Mitigation of ammonia-induced SCC in a cupronickel alloy by additions of MgCl<Subscript>2</Subscript>

The authors carried out failure analysis of bent and branched copper-nickel alloy pipelines that had failed in marine environments. These failures were almost always dominated by a brittle stress-corrosion cracking (SCC) mode and could often be attributed to the presence of ammoniacal byproducts in the operating environment. Attempts were made to reproduce the marine-type field failures in the laboratory by testing a Cu-5.37%Ni alloy, similar to the material used in failed pipelines. The tests were performed under slow strain rate test (SSRT) conditions in aqueous ammonia and ammoniacal seawater. Results revealed that the ammonia-induced brittle SCC failures were predominant and reduced the load-bearing capacity of the alloy. The real-life failures are not simple SSRT-type failures. The operating conditions, in addition to the induced residual stresses from manufacturing/processing, subject the system pipes to external forces and widely varying pressures and fluid flow rates. This combination of stresses can produce both static and cyclic stress conditions, similar to a static load coupled with a low-amplitude cyclic load. Tests conducted under superimposed cyclic stresses on prestressed specimens were found to accelerate the stress-corrosion failures in the present copper-nickel alloy in an ammoniacal environment. During the testing process, it was established that chlorides of sodium and magnesium also had a role to play on the ammonia-induced SCC. Further tests were therefore designed, and this paper summarizes test results, which point to the possible mitigation of ammonia-induced SCC in cupronickels by the addition of MgCl₂.     

Effect of ammonia concentration on environment-assisted failures in a low-nickel copper alloy

Copper-nickel (Cu-Ni) alloys are the materials of choice for many piping applications; however, a number of premature field failures in copper-nickel pipes exposed to the marine environment have been observed by the author. A majority of these failures occurred at the bends in long-span and branched pipes located near water closets and/or bilges that were frequently filled with stagnated water. Failure analysis investigation revealed that the nickel content of the failed pipes was typically less than the specified value. The operative mechanism(s) causing the premature field failures invariably involved corrosion-assisted material cracking. The environment to which the failed pipes were exposed contained magnesium chloride (MgCl₂) and sodium chloride (NaCl), along with biodegradable materials capable of releasing ammoniacal byproducts. Many complex material-environment interactions are possible in marine piping systems, and the operative interaction depends on the stresses encountered and the chemistry and temperature of the exposure environment. While a failure analysis can usually identify the overall operative modes and mechanisms that cause the failures, an understanding of the material-environment interactions is needed to develop the corrective measures necessary to avoid premature field failures in real life applications. A commercial Cu-5.37% Ni alloy very similar to the composition of the field failure pipes was studied under slow strain conditions in air and in solutions containing 3.5 wt.% NaCl + 10.0 wt.% MgCl₂ + either 1.0 wt.% ammonia or no ammonia additions. No deterioration of the mechanical properties of the alloy was observed in the tests conducted in the NaCl/MgCl₂ solution without ammonia. However, with the introduction of 1.0% ammonia, there was a reduction in the mechanical strength of the alloy, and the mode of failure changed from ductile rupture to brittle fracture. The slow strain rate tests of the alloy were conducted in aqueous ammonia at various concentration levels. The failures observed in aqueous ammonia showed a significant loss of strength with increasing ammonia concentration. The failures were predominantly brittle, exhibiting both intergranular and transgranular fracture paths. In general, the propensity for crack formation increased with increasing ammonia. The failures observed in aqueous ammonia were more severe and different than those observed previously in samples tested in ammonia containing 3.5% NaCl + 10.0% MgCl₂ solutions. This paper discusses the aqueous ammonia failures in detail.     

