Effect of additives on nickel-phosphorus deposition obtained by electroless plating:Characterization and corrosion resistance in 3%(mass) sodium chloride medium

N.M’hanni,T.Anik,R.Touir,,,M.Galai,M.Ebn Touhami,E.H.Rifi,Z.Asfari,S.Bakkali

1 Advanced Materials and Process Engineering Laboratory,Faculty of Sciences,Ibn Tofail University,BP 133,14 000 Kenitra,Morocco

2 Regional Center for Education and Training Professions (CRMEF),Kenitra,Morocco

3 Laboratory of Molecular Engineering Applied to the Analysis,IPHC,UMR 7178 CNRS,University of Strasbourg,ECPM,25 Street Becquerel,67087 Strasbourg Cedex 2,France

4 Laboratoire de Chimie Analytique et Moléculaire/LCAM,Université Cadi Ayyad,Faculté Polydisciplinaire,Sidi Bouzid,B.P.4162,46 000 Safi,Morocco

Keywords:Electroless plating Ni-P coating Organic compounds Characterization Corrosion Reaction kinetics

ABSTRACT Nichel-phosphorus(Ni-P)coatings are deposited on mild steel by using an electroless plating process.The effect of three calix[4]arene derivatives,namely tetra methyl ester-P-tertbutyl calix[4]arene(Calix1),tetra acid-P-tert-butyl calix[4]arene(Calix2)and tetra methyl P-tert-butyl-thicalix[4]arene ester(Calix3)on the deposition rate,the deposit composition,and the morphological surface was investigated and the study of growth mechanisms has delivered useful information about the surface properties of deposit.It is found that these additives modify the deposition rate and the nickel crystallization process.In fact,the Calix1 and Calix3 act as an accelerator,while Calix2 acts as an inhibitor for the nickel electroless.Furthermore,it is shown that the chemical bath is more stable with calix[4]arene derivatives addition and the obtained deposits are compact and adherent.It is observed also that the nickel content increases with additives.On the other hand,the X-ray diffraction showed that the orientation peaks are intensified at{1 1 1}in the presence of Calix2,confirming obtained results of EDAX spectrum.The cyclic voltammetry revealed that the tested additives strongly influence the cathodic process and slightly affect the hypophosphite oxidation.Finally,it is found that these compounds improve the anticorrosion efficiency of Ni-P coating on the mild steel substrate in 3%(mass)NaCl,where its polarization resistance increases with Calix2 and Calix3 addition.

1.Introduction

Nickel-phosphorus depositions are widely used for steel and numerous other materials finishing[1].In fact,these coatings offer enormous benefits like wear and abrasion resistance,good corrosion resistance and hardness [2].Electroless nickel deposits are widely used such in automotive [3] and aeronautical industries[4].Usually,the deposit on the material surface can be elaborated via a chemical autocatalytic technique[2,5]or sol–gel coating process[6–8].However,at high temperatures,autocatalytic bath may become critical and lead to sudden decomposition.To improve its stability,many stabilizers like thiourea and lead acetate [9],are used and some of them act by adsorption on the catalytic surface[4,5,10,11].Other compounds such as accelerators [12,13],complexing agent like organic acids or their salts [14] and buffers[12]are also added to the plating solution to improve the bath performance.These compounds influence the kinetics and mechanism of the crystal growth process.They also affect the structure,the morphology,the physical properties,the purity and the anticorrosion proprieties of the deposit [12].

Calixarenes derivatives have wide applications such as ion extraction reagents [15,16],electrochemical and fluorescence sensors,[17] and in medicinal chemistry [18].These compounds are defined as a macrocycle formed by four phenolic units substituted or not,and related by a methylene bridge.They are shaped like a vase or chalice which makes them able to capture molecules.In addition,the strong negative bias generated by oxygen and/or sulphur atoms enable the calix[4]arena to attract and retain the cation in its empty space at the centre.So,a stable complex is then obtained,which can be dissociated to recover the cation [12].The complexing properties can be modulated by varying the substituents grafted at the top and bottom of the calix[4]arene [15].

On the other hand,the chloride ions are some of the greatest rich and aggressive corrosive media for Ni deposit and its alloy[1,6,19,20].Thus,it is found that the corrosion resistance of Ni–P alloys is better than pure Ni [19].Furthermore,this resistance depends on phosphorus content and on the nature of the surface treatment [20].

