CA1242668A - Electrolytically preplating steel with zinc-nickel prior to plating with zinc-nickel - Google Patents

Abstract

Abstract of the Disclosure The adhesion of a corrosion resisting Zn-Ni-alloy plated layer on a steel sheet is improved by pre-plating the steel substrate with a Zn-Ni layer containing Ni in an amount higher than said Zn-Ni-alloy plated layer.

Description

, Electrolytically Preplating Steel with Zinc-Nic~el Prior to Plating With Zinc-Nickel Technical Field of the Invention _ This invention relates to a process for Zn-Ni-a}loy-plated steel sheets which comprises first pre-plating steel sheets with Zn-Ni alloy containing a higher amount of Ni before effecting the principal Zn-Ni-alloy-electroplating in order to imprc,ve the adhesion of said plated layer.

Background of the Invention In recent years there has arisen a demand for Zn-electroplated st~!el sheets with higher corrosion resistance I especially from cnutomobile industry circles. In compliance with this demand, improved Zn-Ni type alloy-plating of steel sheets such as Zn-Ni-alloy plating, Zn-Ni-Co-alloy plating, etc. has been de~eloped.

Although steel sheets with such Zn-Ni type alloy electroplating are excellent in corrosion resistance in the bare state, they are inferior in the susceptibility to the phosphating (phosphate salt treatment) as the pretreatment for electrodeposition coating (electrophoretic painting).
That is, uniform dense phosphate salt crystals are not formed on the plated surface, and therefore adhesion of the electrodeposited coating is not satisfactory.

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~L2~2~6l3 We previously developed improved Zn-Ni-alloy electroplated steel sheets, said alloy containing a slight amount of Ti, Co, etc~ in order to overcome the above-mentioned problem, which is described in United States Patent No. 4,610,937. Said process comprises electroplating steel sheets in an acidic plating bath containing lO - 40 g~l (gram as atom per liter hath) zinc, 15 - 160 g/l nickel, 0.2 - lQ g/l titanium, O.l - 5 g/l cobalt and O.l - 5 g/l aluminum or 0.2 - ~ g/l magnesium, the pH of which is 1.5 ~ 2.5.

The plated steel sheets obtained by this process have remarkably improved adhesion of electrodeposited coatings (electrophoretically painted layer) and are excellent in corrosion resis1:ance in the bare state, since the plated layer contains a slight amount of Ti which is uniformly distribued in the plated layer and represents nuclei for crystalization of phosphate salts in the phosphating.

However, the adhesion of the plated layer to the substrate steel sheet in the above-mentioned plated steel sheets is not quite satisfactory. The plated layer on which a thick electrodeposited coating is applied is liable to be peeled off from the steel substrate by impulsive deformation, sincs the electrodeposited coating is dense and hard per se and is firmly bonded to the plated layer so that the latter suffers strong compressive stress.

i'!` ~` . ~ - . , ~2~i68 For instance, the plated layer on which an electrodeposited coating of more than 15 um applied is easily peeled off when tested by an impulse tester, such as one marketed by "DuPont Company''0 Thus, the above-mentioned improved Zn-Ni-alloy-electroplated steel sheet is not quite satis~actory as a substrate for electrodeposition coatingD

We proceeded with research ~or improving adheslon of said plated Zn Ni alloy layer to the steel substrate and have found that the adhesion can be improved by pre-electroplating substrate steel sheets with a Zn-Ni alloy using an electroplating bath of a speci~ic composition under specific conditions before effecting said Zn-Ni electroplating.

Disclosure of t e Invention That is, this invention provides a process for preparing Zn-Ni-alloy plated steel sheets with excellent adhesion of the plated layer c:omprising ~re-plating a steel sheet in an acidic plating bath contain:ing 7 - 38 g/l Zn and 41 -88 g/l Ni whereby the concentration ratio (Ni2+)/1(Zn2+) + (Ni2+)) is 0.70 - 0085 with an electric current densit~ of

2 - 20 ~/dm2 at a temperature between 55 and 80 C so that the resulting Zn-Ni pre-plated layer contains 12 - 87 wt ~
Ni; and electroplating said pre-plated steel sheet in an acidic electroplating bath containing 25 - 33 g/l Zn, ~1 ~ 88 g/l Ni, 0.2 - 10 g/l , ~2~2~6~

titanium and 0.1 - 5 g/l aluminum or 0.2 - 4 g/l magnesiusT., the pH of which is 1.5 - 2.5.

In the improved Zn~Ni-alloy-plating process which we previously developed, the composition of the electroplated layer which gives products of most stable quality essentially consists of 10 - 12 % by weight nickel, 0.005 - 1% by weight titanium, 0.01 - 0.5% by weight cobalt, 0.001 - 2% by weight aluminum and the balance zinc, or 8 -16% by weight nickel, 0.005 - 1% by weight titanium, 0.05 -0.5% by weight cobalt~ 0.001 - 1% by weight magnesium and .
the balance zinc.

