An Excellent Cost to Corrosion Performance Ratio for Duplex Stainless Steels in FGD Use

Over the past 15 years, the use of duplex stainless steels has increased extensively in many air pollution control  applications. This is the result of continuous improvements in composition and properties. Higher nitrogen contents have increased the duplex stainless steels’ structural stability at high temperature, and consequently, their weldability. The cost advantage of duplex stainless steels over austenitics (which contain more nickel and molybdenum) has become more and more evident during the last four years, with the continuous price increase of alloying elements. Duplex and superduplex grades became increasingly attractive compared to 317LMN and 6 MO in many applications.

J. Peultier, F. Barrau, J. Grocki and J. C. Gagnepain of Industeel have compared the corrosion performance of duplex grade UNS S32205 and superduplex grade UNS S32520 with that of austenitic and superaustenitic stainless steels under conditions encountered in Wet FGD systems. The uniform, pitting and crevice corrosion resistance of the materials was studied using long-term immersion tests with welded assemblies in conditions which simulate process conditions. The results showed that the duplex grade UNS S32205 regularly outperforms the austenitic grade 317LMN. The superduplex UNS S32520/550 is often as resistant as the 6Mo grades. The higher mechanical surface hardness properties of duplex and superduplex stainless steels provide greater resistance to wear, and their higher chromium content increases the rate of repassivation after mechanical damage making these materials much more resistant to erosion by solid particles in aggressive media compared to the austenitic grades. From these results a guideline for the selection of materials of construction for absorber-scrubber applications was developed (see Figure 2).

FIGURE 2.  Material Selection Guide for Absorber Slurry Environments
                  (T = 130˚F/55˚C, Fluoride Content <50ppm)

For the most aggressive conditions where Ni-base alloys are often specified, the super austenitic grade UNS S31266 can also be an alternative. Typical chemical composition of UNS S31266 is:  24 percent Cr – 22 percent Ni – 6 percent Mo – 2 percent W, 1.5 percent Cu and 0.4 percent N.

It is critical that the operator determine in advance the potential for chloride concentration in the recirculating scrubbing liquor. There have been some corrosion problems in systems recently which could have been avoided with adherence to this chart. The factors which determine chloride concentration include not only chlorine in the coal but the bleed rate of  to the wastewater treatment system.

Since 2003, engineering companies and utilities have selected duplex and/or superduplex stainless steel grades for more than 50 new absorber projects (or parts of them) mainly in the US. Among these projects are: Alabama Electric Coop. Lowman; Allegheny Power Hatfield 1, 2, 3 and Fort Martin; AEP Mitchell, Mountaineer, Amos 1, 2, 3, Cardinal 1, 2, Muskingum River and Conesville 4; Detroit Edison Monroe 3, 4; Duke Miami Fort 7, 8, Gibson 1, 2, 3 and Cayuga 1, 2; East Kentucky Power Spurlock 1, 2; First Energy Sammis 1, 2, 3; Kansas City Power  & Light Iatan 2; Ohio Valley Electric Kyger Creek 1, 2, 3, 5, Clifty Creek 1, 6; Progress Energy Roxboro 1, 2, 3, Crystal River 4, 5 and Mayo 1; Reliant Keystone;  TVA Paradise and Kingston; Vectren Warrick and We Energies Elm Road.