GB2212816A - Zn-Ni based composite electroplated material and multi-layer composite plated material - Google Patents

Description

- 1 Zn-Ni BASED COMPOSITE ELECTROPLATED MATERIAL AND MULTI-LAYER COMPOSITE

PLATED MATERIAL

1 BACKGROUND OF THE INVENTION

The present invention relates to a Zn-Ni based composite electroplated material excellent in various properties such as corrosion resistance and processability.

The present invention further relates to a multi-layer composite plated material which comprises said Zn-Ni based composite electroplated material as a substrate and, provided thereon, a composite film layer mainly composed of SiO 2 More particularly, the present invention relates to a high-corrosion resistant multilayer composite plated steel sheet which has excellent corrosion resistance, press workability and spot weldability and which is suitable, for example, for rust proofed steel sheet for automobiles, especially for inner.side of bodies.

As well known, zinc electroplated steel sheets have been widely used as surface treated steel sheets which are improved in corrosion resistance of cold-rolled steel sheet and corrosion resistance after coating and can be mass-produced without damaging workability.

Recently, it is attempted to use zinc plated steel sheet as rust-proof ed steel sheet for automobiles as salts are scattered on roads for prevention of freezing in winter season and thus demand for corrosion resistance in severe corroding atmosphere increases.

1 Increasing the amount of zinc to be plated (deposition amount) has been known for improving corrosion resistance. As other methods, there have been proposed many methods of plating alloys to control dissolution of zinc per se. Many of them use alloys containing iron family elements such as Fe, Ni and Co and especially, Zn-Ni, Ni-Co and Zn-Fe alloy platings are recognized to be useful and practically used. Further, various proposals have been made on composite plating according to which materials other than metals are coprecipitated. For example, there are disclosed Zn-SiO 2 composite plating in Japanese Patent Kokai No. 54-146228, composite plating of Zn- iron family element alloy which contains SiO 21 TiO 2, etc. in Japanese Patent Kokai No. 60-141898 of the present inventors and composite plating which contains alumina in Japanese Patent Kokai Nos. 60-125395 and 61- 270398. Corrosion resistance of these composite platings depends mainly on uniformity of coprecipitated materials such as alumina and Sio 2 in plated layer. Sometimes, coprecipitated materials are agglomerated to mass depending on plating conditions or composition and amount of precipitation becomes unstable. Thus, effect as expected is not exhibited. Furthermore, corrosion resistance is still insufficient under severe corrosion environment such as cold district and besides, workability is also inferior.

page 2a follows Q - 2a The present invention provides a material comprising a ferrous substrate having on at least a portion of the or a surface thereof a layer comprising 1 to 15% by weight of Ni, 0.1 to 10% by weight of Cr, 0.1 to 10% by weight of SiO 2' 0.01 to 3% by weight of Fe, the balance being Zn and incidental impurities and, optionally, one or more incidental ingredients used in plating a ferrous substrate. The ferrous substrate preferable comprises a steel sheet.

The invention also provides a method for making a Zn-Ni based composite plated material, preferably a steel sheet, which comprises subjecting a ferrous substrate to electrolysis in an acidic Zn-Ni electroplating bath containing 5 to 50 g/l of SiO 2 and Fe 2+ and Cr A sparingly soluble chromate film and a coating of agglomerated SiO 2 - containing epoxy resin may if desired also be applied. The invention further provides plated material made by the method, and an automobile body part comprising steel sheet according to the invention.

3 - The invention makes it possible to provide a Zn-Ni based composite electroplated steel sheet substantially free from the aforementioned defects of the conventional techniques and superior in various properties such as corrosion resistance and workability and a highly corrosion resistant multi-layer composite plated steel sheet which includes said Zn-Ni based composite electroplating layer as an underplating layer.

DESCRIPTION OF THE INVENTION

The present invention has been made based on the following findings.

(1) As coprecipitation substance, SiO 2 is the most effective for improvement of corrosion resistance and only when this is uniformly and finely copreci- pitated in the plating layer, corrosion resistance is sharply improved.

(2) As accelerator for coprecipitation of Sio 2' iron family element, especially Ni is effective as shown in Japanese Patent Kokai No. 60141898, but it is difficult with only Ni to stably coprecipitate SiO 2 in the plating layer. For solving this problem, addition of Cr 3+ is very effective and when Cr ion is allowed to coexist in a plating bath, it is possible to stably and uniformly coprecipitate SiO 2 and besides Cr is also inductively coprecipitated together with Si02, resulting in further improvement of corrosion resistance. This is ]c 1 the first point of the present invention. It is considered that this is because repulsion force between S'02 particles is increased by adsorption of Ni ion and Cr ion around Sio 2 and as a result, agglomeration of S'02 is suppressed and SiO 2 is stably and finely collected in plating layer, but the invention is not to be limited to this explanation.

(3) However, coprecipitation of Zn metal and chromium metal is difficult and besides, when SiO 2 to which Ni and Cr are adsorbed is precipitated in Zn metal, workability decreases. In order to solve these problems, addition of Fe is necessary.

That is, effect of Fe is to prevent formation of plating structure which lacks uniformity and smoothness due to inhibition of uniform growth of e lectrodepo sited Zn and Ni caused by Cr ion added for precipitation of Sio 2 The second point of the present invention resides in finding such effect of Fe.

The reason is not clear, but it can be supposed that when Cr ion adsorbed to SiO 2 is electrodeposited, this can have stable structure in Zn-Ni plating layer only through Fe. As a result, Cr and Sio 2 are took into Zn-Ni plating layer without inhibiting uniform growth of Zn-Ni plating layer, resulting in a structure superior in workability.

