S. Vitkova

COMPOSITION OF THE CORROSION PRODUCTS OF GALVANIC ALLOYS ZN–CO AND THEIR INFLUENCE ON THE PROTECTIVE ABILITY

Abstract The composition of the corrosion products of non-chromated and chromated galvanic coatings of Zn and Zn–Co(1–5wt.%) alloys in free aerated solution of 5% NaCl is described and discussed. Corrosion treatment is realized at open circuit conditions for different periods of time. The corrosion products of monolayer galvanic coatings were investigated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). As a result of the treatment the compound zinc hydroxide chloride monohydrat (ZHC)—Zn 5 (OH) 8 Cl 2 .H 2 O—was registered on Zn–Co alloys after the experimental period. ZHC supplementary increases the corrosion resistance due to its low solubility product and ensures better protection. In addition, the protective ability of mono- and bilayer galvanic systems (chromated and non-chromated) was determined using polarization resistance measurements.

Preferred orientation of electrodeposited iron crystallites

Abstract The work of formation of two-dimensional nuclei with a b.c.c. lattice is theoretically calculated, taking into consideration the forces of interaction of up to fourth neighbours. The work of formation is plotted as a function of supersaturation. The orientation axes of iron electrodeposits have been studied experimentally at various overvoltage values. Orientations along the [310] axis were obtained for the first time. The effect of potassium iodide upon overvoltage and crystal orientation was also investigated. The experimental results are in agreement with the theory of preferred orientation of crystallises in electrodeposited metals, developed on the basis of calculations of the work necessary for the formation of two-dimensional nuclei at various supersaturations.

Electrochemical preparation of amorphous Fe-P alloys

Optimum conditions have been established for the electrodeposition of amorphous fe-P alloys from sulphate electrolytes, containing complex-forming additives (glycine and oxalic acid) and sodium hypophosphite. It was shown that the increase of pH, current density and glycine content in the plating solution leads to a decrease of the amount of phosphorus in the Fe-P alloy. The cathodic current yield decreases with the increase of glycine concentration in the electrolyte. Small amounts of Cu2+ and Mn2+ act as brighteners. Sodium hypophosphite exerts a depolarizing effect on the alloy formation process. This effect is more pronounced at low hypophosphite concentrations and low cathodic current densities (CCD). The measurement of the cathodic potential during the deposition of the investigated alloy provides no evidence for a concentration change of phosphorus in the electrodeposited layers. No qualitative alterations of the surface morphology of the amorphous coatings studied have been established as the composition of the alloy and the plating conditions are changed.

Internal stress and magnetic properties of electrodeposited amorphous Fe–P alloys

Amorphous Fe–P alloys were electrodeposited under galvanostatic conditions at a current density of 10Adm–2 from sulfate–chlorine solutions containing β-alanine and glycine, as well as hypophosphoric acid. It was established that an increase in the deposition temperature from 50 to 60°C causes an increase in the deposition rate and a substantial rise in current efficiency and internal stress (IS). With pH increase from 2.0 to 3.0 the phosphorus content decreases, current efficiency almost doubles and there is a substantial drop in IS. It is assumed that a higher electrolyte acidity enhances hydrogen occlusion in the coating, which then desorbs from the deposit creating tensile IS. The same correlation between IS and current efficiency is observed when glycine concentration varies. A profile of residual IS is plotted illustrating the IS distribution throughout the deposit thickness. When the samples are magnetized in the coating plane, the shape of the hysteresis loop indicates perpendicular magnetic anisotropy. This is due to the columnar structure observed in scanning electron micrographs of the coating cross-sections. There is a linear dependence between IS and coercivity of amorphous Fe–P alloys deposited under different conditions.

