Mott–Schottky analysis of passive films on Cu containing Fe–20Cr–xCu (x = 0, 4) alloys

Effect of copper on the defect density of Fe–20Cr–xCu (x?=?0, 4) stainless steel alloys was investigated in deaerated pH 8·5 borate buffer solution at room temperature using Mott–Schottky analysis. Mott–Schottky analysis revealed that the addition of copper increased the acceptor density (N_A, V_Cr^?3), i.e. decreased the Cr⁺³ content of the passive film. Also the donor densities, shallow donor (N_D1, V_O⁺²) and deep donor (N_D2, V_Cr⁺⁶), of the passive films formed were increased. XPS analysis confirmed the decrease in Cr content and enrichment of copper in the passive film of Cu containing alloys, which ultimately dictated their lower corrosion resistance, i.e. decreased film protectiveness and stability.     

The behavior of chromium and molybdenum in the propagation process of localized corrosion of steels

In order to clarify the effect of Cr and Mo on the propagation of localized corrosion of steels, the electrochemical behavior of pure chromium and molybdenum and of some stainless steels was studied under conditions similar to those existing inside occluded corrosion cells of steels. Oxygen-free FeCl₂ and/or CrCl₃ solutions were used for experiments. The surface films formed in these conditions were analyzed by means of AES and XPS. Cr is passive if the pH value >ca. 1.8 and Cr may retard the propagation of localized corrosion of steels in its early stage. The hydrolysis of Cr³⁺ may change the pH to even lower values. At such low pH and high Cl^? concentrations, Cr is active in the potential range of interest for localized corrosion of steels. Mo is passive under these more acidic conditions and the passivity of Mo in this potential range is attributed to the formation of a film of MoO₂ (or hydrated oxide), which is extremely thin but very protective. Mo is thus effective to retard the propagation of localized corrosion of steels in its more advanced stage.     

Bulk glassy Fe-Cr-Mo-C-B alloys with high corrosion resistance

The preparation of bulk glassy alloys with high glass-forming ability and high corrosion resistance in Fe-based system was succeeded by means of copper-mold casting. The temperature interval of supercooled liquid region (ΔT_x) is as large as 53-62 K and the reduced glass transition temperature (T_g/T_m) is as high as 0.62-0.63 for the cast Fe_50−xCr₁₆Mo₁₆C₁₈B_x (x=4, 6, 8 at.%) glassy alloys. The corrosion rates of the Fe_50−xCr₁₆Mo₁₆C₁₈B_x glassy alloys with a diameter of 1.2 mm were in the range of 10⁻³-10⁻² mm year⁻¹ in 1, 6 and 12 N HCl solutions at 298 K. The bulk glassy alloys are spontaneously passivated in 1 and 6 N HCl solutions with wide passive region and low passive current density. They do not suffer pitting corrosion even when polarized anodicly in 12 N HCl solution up to 1.0 V (Ag/AgCl). The high corrosion resistance is due to the formation of chromium-rich passive films during immersion in HCl solutions. In addition, the increase of boron content in alloys improves the corrosion resistance of the bulk glassy alloys within the composition range examined.     

Improvement of corrosion resistance of Cu-based bulk metallic glasses by the microalloying of Mo

(Cu₄₇Zr₁₁Ti₃₄Ni₈)_100−xMo_x bulk metallic glasses (BMGs) with x = 0, 1 and 2 at% were prepared by water-cooled copper mold casting. The effect of the addition of a small amount of Mo on the corrosion resistance of the Cu-based BMGs was examined by electrochemical polarization and weight loss measurement in 1 mol/L H₂SO₄ open to air. It is found that the corrosion resistance of Cu-based BMGs enhanced with increasing Mo content, as indicated by the decrease in passive current density and corrosion rate with the increase in Mo content. X-ray photoelectron spectroscopy (XPS) was employed to study the composition and the state of elements in the passive film, which revealed that the improvement of corrosion resistance of Cu-based BMGs microalloyed with Mo originated from the enrichment of ZrO₂ and TiO₂, but depletion in Cu- or Ni-oxide in the passive films formed during electrochemical polarization. In addition, the galvanostatic-step measurement was performed to investigate the formation kinetics of the passive films. It is demonstrated that the addition of an appropriate amount of Mo can promote the formation of the passive film on the surface.     