(4) Especially, corrosion resistance after coating is improved when Zn or Zn based alloy plating layer is provided on the above-mentioned Zn-Ni plating layer.

(5) When SiO 2 is contained in both the based composite electroplated substrate layer and a ceramics composite film layer cured with an organic resin which is provided on said layer, corrosion resistance is remakably improved.

That is, the present invention more especially provides a Zn-Ni based composite electroplated material, especially a steel sheetsuperior in corrosion resistance and workability which comprises a ferrous substrate and, provided thereon, a Zn-Ni based composite electroplating layer comprising Ni: 1-15% by weight, Cr: 0.1-10% by weight, SiO 2: 0.1- 10% by weight, Fe: 0.01-3% by weight and the balance being essentially Zn; the above Zn-Ni based composite electroplated steel sheet which is further provided thereon, with a Zn or Zn based alloy electroplating layer; a highly corrosion- resistant multi-layer composite plated sheet which comprises said Zn-Ni based composite electroplated steel sheet as a substrate and, provided thereon, a hardly soluble chromate film containing 5% or less of water-soluble matter at a coating weight of 10-150 mg/m 2 in terms of total chromium content as a second layer and a multi-layer film layer containing Sio 2 cured with a solvent type epoxy resin as a third layer at a preferred thickness of 0.3-3p; and said highly corrosiorvresistant multi-layer composite plated steel sheet wherein the composite film layer of the third layer contains v 1 20-80% by weight, based on the resin, of SiO 2 which is preferably a secondary agglonerate preferably forTied by previously agglonierating dry SiO 2 of 2-20 nm in average particle size to a size of 0.3-3 p in a solvent type epoxy resin. 5 The present invention will be explained in more detail below. Ni content in the Zn-Ni composite plating layer is specified to be 1-15% by weight. This is because conditions for adsorbing Ni together with Cr to Sio 2 in a plating bath and easily uniformly dispersing and coprecipitating Sio 2 in the plating layer are aimed at and furthermore, alloying effect of Ni is taken into consideration. When Ni content is less than 1% by weight, especially corrosion resistance before coating is insufficient and when more than 15% by weight, workability is much deteriorated.

Cr content is 0.1-10% by weight and precipitation of such a large amount of Cr becomes possible in the presence of Fe and SiO 2 together. Precipitation amount of Cr determines depending on the contents of Sio 2 and Fe, but when it is less than 0.1% by weight, effect for enhancing the corrosion resistance is not exhibited and when more than 10% by weight, the effect is saturated and workability is deteriorated.

Sio 2 content is specified to be 0.1-10% by weight. In order to stably and uniformly coprecipitate S'02 in plating layer, it is necessary to allow Cr 3+ to exist in plating bath in addition to Ni and Fe.

1- Sio 2 contributes to improvement of corrosion resistance and it is considered that this is because SiO 2 serves as a buffer for pH of atmosphere and further, prevents diffusion of corrosion products. When content of SiO 2 is less than 0.1% by weight, effect to enhance the corrosion resistance is not exhibite and when more than 10% by weight, workability and plate adhesion are deteriorated.

Fe content is specified to be 0.01-3% by weight. Roles of Fe in the composite plated steel sheet are mainly to accelerate bonding of Cr and Zn which is a main component in the plating layer by entering between them and to improve workability. In this sense, if the content is less than 0.01% by weight, the above effects cannot be exhibited and if more than 3% by weight, not only these effects are saturated, but corrosion resistance before coating is deteriorated.

A plating amount of 10-50 g/m 2 is preferred for ensuring corrosion resistance.

This composite electroplated steel sheet may have a single plating layer as mentioned hereabove or it may be additionally plated thereover with a small amount of Zn or a Zn based alloy such as Zn-Ni or Zn-Fe. When the additional Zn or Zn based alloy plating is applied, the amount thereof is preferably 1-5 g/m 2 and thus stable and good corrosion resistance after coating is obtained. This is also advantageous for avoiding defects of coating film at the time of cationic k 1 electrodeposition operation.

The effect of the present invention is not substantially influenced by inevitable coprecipitation of a slight amount of Co, Pb, Sn, Ag, In, Bi, Cu, Sb, 5 As, Al, Ti, Na, P, 0, C or the like besides Zn, Ni, Cr, S'02 and Fe.

With reference to the method of production, the aforementioned composite electroplated steel sheet is preferably produced by subjecting a steel sheet to an electroplating with an acidic electroplating bath containing 30-60 g/l of Zn 2+ and 5-60 g/l of Ni 2+ in which 1-30 g/l of Cr 3+, 5-50 g/l of Sio 2 and 0.1-10 g/1 of Fe 2+. The preferred conditions are a pH of 1-3, a bath temperature of 40-700C and a current density 2 of 50-300 A/dm In addition, electroconducting aids such as salts of Na +, K +, NH 4 + and pH buffer such as boric acid may also be contained in the plating bath.

Chromium sulfate is preferred as a source of Cr in case of a sulfuric acid acidic bath. As SiO 21 there may be used any of aqueous dispersion sol thereof, colloidal silica, dry silica and fumed silica, but desirably f i na particles of 1-100 mi in primary particle size.

In the present invention, corrosion resistance can be further improved by providing an Sio 2 composite film layer cured with an epoxy resin on said Sio 2containing composite plating layer or said Zn or Zn based alloy plating layer through a chromate layer as an intermediate layer therebetween.