Damage in cement pastes exposed to MgCl<Subscript>2</Subscript> solutions

Magnesium chloride (MgCl₂) reacts with cement pastes resulting in calcium leaching and the formation of calcium oxychloride, which can cause damage. This paper examines the damage in different cement pastes exposed to MgCl₂ solutions. Volume change measurement and low temperature differential scanning calorimetry are used to characterize the formation of calcium oxychloride. Thermogravimetric analysis and X-ray fluorescence are used to quantify calcium leaching from Ca(OH)₂ and C-S-H. The ball-on-three-balls test is used to quantify the flexural strength reduction. Calcium oxychloride can form in cement pastes exposed to MgCl₂ solutions with a (Ca(OH)₂/MgCl₂) molar ratio larger than 1. As the MgCl₂ concentration increases, two-stages of flexural strength reduction are observed in the plain cement pastes, with the initial reduction primarily due to calcium leaching from Ca(OH)₂ and the additional reduction due to the calcium leaching from C-S-H (at MgCl₂ concentrations above 17.5 wt%). For the cement pastes containing fly ash, there is a smaller reduction in flexural strength as less Ca(OH)₂ is leached, while no additional reduction is observed at high MgCl₂ concentrations due to the greater stability of C-S-H with a lower Ca/Si ratio. The addition of fly ash can mitigate damage in the presence of MgCl₂ solutions.     

Thermodynamic Assessment of EuCl<Subscript>3</Subscript>–MgCl<Subscript>2</Subscript> and EuCl<Subscript>3</Subscript>–BaCl<Subscript>2</Subscript> Systems

Using the CALPHAD technique, an assessment of the binary EuCl₃–MgCl₂ and EuCl₃–BaCl₂ systems has been carried out in this study. The modified quasi-chemical model was defined to describe the Gibbs energies of the liquid phases, and the model parameters were optimized from the experimental phase diagram data. The phase diagrams and enthalpies of mixing of the EuCl₃–MgCl₂ and EuCl₃–BaCl₂ systems were calculated. The calculated results by the present method agree well with the experimental data. The Gibbs energies of formation of Mg₃Eu₂Cl₁₂, Ba₃Eu₂Cl₁₂, and Ba₂Eu₃Cl₁₃ from the pure components were predicted.     

Effect of La<Subscript>2</Subscript>O<Subscript>3</Subscript> addition on the microstructure of partially remelted Mg-9Al-1Zn alloy

The effect of La₂O₃ addition on the microstructure of partially remelted Mg-9Al-1Zn (AZ91D) alloy was studied. The results indicate that small amounts of La₂O₃ additions to AZ91D alloy refined the partially remelted microstructure and caused the formation of new phase- LaAl₄ in the microstructure. Moreover, the grain size of the partially remelted alloys is decreased with the increasing of La₂O₃ addition. In as-cast microstructure the LaAl₄ phases have two morphologies: needle-like and particle-shape. The presence of more LaAl₄ phases, especially, the particle-shape LaAl₄ which dispersedly distributed within the grains can induce more dislocations, which can result in the occurring of a large amount of recrystallizations. Moreover, these LaAl₄ phases can also restrain the growth and combination of the recrystallizations and partially remelted initial grains during the subsequent heat-treatment.     

Glass-forming ability and thermoplastic formability of a Pd<Subscript>40</Subscript>Ni<Subscript>40</Subscript>Si<Subscript>4</Subscript>P<Subscript>16</Subscript> glassy alloy

The glass-forming ability of a Pd₄₀Ni₄₀Si₄P₁₆ alloy has been investigated. This alloy exhibits a wide supercooled liquid region of 107 K, a high reduced glass transition temperature of 0.596 and a small fragility parameter of 28, indicating that this alloy is a good glass former. Using flux treatment, the Pd₄₀Ni₄₀Si₄P₁₆ alloys can be easily produced as centimeter-scale metallic glasses. The glass transition and crystallization kinetics of this alloy were investigated by means of both differential scanning calorimetry (DSC) and differential isothermal calorimetry (DIC). Thermoplastic forming of the Pd₄₀Ni₄₀Si₄P₁₆ glassy alloy is easily performed due to the high thermal stability and low viscosity of the supercooled liquid. By pressing a “flat” silicon wafer onto a sample within the thermoplastic forming region, the surface of the Pd₄₀Ni₄₀Si₄P₁₆ metallic glasses could be significantly smoothened. The final surface showed a reduced root mean square roughness R _q as low as ~2 nm. This indicates a simple approach to prepare “flat” surfaces for metallic glasses.     

