Showing papers in "Surface & Coatings Technology in 1986"

Hydrogen trapping phenomena in metals with B.C.C. and F.C.C. crystals structures by the desorption thermal analysis technique

Abstract: The types and amounts of lattice defects acting as trapping sites of hydrogen in metals with b.c.c. and f.c.c. structures and those induced by hydrogen during cathodic charging have been determined by the thermal analysis technique. Trapping parameters such as the trap activation energy, trap binding energy and trap density for each trap have been examined by using the mathematical models derived from the existing trap theory. From the results of thermal analysis of b.c.c.-structured metals, it is observed that the interfaces of non-metallic inclusions such as iron oxides, Al2O3, MnS and TiC are strong trapping sites for hydrogen with a high trap activation energy while the lattice imperfections, grain boundaries, dislocations and microvoids are shallow traps. The energy levels of hydrogen around each trapping site are as follows: for a strong trapping site, the saddle point energy is higher than the activation energy of lattice diffusion, which means that trapping and detrapping of hydrogen are difficult but, in a shallow trapping site, the saddle point energy is lower so that trapping and detrapping are easy. In pure nickel with an f.c.c. crystal structure, dislocation and grain boundaries have low trap activation energies compared with the activation energy for lattice diffusion. Equations for lattice solubility and diffusivity under atmospheric hydrogen pressure in pure nickel have been obtained by the thermal analysis technique and are as follows: C((H atom) atom-1)= 1.57 × 10-3exp−11.80 kJRT D(cm2s-1)=7.5 × 10-3exp−39.23 kJRT It is found that internal microcracks or microvoids are generated and are the major trapping sites of hydrogen when pure iron is cathodically charged.

Cathodic arc deposition technology

Abstract: Cathodic arc deposition is a unique technology. An arc spot on a cathode generates a plasma consisting of cathode material with a high degree of ionization. The high concentration of ions allows control of film morphology, high adhesion, efficient reactive deposition of compounds and uniform deposition over a variety of shapes. The composition of the alloys is generally retained from source to substrate. Deposition rates cover a wide range from nanometers per minute to tens of micrometers per minute. Many aspects of the technology warrant further study and its applications are only just beginning.

Surface graphitization of diamond at high temperatures

Abstract: Graphitization of diamond powders has been studied experimentally in a vacuum in the temperature range 1700 – 2100 K. The existence of two graphitization regions with different activation energies has been established. An increase in the specific graphitization rate with an increase in the size of the diamond powder particles, the so-called dimensional effect, has been established at temperatures above 1900 K. The stationary distribution of vacancies in small-size diamond particles explains the observed effect.

Impedance studies of corrosion resistance of aluminium in chloride media

Abstract: The electrochemical behaviour of aluminium in chloride solutions has been studied. Open-circuit impedance measurements reveal that in both acidic and neutral media the dissolution of the oxide formed on aluminium is governed by an empirical relation of the form C m -1 = a − Bt 1 2 where a and B are constants. The rate of the oxide film dissolution in the chloride-containing solutions is found to be markedly lower than that in other halide media, especially in fluoride solutions. The behaviour of the oxide is determined by the pH of the dissolution medium rather than its chloride ion content. Complex plane analysis of the anodic oxide film formed on aluminium indicates that the corrosion resistance is very high in neutral chloride solutions in comparison with acid solutions containing the same amount of chloride ions. Both the charge transfer resistance θ and the Warburg impedance caused by diffusional mass transfer attain markedly higher values in the neutral media which reflect the high passivation properties of the oxide film. It is suggested that many constructions could be coated with aluminium oxide films to protect them from corrosion, especially in marine media.

Magnetic field effects on the electrodeposition of nickel from a high pH watt's bath

Abstract: The crystal orientations of electrodeposited nickel films were determined by means of X-ray diffraction analysis. The quantity of electricity used in the deposition was 50 C cm-2. The current density ranged from 0.5 to 10 A dm-2. The magnetic flux density ranged from 0 to 0.15 T. The crystal orientation of the deposited films is affected by the magnetic fields; the effect tending to a limit as the magnetic flux density is increased. The orientation does not change greatly with the magnetic pole on the cathode side. These effects are attributed to the Lorentz force on the ions which causes visible convection in the electrolyte and to the magnetic properties of nickel.

