The combined action of tensile stress with corrosive environment can lead to chloride stress corrosion cracking cscc. Stresscorrosion cracking of sensitized type 304 stainless. An electrochemical study on stress corrosion cracking of. Failure of 321 stainless steel heater tube in heavy crude oil. Scc was severe in type 304, moderate in types 316l and. The austenitic stainless steel tubes in these services can be susceptible to polythionic acid stress corrosion cracking. The alloy is, however, susceptible to intergranular 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. Pdf stress corrosion cracking scc is the formation and growth of crack. The types 321, 347 and 348 alloys are particularly useful under conditions which cause polythionic acid stress corrosion of nonstabilized austenitic stainless steels, such as type 304. Other forms of scc in stainless steels include caustic cracking and polythionic acid scc. 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. Stressed sensitized 304, 316, and 321 stainless steels were selected and subjected to various environments that.
Resistant to chloride and polythionic acid stress corrosion cracking. Alloy 347 stainless steel plate resists polythionic acid stress corrosion cracking in hydrocarbon service. Polythionic acid may be present in petrochemical environments. Polythionic acid is a generic term used to describe acids of the form h 2 s x o 6. Polythionic acid stress corrosion cracking pascc is a type of intergranular stress corrosion that affects sensitized stainless steels in refineries processing feedstocks containing sulphur. Sensitization was evaluated by double loop electrochemical potentiokinetic reactivation method and igscc studies were carried out using slow strain rate tests in polythionic acid pta. Ae monitoring from various types of stress corrosion cracking scc has been conducted for the past 15 years in our laboratory. Pascc of 304 stainless steel charge heater tubes occurred in a naphtha. Therefore, austenitic stainless steel and other austenitic alloy process equipment that may be exposed to pta should be protected using one or more of the pta scc mitigation methods presented in nacesp1702012see below. The stress corrosion cracking sco of sensitized type 304 stainless steel. The superheater tube material was made of aisi 304h austenitic. Why does chloride stress corrosion cracking and pascc only. 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. Exposure of nonstabilized austenitic stainless steel to temperatures in the sensitizing range will cause the precipitation of chromium carbides at grain boundaries.
Stress corrosion cracking scc is the formation and growth of crack. In the presence of sulfide corrosion products, such conditions are met during shutdown, when moisture originates from general humidity, washing or steam out and oxygen as air enters the unit upon opening to atmosphere. This theory implies that if surface energy is reduced, then. The critical levels of sensitization and tensile stress required to initiate pta scc are not well understood. Stress corrosion cracking of sensitized type 304 steel in low temperature borated water has been observed. Electrochemical characteristics of polythionic acid stress corrosion cracking have been examined mainly on sensitized type 304 stainless steel. Various separations downstream remove the acid gases, particulate matter.
Chloride induced stress corrosion cracking of type 304 and. Subsequently, this particular form of intergranular scc was called polythionic acid stress corrosion cracking pascc. Stress corrosion cracking susceptibility of stainless steels and. The superheater tube material was made of aisi 304h austenitic stainless steel.
How to weld repair 304h flange that has scc due to polythionic acid how to weld repair 304h flange that has scc due to polythionic acid. Microstructure investigation revealed various degrees of stress corrosion cracking. Based on this analysis the leaks were caused by cracks that initiated on the id surface. What are the industrial practices being followed in refineries to mitigate polythionic acid corrosion in furnaces. Ctlpolythionic stress corrosion cracking of stainless. Samples were found to fracture in solutions of more than 3% sulfurous acid. Stress corrosion cracking of sensitized 304 austenitic. Polythionic acid stress corrosion cracking pascc is an affliction of many. In each failure analysis case study, the cracking is characterized using various metallurgical laboratory techniques, including metallography and fractography, in. This study is to identify factors responsible for loss of production due to problems in plant and equipment. Pascc of 304 stainless steel charge heater tubes occurred in a naphtha hydrotreater during. Cracking has also been reported in refinery equipment with lower operating. Presence of cr, carbon, exposure temperatures and presence of polythionic acids play an important role in stress corrosion cracking.
However we have used type 347 for cyclone support rods because of concern about a cyclone dropping if a support rod were to crack. Intergranular and transgranular stress corrosion cracking of the aisi 316l stainless steel at polythionic acid environment 8. It has general corrosion resistance similar to the low carbon 304 304l. Caustic may crack carbon as well as stainless steel. Examples of polythionic acid scc pascc in a sensitized austenitic. It is a very severe, rapid form of intergranular scc that attacks sensitized austenitic stainless steels such a weld hazs in type 304. Scc is highly chemically specific in that certain alloys are likely to undergo scc only when exposed to a small number of chemical environments. Stress corrosion cracking of sensitized type 304 stainless steel in thiosulfate solutions r.