Mechanism of preventing corrosion by SiO 2 has not yet been fully elucidated. However, basic factors therefor are considered that with corrosion of Zn a stable rust of Zn(OH)2-S'02 is formed on the surface and this acts as a protective film and that SiO 2 per se is excellent in alkali resistance. Therefore, for attaining high corrosion resistance, it may be attempted to contain a large ambunt of SiO 2 in plating layer, but it is very difficult to contain a large amount of S'02 by electroplating in view of viscosity of bath and for control of bath. Besides, even if SiO 2 is contained in a large amount, powdering property is markedly deteriorated and the product is practically not satisfactory. According to the present invention, SiO 2 is contained at a practical level in the substrate composite plating layer and thereon can be provided a thin film-like composite film cured with an epoxy resin in which SiO 2 is agglomerated in a suitable size, whereby Sio 2 can be present in a totally sufficient amount to attain superior corrosion resistance and thus corrosion resistance can be remarkably enhanced.

Here, spot weldability and press workability may be questioned because SiO 2 is an insulant and a solid, but it has been found that these properties are rather improved if amount of SiO 2 is properly controlled.

That is, -where an SiO composite film layer higher in 2 electric resistance than plating layer is provided as an upper layer and an SiO 2 based composite plating layer lower than the upper layer and higher than steel sheet in electric resistance is provided as a lower layer, when spot welding is effected, first the Sio 2 composite film layer and then the S'02 based composite plating layer generate heat and are dissolved in succession and as a result, joining between steel sheets is smoothened. When press working is effected, SiO 2 exhibits a kind of lubricating function by proper adjustment of amounts of the resin and Sio 2 in the upper composite film layer, whereby peeling of plating layer and composite film layer and build-up thereof to mold can be reduced.

The optional second and third layers of the multi-layer composite plated steel sheet will be explained below.

The second layer is a hardly soluble chromate film of preferably 5% or less in water-soluble matter content and of a coating weight of 10-150 mg/m 2 in terms of total Cr amount.

The chromate film used in the present invention which is provided between the lower plating layer and the upper coating film improves adhesion of the coating film and thus this is a very important film for imparting high corrosion resistance to organic composite plated steel sheet. Especially, the steel sheet must have water-swelling and dissolution resistance and for this purpose, the chromate film must be made hardly soluble.

When the content of water-soluble matter in chromate film exceeds 5%, much chromium dissolves out 1 due to swelling of chromate film and as a result, adhesion of the upper coating film decreases or it is difficult to further improve corrosion resistance and besides operation becomes complicated because of - pollution of chemical treating solutions with the dissolving-out chromium and disposal of waste liquid. The content of water-soluble matter in the chromate film is preferably 2% or less.

When the coating weight of the hardly soluble chromate film is less than 10 mg/m 2 in terms of total chromium amount, adhesion to the upper coating film is somewhat insufficient or it is difficult to further increase corrosion resistance and life. When the total chromium amount exceeds 150 mg/m 2, adhesion to the upper coating film is markedly deteriorated owing to cohesive failure of chromate film caused by press working and the like and furthermore, a bad effect is exerted on continuous spotting property at spot weldability. Preferred coating weight is 20-100 mg/m 2 in terms of total chromium amount. Cathodic electrolysis methods are most suitable for formation of the hardly soluble chromate film.

The third laye.r is an Sio 2 composite film formed by coating--SiO 2 preferably secondarily agglomerated to a particle size of 0.3-3 p from Sio 2 having a primary particle size of preferably 2-20 nm together with an epoxy resin at a thickness of preferably 0.3-3 p. The epoxy resin acts as a binder resin and is superior to other resins in workability and weldability.

1 As the epoxy resin, there may be used, for example, an epoxy resin'containing at least 50% by weight of phenol in molecule as nonvolatile matter of coating, an epoxy ester resin obtained by reacting said epoxy resin with a dicarboxylic acid in the presence or absence of amine catalyst, an urethanated epoxy ester resin obtained by reacting said epoxy ester resin with a partially blocked isocyanate compound and a bisphenol A type resin. Bisphenol A type epoxy resin is especially effective from the point of corrosion resistance. Number-average molecular weight of the epoxy resin is suitably 300-100,000. When this is less than 300, it is not sufficiently polymerized even by reaction, resulting in insufficient corrosion resistance of the coating film and when more than 100,000, sufficient crosslinking reaction is not attained and corrosion resistance of the coating film is also unsatisfactory.

The SiO 2 composite film cured with epoxy resin acts synergistically with SiO 2 in the substrate plating layer to remarkably enhance corrosion resistance. Sio 2 used here is preferably such that may be dispersed in organic solvents and form secondary agglomeration particles of 0.3-3 p and especially preferred is dry Sio 2 of 2-20 rim in average particle size.

If the secondary agglomeration particle size is less than 0.3 p, effect to improve corrosion resistance is undesirably low and if more than 3 p, press workability and spot weldability of the steel sheet are deteriorated.

To prepare secondary agglomeration particles of 0.3-3 Sio 2 of 2-20 nm in average primary particle size may be most suitably used.

In order to prepare secondary agglomerate particles of Sio 2' a resin of low solvent content is put in a container containing SiO 2 and rough kneading is sufficiently carried out by a high-speed disperser. In this case, adsorption reaction between S'02 and the resin is allowed to proceed at a temperature lower by at least 100C than glass transition temperature of the resin and they are left to stand until the desired secondary particle size is obtained. Then, kneading is effected again by three-roll mill, etc. to make even the particle size distribution and then-the product is dispersed in a solvent without leaving it. In this case, the point is to carry out stirring at a stretch by a high-speed disperser. When such secondarily agglomerated SiO 2 is used, separation of S'02 does not occur even by dilution with solvent and thin-film high-speed roll coating by a roll coater (150 m/min.or higher) becomes possible. As to quality, use of such Sio 2 is advantageous in spot weldability and press workability and besides cationic electrodeposition property is improved, resulting in extension of range of optimum electrodeposition conditions.