Chemical interaction of the Ti<Subscript>90</Subscript>Mg<Subscript>10</Subscript> alloy with ammonia

We have studied the chemical interaction of the Ti₉₀Mg₁₀ alloy with ammonia in the presence of NH₄Cl at temperatures from 150 to 500°C and identified conditions for the formation of fine-particle hydrides and nanocrystalline nitrides of titanium and magnesium.     

Corrosion resistance of amorphous Fe<Subscript>82</Subscript>P<Subscript>16</Subscript>Si<Subscript>2</Subscript> in 0.1 M Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>

A soft-magnetic amorphous Fe-P-Si alloy prepared using ferrophosphorus waste was tested for corrosion in 0.1 M Na₂SO₄. In a nonequilibrium state, the Fe₈₂P₁₆Si₂ alloy interacts with the medium, but annealing and relaxation reduce the interaction, without influencing the magnetic properties of the alloy. The corrosion resistance of the alloy is comparable to that of Finemet (Fe-Si-B-Nb-Cu) materials.     

Synthesis of nanocomposites based on MO-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript> (M = Ca,Sr, Ba) glasses

This work examines the effect of KBF₄ additions on the crystallization behavior of glasses based on the multicomponent systems MO-Bi₂O₃-B₂O₃ with M = Ca, Sr, and Ba. The glass-ceramic composites obtained contain a δ-Bi₂O₃-based crystalline phase with a crystallite size of ≃7 nm, evenly distributed over the glass matrix. The 400°C electrical conductivity of the nanocomposites reaches 2 × 10⁻⁴ S/cm, and the activation energy is 1.1 eV, typical of anion conduction. These values are comparable to those reported for δ-Bi₂O₃ ceramics.     

Structural State and Properties of Nitride Coatings Based on the Highly Entropic Alloy Ti<Subscript>30</Subscript>Zr<Subscript>25</Subscript>Nb<Subscript>20</Subscript>Hf<Subscript>15</Subscript>Ta<Subscript>10</Subscript>Y<Subscript>5</Subscript>

The regularities of the formations of nitride coatings based on the base of high entropy multicomponent alloy Ti₃₀Zr₂₅Nb₂₀Hf₁₅Ta₁₀Y₅ in the initial state without nitrogen and in nitrogen. The initial material of the alloy is solid solution on the base of the bcc-lattice. At the further sputtering of sample in the atmospheres with different contents of nitrogen the coating forms on the base of fcc-lattice. At the optimal conditions of the sputtering the nitride coating on the base of the highly entopic multicomponent alloy has the hardness at the level of 50 GPa and the reduced elastic modulus 350 GPa.     

Glass formation in the Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript>-BaO system

The physicochemical properties of BaO-Bi₂O₃-B₂O₃ glasses have been studied. The effect of KBF₄ additions on the properties of the glasses has been examined. The transmission of the glasses has been correlated with their local structure and composition.     

Study of air oxidation of amorphous Zr<Subscript>65</Subscript>Cu<Subscript>17.5</Subscript>Ni<Subscript>10</Subscript>Al<Subscript>7.5</Subscript> by X-ray photoelectron spectroscopy (XPS)

The oxidation of the bulk amorphous alloy Zr₆₅Cu_17.5Ni₁₀Al_7.5 in air in its amorphous and the supercooled liquid states was studied in the temperature range 573–663 K using X-ray photoelectron spectroscopy (XPS). The oxide film mainly consisted of the oxides of Zr (as ZrO₂) and Al (as Al₂O₃). No Cu or Ni was found in the oxide film formed on the amorphous state of the alloy while significant Cu (as CuO) was present in the oxide film formed on the alloy in its supercooled liquid state. The role of the various alloying elements during oxidation at high temperatures in air is discussed in the paper. The XPS data from oxide film support the previously suggested mechanism for oxidation of this alloy, i.e. the rate controlling process during oxidation of the alloy at low temperatures (in the amorphous state) is the back-diffusion of Ni and Cu, while the oxidation at high temperatures (in the supercooled liquid state) is dominated by the inward diffusion of oxygen.     