Voltammetric studies on glassy carbon electrodes I: Electrochemical behaviour of glassy carbon electrodes in H2SO4, Na2SO4 and NaOH media

Abstract: Voltammetric investigations were carried out on glassy carbon electrodes in 0.5 M H2SO4, 0.5 M Na2SO4 and 1.0 M NaOH over a wide potential region from -2.5 V to +2.0 V versus a saturated calomel electrode at sweep rates up to 640 m V s-1. The results indicate three different processes. (1) In the middle of the potential region reversible surface redox processes take place. Fresh oxidation of surface carbon probably does not occur in this region. (2) In the anodic region surface oxidation as well as oxygen evolution reactions take place. This surface oxidation is probably linked with the activation of glassy carbon electrodes and hence is considered in some detail. (3) In extreme cathodic regions hydrogen adsorption takes place. The surface functional groups undergo spontaneous phase changes in both the middle and the anodic potential regions. The nature of the processes are probably controlled by pH. Preliminary studies in NaCl also indicate a substantial anion effect especially in the anodic region. A detailed discussion of glassy carbon surface behaviour is given with due consideration of earlier work in this field.

Aqueous corrosion properties of austenitic Fe-8.7Al-29.7Mn-1.04C alloy

Abstract: The aqueous corrosion behaviour of the alloy Fe-8.7Al-29.7Mn-1.04C has been investigated by the anodic polarization technique. The anodic polarization behaviour of the alloy was found to be very poor compared with that of conventional 316 stainless steel. Passivation in aqueous solutions only occurs when the pH is in the range 5.55 – 6.10.

Density and surface tension of dioctylphthalate, silicone oil and their solutions

Abstract: The surface tension and the density and their dependence on temperature between 20 °C and 80 °C are reported for the system dioctylphthalate-silicone oil (viscosity, 5 cSt) over the complete composition range.

Influence of the method of preparation on the porous structure of perovskite oxides

Abstract: The results of investigation of the textural properties of perovskite-type binary oxides corresponding to the formulae LaMeO3 (Me ≡ Fe, Co, Ni, Cr) and La0.85Me′0.15CoO3 (Me′ ≡ Ca, Ba, Ce) are presented. The texture of the samples was studied by physical adsorption and capillary condensation of nitrogen, and is discussed in relation to the method of preparation of the samples.

Kinetics of the passivation of molybdenum in acids and alkali solutions as inferred from impedance and potential measurements

Abstract: The electrochemical behaviour of mechanically polished molybdenum electrodes in different acid and alkali solutions was investigated using open-circuit potential and impedance measurements. The effect of the concentration of acid (HCl) and alkali (NaOH) on the electrochemical behaviour of the molybdenum electrodes was also studied. The various kinetic parameters, i.e. capacitance, resistance and potential, controlling the mechanism of oxide film growth were monitored as functions of time. Complex plane analysis reflects the high passivation properties of the naturally formed oxide film on molybdenum irrespective of the dissolution medium. The results showed also that the passive films undergo structural changes during the course of measurements which may be attributed to a further oxidation of the passive MoO 2 film to MoO 3 . The electrode potential was found to be sensitive to variations in pH. In highly concentrated NaOH solution, the passive film is subject to continuous dissolution.

Process and engineering benefits of sputter-ion-plated titanium nitride coatings

Abstract: Titanium nitride (TiN) coatings are established in the metal cutting and metal forming industries where they enhance the operating lives of tools and offer substantial gains in productivity. Several technologies for physical vapour deposition (PVD) of TiN have been developed. Coatings, which are usually 1 – 3 μm thick, are hard, chemically inert and create low friction against mating materials. This paper concentrates on a particular PVD technology called sputter ion plating (SIP). An outline is given of the equipment and the salient features of the process; emphasis is placed on the capability to coat large mixed workloads on stationary fixtures with uniform high quality layers of TiN. The main theme of the paper is to illustrate the engineering benefits of SIP TiN coatings by reference to a number of case histories. Applications include tooling used for metal cutting, metal forming, slitting and punching; applications are also included where the performances of bearings and fasteners are improved.

