CA1211406A

CA1211406A - Plated steel sheet with chromate and composite silicate resin films

Info

Publication number: CA1211406A
Application number: CA000393175A
Authority: CA
Inventors: Tomihiro Hara; Masaaki Yamashita; Masahiro Ogawa; Masakazu Tsukada
Original assignee: Nippon Kokan Ltd
Current assignee: JFE Engineering Corp
Priority date: 1980-12-24
Filing date: 1981-12-23
Publication date: 1986-09-16
Also published as: AU547534B2; DE3151115C2; DE3151115A1; AU7912281A

Abstract

ABSTRACT OF THE DISCLOSURE

A composite coating steel sheet is disclosed which has good corrosion resistance, paintability and corrosion resistance after paint coating. The steel sheet comprises a plated base steel sheet, such as a galvanized, zinc alloy plated or aluminum plated steel sheet, and a chromate film formed on the plated base steel sheet. A composite silicate resin film is formed on the chromate film. The composite silicate film is composed of a reaction pro-duct of a colloidal silica, an organic resin and a silane compound.

Description

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This invention relates to a steel sheet having a composite coating and particularly to a plated base steel sheet provided thereon with multiple coatings to improve corrosion resistance, paintability and corrosion resistance after paint coating.
Chromate treatment has been commonly employed as a rust preventive treatment for galvanized steel sheets, zinc alloy plated steel sheets and aluminum plated steel sheets. In most cases, such treatment is intended merely as a temporary rust preventive measure and the corrosion resistance thereby obtained is at a low level such that white rust appears in 24 to 48 hours when the treated steel sheets are subjected to a salt spray test. ~ccordingly, in a case where the products are intended for use under a severe corrosion environment for a long period of time, there has been no practical way other than applying a thick paint coating at a level of more than 10~ to prevent corrosion.
However, in recent years the price of paint has increased rapidly, reflecting the increase in the price of petroleum products, and it is strongly desired to develop steel sheet products having good corrosion resistance, which can be produced simply by a surface treatment without using paint.

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Having regard to these circumstances, there has been developed in the art a chromate treatment.
A typical one is a coating type chromate treatment in which various binders are added to the chromate treat-ment solution, whereby products having improved cor-rosion resistance are produced. Even in this case, however, the corrosion resistance thereby obtainable is at best at a level of 200 hours against formation of white rust when subjected to a salt spray test.
Even when painting is required to provide an aesthetic outer appearance, it is usual to attempt to cut down the costs for the paint by choosing a low grade paint or by minimizing the thickness of the paint coating. In such a case, the steel sheets are required to have not only high corrosion resistance but also good paintability and corrosion resistance after painting. It is necessary that these properties be well balanced.
However, there have been no chromate treated products which fully satisfy these requirements. Some products which satisfy the requirement for high corrosion resistance, tend to fail in providing adequate paintability or corrosion resistance after painting whereas those having good paintability tend to fail in the requirement for high corrosion resistance.
For example, so-called unichrome treatment is known so as to provide high corrosion resistance with-out painting by means of a reactive chromate.

