Effect of temperature upon stress corrosion cracking of HY-180M steel in 3.5 Pct NaCI

A fracture mechanics and fractographic study of stress corrosion cracking (SCC) of heat treated HY-180 M steel was undertaken over the temperature range 22 to 95 °C at applied potentials of −0.28 V_SHE (−0.48 V_Ag/AgCl) and −0.80 V_SHE (−1.0 V_Ag/Agcl). Particular attention was directed toward Region II behavior, where crack propagation rates were independent of stress intensity(K _l). Region II rates were always higher at the less noble potential of −0.80 V_SHE than at the more noble potential of — 0.28 V_SHE. However, fractography studies suggested that the basic mechanism of cracking at both potentials was the same, and involved hydrogen embrittlement. An Arrhenius analysis of Region II rates showed that crack propagation was under the control of more than one process. Consequently, the mechanistic details remained obscure. Formerly Research Associate in theDepartment of Metallurgical Engineering, University of BritishColumbia.     

A fracture mechanics study of stress corrosion cracking of type-316 austenitic steel

A new test specimen configuration, designated the T-notch double cantilever beam (TNDCB), was developed, calibrated and employed for a fracture mechanics study of stress corrosion cracking (SCC) of cold worked Type-316 austenitic stainless steel exposed to hot aqueous solutions of 44.7 wt pct MgCl₂. The effects of stress intensity (K _I ), temperature (T) and electrochemical potential (E) upon the crack velocity (v) and fractography were investigated. The stress intensity (K _ISCC ) below whichv became immeasurably small was ∼12 MN·m^−3/2. Above this value, three regions of behavior were observed. Region I exhibitedK _I dependent cracking followed by Region II which exhibitedK _I independent cracking and an apparent activation energy of 63 to 67 kJ/mol, followed by Region III where cracking again became dependent uponK _I . The relative proportions of intergranular and transgranular crack paths were markedly dependent upon bothK _I andE, and less sensitive toT. Crack velocity was insensitive to small changes inE with respect to the free corrosion potentials (E _corr), but could be terminated by an applied active potential of ∼−0.35 V_SCE. The pH within the propagating crack was estimated to be <1.0 atE _corr, rising to ∼4.5 at −0.35 V_SCE. The mechanism of SCC was discussed with respect to film rupture events caused by crack tip plastic deformation, adsorption controlled processes on the metal surface, and hydrogen diffusion in the metal lattice. Alan J. RUSSELL, formerly Research Student, University of British Columbia     

Effect of overaging on mechanical properties and stress corrosion cracking of HY-180M steel

The tensile properties, fracture toughness and stress corrosion cracking (SCC) behavior of HY-180 M steel at 22 °C were studied after final 5 h overaging treatments >510 ≤650 °C. SCC tests were conducted for 1000 h with compact tension specimens in aqueous 3.5 pct NaCl solutions at a noble (anodic) potential of −0.28 V_SHE ( −0.48 V_Ag/AgC1) and a cathodic protection potential of −0.80 V_SHE (−1.0 V_Ag/AgC1). The SCC resistance improved at aging temperatures >565 °C, the most significant improvement being at −0.80 V_She, especially after 650 ° aging whereK _ISCC was raised to at least 110 MPa · m^1/2. However, this was at the expense of mechanical properties. Provided low crack propagation rates of ∼3 X 10⁻¹¹ m/s at −0.80V _SHEmay be tolerated, the best compromise between strength, toughness, and SCC resistance was obtained after 594 °C aging. Under these conditions, stress intensities as high as ∼ 110 MPa · m^1/2 can be used, with a yield strength of ∼ 1150 MPa and fracture toughness of ∼ 170 MPa · m^1/2. The retained austenite content after aging increased with aging temperature up to 25 pct by vol at 650 °C. It appeared to correlate with improved SCC resistance, but other microstructural effects associated with aging may be involved. Formerly Research Associate with theDepartment of Metallurgical Engineering , University of BritishColumbia     