The role of molybdenum additions to austenitic stainless steels in the inhibition of pitting in acid chloride solutions

In order to clarify the mechanism for increased resistance to pitting in acid chloride solutions by addition of Me to CrNi stainless steels, the anodic polarization curves, a.c. electrode impedances, ellipsometric parameters and X-ray photo-electron spectra have been measured on the Mo-containing steels passivated in 1N HCl. The results showed that the presence of an adequate amount of Cr is indispensable for the improvement of pitting resistance by the Mo addition. The passive films of the Mo-containing steels were found to be composed of a complex oxyhydroxide containing Cr³⁺ Fe²⁺, Ni²⁺, Mo⁶⁺ and Cl^? and showed a rather higher d.c. resistance in HCl solution than in H₂SO₄ solution. The thickness of the passive film increases with increase in Mo content.     

Elevated temperature deformation of Cr3Si alloyed with Mo

Four-point bend, constant load compressive creep and constant engineering strain rate tests were conducted on arc-melted and powder-metallurgy (PM) processed Cr₄₀Mo₃₀Si₃₀ specimens in the temperature range 1400–1700 K. This is a two-phase alloy consisting of (Cr,Mo)₃Si and (Cr,Mo)₅Si₃ phases. The PM specimens, which were substantially weaker than the arc-melted materials, exhibited a stress exponent, n, of about 2 and an apparent activation energy for creep, Q_a, of 485 kJ/mol. The mechanism in these specimens appeared to be controlled by creep of a glassy phase. In the case of arc-melted specimens for which n ~ 3 and Q_a ~ 430 kJ/mol, the rate-controlling creep mechanism appeared to be that dominant in the (Cr,Mo)₅Si₃ phase. In this case, it is suggested that the Nabarro creep mechanism, where dislocation climb is controlled by Bardeen–Herring vacancy sources, is the dominant creep mechanism. Finally, an analysis of the present and literature data on Cr₃Si alloyed with Mo appeared to suggest that the creep rate decreases sharply with an increase in the Mo/Si ratio.     

High temperature naphthenic acid corrosion and sulphidic corrosion of Q235 and 5Cr1/2Mo steels in synthetic refining media

The corrosion of Q235 and 5Cr1/2Mo steels in synthetic refining media containing naphthenic acid and/or sulphur compounds was studied to evaluate naphthenic acid corrosion (NAC), sulphidic corrosion (SC), and their interaction. Corrosion dependencies on the test duration, temperature, total acid number (TAN) and content of sulphur compound were assessed. Specimens after NAC and SC tests were characterized by SEM/EDX, and XRD. It is found that in liquid phase of media containing only naphthenic acid and at temperature about 230 °C, 5Cr1/2Mo and Q235 steels have almost the same NAC rate, and above 230 °C, 5Cr1/2Mo has a higher NAC rate than Q235 has due to the higher NAC activation energy (63.2 kJ mol⁻¹) of 5Cr1/2Mo than that of Q235 (54.0 kJ mol⁻¹), indicating that increasing temperature accelerates NAC rate of 5Cr1/2Mo more than that of Q235. In oil containing only dimethyl disulphide, the growth of SC film follows parabolic kinetics, and the film of Q235 grows faster than that of 5Cr1/2Mo while SC rate of Q235 is higher than that of 5Cr1/2Mo. In oil containing both naphthenic acid and dimethyl disulphide, 5Cr1/2Mo has a lower corrosion rate than Q235 has. On the basis of “naphthenic acid corrosion index” (NACI), the benefits of 5Cr1/2Mo over Q235 should ascribe to that the pseudo-passive film for 5Cr1/2Mo has better NAC resistance than that for Q235. This is close related to the existing of additional chromium sulphide (Cr₅S₈) on the pseudo-passive film of 5Cr1/2Mo, in contrast with the pyrrhotite (Fe₇S₈) and troilite (FeS) on the film of Q235.     