Amount of SiO 2 is preferably 20-80% by weight of resin in the composite film. When the amount is less than 20% by weight,. corrosion resistance is undesirabiy low and when more than 80% by weight, press workability and spot weldability are deteriorated. More preferred range is 30-70% by weight. When thickness of the coating film is less than 0.3 P, corrosion resistance is undesirably low and when more than 3 p, spot weldability is deteriorated.

The third SiO 2 composite film may additionally contain a curing agent for acceleration of curing by baking and/or a lubricant to further improve press work- ability.

Useful curing agents are, e.g. melamine resins, reS01 type phenolic resins.,polyisocyanate compoundsand blocked polyisocyanate compounds. Amount of the curing agent is suitably such as curing agent/epoxy resin=1/10-1/1 in weight ratio of solid content. Curing at low temperature and for short time becomes possible by use of these curing agents and thus use of them is advantageous for production on high-speed continuous coating and baking line. As the lubricants, preferred are those which can be readily separated above in SiO 2 composite film such as polyolefins, carboxylic acids, polyesters, metal salts of carboxylic acids, polyalkylene glycols, molybdenum disulfide, silicon compounds and fluorine compounds. Especially effective is polyethylene wax. Addition amount of the lubricant is suitably 0.1-10% by weight of Sio 2 Organic solvents in resin composition used for formation of the Sio composite film layer as a 2 1 third layer are not critical. However, for exa..-nple, when 1 bisphenol type epoxy resin is used as epoxy resin and blocked polyisocyanate is used as a curing agent, ketone organic solvents such as, for example, methylisobutyl ketone, acetone, cyclohexanone, isophorone, etc. are 5 especially suitable.

Baking conditions for Sio 2 composite film in the present invention are not critical, but short-time baking treatment is p6ssible with keeping good properties in a wide range of 100-200C as final baking temperature of sheet. Coating method may be any suitab-Le method such as roll coating method, curtain flow coating method and the like.

The construction as mentioned hereabove need not necessarily be applied onto both surfaces of a substrate steel sheet and may be applied onto only one surface and another surface may remain as such or may be provided with other plating layer with or without an organic film thereon.

When the steel sheet according to the present invention is used as rustproofed steel sheet-for automobiles, it is most preferred that one surface of steel sheet is provided with the Zn-Ni based composite electroplating layer or additionally with upper p lating layer (two plating layers) and this side is used as outer surface of body and another surface of the sheet is provided with the multi-layer composite plating layer having said Zn-Ni based composite electroplating layer as a substrate and this side is used as internal surface 1 of the body. Such steel sheet can be produced by first applying Zn-Ni composite electroplating layer on both surfaces of a steel sheet and then applying chromate film and SiO 2 composite film onto only one surface. If necessary, the surface applied with no chromate film and SiO 2 composite film may be applied with Zn or Zn based alloy plating as upper plating layer.

Substrate steel sheet to which the present invention is applied is ordinarily a dull finish rolled mild steel sheet, but the present invention may also be applied to bright finish rolled mild steel sheets, high tensile steel sheets containing Mn, S, P or the like in a large amount as steel components, highly corrosion- resistant steel sheets low in corrosion rate containinq Cr, Cu, Ni, P or the like in a large amount, or any other suitable steel sheet. Other ferrous substrates may also be used.

The following Examples illustrate the invention.

Examples 1-26 and Comparative Examples 1-11 Cold rolled steel sheets were subjected to degreasing with alkali, pickling with 5% sulfuric acid, washing with water and electroplating under the following conditions. A sulfuric acid bath having a temperature of 60C and a pH of 2 and stirred by pump to keep a flow rate of 90 m/min was used. Interval between electrodes was 10 mm. Colloidal silica of 10-20 mv in average particle size and dry silica of 30-50 mi in average particle size were used as SiO 2 added to the plating bath. Contents of Ni, Cr, SiO 2 and Fe in plating layer 17 - 1 were controlled by controlling the addition amounts thereof and current density and plating amount was 2 g/m Zn or Zn based alloy plating was applied onto some of the steel sheets at 3 g/m 2 as an upper layer.

Plating conditions are shown in Table 1 and composition of the plated steel sheets and results of evaluation for corrosion resistance and workability are shown in Table 2.

Notes 1) - 5) in Tables 1 and 2 mean as 10 follows: 1) Steel sheet:

-15 A: Dull finished steel sheet (low-carbon steel) B: Bright finished steel sheet (low-carbon steel) C: High tensile steel (low-carbon steel containing 0.1% P) D: Low corrosion rate steel sheet (low-carbon steel containing 0.2% Cu, 0.1% Ni and 0.08% P) E: Low corrosion rate steel sheet (low-carbon steel containing 2% Cr) 2) Composition of plating bath: A sulfate bath; pH 2, bath temperature 600C and containing 50 g /1 of Na 2 so 4 as a conducting aid in addition to the components enumerated in Table 1. The upper layer plating was performed in a known plating bath at a current density of 100 A/dm 2 for both the Zn and Zn based alloy platings.

3) Sio 2:

A: Colloidal silica of 10-20 mp in average particle size 1 B: Dry silica of 30-40 mp in average particle size 4) Method of evaluation of corrosion resistance:

(i) Corrosion resistance before coating:

The plated samples as such were subjected to the following cyclic corrosion test of 30 cycles.

