Corrosion behaviour of nitrogen ion implanted AISI type 304L stainless steel in nitric acid medium

The effects of nitrogen ion implantation on corrosion behaviour of 304L stainless steel in 1 N HNO₃ medium were investigated using surface analytical and electrochemical techniques. Nitrogen ion was implanted at 70 keV in the dose range of 1 × 10¹⁵, 1 × 10¹⁶, 1 × 10¹⁷ and 2.5 × 10¹⁷ N⁺/cm², respectively. Grazing incidence X-ray diffraction results for unimplanted and up to dose of 1 × 10¹⁶ N⁺/cm² showed co-existence of γ-Fe and α′-Fe and, at higher doses (1 × 10¹⁷ and 2.5 × 10¹⁷) preferential formation of chromium nitride was observed. X-ray photoelectron spectroscopy investigation confirmed the formation of chromium nitride at higher doses. Electrochemical corrosion investigation revealed nobler open circuit potential, decrease in corrosion current densities, passive current densities and increase in polarization resistance with increase in dose rate. Surface morphology analysis after polarization study using atomic force microscope showed grain boundary dissolution for unimplanted specimens and resistance to surface dissolution with increase in dose rate for implanted specimens.     

Oxidation of stainless steel 304L in carbon dioxide

Oxidation of 304L stainless steel in a carbon dioxide atmosphere at 10⁵ Pa has been studied. Between 1193 and 1293 K the oxidation kinetics exhibit first a rapid increase, then a parabolic behaviour with apparent activation energy of (209 ± 8) kJ mol⁻¹ and obeys a Langmuir pressure law. After 1.15 mg cm⁻², the kinetics become almost linear.The reaction products are chromia at the grain boundaries, wüstite (Fe_1−xO) on the surface for weight gains greater than 0.30 mg cm⁻² and chromite. The very complex reaction mechanism takes into account random buckling for weight gains >1.15 mg cm⁻².     

Corrosion resistance of welds in type 304L stainless steel made with a nickel-copper-ruthenium welding consumable

Ni-Cu-Pd welding consumables have been recently developed for 300-series austenitic stainless steels such as Type 304L (SS304L) to reduce the amount of Cr(VI) in the welding fumes. In this study, a modified filler metal that replaces Pd with Ru was evaluated. Initial tests conducted on button-melted samples and bead-on-plate welds indicated that Ni-Cu-Ru exhibited good corrosion properties. Actual Ni-Cu-Ru arc welds made on SS304L were successfully produced and the corrosion performance was comparable to or better than that of Ni-Cu-Pd welds. These welds are a suitable replacement for welds made with standard 300-series welding consumables, such as SS308L.     

Corrosion assessment of nitric acid grade austenitic stainless steels

The corrosion resistance of three indigenous nitric acid grade (NAG) type 304L stainless steel (SS), designated as 304L1, 304L2 and 304L3 and two commercial NAG SS designated as Uranus-16 similar to 304L composition and Uranus-65 similar to type 310L SS were carried out in nitric acid media. Electrochemical measurements and surface film analysis were performed to evaluate the corrosion resistance and passive film property in 6 N and 11.5 N HNO₃ media. The results in 6 N HNO₃ show that the indigenous NAG 304L SS and Uranus-65 alloy exhibited similar and higher corrosion resistance with lower passive current density compared to Uranus-16 alloy. In higher concentration of 11.5 N HNO₃, transpassive potential of all the NAG SS shows a similar range, except for Uranus-16 alloy. Optical micrographs of all the NAG SS revealed changes in microstructure after polarization in 6 N and 11.5 N HNO₃ with corrosion attacks at the grain boundaries. Frequency response of the AC impedance of all the NAG SS showed a single semicircle arc. Higher polarization resistance (R_P) and lower capacitance value (CPE-T) revealing higher film stability for indigenous NAG type 304L SS and Uranus-65 alloy. Uranus-16 alloy exhibited the lowest R_P value in both the nitric acid concentration. Auger electron spectroscopy (AES) study in 6 N and 11.5 N HNO₃ revealed that the passive films were mainly composed of Cr₂O₃ and Fe₂O₃ for all the alloys. The corrosion resistance of different NAG SS to HNO₃ corrosion and its relation to compositional variations of the NAG alloys are discussed in this paper.     