Schiff bases as corrosion inhibitors of copper and its alloys in acid media

Abstract: The inhibiting action of some Schiff bases on the corrosion of copper and its alloys in hydrochloric and sulphuric acid solutions has been studied. The Schiff bases were synthesized by reacting salicyl aldehyde with aliphatic or aromatic amines. The behaviours of electrolytic copper, Al brass and cupronickels (90wt.%Cu-10wt.%Ni and 70wt.%Cu-30wt.%Ni with 0.4 wt.% Fe) have been examined. The aggressive media were provided by 5% solutions of hydrochloric acid and of sulphuric acid in the temperature range 35 – 60 °C and 40 – 80 °C respectively. Weight loss determinations and the recording of anodic and cathodic polarization curves were performed. The Schiff bases prepared from aliphatic amines stimulate copper corrosion, whereas those obtained from aromatic amines inhibit the corrosion. In both the acid solutions considered N -2-thiophenylsalicylidenimine was particularly efficient. The inhibiting action is mainly exerted on the cathodic reduction reaction. The effect of the various Schiff bases is explained on the basis of their stability and by considering the influence of the different substituents in the aromatics rings.

4-amino-3-substituted-5-mercapto-1,2,4-triazolines as inhibitors for the acid corrosion of steel

Abstract: Corrosion inhibition by 4-amino-3-H-5-mercapto-1,2,4-triazoline and some related compounds on the dissolution of mild steel in sulphuric acid solutions was measured using chemical and electrochemical methods. Polarization curves indicated that these compounds act as mixed-type inhibitors, that is, both the cathodic and anodic curves were affected. The effect of the structural changes in these compounds on their inhibition efficiency has been studied. Mass-loss measurements indicated that the rate of corrosion of mild steel rapidly increases with temperature over the range 25 – 60 °C both in the absence and in the presence of inhibitors, with an apparent activation energy in all instances of about 15 kcal mol -1 .

Inhibitive action of some schiff bases and amines towards the corrosion of copper in an aqueous alcoholic medium

Abstract: Some Schiff bases and relative amines were tested as copper corrosion inhibitors in aqueous 25 vol.% ethanolic 0.1 M NaCl solution at 30 °C. Polarization curves and weight loss measurements allowed the inhibitor efficiencies of the additives to be screened. The data showed a remarkable decrease in the corrosion rate only in the presence of N-2-mercaptophenyl-salicylideneimine (BSS) or 2-mercaptoaniline (AS). Nevertheless, better results were always obtained with the Schiff bases than with the amines. In order to study the nature of the solid products formed in free corrosion conditions or, in the case of BSS, in anodic galvanostatic conditions, their IR spectra were compared with those of synthetic complexes obtained with these organic substances and copper(I) or copper(II). The surface analysis of the specimens at the end of the immersion tests was carried out using scanning electron microscopy and photopotential measurements. Further investigation showed that the inhibition of copper corrosion by BSS and AS followed the Langmuir and the Frumkin adsorption isotherms, respectively, over a wide range of concentrations. The computed adsorption energies together with some tests on the pre-filming action of the two substances indicated chemisorption as the inhibition mechanism.

A study of the effects of variations in parameters on the strength and modulus of plasma sprayed zirconia

Abstract: Thirty three plasma spray runs of ZrO2 (with 20% Y2O3) were made in which processing parameters were intentionally varied. Strength and modulus were measured using four-point bend testing. Strength values ranging from 2110 to 7690 lbf in-2 and modulus values ranging from 2.88 to 6.80 Mlbf in-2 were observed. An elementary model was developed to predict the strength and modulus based on the powder particle heating and acceleration as well as the residual stress of the substrate and the thermal shock induced during spraying. The model explains 88% of the variation in strength values and 72% of the variation in modulus values.

