The stress corrosion cracking of high strength CrMnSiNi and CrMo steels

Practically all constructional steels are working under applied loads and environments. Below some stress levels the deterioriation of the material occurs by typical corrosion modes. Modern constructions are often loaded with enough high stresses to promote catastrophic failures due to stress corrosion crack propagation. Actually the whole range of metallic materials used in reliable constructions which are exposed to corrosive environments should be tested for their sensitivity to stress corrosion cracking. By measuring the material constant K_ISCC (critical stress intensity factor for stress corrosion cracking) it is possible to construct the reliable parts working in a safe range of stresses, which cannot be computed knowing yield strength of the material only. For measuring K_ISCC values of high strength CrMnSiNi and CrMo steels, original stands were built and long-term (> 10³h) tests applied by means of the cantilever beam method. Sensitivity of tested steels to stress corrosion cracking was expressed as a ratio K_ISCC:K_IC. Some other observations concerning kinetics of crack propagation and other properties of the materials have been carried out.     

Laser-processed corrosion-resistant amorphous NiCrPB surface alloys on a mild steel

Laser processing is applied to crystalline surface alloys which are suitable for vitrification by conventional melt spinning and are bonded previously to a mild steel substrate. If amorphous surface alloys are processed, they passivate spontaneously in 1 N HCl at 30°C, and exhibit almost the same polarization curves as those of conventionally processed extremely corrosion-resistant amorphous counterparts. Among various alloys vitrification of a wide surface by the laser processing requires that the NiCrPB alloys contain 14–17 at. % chromium, 15–18 at. % phosphorus, 2–5 at. % boron and 19–20 at. % of the sum of phosphorus and boron. This composition range is quite narrow in comparison with that vitrifiable by conventional methods for the formation of the amorphous alloys such as melt spinning.     

Oxidation mechanism of FeNi2025Cr5Al alloys-influence of small amounts of yttrium on oxidation kinetics and oxide adherence

S.I.M.S. and optical spectrometry were used to determine the composition of oxide scales developed on several FeNi2025Cr5AlY alloys, oxidized at high temperature (1273–1573K) under a pure oxygen atmosphere. The oxide scales were made up of an alumina layer containing a certain amount of spinel, independently of the iron and nickel content; yttrium enters in solid solution in the alumina layer. The growth of the alumina layer depends mainly on anionic diffusion, but does not obey a parabolic rate. The amount of yttrium in the alloys must be limited to 300 ppm (approximately the solubility of this element in the alloys), in order to have an optimum effect on oxidation kinetics and oxide adherence. This improvement in adherence is due to yttrium solute atoms in the alumina, rather than to mechanical keying by yttria particles.     

Pre-exposure embrittlement and stress corrosion failure in AlZnMg Alloys

It has been found that a high purity Al-6%Zn-3%Mg becomes embrittled if pre-exposed to moist gases prior to tensile testing. The degree of embrittlement increases with the time of preexposure and with the temperature and relative humidity of the pre-exposure environment. The alloy is most sensitive to embrittlement when solution treated at 475°C but this sensitivity can be reduced considerably if the surface film formed at 475°C is removed by electropolishing. The embrittlement is not strain-rate sensitive and the ductility of the pre-exposed alloy cannot be recovered by storing unstressed in dry air or in vacuo. However, the ductility of embrittled specimens can be fully restored if tensile testing is carried out under vacuum. If 1.7% copper or 0.14% chromium are added to the high purity alloy the rate of embrittlement is reduced and is even more reduced in the commercial 7075 alloy. Also, both the chromium containing alloy and the commercial alloy recover their ductilities during storage in laboratory air at room temperature—the rate of recovery being much higher than the rate of embrittlement.It is proposed that embrittlement is due to the deep penetration of an agent such as atomic hydrogen which reduces the grain boundary cohesion. It is also proposed that a similar effect must occur during the intergranular stress corrosion failure of AlZnMg alloys.     

The oxidation behaviour of austenitic FeCrNi alloys containing dispersed phases

The high temperature oxidation behaviour of FeCrNi austenitic alloys containing 1% Ti which, in some cases, had been converted into an oxide dispersion has been examined. The oxide dispersions were produced by an internal oxidation treatment using a 50/50 Cr/Cr₂O₃ powder mixture in a sealed quartz capsule at 1100°C: the samples were not in direct contact with the powders. Generally, the effect of the dispersed oxide was much less pronounced than in corresponding nickel-free, ferritic alloys. Nevertheless, the time-to-breakaway of the protective Cr₃O₃ scale which developed on Fe18CrNi alloys was substantially increased, although the differences between the untreated and the internally oxidized alloys reduced with increasing nickel content. An Fe14Cr20Ni alloy did not show any improvement after internal oxidation. Unlike the ferritic alloys, no coarsening of the dispersoid phase was observed during exposure.     

