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The materials for use in nitric acid service have to be resistant to intergranular corrosion and have low general corrosion rates. End grain corrosion resulting from inclusions and segregation along dislocation bands needs to be minimised. The Nitric Acid Grade of stainless steel resistant to these corrosion problems has been indigenously produced. It is emphasised that proper fabrication practices have to be employed for NAG grade to take full advantage of its cleanliness, controlled chemistry and microstructure. The problems associated with fabrication of large diameter pipes, oxide embedment and welding are described with our experience in its production and quality assessment.
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... Одной из главных технологических характеристик аустенитных хромоникелевых нержавеющих сталей является их коррозионная стойкость. При воздействии агрессивной среды сталь подвержена таким локальным видам коррозии, как: межкристаллитная коррозия (МКК), коррозионное растрескивание (КР), точечно-язвенная (питтинговая) коррозия и щелевая коррозия [1,2]. ...
Изучено влияние концентрации кремния (в пределах 0,14-0,78 мас. %), добавок бора и редкоземельных металлов на коррозионную стойкость низкоуглеродистой аустенитной хромоникелевой стали типа Х18Н11. Показано, что увеличение концентрации кремния в стали с 0,14 до 0,78 мас. % увеличивает скорость межкристаллитной коррозии в сильно окислительной среде (азотная кислота): в закаленном состоянии только при испытании в растворе 27 % HNO3 + 4 г/л Cr+6 и в сенсибилизированном состоянии. При этом повышение температуры нагрева под закалку до 1150 °С позволяет значительно снизить скорость коррозии у стали с 0,78 мас. % Si, мало влияя на скорость коррозии низкокремнистой стали. Установлен различный характер травления образцов при проведении испытаний. Низкокремнистая сталь при испытаниях в азотной кислоте корродирует преимущественно по границам двойников, а высококремнистая сталь преимущественно по границам аустенитных зерен. Исследовано совместное влияние увеличения концентрации азотной кислоты (от 20 до 65 %) и температуры испытаний (от 100 до 130 °С). С поверхности образцов высококремнистой стали значительное выпадение зерен происходит в условиях испытаний в 56 и 65 % HNO3 при 120 и 130 °С, что резко увеличивает коррозионные потери образцов этой стали, в то время как выпадения зерен у стали с низкой концентрацией кремния не наблюдали. При меньших температурах и концентрациях азотной кислоты коррозионные потери всех сталей сблизились. Показано, что микролегирование редкоземельными элементами не ухудшает коррозионную стойкость сенсибилизированной стали. В отличие от РЗМ, легирование хромоникелевой стали даже небольшой добавкой бора (0,0015 %) на порядок уменьшает коррозионную стойкость стали. Повышение температуры нагрева под закалку бористой стали привело к увеличению скорости коррозии.
(The effect of silicon concentration (in the range of 0.14–0.78 wt. %) and microalloying of boron and rare-earth metals (REM) on the corrosion resistance of low-carbon austenitic nickel-chromium
steel type CH18N11 (AISI 304L) was studied. Increasing the concentration of silicon in the steel with 0.14 to 0.78 wt. % increases the rate of intergranular corrosion in a highly oxidizing environment (nitric acid): in the quenched state only when tested in a solution of 27 % HNO3 + 4 g/l Cr+6 and in a sensitized state. At the same time, an increase the quenching temperature to 1150 °С can significantly reduce the corrosion rate of steel with 0.78 wt. % Si, little effect on the corrosion rate became in steel with 0.14 wt. % Si. Showed a different nature of etching samples during testing.
When tested in nitric acid, low-silicon steel corrodes mainly along the boundaries of twins and highsilicon steel mainly along the boundaries of austenitic grains. The combined effect of increasing the concentration of nitric acid (from 20 to 65 %) and the test temperature (from 100 to 130 °C) was investigated. From the surface of high-silicon steel samples, significant grains loss occurs under test conditions of 56 and 65 % HNO3 at 120 and 130 °C, which increases the corrosion loss of samples of this steel. At the same time, grains loss from steel with a low silicon concentration wasn’t observed.