Effect of temperature on the inhibition of the acid corrosion of steel by benzaldehyde thiosemicarbazone: Impedance measurements

Abstract: An impedance study has been carried out of the corrosion of steel in sulphuric acid containing benzaldehyde thiosemicarbazone as an inhibitor, over the temperature range 25 – 60 °C. The impedance data indicate that the corrosion of steel is controlled by a charge transfer process at all the temperatures examined and in solutions containing various concentrations of the inhibitor. At a critical concentration of the inhibitor (5 × 10-4 M) a high protection efficiency is maintained over the temperature range examined. This is thought to relate to a high surface coverage of a relatively compact adsorbed film, which resists desorption at elevated temperatures, unlike the behaviour evident at lower inhibitor concentrations.

Some characteristics of Ni-Co alloy powders electrodeposited from dilute sulphate baths

Abstract: Electrodeposition of Ni-Co alloy powder was carried out utilizing dilute sulphate electrolytes containing concentrations of Ni2+ and Co2+ ions within the range found in some industrial effluents. The electrolytes examined had the composition 0.0125 – 0.0750 mol l-1 NiSO4·6H2O, 0.0125 – 0.0500 mol l-1 CoSO4·7H2O, 0.23 – 1.00 mol l-1 (NH4)2SO4, 0.2 – 0.4 mol l-1 H3BO3 and 0.07 mol l-1 Na2SO4·10H2O. The effects of the bath constituents, pH and current density on some characteristics of the alloy were investigated. Increased concentration of NH+4 ion in the bath was found to increase both the cathode potential and current efficiency and decrease the nickel content of the alloy. The cathode potential decreased with increasing concentration of either of the metal ions in the bath; a corresponding increase in the current efficiency and the metal content in the alloy were also observed. An increase in pH increased the cathode potential and current efficiency without appreciably affecting the alloy composition. The catalytic activity of the alloy powder towards the decomposition of 0.4% H2O2 solution increased with increase in the concentrations of NH+4 and/or CO2+ ions in the bath and with raising the pH. The surface morphology of the alloy powder was greatly influenced by the above-mentioned variables. The structure of the alloy, as revealed by X-ray diffraction studies, was dependent on its composition.

Inhibition by magnetic fields of the growth of zinc dendrites from alkaline zincate baths

Abstract: Crystals of electrodeposited zinc films were observed directly and indirectly by metallurgical microscopy. The quantity of electricity used in the deposition was 18 C cm -2 , the current density ranged from 1.0 to 10.0 A dm -2 and the magnetic flux density ranged from 0 to 0.125 T. The temperature of the test solutions ranged from 5 to 60 °C. The growth of dendrites was inhibited by the magnetic fields. Compared with the efficiencies in the absence of the magnetic field, the cathodic efficiency increased by about 5% – 10% and the anodic efficiency decreased by about 5% – 10%.

Impurity effects on the macromorphology of electrodeposited zinc II. Causes, appearances and consequences of spongy zinc growth

Abstract: The effects of minor impurity components in zinc electrodes (metals which are more electropositive than zinc or which exhibit rather low hydrogen overpotentials) were investigated to obtain information regarding the initiation and development of the spongy growth of zinc and its suppression in the electrowinning of zinc from binary chloride systems. Several types of metallic impurity result in the appearance of spongy zinc: (a) impurities such as copper which grow independently and locally and further perturb the growth surface; (b) impurities such as nickel and cobalt which are codeposited as alloys with zinc and, after attaining a critical content in the growth surface, affect either the mossy growth or dissolution of zinc with the evolution of hydrogen; (c) ionic species such as iron which neither codeposit locally nor alloy with zinc but have a catalytic effect on the spongy growth of pure zinc; (d) impurities such as molybdate which create dense compact bright zinc deposits and suppress the detrimental effects of other impurities, and in particular neutralize the synergic effects of iron, nickel and cobalt.