The dissolution and passivation kinetics of stainless alloys containing molybdenum—1. Coulometric studies of FeCr and FeCrMo alloys

The evolution of active current density with time has been measured on new surfaces of FeCr and FeCr-1.5 at. % Mo alloys. The solutions used were 1M and 4M HCl. At several potentials, and at steady-state current densities as high as 150 mA/cm¹, the inhibiting effect of molybdenum was established after the passage of a charge density equivalent to less than three monolayers of alloy. The results suggest that molybdenum acts by enriching at surface kink or step sites in the active state.     

Strengthening mechanisms of MgSi alloys

The origins of high room temperature strength of rapidly solidified () Mg-high Si alloys have been analysed using various strengthening mechanism models. The analysis revealed that the high strength was attributed to the fine-grain strengthening mechanism as well as the strengthening mechanisms due to Mg₂Si particles. A large contribution of the fine-grain strengthening mechanism results from the strong dependence of strength on grain size for h.c.p. metals. However, the strength of the alloys containing small particles (?1 μm) rapidly decreased at 473 K. This is explained by the critical particle size concept.     

An investigation of thin oxide films thermally grown in situ on Fe24Cr and Fe24Cr11Mo by auger electron spectroscopy and X-ray photoelectron spectroscopy

AES depth profiling and XPS have been used for the characterization of thin oxide layers thermally grown in situ in the UHV-analysis chamber on pure iron, chromium and the alloys Fe24Cr and Fe24Cr11Mo at a temperature of 384°C. The apparent oxide film thickness and the film composition were monitored as a function of oxygen exposure. The oxidation rate of the Fe24Cr alloy was found to lie in between that of pure iron and chromium. The films formed have a duplex structure, the outer part being iron oxide, the inner part mostly chromium oxide. Alloying with molybdenum decreases the rate of oxidation by a mechanism involving the formation of a barrier layer rich in molybdenum at the oxide-metal interface. No molybdenum is found in the outer part of the oxide film.     

Potentiodynamic polarization behaviour of two 17Cr13Ni2·5 Mo austenitic steels with different N contents

Potentiodynamic polarization measurements have been made on two 17Cr13Ni2·5Mo austenitic steels differing from each other in the content of N. The electrolyte solutions have been 1·0M H₂SO₄, mixtures of HCl and H₂SO₄ and 1·0M HCl at 20°C. In 0·9 and 1·0M HCl the polarization curves of the two austenitic steels possess two peaks which are difficult to explain. Experiments indicate that the steel containing N is less corrosion resistant than the steel without N.     

Distribution of intercalates in CrO3H2SO4-graphite and CrO3HClO4-graphite bi-intercalation compounds

Binary graphite intercalation compounds with sulfuric acid (H₂SO₄GICs), perchloric acid (HClO₄GICs) as well as chromium trioxide (CrO₃GICs) are known to be readily de-intercalated due to cathodic reduction and/or washing with aqueous solutions. The subsequent electrochemical intercalation of sulfuric and perchloric acid into the host CrO₃GICs has resulted in the ternary CrO₃H₂SO₄GICs and CrO₃HClO₄GICs, respectively. The products of the subsequent intercalation composed of the bi- and co-intercalation domains appeared to be very resistant to de-intercalation. The comparison of the results of the X-ray microprobe and X-ray diffraction analysis obtained for the original CrO₃GICs and the cathodically reduced ternary compounds have allowed insight into the structural and chemical changes developing during the subsequent intercalation/de-intercalation process. The knowledge of these properties is helpful for both the better understanding of the intercalation mechanism of the acids into the host CrO₃GIC and the elucidation of the extremely high stability of the ternary compounds.     

The characterization by raman spectroscopy of oxide scales formed on a 20Cr25NiNb stabilized stainless steel

Laser Raman spectroscopy (LRS) has been used to examine the composition of 0.3–2 μm thick scales formed on the 20Cr25NiNb stabilized stainless steel during oxidation, at 600–950°C, in CO₂ and in CO₂ + 4%CO + 350 vpm CH₄ + 300 vpm H₂O + 400 vpm H₂. All the scales contained iron rich spinel oxides, except in carbon dioxide at 600°C, where Fe₂O₃ predominated. This difference was responsible for the greater attack of the stainless steel at 600°C in carbon dioxide than in the mixed gas environment. Increasingly at ≥800°C the spinel composition was modified by manganese incorporation and Cr₂O₃ was also a major component of the scales formed in both environments. The principal scale constituents identified by LRS accord both with thermodynamic predictions and current understanding of scale development on the 20/25/Nb stainless steel.     

