M. G. Pujar

Evaluation of microstructure and electrochemical corrosion behavior of austenitic 316 stainless steel weld metals with varying chemical compositions

Austenitic stainless steel weld metals have, in general, inferior corrosion resistance compared with the base metals. This is due to the fact that the weld metal has an inhomogeneous and dendritic microstructure with microsegregation of major elements (i.e., Cr, Mo, and Ni) as well as minor elements (i.e., S and P) at the δ-γ interface boundaries. The nonuniform alloying element concentration around ferrite particles plays a major role in determining the electrochemical corrosion behavior of such weld metals. Although the presence of ferrite is considered to be detrimental as far as the localized corrosion is considered, its exact role in uniform corrosion is still not clear. The uniform corrosion behavior of an alloy is determined by the fundamental electrochemical parameters of the major alloying elements. In this study, an attempt has been made to correlate the microstructure and uniform corrosion behavior of type 316 stainless steel weld metals with varying concentrations of Cr and Mo, and different ferrite contents. From the empirical equations obtained during the analysis of the electrochemical corrosion data, an attempt has been made to understand the role of Cr, Mo, and ferrite in altering the electrochemical corrosion parameters of the weld metal. Based on the extensive microstructural characterization, a dissolution model for the weld metal in the moderately oxidizing medium has been proposed.

Effects of laser surface melting on the pitting resistance of sensitized nitrogen-bearing type 316L stainless steel

Laser surface melting of sensitized nitrogen-bearing type 316L austenitic stainless steel was carried out using a pulsed ruby laser. The sensitization heat treatment was carried out at 923 K for 50, 200, 1000, and 2500 h, and the sensitized microstructure was classified according to ASTM A 262 practice A. The degree of sensitization was assessed by the electrochemical potentiokinetic reactivation (EPR) test. The critical pitting potentials of as-sensitized as well as sensitized-laser melted specimens were determined by potentiodynamic anodic polarization method in a medium containing 0.5 M NaCl and 0.5 M H2SO4 at room temperature. Results indicated that upon laser melting the pitting resistance increased significantly. This increase was attributed to the elimination of the sensitized heterogeneous microstructure by laser melting. The microscopic examination of the pitted specimens showed only micropits that developed at the interfaces of oxide/sulfide inclusions of titanium and matrix.

Susceptibility of As-Welded and Thermally Aged Type 316LN Weldments Toward Pitting and Intergranular Corrosion

Abstract In this study, an attempt was made to investigate the pitting and intergranular corrosion susceptibility of as-deposited and thermally aged weldments of type 316LN stainless steel (SS) made by shielded metallic arc (SMA) welding process using 316 SS filler wires having a carbon content of 0.059 wt%. The delta-ferrite content, which was 4.5 FN (Ferrite Number) for the as-welded samples, changed to 0.3 FN after the aging treatment (1023 K/5 h). Initial delta-ferrite on thermal aging decomposed to various secondary phases like sigma and M23 C6 and chi. These phases were confirmed metallographically and identified by the powder x-ray diffraction method. Pitting corrosion resistance was determined at ambient temperature in neutral chloride and acidic chloride media on both as-welded and thermally aged weld metal samples by anodic polarization technique. The results indicated that in neutral and acidic chloride media, the pitting corrosion resistance of thermally aged samples was lower compared to that...

Enhancement of Corrosion Resistance of Type 304 Stainless Steel Through a Novel Thermo-mechanical Surface Treatment

The paper describes a novel thermo-mechanical surface treatment approach, involving conventional shot blasting followed by laser surface heating, to engineer microstructural modification in type 304 austenitic stainless steel for enhancing its corrosion resistance. Thermo-mechanical surface treatment resulted in the formation of fine recrystallized grains with some strain-induced martensite on the modified surface. Surface treatment of type 304 stainless steel brought about significant improvement in its resistance against uniform as well as pitting corrosion. Electrochemical impedance spectroscopic studies showed improved polarization resistance (Rp) value for thermo-mechanically treated surface indicating formation of a more protective passive film than that formed on the untreated surface. In contrast to untreated type 304 stainless steel specimens where pits preferentially initiated at the site of Al2O3 inclusions, thermo-mechanically treated specimen exhibited only general dissolution with a few repassivated and shallow pits. Grain refinement and dispersion of alumina inclusions on the modified surface are considered to be the key factors responsible for improvement in uniform and pitting corrosion resistance of type 304SS.

Microbiologically Influenced Corrosion in UNS S31653: Detection and Analysis Using Electrochemical Noise Technique

Abstract Austenitic stainless steels are susceptible to microbiologically influenced corrosion (MIC), and biofilms of slime-forming bacteria can affect the integrity of the passive film of the stainless steel components. An attempt was made to detect and analyze the MIC on UNS S31653 using electrochemical noise (EN) technique. The progress of MIC was monitored in natural reservoir water, concentrated with biofilm-forming microbes along with 0.5 M sodium chloride (NaCl) and nutrients, as well as in a similar media minus microbes (made sterile by autoclaving) using two nominally identical UNS S31653 cylindrical specimens for about 3 weeks, where current and potential noise signals were collected at 1 Hz sampling frequency. Analysis of the shot-noise parameters like frequency of corrosion events, average charge in each event, true coefficient of variation, and noise resistance showed that initiation and propagation of MIC was marked by significant changes in these parameters. The presence of crevice corrosio...