The corrosion behaviour of zinc metal in acidic solutions of polyvinylpyrrolidones and polyvinylpyridines

Abstract: The effect of polyvinylpyrrolidone (PVP) of various average molecular weights ( M = 10 000, 24 500 and 40 000), poly-2-vinylpyridine ( M = 2.85 × 10 5 ) and poly-4-vinylpyridine as inhibitors of the corrosion behaviour of zinc metal in 1.0 M H 2 SO 4 solutions has been studied. It was shown using the weight-loss technique that the polymers studied impart a significant inhibiting effect on the corrosion rate of zinc metal. The protection efficiency in the presence of such polymers reached about 87% at an inhibitor concentration of 0.1 M. It was also observed that the inhibitive effect of PVP was increased by decreasing its molecular weight; the protection increased in the order: PVP (10 000) > PVP (24 500) > PVP (40 000) where the figures in paremtheses represent the respective molecular weights. The corrosion rate of zinc using poly-4-vinylpyridine as an inhibitor is lower than in the case using a comparable concentration of poly-2-vinylpyridine. From potential-time measurements it is concluded that the corrosion process is partially cathodically controlled. The results are analysed in terms of the formation of a protective film on the metal surface.

Electrochemical behaviour of the tin anode in alkaline-stannate plating baths

Abstract: In order to determine the conditions for tin plating, the anodic oxidation and passivation of tin have been investigated in sodium stannate solution in the absence and in the presence of sodium hydroxide. Both galvano-static and potentiostatic techniques were used. It was found that the passivation of this electrode is a complex process consisting of several steps. The passivation time increases with increasing stannate concentration and with decreasing current density. Addition of sodium hydroxide to the bath increases the passivation time. At low current densities and high alkali concentrations, passivation does not occur and the anode remains active and dissolves continuously as stannite. X-ray diffraction and electron spectroscopy for chemical analysis (X-ray photoelectron spectroscopy) examination confirmed the existence of tin(II) and tin(IV) oxides and hydroxides in the passive films formed potentiostatically in sodium stannate solutions in the absence and in the presence of sodium hydroxide.

On equilibrium in the Zr-H system

Abstract: Critical assessments have been made of phase relationships of the Zr-H system involving the α-ZrH region and its boundaries.

Knot stability and tensile strength of an absorbable suture material

Abstract: The tensile strengths of different gauges of two smooth-surface suture materials, the absorbable material polydioxanone (PDS) and the nonabsorbable material polypropylene (Prolene) are determined. The knotting regimen required to provide knot security in vitro and following implantation in an animal model is also studied for both materials. The tensile strength of both materials is significantly reduced by knotting and it is found that the number of throws required to ensure non-slippage of the knot under tension varied with both the suture material and its gauge. In vivo, Prolene suffered no loss in tensile strength over 56 days whilst PDS lost some 30% of its tensile strength over 28 days.

Surface roughening and dendritic growth in pulsating overpotential copper electrodeposition

Abstract: The mechanism of surface roughening and the initiation of dendritic growth in pulsating overpotential electrodeposition of metals is proposed. Good agreement between theory and experiment is obtained for copper electrodeposition.

Surface enrichment of rhodium in Pd-Rh alloys after high temperature air oxidation

Abstract: Foils of Pd-Rh alloys of two different compositions (85% Pd and 95% Pd) were heated in air in the temperature range 600–1100 °C, and their surface composition was determined by Auger electron spectroscopy and argon ion sputtering-depth profiling. Apparent surface enrichment of palladium as an oxide was observed for foils heated at or below 800 °C, whereas rhodium enrichment was found for foils heated between 850 and 950 °C. When the foils were heated at 1000 and 1100 °C, a slight (monolayer) enrichment of rhodium was observed. These changes in the surface composition can be attributed to the thermodynamic differences in the formation and decomposition of palladium and rhodium oxides and to diffusional processes in thick surface oxides formed on palladium-rich alloys.