Synthesis of Fe–Cr–Mo–C–B–P bulk metallic glasses with high corrosion resistance

Bulk metallic glasses with a maximum thickness (t_max) of 1.0–2.7 mm were synthesized in the Fe₄₃Cr₁₆Mo₁₆(C, B, P)₂₅ system over a wide composition range by copper mold casting. They exhibit a large supercooled liquid region (ΔT_x) of 40–90 K and a high reduced glass transition temperature (T_g/T_l) of 0.54–0.60, indicating high glass-forming ability (GFA) and high thermal stability of the supercooled liquid. The critical cooling rate for glass formation was evaluated to be of the order of 10² K s⁻¹. The bulk metallic glasses exhibited high corrosion resistance in aggressive HCl solutions. The alloying element P has a beneficial effect on corrosion resistance.     

Influence of pH on the passivation behavior of 254SMO stainless steel in 3.5% NaCl solution

The potentiodynamic polarization measurement of 254SMO stainless steel (UNS 31254) was conducted in 3.5% NaCl solutions with pH ranging from 0.1 to 5. The results indicated that this stainless steel offered excellent pitting corrosion resistance in corrosive environments. Further, it also exhibited various features on the polarization curves in different pH solutions. The electrochemical constant-potential passivation treatment performed at different pH followed by XPS analysis revealed that the primary constituents of the outermost layer of the passive films formed in the weak (pH 5) and strong (pH 0.8) acid solutions are iron oxides and Cr₂O₃ and Cr(OH)₃, respectively. Molybdenum oxides, primarily in the six-valence state, existed in the outermost layer of the passive film. Only very weak signals corresponding to that of nickel oxides were detected in the film formed in the weak acid (pH 5) solution. The ICP-MS analyses indicated selective dissolution of a significant amount of Fe and a few Mo and Ni ions during the passivation treatment in the strong acid (pH 0.8) solution. No Cr dissolution was observed; this indicated that the Cr in the film is relatively stable. XPS depth profiling results showed that a similar bilayer-structured film was formed in both the solutions (pH 0.8 and 5); the outer layer of this film is primarily composed of Cr(OH)₃ and Mo(VI), and the inner layer, Cr₂O₃ and Mo(IV). The results of the examinations of passive film formations and dissolution by XPS and ICP-MS were consistent with the polarization curves.     

Effect of Mo on corrosion behavior of Ni20Cr–xMo alloys in air with NaCl–KCl–CaCl2 vapor at 570°C

The effect of Mo on the corrosion behavior of Ni20Cr–xMo alloys in an oxidizing chlorine-containing atmosphere using air mixed with the salt-vapor mixture of NaCl–KCl–CaCl₂ at 570°C was investigated. The results revealed that the corrosion performance of the Ni20Cr alloys in the oxidizing chlorine atmosphere was improved by Mo addition of up to 3 wt%. The Mo-free alloy formed a potassium chromate during corrosion as a result of the reaction between the Cr₂O₃ scale and KCl vapor. The chromate formation increased the chlorine potential at the scale surface and induced the breakdown of the protective Cr₂O₃ scale, resulting in internal chromium chloride precipitates and a Cr-depleted zone. In contrast, the presence of Mo resulted in the formation of a NiO scale, which did not react with the salt vapors and, therefore, prevented the formation of chromates. The beneficial effect of Mo on the high-temperature chlorination of Ni–Cr alloys in salt-vapor-containing atmospheres was ascribed to the suppression of chlorine generation due to NiO scale formation.     

Role of Mo in the local configuration and structure stabilization of amorphous steels,a Synchrotron X-ray diffraction and Mössbauer study

Amorphous steels are promising materials with potential structural applications. The glass-forming ability (GFA) and mechanical properties of metallic glasses are intimately related to the local structure. In Fe-based alloys, Cr and Mo content seem to play a key role in stabilizing the amorphous atomic-level structure. Here we present a study on the effects of changing Mo content in Fe_72?xC₇Si_3.3B_5.5P_8.7Cr_2.3Al₂Mo_x amorphous steels. We study the local structure of these alloys by Synchrotron X-ray diffraction and Mössbauer spectroscopy. The results show how the amorphous phase evolves from a ferromagnetic Fe-rich structure to a structure with predominance of paramagnetic environments with the increase of Mo content. The changes in the distribution of magnetic environments cannot be attributed only to the Fe–Mo substitution but to a change of local configuration in the amorphous phase.     