Electrodeposition of low tin content zinc-tin alloys

Abstract Znue5f8Sn alloy coatings containing 10%–20% tin with good appearance were deposited from slightly acidic sulphate electrolytes. The effect of the concentration of tin sulphate and the cathodic current density on the composition and the structure of the alloy deposits was investigated. The partial deposition rates for Zn and Sn from electrolytes containing various brightener concentrations at a definite potential were determined. An increase in the concentration of organic additive from 1 to 5 ml 1 −1 leads to an abrupt decrease in zinc deposition rate, while the tin deposition rate decreases slowly. A further increase in brightener (organic additive) concentration does not affect particularly the deposition rate of either metal. The protective properties of Znue5f8Sn coatings were investigated by means of the Paatsch test. It is established that the corrosion resistance of Znue5f8Sn alloy deposits is better than the corrosion resistance of Zn and Znue5f8Co (0.3%–1.3% Co) coatings.

Relaxation and crystallization in electrodeposited Fe31Ni45P23 amorphous alloys

Abstract X-ray diffractometry, Mossbauer spectroscopy and DTA measurements have been used to study the relaxation and crystallization processes in electrodeposited Fe 31 Ni 45 P 23 amorphous alloys. The changes in the Mossbauer spectra and in the hyperfine field distributions due to aging at 205°C have been associated with changes in the short range ordering in amorphous state. Ni, (NiFe) 3 P and (NiFe) 2 P phases have been identified during the crystallization.

The effect of sulphur and carbon inclusions on the corrosion resistance of electrodeposited Ni-Fe alloy coatings

Abstract The effect exerted by sulphur and carbon inclusions from the brightener on the alteration of the structure of electrodeposited bright nickel-iron coatings is investigated. Different possible states of the included sulphur (in an organic chain or as a sulphide) are considered. Along with the structural changes of the deposit owing to the inclusions, the alteration of the corrosion rate in the active state in 1N H 2 SO 4 is studied.

Protective ability of chromate passive films on electrodeposited zinc-tin alloys

Abstract The protective ability of passive chromate films on electrodeposited Znue5f8Sn alloys (tin content of up to 20%) was studied using a double-cell electrochemical method according to Paatsch. Samples of the same chromate films on zinc deposits were studied for comparison. Three new compositions of chromating solution for electrodeposited Znue5f8Sn(15%) alloy are suggested and appropriate ratios of Cr VI to activating ion are proposed. Tests of the chromate films, revealing the best protective ability according to the Paatsch method, as well as of non-passivated zinc and Znue5f8Sn(15%) deposits, were carried out in a salt spray chamber. The results of the two methods show good agreement. The colourless and dark yellow films on the alloy reveal better corrosion stability than the films on pure zinc deposits. The surface morphology of the passive films and changes in the morphology after corrosive treatment in 5% NaCl were also studied.

Electrocrystallization of nickel coatings with 〈211〉 texture

Abstract Transmission electron microscopy (TEM) and X-ray structural analysis were used to investigate the structural characteristics of polycrystalline nickel electrodeposits of different thicknesses and with a 〈211〉 texture. The results show unambiguously that (111) twinning planes perpendicular to the substrate and characteristic of the 〈211〉 orientation are observed even in very thin coatings (about 60 A thick) which can be examined by TEM. When the film thickness is gradually increased to 60 μm only the size of the crystallites, the dimensions of the coherent scattering regions and the twinning probability β are altered; the structure of the individual crystallites is unaffected. The defect planes were observed at all film thickness investigated. Comparison of the X-ray and TEM data suggests that the height of the coherent scattering regions and their dimensions perpendicular to the twinning planes are almost equal. Other types of twinning defects were observed along the (111) plane oblique to the substrate, and our studies have proved that these defects are rare and are not related to the initiation of 〈211〉-oriented crystallites. The final results of the investigations are in good agreement with the hypothesis that the 〈211〉 texture initiates as a result of the presence of paracrystalline clusters oriented parallel to the substrate which contain (111) twinning planes. Data which show that some additional factors may affect the formation of nickel polycrystalline coatings with this texture are also presented.

Structure and magnetic properties of electrodeposited cobalt

Abstract The structure of electrodeposited cobalt coatings has been studied by means of quantitative X-ray methods to determine the amounts of h.c.p. and f.c.c. phases in the presence of texture, the perfection, the particle size and the microstrain. The influence of the different structural features on the various magnetic properties has been studied and the reasons for this influence are discussed.