Hydriding of Mg<Subscript>2</Subscript>Ni in ammonia

The chemical interaction between the intermetallic compound Mg₂Ni and ammonia in the presence of NH₄Cl as an activator is investigated at temperatures from 100 to 450 °C, and the reaction scheme is presented. The results demonstrate that the use of ammonia for hydriding/nitriding the intermetallic compound makes it possible to prepare various magnesium compounds (Mg₂NiH₄, Mg₃N₂, and Mg (NH₂)₂) in a highly dispersed state.     

Effect of nano-TiO<Subscript>2</Subscript> addition on the microstructure and bonding strengths of Sn3.5Ag0.5Cu composite solder BGA packages with immersion Sn surface finish

The effects of nano-TiO₂ particles on the interfacial microstructures and bonding strength of Sn3.5Ag0.5Cu composite solder joints in ball grid array packages with immersion Sn surface finishes have been investigated. Metallography reveals that addition of nano-TiO₂ particle retarded wicker-Cu₆Sn₅ IMC formed in the Sn3.5Ag0.5Cu composite solder joints. The thickness of the interfacial intermetallic compounds of the solder joint was reduced with increased additions of nano-TiO₂ particles (0.25–1.0 wt%), but further additions up to 1.25 wt% decreased the beneficial influence. This indicates that the presence of a small amount of nano-TiO₂ particles is effective in suppressing the growth of the intermetallic compounds layer. In addition, the shear strength of the soldered joints was improved by larger nano-TiO₂ particle additions, with the peak shear strength occurring at 1.0 wt% of nano-TiO₂ particles into the Sn3.5Ag0.5Cu solder. The fracture mode also changed with increased amounts of nano-TiO₂ particles.     

Induced NH<Subscript>2</Subscript> bonding of carbon nanotubes using NH<Subscript>3</Subscript> plasma-enhanced chemical vapor deposition

Plasma-enhanced chemical vapor deposition was used to modify the multiwall carbon nanotubes (MWCNTs) using ammonia (NH₃) plasma. For various durations of NH₃ plasma treatment, a scanning electron microscope, X-ray, Raman spectroscopy and contact angle measurement were used to ascertain several characteristics of the MWCNTs. The experimental results show that: (1) the length of the MWCNTs is reduced, if the duration of the plasma treatment is increased; (2) the NH₃ plasma treatment can incorporate amine (NH₂ ⁻) or amino (NH⁻) functional groups onto the MWCNT surface; (3) the plasma treated carbon nanotubes become more hydrophilic.     

Microwave dielectric properties and its compatibility with silver electrode of LiNb<Subscript>0.6</Subscript>Ti<Subscript>0.5</Subscript>O<Subscript>3</Subscript> with B<Subscript>2</Subscript>O<Subscript>3</Subscript> and CuO additions

The influences of B₂O₃ and CuO (BCu, B₂O₃: CuO = 1:1) additions on the sintering behavior and microwave dielectric properties of LiNb_0.6Ti_0.5O₃ (LNT) ceramics were investigated. LNT ceramics were prepared with conventional solid-state method and sintered at temperatures about 1,100 °C. The sintering temperature of LNT ceramics with BCu addition could be effectively reduced to 900 °C due to the liquid phase effects resulting from the additives. The addition of BCu does not induce much degradation in the microwave dielectric properties. Typically, the excellent microwave dielectric properties of ε_r = 66, Q × f = 6,210 GHz, and τ _f  = 25 ppm/^oC were obtained for the 2 wt% BCu-doped sample sintered at 900 °C. Chemical compatibility of silver electrodes and low-fired samples has also been investigated.     

Effects of metal oxides addition on the electrochemical performance of M1Ni<Subscript>3.5</Subscript>Co<Subscript>0.6</Subscript>Mn<Subscript>0.4</Subscript>Al<Subscript>0.5</Subscript> hydrogen storage alloy

