What Is the Maximum Service Life of a 316 Stainless Steel Sheath with 1.2 Millimeter Wall ThicWhat Is the Maximum Service Life of a 316 Stainless Steel Sheath with 1.2 Millimeter Wall Thickness in Continuous 15% Phosphoric Acid Service at 110 Degrees Celsius?kness in Continuous 15% Phosphoric Acid Service at 110 Degrees Celsius?

In choosing the sheath material for electric immersion heaters used in chemical process applications such as phosphate fertiliser production, metal treatment baths and food processing equipment handling phosphoric acid, both corrosion resistance and thermal performance need to be considered. Phosphoric acid is not as aggressive as hydrochloric acid or sulphuric acid, however at raised temperatures above 80oC and high concentrations above 10% accelerated attack on 316 stainless steel can occur notably at weld zones and places of high residual stress. The sheath wall thickness sets the corrosion allowance for the heater during its service life, however the connection is nonlinear in that corrosion rates tend to increase with wall thinning and increased local current density. This paper presents a quantitative service life forecast for a 1.2 mm 316 sheath in 15% phosphoric acid at 110°C, based on known corrosion data and field experience in chemical facilities.

Corrosion Behaviour of 316 Stainless Steel in Hot Phosphoric Acid 
In phosphoric acid concentrations between 10% and 30% at temperatures above 80°C, 316 stainless steel has general corrosion rates from 0.1 to 0.5 mm/yr depending on acid purity, temperature, and flow velocity. The corrosion mechanism is mostly uniform thinning rather than pitting since phosphoric acid is not a powerful oxidising agent and does not contain chlorides responsible for localised attack. Commercial phosphoric acid contains contaminants such as fluorides, sulphates and chlorides from phosphate rock which can accelerate attack and give rise to pitting. The general corrosion rate of 316 in 15% phosphoric acid at 110°C with normal industrial purity is about 0.25 mm/year. This rate is based on the sheath surface temperature being within 5°C of the bulk acid temperature. Actually, the heat flux causes a greater temperature of the heater sheath. For a 1.2 mm wall 316 sheath at 6 W/cm² in 110°C acid, the outer surface temperature is around 125°C. At this temperature, the corrosion rate increases to about 0.35 mm/y. The temperature is increased by the heat flux, and an additional 40% is added to the corrosion rate relative to the bulk acid temperature.

1.2 Estimation of service life for wall thickness of millimetre
316 sheath has a nominal thickness of 1.2 mm . However , production tolerances are ±0.1 mm . This means the minimum thickness could be 1.1 mm . The conservative service life estimation is based on the minimum thickness. It uses 0.35 mm/year and corrosion rate is 0.35 mm/year. But failure will occur before the wall can be totally eaten through. Minimum structural thickness is 0.5 mm to resist internal pressures due to MgO expansion and external process pressure. Thus, the useful corrosion allowance is 1.1 mm – 0.5 mm = 0.6 mm. The predicted service life is 0.6/0.35 = 1.7 years at 0.35 mm per year . This is the moment for sheath to attain its minimum structural thickness. In fact, heaters are often replaced at a remaining wall thickness of 0.7–0.8 mm, to allow for a safety margin. At this replacement threshold the service life falls to about 1.2 years. The initial wall thickness for a 5-year service life would need to be 0.35 x 5 years + 0.5 (structural margin) + 0.1 (tolerance) = 2.35 mm or around 2.5 mm. Therefore, a 1.2 mm wall 316 sheath in this service is only suited for applications where annual replacement is acceptable.

Suggested Phosphoric Acid Service Wall Thickness
The following table lists acceptable sheath wall thicknesses for electric immersion heaters in phosphoric acid service as a function of concentration, temperature and desired service life. Values are for industrial grade acid with normal impurities and a watt density of 4 to 6 W/cm2.

Phosphoric Acid Concentration | Bulk Acid Temperature | Target Service Life | Recommended Minimum 316 Wall Thickness |Service Life at Recommended Thickness Anticipated Corrosion Rate at Surface
10 – 15 % 80 – 100 °C 2 years 1.2 mm 0.20 – 0.30 mm/year 1.5 – 2.5 years
10 – 15% 80 – 100°C 5 years 2.0 mm 0.20 – 0.30 mm/year 4-6 years
10–15% 100–120°C 2 years 1.6 mm 0.30–0.45 mm/year 2–3 years
10 – 15% 100 – 120°C 5 years 2.5 mm 0.30 – 0.45 mm/year 5 – 7 years 
15 – 25% 80 – 100°C 2 years 1.6 mm 0.25 – 0.40 mm/year 2 – 3 years 
15 – 25% 80 – 100°C 4 years 2.5 mm 0.25 – 0.40 mm/year 4 – 6 years
15 – 25% 100 – 120°C 1 year 1.6 mm 0.40 – 0.60 mm/year 1 – 2 years
15 – 25% 100 – 120°C 3 years 2.5 mm 0.40 – 0.60 mm/year 3 – 4 years
> 25% Any temperature Any Not recommended > 0.60 mm/yr Unpredictable; use higher alloy
316 stainless steel is not normally suited for phosphoric acid concentrations greater than 25% at temperatures above 100°C, regardless of wall thickness. In this scenario, engineers might consider upgrading to a high-moly alloy such as Alloy 20 (Carpenter 20) or Alloy 625, which have corrosion rates two orders of magnitude lower than 316 in hot phosphoric acid.

Design changes to extend service life without increasing wall thickness
When space or thermal response considerations dictate the use of a 1.2 mm 316 sheath for hot phosphoric acid duty, three design adjustments are available to enhance service life much beyond the estimated 1.7 years. First is reduction of watt density. When the operating level is reduced from 6 W/cm² to 3 W/cm², the sheath surface temperature is reduced from 125°C to about 116°C and the corrosion rate from 0.35 to around 0.22 mm per year. This extends the service life from 1.7 years to 2.7 years. The second change is to passivate the sheath surface with 20% nitric acid at 50°C for 60 minutes before installation. Passivation eliminates imbedded iron particles and increases the thickness of the chromium oxide layer, reducing the initial corrosion rate by 20–30% over the first few months of service. The third change is to manage the purity of the acid. If the fluorides can be kept under 50 ppm and the chlorides under 100 ppm, the corrosion can be limited to 0.15–0.20 mm per year and the service life will be 3–4 years. In important applications where an unforeseen heater failure would result in considerable loss of production, engineers should incorporate a spare heater or build the tank with twin heaters so that one may be replaced and the other maintains temperature. The cost of a spare heater is negligible compared to the cost of an unforeseen stoppage of continuous chemical manufacturing. Specify corrosion coupon testing with the real acid in the process stream when selecting the heaters for phosphoric acid service. Real world acid purity varies widely from plant to plant and even from production batch to production batch, so published data provide estimations. A three month coupon test of a 1.2 mm 316 sample exposed to the process acid at the anticipated sheath temperature is the best indicator of service life. Typically, the cost of this test is less than the cost of one failed heater.