Oxidation behavior of TiC particle-reinforced 304 stainless steel

TiC particle-reinforced 304 stainless steels were prepared using a new developed in situ technology and exhibited the uniform distribution of TiC particles in the matrix. The oxidation behavior of 304SS-2TiC and 304SS-6TiC (all in weight percentage) was compared with that of 304SS at 850 °C in air for 96 h using thermogravimetry analysis. For 304SS, the rate of weight gain was very slow initially, but accelerated suddenly to a very high level, forming breakaway oxidation. The addition of TiC particles to 304SS resulted in no breakaway oxidation and maintained a low oxidation rate in the whole reaction time investigated. Examination of oxide scale morphology and cross-section analysis by scanning electron microscopy and optical microscopy showed a significant scale spallation and a deep oxide penetration in the case of 304SS, but a rather continuous, dense and adherent oxide layer formed on the surface of TiC particle-reinforced alloys. XRD analysis revealed the presence of Cr₂O₃ together with spinel-type oxides in the oxide scale. For TiC-containing alloys, fine TiO₂ was also found on the surface and the amount of this oxide increased with TiC addition. The TiC addition developed finer matrix structure before oxidation, which accelerates chromium diffusion. As a result, scale adherence was improved and oxidation resistance was increased.     

AE response of type 304 stainless steel during stress corrosion crack propagation

Acoustic emission (AE) signals generated by transgranular stress corrosion cracking (TGSCC) and intergranular stress corrosion cracking (IGSCC) have been compared by means of slow strain rate tensile tests (SSRT) performed using both solubilised and sensitised type AISI 304 stainless steel in a 1 M NaCl + 1 M HCl solution. Results show that the AE activity during the propagation of TGSCC is much higher than the AE activity during the IGSCC propagation. However, no significant difference was found between the mean amplitude and rise-time of the AE signals registered during the propagation of TGSCC and those measured for IGSCC propagation.     

A comparative study on the corrosion behaviour of 304 austenitic stainless steel in sulfamic and sulfuric acid solutions

Potentiodynamic polarization and EIS have been employed to compare the corrosion behaviour of 304 stainless steel in NH₂SO₃H and H₂SO₄ solutions. Corrosion tests were carried out as a function of the acid’s concentration (0.1-0.5 M) and solution temperature (20-60 ^oC). The corrosion rate is higher in H₂SO₄ than in H₂NSO₃H in all concentrations and temperatures. Values of the activation energy (E_a) revealed that the corrosion process is faster in H₂SO₄ than in NH₂SO₃H solution. EIS data showed that the display of Nyquist plots, and hence the mechanism of corrosion, depends not only on the acid concentration but also on the solution temperature. In 0.1 M concentration, the equivalent circuits R_e(R_ctQ_dl) and R_e(R_ctQ_dl)(RQ)_ads describe the corrosion systems in H₂NSO₃H and H₂SO₄ solutions respectively. At concentrations ?0.2 M, the equivalent circuit R_e(R_ctQ_dl)Q_diff is applicable. Adsorption of the counter ion of the acid on the steel surface and the stability of the surface complex may explain the observed corrosion rates.     

SKFM observation of SCC on SUS304 stainless steel

We have developed a new SCC test device that enables the SKFM (super Kelvin force microscope) observation. By using this test device, SCC test was done at 343 K and 28% RH with attaching MgCl₂ droplets. The crack tip deformation was observed by both KFM and SKFM. The results show that cracks initiated from the pit were usually found within half day. The negative potential parts were observed on both cracked parts and some of slip deformation parts and disappeared with time when the specimen was kept in the air. Discontinuous cracks were often found near the tip of the slip deformation part. These discontinuous cracks joined together and the amount of slip deformation increased with the progress of the crack. The negative potential part is considered to be the part where hydrogen exists. The movement of hydrogen generated by the cathodic reaction plays the important role to create the crack.     

Effect of hydrochloric acid on pickling of hot-rolled 304 stainless steel in iron chloride-based electrolytes

Weight loss, corrosion potential and EIS measurements were performed to understand the role of HCl in the pickling of oxidised 304 stainless steel in iron chloride-based electrolytes. The surface finish was analysed with SEM-EDX. The oxidised 304 stainless steel is active on immersion, resulting in a low corrosion potential and a high weight loss. After certain duration the stainless steel either remains active or becomes passive depending on HCl content. At low HCl contents, an ongoing active-to-passive transition results in localised corrosion at pits, grain boundaries and honeycombed recesses. The corrosion potential becomes high and the weight loss is suppressed. The weight loss decreases in the initial stage and rises on extended pickling with adding HCl. Because of anodic brightening, the 304 stainless steel is always active as HCl is concentrated. In contrast with the material that is passivated, the charge transfer resistance is considerably low and the double layer capacitance is large during that brightening.     

Influence of ultrasound on pitting corrosion and crevice corrosion of SUS304 stainless steel in chloride sodium aqueous solution

The change of polarization curves and surface morphologies of SUS304 stainless steel was investigated in 3.5 mass% NaCl solution with or without the application of ultrasound (US). As the result, both the pitting corrosion and the crevice corrosion were largely suppressed by the application of US. The reason is attributed to the decrease in the concentration of hydrogen and chloride ions in pits or in the crevice by removing the corrosion product and stirring the liquid there.     