. In accordance with this invention, the adhesion of the st.eel substrate and the above-mentioned principal Zn-Ni-alloy-plated layer is improved by pre-plating the substrate with another Zn-Ni alloy.

The pre-electroplated layer must contain a higher amount of Ni than the principal Zn-N.i-alloy-electroplc.ted layerO According to our study, the pre-plated layer must contain more than 12 wt % and not more than 87 wt % Ni.

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1 For the pre plating bath, a chloride bath, a sulfate bath or a mixed chloride/sulfate bath can be employed.
The bath should contain 7 - 38 g/Q Zn ions and 41 - 88 g/~
Ni ions, whereby the concentration ratio [Ni+2]/([Zn2+] ~
~Ni2 ]) must be 0.70 - 0.85. By employment of such a bath the Ni content of 12 wt % to 87 wt % is achieved.
The plating bath preferably contains 11 - 34 g/Q Zn and 62 - 72 g/Q Ni, more preferably 15 - 30 g/Q Zn and 85 -70 g/Q Ni.
When electroplating is effected in a bath contain-ing Zn2+ and Ni2+, adhesion between the preplated layer and the steel substrate and adhesion between the pre-plated layer and the principal plated layer are influenced by electric current density, and smaller densities give better adhesion and increase Ni content in the resulting plated layer. Although it is preferred to carry out the electroplating with lower current density in order to improve adhesion, the Ni content in the pre-plated layer exceeds 87 wt ~ when current density of less than 2 A/dm2 is employed.
In this condition, if crackin~ occurs in the principal plated layer, corrosion of the principal plated layer is promoted although it is temporarily protected. This is probably because the corrosion electrolytic potential of the pre-plated layer is higher than that of the principal 1 plated layer. As the result, adhesion between the principal plated layer and the electrodeposited coating formed thereon gradually is deteriorated. Therefore, electrop].ating must be carried out with a current density of not less than 2 A/dm2.
On the other hand, when current density is .in excess of 20 A/dm2, the Ni content of the pre-plated layer becomes close to that of the principal plated layer~ This means that adhesion of the pre-plated layer to the steel sub-strate is weakened to the same level as that of said Zn-Ni-alloy plating. Therefore, adhesion is improved by carrying out the pre-electroplatina with a current density of between 2 and 20 A/dm2.
If the Ni content of the pre-plated layer is lower than or of the same level as that of the principal plated layer, adhesion of the principal plated layer to the steel substrate is not sufficient, and the pre-plated layer is preferentially aorroded when cracking occurs in the principal plated layer and thus the plated layers are peeled off. Therefore, the Ni content of the pre-plated layer must be higher than that of the principal plated layer, that is, higher than 12 wt % and not higher than 87 wt ~ when the pre-plating is carried out under the above-mentioned currency condition. Thus, the corrosion potential of the pre-plated layer is higher than the principal plated layer so that the difference in the corrosion potential 2qS6~3 1 from that of the principal plated layer is not too large, and corrosion between the steel substrate and the principal plated layer is inhibited and good adhesion and endurance of coatings are brought about. The Ni content of the pre-plated layer more preferable for inhibiting corrosionbetween the steel substrate and the principal plated layer is 17 - 42 wt %, that is, higher than that of the principal plated layer by 5 - 30 wt %.
In order to make the Ni content of the pre-plated layer more than 12 wt % and not less than 87 wt %, further the pre-electroplating should preferably be carried out at a temperature between 55 and 80C. If the concentra-tion ratio [Ni ~ Zn ~ + ~Ni ~) is less than 0.70 and the bath temperature is lower than 55C, the Ni content in the pre-plated layer becomes less than 12 wt ~. If the ratio is in excess of 0.85, the Ni content of the resulting plated layer fluctuates and pre-plated layers of uniform quality cannot be produced. If the bath temperature is in excess of 80C, there is the disadvantage of requiring a special material for constructing vessels and plenty of energy for warming the bath.
In order to achieve the preferred Ni con-tent of 17 -42 wt ~ in the pre-plated layer as mentioned above, the electroplating should be conducted in a bath of which 25 said concentration ratio is 0.70 - 0.77 at 65 - 80C or 2~

the ratio is 0O77 - 0.80 at 55 ~ 65C.

The thickness of the pre-plated layer should be not less than 0.05 umO With pre-plating thinner than this, no improvement in corrosion resistance is expected. If the thickness is in excess of 1 um, it will invite cracking in the pre-plating layer when the sheet is subjected to deformation, which is undesirable for corrosion prevention. The.refore the thickness of the pre-plated layer should be 0.05 ~ 1 um.

