Corrosion Properties of HNS

The effect of nitrogen alloying on the pitting corrosion resistance of duplex and austenitic stainless steels has been examined. In order to avoid alteration of the phase ratio as a result of nitrogen alloying of the duplex steels, a simultaneous decrease has been made in the nickel content. Austenitic alloys of compositions corresponding to the austenite phase of the duplex steels have been investigated and compared to the behaviour of austenitic steels in which the nitrogen content or the nickel content alone has been varied. Nitrogen has a beneficial effect on pitting and crevice corrosion resistance in all cases but the duplex stainless steel exhibit a lower resistance to pitting and a higher resistance to crevice corrosion than predicted from the austenite nitrogen content.

The resistance of X5CrNi18-10 stainless steel to pitting corrosion in a solution containing chlorides and sulphates was tested using the potentiodynamic polarization method. The obtained results show that the stainless steel is significantly resistant to pit formation, but it is susceptible to pit growth and crevice corrosion. Pits formed at the corrosion potential grow continuously. Statistical analysis of the results obtained during pitting corrosion testing was performed. It can be assumed with a probability of 95% that values of indicators of resistance to pit formation (the pitting potential Epit, the metastable pitting potential Empit and the difference Epit-Ekor) will be within the range of several percents. Values of indicators of resistance to pit growth (the amount of charge required for the pit growth q, the protective potential Eprot and the difference Epit-Eprot) will be within the broader range. In addition, the appearance of pits on the surface of the stainless steel, as well as the appearance of the pits bottom, were analyzed. It was shown that the structure of the stainless steel tested was not sensitized to pitting and intergranular corrosion, which means that the stainless steel was not previously thermally treated.

The influence of alloying levels in the range (in mass%) of 0.01-0.5 nitrogen and 0.2-10 manganese on pitting corrosion resistance has been investigated for austenitic stainless steels with a base composition (in mass%) of 20Cr-18Ni-4.5Mo. The beneficial effect of nitrogen alloying is confirmed, while the detrimental effect of manganese alloying can be offset by the higher nitrogen contents attainable in such steels, The effect of sensitizing heat treatments at 800 and 900°C on resistance to pitting and intergranular corrosion is also evaluated and correlated to the precipitation of secondary phases.

This report examines the degradation modes of four Ni-Cr-Mo alloys under conditions relevant to the Yucca Mountain Site Characterization Project (YMP). The materials considered are Alloys C-276, C-4, C-22, and 625 because they have desirable characteristics for the conceptual design (CD) of the high-level radioactive-waste containers presented in the YMP Site Characterization Plan (SCP). The types of degradation covered in this report are general corrosion; localized corrosion, including pitting and crevice corrosion; stress corrosion cracking in chloride environments; hydrogen embrittlement (HE); and undesirable phase transformations due to a lack of phase stability. Topics not specifically addressed are welding concerns and microbiological corrosion. The four Ni-Cr-Mo alloys have excellent corrosion resistance in chloride environments such as seawater as well as in more aggressive environments. They have significantly better corrosion resistance than the six materials considered for the CD waste container in the YMP SCP. (Those six materials are Types 304L and 3161L stainless steels, Alloy 825, unalloyed copper, Cu(70)-Ni(30), and 7% aluminum bronze.) In seawater, the Ni-Cr-Mo alloys have negligible general corrosion rates and show little evidence of localized corrosion. The four base materials of these alloys are expected to have nearly indistinguishable corrosion resistance in the YMP environments. The strength requirements of the SCP-CD waste container are met by these materials in the annealed condition; in this condition, they are highly resistant to HE. Historically, HE has been noted when these materials have been strengthened (cold-worked) and used in sour gas (H{sub 2}S and CO{sub 2}) well service -- conditions that are not expected for the YMP. Metallurgical phase stability may be a concern under conditions favoring (1) the formation of intermetallics and carbides, and (2) microstructural ordering.

Pitting corrosion and crevice corrosion behaviors of high nitrogen austenitic stainless steels

Specialty steel producers have expended considerable effort to develop stainless steels that are resistant to pitting and crevice corrosion in seawater. Stainless steels with resistance to crevice corrosion and pitting combined with their excellent resistance to general corrosion and erosion, good formability, and good weldability would be ideal for use as plate and tubing fabricated into tanks, condensers and heat exchangers. Many applications for heat exchangers involve intermittent or continuous exposure to polluted seawater where the crevice corrosion and pit resistant stainless steels may be a viable replacement for copper based materials.

Dilation-balloon expandable coronary stents are made of implant grade stainless steels, UNS S31673, e.g., BioDur® 316LS. Boston Scientific/Interventional Technologies (BS/IVT) determined that addition of platinum to UNS S31673 could produce a stainless steel with enhanced radiopacity, which made such stents more visible radiographically. A goal of the program was to ensure the platinum additions would not adversely affect the corrosion resistance of the resulting 5–6 wt % PERSS® alloys. Corrosion resistance of PERSS and BioDur 316LS was determined using electrochemical tests for general, pitting, crevice and intergranular corrosion. Experimental methods included A262E, F746, F2129, and potentiodynamic polarization. The ∼ 6 wt % PERSS alloy (IVT 78) had a resistance to pitting, crevice and intergranular corrosion similar to base materials. IVT 78 was a single-phase austenitic PERSS alloy with no evidence of inclusions or precipitates; it was more resistant to pitting corrosion than the ∼ 5 wt % PERSS alloys. PERSS performance was not a function of oxygen content in the range 0.01 to 0.03 wt %.

