Pdf polythionic acid corrosion in refinery hydroprocessors. Polythionic acid stress corrosion cracking pascc is an affliction of many refineries. The most common fluid causing intergranular corrosion in hydrocarbon plants is polythionic acid. This damage mechanism primarily affects standard and high carbon stainless steels, but low carbon and chemically stabilized stainless steels may also be vulnerable under certain conditions. Photomicrographs will be shown that demonstrate the lack of grain boundary sensitization, and also the lack of grain boundary ditching in the oxalic acid test.
Stress corrosion cracking scc is the formation and growth of crack. Presence of cr, carbon, exposure temperatures and presence of polythionic acids play an important role in stress corrosion cracking. Alloy for resistance to polythionic acid stress corrosion. Intergranular and transgranular stress corrosion cracking of the aisi 316l stainless steel at polythionic acid environment 8. Polythionic acid corrosion in refinery hydroprocessors. Stress corrosion cracking by krittin rattakham youtube. Ctlpolythionic stress corrosion cracking of stainless. Developed as a material to resist corrosion attack in phosphoric acid processing, the grades chemical composition ensures superior performance in a wide range of demanding conditions. A shutdown includes the actual down time period and the contiguous periods required to shut down and start up the equipment. Stress corrosion cracking issues in syngas plants that may. This theory implies that if surface energy is reduced, then.
This paper discusses ae results of activepath dissolutiontype scc of austenitic stainless steel in chloride solution and fused salt, fluoride and polythionic acid solutions, brass. Alloy for resistance to polythionic acid stress corrosion cracking for. Stress corrosion cracking of sensitized austenitic stainless steels in kuwait petroleum refineries. Electrochemical characteristics of polythionic acid stress corrosion cracking have been examined mainly on sensitized type 304 stainless steel. The probable role of low levels of chloride ions or sulfurcontaining ions is described including the relationship to polythionic acid cracking. If not specified earlier, meeting a 262 practice c would be really tough, while practice e could be relatively easy. The alloy is, however, susceptible to intergranular polythionic acid stress corrosion cracking. If the cracking has been correctly diagnosed as chloride stress corrosion cracking, and the problem is confined to small drain, flush fittings, etc. Tcr engineering mumbai corrosion testing, nace, astm. However, to avoid scc in austenitic ss steels, exposure to conducive environment polythionic acid formation in presence of oxygenwaterchlorides along with metal sulfidesh2s to be reduced by adopting.
This practice can be applied to wrought products, castings, weld metal of stainless steel or other materials that are used in environments. What are the industrial practices being followed in. Trithionic acid h 2 s 3 o 6, tetrathionic acid h 2 s 4 o 6 are simple examples. The austenitic stainless steel tubes in these services can be susceptible to polythionic acid stress corrosion cracking. Stress corrosion cracking of sensitized 304 austenitic. It is twis wealth of knowledge in corrosion and the effects of hydrogen on the properties of materials, combined with our testing facilities and broad experience in the field of metallurgy and corrosion which sets us apart from our competitors in stress corrosion cracking scc testing. Any acid of the form h2sno6 having a chain of sulfur atoms. Stress corrosion cracking can lead to catastrophic brittle failure. This standard practice provides mitigation methods to protect austenitic stainless steels and other austenitic alloys from polythionic acid pta stress corrosion cracking scc that can occur during a shutdown of refinery equipment. Severe stress corrosion cracking was observed in the csbp ltd, western australia 304l unlagged stainless steel absorber column of a nitric acid plant after 15 years of plant operation. This phenomenon is usually an internal problem, occurring on the processexposed side of a piping run, vessel shell, exchanger bundle, heater tube.
Stress corrosion cracking of sensitized austenitic. This paper discusses ae results of activepath dissolutiontype scc of austenitic stainless steel in chloride solution and fused salt, fluoride and polythionic acid solutions, brass in ammonia mattossons solution. Preface thispresentvolumeisthefirstofatwopartseriesonstresscorrosioncrackingplannedbythe advancedresearchprojectsagencyforthedesigner. Decreasing the carbon content shifted the sensitization curve to longer times. Stress corrosion cracking susceptibility of stainless. Protection of austenitic stainless steels and other austenitic alloys from polythionic acid stress corrosion cracking during shutdown of refinery equipment.
At lower ph the dissolved hydrogen cyanide hcn and ammonium cyanide. Polythionic acids form from sulfide scales exposed to oxygen and water in the stainless steel that are sensitized which can occur in most stainlesssteel tube materials after exposures to temperatures in excess of 700f to 1500f 371c to 815c during manufacturing, fabrication or in service. Stress corrosion cracking investigations on ubend samples of sensitized 304 austenitic stainless steel have been conducted in sulfurous acid solutions in the concentration range, 0. How to reduce the risk of stress corrosion cracking scc the risk of stress corrosion cracking scc can be minimized through plant and equipment design. Polythionic acid stress corrosion cracking deterioration. Both austenitic and straight chromium grades of stainless steels can be attacked by polythionic acid.
The next line of defence in controlling stress corrosion cracking is testing to be sure that materials are safe on a batchbybatch. Stress corrosion cracking scc caused by polythionic acid andor chlorides has. Stress corrosion cracking scc can occur in a most metals and alloys due to the combined action of corrosion, tensile stress residual or applied and a susceptible microstructure. Pascc is fairly easy to find with penetrant testing. Learn vocabulary, terms, and more with flashcards, games, and other study tools. The importance of a conjoint action of corrosion and stress as well as the nature of. Why does chloride stress corrosion cracking and pascc only. Stress corrosion cracking facts and how to reduce the. Stressed sensitized 304, 316, and 321 stainless steels were selected and subjected to various environments that. In certain process units, such as hydrocracking, soda ash washing neutralization of austenitic stainless steel is required during turnarounds to mitigate the potential for polythionic acid stress corrosion cracking pta scc.