Corrosion resistance was evaluated by decrement of thickness and the results were indicated by the follow ing grade.

Decrement of thickness 0 - 0.1 mm 0.1 - 0.2 mm 0.2 - 0.3 mm 0. 3 mm ^.

0 A x (ii) Corrosion resistance after coating:

The samples were subjected to phosphate treat- ment by dipping and cationic electrodeposition coating (20 W and crosscuts which reached the substrate steel sheet were made on the surface and these samples were subjected to the following cyclic test of 50 cycles.

Corrosion resistance was evaluated by width of blistering and the results were indicated by the following grade.

Width of blistering 0 - 1 mm @ 1 - 3 mm 0 3 - 5 mm mm. or more A X 19 (iii) Cyclic corrosion test: Salt spray (JIS Z2371) Drying (60IC) 1 Wetting (50C, RH 95%) Cooling (-20OC) 3 hours 2 hours 2 hours 1 hour 5) Method of evaluation of workability:

The plated samples were press molded into cylinder of 50 0 x 25H and then the cylinder was subjected to adhesive tape peel test. Workability was evaluated by decrement in weight and the result is indicated by the following grade.

Decrement in weight 0 - 2 mg @ 2 - 5 mg 0 - 8 mg 8 mg or more is A X Table 1 Plating Conditions Steel 1) Composition of plating bath 2) (g/1) 3) Current Sio sheet 2+ 2+ 3+ Sio 2 2+ 2 density Zn Ni Cr Fe (A/dm2) Example 1 A 30 60 1 5 0.1 A so 11 2 A 40 40 5 10 0.1 A 100 we 3 A 40 40 15 20 0.5 A 150 19 4 A 40 40 20 30 0.7 A 200 99 5 A 60 20 1 5 3 B so If 6 A 50 30 5 10 0.1 B 100 7 A 50 30 15 20 0.5 B 150 8 A so 30 20 30 0.7 B 200 9 B 60 10 2 5 5 A 50 19 10 c 40 30 10 15 0.1 A 100 D 40 30 20 30 0.5 A 150 12 E 40 30 30 5 0.7 A 200 13 A 60 5 1 15 10 A 50 (to be cont'd.) 1 m C 1 1 Table 1 (ContId.) Example 14 A 50 30 5 30 0.1 A ' 100 to 15 A so 30 10 50 0.5 A 150 01 16 A 50 30 20 10 0.7 A 300 ke 17 A 30 40 10 15 1 A 100 18 A 40 30 5 20 5 A 150 of 19 A 30 40 15 20 1 A 150 99 20 A 40 30 15 15 5 A 150 21 A 30 40 20 5 1 A 100 91 22 A 40 30 10 30 5 A 200 cc 23 B 30 40 20 30 1 B 150 of 24 c 40 30 20 30 1 B 150 91 25 D 40 30 30 30 1 B 150 it 26 E 40 30 20 50 1 B 300 Comp. Ex. 1 A 30 40 - - - 50 2 A 30 40 1 - - - 100 (to be cont'd.) 11 1 f--, 1 1 Table 1 (Cont'd.) Comp. Ex. 3 A 30 40 20 A 150 4 A 30 40 50 fl 5 A 30 40 1 - 1 - 100 6 A 80 2 5 10 1 A 150 7 A 20 80 5 10 1 A 50 8 A 40 30 0.5 2 3 A 20 to 9 A 40 30 50 100 3 A 200 be 10 A 40 30 5 10 - B 100 11 A 40 30 5 10 20 B 150 1 1 N) N) 1 Table 2

Composition of plating layer (wt%) Ni 12 12 12 7 9 9 9 9 9 9 S'02 0.1 1 3 5 0.1 1 3 5 0.1 1 2 5 Cr 0.1 1 3 5 0.1 1 3 5 0.2 2 5 10 Fe 0.01 0.01 0.03 0.05 1 0.01 0.03 0.05 2 0.01 0.03 0.05 Upper plating layer Evaluation of properties Corrosion resistance before coating4)(i) Corrosion resistance after coating4)(ii) Workability 5) Example 1 to 2 11 of It 91 19 19 91 11 11 19 3 4 6 7 8 9 12 0 (5) 00 (0) 00 00 0 00 (9) 0 OP 00 0 (0) 0 00 (8) 00 00 00 OP 0 (to be contld.) 1 W 1 Example 13

11 14 00 15 go 16 to 17 18 19 er 20 91 21 29 22 90 23 he 24 %1 25 go 26(Cont 1 d.) 1 0.1 9 1 9 2 9 5 12 2 9 1 12 3 9 3 12 5 9 2 12 5 9 5 9 10 9 5 0.2 2 5 10 1 2 3 3 2 5 5 5 5 10 3 0.01 0.03 0.05 0.1 2 0.1 2 0.1 2 0.1.

0.1 0.1 0.1 Zn Zn Zn-10Mi 91 Zn-80%Fe If Z n Zn-11Mi of Zn-85%Fe 0 00 CO) (8) 00 C0) (0) 00 00 0 (8) 00 00 (3) 0 (C5) 00 (0) 02 C0) (G) CO) 00 0 0 (to be cont'd.) 1 $C:.

