Stress corrosion cracking study of microalloyed pipeline steels in dilute NaHCO3 solutions

Studies were carried out to evaluate the stress corrosion cracking (SCC) behavior of a X-70 microalloyed pipeline steel, with different microstructures by using the slow strain rate testing (SSRT) technique at 50 °C, in NaHCO₃ solutions. Both anodic and cathodic potentials were applied. Additionally, experiments using the SSRT technique but with pre-charged hydrogen samples and potentiodynamic polarization curves at different sweep rates were also carried out to elucidate hydrogen effects. The results showed that the different microstructures in conjunction with the anodic applied potentials shift the cracking susceptibility of the steel. In diluted NaHCO₃ solutions cathodic potentials close to their rest potential values decreased the SCC susceptibility regardless the microstructure, whereas higher cathodic potentials promote SCC in all steel conditions. Certain microstructures are more susceptible to present anodic dissolution corrosion mechanism. Meanwhile concentrated solution did not promotes brittle fracture.     

Hydrogen embrittlement properties of heat affected zone of high strength steel in shielded metal arc welding

Corrosion resistance, mechanical properties and hydrogen embrittlement were investigated from an electrochemical view, with the slow strain rate test (SSRT) method with applied constant cathodic potential. Fracture surface was analyzed by SEM. Corrosion resistance and mechanical properties were increased by post-weld heat treatment (PWHT) compared to those in the as-welded condition. Elongation and time-to-fracture were decreased with shifting cathodic polarization potential to the low potential direction. On analysis of SEM fractography, the quasi-cleavage (Q.C) fracture mode was also observed with an increase of susceptibility to hydrogen embrittlement. At the applied cathodic potential between −770 mV and −875 mV (SCE; saturated calomel electrode), the fracture morphology was of the dimple pattern with ductile fracture, while it changed to the transgranular pattern at under −900 mV (SCE). Eventually it is suggested that an optimum cathodic protection potential range was from −770 mV to −875 mV (SCE) without regard to PWHT condition.     

Electrochemical study of hydrogen embrittlement and optimum cathodic protection potential of welded high strength Steel

Electrochemical properties and mechanical properties of welded high strength steel were investigated by the slow strain rate test (SSRT) method with an applied constant cathodic potential. There was no correlation between maximum tensile strength, yield strength, stress at failure, and hydrogen embrittlement. However, the elongation, time-to-fracture, and strain-to-failure ratio decreased with shifting potential of the negative direction. Elongation, time-to-fracture, and strain-to-failure ratio for the various welding methods displayed the highest values when the potential was −770 mV regardless of post-weld heat treatment (PWHT) conditions. The elongation, time-to-fracture, and amount of dimples by PWHT were increased. The optimum potential region for cathodic protection without hydrogen embrittlement was observed between −770 and (above) −850 mV (SCE) in the post-weld specimens heat treated at 550°C and the as-welded specimens.     

Effect of cathodic polarization and sulfate reducing bacteria on mechanical properties of different steels in synthetic sea water

At the slow strain rate tensile tests done using the specially designed facility, the decrease in the elongation to fracture, reduction of area, fracture energy and no effect on the strength have been stated for the low alloy ferrite‐pearlite and sorbite steels, polarized in synthetic sea water at potentials corresponding to the cathodic protection (? 800 to ? 1400 mV_SCE). Presence of SRB promotes the plasticity loss, being especially pronounced at potentials ? 1100 to ? 1200 mV_SCE. At higher cathodic polarization, the plasticity estimated in inoculated and in sterile water equalizes. The effects have been correlated with the contents of absorbed and of permeable hydrogen. The promotion of hydrogen charging and the plasticity loss by SRB at the low and medium applied cathodic polarization has been accounted for the observed production of S^?2 ions and inhibition of deposit formation. The negligible effect of SRB at the high cathodic polarization has been suggested to be a result of the suppression the SRB growth due to the high alkalization of the near surface solution. The same amount of hydrogen produces the less detrimental effect on the sorbite than on the ferrite‐pearlite steel. However, at the similar cathodic polarization, the sorbite steel absorbs the highest amount of hydrogen and reveals the most pronounced degradation. Cathodic protection of constructions subjected to the action of SRB in the sea water should provide the conditions, under which no fragment of marine construction could be polarized by potential corresponding to the maximum degradation of the plastic properties of steels (? 1100 to ? 1200 mV_SCE).     