Stress corrosion cracking of high strength HY-180M steel in 3.5 Pct NaCl

Stress corrosion cracking of HY-180M steel was studied at 22°C in an aqueous solution of 3.5 pct NaCl (pH = 6.5). The steel had a nominal weight percentage composition of 10Ni-14Co-2Cr-lMo-0.16C and was heat treated to yield a fracture toughness value ofK _Ic ≃ 160 MPa . m^1/2. The SCC velocity (v) was studied as a function of stress intensity (K _I) and electrochemical potential (E) using precracked compact tension specimens, a Ag/AgCl reference electrode and a 1000 h exposure test. Also, the polarization behavior, microstructure, fractography and corrosion products were studied. The results showed that SCC was markedly dependent uponE, and did not occur whenE =-0.52 V_SHE (-0.72 V_Ag/AgCl), which corresponded closely to the thermodynamically reversible potential of iron. However, SCC occurred at a more noble potential of-0.28 V_SHE (-0.48 V_Ag/AgCl ) and at a less noble potential of-0.80 V_SHE (-1.00 V_Ag/AgCl). The stress intensity below which SCC was not observed was K_ISCC ≃ 5.5 MPa . m^1/2 at -0.28 V_SHE and K_ISCC ≃ 60 MPa . m^1/2 at -0.80 V_SHE . Also, Region I behavior (v dependent uponK ₁) and Region II behavior (v independent ofK ₁) were observed. Cracking was considered to occur solely by hydrogen embrittlement at -0.80 Vshe, whereas anodic dissolution processes played a necessary role, either directly or indirectly, in SCC at -0.28 V_SHE . The indirect effects were discussed in relation to hydrolysis effects in the crack promoting hydrogen embrittlement and/or corrosion product wedging stresses.     

Stress Corrosion Cracking of HY-180 Steel in Aqueous 3.5 Pct NaCI

Stress corrosion cracking of HY-180 steel (Fe-10 Ni-2 Cr-1 Mo-8 Co-0.12 C) was studied in aqueous 3.5 pct NaCI (pH = 6.5) at 22 °C. The alloy was austenitized, water quenched and aged at 510 °C for 5 h. Specimens were of the precracked, double cantilever beam (DCB) variety and exposure times extended up to 1000 h. The crack propagation rates (v) were studied as a function of stress intensity(K,) under both freely corroding potentials(E ≈-0.36 V_SHE) and potentials produced by coupling to Zn(E ≈ -0.82 V_SHE. Crack fractography was studied by scanning electron microscopy and corrosion products were identified by electron diffraction analysis. The stress intensity, K_ISCC, below which SCC could not be detected was ~45 MPa m^1/2 for both freely corroding and Zn-coupled conditions. Analysis of the results showed that cracking was consistent with a hydrogen embrittlement mechanism, irrespective of potential. Furthermore, comparison of the data with previous studies on a similarly heat treated and closely related alloy (HY-180 M), containing 14 Co-0.16 C, showed no significant difference in SCC behavior, provided comparison was made at similar electrochemical potentials.     

Slip-step dissolution and micromechanical analysis to model stress-corrosion crack growth of type 321 stainless steel in boiling mgci2

It is hypothesized that for ductile austenitic stainless steels exposed to boiling MgCl₂ solution, the relevant crack propagation mechanism is slip dissolution. This model relates crack advance to oxidation or anodic dissolution that occurs on the bare surface that is created when a thermo-dynamically stable, protective film at the crack tip mechanically ruptured. Based on the model of slip-bare metal dissolution repassivation and crack-tip strain analysis, a theoretical equation of stress-corrosion crack growth rate as a function of crack-tip strain rate and potential for 321 stainless steel in boiling 42 pct MgCl₂ solution is proposed. The theoretical prediction shows that when the crack-tip strain rate changes from 10⁻⁴ to 10⁻² s⁻¹ the crack propagation rate changes from 0.01 to 3 mm/h at the free corrosion potential (−0.35 V_SCE). If the crack-tip strain rate is above 10⁻²/s, the crack propagation rate should correspond to the upper bound determined by the maximum metal dissolution rate. When the crack-tip rate is below 10⁻⁴/s, the crack propagation rate is below 0.01 mm/h. The slip-step dissolution model predicted that there exists a critical potentialE _c, above which the crack propagation rate is independent on potential, but below which the crack propagation rate decreased with decreasing potential. The theoretical prediction has been verified by slow strain rate tests of 321 stainless steel under potential control (above −0.35 V_SCE) in 42 pct MgCl₂ solution.     