Citeseerx document details isaac councill, lee giles, pradeep teregowda. Effect of the degree of rolling reduction on the stress corrosion. Experience with polythionic acid stress corrosion cracking. 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. Stress corrosion tests using ubend specimens were carried out in samans solutions containing polythionic acid concentrations from 0. Chloride stress corrosion cracking scc of type 304 and 304l austenitic stainless steels ss was studied by a constant load method in air at a temperature of 353 k with relative humidity rh of. 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. 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 pascc occurs in refinery units processing sulfurous streams. Corrosion under insulation cui is a severe problem for 304 stainless steel pressure vessel. An engineering approach to stresscorrosion cracking of austenitic stain less steelhow it can. 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 alloys 321 and 347 are particularly useful under conditions which cause polythionic acid stress corrosion of nonstabilized austenitic stainless steels, such as alloy 304. The cracking was found when rust stainingleaks were observed. Two examples of stress corrosion cracking scc in refinery equipment are presented to highlight the lessons learned to prevent recurrence of such failures. All of the cracks observed were found underneath the coke deposits. The test solution was prepared by passing sulfur dioxide gas into deionized water for 4 hours and then bubbling hydrogen sulfide for 3 hours. Effect of polythionic acid concentration on stress. Protection of austenitic stainless steels and other austenitic alloys from polythionic acid stress corrosion cracking during shutdown of refinery equipment. Stress corrosion cracking scc is the growth of crack formation in a corrosive environment. Corrosion behavior of the stressed sensitized austenitic. 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. Heat treatment method for reducing polythionic acid stress.
Alloy 347 resists polythionic acid stress corrosion cracking in hydrocarbon service. Stress corrosion cracking of sensitized austenitic. The alloy can be used in nitric solutions, most diluted organic acids at moderate temperatures and in pure phosphoric acid at lower temperatures and up to 10% diluted solutions at elevated temperatures. What are the industrial practices being followed in. A constant tensile stress has been applied to sensitized specimens types 304, 316l, 304ln, 304nh, and 316nh stainless steels. The commonly used types of stainless steels, 304304h and 316316h are. Type 304l shares disadvantages of 304h it does sensitize at high temperature, however and can be susceptible to polythionic acid stress corrosion cracking during shut down, especially if the regenerator is water washed. Corrosion potentials of types 304l stainless steel and alloy 825 in longterm boildown. Stress corrosion cracking issues in syngas plants that may. Pdf stress corrosion cracking damages researchgate. They have high strength, excellent fabrication characteristics, and can readily meet a wide range of design demands load, service life, low maintenance, etc. Solomontransgranular, granulated, and intergranular stress corrosion cracking in aisi 304 ss. The probable role of low levels of chloride ions or sulfurcontaining ions is described including the relationship to polythionic acid cracking.
Results reveal that the stress corrosion life decreases rapidly on increasing the concentration from 0. The applications for stainless steels in petroleum refining are many and varied. Polythionic acid stress corrosion cracking pascc is caused by sulfur. Carbon steel and type 304 stainless steel samples were exposed in thirteen corrosion tests conducted in solutions with varying ph, oxygen content, so2 content and chloride ion content in a simulated sour water solution produced in the laboratory.
However, it is subject to carbide precipitation in the heat affected zone haz of welds. The radiolysis reactions in acidic, reducing solutions. The cracks are intergranular and narrow with no significant branching. Corrosion rates were calculated, and ubends were examined for stress corrosion cracking. Polythionic acid scc true or false cracking of 304 cyclones is rare enough that upgrading to type 347 has not been considered necessary.
Stress cracking agents in process streams are not the only cause of scc. These characteristics are consistent with stress corrosion cracking due to polythionic acids. Isaacs the stress corrosion cracking of a sensitized type 304 stainless steel has been studied at room temperature using controlled potentials and two concentrations of sodium thiosulfate. Cracking of sensitized stainless steels inspectioneering.
Standard recommended practice austenitic stainless steels. Weldments may be sensitive to haz intergranular corrosion in oxidizing acid environments, and to polythionic acid stress corrosion cracking. In this chapter, the conditions for the occurrence of scc are first introduced. How to weld repair 304h flange that has scc due to. 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. It can lead to unexpected sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature. Two examples of stress corrosion cracking scc in refinery equipment are presented to. 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 this report, the literature on stress corrosion cracking scc of the. Does anyone have experience with polythionic acid stress corrosion cracking pascc in boltedremovable type 304 reactor internals e. Highnickel alloys, such as alloy 600 or, better, commercially pure nickel uns n02201, are used for hot concentrated caustic. Exposure of nonstabilized austenitic stainless steel pipe to temperatures in the sensitizing range will cause the precipitation of chromium carbides at grain boundaries. Polythionic acid stress corrosion cracking pascc occurs in.
The purpose of this paper is to investigate stress corrosion cracking scc for 304, 316, and 321 stainless steels in petroleum. Then, the stress corrosion cracking mechanism for various materials in conditions that are susceptible is discussed in detail. An austenitic stainless steel for sulfuric acid corrosion environments. However, due to sensitisation, its corrosion resistance reduces, rendering it more. Polythionic acid stress corrosion cracking deterioration.
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