Electroplating of catalytically active nickel coatings from baths of various anionic compositions

The effect of the nature of bath anions (Cl⁻, SO₄²⁻ and CH₃COO⁻) on the characteristics of the electrochemical plating of nickel coatings, namely, parameters of the cathodic polarization curves, bath stability, and current efficiency, as well as the composition and morphology of deposits and their catalytic activity in the anodic oxidation of hypophosphite ions and cathodic evolution of hydrogen, is studied. The anionic composition of the baths is found to substantially affect not only the kinetics of the deposit growth, but also the properties of the deposit. The differences observed are determined by the complex-forming and buffering properties of the anions.     

Investigation of calcareous deposits formation on copper and 316L stainless steel under cathodic polarization in artificial seawater

The formation and growth of calcareous deposits on 316L stainless steel and copper under cathodic polarization in artificial seawater were investigated by electrochemical tests and analytical techniques such as SEM, EDX and XRD. The deposits mineral compositions were related to the types of metallic materials and were different on each substrate. On 316L stainless steel at potentials less negative than the water reduction potential (−1100 mV/SCE), the deposits were composed of aragonite with low amounts of brucite; at the potentials more negative than the water reduction potentials, only of brucite. Around the water reduction potentials, the deposits were composed of both aragonite and brucite together. However the formation of brucite was noted before activating the water reduction processes. The results were moderately similar to that on mild steel and the type of deposited phases depended on potentials, at which the cathodic protection was carried out. The deposits formed on copper differed from those on 316L stainless steel and the types of deposited phases were independent from water reduction potential on copper (−1150 mV/SCE). Hence, the deposits were composed only of aragonite at all potential ranges. Due to the low current densities observed in chronoamperometric curves during cathodic polarization and in sufficient alkalinity, it seemed that the brucite could not deposit on this metallic substrate. The article is published in the original.     

Hydrogen absorption resulting from the simulated pitting corrosion of carbon-manganese steels

Hydrogen permeation measurements were performed on membranes of BS4360 Grade 50D C-Mn steel in the quenched and tempered condition. The rates of hydrogen absorption resulting from exposure to FeCl₂ solutions in a simulated corrosion pit were measured and found to be lower than those occurring in artificial sea water at applied potentials in the range commonly used for cathodic protection. A progressive decrease in the hydrogen permeation flux was recorded during simulated pitting and was attributed to the formation of a partially protective film of magnetite on the steel surface. At cathodic applied potentials iron plating was observed on the membranes. It is suggested that a similar process occurs in the cathodic protection of steel containing real corrosion pits and leads to a lowering of the Fe²⁺ ion concentration within the pits and a decrease in the aggressiveness of the local environment.     

Mechanical properties of injection molded Fe-6% Ni-0.4% C steels with varying Mo contents of 0.5 to 2%

In our previous studies, sintered and heat-treated alloy steels (Fe−6Ni−0.5Mo−0.4C (mass%)) produced by a MIM process showed excellent mechanical properties of 2000 MPa tensile strength and 5% elongation. This was attributed to the solid solution strengthening and the mezzo-heterogenous microstruture, which consisted of martensite or retained austenite (Ni and Mo rich phases) surrounded by a network of tempered martensite. This study has been performed to clarify the effect of Mo on the mezzo-heterogeneous microstructure and the mechanical properties of MIM processed and sintered alloy steels (Fe−6Ni−0.4C) with varying Mo content (0.5–2 mass%). The tensile properties of the heat-treated steels with added 2 mass% Mo were lower than those of the steels with added 0.5 mass% Mo. The reduction in the tensile properties, particularly the appearance of large pores formed at the original location of Mo power through the transient liquid phase formation and the low hardness of the matrix, was due to the low sintered density. By using mechanically milled fine Ni and Mo powders, the heat-treated steel (Fe−6Ni−2Mo−0.4C) showed excellent properties, including tensile strength of 1800 MPa and ductility of 2.2% elongation. This article is based on a presentation made in the symposium “The 3rd KIM-JIM Joint Symposium on Advanced Powder Materials”, held at Korea University, Seoul, Korea, October 26–27, 2001 under auspices of The Korean Institute of Metals and Materials and The Japan Institute of Metals.     