This standard is directed toward preventing stress corrosion. Chemical studies of polythionic acid stresscorrosion cracking. C for one hour, no intergranular corrosion was observed in tp321 and tp347 welded joints. A precursor of stress corrosion cracking in chloridebearing environments is pitting corrosion, occurring if the stainless steel is not sufficiently resistant to pitting.
Samans, stress corrosion cracking susceptibility of stainless steels and nickelbase alloys in polythionic acids and acid copper sulfate solution, corrosion. The polythionic acid sulfurous acid and hydrogen sulfide environment provides a way of evaluating the resistance of stainless steels and related alloys to intergranular stress corrosion cracking. Stresscorrosion cracking, materials performance and. The higher temperature limit for acid copper sulfate was significantly higher than for polythionic acid so the more standard acid copper sulfate test can be used for control. Chemical studies of polythionic acid stress corrosion cracking. An austenitic stainless steel for sulfuric acid corrosion environments. Abstractlow temperature stress corrosion cracking of sensitized stainless steel, promoted by aqueous solutions of polythionates sxo, and related sulfur compounds. Polythionic acid stress corrosion cracking pascc is an affliction of many refineries processing sulfur containing feedstocks, and since that is the norm these days, most refiners reduce their susceptibility to pascc by selecting resistant alloys orby neutralizing exposed surfaces during shutdowns.
The chemical environment that causes scc for a given alloy is often one which is only mildly corrosive to the metal. Stress corrosion cracking scc may be defined as the delayed failure of alloys by cracking when exposed to certain environments in the presence of a static tensile stress. These characteristics are consistent with stress corrosion cracking due to polythionic acids. The test solution was prepared by passing sulfur dioxide gas into deionized water for 4 hours and then bubbling hydrogen sulfide for 3 hours. Various separations downstream remove the acid gases, particulate matter. Carbonate cracks are suface breaking flaws that are predominantly intergranular in nature and typically occur in aswelded carbon steel fabrications as a network of very fine, oxidefilled cracks similar in appearance to caustic stress corrosion cracking and amine stress corrosion cracking.
Heat treatment method for reducing polythionic acid stress. In this chapter, the conditions for the occurrence of scc are first introduced. Stress corrosion cracking susceptibility of stainless steels and. Refinery environment assisted cracking flashcards quizlet. Figure 4 shows the results of polythionic acid stress corrosion cracking test following astm g35 on 347ap, tp321, and tp347 without stabilization. Exposure of nonstabilized austenitic stainless steel pipe to temperatures in the sensitizing range will cause the precipitation of chromium carbides at grain boundaries. The stress corrosion cracking sco of sensitized type 304 stainless steel in thiosulfate. It can lead to unexpected sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature. Polythionic acid is an oxoacid which has a straight chain of sulfur atoms and has the chemical formula s n so 3 h 2 n 2. The cracks are intergranular and narrow with no significant branching.
The terminology sensitization, which is generally used to discuss intergranular corrosion cracking behaviors, has different metallurgical meanings depending on intergranular corrosion test methods such as strauss, electrochemical potentiokinetic reacitivationepr and wackenroeder test. Polythionic acid stress corrosion cracking pascc is an affliction of many refineries processing sulfur containing feedstocks, and since that is the norm these days, most refiners reduce their susceptibility to pascc by selecting resistant alloys. The purpose of this paper is to investigate stress corrosion cracking scc for 304, 316, and 321 stainless steels in petroleum. Stress corrosion cracking scc is the growth of crack formation in a corrosive environment. Stress corrosion cracking is the growth of crack formation in a corrosive environment. Then, the stress corrosion cracking mechanism for various materials in conditions that are susceptible is discussed in detail. Cracking of sensitized stainless steels inspectioneering. The astm standard for a376 and a 312 gives this treatment for the stabilzed grade austenitic stainless steels as a supplementary requirement without. Low temperature stress corrosion cracking of sensitized stainless steel, promoted by aqueous solutions of polythionates s xo 6 2. Pdf stress corrosion cracking of sensitized austenitic. Ae monitoring from various types of stress corrosion cracking scc has been conducted for the past 15 years in our laboratory. The test may not be relevant to stress corrosion cracking in polythionic acid or caustic environments. On the other hand, polyesters are readily degraded by acids, and scc is a likely failure mechanism. What are the industrial practices being followed in refineries to mitigate polythionic acid corrosion in furnaces.
Scc is the brittle cracking of a metal due to the result of combined effects from localized corrosion and tensile stress. Resistant to chloride and polythionic acid stress corrosion cracking. Effects of sensitization and service fluid chemistry on. Polythionic acid stress corrosion cracking pascc is a type of intergranular stress corrosion that affects sensitized stainless steels in refineries processing feedstocks containing sulphur. All of the cracks observed were found underneath the coke deposits. The cracking was found when rust stainingleaks were observed. A thermal stabilization treatment at 899 900 deg c for two hours is specified for increasing the corrosion resistance to intergranular corrosion and polythionic acid stress corrosion cracking. Polythionic acid may be present in petrochemical environments. Stress corrosion cracking scc caused by polythionic acid andor chlorides has occurred in coal liquefaction pilot plants. Soda ash washing can be a costly and time consuming endeavor for the refiner. The alloys 321 and 347 are particularly useful under conditions which cause polythionic acid stress corrosion of nonstabilized austenitic stainless steel, such as alloy 304.