Advanced Materials Research Vol. 794

Abstract: Stainless steels are among the most investigated materials on biofouling and microbially-influenced corrosion (MIC). Although, generally corrosion-resistant owing to tenacious and passive surface film due to chromium, stainless steels are susceptible to extensive biofouling in sub-soil, fresh water and sea water and chemical process environments. Biofilms influence their corrosion behavior due to corrosion potential ennoblement and sub-surface pitting. Both aerobic and anaerobic microorganisms catalyse microbial corrosion of stainless steels through biotic and abiotic mechanisms. MIC of stainless steels is common adjacent to welds at the heat-affected zone. Both austenite and delta ferrite phases may be susceptible. Even super stainless steels are found to be amenable to biofouling and MIC. Microbiological, electrochemical as well as physicochemical aspects of MIC pertaining to stainless steels in different environments are analyzed.

Abstract: The relative superiority of duplex stainless steels (DSS) over austenitic grades with regards to stress corrosion cracking (SCC) is discussed. The benefits of N to SCC resistance of DSS are provided. The selective dissolution of phases and its impact on corrosion SCC is reviewed. The hydrogen embrittlement of DSS is reviewed with emphasis on the ferrite participation, the role of environments and fracture morphology. The evolution of secondary phases and precipitates and the resultant change in corrosion resistance and SCC in DSS is discussed.

Abstract: The susceptibility of non sensitized 304L stainless steel (SS) components towards stress corrosion cracking (SCC) has been studied here in the light of the significant role played by surface working operations. The plant experience shows that the fracture surfaces of non sensitized 304L stainless steel components have no signs of carbide precipitation. However, heavy plastic deformation has been evidenced in the form of high density of slip bands on the surface up to a depth of about 100 μm with high tensile residual stresses near the surface. The present study has established that the primary cause of the increase in SCC susceptibility is the heavy plastic deformation near the surface and high magnitude of tensile residual stresses which is a consequence of the surface finishing operations like machining and grinding. In this study, solution annealed 304L stainless steel has been subjected to a) surface working operations like machining and grinding and b) bulk deformation operations such as 10 % cold rolling operation. The materials in different conditions where then subjected to detailed a) microstructural characterisation, b) electrochemical characterisation and c) tests for determining the stress corrosion cracking susceptibility. The distinct differences in the micro structure as a result of bulk deformation vs. surface deformation of 304L austenitic stainless steel were highlighted and correlated to the susceptibility towards stress corrosion cracking. The effect of surface working on the nature and composition of high temperature (300 °C and 10 MPa) oxide formed on 304L stainless steel has been studied in-situ by contact electric resistance (CER) and electrochemical impedance spectroscopy measurements using controlled distance electrochemistry technique in high purity water (conductivity < 0.1 μScm^-1) at 300 °C and 10 MPa in an autoclave connected to a recirculation loop system. The results highlighted the distinct differences in the oxidation behaviour of surface worked material as compared to solution annealed material in terms of specific resistivity and low frequency Warburg impedance.

Abstract: 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.

Abstract: In the present study macro electrochemical (anodic polarization) and micro electrochemical (scanning electrochemical microscopy (SECM) area scan measurements at passive potential) techniques have been used to study the influence of sigma phase and/or the resultant chromium depletion regions on localized corrosion behavior of aged type 2205 duplex stainless steel (DSS) in neutral chloride ion solution. DSS type 2205 was subjected to aging at 750 °C for 30 min, 10 h and 48 h. The formation and growth of the sigma phase with heat treatments was assessed by optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction. The influence of formation of sub-microscopic and bulky sigma phase on intergranular corrosion (IGC) and pitting corrosion was investigated by various electrochemical techniques including electrochemical potentiokinetic reactivation (EPR), potentiodynamic polarization and SECM. Apart from EPR tests, ASTM A 262 Practice B test was carried out to evaluate the presence of chromium depletion regions with heat treatments. The results showed that with increasing aging duration, the degree of sensitization and IGC rates initially increased and then decreased with heat treatment. The pitting potentials decreased continuously with increase in aging duration up to 10 h as assessed by potentiodynamic polarization tests. The SECM area scan measurements showed more metastable pitting corrosion events for 30 min and 10 h aged specimens compared to the 48 h aged specimen at passive potential in 0.1M neutral chloride ion solution.

Abstract: With rising demands, oil and gas exploration of high-pressure high-temperature (HPHT) wells are increasing worldwide. Due to aggressiveness of HPHT environments, piping and equipments are constructed with high-strength corrosion resistant alloys (CRAs). Duplex stainless steel is one of the candidate alloys that offer high strength along with corrosion resistance. It possesses the advantages of both austenitic and ferritic stainless steels and hence, the name duplex or dual phase stainless steel. In order to control corrosion, cathodic protection is commonly being employed on the structures and equipment. Cathodic protection is accomplished by applying a direct current to the structure which causes the structure potential to change from the natural corrosion potential (E_corr). The required cathodic protection current is supplied by sacrificial anode materials or by an impressed current system. Hydrogen embrittlement (HE) is an associated phenomenon, which results in the production of hydrogen ions, leading to its absorption in the protected metal and subsequent hydrogen embrittlement of metals and welds. To prevent this embrittlement, cathodic protection is closely studied in terms of finding the critical potential, pH, temperature etc. that does not cause hydrogen embrittlement. This paper describes the study carried out to find the role of pH on the absorption of hydrogen in Duplex Stainless steel. It has been observed that at a critical pH, hydrogen intake in the sample is very high, as compared to the pH below and above the critical pH. Critical pH observed for duplex stainless steel is a trade of between hydrogen evolution and absorption for given duplex structure.