At lower temperatures and the nitric acid concentrations of corrosive loss steels closer. It is shown, that microalloying with rare-earth elements does not impair the corrosion resistance of sensitized
steel. Unlike REM, alloying of chromium-nickel steel even with a small addition of boron (0.0015 %) reduces the corrosion resistance of steel by an order of magnitude. Increasing the quenching temperature of boron steel has led to an increase in corrosion rate)
... It is shown that both phosphorus [23,25,30, and silicon [3,23,25, have an impact on the occurrence and kinetics of IGC. These conclusions motivate some authors to produce SS with optimized impurities content [43,. On the other hand, the specific reactivity of grain boundaries is explained by the particular metallurgy of grain boundaries [3,18,23,31,32,69]. ...
This paper studies the corrosion behaviour of non-sensitized stainless steels in oxidizing environments, where they can suffer intergranular corrosion. In this case, corrosion rate estimated by gravimetric measurement is not constant as a function of time. This paper proposes a quantitative modelling of the IGC kinetics. Two models were developed: the first one is based on the geometrical simulation of the groove penetration; the second one uses a semi-empirical approach based on the typical shape of the corrosion kinetics. Both models reproduce successfully the experimental corrosion kinetics observed for AISI 310L SS corroded in nitric acid containing oxidizing ions.
Austenitic stainless steels type 304L, 316L and 310Nb are largely used as structural materials for equipments handling nitric acid media in reprocessing plants. In almost all nitric media, these materials, protected by a chromium(III) oxide rich layer, remain in their passive state. However, in some particular nitric media, their corrosion potential may be shifted towards their transpassive domain. In this domain, they can suffer intergranular corrosion, even though they are not sensitized owing to their very low carbon content. The corrosion potential of the steel depends greatly on the cathodic reaction involved in the oxido-reduction process between the elements Fe, Cr, Ni of the steel and the oxidizing species of the medium. Three cases of an increase in the corrosion potential can be found in reprocessing media: pure nitric acid–water solutions, in which the cathodic reaction is the reduction reaction of HNO3; nitric acid media containing oxidizing species, in which the cathodic reaction is the reaction of reduction of the oxidizing species into the reduced one; nitric media containing metallic elements electrochemically more noble than the steels, causing galvanic coupling. In each case, the mechanism and the relevant situations we experimentally studied are described.
A novel laser surface modification approach to suppress sensitization in AISI 304 (UNS S30400) austenitic stainless steel is described. Surface modification of austenitic stainless steel was carried out with a 10-kW carbon dioxide (CO2) laser in both continuous wave and pulse-modulated modes. After laser surface modification, the material was subjected to a sensitization heat treatment at 923 K for 9 h. Me degree of sensitization was determined by electrochemical potentiokinetic reactivation test while the susceptibility to intergranular corrosion was determined using the ASTM A262, practice B test. The results of the study demonstrated that the laser-melted surface exhibited significantly higher resistance against sensitization and intergranular corrosion than the base metal. The laser-melted surface, even after being subjected to severe sensitization heat treatment, developed comparable or even a lower degree of sensitization than the base metal in the as-received condition. Enhanced immunity against sensitization of the laser-treated surface is attributed to its duplex microstructure and higher fraction of low-angle grain boundaries. The highlight of the investigation was that a laser surface melting treatment of unstabilized austenitic stainless steel brings about signficant reduction in its susceptibility to sensitization during subsequent exposure to a susceptible temperature region and to intergranular corrosion during service.
Radiation-induced segregation (RIS) in desensitized type 304 stainless steel (SS) was investigated using a combination of electrochemical potentiokinetic reactivation (EPR) test and atomic force microscopy (AFM). Desensitized type 304 SS was irradiated to 0.43dpa (displacement per atom) using 4.8MeV protons at 300°C. The maximum attack in the EPR test for the irradiated desensitized SS was measured at a depth of 70μm from the surface. Grain boundaries and twin boundaries got attacked and pit-like features within the grains were observed after the EPR test at the depth of 70μm. The depth of attack, as measured by AFM, was higher at grain boundaries and pit-like features as compared to twin boundaries. It has been shown that the chromium depletion due to RIS takes place at the carbide–matrix as well as at the carbide–carbide interfaces at grain boundaries. The width of attack at grain boundaries after the EPR test of the irradiated desensitized specimen appeared larger due to the dislodgement of carbides at grain boundaries.