Corrosion behaviour of MoSx-based coatings deposited onto high speed steel by magnetron sputtering

MoS_x-based films were deposited using magnetron sputtering from a pure MoS₂ target. Alloying was accomplished by “co-deposition” from separate targets onto substrates having a two-fold rotation. An additional experiment had also a Cr⁺-ion etch for surface preparation, followed by a Cr adhesion layer, made using a Cr target mounted on a cathodic arc evaporation source. MoS_x and Al- and Ti-alloyed MoS_x coatings have been deposited onto high speed steel (HSS) and glass substrates for corrosion investigations.The coatings were characterised by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, hardness and adhesion measurements. The corrosion behaviour of the samples was electrochemically measured by open-circuit-potential (OCP) measurements and by potentiodynamic corrosion tests in 0.8 M NaCl solution (pH 7). Additionally the MoS_x-based coatings on HSS have been exposed to salt spray tests. The corrosion investigations revealed that the addition of Al and Ti to MoS_x shifts the open-circuit-potential of about 80 to 110 mV to lower values, i.e. the alloying elements make the MoS_x coating a little bit less noble. In agreement with the OCP measurements, the corrosion potential E_corr in potentiodynamic corrosion tests was the highest for non-alloyed MoS_x coatings on HSS substrates. After the potentiodynamic corrosion tests, a strong corrosive attack could be observed for all coated samples. In salt spray tests the lowest number of corrosion pits was found for the MoS_x-Al (Cr⁺) coating on HSS.     

Influence of metalloids and annealing on the fundamental magnetic properties of bulk Fe–(Cr,Mo,Ga)–(P,B,C) metallic glasses

Although bulk ferromagnetic metallic glasses have been synthesized in many alloy systems, the dependence of fundamental magnetic properties – saturation moment and Curie temperature – on annealing treatments and the content of alloying metalloids has not been well understood. In the present work, bulk ferromagnetic glassy alloys of Fe_77.5−x(Cr_0.33Mo_0.33Ga_0.33)_xP₁₂B_5.5C₅ (x = 8–14), (Fe₆₆Cr₄Mo₄Ga₄P₁₂C₅)_(100−x)/95B_x (x = 3–7), and (Fe₆₆Cr₄Mo₄Ga₄P₁₂B₅) _(100−x)/95C_x (x = 3–8.5) were prepared by a flux-melting and water-quenching technique. The effects of alloying metals (Cr, Mo, and Ga), metalloids (P, B, and C), and annealing treatments on the saturation moment and Curie temperature of these bulk glasses were investigated. The saturation moment and Curie temperature decrease linearly with increasing metals content. However, the saturation moment and Curie temperature change complicatedly with metalloids content. The Curie temperature increases with both annealing temperature and annealing time. The dependence of magnetic properties on the composition and annealing treatment for our bulk glasses was compared with that for glassy ribbons. The results suggest that the dependence found in our bulk glasses can be explained by the theoretical models proposed for glassy ribbons.     

Microstructure and corrosion resistance of Cr/Cr2N multilayer film deposited on the surface of depleted uranium

Depleted uranium is widely used in national defence and nuclear energy fields. However, the inferior corrosion resistance limits its application. A Cr/Cr₂N film was prepared by magnetron sputtering on the uranium to improve its corrosion resistance. The Cr/Cr₂N film exhibits modulation structure. The introduction of the Cr/Cr₂N increases the corrosion potential; the corresponding current density decreases about three orders of magnitude. After polarization corrosion, the surface morphology of the Cr/Cr₂N-coated on uranium keeps integrated. Only a thin layer of film (∼40 nm) is oxidized. The Cr/Cr₂N film shows great potential in improving oxidation and corrosion resistance of depleted uranium.     

