the effect of cyanide cadmium plating bath compositions on steel hydrogenation

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  • Electrodeposition and Surface Treatment Elsevier Sequoia S.A., Lausanne - Printed in Switzerland

    213

    THE EFFECT OF CYANIDE CADMIUM PLATING BATH COMPOSITIONS ON STEEL HYDROGENATION

    V. N. KUDRYAVTSEV, K. S. PEDAN and A. T. VAGRAMYAN

    Institute of Physical Chemistry, Academy of Sciences of the U.S.S.R,, Moscow (U.S.S.R.)

    (Received June 20, 1972)

    SUMMARY *

    The effect of cyanide cadmium electrolyte main components in a wide range of concentrations (Cd 15-40 g/l; NaCN 52-157 g/l; NaOH 10-120 g/l; Ni O-O.3 g/l) on steel hydrogenation has been studied. It is recorded that at high cadmium concentration in the solution the quantity of hydrogen absorbed by the steel sharply decreases. The introduction of nickel brightener (up to 0.3 g/l) in the solu- tion leads to a significant hydrogenation increase. It is shown that a change of NaCN concentration (in most cases) (52-157 g/l) and NaOH (lo-40 g/l) in the conventional limits produces a negligible effect on hydrogen absorption. On the basis of the obtained experimental data a tentative calculation method is suggested, with the help of which it is possible to estimate the relative influence of each component of the electrolyte on hydrogenation.

    INTRODUCTION

    It is well known that significant hydrogenation and a deterioration in the mechanical properties of steel parts may occur when they are electroplated with cadmium from cyanide electrolytes. Many attempts have been made to replace these last-named by less hydrogenating, but equally effective electrochemically non-cyanide electrolytes, but none can be said to have been completely successful. So the problems of hydrogenation and hydrogen embrittlement elimination under cadmium cyanide electroplating are still very acute. In order to solve these prob- lems it is necessary to know how a change in the content of the main components in the solution influences hydrogenation. The available information relating to the influence of separate components of cyanide cadmium electrolyte on hydrogena- tion and the mechanical properties of steel is scarce, and it is contradictory. Thus, Sachs and Melbourne, in a study of bend ductility on spring steel specimens,

    * R&urn& en fran$ais & la fin de larticle. Deutsche Zusammenfassung am Schluss des Artikels.

    Electrodepos. Surface Treat., I (1972173)

  • 214 V. N. KUDRYAVTSEV, K. S. PEDAN, A. T. VAGRAMYAN

    discovered that hydrogenation during cadmium electroplating is lower from an

    electrolyte with a small content of free sodium cyanide (2.4 g/l), than from one

    with a large content of sodium cyanide (22.5 g/l). According to Sink2, however, at

    constant cadmium concentration (15, 22.5, 30 and 37.5 g/l) an increase of sodium

    cyanide in the electrolyte of from 50 to 100-125 g/l decreases rather than increases

    hydrogenation, and it is only when the NaCN concentration is raised to over

    125 g/l that hydrogenation increases. At a cadmium concentration of 45 g/l,

    too, an increase in the NaCN content causes the hydrogenation to increase.

    Cotton3 and also Geyer, Lawless and Cohen4 indicate that an increase of

    cadmium concentration in the electrolyte leads to the lowering of hydrogenation.

    The data obtained by Sink2 show that at NaCN concentrations of 100, 125 and

    150 g/l an increase of Cd content in the electrolyte from 15 to 30 g/l results in a

    decrease of hydrogenation, while further increases in concentration lead to an increase of hydrogenation.

    Dingley, Bednar and Roger& present data demonstrating a significant

    influence of alkali on hydrogenation during cadmium electroplating processes. They believe that the origin of hydrogen embrittlement is mainly connected with

    the unstable nature of cyanide electrolytes, which is due to the low alkali concentra-

    tion in conventional cyanide electrolytes. To diminish hydrogenation it is recom-

    mended that the alkali content in cyanide cadmium electrolytes be increased so

    that the OH- and the CN- ion concentrations are equal. However, it should be

    noted that the comparative compositions of stable and unstable electrolytes,

    given by the authors, had unequal cadmium content. In stable electrolytes the

    cadmium concentration was higher5, which as some other authors believe3,4

    exerted a decisive influence on hydrogenation.

