Hyperduplex stainless steel as alternative to Ni-based alloys
In the early 1990s higher alloyed superduplex stainless steels such as UNSS32750 were developed, and has since then gained increased use. This development has now continued to even higher alloying levels, giving a performance close to that of Ni-based alloys and Titanium in various critical applications.
The hyperduplex stainless steel UNSS32707 is characterised by a ferritic-austenitic (duplex) microstructure, having an alloying content giving a Pitting Resistance Equivalent (PRE) value of nominal 49. The PRE is typically noted as PRE=%Cr+3.3x%Mo+16x%N, and give an indication on a materials resistance to pitting corrosion. As a comparison the 22%Cr duplex grades have PRE up to approximately 35, while the superduplex grade UNSS32750 is around 42.
The main elements of UNSS32707 is shown below (nominal wt-%):
Heat exchangers are typically operating under highly corrosive conditions, and offer many challenges when it comes to materials selection. Hyper duplex stainless steels are suitable for many different heat exchanger services within chemical- and petrochemical plants. In the following a few typical examples are given:
Seawater cooled heat exchangers
The chlorides in the seawater have a significant effect on the corrosivity, and can give severe localised corrosion on low alloyed steels and on stainless steels. Even at low temperatures there are limits on what the materials of construction can handle, and when temperatures are increased such as in heat exchangers using seawater as cooling media, then higher alloyed stainless steels, Ni-based alloys or Titanium grades will be needed.
Besides the chloride content, main factors affecting the corrosivity of a cooling water are temperature, the acidity (pH) and the oxidising potential. Chlorination used to kill biofilm can have severe negative effect on lower grades, as it increases the oxidising potential. Pollutants, fouling behaviour, entrained solids and micro-organisms are additional factors of consideration. Erosion on low hardness materials, such as eg copper based alloys, can be very high in cooling waters, particularly those containing solid sand particles.
For hot seawater service, hyper duplex UNSS32707 heat exchanger tubing can be an alternative to the often specified Titanium, due to its very high resistance to seawater corrosion combined with its fabrication compatibility and economical advantages.
Crude oil distillation
In the overhead systems of the atmospheric- and vacuum distillation units, rapid corrosion on carbon steels can occur as a result of periodically inferior control of the process parameters, contamination of the crude or malfunctioning of the corrosion inhibition system.
Acidic conditions arise from chloride salts entering the refinery in the crude oil. Even after the desalting operation there are some salts left in the crude that can hydrolyse to HCl when the crude is heated.
Tubes in hyper duplex stainless UNSS32707 have been installed in atmospheric distillation overhead condensers and air coolers, and results have so far been very promising. This upgrade will give more uptime of equipment and lower costs for maintenance.
Organic acids can, especially if impurities are present, be relatively corrosive. Standard and high alloyed austenitic grades are being used where CRA´s are applicable, but duplex stainless steels are now more frequently used due to their suitability for this type of service. The highest alloyed hyper duplex stainless steel UNSS32707 has properties that are equal or better than many of the more expensive Ni-based alloys.
In production of chlorinated hydrocarbons (EDC, VCM, etc) hydrolysis where HCl is formed typically occurs. Being a very strong acid, this can give rapid attack on carbon steel. In true service it is difficult to keep control of the HCl dew point, and acid concentrations can be very high locally. Selection of hyper duplex stainless steel could improve the situation significantly, and provide a long lasting upgrade solution improving the overall economics of the unit.
Martin Holmquist is with Sandvik Materials Technology, Schiedam, The Netherlands. www.smt.sandvik.com