Role of potassium ferrocyanide in electroless copper baths

Abstract: The influence of potassium ferrocyanide on the electroless copper plating rate and the structure of the deposit is investigated. It is established that, in contrast to free ferrous ions, the ferrocyanide complex, even in low concentrations, reduces the plating rate at room temperature. It is assumed that this behaviour of the ferrocyanide complex is due to the presence in the solution of some free cyanide ions. Good surface levelling is observed in electroless copper plated in solutions containing potassium ferrocyanide. This effect is enhanced by the presence of poly(ethylene glycol).

The hydrogen evolution reaction on Ni-Sn alloys and intermetallics

Abstract: Ni-Sn alloys containing 0%, 1.1%, 11.6%, 25.5%, 40.3%, 63.5%, 77.9%, 83.5%, 98% and 100% tin were prepared and examined regarding their activity towards the hydrogen evolution reaction in sulphuric acid solutions. The composition of these alloys and intermetallics was determined from the initial weight of the constituents used in the alloy preparation. The identification of various phases was carried out by X-ray diffraction analysis using ASTM standard microfiles. The surface composition of these alloys was determined by Auger electron spectroscopy for several typical cases, both before and after the cathodic polarization. The electrochemical measurements consisted of steady state potentiostatic polarization curves at various temperatures and potentiodynamic profiles. The electrochemical data deduced include Tafel slopes, exchange current densities, apparent heats of activation and potentiodynamic behaviour. On nickel and nickel-rich intermetallics, electrochemical desorption is indicated as the rate-determining step whereas the hydrogen evolution reaction appears to proceed by the initial dischange mechanism on tin and tin-rich alloys. Also the activity of nickel-rich intermetallics approaches that of nickel whereas the tin-rich alloys tend to exhibit activity similar to that of tin.

Impedance studies of the inhibition of the corrosion of mild steel in 0.1 M sulphuric acid with 10% methanol by thiosemicarbazide derivatives

Abstract: Selected substituted thiosemicarbazides and thiosemicarbazones have been studied as corrosion inhibitors for mild steel in sulphuric acid using electrochemical techniques. The impedance measurements typically gave a single relaxation or semicircle comprising the charge transfer resistance in parallel with the double-layer capacitance. Inhibitor efficiencies and the adsorption isotherms were calculated from the charge transfer resistance values, whilst the capacitance-potential diagrams provided mechanistic information on the adsorption of these types of compounds. Thiosemicarbazide and its derivatives act as mixed adsorption type inhibitors, increased adsorption resulting from an increase in the electron density at the reactive C=S group by suitable substitution of a terminal hydrogen in the hydrazino or thiamido groups of the thiosemicarbazide molecule. The substituted compounds were classified into three distinct groups according to their molecular structure and electrochemical response.

Potentiodynamic behaviour of the nickel electrode in acid media

Abstract: The electrochemical behaviour of nickel has been investigated in H 2 SO 4 and HCl solutions of various concentrations by means of the potentiodynamic method. In very dilute solutions, the low anodic current is controlled by the presence of pre-existing film. At higher concentrations the film is removed and dissolution occurs. In H 2 SO 4 solutions, pseudopassivity, primary passivity, transpassivity and secondary passivity are observed prior to oxygen evolution. In all these regions, the rate of dissolution increases with acid concentration. In HCl solutions, the anodic polarization curves exhibit a current peak followed by a passivation region; the latter may not involve complete passivity. In the active region, the uniform dissolution is controlled by diffusion processes. In the passive region, local attack in the form of pitting occurs. In both acid solutions, the cathodic and anodic polarization curves satisfy Tafel's relation.

Electrode behaviour of spinel-type Co-Fe cobaltites

Abstract: Anodic and cathodic behaviour of spinel-type Fe x Co 3− x O 4 (0 ⩽ x ⩽ 2) prepared in thin film form by pyrolysis of Fe(NO 3 ) 3 and Co(NO 3 ) 2 aqueous mixtures spread on a titanium plate was investigated. Catalytic activities for both the oxygen evolution and the oxygen reduction reactions increase with increasing cobalt content in the oxides. The iron component in the oxides inhibits oxidation of the oxides but accelerates reduction, causing changes in the surface composition.