1 Table 2 (Cont'd.) Comp. Ex. 1 12 - X X 0 11 2 12 0.1 X A A of 3 12 3 0 A A of 4 12 0.1 X X @ it 5 12 0.1 - 0.1 X A @ of 6 0.5 1 1 0.1 X A If 7 20 1 1 0.1 A 0 X go 8 9 0.05 0.05 1 A A @ if 9 9 15 15 1 @ X if 10 9 1 1 - 0 0 X 11 9 1 1 5 X 0 @ 1 1 1 K) ul 1 - 26 1 Examples 27 - 43 and Comparative Examples 12 - 42 Multi-layer composite plated steel sheets were made as follows:

Cold rolled steel sheets were subjected to electroplating in the same manner as in the above examples. Aft:er washing with water, the plated steel sheets were subjected to cathodic electrolysis in a chromate solution mainly composed of Cr 6+ to form thereon a chromate film 3+ mainly composed of Cr followed by washing with water and drying. Thereafter, S'02 composite film having specific composition was formed on one surface by roll coating-at a given thickness and then immediately, the samples were subjected to baking treatment so that ultimate maximum sheet temperature reached 150C in 20 minutes. Plating conditions and properties of the thus obtained multi- layer composite plated steel sheets are shown in Table 3 and Table 4, respectively.

Notes 1) - 3) in Table 3 are the same as in Table 1.

Notes 1) - 7) in Table 4 mean as follows:

Content of water-soluble matter in chromate film which is expressed by ratio of difference in amount of chromium deposited before and after dipping in boiling distilled water for 30 minutes and initial amount of 25 chromium deposited.

1 2) Main resins:

(Kind of main resins) A: Bisphenol A type epoxy resin (number -average molecular weight 2,900) B: Urethanated epoxy ester molecular weight 10,000) resin (number-average C: Oil free polyester resin (number-average molecular weight 15,000) D: Carboxylated polyethylene resin (number-average molecular weight 100,000) (Curing agent) A: Hexamethylene diisocyanate B: Resol type phenolic resin C: Hexamethylene diisocyanate D: (20% aqueous solution) Main resin/curing agent=10/1-10/10 3) Indication of particle size of Sio 2 Average primary particle size 1 nm 2 nm 10 nm 1 20 nm 50 0.211 ss MS RS NS LS Secondary agglomeration 0.3 -3p SM MM RM NIX LM particle size 4p SL MI, RL NL LL 1 1 4) Corrosion resistance:

Corrosion resistance was evaluated by decrement of thickness after subjecting the samples to the following cyclic corrosion test of 200 cycles.

grade.

Salt spray (JIS Z-2371) Drying (70IC) Wetting (500C, RH 95%) 4.

L-Cooling (-20IC) 2 hours 2 hours 2 hours 2 hours The results are indicated by the following Decrement in thickness Less than 0.1 mm Not less than 0.1 mm and less than 0.2 mm Not less than 0.2 mm and less than 0.3 mm Not less than 0.3 mm 0 A X 5) Dissolving-out of chromium:

This was evaluated by total amount of chromium dissolved out into degreasing solution after the samples were subjected to degreasing with alkali (spraying treatment with 20 g/1 of degreasing solution L-440 manufactured by Japan Parkerizing Co. at 600C for 5 minutes.) and indicated by the following grade.

Less than 5 mg/m 2 Not less than 5 mg/m 2 and not more than 10 mg/m 2 0 More than 10 mg/m 2 and not more than 30 mg/m 2 A More than 30 mg/m 2 X 29 1 6) Press workability:

The samples were press molded into a cylinder of 500 x 25H and the worked surface was subjected to adhesion tape peel test. Workability was evaluated by decrement in weight and the results are indicated by the following grade.

Decrement in weight 0 - 2 mg 2 - 5 mg More than 5 mg Breaking of sheet 0 A X Spot weldability:

Electrode chip: Tip diameter 6 mmO, type FC Applied pressure: 200 kg Current: 90% of critical current value of dust formation Time: 10 cycles Number of continuous spot welding More than 5,000 @ More than 4,000 0 More than 3,000 A Less than 3,000 X 1 Table 3 Plating Conditions Steel 1) Composition of plating bath 2) (g/1) 3) Current Sio sheet 2+ 2+ 3+ S'02 2+ 2 density Zn Ni Cr Fe (A/dm2) Example 27 A 30 60 1 5 0.1 A 50 go 28 A 60 5 30 10 3 A 100 te 29 A 60 10 5 20 5 A 150 we 30 A 30 35 15 30 10 A 200 31 A 60 10 20 50 3 A 300 of 32 A 40 35 20 20 1 A 100 el 33 A 40 35 20 20 1 A 150 91 34 A 40 35 5 20 5 A 100 A 40 35 20 20 1 A 150 of 36 A 40 35 5 20 5 B 100 37 A 40 35 20 20 1 B 150 of 38 A 40 35 5 20 5 B 100 39 A 40 35 20 20 1 B 150 (to be contId.) 1 W C) 1 Table 3 (Contld.) Example 40 A 40 35 5 20 5 A ' 100 It 41 A 40 35 20 20 1 A 150 1$ 42 B 40 35 5 20 5 A 100 It 43 c 40 35 20 20 1 A 150 If 44 D 40 35 5 20 ' 5 A 100 E 40 35 20 20 1 A 150 Comp. Ex. 12 A 40 35 - - - 50 19 13 A 40 35 - - - - 100 19 14 A 40 35 - A 150 01 15 A 40 35 15 20 1 A 200 11. 16 A 40 35 5 20 5 A 100 OR 17 A 40 35 2 A 50 91 18 A 40 35 100 A 200 of 19 A 70 3 20 A 100 go 20 A 20 80 - A 100 01 21 A 80 1 1 5 1 A 20 (to be contId.) 1 W f-i 1 Table 3 (Contld.) Comp. Ex. 22 A 40 35 15 20 20 A 150 of 23 A 40 35 - 20 - A 100 1# 24 A 40 35 - 20 - A 100 er 25 A 40 35 - 20 1 A 100 - 11 26 A 40 35 A 100 00 27 A 40 35 - 20 - A 100 It 28 A 40 35 - 20 - A 100 we 29 A 40 35 5 A 20 @1 30 A 40 35 - 20 - A 100 31 A 40 35 20 B 100 32 A 40 35 20 B 100 er 33 A 40 35 50 200 99 34 A 40 35 20 B 100 of 35 A 40 35 20 B 100 00 36 A 40 35 - 20 - B 100 (to be contId.) 1 W 1 1 1 Table 3 (Contld.) Comp. Ex. 37 A 40 35 20 100 38 A 40 35 20 B 100 39 A 40 35 20 B 100 $c 40 A 40 35 20 B 100 41 A 40 35 20 B 100 42 A 40 35 20 B 100 111 1 W W 1 1 I Sio 2 Example 27