Augerspektroskopische Untersuchungen zur Lochkorrosion von Fe-Cr-, Fe-Mo- und Fe-Cr-Mo-Legierungen

Auger-spectroscopic investigations into pitting corrosion of FeCr, FeMo and FeCrMo alloys A single pit was produced on binary iron-chromium (Fe17Cr), iron-molybdenum (Fe1Mo, Fe3Mo, and Fe5Mo) and ternary iron-chromium-molybdenum alloys (Fe17CrxMo, x = 1, 3, 7 wt.%) in 0.5 M H₂SO₄ with Cl^? additions (0.02 to 0.08 molar) at room temperature. Compositional changes at the bottom of the pit and in the surrounding passive film were determined using high resolution Auger-Electron Spectroscopy (AES). The chromium and molybdenum content of the passive layers are not changed in the presence of Cl^? and there is no incorporation of chlorine in the passive film as long as the films were formed in a chlorine-free electrolyte and Cl^? was added later. At the bottom of the pits salt films could be determined in all cases after removing the electrolyte with filter paper. These salt films are strongly enriched in chromium, molybdenum and chlorine. The salt film was not detectable for Fe-Cr after rinsing the samples with distilled water, whereas in the case of molybedenum additions the salt film could not be washed away with water.     

Effect of partial substitution of Cr for Ni on densification behavior, microstructure evolution and mechanical properties of Ti(C,N)-Ni-based cermets

Four cermets of composition TiC-10TiN-16Mo-6.5WC-0.8C-0.6Cr₃C₂-(32 − x)Ni-xCr (x = 0, 3.2, 6.4 and 9.6 wt%) were prepared, to investigate the effect of the partial substitution of Cr for Ni on densification behavior, microstructure evolution and mechanical properties of Ti(C,N)-Ni-based cermets. The partial substitution of Cr for Ni decreased full densification temperature, and the higher the content of Cr additive was, the lower full densification temperature was. The partial substitution of Cr for Ni had no significant effect of the formation of Mo₂C and Ti(C,N) and the dissolution of WC, and however, it had a significant effect on the dissolution of Mo₂C. Cr in Ni-based binder phase diffused into undissolved Mo₂C to form (Mo,Cr)₂C above 1000 °C at 6.4-9.6 wt% Cr additive, and a small amount of (Mo,Cr)₂C did not dissolve after sintering at 1410 °C for 1 h at 9.6 wt% Cr additive. In the final microstructure, Cr content in Ni-based binder phase increased with increasing the content of Cr additive, and however, regardless of the content of Cr additive, coarse Ti(C,N) grains generally consisted of black core, white inner rim and grey outer rim, and fine Ti(C,N) grains generally consisted of white core and grey rim. The partial substitution of Cr for Ni increased hardness and decreased transverse rupture strength (TRS). Ni-based binder phase became hard with increasing the content of Cr additive, therefore resulting in the increase of hardness and the decrease of TRS. TRS was fairly low at 9.6 wt% Cr additive, which was mainly attributed hardening of Ni-based binder phase and undissolved (Mo,Cr)₂C.     

Towards improved integrated properties in FeCrPCB bulk metallic glasses by Cr addition

Fe-based soft-magnetic metallic glasses (MGs) of Fe_80−xCr_xP₉C₉B₂ (x = 0, 2, 5, 8 and 16 at.%) with high glass-forming ability (GFA), good soft-magnetic properties and high corrosion resistance are fabricated. With the addition of Cr to FePC-based alloys, the GFA and saturation magnetization (M_s) slightly decrease while the corrosion resistance effectively increases. The Fe–Cr–P–C–B BMGs exhibit good GFA and fully glassy rods can be produced up to 1.8 and 1.5 mm in diameter for the 2 and 5 at.% Cr added alloys, respectively. The alloys with 2 and 5 at.% Cr addition also show good soft-magnetic properties featured by high M_s of 1.16 and 1.04 T, low coercivity of 2.7 and 2.2 A/m, respectively. Besides, the corrosion behavior of the alloys was studied by immersion tests and potentiodynamic polarization measurements. It was found that the addition of Cr efficiently enhances the corrosion resistance of Fe–Cr–P–C–B alloys and the glassy alloy with 5 at.% Cr addition exhibits better corrosion resistance in comparison with the stainless steel SUS304 in 3 mass% NaCl solution. The combination of large GFA, good soft-magnetic properties, high corrosion resistance as well as low cost makes the Fe–Cr–P–C–B alloys as promising soft-magnetic and anti-corrosive materials for industrial applications.     