    To improve the quality of the deposit it is recommendeda, ! that a nickel salt

    additive be introduced into the cyanide electrolyte. The effect of such an additive

    on hydrogenation has not been studied.

    The present paper is devoted to the study of the influence of the concentra-

    tion of cyanide cadmium electrolyte main components on steel hydrogenation.

    EXPERIMENTAL PROCEDURE

    To investigate hydrogenation a direct method was used for determining the quantity of hydrogen adsorbed by the steel base during electroplatinglo.

    The procedure used was as follows. Steel specimens were chemically stripped

    of Cd deposit in 40-50% NH,NO, solution, cooled with ice to 5-10C, washed in distilled water, degreased in acetone and then placed in the device for vacuum

    extraction. The content of electrolytic hydrogen in steel was estimated at 400C and residual pressure of 1O-6 mm Hg.

    The specimens used throughout this investigation had been produced from a quenched spring steel Type Y-8A of the following composition (%): C 0.8; Si 0.2;

    Electrodepos. Surface Treat., I (1972173)

  • STEEL HYDROGENATION 215

    Mn 0.22; P 0.018; S 0.02; Cr 0.15; Ni 0.12. The Rockwell C hardness was 50.

    This steel was chosen on account of its wide use in industry, inclination to hydrogen

    embrittlement, good reproducibility of results (the value of relative error being not

    more than 5-10%) and, also, because of the constant and minimum content of

    metallurgical hydrogen in the steel (at an extraction temperature of 400C it does

    not exceed 0.05 cm3/100 g). The specimens used in the estimation of hydrogen in

    steel after cadmium electrodeposition were 90 x 8 x 0.3 (mm) in size.

    The main plating solutions were prepared from Cd0 and NaCN by way of

    their dissolution in distilled water. NaOH or Ni (in the form of NiS0,.7H,O salt)

    was added to the main solutions in sufficient quantities. All the salts were chemi-

    cally pure. Prior to the experiment the solutions were subjected to pre-electrolysis

    (at c.d. = 0.75 amp/dm2) for a time sufficient to allow the passage of at least 5

    amp-h per 1 1 of the solution.

    Cadmium electrodeposition was performed in 0.8 I of bath in a thermostatic

    glass cylindrical cell at 20C without agitation, the electroplated area being 15 cm2.

    The anodes were made of cadmium type Cd-00. To avoid anode passivation in

    the process of electrolysis the anodic current density was not more than 1.5 amp/

    dm2. Cadmium current efficiency was determined with the help of a cupric coulo-

    meter. From time to time the electrolytes were filtered and analysed to detect any

    changes in composition and, where necessary, a content correction was made.

    After each correction the solution was again subjected to pre-electrolysis (cd. =

    0.75 amp/dm2) for a time sufficient to allow the passage of at least 1 amp-h per

    1 1 of solution.

    Prior to the electroplating the specimens were degreased in alkaline solution

    of the following composition (g/l): Na,PO, 30; NaOH 10; Na,CO, 30; OP 7 = 3

    at 40C in an ultrasonic field. The degreased specimens were thoroughly rinsed in

    hot running water and then in distilled water, and electroplated with cadmium,

    the thickness of the deposit being 0.4 mil in all experiments. Prior to cadmium

    electrodeposition the degreased and rinsed specimens were kept in cyanide electro-

    lyte for 40-60 set in order to improve the adhesion of the deposit. No other pre-

    plating operations which could lead to additional hydrogenation of steel were

    conducted. The influence of sodium cyanide (total and free) on steel hydrogena-

    tion was studied in the solutions with constant cadmium concentration (Table 1).

    Solutions containing 104.8 g/l NaCN and 15 and 30 g/l Cd were used for

    the study of Ni and alkali influence on hydrogenation.

    EXPERIMENTAL RESULTS AND DISCUSSION

    1. E#ect of sodium cyanide

    The dependences of steel hydrogenation on total cyanide concentration

    during electroplating from electrolytes with cadmium contents of 15 and 30 g/l

    Electrodepos. Surface Treat., I (1972/73)

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