Surface modification of type 18/8 stainless steel was carried out using a 3 kW CO2 laser with continuous and pulse wave and with 25%, 50% & 70% overlapping. After laser surface modification, the material was sensitized at 650°C for 9 hours. The degree of sensitization (DOS) was determined by the double loop electrochemical potentiokinetic reactivation (DL-EPR) test and the susceptibility to intergranular corrosion (IGC) was determined by practice B, A 262, ASTM. The DL-EPR value of base material was 4.52 whereas on the laser modified surfaces it was in the range of 0.11 to 0.91. After the IGC test, the microstructure was examined over the cross section of the sample which revealed much less attack on laser melted side compared to the base material. X-ray diffraction revealed presence of about 4% ferrite in the laser melted regions. As sensitization is mainly a grain boundary phenomena, the nature of grain boundaries were examined by orientation imaging microscopy (OIM). The results are discussed to explain the reasons for the low DL-EPR values on the laser melted surfaces. The present study demonstrates a clear possibility of remarkable improvement in DOS and resistance to IGC by laser surface modification.
Thermomechanical processing (TMP) and strain annealing were performed on alloy 600. The degree of sensitization (DOS) and susceptibility to intergranular corrosion (IGC) were assessed by electrochemical potentiokinetic reactivation and weight loss tests respectively. The grain boundary character distribution was established by orientation imaging microscopy. It was shown that as the fraction of special boundaries increased with TMP/strain annealing, the DOS and IGC rates decreased drastically. Alloy 600 with fraction of special boundaries above 85% showed only uniform corrosion. The DOS and IGC values indicating resistance to sensitization and IGC and the methods to achieve such values have been highlighted.
Sensitisation in twin boundary engineered type 304 austenitic stainless steel (SS) was investigated using electrochemical potentiokinetic reactivation technique. As-received (AR) specimens of type 304 SS were given two solution-annealing treatments at 1000 °C for 0.5 and 2 h. Solution-annealed specimens were given a small strain of 5% using cold-rolling followed by strain–annealing (strain–annealing) at 927 °C for 24, 48, and 72 h. Electron backscatter diffraction analyses have shown increase in fraction of special boundaries (Σ⩽29)(Σ⩽29) in two solution-annealed specimens. Twin-boundary engineered specimens were given two potential sensitisation treatments at 575 and 675 °C for 1 h. The increase in twin fraction and decrease in grain average misorientation (GAM) always resulted in reducing the extent of sensitisation. An exponential correlation between GAM and twin-fraction was established and increase in GAM always associated with increase in twin-fraction.
Nuclear spent fuel reprocessing and waste management plants use nitric acid as process fluid and type 304 L stainless steel as construction material. Tubular products like bars, tubes and pipes are prone to End Grain Corrosion from the exposed cross-sectional surfaces. In this study, type 304 L stainless steel is subjected to different heat treatment to induce selective segregation of phosphorus. The susceptibility to End Grain Corrosion was established in tests using boiling nitric acid containing oxidizing Cr(VI) ions. A clear effect on End Grain Corrosion was found for heat treatment reported to induce phosphorus segregation. Finally, specific annealing heat treatment is developed that erases out the segregation, without affecting the grain size or sensitization.