Synopses of Papers in this Issue

Abstract

A variety of techniques involving electrochemical, hydrogen permeation and acoustic emission measurements have been used in studying the stress corrosion cracking of initially smooth and pre-cracked specimens of a maraging steel in different chloride-containing solutions. In solutions having pH's in excess of 2, cracking of smooth specimens occurred in two regimes of potential separated by a region in which cracking did not occur, although cracking was induced in this latter region if the specimens were precracked or pre-pitted, or pitting was facilitated by non-metallic inclusions emerging at the test specimen surfaces by stressing the specimens transversely to the rolling direction. It is considered that such geometrical discontinuities are more important in facilitating cracking by fostering local changes in solution chemistry than because of their effect in terms of stress intensification. However, the evidence, viewed in its entirety, does not support the hypothesis that failure invariably results from the ingress of hydrogen into the steel following the creation of acidic conditions within an initiating pit or pre-crack. Rather does it support crack growth by dissolution at high potentials, and by a hydrogen induced process at low potentials, with possibly both processes involved at some intermediate potentials, including the free corrosion potential. This conclusion is supported by various electrochemical measurements, which show good correlation with the potential dependence of cracking and with the effects upon cracking of additions of chloroplatinic acid, sodium arsenate or thiourea to the chlQride solutions, with or without applied polarisation. Thus, the effects of these additives upon the hydrogen and dissolution reactions are in agreement with their influence upon stress corrosion cracking, as are the effects of applied polarisation. Hydrogen permeation measurements under conditions of cathodic polarisation confirmed the effects of these additions upon the uptake of hydrogen by the steel, but when permeation membranes were subjected to anodic polarisation they were perforated by dissolution and the apparent hydrogen permeation was due to the passage of solution through the membrane. Further support for hydrogen induced cracking dominating at lower potentials and dissolution being controlling at higher potentials was derived from acoustic emission experiments, which showed emissions to be enhanced by cathodic but not by anodic polarisation as crack growth occurred, implying different mechanisms of growth in the different potential regimes.     

Effect of hydrogen on pitting susceptibility of 2507 duplex stainless steel

The effect of hydrogen on pitting corrosion susceptibility of duplex stainless steel was investigated. Pits are observed on the hydrogen-charged specimen after 6 days of immersion in 6% FeCl₃ solution, while no pits on the uncharged specimen even after more than 30 days of immersion, which indicates that hydrogen promotes pitting initiation and pit growth. Moreover, pitting susceptibility increases with hydrogen charging current density. The pitting tends to nucleate initially inside the austenite or at ferrite/austenite boundaries, and then appears in the ferrite, because of different behaviors of hydrogen in two phases, such as solubility and diffusivity of hydrogen.     

Hydrogenation properties and crystal structures of Ti−Mn-V BCC solid solution alloys

We have proposed new hydrogen absorbing alloys of the ‘Laves phase related BCC solid solution alloy’, the hydrogen capacity of which reaches almost double that of conventional rare-earth based AB₅ alloys. We have reported the hydrogen absorbing properties of Ti−V−Mn, Ti−V−Cr and T−V−Mn−Cr alloys. It has been accepted that the crystal structural change of BCC hydrogen absorbing alloys is the same as that of V metal. The mono-hydride (H/M=1) of V metal has a BCT structure and the di-hydride (H/M=2) has an FCC structure. However, we recently found that the Ti−V−Mn alloy shows different behaviors in phase transformation with hydrogenation to V metal. We found three hydride phases with a BCC, a deformed FCC and an FCC structure in the Ti−V−Mn solid solution alloy-H₂ system. The deformed FCC hydride phase has not yet to our knowledge been reported. The lattice constant of the deformed FCC was 0.407 nm, one axis of which is reduced by about 4%. Its single-phase region appeared at a hydrogen content between 0.8 H/M and 1.0 H/M in absorption at 298 K. The lower plateau observed due to formation of the deformed FCC hydride phase gives an increase of effective hydrogen capacity by decreasing hydrogen remaining in the alloy in the desorption process. This article based on a presentation made in the symposium “The 2nd KIM-JIM Joint Symposium: Hydrogen Absorbing Materials”, held at Hanyang University, Seoul, Korea, October 27–28, 2000 under the auspices of The Korean Institute of Metals and Materials and The Japan Institute of Metals.     