Abstract: The in-cell stainless steel piping and erection works require extensive welding. In many instances the approach for gas tungsten arc welding (GTAW) is limited and it is not possible to provide a cover of high purity inert Argon gas (backing shield) and in some instances, oxidation of the weldment takes place. The oxide forms over the weld fusion zone (root pass) as well as a heat tint forms over the surfaces of the adjacent base material. In reprocessing and waste management plants, the welded pipes come in contact with the process fluid which is nitric acid of concentration up to 6 M and at temperatures up to boiling point. The present study was focused on preparing induced oxidized welds of type 304L using filler wire of type 308L, using gas tungsten arc welding (GTAW) process and studying their corrosion behavior in nitric acid environments. Sample welds were prepared under proper welding conditions and also with conditions in which deliberately Argon gas was not purged or partially purged. The weldments with no oxides, partial oxides and excess oxides on the weld root pass were used for corrosion and characterization studies. Micro Laser Raman spectroscopy established the oxide to be hematite. Metallographic examination of the cross-section of the weldment showed the thickness of oxide to be 200-300 mm. Corrosion tests of the weldments as per practice C, A262, ASTM were done for five periods. Metallographic examination was done after the practice C exposures and showed absence of oxides on the weld root pass. Type 304L specimens were heat treated at 500 – 900 °C for 5 minutes to generate heat tints. These specimens were tested as per practice C, A262, ASTM for 5h and four periods of 48 h each. The corrosion rate in the first five hours exposure was high for the specimen heat treated at 900 °C but it came down to normal values in subsequent exposures. To confirm the corrosion behavior of hematite and magnetite in boiling nitric acid, powders of pure Fe₂O₃ and Fe₃O₄ were tested in boiling 65% nitric acid. The results are analyzed to establish the behavior of oxides on the stainless steel welds in nitric acid.

Abstract: Austenitic stainless steels are susceptible to microbiologically influenced corrosion (MIC) when they are in contact with sea water. This is due to the changes in the chemistry of the environment at the metal surface because of the settlement and activities of microorganisms. The thrust of our work was in understanding the changes in the electrochemical behaviour of a type 316L stainless steel in the presence of a natural biofilm as well as the influence of metallurgical characteristics on microbial adhesion and MIC. The presence of a biofilm on material surface can influence the corrosion behaviour since the value of a given parameter such as temperature, pressure, concentration of a solute and pH at the water /substrate interface under the biofilm may be different from that in the bulk environment. The non-uniform nature of biofilm thus helps in generating heterogeneity in the environment at the surface. Thus, biofilms are known to aid in the initiation of corrosion, change the mode of corrosion or cause changes in the corrosion rate. Bacteria Arthobacter nicotinae (An) and algae Chlorella pyrenoidosa (Cp) were used for the study and bio film formed due to these showed pit initiation and increase in corrosion rate as time proceeds. 316L base metal (BM) and weld metal (WM) as received and after heat treated at 450°C for 10000 hours were studied and corrosion evaluation was done. Heat treated WM showed severe response to corrosion compared to as received WM. Key Words: MIC, AISI 316L SS, biofilm, weld metal, localized corrosion.

Abstract: Metallurgical investigations were directed to probe into the incidence of inordinate rusting and pitting in imported AISI 430 grade hot-rolled ferritic stainless steel sheet coils. Visual examination, electron microprobe analyses (EPMA), scanning electron microscopy (SEM) and electrochemical potentiokinetic reactivation (EPR) were concomitantly employed to investigate the problem. Studies revealed that the unprecedented degree of corrosion in ferritic stainless steel coils, during the short span of shipment time, was attributable to the ingress of sea water and its retention within the tight folds/ wraps of the steel coils during their shipment. The abundance of moisture and chloride (from the entrapped saline electrolyte) on the steel surface together with depleted O2 supply within the tight folds are presumed to have created conditions akin to an actively-corroding crevice, by way of passive film instability and its eventual breakdown on the stainless steel surface. As a consequence, the coils are believed to have suffered an accelerated and intensified chloride-induced corrosion attack and damage within the short span of shipment time. The investigations also revealed that the corrosive conditions were further exacerbated by the vulnerability and susceptibility of ferritic stainless steel to intergranular corrosion (IGC) due to its inherent sensitized condition. The paper thus throws light on an unusual precedent of chloride-induced corrosion in ferritic stainless steel and highlights the investigative metallographic work and corrosion failure analysis that led to above revelations.

Abstract: The semiconducting property of passive films formed on Alloy 1 (18Cr-2Mo-1N)], Alloy 2 (17.5Cr-3Mo-0.5N)] and Alloy 3 (Type 316 SS) were studied by using the Mott-Schottky (M-S) approach in 3.5 wt. % NaCl solution of pH 2, 7 and 12. The M-S analysis shows that the film acts as n - type and p - type semiconductors across the potential range. The donor density of Alloy 1 has been found to be lower by about 31 %, 11 % and 6 % as compared to that of Alloy 2 at pH 2, 7 and 12 respectively. However; Alloy 3 has higher donor density 44 %, 27 % and 30 % in comparison with Alloy 1. The donor density of Alloy 3 found to be greater about 21 %, 18 % and 25 % to the Alloy 2 at pH 2, 7 and 12 respectively. These results indicate that the sensitivity of electrolyte composition and the presence of alloying elements like nitrogen and nickel on the donor density of passive film. High nitrogen stainless steels exhibited a lower donor density that corresponds to good protectiveness, more stable passive film which is in agreement with a low passive current density (i_pass), higher pitting potential (E_pit) and polarization resistance (R_p).