We have experienced repeated failures on seal flush piping on the naphtha and distillate reflux pumps in our crude fractionation unit. Stress corrosion cracking issues in syngas plants that may be. Pdf polythionic acid corrosion in refinery hydroprocessors. The most common fluid that causes inter granular stress corrosion cracking. Stress corrosion tests using ubend specimens were carried out in samans solutions containing polythionic acid concentrations from 0. 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. The sensitization curves for solutionannealed wrought stainless steels, which had been held for varying times at temperatures. Experience with polythionic acid stress corrosion cracking. Samans, stress corrosion cracking susceptibility of stainless steels and nickelbase alloys in polythionic acids and acid copper sulfate solution, corrosion. Sensitization is the condition where chromium carbide precipitation at the grain boundaries from a heating process, e. Astm g35 982015 standard practice for determining the. The austenitic stainless steel tubes in these services can be susceptible to polythionic acid stress corrosion cracking. The astm standard for a376 and a 312 gives this treatment for the stabilzed grade austenitic stainless steels as a supplementary requirement without. The most common fluid that causes inter granular stress corrosion cracking igscc in petrochemical plants and refineries is polythionic acid.
Stress corrosion cracking susceptibility of stainless steels. It can lead to unexpected sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature. Rp0170hd1985sg protection of austenitic stainless steel from polythionic acid stress corrosion cracking during shutdown of refinery equipmenthd1985 available for download methods to protect austenitic stainless steel from polythionic acid stress corrosion cracking scc found to occur during downtimes and contiguous shutdown and startup. Stress corrosion cracking of gas pipelines 3 minute 2014 thesis. Stress corrosion cracking susceptibility of stainless. 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. Once the metal is sensitized, sulfide scale on the metal can react with moisture and air, typically during shutdowns, to form polythionic acid. Alloy for resistance to polythionic acid stress corrosion cracking for. Nace sp0170 protection of austenitic stainless steels.
Are there any methods that can be used to give a 100% picture of the status of the pipes whilst they are mounted in the heater. Intergranular and transgranular stress corrosion cracking of the aisi 316l stainless steel at polythionic acid environment 8. The alloys 321 and 347 are particularly useful under conditions which cause polythionic acid stress corrosion of nonstabilized austenitic stainless steels, such as alloy 304. An electrochemical study on stress corrosion cracking of. Introduction the 304 stainless steel charge heater coil of a naphtha hydrotreater unit nht was found to be extensively cracked following unit shutdown, when soda ash washing was applied.
The piping is currently constructed of 316l tubing. 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. Based on this analysis the leaks were caused by cracks that initiated on the id surface. Resistance to corrosion stainless steel tubing manufacturer.
The item was exposed to sulfur containing natural gas in a continuous flare. Alloy 347347h heat resistant stainless steel plate. Polythionic acid stress corrosion cracking of 310 stainless steel. Exposure of the austenitic stainless steel piping or equipment to moisture and air can lead to a potential for this type of cracking if the ss is sensitized. Effect of polythionic acid concentration on stress corrosion. The cracking was found when rust stainingleaks were observed. Sp01702018 protection of austenitic stainless steels and. Exposure of nonstabilized austenitic stainless steel pipe to temperatures in the sensitizing range will cause the precipitation of chromium carbides at grain boundaries. General corrosion is used to describe the alloy surface are spoon way to compare the corrosion occurred in the terminology. In the process units with polythionic acid stress corrosion cracking ptascc such as hydrotreater or hydrocracker, the neutralization treatment like soda ash washing andor pwht, post weld heat treatment, are conducted for stabilized austenitic stainless steels of tp321h or tp347h to mitigate the potential of ptascc. The purpose of this booklet, therefore, is to help materials engineers identify those applications and to provide data to support the use of stainless steels in the hightemperature, corrosive environments that may be encountered. In this chapter, the conditions for the occurrence of scc are first introduced. Igscc is most common in sensitized 300 series stainless steel and higher nickel base austenitic alloys.
Other forms of scc in stainless steels include caustic cracking and polythionic acid scc. Results for the polythionic acid test have not been correlated exactly with those of intergranular corrosion tests. Polythionic acid stress corrosion cracking deterioration mechanisms. Chloride stress corrosion cracking of an austenitic stainless. These characteristics are consistent with stress corrosion cracking due to polythionic acids. This damage mechanism primarily affects standard and high carbon stainless steels, but low carbon and chemically stabilized stainless steels may also be. Intergranular cracking tendencies of fourteen stainless steels and two highnickel alloys have been evaluated by exposing numerous specimens to polythionic acid and standard acid copper sulfate solutions. Effect of polythionic acid concentration on stress. This standard is directed toward preventing stress corrosion. Nace sp0170, protection of austenitic stainless steels and other austenitic alloys from polythionic acid stress corrosion cracking during a shutdown of refinery equipment, nace international, houston, tx. Stress corrosion cracking of austenitic stainless steels can occur in refinery environments where chlorides or sulfidic scales exist. For polythionic acid stress corrosion cracking during shutdown of refinery austenitic stainless steel equipment the good practice is to follow standard recomended practice nace rp01702004 item n.