The austenitic stainless steels are used in nuclear spent fuel reprocessing and waste management plants and the process fluid is nitric acid at temperature up to boiling point. However incorporation of oxidizing ions e.g. fission products as well as corrosion products, in nitric acid stream make the environment highly corrosive to stainless steels. Present work aims to investigate role of process parameters and material parameters (composition and microstructure) on corrosion behaviour of stainless steels. The process parameters studied are temperature, acid concentration and oxidizing ions. It has been shown that the potential attained on stainless steel is a function of acid concentration and temperature and is further strongly affected by addition of oxidizing ions. This developed potential determines the corrosion behaviour of stainless steel. Increasing the temperature and concentration of nitric acid and concentration of oxidizing species increased the developed potential. Potentials were applied to types 304 L (nitric acid grade - NAG), 304 L (commercial purity) and 310 L stainless steels in boiling 6 M nitric acid for a period of 48 h. The corrosion rates measured in such experiments were plotted as a function of applied potential. The form of corrosion was established by microstructural examination. A clear demarcation was observed between uniform corrosion and intergranular corrosion at a potential of 960-980 mVSCE. Above this potential range corrosion rate increased exponentially and the form of corrosion is shown to be intergranular corrosion. Below this potential range, uniform and low rate of corrosion occurred. The influence of microstructure (step, dual and ditch) of type 304 L was also studied and is described in this paper.
An oxide dispersion strengthened steels are one of the most promising high temperatures, and high performance advanced structural material being developed for future fast reactors and high-temperature Generation IV reactors. In the present work, the corrosion resistance and its correlation with the passive film compositions of 11% Cr F/M and 9-15% Cr (with Zr or Hf) ODS steels is examined and compared with AISI type 304L stainless steel in boiling 60 - 62% (~13 M) HNO3. The corrosion rate measured in 62% HNO3 for 240 h of 11% Cr F/M, 9% Cr and 15% Cr (Zr) ODS steels show high corrosion rate. On the other hand, low corrosion rate was observed in 304L stainless steel (0. 21 to 23 mm y-1). However, severe intergranular corrosion attack was revealed in type 304L SS after 240 h exposure, but none in ODS steels. Such an intergranular corrosion attack seen in type 304L stainless steel is undesirable. On the contrary, low corrosion rate (0.04 0.15 mm y-1) of 15% Cr (Hf) ODS steel in 3 M, 6 M and 9 M HNO3, comparable to that of type 304L stainless steel was observed. The improved corrosion resistance of 15% Cr (Hf) ODS steel was attributed to enrich (20 at. %) and protective Al2O3 layer formation in addition to Cr2O3 in the passive film.
Austenitic stainless steels like SS 304L are prone to intergranular corrosion (IGC) in boiling nitric acid media. A major cause is sensitisation induced in the heat affected zone of the weldments. Developments in sensitisation resistant materials have lead to the minimisation of degradation due to IGC. Corrosion in nitric acid medium is essentially controlled by oxidising potentials in the transpassive potential regime. The corrosion potential of stainless steel in nitric acid is a function of the concentration of acid, temperature and concentration of specific oxidising ions. In this work, a 'master curve' for the dependence of corrosion rates on the applied potentials was generated for type 304L stainless steel (nitric acid grade) in 6M nitric acid at near boiling (95°C) condition. The master curve was validated by measuring the corrosion rates at 48 h exposure in 6M near boiling nitric acid, in which various oxidising ions had been added. It was illustrated that there is a threshold potential (at transition to transpassivity) above which the non-sensitised stainless steel also undergoes IGC. A methodology for the prediction of corrosion rate has thus been proposed that can be applied to austenitic stainless steels at any given operating condition/component in operating plants where nitric acid media are used as process fluid. The effect of the temperature/area ratio of stainless steel on corrosion has been illustrated.
Austenitic stainless steels are the main materials of construction in nuclear spent fuel reprocessing and waste management plants that use nitric acid (HNO3) as the process fluid. In this study, it has been shown that the potential, attained either by addition of oxidizing ions or by external application, determines the corrosion behavior of stainless steels. An equation derived from curve fitting of the measured corrosion data is proposed for predicting the corrosion rates of a specific stainless steel at any given operating potential. The influence of microstructure ("step," "dual," and "ditch") of Type 304L (UNS S30403) stainless steel was also studied.