Pitting Corrosion Studies on Solution-Annealed Borated Type 304L Stainless Steel Using Electrochemical Noise Technique

The pitting corrosion resistance of Type 304L (UNS S30403) stainless steel (SS) with (1.2 wt%) and without boron in as-received and solution-annealed (1,423 K for 2 h and 4 h) conditions was evaluated using the electrochemical noise (EN) technique in neutral 0.5 M sodium chloride (NaCl) solution for 72 h at the corrosion potential (Ecorr). EN data were analyzed using visual records, statistical parameters, spectral analysis, and shot-noise parameters. Weibull and Gumbell plots were prepared to study the distribution of pitting and passivation events and metastable pit radii, respectively. Current transient analysis showed the maximum number of pit nucleation and metastable pitting events in an as-received specimen, which decreased gradually with solution annealing. Pit radii calculated using Gumbell distribution showed the lowest metastable pit radii in solution-annealed specimens compared to as-received specimens. Therefore, EN analysis of the data revealed the improvement in pitting corrosion resistance...

Pitting Corrosion of Austenitic Stainless Steels and Their Weldments

Abstract Pitting corrosion is a major problem associated with the application of austenitic stainless steels in industries. The naturally formed protective passive film on stainless steels is damaged by halide ions leading to the formation of pits. Pits provide sites for cracks to initiate and propagate, and thus reduce the useful life of engineering components in service. Passivity and its significance, influence of alloy composition, microstructure, cold working, grain size etc., and the parameters of environments are discussed in detail with respect to pitting corrosion. Influence of welding and associated microstructural changes on pitting corrosion are also discussed. Various morphologies of pits are highlighted.

Corrosion behaviour of 304LN activated tungsten inert gas and flux-cored arc weld metals

Activated tungsten inert gas (A-TIG) and flux-cored arc (FCA) weld metals were prepared using 304LN stainless steel plate. The weld metals were thermally aged at 923, 973 and 1023 K for 100 h to study the decomposition of initial δ-ferrite in A-TIG (∼10 ferrite number (FN)) and FCA (∼5 FN) weld metals into secondary phases like M23C6 carbides, χ and σ. Ferrite number is the measurement of δ-ferrite based on the principle of magnetic property using ferritescope. Preliminary microstructural studies revealed the formation of carbides in FCA weld metals aged at 923 K for 100 h, which was correlated with higher carbon content (0.04 wt-%), and also ageing at higher temperature transformed δ-ferrite into χ/σ phases. However, A-TIG weld metals showed the transformation of δ-ferrite mainly into χ/σ phases. The δ-ferrite transformation kinetics was found to be sluggish in A-TIG weld metals compared to FCA weld metals. This difference was attributed to the difference in the carbon contents of A-TIG and FCA welds. Activated tungsten inert gas weld metals showed better uniform and pitting corrosion resistance compared to FCA weld metals in as-deposited and thermally aged conditions. Presence of higher amount of initial δ-ferrite content in A-TIG weld metal helped diffusion of minor alloying elements like sulphur and phosphorous into it, thereby reducing their microsegregation at the δ/γ interface boundaries and subsequent pitting corrosion attack. Thus, A-TIG welding process was found to be superior compared to FCA welding process.

Studies on Dissolution Behavior of the Surface Layer of Sodium-Exposed SS 316LN in Decontaminating Formulation Using PEMHS

The Prototype Fast Breeder Reactor (PFBR) is nearing completion at Kalpakkam, India. Sodium is the heat transfer medium for PFBR, and austenitic steel SS 316LN is the material of construction for the sodium circuits of the reactor. During reactor service, the inner surfaces of the sodium circuit pipelines undergo corrosion by interacting with liquid sodium, forming ferritic layers. Radioactive nuclides formed by the activation of corrosion products are deposited on the ferritic surface, resulting in a radioactive burden on maintenance personnel. Chemical decontamination is generally carried out by dissolving the surface ferritic layer on the inside surface of the sodium circuit. In this context, a study of the dissolution behavior of the ferritic layer on SS 316LN samples formed by exposure to liquid sodium at 823 K was carried out by monitoring the H2 released during the chemical interaction with decontamination formulation. The decontamination chemical formulation was a mixture of sulfuric acid and phosphoric acid. This paper discusses the sample preparation, formation of the ferritic layer, and studies carried out on its dissolution behavior in decontamination formulation by monitoring the hydrogen released during the reaction using a proton exchange membrane–based hydrogen sensor.

Characterisation of microstructural damage due to corrosion fatigue in AISI type 316 LN stainless steels with different nitrogen contents

Corrosion fatigue (CF) behaviour of AISI type 316 LN stainless steels (SS) with three different nitrogen contents was evaluated in a boiling aqueous solution of 5 M NaCl+0·15 M Na2SO4+2·5 ml l−1 HCl at a stress ratio of 0·5 and a frequency of 0·1 Hz. After the CF tests, the specimens were observed under a field emission gun scanning electron microscope (FEG-SEM) as well as an atomic force microscope (AFM) to understand the deformation mechanism which led to the failure. Slip character could be explained based on the surface deformation features observed using FEG-SEM and AFM. A slip irreversibility relation has been proposed which when applied could explain the CF behaviour of these steels with varying nitrogen contents. Increase in the nitrogen content increased the slip reversibility up to 0·14 wt-% nitrogen; however, further increase in nitrogen content had no beneficial effect on the slip reversibility.