So,in this work,we have investigated the effect of three calix[4]arenes compounds on electroless Ni-P deposits structure,morphology,coating rate and deposit composition using electrochemical measurements,SEM/EDAX analysis and X-ray diffraction.The effects of the additives on the anticorrosive performances of Ni-P deposits were investigated and evaluated in 3% (mass) by electrochemical methods.

2.Experimental

All solutions used in these experiments were prepared from analytical grade reagents and distilled water.The obtained Ni-P alloy coatings were prepared using the components offered in Table 1.The used calix[4]arene derivatives were synthesized according to literature[21],and their molecular structures are presented in Table 2.Thus,such as indicated in the introduction section,the calix[4]arena is ready to capture cations in its empty space at the center,due to its vase shape and the strong negative bias generated by oxygen and/or sulphur atoms.For this in order to enhance its potential acceptor–donor,the hydrogen atom in Calix 2 (acid function) was substituted by inductive donor -CH3group to obtain Calix 1 (ester function).In addition,it is indicated that the richness of these calix[4]arene is that they have many functional sites,giving access to real bunches of ligands arranged around a single central processing unit.For this,the methylene bridge in Calix 1 was substituted by–S atom to obtain Calix3(ester function with sulphur bridge).However,before each experiment,these compounds were dissolved in dichloromethane solvent(5 ml) and added with a small concentration to the electrolytic bath.The temperature bath was set at(358±2)K using isothermal cell and the pH was adjusted in the range from 3.5 to 7 using NaOH or acetic acid.In addition,the plating solution was continuously stirred with a magnetic bar (500 r?min-1).

Table 1 Chemical composition of the plating bath (pH=5.5±0.1; T=(356±2) K)

Table 2 Names,molecular structures,abbreviation and molar mass of the used additives

The electrolysis cell (Pyrex?) closed by cap with five apertures.The used mild steel samples(2 cm×2 cm×0.1 cm)were prepared such as presented in our preceding work [12].So,these samples were polished with emery paper (from 200 to 1200 grade) to remove the oxide layer.Then,they were cleaned with acetone to dislocate the adsorbed impurities.Afterwards,they were immersed in dilute sulphuric acid(10%)for two seconds to engrave their surface,and they were rinsed with distilled water.After these preparation,the sample was weighed using an electronic balance and was immersed in the deposit bath for one hour of immersion time.The deposition rate was calculated from the weight-gain as follows:

where ωdis the deposition rate(μm?h-1),mfis the final mass of the coated electrode (kg),miis the initial mass of the uncoated electrode (kg),Ais the electrode surface (m2),Δt=0.5 h is the immersion time and ρ is the nickel density (kg?m-3).

The activation energy was determined according to the Arrhenius law:

wherekis pre-exponential factor,Eais the activation energy(J?mol-1),Ris the universal constant gas (J?mol-1?K-1) andTis the temperature (K) of the solution.

The surface morphologies and the chemical composition of the obtained coatings were carried out by the scanning electron microscopy(SEM)instrument(JOEL JSM-5500)coupled with EDAX analysis.Their microstructures were made by an X-ray diffraction analyser (XRD) (Philips Xpert) with radiation Cu Kα(λ=0.154 nm).In addition,to estimate the degree of crystallinity,DC,of the obtained deposits,XRD peak was de-convoluted using PsdVoigt1 function from ORIGIN PRO 2015 software,and theDCwas calculated as follows:

whereACrepresents the area under crystalline peak andAAis area under amorphous region.

However,the cyclic voltammetry measurements for the coating process were performed using the glassy carbon,the platinum plate and the Ag?AgCl as working,auxiliary and reference electrodes,respectively.All given potentials areversusAg?AgCl (reference electrode).So,it was made in the range of 1000 mV to-1200 mV with a scan rate of 10 mV?s-1.

The anticorrosion propriety of the obtained coatings in 3%(mass) NaCl solution was tested using current–potential curves with a scan rate of 0.2 mV?s-1and electrochemical impedance spectroscopy technique.This last technique was supported out at the open circuit potential(EOCP)using a transfer function analyser,over the frequency range from 100 kHz to 10 MHz with 10 points per decade and with an applied amplitude of AC signal 10 mVrms.The obtained data were fitted using Z-view program with the appropriate electrical equivalent circuit.