The corrosion behavior of Type 304L (UNS S30403) nitric acid grade (NAG) stainless steel in boiling 9 M and 1 M nitric acid (HNO 3) and with the addition of 0.03 M sodium fluoride (NaF) and with fluoride complexed with 0.06 M to 0.15 M aluminum nitrate (Al[NO 3] 3) and 0.015 to 0.0375 M zirconium nitrate (Zr[NO 3] 4) has been established. The corrosion behavior has also been studied using potentiodynamic polarization in the same environments at room temperature and at near boiling temperature and correlated to the measured corrosion rates. It was shown that the fluoride addition accelerates anodic dissolution. The surface film formed on stainless steel was analyzed using x-ray photoelectron spectroscopy to show that fluorides do not remain attached to the surface. The effect of different ratios of complexing with Al(NO 3) 3 and Zr(NO 3) 4 has been studied, showing more effectiveness of zirconium. The corrosion behavior of Types 304L, 304L (welded), 310L (UNS S31000) stainless steels and Alloy 690 (UNS N06690) was established in boiling HNO 3 and HNO 3 with complexed fluoride and in its vapor and condensate phases. The corrosion rates were shown to be higher in the vapor phase than in the concentrate phase for all the materials tested. The differences in the corrosion rates measured in 48 h or 120 h exposures have been explained.
The degradation of materials involving corrosion in handling nitric acid in the spent fuel nuclear reprocessing plant is a serious issue. In the present work, the corrosion resistance of American Iron and Steel Institute (AISI) type 304L stainless steel (SS) and nitric acid grade (NAG) type 310L SS in 1 to 11.5 M HNO3 and boiling 15.65 M HNO3 was evaluated. In both the alloy steels, the open circuit potential and corrosion potential are shifted to more noble potential with increasing concentrations. However, the passive current density was not affected, and the transpassive potential was shifted to higher potential with increasing concentrations. The corrosion rate measured in boiling 15.65 M HNO3 after 240 h shows a much lower corrosion rate in type 310L SS (∼0.06 ± 0.012 mm/y) then type 304L SS (∼0.18 ± 0.02-0.2 ± 0.001 mm/y). These observations are corroborated with the scanning electron microscope (SEM) morphologies that show severe intergranular corrosion (IGC) attack in type 304L SS then in type 310L SS. The X-ray photoelectron spectroscopy (XPS) study of the passive oxide films of both alloy steels shows the presence of Cr2O3 and SiO2, and the depth profile indicated predominant Si enrichment.
The paper evaluates laser surface melting treatment for rejuvenation of inter-granular corrosion damaged type 304 stainless steel. Surface melting of inter-granular corrosion damaged specimens with CW CO2 and pulsed Nd:YAG lasers was quite effective in sealing surface damage. However, inter-granular corrosion susceptibility of laser-rejuvenated surface was strongly influenced by associated thermal exposure. With respect to CW CO2 laser, pulsed Nd:YAG laser-rejuvenated surface demonstrated significantly suppressed micro-structural damage and IGC-susceptibility. Compact pulsed Nd:YAG laser, with flexible beam transportation, presents an effective tool for in-situ rejuvenation of inter-granular corrosion damaged in-service stainless steel components operating in susceptible environments.
Corrosion is termed as the chemical or electrochemical reaction between a material and its environment that leads to deterioration of the material and/or its properties. Corrosion has been traditionally classified as uniform corrosion and localized corrosion. Uniform corrosion is the form of corrosion in which the anodic reaction proceeds uniformly over the entire exposed surface. Uniform corrosion leads to thinning of materials. Rusting of iron is the most common example of uniform corrosion. The uniform corrosion rate of most of the materials in commonly used environments is already established and is available in numerous handbooks. Iso-corrosion charts are a simple way to illustrate the regime of environmental variables at which a given corrosion rate would be encountered. Depending on the application, one can determine the corrosion rate for a candidate material from such databases/iso-corrosion charts. Materials resistant to uniform corrosion for specific applications are available. Like steels are prone to uniform corrosion in atmospheric exposure conditions, but weathering steels (C, 0.10; Mn, 0.35; Si, 0.50; Cu, 0.40; P, 0.10; Ni, 0.40; and Cr, 0.80) are generally resistant to atmospheric exposures and usually do not need painting for corrosion protection. These weathering steels have a ferrite-pearlitic microstructure and derive strength mainly from solid solution strengthening.