Effect of Magnesium Content on the Corrosion Behaviors of Grf/Al Composite

Graphite-fiber-reinforced aluminum composites (Gr_f/Al) with different magnesium content were fabricated with the pressure infiltration method. Their corrosion behaviors were investigated by potentiodynamic polarization measurements, immersion tests, electrochemical impedance spectroscopy (EIS) analysis, and scanning electron microscopy (SEM). Both the corrosion potential (E _corr) and the pitting potential (E _pit) decreased with the increase of magnesium content, whereas the corrosion current density (i _corr) decreased sharply at first and then increased slightly. The i _corr of Gr_f/Al-3.2Mg was the lowest among the four composites with different magnesium contents, which indicated that Gr_f/Al-3.2Mg had the best corrosion resistance. EIS showed that the capacitive reactance of Gr_f/Al-8.5Mg was 1682 Ω × cm⁻², which was the worst, whereas that of Gr_f/Al-3.2Mg was 3498 Ω × cm⁻², which was the best. SEM results revealed that magnesium and silicon formed the Mg₂Si phase in Gr_f/Al-3.2Mg, which hindered the extension of corrosion crack and improved the corrosion resistance.     

A critical evaluation of the stress-corrosion cracking mechanism in high-strength aluminum alloys

Attempts have been made to elucidate the mechanism of stress-corrosion cracking (SCC) in high-strength Al-Zn-Mg and Al-Li-Zr alloys exposed to aqueous environments by considering the temperature dependence of SCC susceptibility based upon the anodic dissolution and hydrogen embrittlement models. A quantitative correlation which involves the change of threshold stress intensity,K _ISCC, with temperature on the basis of anodic dissolution has been developed with the aid of linear elastic fracture mechanics. From the derived correlation, it is concluded that the threshold stress intensity decreases as the test temperature increases. This suggestion is inconsistent with that predicted on the basis of hydrogen embrittlement. It is experimentally observed from the Al-Zn-Mg and Al-Li-Zr alloys that the threshold stress intensity,K,_ISCC, decreases and the crack propagation rate,da/dt, over the stress intensity increases with increasing test temperature. From considering the change in SCC susceptibility with temperature, it is suggested that a gradual transition in the mechanism for the stress-corrosion crack propagation occurs from anodic dissolution in stage I, where the crack propagation rate increases sharply with stress intensity, to hydrogen embrittlement in stage II, where the crack propagation rate is independent of stress intensity.     

Crack Propagation During Sustained-Load Cracking of Al-Zn-Mg-Cu Aluminum Alloys Exposed to Moist Air or Distilled Water