The new Fe-Cr-Mo ferritic stainless steels are reviewed with an emphasis on corrosion behavior. The properties of commercial steels are outlined showing how these alloys can be divided into general purpose (18Cr-2Mo), moderately severe service (26Cr-1Mo), and premium service (29Cr-4Mo) materials. The stress corrosion cracking (SCC), intergranular corrosion, pitting, crevice corrosion, and general corrosion of the Fe-Cr-Mo ferritic stainless steels in general is discussed. The influence of nickel on the corrosion resistance and toughness of ferritic stainless steels is considered.

MATERIAL DESCRIPTION Duplex (Ferritic-austenltic) stainless steel combines many of the good properties of tfie ferritic and the austenitic steels. Owing to its high chromium and molybdenum contents, the steel has very good resistance to pitting and general corrosion. The duplex microstructure gives the steel both high resistance to stress corrosion cracking and high mechanical strength. Duplex stainless steel possesses good weldability.

Simulation of Crevice and Pitting Corrosion Using the Orthogonal Collocation Method

Martensitic stainless steels are used in a large number of various industrial applications, e.g. molds for plastic injections and glass moldings, automotive components, cutting tools, surgical and dental instruments. The improvement of their tribological and corrosion properties is a problem of high interest especially in medical applications, where patient safety becomes a priority. The paper covers findings from plasma nitrided AISI 440B (PN-EN or DIN X90CrMoV18) stainless steel corrosion resistance studies. Conventionally heat treated and plasma nitrided in N 2 :H 2 reaction gas mixture (50:50, 65:35 and 80:20, respectively) in two different temperature ranges (380 or 450°C) specimens groups were examined. Microscopic observations and electrochemical corrosion tests were performed using a variety of analytical techniques. As obtained findings show, plasma nitriding of AISI 440B stainless steel, regardless of the process temperature, results in reduction of corrosion current density. Nevertheless, applying thermo-chemical process which requires exceeding temperature of about 400°C is not recommended due to increased risk of steel sensitization to intergranular and stress corrosion. According to the results, material ion nitrided in 450°C underwent leaching corrosion processes, which led to significant disproportion in chemical composition of the corroded and corrosion-free areas. The authors suggest further research into corrosion process of plasma nitrided materials and its degradation products.

Lean duplex stainless steel (LDSS) has been used in various applications, including flexible pipelines in offshore and other industrial settings. In recent years, LDSS has become the preferred choice over standard duplex stainless steel (DSS) for flexible pipeline applications due to its lower costs, achieved by reducing nickel and molybdenum content, while still providing comparable corrosion resistance and mechanical strength properties to DSS. However, there is still limited reporting on the corrosion effects of reducing these alloys on the behaviour of lean duplex stainless steel in flexible pipelines. This comparative study investigates the corrosion resistance of lean duplex stainless steel, LDX 2101 and duplex stainless steel, UNS S32205 in flexible pipeline applications using linear polarization resistance (LPR). The research focuses on assessing material performance in environments containing CO2 and H2S, commonly found in oil and gas production, by conducting short-term and long-term tests to evaluate pitting and selective corrosion. The samples, LDX 2101 and UNS S32205 were immersed in a 3.5M NaCl solution, and corrosion measurements were performed using the Metrohm Autolab potentiostat. The results indicate that both materials exhibit good corrosion resistance, but there are differences in their performance under specific conditions. While lean duplex stainless steel, LDX 2101, can be used as a substitute for duplex stainless steel UNS S32205, its corrosion resistance and mechanical properties gradually decrease over time due to the reduced nickel and molybdenum content. As a result, it would not be as effective as duplex stainless steel UNS S32205 in withstanding corrosion in aggressive conditions over a prolonged period.

Effect of W, Mo and N contents on pitting corrosion resistance of 25Cr and 23Cr base duplex stainless steels was investigated using the ferric chloride test according to ASTM G48. The improving effect by W addition for pitting corrosion was found out, and the modified pitting resistance equivalent(PREW) of Cr+3.3(Mo+0.5W)+16N was determined by using the least-squares method. In addition, effect of W, Mo and N contents on pitting corrosion at heat-affected zone in weldment was studied to decide the proper chemistry of the base metal. From these results, 25Cr duplex stainless steel with 2mass% W(25Cr-7Ni-3Mo-2W-0.3N), and high Mo-23Cr duplex stainless steel(23Cr-7Ni-4.5Mo-0.3N) have been developed as high pitting corrosion resistance materials. Further, corrosion behavior of the new developed-steels was investigated in H2S containing CO2 environment and these steels showed the improved-corrosion resistance compared with conventional 25Cr duplex stainless steel. Finally, pitting corrosion resistance of the duplex stainless stainless steel with high N content was discussed based on the corrosion test results and the calculation result of PREW in each phase of α and γ, and it was indicated that the duplex stainless steel with 40-45% α content has the most excellent pitting corrosion resistance.

Although a wide variety of degradation modes can occur in aqueous environments for corrosion-resistant metallic materials, localized corrosion such as pitting corrosion, crevice corrosion, SCC, and hydrogen embrinlement (HE) is considered to be the primary mode. The evaluation of the susceptibility of candidate corrosion-resistant container materials to pitting and crevice corrosion is well underway using electrochemical polarization techniques described in the Activity Plan E-20-43144. The proposed activity (E-20-56) is aimed at evaluating the SCC behavior of these materials in susceptible environments using the linearelastic-fracture-mechanics (LEFM) concept. The mechanical driving force for crack growth, or the stress distribution at the crack tip is quantified by the stress intensity factor, K, for the specific crack and loading geometry. The critical stress intensity factor for SCC, KISCC for candidate materials will be evaluated in environments of interest, and their comparisons will be made to select the waste package inner container material having an optimum SCC resistance.

Corrosion performance of a high-nickel alloy