Effect of the austenite stability on the physicomechanical properties of invars strengthened by combined treatments

This work is devoted to the investigation of an N30K10T3 invar alloy with metastable austenite (martensite point M _s ≈ −80°C) and an N40K10T3 invar with stable austenite (M _s < −196°C). The Curie points of the alloys are θ_C ≈ 200 and 310°C, respectively. Effects of aging of preliminarily deformed invars on the hardness, thermal-expansion coefficient, stress-corrosion resistance, and coercive force have been studied. It has been demonstrated that these properties of quenched alloys can be affected by both deformation and decomposition of the supersaturated solid solution. In the metastable alloy, the coefficient of linear expansion depends on temperature and aging time; no such dependence is observed in the stable alloy.     

Effect of bis(hydroxyethyl)triethylenetetramine on a hydrogen diffusion flow across a steel membrane from chloride solutions containing H2S and CO2

The effect of bis(hydroxyethyl)triethylenetetramine on hydrogen transfer across a steel membrane from H₂S−CO₂-containing weakly acidic and neutral salt solutions simulating stratal waters was studied. This effect was found to depend on the inhibitor’s concentrations, temperature, test duration, cathodic and anodic polarization, and the nature of hydrogenation stimulators.     

Electrochemical kinetics of the high entropy alloys in aqueous environments—a comparison with type 304 stainless steel

A high entropy alloy (HEA) is a multi-component alloy containing several major alloying elements, which has a high degree of atomic disorder that leads to various unique magnetic, mechanical, and electrochemical properties. It is known that one HEA, evaluated previously, is more resistant to general corrosion than type 304 stainless steel (304s), both in H₂SO₄ and in NaCl solutions at room temperature, but pitting corrosion resistance of the HEA is less than that of 304s.The anodic polarization curves determined for the HEA in aqueous environments showed that the general corrosion resistance of both HEA and 304s decreases as the temperature increases above room temperature. The decrease in the corrosion resistance of the HEA with an increase in temperature is less in NaCl than in H₂SO₄. The general corrosion rate for the HEA is lower than that for 304s in H₂SO₄, but higher than that for 304s in NaCl. The activation energies are: 94.06 kJ/mole for the HEA and 219.97 kJ/mole for 304s in 1 N H₂SO₄, and 310.43 kJ/mole for the HEA and 343.18 kJ/mole for 304s in 1 M NaCl. In addition, it was observed that concentration polarization occurred in cathodic reduction processes in deaerated 1 M NaCl at various temperatures.The polarization curves for the HEA and 304s indicated there is mixed control in 1 N H₂SO₄ and anodic control in 1 M NaCl based on the assumption that the surface chemical compositions of the two alloys are similar to their bulk chemical compositions. However, the decrease in the cathodic current with time for the HEA at more negative applied potentials is attributed to the high hydrogen overvoltage. In addition, the weak endothermic reaction revealed by differential scanning calorimetry (DSC) analysis showed that only small amounts of Cu-rich phases are segregated in the interdendritic phases; and the wide range of temperatures over which the endothermic reaction occurs indicates the formation of pro-eutectoid phases.     

Corrosion behavior of titanium-clad carbon steel in weakly alkaline solutions

The electrochemical polarization curves and corrosion potentials during long-term immersion of Ti, carbon steel, carbon steel tightened-Ti and Ti-clad carbon steel specimens were investigated in bentonite-contacting solution (mixture of sodium sulfate and sodium carbonate solutions), sodium sulfate solution and borate solution adjusted to pH=9.0-9.84. Ti and carbon steel were passivated during immersion in borate solution, while carbon steel was corroded in the solutions containing SO₄²⁻ ions. The immersion potentials of Ti-clad steel and steel-tightened Ti specimens were controlled by the corrosion potential of carbon steel (about 0 V vs. RHE at 298 K). The Ti side of the clad specimen was, therefore, polarized cathodically at this immersion potential, and this caused hydration of and/or hydrogen penetration into the oxide film, resulting in degradation of its barrier property. Furthermore, the cathodic current on the Ti side was partially coupled with the anodic current in the corrosion reaction on the carbon steel side, resulting in acceleration of the overall corrosion rate.     

