Development of the High Speed Black Electroplating Process

DEVELOPMENT OF THE HI GH SPEED BLACK ELECTROPLAT I NG PROCESS Katsushi Saito Nippon Steel Corp., Kimitsu R & D Laboratory Chiba Kimitsu city ,Japan

ABSTRACT High speed black electroplating process have been developed . The black coatings are deposited on cathode surface by electrolysis in acid solution containing Znzt,NiZ+,Cr3f and NO,- . Quantities of electricity to obtain the black surface are only 2 X lO’to 3 X lo3 coulomb/mz. The black coatings are mainly composed of hydrated oxides , metallic Zn and Zn-Ni alloy.The blacked Zn-Ni alloy plated steel sheet with chromate and resin- silica coat has exellent properties ,for examplescratch resistance ,corrosion reistance , adhesion and oeldabilit? .

1. Introduction Many processes1’ ’’ related to the biack electroplating are disclosed , however the high speed electrodeposition process applicable for the continuous electrogalvanizing lines has not been established. The fig.1 isthe illustration of the new black electroplating steel sheet and a conventional prepaint steel sheet.

The left figure is a schematic cross Resin . section view of the new black electroplating steel sheet (Inorganic type). Chemjcai Black soatlng7 The inorganic type is composed of Ni-Zn plate 3 micrometers in thj.ckness of Zn-h‘i alloy plating , about 0.3 micrometers Steel sheet - in thickness black electroplating ,very thin chromating and 1 micrometer in Inorganic type prepsint type thickness of the resin-silica coating . The right one is a view of Fig.1 Schematic diagram of black coated steel sheet. a prepaint steel sheet (Prepaint type). Table 1 shows the comparison of the each black coated electrogalvanized steel sheet. The prepaint type has exellent appearance ,pressformability and corrosion resistance, but poor finger print proof ,scratch resistance and spot weldability. On the other hand, the inorganic type shob-s opposite performance to prepaint type . The performance difference between two types depend on coating structure. Table.] Comparison of the black coated steel. sheets

black typo Inorganic black type

Very good > Good > No-good 1

643 2. Experimental method A sheet of steel is electroplated in Zn-Ni sulphate solution and water rinsed then black electroplated ,chromated ,finaly acrylic resin-silica coated and baked at 12OC. The black coatings are electrodeposited in sulphate solution by a vertical type plating cell with Pb-Sn anode as shown in Fig.2 .Lightness value of the coatings are measured by the color difference meter as Lab. The black coatings are investigated by SEN , X-ray (on steel) and others .

Experimental method

0.80 i100 x ZOOmm

IZSNi-Zn alloy plating: ZOgIm' Zinc plating : 2op/,::' r-7

Electrolyte p-

3. corrnion ,Cfilt."C. 4. Scrwh feiiiianc( Fig.?. The experimental vertical 3. Result 3.1 Black plating solution Nickel oxides [NiO and Ni,O,] are black compounds . 'n'e undertook studies. of deposition methods of these compounds electrolytically on steel. We found that oxidj.zing ion and zinc ion with nickel ion are very effective to deposit the black coatings. This is a characteristic point of this process. HaKO,is an exellent compound for use as an oxidizing agent in this process. The relation between lightness value of the coatings and NaNOO, concentration are shown in Figure 3 . The valuahle black appearance needs lightness value below 7.0 . NaNO, reduce lightness value of the coating and its concentration should be maintained at more than 4 grams per liter . Higher current density is effective to reduce lightness value. Coating with a acrylic resin-silica significantly reduces lightness value. Fig.4 shows zinc ion effect on lightness value of the coatings. Because o€ a great deal of hydroKen gas generated at cathode in the solution without zinc ion ,low lightness value coating could not be deposited in a short time* Zinc jon more than TO g/D as ZnSO,*TH,O is neccessary in this process. n ,,OK = 2 X I 01Afm2 2 X 10%f m2 c ._ I,_Without resin COBL

0 2 4 6 8 0 10 20 30 NO,- concentration (g/L) Zn" concentration (gf!) Fig.3 The effect of KO,- on L-value Fig.4 The effect of NO,- on L-value 3.2 Electrolytic condition Relation between electrolytic conditions and lightness value without resin coatings are shown in fig.5 as solid line with open marks .It needs only 30 coulomb/dm' to deposit black coating below lightness value 20. Preferable current density ranges are within 20 to 40 A/dmz. Therefore neccessary time is only 1 second . Solid line with solid marks in fig.5 shows the black coating weight deposited. The coating weight of the black coating is about 1 g/mZ . The effects of current density on lightness value are shown in fig. 6 . In this experiment we used solutions composed of different concentrations of zinc ,nickel and chromium ion. Lightness value of the coatings electrodeposited under low current density are high and unstable. Whereas lightness value of the coatings electrodeposited under high current density is low and stable .

I 0 I 2 3 4 5 10 15 20 25 30 Ouanuty of electroricity ( x 10'Cfm') Current density ( X IO' Aim')

Fig.5 Relation between coulomb and Fig.6 The effect of current density on L-value of the coating the black coatings 3.3 1:mprovement The above method did not substantially improve performance of black electroplated steel sheet especially corrosion resistance and adhesion However, we found that the addition of small amounts of trivalent chroeium ion improved adhesion and corrosion resistance of the black electroplating s tee1 sheet.