Crack initiation model for sensitized 304 stainless steel in high temperature water

To investigate the initiation behavior of stress corrosion cracking (SCC) for sensitized Type 304 stainless steel in high temperature water, a constant load SCC test method combined with in situ crack observation technique was employed. The in situ crack observation system allowed us to detect small cracks of at least 100 μm. As a result, a fracture time decreased with an increase in an applied stress. The first cracks were observed at most 3 h before the specimen was fractured under all the stress conditions. After that, many cracks were initiated in a short time to fracture. The fracture was caused by coalescence of multiple cracks rather than by growth of some primary cracks. The simulation model for surface crack initiation was newly developed using a Monte Carlo method, which was based on damage mechanics and stress analysis around the existing cracks. The simulation could represent the empirical results of changes in the crack distribution and the cumulative number of cracks during the SCC tests. It was concluded, therefore, that the crack initiation process should be considered in simulating the life prediction of the material in this SCC system.     

Mechanism of improved corrosion resistance of type 304l stainless steel,nitric acid grade,in nitric acid environments

Materials for critical components in nuclear fuel reprocessing plants are required to have low corrosion rates and long designed life because access for repairs is not possible. Stainless steel type 304L, nitric acid grade (NAG), is the new material suitable for such applications. It has guaranteed low corrosion rates and is not susceptible to intergranular corrosion (IGC) in nitric acid environments. The corrosion behavior of type 304L stainless steel, NAG, and type 304L stainless steel, commercial purity (CP), in nitric acid environments is investigated in detail. Studied are: microstructural mapping in the three directions (longitudinal, long transverse, and short transverse), effect of sensitization heat treatment, resolution annealing and sensitization heat treatment for the as-received and cold-worked samples of the two varieties on the resultant microstructures. The anodic polarization characteristics along the three directions for both varieties in 1N HNO₃ are compared. The susceptibility of both varieties to end grain corrosion in 9N HNO₃ + 1 g Cr⁺⁶/liter boiling solution is assessed, and microstructural examination of the exposed sample is carried out to compare the degree of end grain corrosion. Their susceptibility to IGC due to segregation of impurity elements to grain boundaries is also compared. It is shown that controlled microstructure (fine grain size, retained cold work, and discrete precipitation at grain boundaries) along with controlled chemical composition is responsible for improved corrosion resistance of the NAG variety. The NAG variety has much less susceptibility to corrosion along the long- and short-transverse directions and, therefore, less susceptibility to end grain corrosion. The means and consequences of controlling chemical composition are also discussed.     

Determination of the sensitized zone extension in welded AISI 304 stainless steel using non-destructive electrochemical techniques

Extension of sensitized zone (SZ) in welded AISI 304 stainless steel was determined by two non-destructive electrochemical tests: double loop electrochemical potentiokinetic reactivation technique (DLEPR) and local electrochemical impedance spectroscopy (LEIS). Welding was carried out using the shielded metal arc with two selected welding energies: the first one (0.7 kJ mm⁻¹) does not promote the sensitization of the 304 steel and it constitutes the reference sample and the second one (2.2 kJ mm⁻¹) which leads to the precipitation of chromium carbides in the grain boundaries after the welding process. The non-destructive DLEPR and LEIS tests allowed the length of the SZ to be determined and a good agreement between the two techniques and the microstructure of the two welded samples was shown. The presence of an inductive loop on the local impedance diagrams seems to reflect a galvanic coupling between the weld string (anode) and the welded stainless steel plates (cathode) which will be very prejudicial to a good corrosion resistance of the welded system. The results showed that the two electrochemical tests could be applied in practical cases in industrial field.     

Effects of dithionite ion on corrosion of type 304 stainless steel in sulphuric acid solution

Corrosion behaviour of type 304 stainless steel was investigated, with particular attention to additive effects of hydrosulphite (Na₂S₂O₄) on corrosion in 0.1 mol/l H₂SO₄ solution with various amounts of Na₂S₂O₄ up to 60 mmol/l.Corrosion of SUS304 occurred below pH 3.0 at 30 °C in a 0.1 mol/l H₂SO₄ solution in which Na₂S₂O₄ was added to 0.1-20 mmol/l. The maximum corrosion rate at 30 °C was measured as 7.2 g/m² h (7.9 mm/y) in 0.1 mol/l H₂SO₄-10 mmol/l Na₂S₂O₄ at pH 1.2. Microscopic surface observation revealed that active dissolution was accompanied by intergranular corrosion at the metal surface.The SUS304 was easily passivated in 0.1 mol/l H₂SO₄ solution with more than 30 mmol/l Na₂S₂O₄. NiS was detected in the passivated film.     