All electrochemical measurements were made using Potentiostat/Galvanostat/VoltaLab PGZ100 monitored by a personal computer and Voltalab 4.0 software.

3.Results and Discussion

3.1.Effect of additives on the coating deposition rate

Fig.1 shows the effect of the calix[4]arenes derivatives concentration in the range from 10-6mol?L-1to 10-3mol?L-1on the deposition rate.In the absence of calix[4]arenes derivatives,it is noted that the bath doesn’t stable,the obtained coating is dark brown and the deposition rate is 11 μm?h-1.So,in their presence,the bath stability and the obtained deposit quality improve.It is obtained also that the deposition rate increases with Calix1 or Calix3 addition,while it decreases with the presence of Calix2.In fact,it is known that the additives can act as accelerators[22]or inhibitors versus of their concentrations in the coating bath[22].In addition,the nature of the functional groups in the calix[4]arenes derivatives determines their mechanism action.So,the -S-Sgroup favors polarization of the P-H bond in hypophosphite ions and the increase in the number of active centers,which affect the surface reaction.An introduction of and -OCH3groups promotes inhibiting the formation of surface complexes.Throughout their concentration range,Calix1 or Calix3 acts as an accelerator while Calix2 acts as an inhibitor.From Fig.1,at the concentration of 10-4mol?L-1,10-6mol?L-1and 10-4mol?L-1were selected as the optimum concentrations of Calix1,Calix2 and Calix3,respectively.Their corresponding deposition rates are 13.86 μm?h-1,8.35 μm?h-1and 7.81 μm?h-1.Furthermore,it is noted that the bath doesn’t stable and the obtained coating is dark brown for other concentration.

3.2.Apparent activation energy

Fig.2 shows the Arrhenius plots of Ni-P deposition rate obtained in the absence and the presence of calix[4]arenes derivatives at the optimum concentrations.From this Figure,it is observed that the deposition rate increases linearly withT-1×103for all plating solutions.So,the obtained apparent activation energy values decrease from 71.93 kJ?mol-1in the absence of additives to 40.73 kJ?mol-1,50.58 kJ?mol-1,and 53.21 kJ?mol-1in the presence of 10-4mol?L-1of Calix1,10-6mol?L-1of Calix2,and 10-4mol?L-1Calix3,respectively.This decrease ofEaindicated that the presence of these compounds favours the surface reaction(polarization of the P-H bond in hypophosphite ions for example as indicated above).These finding confirm also the gravimetric results since the deposition rate increases with the decrease in the apparent activation energy (where 13.86 μm?h-1,8.35 μm?h-1and 7.81 μm?h-1for 10-4mol?L-1of Calix1,10-6mol?L-1of Calix2 and 10-4mol?L-1of Calix3,respectively).

Fig.1.Deposition rate of Ni-P deposit versus.calix[4]arenes derivatives(pH=5.5±0.1; T=(358±2) K).

3.3.Deposits characterizations

3.3.1.Surface morphology

The obtained SEM micrographs of deposits in the absence and presence of calix[4]arenes derivatives are offered in Fig.3.The micrographs showed that the Ni-P deposits have a nodular aspect and are relatively homogenous.In addition,it is noted that the grain size decreases in the presence of Calix1,Calix2 or Calix3.This can be mainly attributed to the stability role of these additives,which they can stabilize nucleation and grain growth.However,it is known that the high over-voltage,which is the main problem of deposition process at the early stage,is removed by the initial catalytically-active Ni nuclei formation or by the use of stabiliser.It also observed that the surface of the obtained coatings in the presence Calix1 or Calix3 presents some micro-pore,which can be attributed to the hydrogen evolution.In addition,the presence of this micro-pore can be occurred by acid pickling on the alloy surface to replace the natural loose oxides/hydroxides film with an adherent composite layer.