Intergranular sustained-load cracking of Al-Zn-Mg-Cu (AA7xxx series) aluminum alloys exposed to moist air or distilled water at temperatures in the range 283 K to 353 K (10 °C to 80 °C) has been reviewed in detail, paying particular attention to local processes occurring in the crack-tip region during crack propagation. Distinct crack-arrest markings formed on intergranular fracture faces generated under fixed-displacement loading conditions are not generated under monotonic rising-load conditions, but can form under cyclic-loading conditions if loading frequencies are sufficiently low. The observed crack-arrest markings are insensitive to applied stress intensity factor, alloy copper content and temper, but are temperature sensitive, increasing from ~150 nm at room temperature to ~400 nm at 313 K (40 °C). A re-evaluation of published data reveals the apparent activation energy, E _a for crack propagation in Al-Zn-Mg(-Cu) alloys is consistently ~35 kJ/mol for temperatures above ~313 K (40 °C), independent of copper content or the applied stress intensity factor, unless the alloy contains a significant volume fraction of S-phase, Al₂CuMg where E _a is ~80 kJ/mol. For temperatures below ~313 K (40 °C) E _a is independent of copper content for stress intensity factors below ~14 MNm^−3/2, with a value ~80 kJ/mol but is sensitive to copper content for stress intensity factors above ~14 MNm^−3/2, with E _a , ranging from ~35 kJ/mol for copper-free alloys to ~80 kJ/mol for alloys containing 1.5 pct Cu. The apparent activation energy for intergranular sustained-load crack initiation is consistently ~110 kJ/mol for both notched and un-notched samples. Mechanistic implications are discussed and processes controlling crack growth, as a function of temperature, alloy copper content, and loading conditions are proposed that are consistent with the calculated apparent activation energies and known characteristics of intergranular sustained-load cracking. It is suggested, depending on the circumstances, that intergranular crack propagation in humid air and distilled water can be enhanced by the generation of aluminum hydride, AlH_3, ahead of a propagating crack and/or its decomposition after formation within the confines of the nanoscale volumes available after increments of crack growth, defined by the crack arrest markings on intergranular fracture surfaces.     

Aqueous environmental crack propagation in high-strength beta titanium alloys

The aqueous environment-assisted cracking (EAC) behavior of two peak-aged beta-titanium alloys was characterized with a fracture mechanics method. Beta-21S is susceptible to EAC under rising load in neutral 3.5 pct NaCl at 25 °C and −600 mV_SCE, as indicated by a reduced threshold for subcritical crack growth (K _TH ), an average crack growth rate of up to 10 μms, and intergranular fracture compared to microvoid rupture in air. In contrast, the initiation fracture toughness (K _ICi ) of Ti-15-3 in moist air is lower than that of Beta-21S at similar high σ_YS (1300 MPa) but is not degraded by chloride, and cracking is by transgranular microvoid formation. The intergranular EAC susceptibility of Beta-21S correlates with both α-colonies precipitated at β grain boundaries and intense slip localization; however, the causal factor is not defined. Data suggest that both features, and EAC, are promoted by prolonged solution treatment at high temperature. In a hydrogen environment embrittlement (HEE) scenario, crack-tip H could be transported by planar slip bands to strongly binding trap sites and stress/strain concentrations at α colony or β grain boundaries. The EAC in Beta-21S is eliminated by cathodic polarization (to −1000 mV_SCE), as well as by static loading for times that otherwise produce rising-load EAC. These beneficial effects could relate to reduced H production at the occluded crack tip during cathodic polarization and to increased crack-tip passive film stability or reduced dislocation transport during deformation at slow crack-tip strain rates. High-strength β-titanium alloys are resistant, but not intrinsically immune to chloride EAC, with processing condition possibly governing fracture. Formerly Graduate Research Associate, University of Virginia Formerly Graduate Research Associate, University of Virginia     

Lead-induced solid metal embrittlement of an excess silicon Al−Mg−Si alloy at temperatures of −4°C to 80°C