Stainless steel corrosion pitting corrosion crevice. Stress corrosion cracking by krittin rattakham youtube. Polythionic acids form from sulfide scales exposed to oxygen and water in the stainless steel that are sensitized which can occur in most stainless steel tube materials after exposures to temperatures in excess of 700f to. Polythionic acid stress corrosion cracking stainless steel. Chloride stress corrosion cracking of an austenitic. Stresscorrosion cracking of sensitized type 304 steel in low temperature borated water has been observed. Cracking of sensitized stainless steels inspectioneering. Protection of austenitic stainless steels and other. In case of general corrosion, the village gradually thinner material because of corrosion, corrosion or failure.
Polythionic stress corrosion cracking of stainless steel tube. Electrochemical characteristics of polythionic acid stress corrosion cracking have been examined mainly on sensitized type 304 stainless steel. Oct 26, 2017 in the process units with polythionic acid stress corrosion cracking ptascc such as hydrotreater or hydrocracker, the neutralization treatment like soda ash washing andor pwht, post weld heat treatment, are conducted for stabilized austenitic stainless steels of tp321h or tp347h to mitigate the potential of ptascc. The test solution was prepared by passing sulfur dioxide gas into deionized water for 4 hours and then bubbling hydrogen sulfide for 3 hours. Polythionic acid stress corrosion cracking pascc is a type of intergranular stress corrosion that affects sensitized stainless steels in refineries processing feedstocks containing sulphur. The probable role of low levels of chloride ions or sulfurcontaining ions is described including the relationship to polythionic acid cracking. 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. They can be subject to polythionic acid stress corrosion cracking during shut down. Development of fitforpurpose austenitic stainless steels. What are the industrial practices being followed in. Stainless steel in strong acid and alkali corrosion may be present round. Standard recommended practice austenitic stainless steels and.
A hydrotest at 735 psi revealed numerous leaks in four tubes from a heater. Polythionic acid stress corrosion cracking deterioration. Polythionic stress corrosion cracking of stainless steel tube environment. The most commonly available duplex in north america is 2205, at a cost roughly 20% above that of 316l.
Chemical studies of polythionic acid stresscorrosion cracking. Polythionic acid is formed in the presence of sulfur, moisture and oxygen. The cast stainless generally did not polythionic crack but, the ferrite content were susceptible to sigma phase embrittlement. Stress corrosion cracking issues in syngas plants that may. Heaters used in hydrodesulfurization, hydroforming, hydrocracking, and similar processes often have austenitic stainless steel tubes and usually process reactor feed or recycled. This standard recommended practice provides methods to protect austenitic stainless steels and other austenitic alloys from polythionic acid stress corrosion cracking pta scc occurring during downtimes and contiguous shutdown and startup periods. Stress corrosion cracking scc is the growth of crack formation in a corrosive environment. May 27, 2011 duplex stainless steel is a practical solution to most 304l or 316l scc failures. Caustic may crack carbon as well as stainless steel.
Experience with polythionic acid stress corrosion cracking in. This recommended practice provides methods to protect austenitic stainless steel from polythionic acid stress corrosion cracking scc found to occur during downtimes and contiguous shutdown and startup periods. Sulfur can come from feed stock, additives or fuels. Anybody aware of any ndt method that can be used to test for the presence of polythionic acid stress corrosion cracking in heatr pipes in a fired heater. Low temperature stress corrosion cracking of sensitized stainless steel, promoted by aqueous solutions of polythionates s x o 6 2. Exposure of nonstabilized austenitic stainless steel to temperatures in the sensitizing range will cause. In the welded condition many stainless steels are susceptible to rapid intergranular corrosion or stress corrosion cracking. Stress corrosion cracking in stainless steel question. Polythionic acid stress corrosion cracking in heater pipes. Refer also to nace rp0170, protection of austenitic stainless steels and other austenitic alloys from polythionic acid stress corrosion cracking during shutdown of refinery equipment.