The principal plated layer is formed by elecl:roplating steel sheets with an acidic p:lating bath containi.ng 25 - 33 g/l ~gram as atom per liter bath) zinc 41 - 88 g/l nickel, 0.2 - 10 g/l titanium, 0.1 - 5 g/l cobalt and 0.1 - 5 g/l aluminum or 0.2 - 4 g/l magnesium, the pH of which is 1.5 - 2.5O

As has been explained in the above, this lnvention improves adhesion of the plated Zn-Ni-alloy layer by applying a pre-plated layer in which the Ni content is higher than l:hat of the principal plated layer with relatively low current density. Although the reason why electroplating with .such low current density brings about better adhesion is not quite clearly understood, .it is surmized that the pre-plated layer of which the Ni content is higher than that of the principal plated layer causes a crystall growth different from that of the principal plated layer which contributes to the better adherence of the plated layers.

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It has been confirmed that adhesion of the plated layer is not impaired if a slight amount of another metal such as Co, Cr, Ti Fe, etc. is codeposi-ted in the pre-plated layer when a pre-plated layer contains more than 12 wt % and not more than 87 wt % Ni.

D ailed Description of the Embodiments Examples Cold-rolled steel sheets 0.8 mm in thickness were degreased and pickled by the conventional method, and were pre-plated with a Zn-Ni alloy and thereaf-ter was subjected to the Zn-Ni-alloy-pla-ting using baths as indica-ted in Table 1.

Cation The plated sheets were pre-treated wi-th a commercially available phosphating solution (DT-3030*) and sub~ected to electrodeposition coating (20 um thick coating using a cation type electrodeposition solution (Power-Top U-30**) at 200V for 3 minutes, the coa-ted sheets were *Trade Mark of Nippon Parkerizing Corporation, for phosphating solutions ** Trade Mark of Nippon Paint Co. for cation type electrodeposition solutions ,~ s. .

- lo - ~2~68 1 baked at 180C for 20 minutes. Adhesion between the steel substrate and the preplated layer and adhesion bQtween the principal plated layer and the coated film were checked. The tests were carried out using a Du Pont impact tester (1 kg weight, 50 cm dropping distance).
An adhesive tape was applied to the deformed part and the tape was forcibly detached, and peel-off of the coated layer or the coated layer with plated layer was observed. The test of adhesion of coating was carried out with samples as coated (primary adherence test) and samples after having been soaked in water of 40C for 240 hours (secondary adhesion test). The results are shown in Table 2 together with compositions of the plated layers. The evaluation standard is shown in Table 3.
The pre-plating baths of this invention and comparative baths contai`ned 85 - 70 gNi/Q nickel sulfate, 15 -30g Zn/Q zinc sulfate and 36 g~Q sodium sulfate~ and the pH of the baths was 2. Within the above ranges, the [Ni ~/([Zn ] ~ ~Ni ~) ratio was varied.
The principal plating baths contained 49 gNi/Q nickel sulfate, 25 gZn/Q zinc sulfate, 72 g~Q sodium sulfate, 1 gCo/Q cobalt sulfate, 5O2 g~l/Q alumi~num sulfate and

3 gTi/Q. The pH of the baths was 2.

It is apparent from Table 2 that coated steel sheets 6~

1 prepared using the plated steel sheets with pre-plating in accordance with this invention are remarkably superior to the above-mentioned Zn-Ni plated steel sheets in both the primary adhesion test and the secondary adhesion test.
As has been explained above, the adhesion of Zn Ni-alloy electroplated layer is remarkably improved by pre-electroplating another Zn-Ni alloy in accordance with this invention. The plated layer is fully resistant to impulsive deformation with electrodeposition coating thereon.

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Table 3 Rating Area of Peeling ____ No peeling

4.5 less than 1.0 %
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Claims (3)

The embodiments of the inventions in which an exclusive property or privilege is claimed are defined as follows:

1. A process for preparing Zn-Ni-alloy plated steel sheets with excellent adhesion of the plated layer comprising pre-plating a steel sheet in an acidic plating bath containing 7 - 38 g/l Zn and 41 - 88 g/l Ni whereby the concentration ratio (Ni2+)/((Zn2+) + (Ni2+)) is 0.70 - 0.85 with an electric current density of 2 - 20 A/dm2 at a temperature between 55° and 80°C so that the resulting Zn-Ni pre-plated layer contains 12 - 87 wt % Ni, the Ni being in a higher percentage than in a principal electroplated layer to be formed thereon in the range of 12 - 87 wt % Ni; and electroplating said pre-plated steel sheet in an acidic electroplating bath containing 25 - 33 g.l Zn, 41 - 88 g/l Ni, 0.2 - 10 g/l titanium and 0.1 - 5 g/l aluminum or 0.2 -4 g/l magnesium, the pH of which is 1.5 - 2.5.

2. The process as described in Claim 1, wherein the Zn content of the pre-plating bath is 11 - 34 g/l and the Ni content thereof is 62 - 79 g/l.

3. The process as described in Claim 1, wherein the Zn content of the pre-plating bath is 15 - 30 g/l and the Ni content thereof is 85 - 70 g/l.