It is shown that minute (e. g., <500 ppm) additions of heavy metal impurities (e. g., Pb, Bi) can induce sustained load cracking at ambient temperatures in Al−Mg−Si alloys. This article describes experiments designed to elucidate the role of Pb level, test temperature, strain rate, and stress state on the observed cracking phenomena. Sustained load cracking was observed at temperatures as low as −4°C in either air or vacuum with the rate of crack growth and the apparent threshold value for crack growth being strongly influenced by the amount of lead (i.e., internal) in the alloy. The fracture mode was strongly affected by the test temperature, Pb level, strain rate, and the imposed stress intensity level,K. Fracture in the low-lead (i.e., <10 ppm) alloys was predominantly by intergranular microvoid coalescence (IGMVC), while fracture in the higher lead alloys was predominantly by low ductility intergranular fracture (LDIGF) when the crack-tip strain rate was sufficiently low. High-resolution scanning electron micrographs taken from LDIGF surfaces suggested minimal deformation, while surface analyses of these surfaces performed using both laser microprobe mass spectroscopy (LMMS) and high-resolution scanning auger microscopy indicated that lead was primarily responsible for the LDIGF cracking phenomenon. Lead was observed both on the surfaces of fractured specimens as well as in subsurface cracks not contiguous with the macroscopic cracks. An external supply of Pbvia the application of either solid Pb or a Pb−Bi alloy to the external surfaces of specimens held under sustained load promoted LDIGF, decreased thresholdK’s, and increased the rate of crack growth. The phenomena and possible mechanisms of heavy metal impurity-induced cracking at ambient temperatures in aluminum alloys are discussed. Y.S. KIM, formerly with the Department of Materials Science and Engineering, Case Western Reserve University, is with the Welding Research Center, Research Institute of Industrial Science and Technology (RIST), Pohang 790-600, Korea.     

The role of heat treating on the sour gas resistance of an X-80 steel for oil and gas transport

In this work, the role of the microstructure in the stress sulfide cracking (SSC) resistance of an API X-80 steel was investigated by exposure of as-received and heat-treated specimens to a H₂S-saturated aqueous National Association of Corrosion Engineers (NACE) solution. It was found that for similar corrosive environments and applied stress intensity factors of 30 to 46 MPa√m, crack growth in LEFM (linear elastic fracture mechanics) compact specimens is strongly influenced by heat treating. In the as-received alloy, crack growth in the direction normal to rolling was controlled by metal dissolution of the crack tip region in contact with the corrosive environment, with crack growth rates of the order of 1/W(da/dt)∼8.3×10⁻⁴ h⁻¹. Alternatively, crack growth in the direction parallel to the rolling direction did not show metal dissolution, but instead hydrogen embrittlement along segregation bands. In this case, crack growth rates of the order of 1.2×10⁻³ h⁻¹ were exhibited. In the martensitic condition, the rate of crack propagation was relatively fast (1/W(da/dt)∼4.5×10⁻² h⁻¹), indicating severe hydrogen embrittlement. Crack arrest events were found to occur in water-sprayed and quenched and tempered specimens, with threshold stress intensity values (K _ISSC) of 26 and 32 MPa√m, respectively. Apparently, in the water-sprayed condition, numerous microcracks developed in the crack tip plastic zone. Crack growth occurred by linking of microcracks, which were able to reach the main crack tip. In particular, preferential microcrack growth occurred across carbide regions, but their growth was severely limited in the ferritic matrix. Quenching and tempering (Q&T) resulted in a tempered martensite microstructure characterized by fine distribution carbides, most of which were cementite. In this case, the crack path continually shifted to follow the ferrite interlath boundaries, which contained mostly fine cementite precipitates. As a result, the crack was tortuous with numerous bifurcations along ferrite grain boundaries. Most of the tests were carried out in NaCl-free NACE solutions; the only exception was the as-received condition where 5 wt pct NaCl was added to the sour environment. In this case, crack growth did not occur after exposing the specimen to the salt-free NACE solution for 30 days, but addition of 5 pct NaCl promoted crack propagation.     

Stress corrosion cracking relation with dezincification layer-induced stress

Brass foil with a protective layer formed on one side was deflected during corrosion in an ammonia solution under various applied potentials, and then corrosion-induced stress generated at brass/dezincification layer under different potentials could be measured. At the same time, susceptibility to stress corrosion cracking (SCC) of brass in the ammonia solution under various applied potentials was measured by using a single-edge notched specimen. At open-circuit potential, both corrosion-induced tensile stress and susceptibility to SCC (I _σ) had a maximum value. Both tensile stress σ _p and susceptibility I _σ decreased slightly with decreasing potential under anodic polarization, but reduced steeply with a decrease in potential under cathodic polarization. At the cathodic potential of − 500 mV_SCE, corrosioninduced stress became compressive because of the copper-plating layer; correspondingly, susceptibility to SCC was zero. Therefore, the variation of SCC susceptibility with potential is consistent with that of the corrosion-induced additive stress.     