Exposure of nonstabilized austenitic stainless steel to temperatures in the sensitizing range will cause the precipitation of chromium carbides at grain boundaries. Pascc occurs when sensitized stainless steels that have been in service, crack intergranularly after exposure to air and moisture, often during shutdowns. Polythionic acid stress corrosion cracking of type 310 stainless steel. Please let us know here why this post is inappropriate. Effect of carbon content on polythionic acid stress corrosion.
Standard recommended practice austenitic stainless steels. Protection of austenitic stainless steels and other austenitic alloys from polythionic acid stress corrosion cracking during a shutdown of refinery equipment this standard practice provides mitigation methods to protect austenitic stainless steels and other austenitic alloys from pta scc that can occur during a shutdown of refinery equipment. Polythionic acid stress corrosion cracking pascc is a type of intergranular stress corrosion that affects sensitized stainless steels in refineries processing. This acidic condition, along with temperatures high enough to sensitize stainless steel, often led to polythionic acid stress corrosion cracking of the valve bodies. Examples of polythionic acid scc pascc in a sensitized austenitic stainless steel and caustic scc in carbon steel equipment are presented. In each failure analysis case study, the cracking is characterized using various metallurgical laboratory techniques, including metallography and fractography, in order to identify the mode of failure. A shutdown includes the actual down time period and the contiguous periods required to shut down and start up the equipment. Abstract polythionic acid stress corrosion cracking pta scc of conventional austenitic stainless steel is a serious problem in refinery desulfurization units. Igscc is most common in sensitized 300 series stainless steel and higher nickel. Alloy 347 stainless steel plate does not perform well in chloride solutions, even in small concentrations, or in sulfuric acid.
The ptascc of stainless steels is generally caused by. It can also be utilized in chloride or fluoride free caustic solutions at moderate temperatures. Results show that alloys 304 and 304l, in the solution. Protection of austenitic stainless steels and other austenitic alloys from polythionic acid stress corrosion cracking during a shutdown of refinery equipment. The applications for stainless steels in petroleum refining are many and varied. The process stream in low in organic chlorides, but high in hydrogen sulphide. Sem fractograph of intergranular scc of type 316 stainless steel in polythionic acid showing brittle features. Abstractlow temperature stress corrosion cracking of sensitized stainless steel, promoted by aqueous solutions of polythionates sxo, and related sulfur compounds. They also compared specimens of the steel as received to others heat treated at 50c and air or water cooled.
Sp01702018 protection of austenitic stainless steels. The coil carries sour naphtha 350 ppmw s feed and hydrogen. All of the cracks observed were found underneath the coke deposits. Then, the stress corrosion cracking mechanism for various materials in conditions that are susceptible is discussed in detail. Stress corrosion cracking scc may be defined as the delayed failure of alloys by. Failure is accelerated by the presence of increasing amounts of intergranular precipitate. 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. Polythionic acid stress corrosion cracking occurs on sensitized stainless heater tubes in hydroprocessing units. In certain process units, such as hydrocracking, soda ash washing neutralization of austenitic stainless steel is required during turnarounds to. Low temperature stress corrosion cracking of sensitized stainless steel, promoted by aqueous solutions of polythionates s xo 6 2.
Rp0170hd1985sg protection of austenitic stainless steel. 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. Type 321 is particularly useful under conditions which cause polythionic acid stress corrosion of nonstabilized austenitic stainless steels such as type 304. Polythionic acid stress corrosion cracking stainless. Exposure to oxygen and water during turnaround forms polythionic acid and that leads to pta scc on sensitized stainless steel, where soda ash washing, nitrogen purge and pwht is required as a nace countermeasure. The stress corrosion cracking sco of sensitized type 304 stainless steel. Results reveal that the stress corrosion life decreases rapidly on increasing the concentration from 0. Investigation of the effect of low temperature aging on.
The cracks are intergranular and narrow with no significant branching. 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. Low temperature stress corrosion cracking of sensitized stainless steel, promoted by aqueous solutions of polythionates sxo62. 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. Siconversionunits inviewofpresentacceptedpracticeinthistechnologicalarea,u. Among them, the scc due to polythionic acid h 2sxo 6, x 3 6 is one of the most serious corrosion issue affecting equipment for hydrotreater and hydrocracker in complex refinery. Duplex stainless steel is a practical solution to most 304l or 316l scc failures. Stress corrosion cracking scc in stainless steels request pdf. Alloy 347 stainless steel plate resists polythionic acid stress corrosion cracking in hydrocarbon service. The temperature of the column in the region of the cracking is approximately 70c 158f. This is because the heat from welding sensitizes the base metal heat affected zone haz and the weld.
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