28 29 31 10 32 3 33 3 34 3 3 36 3 37 3 U to It to g# It #q 11 1 1 Table 4

Plating layer (1st layer) Composition of plating excluding Zn (wt%) 0.1 1 3 Ni is 1 5 10 5 10 10 10 10 10 Fe 0.01 1 2 3 1 0.1 0.1 2 0.1 2 0.1 Cr 0.1 10 1 3 5 3 3 1 3 1 3 Chromate layer (2nd layer) Plating amount (g/m 2) Content of watersoluble matter (%) 1) Plating amount of Cr (mg/m 2 20 20 20 20 20 20 20 20 20 0.5 0.5 0.5 0.5 0.5 0.5 3 5 0.5 0.5 0.5 (to be contld.) 45 45 45 45 45 45 45 10 20 100 1 1 (j J:1 1 Table 4 (Contld.) Organic solvent type coating (3rd layer) 4) 5) 6) 7) Main resin Sio 2 Thick- Corrosion Dissolving- Press Spot resistance out of Cr work- weld2) Amount 3) Amount ness ability ability Kind (Wt%) Particle (wt%) 01m) size A 50 RM 30 1 @) 00 A 50 RM 30 1 00 02 (:c) 00 A 50 RM 30 1 00 00 00 09 A 50 RM 30 1 @ 00 00 @ B 50 RM 30 1 CR) 00 @ 0? A 50 RM 30 1 0-1 00 (P) 09 A 50 MM 30 1 00 00 09 09 A 50 MM 30 1 09 @) A 50 LM 30 1 01 00 CC) 09 A 50 LM 30 1 09 00 @ 00 A 50 LM 30 1 ng 09 00 @ (to be cont'd i 1 W Ln 1 1 11 Example 38 it 39 to 40 41 42 43 44 Comp. Ex. 12 of 13 of 14 16 17 If it of we If Table 4 (Contld.) go 91 91 of 99 3 3 3 3 3 3 3 3 18 19 3 3 3 0.05 15 3 10 10 10 10 10 10 10 10 10 10 10 10 10 0.5 2 0.1 2 0.1 2 0.1 2 0.1 0.1 2 1 20 20 20 20 20 20 20 20 20 20 20 20 20 20 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 (to be contId.) 45 45 45 45 45 45 45 45 45 45 45 45 45 1 W m 1 1 Table 4 (ContId.) B A A A A A B B A A A c D A A A v i so 50 50 LM RM RM RM RM mm mm mm RM RM RM RM RM 10 15 35 40 40 40 40 RM 40 40 40 40 1 1 1 1 1 0.3 2 3 1 1 1 1 1 1 1 1 0 00 C) 00 00 X X X A 00 A 00 00 00 R @ 00 00 0I C) 00 C) 0 X (to be contId.) 0 0 C9) CO) 0 00 0 00 1 1, -1 Table 4 (ContId.) Comp. Ex. 20 21 22 23 24 I@ 11 If 26 27 28 29 30 31 32 33 34 3 0.1 3 3 3 3 3 3 3 0.1 3 3 3 3 - 0.1 10 10 10 10 10 10 10 10 10 10 10 0.1 3 20 20 3 20 - 20 - - 20 - - 20 - - 20 - - - 20 - - 20 - - 20 - - 20 - - 20 0.1 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 45 45 45 200 45 45 45 45 45 45 45 (to be contld.) 1 W co 1 1 Table 4 (Contld.) A A A A A A A B A A A A A A A p 50 50 so so 50 50 50 50 50 50 so so RM RM RM RM RM RM RM RM RM RM RM ss sm SL ms 1 40 40 40 40 40 40 5 90 40 30 30 30 1 1 1 1 1 1 1 1 1 0.1 4 1 1 1 1 A A A X A A 00 A 00 X 00 0 0 A 0 00 00 00 X 0 (0) 02 0 00 02 00 01 CO) 0 A A A X X X X A X A A X (to be cont'd.) 00 0 OP A A 00 X X X A A X A 11.1 1 W ko 1 Table 4 (ContId.) Comp. Ex. 35 3 10 20 0.5 45 to 36 3 10 - - 20 0.5 45 if 37 3 10 - - 20 0.5 45 of 38 3 10 - - 20 0.5 45 go 39 3 10 - - 20 0.5 45 it 40 3 10 - - 20 0.5 45 go 41 3 10 - - 20 0.5 45 of 42 3 10 - - 20 0.5 45 (to be contld.) 1 1 lt-l CD 1 1.