Improvement of the resistance to stress corrosion cracking in austenitic stainless steels by cyclic prestraining

Austenitic stainless steels are known to be sensitive to stress corrosion cracking (SCC) in hot chloride solutions. The aim of the present study is to find improvements in the SCC behavior of 316L-type austenitic stainless steels in 117°C MgCl₂ solutions. Previously, the authors have proposed the “corrosion-enhanced plasticity model” (CEPM) to describe the discontinuous cracking process which occurs in SCC. This model is based on localized corrosion (anodic dissolution, and hydrogen absorption)-deformation (dislocations) interactions (CDI). From the framework of this model, it is proposed that a prestraining in fatigue at saturation decreases the SCC sensitivity. This idea is experimentally confirmed for both crack initiation and crack propagation, through the analysis of the SCC behavior by slow-strain-rate tests of single and polycrystals after different prestraining conditions.     

The effect of crack- tip strain rate and potential on the propagation rate of stress corrosion crack for 321 stainless steel in boiling 42 Pct MgCl2 solution

The decay law of current with time on the bare surface of scratched 321 stainless steel in boiling 42 pct MgCl₂ solution has been studied by the rapid scratching technique under potentiostatic condition. Based on the decay law and the model of slip-bare metal dissolution-repassivation, a theoretical equation of stress corrosion crack propagation rate as a function of crack-tip strain rate and potential for 321 stainless steel in boiling 42 pct MgCl₂ solution has been proposed as da/dt = 3.6 × 10⁴ M/zFρ ε/Nnbφ 4+6E/0.03-E [1-exp(-Nnbφ(0.03-)/ε)] The theoretical calculation shows that when the crack-tip strain rate changes from 10^-4/s to 10^-2/s, the crack propagation rate changes from 0.01 mm/h to 3 mm/h at natural corrosion potential (-0.35 V(SEC)). If the crack-tip strain rate is above 10^-2/s, the crack propagation rate should correspond to the upper bound determined by the maximum metal dissolution rate. When the crack-tip strain rate is below 10^-4 s^-1, the crack propagation rate is below 0.01 mm/h. The sensitive potential E_c to cracking is -0.35 V(SEC); above the sensitive potential E_c, the crack propagation rate varies slowly with potential. However, below E_c, the crack propagation rate decreases rapidly with potential decreases. The crack propagation rate of 321 stainless steel in 42 pct MgCl₂ solution has been measured using a slow strain rate test technique. The the- oretical calculation is consistent with the experimental results.     

Limitations of potentiostatic control in stress corrosion crack growth measurements

The electrode potential distribution along a crack in a potentiostatically polarized specimen has been derived analytically by including polarization behavior and solution conductance considerations. The analysis has been applied to the stress corrosion cracks within low alloy steels in an 8M sodium hydroxide solution at 373 K and shows that the electrode potential at the tip falls to the normal equilibrium corrosion potential as the crack length increases. These results show that potentiostatic control at the tip of a stress corrosion crack is subject to large varying systematic errors. Consequently the validity of stress corrosion mechanisms based on potentiostatically controlled crack growth measurements which do not take into account such errors should be reexamined. List of Symbolsα the Tafel constant for the anodic dissolution reaction on a natural logarithm scale, V,β the Tafel constant for the cathodic reduction reaction on a natural logarithm scale, V,C the specific conductance of a solution, Cl'1 mδ the crack opening displacement, m,E _c the free corrosion potential, V,E _app the potentiostatically applied potential, V,E _x the potential at position x within a crack, VE _y Young's modulus of elasticity, MNm^-22,i _c the corrosion current density at the free corrosion potential, Am^-2,i _app the net anodic current density supplied to polarize the specimen to a potential E _app , Am^-2,i ₁ the cathodic current density at potential E_app, Am^-2,i ₂ the anodic current density at potential E, Am-2,i _x the net anodic current density at potential Ex, Am-2,i f the current flow per unit length along the crack at position x, Am-2,ρ _y the yield stress, MNm-2,K the stress intensity, MNm-3/2, w the width of the stress corrosion crack, m, and x the length of the stress corrosion crack, m.     