The effect of phosphorus addition on the corrosion behavior of ARC-MELTED Ni10TaP alloys in 12 M HCl

The corrosion resistance of arc-melted Ni10TaP alloys containing 0, 10 and 20 at% phosphorus in 12 M HCl solution at 30 °C was investigated. The alloys containing 0 and 10 at% phosphorus suffer severe corrosion. The addition of 20 at% phosphorus to crystalline Ni10Ta alloy results in a three-orders-of-magnitude decrease in the corrosion rate. The open circuit potentials of the Ni10Ta alloys containing 0 and 10 at% phosphorus stay almost constant in the active region of nickel, while the open circuit potential of the Ni10Ta20P alloy increases almost linearly in the initial 2 h. The Ni10Ta alloy consists of intermetallic Ni₈Ta and immersion in 12 M HCl results in faceting dissolution. Ni10Ta10P alloy is composed of major Ni₈Ta and Ni₃P phases and minor Ni₂Ta and Ni₂P phases. Immersion of Ni10Ta10P alloy leads to preferential dissolution of the Ni₈Ta phase and to continuous thickening of the corrosion product film consisting mostly of tantalum as cations. Ni 10Ta20P alloy consists of Ni₂Ta, Ni₃P, Ni₂P and NiP phases. Immersion of Ni10Ta20P alloy gives rise to initial increase in elemental phosphorus on the surface as a result of selective dissolution of nickel and selective oxidation of tantalum. The formation of elemental phosphorus with a high cathodic activity is responsible for the initial ennoblement of the open circuit potential and for the formation of the passive film in which tantalum is highly concentrated. The higher corrosion resistance of Ni10Ta20P alloy than Ni10Ta10P alloy is attributable to the formation of the Ni₂Ta phase with a higher tantalum content than the Ni₈Ta phase which is the readily corroded major intermetallic phase in the Ni10Ta10P alloy.     

The effect of oxygen on the open-circuit passivity of Fe18Cr

Electrochemical studies were conducted to determine the critical amount of oxygen necessary to prevent corrosion by maintaining the open-circuit passivity of Fe18Cr samples initially passivated at 0.6 V(NHE) in 1N H₂SO₄ solutions. Samples passivated at 0.6 V(NHE) and then released to open circuit in O₂-saturated (29.4 ppm O₂ dissolved) solution maintained a state of passivity. Samples passivated and released to open circuit in N₂-purged solution decayed to a state of active corrosion in 800–2000 min. A passive state, however, could be maintained if O₂ were added to the N₂ flow so that a minimum of 1.7 ppm O₂ was present in solution. Furthermore, this critical amount of O₂ had to be added before open-circuit decay reached 0.45 V(NHE). Auger electron spectroscopy measurements indicated that following open-circuit stabilization of passivity by O₂ the percentage chromium in the film increased with increasing open-circuit potential. X-ray photo-electron spectroscopy measurements indicated that the film thickness decreased with increasing open-circuit potential.     

The effect of Cl− ions on the passivation of Fe26Cr alloy

Anodic oxide films formed on Fe26Cr in pH 2.0 H₂SO₄ solution in the presence and absence of Cl⁻ ions have been investigated using electrochemical techniques and Auger electron spectroscopy (AES) combined with ion sputtering. It is possible to incorporate Cl⁻ ions into passive oxide films formed over the entire passive potential range only when Cl⁻ is present in the solution from the very beginning of film formation. Cl⁻ ion incorporation does not cause any change in film thickness or Fe/Cr ratio, or any film thinning or film breakdown. A relatively short anodization in Cl⁻-free solution is sufficient to prevent any subsequent Cl⁻ ion incorporation. The susceptibility of the passive film to Cl⁻ attack appears to depend on the presence of small amounts of impurity in the alloy. A 99.97% pure alloy does not pit, whereas a 99.93% pure alloy, with larger concentrations of C, S, Mn, Co and Ni, does suffer intergranular attack in Cl⁻ solution.     

In-depth profiles of anodic oxide films on FeNi alloy in boric acid-sodium borate solutions

The in-depth profiling of anodic oxide films formed for 1 h on a 55Fe-45Ni alloy in boric acid-sodium borate solutions was made by the simultaneous use of AES (Auger Electron Spectroscopy) and sputter-etching with Ar⁺ ion to determine the thickness and composition of anodic oxide films and to elucidate the experimental variables which affect them. The anodic oxide film was thicker and richer in the Fe component in pH 6.48 solution than in pH 8.45 solution. It was estimated from the in-depth profile of the anodic oxide film that a significant part of the Fe component in the anodic oxide film was in the di-valent state. Enrichment of the Ni component at the oxide/substrate interface and depletion of Ni component in the film were measured as a function of pH and anodic potential. These results are explained in terms of the combination of the following factors: (i) preferential dissolution of the Fe component, (ii) preferential oxidation of the Fe component, and (iii) preferential anodic deposition of the Fe or Ni component.     