3.3.2.Chemical composition and microstructure

The EDAX analyses of the obtained deposits in the absence and presence of each additive are presented in Fig.4.It is obtained that the deposits consist mainly of nickel peaks with a small phosphorus peak.In addition,it is shown from Table 3 that the low phosphorus content 2.39% (mass),was obtained from the bath with Calix2.So,in the presence of the Calix1 and Calix3,the percentages of phosphorus are quite similar 3.2%and 3.3%(mass),respectively.In fact,the increase of the nickel content in the presence of the additives can be elucidated by the fact that,calix[4]arenes additives complex is combined mostly of the free Ni2+ions.Only those complex combined ions adsorbed on substrate surface,and catalyzed by nickel atoms to deposit from the bath on the substrate surface.In addition,it indicated that some additives inhibit the bulk reaction and exert the catalytic effect on the surface reaction in a certain concentration range,due to the formation of complexes which results in a decrease in the polarizing ability of some atoms and the formation of the less active surface [23].

In contrast,Fig.5 shows the XRD diffraction patterns of Ni-P deposits without and with the various calix[4]arenes additives.It is shown that all the patterns show a peak at about 2θ=44.5°which is the characteristic peak of (1 11) crystal face in Nickel crystal [24].This peak is wider in the absence of additives than in their presence.

Fig.2.Arrhenius plots of Ni-P deposit rate without and with the optimum concentration of the additives (pH=5.5±0.1).

Fig.3.Micrographs of the obtained Ni-P deposits(a)without and with(b)10-4 mol?L-1 of Calix1,(c)10-6 mol?L-1 of Calix2 and(d)10-4 mol?L-1 of Calix3(pH=5.5±0.1;T=(358±2) K).

Additionally,the highest and lowest intensities are recorded in the presence of Calix2 and without additives respectively.Thus,in the presence of Calix1 and Calix3 the intensities of the peaks are nearly the same.It was known that the lower is the phosphorus content;the higher is the intensity of the peak [25].For this,the amount of phosphorus in deposits is in the following order:without additive>Calix3 ≈Calix1>Calix2.This is in agreement with EDAX analyses (Table 3).Further insight into the microstructure of the Ni-P coating is given by the de-convolution of XRD (1 1 1)peak (Fig.6).From this figure,the peak is the cumulative peak of a broad peak and a sharp peak.It is noted that in the absence of additive,the contribution of the broad peak to the recorded peak is higher than in their presence.In addition,Table 4 gives the degree of crystallinity values of each Ni-P deposits.It can be clearly observed that the crystalline character follows the order:without additive

Table 3 Chemical composition of Ni-P deposit without and with additives(pH=5.5±0.1;T=(358±2) K)

3.3.3.Cyclic voltammetry studies

To elucidate the effects of calix[4]arenes derivatives addition on the different reactions,occurring during the electroless deposition process of Ni-P,cyclic voltammetry was performed.The obtained results are shown in Fig.7.It is remarked that the obtained voltammograms show a nucleation loop and reveal the existence of several peaks in the negative scan:the cathodic reaction begins at-0.083 V which can be associated with the reduction of HCit2-[28].In fact,at pH=5.5,the HCit2-ion is the predominant specie in solution [29].After that,a single peak K was observed which can be attributed to the reduction reactions of Ni2+,H2PO2-ions and H+protons according to the following reactions [30]:

In the positive scan,three peaks were observed.A first small anodic peak A related to the oxidation of hydrogen atoms adsorbed on the electrode surface[31].A second peaks B which is attributed to the oxidation of hypophosphite ions [23].A third wide peak C was observed.

In order to elucidate the nature of this last peak its deconvolution was making.So,the de-convolution of the peak C was presented in Fig.8.It is revelled from this figure,the existence of two peaks C1 and C2 which are attributed respectively to the deposit dissolution.Some researchers have mentioned that the existence of two peaks during the dissolution of the coating is linked to its nature which includes crystalline (rich in Ni) and amorphous (rich in P) phases [31].However,such a feature is also indicated in the oxidation of pure nickel electrodeposited [32].It can also be seen that the presence of calix[4]arenes derivatives increases both the cathodic and anodic current densities of peak K and peak C in the following order:Calix1>Calix2>Calix3.In addition,the presence of Calix1 and Calix2 shift the potential of the deposits dissolution to more anodic value.This finding can be interpreted by that the Ni-P deposit became more resistant with the presence of additives.

Fig.4.EDAX profile for the obtained Ni-P deposits(a)without and with(b)10-4 mol?L-1 of Calix1,(c)10-6 mol?L-1 of Calix2 or(d)10-4 mol?L-1 of Calix3(pH=5.5±0.1;T=(358±2) K).