In spent fuel reprocessing plants, various nitric media are met throughout the PUREX process that leads to the separation of fission products, uranium and plutonium. Structural materials for the equipment are mainly austenitic stainless steels and zirconium. They are protected by a stable passive layer in nitric acid. At first, corrosion mechanisms of stainless steels are presented. Special attention is paid to the cathodic reactions that are specific to nitric media. Secondly, the mechanism of passivity and the behaviour of zirconium are described.
The austenitic stainless steels are used in nuclear spent fuel reprocessing and waste management plants and the process fluid is nitric acid at temperature up to boiling point. However incorporation of oxidizing ions e.g. fission products as well as corrosion products, in nitric acid stream make the environment highly corrosive to stainless steels. Increasing the temperature and concentration of nitric acid and concentration of oxidizing species push the operating potential to the transpassive potential range and cause severe intergranular corrosion of even solution annealed stainless steel. In this study it has been shown that the potential attained either by addition of oxidizing ions or by external application through a potentiostat, determines the corrosion behavior of stainless steel. Potential were applied to types 304 L (nitric acid grade - NAG), 304 L (commercial purity) and 310 L in boiling 6 M nitric acid for a period of 48 h. The corrosion rates measured in such experiments were plotted as a function of applied potential. The form of corrosion was established by microstructural examination. A clear demarcation was observed between uniform corrosion and intergranular corrosion at a potential of 960-980 mV SCE. In this potential range corrosion rate increases exponentially and the form of corrosion is intergranular corrosion. Below this potential range, uniform and low rate of corrosion occurred. An equation derived from curve fitting of these data is proposed to be used for predicting the corrosion rates of a specific stainless steel at any given operating potential. The influence of microstructure ("step", "dual" and "ditch") of type 304 L was also studied and is described in this paper.
SS 304L is widely used as a structural material in applications handling nitric acid such as nuclear fuel processing plants
and nuclear waste management facilities. Bar, wire, and tubular products of this material are especially susceptible to end-grain
corrosion in nitric acid environment. Such an attack takes place on the tubular and forged surfaces that are perpendicular
to the hot-working direction and occurs as localized pitting type attack. This study shows that the possible reasons for the
directional nature of end-grain attack are the manganese sulfide inclusions aligned along the hot-working direction and/or
segregation of chromium along the flow lines during the fabrication stage itself. It has been shown in this study that controlled
solution annealing, laser surface remelting, and weld overlay can be used to avoid/minimize end-grain corrosion. Different
annealing heat-treatments were carried out on two heats of SS 304L tube and susceptibility to corrosion was measured by ASTM
A 262 practice C and electrochemical potentiokinetic reactivation (EPR) test. Solution annealing at 950°C for 90min has
been shown to increase the resistance to end-grain corrosion. Laser surface remelting using continuous wave CO2 laser under argon shield and weld deposition (overlay) using SS 308L material were done on the end faces of the tubes. These
samples were completely resistant to end-grain corrosion in nitric acid environments.
Corrosion engineering has played a crucial role in chemical process industries in ensuring safe, reliable operation in spite of the many corrosive and difficult chemicals that are handled. Corrosive chemicals including sulfuric acid, nitric acid, and hydrogen chloride are discussed. The importance of cooperative efforts such as the Materials Technology Institute of the Chemical Process Industries and the need for record keeping are emphasized.
Three important problems concerning the corrosion of stainless steels by nitric acid are addressed: vapor-phase corrosion of type 304 (UNS S30400) and its variants over very strong acid, intergranular corrosion of the molybdenum-bearing grades (such as type 316L [UNS S31603]) unrelated to carbide precipitation, and the effects of contaminants and admixtures.