645 Fig.7 shows the effect of added 7 trivalent chromium ion on corrosion z"" 35g/t NI'* 452/t NO>- 3.65!! resistance evaluated by salt spray 'O0-, ~~t,~~=~x~~~~/m? fog test. Chromium hydroxides such SST. 168hrs. en '."~ {DK=ZX lo'A/m' as Cr(OH), codeposited in the coatin5 - Y:~< did improve the performance. 60 - ;I \. 3.4 Typical Performance L. o?r/DK=3 X IO' A/m' - ", (1) Adhesion +a\ 40 ".. .i ", We evaluated by cellophane taping g 20. after erichsen 8mm forming and lmm \& ~ -u ...... -- ...... " 0 cross cutting; Adhesion of the 0 0.2 0.4 0.6 0.8 1.0 black Zn-Ni electroplating steel Cr3+ concentration sheet was excellent. Fig. I Effect of Cr'+on corrosion resistance (2) Scratch resistance Scratch resistance is evaluated by commercial scratch resistance machine which evaluated minimum weight injury by a sapphire tip.0.l mm in dia. It depends on base metal hardness(fig.8). The hard Zn-Ni plating is exellent . (3) Weldability The black electroplating steel sheet is weldable .Fig.9 shows suitable current range under spot welding. Zn-Hi plated steel sheet has wide current range(6.5-12.0 KA). Current ranges of the black coated Zn-Ni plated steel sheet depending on pH in the solution.

'I..,,,., :.' ,,,<,~,, 1 I,l .Shec;3 0.8mmj Sapphira lip pH=3.5 Tip lace 4.0" O.1"d Electrosalvanized sted rZ3.'Cycles IO .. .. 1 -4,5fistick , Electrode Iorcc, Z00W I: Black Prepsinted E G I , . , 4 /50 100 150 200 11 Welding current (KA) ~icrovicken hadnes (Hut Fig.8 Relation between scratch resistance Fig.9 Weldable current ranges of the and base metal hardness black plated steel sheet

3- 3- Structure of the black coatings deposited are investigated . Fig.10 is the X-ray analysis of the powder removed and the coatings deposited on steel sheet . The coatings are composed of metal and hydrated oxides. The coatings deposited under higher current densjty and lower pH solution include higher metal content than the coatings under lower current density and higher pH solution . This is also observed by glow discharge spectroscopy analrsis(GDS). Fig.11 shows intensity ratio Oxgen/Zinc of the deposited coating in depth. 4

646 The coatings deposited under high current density have the lower O/Zn ratio . Surface of the black electroplated. coatings are shovn in photo.1 observed by a field emission scanning electron microscope (FE-SEM). The black coating is fine and has needle-like crystals .

DK= I X IO’CJm‘

DK = 1 x 10’Alm’

0 50 100 , 50 IOD 0 2 4 6 8 10 12 14 16 18 23 Sputtering time lrec.) pH=Z.Z pH=3.5 Fig.10 The X-ray analysis of the black coatings Fig.11 GDS analysis of the on steel black coatings in depth

DK20 A/dmz 0 couIomb/dmz DK 20A/dmz coulomb/dmz

Photo 1. FE-SEM observation of the black coatings on Zn-\1 plated steel sheet 5

647 4.Discussion I) Black appearance , The black coatings are composite plating including oxides and metals and the degree of blackness is proportional to Nj content in the coating . It is well known some nickel oxides are black . This is one reason why coatings are black Another reason is relation to surface morphology The coatings such . ._ . as needle-like is easy to absorb light. 2)Deposition of the composite plating The NOJ- is reduced with ZnZt and NiZt and solution pH is also increased at cathode surface . Therefore deposition of insoluble hydroxides are possible with metal platings . However the consumption of NO,- is a little and NO,- is little in the solution It is considered that NO,- is regenerated by oxidizing at anode.

5. Conclusion (1)The.. black coatings are deposited on cathode surface by electrolysis in acid solution containing Zn",NiZ+,Cr'+ and KO3- .Quantities of electricity to obtain the black surface are only 2 X 10ato 3 X lo3 coulomb/mz. (2)The black coatings are mainly composed of hydrated oxides , metallic Zn and Zn-Ni alloy. (3)The blacked Zn-Ni alloy plated steel sheet with chromate and resin-silica coat has exellent properties,for example scratch resistance ,corrosion resistance adhesion and weldability .

The black electroplating steel strips have been produdd in a continuous electrogalvanizing line with Some $mprovements. Fig.12 is a schematic diagram: of the electrolytic cell for black plating in the continuous electrogalvanizing line. Electrolyzing is only executd..'in the up Pass section in order to obtaln high current density electrolyzing . The predipping in the down pass section improve uniforaity of the black coatings.

Fig.12 A electoplating cell for the black electroplati.ng in the continuous E.G.L. 6. Refference 1) E.Barth, R.Kamme1 , U. Landau , .I. Osterwald and M. !layer ; Inter Finish '82 (1964) , p85 -- 2) M.S.E.Abdo and T. Sahhf : Plating and Surface Finishing , 13 ,(1966) 6 ,p108 6

648