Fig.5.XRD patterns for the obtained Ni-P deposits (a) without and with (b) 10-4 mol?L-1 of Calix1,(c) 10-6 mol?L-1 of Calix2 or (d) 10-4 mol?L-1 of Calix3(pH=5.5±0.1; T=(358±2) K).

3.4.Corrosion resistance of Ni-P deposit

It is known that the NaCl solution is one of the most common and used corrosion medium for the electrochemical tests.It can significantly accelerate pitting corrosion of Ni-P coatings.In fact,Cl-ions have the ability to replace the water and/or oxygen molecules adsorbed on the Ni-P coating surface and form a soluble NiCl2products according to the following Eq.(7) [33]:

For this,the effect of calix[4]arenes additives used for the Ni-P deposits on the mild steel corrosion in 3%(mass) NaCl solutions using potentiodynamic polarization measurements.The obtained results are presented in Fig.9 and their extracted parameters are illustrated in Table 5.

As it can be seen,both anodic and cathodic branches do not exhibit well-defined Tafel region.Additionally,the cathodic branches of the coated mild steel do not show a diffusion region but the anodic regions have a pseudo-passivation behaviour which its interval depends to the nature of the additive.These results indicated that the Ni-P deposits change both the dissolution of metal and the oxygen reduction mechanisms.It can be also seen from Table 5 that the Ni-P deposit shifts positively the corrosion potential of the mild steel and decreases the anodic and the cathodic current densities.These findings indicate that the Ni-P coatings act as an anodic protector and improve the corrosion resistance of the mild steel in 3%(mass) NaCl solution.Furthermore,the corrosion current density decreases following the order:icorr(Calix1)>icorr(no additive)>icorr(Calix3)>icorr(Calix2).Consequently,the corrosion resistance of the Ni-P deposit increases as follows Calix1

Table 4 Degree of crystallinity of Ni-P deposit without and with additive (pH=5.5±0.1; T=(358±2) K)

Fig.6.De-convolution of XRD(1 1 1)peak for the obtained nickel deposits(a)without and with(b)10-4 mol?L-1 of Calix1,(c)10-6 mol?L-1 of Calix2 or(d)10-4 mol?L-1 of Calix3 (pH=5.5±0.1; T=(358±2) K).

Fig.7.Cyclic voltammograms of the Ni-P deposits on glassy carbon without and with 10-4 mol?L-1 of Calix1,10-6 mol?L-1 of Calix2 or 10-4 mol?L-1 of Calix3(v=10 mV?s-1;pH=5.5±0.1; T=(358±2) K).

On the other hand,to gain deeper insight into anti-corrosive behavior of the obtained deposit,electrochemical impedance spectroscopy measurements were performed on uncoated and coated mild steel,from solutions without and with additives,in 3%(mass)NaCl solution.The obtained Nyquist and Bode plots are presented in Fig.10 and their extracted parameters,using the electrical equivalent circuit presented in Fig.11,are illustrated in Table 6.

It is noted that the obtained plots were composed by two slightly depressed semicircles(two times constant in the Bode representation) where their total diameter corresponds to the polarization resistance (Rp).So,the first one loop was attributed to the adsorbed species on the metal surface (Rad,Cad) and the second loop was attributed to the double layer capacitance in parallel with the charge transfer resistance (Rct,Cdl).However,during this measurement,the constant phase element (CPE) can designate the inhomogeneity of surfaces,while in solid state it defines the inhomogeneity of the charge distribution.So,this constant was used to derive the value of the effective calculated double layer capacitance (C) using the following equation [34]:

For the former,the capacitance has the unit of F (or F?cm-2),whereas the latter involves the remaining time dimension and CPE has the unit of F?Sn-1or(F?Sn-1?cm-2).So,nis a coefficient representing the frequency dispersion of capacitive time (or surface irregularity).

The polarization resistance(Rp)gives insight into the protective capacity of the deposits since the high values ofRpvalues are associated with good anti-corrosion performances.So,the protective ability of the obtained coating follows the order:Calix2>Calix3>no additive>Calix1.