Table 4 (ContId.) A 50 RS 30 1 0 00 X, A A 50 ms 30 1 (1) @ X A A so M 30 A @ A X A 50 RL 30 A 00 A X A 50 M 30 A 00 A X A 50 LS 30 C) 00 X A A 50 LM 30 1 C) 00 A A A 50 LL 30 1 A CP) A X 0 1 Ir -i 1 11 1, 1 As is clear from Table 4, results of Examples 27-45 of the present invention are especially superior in corrosion resistance as well as resistance to dissolv ing-out of chromium, press workability and spot weldabilitv to those of Comparative Examples 12-14 where SiO 2 was not contained in both or either of the plating layer and the film, Comparative Examples 15 and 16 where epoxy resin was not used as main resin of coating film and Comparative Examples 17-42 where the components -are 10 outside the ranges of the present invention.

Claims (23)

WHAT IS CLAIMED IS:

1. A Zn-Ni based composite plated steel sheet excellent in corrosion resistance and workability which comprises a f errous substrate and, provided thereon, a Zn-Ni based composite electroplating layer comprising 1-15% by weight of Ni, 0.1-10% by weight of Cr, 0.1-10% by weight of SiO 2' 0.01-3% by weight of Fe and the balance being essentially Zn.

2. A ZnNi based composite plated steel sheet according to claim I which additionally comprises a Zn or Zn based alloy electroplating layer provided as upper layer on the composite electroplating layer.

3. A Zn-Ni based composite plated steel sheet according to claim 2 wherein amount of the upper Zn 2 or Zn based alloy electroplating layer is 1-5 g/m

4. A Zn-Ni based composite plated steel sheet according to claim 1 which additionally comprises a hardly soluble chromate film layer of 10-150 mg/m 2 in total Cr content provided on the composite electroplating layer and an epoxy resin thin film layer containing agglomerated Sio 2 provided on said chromate film layer.

5. A Zn-Ni based composite plated steel sheet according to claim 4 wherein the chromate film layer has a content of water-soluble matter of 5% or less.

6. A Zn-Ni based composite plated steel sheet according to claim 2 which additionally comprises a hardly soluble chromate film layer of 10-150 mg/m 2 in total Cr content provided on the Zn or Zn based alloy electroplating 1 1 layer and an epoxy resin film layer containing agglo merated SiO 2 provided on said chromate fil- layer.

7. A Zn-Ni based composite plated steel sheet according to claim 4 wherein content of the agglomerated Sio 2 in the epoxy resin film layer is 20-80% by weight of the epoxy resin.

8. A Zn-Ni based composite plated steel sheet according to claim 4 wherein the SiO 2 is a secondary agglomerate having a particle size of 0.3-3 i which is formed by agglomeration in the epoxy resin.

9. A Zn-Ni based composite plated steel sheet according to claim 4 wherein the epoxy resin film layer has a thickness of 0.3-3 p.

10. A Zn-Ni based composite plated steel sheet according to claim 1 wherein the ferrous substrate is a dull finished steel sheet, a bright finished steel sheet, a high-tensile steel sheet or a low corrosion rate steel sheet.

11. A method for making a Zn-Ni based composite plated steel sheet which comprises subjecting a ferrous substrate to electrolysis in an acidic Zn-Ni electroplating bath containing 5-50 g/1 of SiO 2 and Fe 2+ and Cr 3+ which are adsorbed to SiO 2 to positively charge the S'02.

12. A method for making a Zn-Ni based composite plated steel sheet which comprises subjecting a ferrous substrate to electrolysis in an acidic Zn-Ni based composite electroplating solution containing 5-50 g/l of Sio 2 and Fe 2+ and Cr 3+ which are adsorbed to Sio 2 to 11.

- positively charge the S'021 then to formation of a hardly soluble chromate film and finally to coating of agglomerated Sio 2 -containing solvent type epoxy resin thin film.

13. A method according to claim 11 wherein the Zn-Ni based composite electroplating solution containing 30-60 g/l of Zn 2+, 5-60 g/l of Ni 2+, 1-30 g/l of Cr 3+ 2+ 5-50 g/l of SiO 2 and 0.1-10 g/l of Fe

14. A method according to claim 11 wherein the electrolysis is carried out at a current density of 50-300 A/dm 2.

15. A method as claimed in claim 11 or claim 12 carried out substantially as described herein.

16. Plated steel sheet whenever prepared by a method as claimed in any one one of claims 11 to 15.

17. Material comprising a ferrous substrate having on at least a portion of the or a surface thereof a layer comprising 1 to 15% by weight ot' Ni, 0.1 to 10% by weight of Cr, 0.1 to 10% by weight of Sio 2' 0.01 to 3% by weight of Fe, the balance being Zn and incidental impurities and, optionally, one or more incidental ingredients used in plating a ferrous substrate.

18. Material as claimed in claim 17, also having any one of the features specified in any one of claims 2 to 10, or any combination of such features.

19. Material as claimed in claim 17 or claim 18, wherein the ferrous substrate comprises a steel sheet.

4 46 -

20. Material as claimed in clAim 17 substantially as described herein.

21. Material as described in claim 17 substantially as described in any one of the Examples herein.

22. An automobile body part comprising steel sheet as claimed in any one of claims 1 to 10 and 16 or material as claimed in any one of claims 17 to 21.

23. Any novel feature described herein or any novel combination of herein described features.

PublIshed 1989 at The Patent=oe. State House, 66171 High Holborn. London WC1R 4TP. Further copies =Lay be obtained &om The PatentoffLoe.