The AB₅-type M1Ni_3.5Co_0.6Mn_0.4Al_0.5 alloy (where M1 denotes mixed lanthanide) was modified with different additives (ZnO and MnO₂), and the effects of metal oxides on the electrochemical properties of the M1Ni_3.5Co_0.6Mn_0.4Al_0.5 − x% M (x = 5, 10; M = ZnO, MnO₂) alloy were studied. The results showed that the addition of metal oxides had a positive effect on the activation property of the alloy electrode. With the addition of ZnO, the maximum discharge capacity of the alloy increased from 315 to 334 mAh/g (x = 5) and 341 mAh/g (x = 10) with good cycle capability (C ₃₀/C _max) (87% for x = 5 and 85% for x = 10), while the maximum discharge capacity remained invariable and the cyclic stability was deteriorated by the addition of MnO₂. Linear polarization (LP), cycle voltammetry (CV), and electrochemical impedance spectroscopy (EIS) measurements were also performed to investigate the electrochemical kinetics of alloy electrodes.     

Low temperature sintering and dielectric properties of Ba<Subscript>3</Subscript>(VO<Subscript>4</Subscript>)<Subscript>2</Subscript> microwave ceramics using Co<Subscript>2</Subscript>O<Subscript>3</Subscript> additives

The Ba₃(VO₄)₂–x wt% Co₂O₃ (x?=?0.5–5) ceramics were prepared by the solid state reaction method in order to reduce the sintering temperature. The effects of the Co₂O₃ additions on the phase composition, microstructures, sintering characteristics and microwave dielectric properties of Ba₃(VO₄)₂ ceramics are investigated by an X-ray diffractometer, a scanning electron microscope and a network analyzer. As a result, the Q?×?f value of 54,000 GH, the ε _r of 14.6 and the τ_f value of +58.5 ppm/°C were obtained in the sample of the Ba₃(VO₄)₂–3 wt% Co₂O₃ ceramic sintered at the temperature of 925 °C, which is capable to co-fire with electrode metal of high conductivity such as Ag (961 °C). Moreover, the Q?×?f values of the sample with Co₂O₃ higher than that of 3 wt% additions decreased because of the formation of Ba₂V₂O₇ phase.     

Metastable phases in the Ni<Subscript>75</Subscript>Nb<Subscript>12</Subscript>B<Subscript>13</Subscript> alloy prepared under nonequilibrium conditions

The structure of Ni₇₅Nb₁₂B₁₃ alloys prepared by liquid quenching (LQ) and mechanical alloying (MA) has been studied by x-ray diffraction. The alloy prepared by LQ at a cooling rate of ～10⁶ K/s is shown to be fully amorphous, while MA yields an amorphous-crystalline material in which the predominant phase is an fcc Ni〈Nb,B〉 solid solution. The thermal stability of the alloys and their structural transformations on heating have been studied by differential scanning calorimetry. The amorphous phase obtained by LQ is shown to crystallize at 490°C. After heating to 720°C, the alloy consists of two equilibrium phases: Ni₂₁Nb₂B₆ (τ) and Ni₅Nb₃B₂ (z). Heating the MA alloy to 720°C leads to the formation of a stable τ-phase, while the Ni-based fcc solid solution remains supersaturated and, hence, metastable. Increasing the milling time leads to the formation of nanocrystalline τ and Ni₃B phases, in addition to the Ni-based fcc solid solution, which corresponds to the equilibrium phase composition of the Ni₇₅Nb₁₂B₁₃ alloy in the Ni-Nb-B phase diagram. The effect of high-energy milling on the phase composition of the alloy is similar to that of heat treatment.     

Preparation of Mo<Subscript>2</Subscript>C by reduction and carbonization of MoO<Subscript>2</Subscript> with CH<Subscript>3</Subscript>OH

Molybdenum hemicarbide (Mo₂C) can be widely used in the fields of steel, metal ceramics, and catalyst of hydrogen-involved reactions. This work reported a new and simple method for preparing molybdenum hemicarbide by reducing and carbonizing MoO₂ with methanol, and both thermodynamic and experimental studies were carried out. Based on the results, the reduction and carbonization routes are obtained: MoO₂ was reduced and carburized directly to Mo₂C in one step at lower temperatures, and first reduced to metallic Mo and then carburized to Mo₂C at higher temperatures. The reduction mechanisms at low and high temperatures were proposed as well: The pseudomorphic transformation mechanism dominated the reaction at low temperatures, but at high temperatures, the reaction obeyed the chemical vapor transport mechanism.