Fig.8.De-convolution of the anodic peak C for the obtained Ni-P deposits on glassy carbon(a)without and with(b)10-4 mol?L-1 of Calix1,(c)10-6 mol?L-1 of Calix2 or(d)10-4 mol?L-1 of Calix3 (pH=5.5±0.1; T=(358±2) K).

Fig.9.Potentiodynamic polarization curves of uncoated and coated mild steel by Ni–P deposit without and with the optimum concentration of additives in 3%(mass)NaCl solution.

However,it is known that the corrosion appearance was closely linked to the quality of coated surface(coated surface morphology and/or defects (e.g.porosity)).This directly affects the anticorrosion performance of coatings,indicating that the electrolyte has grasped the substrate through micro-pores in the deposit and initiated its dissolution.So,the total estimated coating porosity (Pc)relates to the ration of polarization resistance of the uncoated(Rs) and the coated surface (Rcs) as follows [35]:

It is found that the estimated porosity of the obtained coating follows the order (Table 6):Calix2>Calix3>no additive>Calix1.These results are in agreement with the calculated polarization resistance,while it is not with the SEM observations of coated mild steel in Fig.3,which can be attributed to the chemical composition of the deposit.Finally,these results confirm those obtained from current-potential measurements.

Table 5 Corrosion characteristics of electroless nickel obtained in the presence of different additives in 3% (mass) NaCl solution at ambient temperature

Table 6 Electrochemical parameters of electroless nickel obtained in the presence of different additives in 3% (mass) NaCl solution at ambient temperature.

4.Conclusions

In this study,autocatalytic deposition of Ni–P alloy is carried out using three different calix[4]arenes derivatives.It is found that the deposition rate increases with Calix1 or Calix3,while it decreases with Calix2.It is also found also that these compounds improve the bath stability.In addition,the presence of additives improves the coating quality,where the particles sizes become smaller.However,the cyclic voltammetry showed that these calix[4]arenes derivatives act on both anodic and cathodic reactions mechanisms.In fact,they increase both the cathodic and anodic current densities of cathodic and anodic reactions,with improving the deposit resistance dissolution.The XRD and EDAX analyses indicated that the phosphorus content and the structureof deposit depend to calix[4]arenes derivatives addition.Finally,the polarization curves and impedance spectroscopy diagrams showed that the Ni-P deposit in the presence of Calix2 and Calix3 have good resistance in 3%(mass)NaCl solution compared to these obtained in the absence and presence of Calix1.These results are in agreement with the calculated porosity of the coating,which is not with the SEM observations.

Fig.10.(a)Nyquist and(b and c)Bode plots of uncoated and coated mild steel by Ni–P deposit without and with the optimum concentration of additives in 3%(mass)NaCl solution.

Fig.11.Proposed electrical equivalent circuit for corrosion resistance.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank ‘‘University Center for Analysis,Technology Transfer and Incubation Expertise,Kenitra Morocco”and the ‘‘Moroccan Ministry of Higher Education,Morocco’’.We thank also ‘‘Faculty of Science,Ibn Tofail University,Kenitra,Morocco”for giving materials of all test.

Nomenclature

Asurface electrode,m2

ACarea under crystalline peak

baanodic Tafel slope,V?dec-1

bccatodic Tafel slope,V?dec-1

Cadadsorption species capacity,F?m2

Cdldouble layer capacity,F?m2

DCestimate degree of crystallinity

Eaactivation energy,J?mol-1

Ecorractivation energy,V

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icurrent density,A?cm-2

icorrcorrosion current density,A?cm-2

kpre-exponential factor

mffinal mass of the coating,kg

miinial mass of the coating,kg

nadcoefficient of homogeneity of adsorbed layer

ndlcoefficient of homogeneity of double layer

PCestimated coating prosity,%

Qcoefficient constant of CPE,F?Sn-1?m-2

Runiversal constant gas,J?mol-1?K-1

Radadsorbed species resistance,Ω?cm2

Rcscoated surface resistance,Ω?cm2

Rctcharge transfer resistance,Ω?cm2

Reelectrolyte resistance,Ω?cm2

Rppolarisation resistance,Ω?cm2

Rsuncoated surface resistance,Ω?cm2

Ttemperature,K

ρ nickel density,kg?m-3

ωddeposition rate,m?h-1

Δtimmersion time,h