Moisture-induced delayed spallation and interfacial hydrogen embrittlement of alumina scales

While interfacial sulfuris the primary chemical factor affecting Al₂O₃ scale adhesion, moisture-induced delayed spallation appears as a secondary, but impressive, mechanistic detail. Similarities with bulk metallic phenomena suggest that hydrogen embrittlement from ambient humidity, resulting from the reaction Al_alloy+3(H₂O)_air=Al(OH)⁻ ₃+3H⁺ may be the operative mechanism. This proposal was tested on pre-oxidized René N5 by standard cathodic hydrogen charging in 1N H₂SO₄, as monitored by weight change, induced current, and microstructure. Cathodic polarization at −2.0 V abruptly stripped mature Al₂O₃ scales at the oxide-metal interface. Anodic polarization at +2.0V, however, produced alloy dissolution. Finally, with no applied voltage, the acid electrolyte produced neither scale spallation nor alloy dissolution. Thus, hydrogen charging was detrimental to alumina scale adhesion. Moisture-induced interfacial hydrogen embrittlement is concluded to be the cause of delayed scale spallation and desktop thermal barrier coating failures.     

Effect of Plastic Deformation on the Corrosion Behavior of a Super-Duplex Stainless Steel

The role of plastic deformation on the corrosion behavior of a 25Cr-7Ni super-duplex stainless steel (SDSS) in a 3.5 wt.% sodium chloride solution at 90 °C was investigated. Different levels of plastic strain between 4 and 16% were applied to solution annealed tensile specimens and the effect on the pitting potential measured using potentiodynamic electrochemical techniques. A nonlinear relationship between the pitting potential and the plastic strain was recorded, with 8 and 16% causing a significant reduction in average E _p, but 4 and 12% causing no significant change when compared with the solution-annealed specimens. The corrosion morphology revealed galvanic interaction between the anodic ferrite and the cathodic austenite causing preferential dissolution of the ferrite. Mixed potential theory and the changing surface areas of the two phases caused by the plastic deformation structures explain the reductions in pitting potential at certain critical plastic strain levels. End-users and manufacturers should evaluate the corrosion behavior of specific cold-worked duplex and SDSSs using their as-produced surface finishes assessing in-service corrosion performance.     

Effect of heat treatment on the structural transformations and properties of high-nitrogen chromium steels

Conclusions  

Characterization of Hydrogen Permeation in Armco-Fe during Cathodic Polarization in Aqueous Electrolytic Media

The study of hydrogen permeation behavior in Armco-Fe showed that 0.1 M H₂SO₄ was a more effective medium for cathodic polarization compared to 0.1 M NaOH. When both electrolytes were “poisoned” with 1.00 g/L Na₂HAsO₄ · 7H₂O, as hydrogen recombination inhibitor, the corresponding hydrogen permeation levels were 3.5 × 10⁻⁵ A/cm² in 0.1 M H₂SO₄ while 0.75 × 10⁻⁵ A/cm² in 0.1 M NaOH. The breakthrough times were less than 30 s in 0.1 M H₂SO₄, while about 100 s in the NaOH. With varying amounts of “poisons”, peak permeation of hydrogen (1.75 × 10⁻⁵ A/cm²) was achieved with 10 g/L Na₂HAsO₄ · 7H₂O in 0.1 M H₂SO₄, while the least permeation resulted with 10 g/L (NH₂CSH₂) Thiourea addition for same level of 1.00 mA/cm² cathodic polarization.     

Nanostructure and energy properties of the stainless steel X18N10T surface studied by scanning tunneling microscopy and scanning tunneling spectroscopy. 2. Measurements at different potentials in HCl and H2SO4 solutions

Stainless steel Kh18N10T is studied by electrochemical scanning tunneling microscopy (ESTM) and electrochemical scanning tunneling spectroscopy (ESTS) at the interface with solutions of 0.01 N HCl and 0.1 N H₂SO₄ at the controlled sample and tip potentials. The sample potential varied from −0.46 to +1.14 V in 0.01 N HCl and from −0.06 to +0.74 V (NHE) in 0.1 N H₂SO₄. It is shown that although in the studied acid solutions at the controlled potential the surface is smother than in air, this does not allow one to determine the extremal values of parameters α, β, and γ corresponding to individual properties of Fe, Cr, and Ni or their oxides. It is shown that compared to the steel nanorelief, the sample potential has a stronger effect on the electrophysical properties that determine the tunneling transfer of electrons. Original Russian Text ? Yu.M. Stryuchkova, E.V. Kasatkin, 2008, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2008, Vol. 44, No. 6, pp. 620–626.