Use of inkjet printing to deposit magnesium chloride salt patterns for investigation of atmospheric corrosion of 304 stainless steel

Inkjet printing was used to deposit MgCl₂ salt patterns on 304 stainless steel foils to investigate atmospheric corrosion. Results were found to be more consistent if initial hydration (1 h at ∼90% RH) of the printed salt pattern was carried out. The pit diameter following exposure at 45% RH and 300 K for 24 h was found to increase with the diameter of the original salt deposit, which is consistent with the idea of cathodic limitation of the pit current. For a constant deposition area, the pit diameter increases with increased salt deposition density, which may be associated with a lower ohmic drop resulting from a higher droplet, or could be influenced by enhanced corrosion during the initial hydration stage.     

Development of high Mn-N duplex stainless steel for automobile structural components

A new high Mn-Ni free (duplex stainless steel) DSS containing 18Cr-6Mn-1Mo-0.2N has been developed by examining the effects of manganese on the corrosion and mechanical properties of high Mn SSs containing 18Cr-4 ∼ 11Mn-0 ∼ 2Ni-0 ∼ 1Mo-0.2N. The alloy with 45% ferrite is found to be an optimum alloy with much higher mechanical strength and similar corrosion resistance compared with those of standard SS304. In addition, the alloy was free of precipitation of sigma phase and Cr-nitride when exposed to high temperatures due primarily to relatively low contents of Cr, N and Mo. With an increase in Mn content, the resistance to pitting and metastable pitting corrosion of high Mn DSS decreased since the number of (Mn, Cr) oxides, acting as preferential sites of pitting, increased with the Mn content.     

Finite element calculation of crack propagation in type 304 stainless steel in diluted sulphuric acid solutions

The crack propagation rate of Type 304 stainless steel in diluted sulphuric acid solutions was modelled based on finite element calculations of the chemical and electrochemical conditions within the crack and an analytical expression for the crack-tip strain rate. The slip/dissolution/repassivation mechanism was assumed. The influence of various variables - dissolved oxygen, sulphuric acid content, stress intensity, crack length, temperature, flow rate, sensitization and yield strength - was studied and found to be in agreement with experimental observations. The model was calibrated against one data point. The effect of temperature on thermal conductivity, electrode reaction kinetics, equilibrium constants, yield strength and crack geometry was considered and the thermal activation energy for the crack propagation rate could almost fully be accounted for.     

Galvanic corrosion between the constituent phases in duplex stainless steel

The exclusive single-phase with the exact chemical composition of the constituent phase in 2205 duplex stainless steel (DSS) could be prepared using selective dissolution method. The respective electrochemical behavior of each constituent phase could then be measured. The experimental results showed that the two distinct peaks in the active-to-passive transition region of the polarization curve determined in 2 M H₂SO₄ + 0.5 M HCl mixed solution were actually corresponded to the respective anodic peaks of the single austenite and ferrite phases. A polarity reversion was found between austenite and ferrite phases in mixed H₂SO₄ + HCl solution and HNO₃ solution. Galvanic current measurements also revealed that austenite was anode in HNO₃ solution, but became cathode when exposed in 2 M H₂SO₄ + 0.5 M HCl mixed solution.     

Corrosion behaviour of low temperature plasma carburised 316L stainless steel in chloride containing solutions

The electrochemical corrosion behaviour of the carburised (expanded austenite) layer on 316L austenitic stainless steel produced by low temperature plasma carburising has been studied in 0.5 M NaCl and 0.5 M HCl + 0.5 M NaCl solutions. The present work focuses on the variation of the corrosion behaviour of the carburised layer with depth from the surface and the effect of carbon concentration on electrochemical behaviour. The results show that the carburised layer has excellent resistance to localised corrosion. There exists a critical carbon concentration, above which the expanded austenite possesses excellent resistance to both metastable pit formation and pit growth.     

In situ AFM detection of pit onset location on a 304L stainless steel

A set-up combining an AFM and an electrochemical cell has been used to study in situ the local corrosion of a 304L stainless steel in an aqueous chloride-containing solution. The focus was made on the sites where pits were initiated under controlled potential in order to establish whether or not the pits were randomly distributed at the nanoscale. Grain boundaries and surface stoichiometric inhomogeneities appeared to concentrate pit onsets significantly. The influence of the mechanical history of the material, especially soft surface strain hardening, on the location of the first pits has been shown. The study revealed that 70% of the pits initiated at strain hardened areas resulting from mechanical polishing. A plausible model has been suggested to explain such a behaviour.