不同(Mo+B)/(Ni+Cr)质量比的原位合成MoB/NiCr涂层的组织结构与性能

采用超音速火焰喷涂技术沉积含3种不同(Mo+B)/(Ni+Cr)质量比（1:1,2:1和3:1）的Mo-B-Ni-Cr球磨复合粉末以原位反应制备获得MoB/NiCr涂层。采用扫描电子显微镜（SEM）和X射线衍射仪（XRD）分析了MoB/NiCr涂层的组织结构和物相。同时讨论了不同(Mo+B)/(Ni+Cr)质量比对涂层的组织结构、硬度、结合强度和耐腐蚀性能的影响。研究结果表明,(Mo+B)/(Ni+Cr)质量比为1：1的MoB/NiCr涂层孔隙率最低及涂层厚度最大。在3种涂层中均原位反应生成了Mo₂NiB₂三元硼化物,且随着(Mo+B)/(Ni+Cr)质量比的增加,涂层中三元硼化物含量随之增加,涂层的硬度值增加,结合强度反而随之降低;由于涂层中三元硼化物的原位生成,MoB/NiCr涂层的硬度值均高于316L不锈钢基体。通过能谱和XRD分析发现,经过360 h熔融锌腐蚀试验后,涂层表层中没有发现锌元素及其金属间化合物,然而随着(Mo+B)/(Ni+Cr)质量比的增加,涂层的孔隙率增加及厚度降低。最后,综合分析可得,相比其他涂层,(Mo+B)/(Ni+Cr)质量比为1:1的MoB/NiCr涂层具有更好的耐熔融锌腐蚀能力。     

Passivity of stainless steel in sulphuric acid under chemical oxidation

The main purpose of the present work was to study the passivity of stainless steel in sulphuric acid under chemical oxidation with H₂O₂ solution. Potentiodynamic polarisation and open circuit potential (OCP) measurements indicated that H₂O₂ facilitates the increase of OCP of stainless steel which shifts from active region into passive region. The X-ray photoelectron spectroscopy results suggested that a passive film, composed of oxyhydroxides, Cr₂O₃, Cr(VI) species, NiO, and sulphate (FeSO₄, Cr₂(SO₄)₃·xH₂O), is achieved after H₂SO₄–H₂O₂ passivation. The passive mechanism under H₂O₂ oxidation was discussed and the corrosion resistance of passive film was compared with that of the passive films produced by HNO₃ passivity and H₂SO₄ potentiostatic passivity. The results of electrochemical impedance spectroscopy, cyclic voltammetry, and anodic polarisation experiments confirmed that the chemical oxidation with H₂O₂ solution is capable of improving the corrosion resistance of stainless steel significantly and the passive film is more stable than those produced by other passivating methods.     

Improvement of the oxidation-proof property and the scale structure of Mo3Si intermetallic alloy through the addition of chromium and aluminum elements

The influences of chromium and aluminum additions to Mo₃Si intermetallic alloy on the microstructure, mechanical and oxidation properties as well as the scale evolution were investigated for alloys with the composition of Mo_75 − yCr_ySi_25 − xAl_x (y = 0, 15, 30, 45, 60, 75 mol%, x = 0, 5, 10 mol%). Cr and Al were added for the aim of not only improving the oxidation property but also changing the compound type from Daltonide to Berthollide ones. The alloys were prepared by arc-melt method and the button ingots were homogenized at 1873 K for 10 h. It was found that the homogenized (Mo,Cr)₃Si alloy consists of continuous solid solution. During oxidation test at 1173 K, the mass of Mo₃Si alloy decreased remarkably due to the evaporation of molybdenum oxides. However, the improvement of the oxidation resistance was confirmed when chromium was added up to at least 15 mol%. In particular, the (Mo,Cr)₃Si alloys with above 30 mol%Cr showed excellent oxidation resistance through the formation of the passive oxide-scale layer. Although the effect is smaller, aluminum addition also contributed to the improvement of oxidation resistance.