Environmental fatigue of an Al-Li-Cu alloy: Part II. Microscopic hydrogen cracking processes

Microscopic fatigue crack propagation (FCP) paths in peak-aged unrecrystallized alloy 2090 are identified as functions of intrinsicda/dN- ΔK kinetics and environment. The FCP rates in longitudinal-transverse (LT)-oriented 2090 are accelerated by hydrogen-producing environments (pure water vapor, moist air, and aqueous NaCl), as defined in Part I. Subgrain boundary cracking (SGC) dominates for ΔK values where the cyclic plastic zone is sufficient to envelop subgrains. At low ΔK, when this crack tip process zone is smaller than the subgrain size, environmental FCP progresses on or near (100) or (110) planes, based on etch-pit shape. For inert environments (vacuum and He) and pure O₂ with crack surface oxidation, FCP produces large facets along 111 oriented slip bands. This mode does not change with ΔK, andT ₁ decorated subgrain boundaries do not affect an expectedda/dN- ΔK transition for the inert environments. Rather, the complex dependence ofda/dN on ΔK is controlled by the environmental contribution to process zone microstructure-plastic strain interactions. A hydrogen embrittlement mechanism for FCP in 2090 is supported by similar brittle crack paths for low pressure water vapor and the electrolyte, the SGC and 100/110 crystallographic cracking modes, the influence of cyclic plastic zone volume ( ΔK), and the benignancy of O₂. The SGC may be due to hydrogen production and trapping atT ₁ bearing sub-boundaries after process zone dislocation transport, while crystallographic cracking may be due to lattice decohesion or hydride cracking. Robert S. Piascik, formerly Graduate Student, Department of Materials Science, University of Virginia.     

Dissolution of columbite and tantalite in acidic fluoride media

The dissolution reactions of Nb and Ta from columbite and tantalite in the aqueous solutions of HF, HF-HC1, NH₄F-HCI, HF-H₂SO₄, and NH₄F-H₂SO₄ were kinetically studied in the temperature range of 333 to 353 K. The presence of both H⁺ and F^- in the leachant is necessary for the fast dissolution of columbite and tantalite. The increase in these ion concentrations and the elevation of temperature are effective in increasing the dissolution rate. The initial dissolution rate of Nb from columbite under the conditions employed in this work can be expressed as follows:R _i =k _o a(H⁺)^1.2 C(F^2212;)^1.1 exp(−E _a /RT) where an apparent activation energy,E _a , ranges from 53.9 kJ mol⁻¹ to 65.5 kJ mol⁻¹. Formerly Graduate Student, Department of Metallurgy, Kyoto University, Japan     

Dissolution of columbite and tantalite in acidic fluoride media

The dissolution reactions of Nb and Ta from columbite and tantalite in the aqueous solutions of HF, HF-HC1, NH₄F-HCI, HF-H₂SO₄, and NH₄F-H₂SO₄ were kinetically studied in the temperature range of 333 to 353 K. The presence of both H⁺ and F^- in the leachant is necessary for the fast dissolution of columbite and tantalite. The increase in these ion concentrations and the elevation of temperature are effective in increasing the dissolution rate. The initial dissolution rate of Nb from columbite under the conditions employed in this work can be expressed as follows:R _i =k _o a(H⁺)^1.2 C(F^2212;)^1.1 exp(−E _a /RT) where an apparent activation energy,E _a , ranges from 53.9 kJ mol⁻¹ to 65.5 kJ mol⁻¹. Formerly Graduate Student, Department of Metallurgy, Kyoto University, Japan