The salt-induced corrosion behaviour of FeCr alloys at elevated temperatures—I. Alloys dilute in chromium

The effect of sodium sulphate coatings on the oxidation behaviour of pure iron and iron-5% chromium in oxygen at 1 atm. pressure has been investigated using a conventional thermobalance technique. The oxidation kinetics and the scale morphology of pure iron were relatively unaffected by the presence of Na₂SO₄, whereas Fe-5Cr was found to undergo an initial enhanced oxidation. However, when the limited supply of salt was exhausted the rate became similar to that of the uncoated alloy. In the absence of porous scale it is concluded that fluxing processes, such as those postulated for the appropriate Ni and Co based alloys, are not a feature of the Na₂SO₄ induced corrosion of the present alloy. Experiments designed to investigate the singular roles of sodium oxide and sulphur indicate that it is sulphide formation which is the dominant cause of the initial accelerated oxidation. There is also evidence to suggest that reactions involving scale and salt may contribute to the acceleration in rate. Additions of sodium chloride do not drastically influence the overall level of attack of either of the Na₂SO₄-coated materials.     

Hot corrosion of NiCr alloys in SO2+O2 atmospheres—I. Corrosion kinetics

The corrosion of Ni, Cr and NiCr (0·1–50 at.%) has been studied in the temperature range 600–1000°C using a microgravimetric apparatus with a liquid seal. The corrosion rate for 0·1 and 1% CrNi alloys was higher than that for Ni under similar conditions. Higher proportions of Cr decreased the corrosion rate considerably. All the alloys passed through a maximum corrosion rate between 700 and 800°C in maximum p_SO3 conditions. In situ visual observations were made to view nodular, dendritic and needle-growths and the liquid surface during corrosion. Au and Pt markers were used in marker studies. Electron probe and X-ray diffraction analyses indicated that the corrosion product was mainly NiO, with some Cr₂O₃, Ni₂S₃ and CrS.     

The preparation and thermodynamic properties of Fe(II)Fe(III) hydroxide-carbonate (green rust 1); Pourbaix diagram of iron in carbonate-containing aqueous media

Carbonate-containing green rust 1, GR1(CO₃²⁻), is prepared by oxidation of Fe(OH)₂ in aqueous solution. Ferrous hydroxide is precipitated from NaOH and FeSO₄·7H₂O solutions and carbonate ions are added as a Na₂CO₃ solution. For sufficiently large concentrations of sodium carbonate, SO₄²⁻ ions do not play any role during the oxidation process and, at the end of the first stage of reaction, Fe(OH)₂ oxidizes into GR1(CO₃²⁻). In the second stage of reaction, GR1(CO₃²⁻) oxidizes into α-FeOOH goethite except when the transformation of ferrous hydroxide is partial, which leads to the formation of magnetite. From the X-ray diffraction analysis of GR1(CO₃²⁻), lattice parameters of its hexagonal cell are found to be . From the Mössbauer analysis of the stoichiometric GR1(CO₃²⁻), which leads to a Fe²⁺:Fe³⁺ ratio of 2:1, the chemical formula is established to be: [Fe₄^(II)Fe₂^(III)(OH)₁₂][CO₃·2H₂O]. The 78 K Mössbauer spectrum of the compound can be fitted with three quadrupole doublets, two Fe²⁺ doublets d₁ and D₂ corresponding to isomer shifts (IS) of 1.27 and 1.28 mm s⁻¹ and quadrupole splittings (QS) of 2.93 and 2.67 mm s⁻¹, respectively, and one Fe³⁺ doublet D₃ with an IS of 0.47 mm s⁻¹ and QS of 0.43 mm s⁻¹. These three doublets were already used to fit the Mössbauer spectrum of chloride-containing GR1(Cl⁻) [see J.M.R. Génin et al., Mat. Sci. Forum8, 477 (1986) and J.M.R. Génin et al., Hyp. Int. 29, 1355 (1986)]and therefore are characteristic of GR1 compounds. From the recording of electrode potential E and the pH of the suspension versus time during the oxidation, the standard free enthalpy of formation of stoichiometric GR1(CO₃²⁻) is estimated to be ΔG °_f = − 966.250 calmol⁻¹. Knowing the chemical formula and ΔG °_f of GR1(CO₃²⁻) the Pourbaix diagram of iron in carbonate-containing aqueous solutions is drawn.