissolution 0.15-0.40 1,250-3,500 Unacceptable <5 >10.5 Any Rapid chromite >0.40 <1,250 Unacceptable <5 5.5-6.5 >3 Cr3+ solubility 0.10-0.30 1,700-5,000 Unacceptable <5 <5.5 Any Rapid Cr3+ >0.30 <1,700 Unacceptable
5-20 (low) 8.5-9.5 >3 Reduced solubility 0.03-0.08 6,000-17,000 Marginal 
20-100 (moderate) 8.5-9.5 >5 Passive film stabilized <0.02 >25,000 Yes 20-100 9.5-10.5 >5 Some stabilization 0.02-0.05 10,000-25,000 Acceptable >100Any Any Fully passive <0.02 >25,000 Yes 
The FAC of 316 is very dependent on flow velocity.

Flow velocity is the scaling factor that transforms passive film solubility into measured wall thinning. The table below shows the velocity dependency at 200° C, deaerated conditions, pH 9.2 (marginal zone).

Flow Velocity (m/s) Wall Shear Stress (Pa, estimated)FAC Rate @ DO<5 ppb, pH 9.2 (mm/yr)Time to 50% of Original 1.5 mm Wall Loss(hours) Recommended Maximum Velocity for 10-Year Life <1 <10 <0.01 >75,000 Safe 1-2 10-20 0.01-0.03 25,000-75,000 Safe 2-3 20-35 0.03-0.06 12,500-25,000 Acceptable 3-5 35-60 0.06-0.12 6,000-12,500 Marginal (monitor) 5-8 60-100 0.12-0.20 3,750-6,000Not recommended 8-12 100-150 0.20-0.35 2,000-3,750 Unacceptable >12 >150 >0.35 <2,000 Not acceptable
Design Specification and Practical Water Chemistry of Feedwater Heaters at 200°C

316 encased heaters at 200°C in high velocity boiler feedwater are protected from flow accelerated corrosion by the following requirements.

Feedwater Type Target Dissolved Oxygen (ppb) Recommended pH, 200°C Maximum Flow Velocity at Sheath (m/s) Expected Heater Life (years)
All volatile therapy (AVT) - reduction 2-5 8.0-8.5 3.0 8-10
All-volatile treatment (AVT) - oxidizing 50-100 8.0-8.5 5.0 10-15 
Oxygenated therapy (OT) 100-300 8.0-8.5 6.0 >15 
Phosphate treatment 5-20 8.5-9.0 3.0 5-8 (monitor) 
Any treatment with pH >9.5 N/A >9.5 <2.0 5-10 (316 marginal)
Field Identification of FAC on 316 Heater Sheaths

The FAC on a 316 heater will produce unique surface features that are different from conventional corrosion mechanisms in high temperature, high velocity feedwater. The surface is smooth, polished or "orange-peel" in appearance with no pitting or scale. The wall thinning is homogeneous at the high velocity zones (e.g. pipe entry, directly facing the flow). The thinning is often a steady drop of outside diameter throughout the heater length. Scanning electron microscopy (SEM) shows a smooth, featureless surface, with no attack at the grain boundaries. Water chemistry log will show pH >9.5 or <6.0 and dissolved oxygen <5-20 ppb for prolonged durations. For such failures the answer is to adjust water chemistry to keep pH in the 8.0-8.5 range, and dissolved oxygen >20-50 ppb (if compatible with overall system), or to limit flow velocity by placing the heater in a bypass or stilling well. FAC susceptibility can be eliminated by changing to titanium or Alloy 825.

Summary: Feedwater Chemistry and Velocity Management to Prevent FAC

The flow accelerated corrosion of 316 stainless steel encased heaters in 200°C boiler feedwater takes place in deaerated conditions (DO <5-10 ppb) in combination with alkaline pH >9.5 or acid pH <6.0 and flow velocities >3-5 m/sec. Under these conditions, the passive film is dissolved as chromite or chromium ions, resulting in wall thinning rates of 0.1-0.5 mm/year. For high temperature feedwater service with 316 sheaths, engineers should stipulate pH of 8.0-9.0, dissolved oxygen >20 ppb (ideally 50-100 ppb) and a flow velocity below 3-4 m/s at the sheath surface. If high velocity (>5 m/s) cannot be avoided, oxidising water treatment (oxygenated treatment, OT) with DO >100 ppb will stabilise the passive film and avoid FAC. The methodology given here relates dissolved oxygen, pH, and flow velocity to measurable FAC rates at 200°C, and allows the buyer to define water chemistry and hydraulic parameters that will maintain the passive film on 316 heater sheaths in high velocity boiler feedwater operation.