Stress Corrosion Cracking in Coal Liquefaction Equipment
Publication Date: 2012-Jul-17
The IP.com Prior Art Database
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STRESS CORROSION CRACKING IN COAL LIQUEFACTION EQUIPMENT
V. B o Baylor and L. A. Zeis
Introduction Commercialization of coal liquefaction processes depends on the
relihble operation ofi.-,ch larger scale plants than currently exist.
Basing materials selection for large plants on experience in pilot plants involves certain risks, considering the influence of coal chemistry and design on materials performance; however, certain problems will be the same in both large- and small-scale plants. One such problem may be the stress, corrosion .cracking of austenitic stainless steels-observed In the-- reaction and vapor-liquid sep~ration areas in the coal liquefaction pilot plants.
Stress corrosion cracking failures are due to the combined action of a tensile stress and a corrosive agent. Stresses can be applied or residual; cracking may be Intergranular or transgranular. In coal liquefaction systems, ~he fo~mer~is often attributed to the actlou of polythlonlc acid
on sensitized stainless steel~ whereas t~he latte~ is often caused by chlorides ~ either sensitized or annealed stainless steel. Polythlonlc acids, H2SzO6 (x - 3-5), are formed by exposure of metal sulflde scales to moist air,during shutdowns. Stresses can result from applied loads but can also be residual, as produced by welding or mechanical deformation during metal processing and assembly.
The austenltic stainless steels have been used in the coal liquefac- tion environments to provide good corrosion resistance. They are used primarily for piping and as vessel claddlnE or overlays in the reactor area. The reactor vessel shell is generally a low-alloy steel such as
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2~4 Cr-1Mo, and the stainless steel is used to protect the shell from t~ hydrogen, sulfur, and other corrosive species. Unfortunately, nonstabilized
or higher-carbon grade austenitic stainless steels can become sensitized
durin~ welding or high temperature operation and are thus susceptible to
polyth£onic acid attack. A/1 300-series stainless steels are also prone ~o
chloride stress corrosion cracking in aqueous solutions.
The view generally held by process developers was that the coal liquefaction streams after dlssolutlon should be very s~milar to crude oLl refinery streams (except for the presence of solids), and that, therefore, the materials problems should be amenable to current oll industry prac- .tlces.: En~ughd/fferences ex/st between the process stream chem~strles and observed materials behavior, however, to ~arrant caution in d~rect appllcatlon of refinery practices. Although the materials data base for refineries is certainly appllcable to coal llquefactlon processes, it
should be modified by testing and experience.
Review of Pilot Plant Ex~erlence
Incidents of stress corrosion cracking of austeultln stainless steels
observed at the SRC plants are described below. Most failures were attri- buted to either polythlonlc acids or chlorldes. The polythlonlc.acld stress corrosion cracking occurred in...