~ILZ~L~4~6 According to this method , a plated steel sheet is dipped in a trea-tment solution composed of chromic acid and a mineral acid for a long period of time to form a chromate film coating having a thickness of from about 500 to about 700 mg/m2 calculated as the amount of metal chromium.
However , such a treatment has drawbacks that the dipping process requires a long period of time and the degradation or aging of the treatment solution is rapid , and it is totally unsatisfactory as a strip coating technique. Further , as the chromate fi~m is rather thick , it is susceptible to cracking and its paintability is not good. If the thickness of the chromate film is decreased to a level of 100 mg/m2 , the film cracking may be avoided , but the corrosion resistance is reduced to a level of 100 hours , thus losing the high corrosion resistant char t r ti ac e lS CS.
On the other hand , in coating type chrcnate treatment , a binder is used to fix or trap a substantial amount of chromium therein and to prevent the film cracking by the binder. However , it has the drawback that the solution is susceptible to gelation, and even when it is not susceptible to gelation , it tends to form a thick film which requires a special treatment for d~ying.
In recent yeæs , it has been proposed , for the purpose of avoiding the above mentioned difficulties , to form a double layer coating by duplex plating treatment. Namely , a thin chromate -film is formed as the first layer in a short period of time , and then ~n inorganic or organic substance is coated thereon to form a film as the seoond layer which protects the first layer of the chromate film.
Use of inorganic materials is 4~6 disclosed in Japanese Patent Application 60283 filed May 31, 1973 published under No. 9545/75 on January 31, 197S, Saburo Ayusawa et al and Japanese Patent Publi-cation No. 19981/78 published June 23, 1978 (Applic-ation No. 50791, filed May 9, 1973) Saburo Ayusawa et al, in which a chromate film is electrolytically formed as the first layer and a treatment solution com-prising a chromic acid and silica sol is applied and then dried to-form a film as the second layer. High 10, corrosion resistance is obtainable by forming the second layer sufficiently thick. However, the film there~y obtained is apt to be peeled off because of the thick silica layer and thus lacks in durability after processing. Further, as the silica sol has poor affinity ~o a paint, the treated surface is not satis-factory as a substrate for paint coating.
A typical example in which an organic mate-rial is used for the second layer, is Japanesepatent Publication ~o. 35620/77 published September 10, 1977 20 (Application ~o. 4646, filed January 22, 1969) Hiroshi Uchida et al, in which a chromium hydrated oxide layer is formed as the first layer and then a water soluble organic resin layer is formed as the second layer.
However, no adequate effectiveness cannot be expected from the organic resin layer disclosed therein, since the functional groups present in the organic resin of _ ~ _ , .
; ~, 4~6 the second. layer tend to attract water and therefore it is impossible to obtain a hlgh level of corrosion resistance. Even if the thickness of the second layer is increased as much as 1~, a high level of corrosion resistance will not be obtainable, and in such a case, it will be difficult to completely dry the film by hot air only and a special apparatus for drying will be required.
Further, in ~apanese Patent Publications Nos.
10 36100/74 published Septe~ber 27, 1974 (Application ~o. 48605, filed July 19, 1960) Saburo Ayusawa et al, ~o. 18445/75, published June 28, 1975 (Application 117003, filed December 23, 1970) Kohi Yama~a ~t al, ~o. 4611/74 published February i, 1974 (Application ~o. 32177j filed April 15, 197G) Akimi Umezono et al and ~o. 1986/74, published January 17, 1974 (Appli-cation No. 23755, filed March 20, 1970) Akimi Umezono et al, it is proposed to form a chromate film as the first layer and an organic film as the second layer.
~owever, in each of these cases, the functional groups in the organic resins in the second layer tend to attract water and it is impossible to obtain a high level of corrosion resistance. These products are pre-sumably effective as a substrate for painting taking advantage of the functional groups.
As menticned above, the products having a second layer formed with an inorganic or organic mate-rial are effective either for corrosion resistance without painting or for paintability, but they can not satisfy both of the required properties at the same time. One possibility might be to form the second layer as a mixture of an organic material and in inor-ganic material. However, a mere mi~ture does not pro-vide better results, and to the contrary, it is likely in many cases that desirable properties of the individual components will be impaired.
Thus, it has been difficult to produce chromate treated steel sheets which are substrates for painting and which at the same time have superior corrosion ~ resistance.
Accordingly, the present invention seeks to overcome the above mentioned difficulties with con-ventional surface treated steel sheets, and to provide steel sheets having good corrosion resistance, paint-ability and corrosion resistance after painting.
It has now been found that superior corrosion resistance, paintability and corrosion resistance after painting are obtainable by forming, on a plated base steel sheet, for example, a galvaniæed steel sheet, a zinc alloy plated steel sheet or an aluminum steel sheet, a first layer of a chromate film by chro-mate treatment of either reaction type or coating type and then forming, on such first layer, a second layer of a composite silicate resin film composed of a silica sol and an organic resin. If either one of the i, . ~
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first and second layers is omitted, the desired level and balance of the properties are not obtainable.
Thus, the present invention provides a com-posite coating steel sheet which comprises a plated base steel sheet, a chromate film formed on the sur-face o~ the plated steel sheet and a composite silicate resin film formed on the chromate film and comprising a colloidal silica, an organic resin and a silane com-pound.
In another aspect of the invention there is provided a process for forming the composite coated steel sheet.
The invention is illustrated by reference to the accompanying drawings in which:
Figs. 1 and 2 are graphs illustrating the relationship between the coating amount of chromium and the thickness of the composite silicate resin film, and the corrosion resistance thereby obtained.
As the plated base steel sheet there may be 2Q used a galvanized steel sheet, a zinc-iron alloy plated steel sheet, a zinc-nickel alloy plated steel sheet, a zinc-manganese alloy plated steel sheet, or an aluminum plated steel sheet. The steel sheet may also be a multi-layer plated steel sheet, such sheets having become popular in recent years, and which have two or more such plated layers. In the case of the zinc-iron alloy plated steel sheet, the iron content in the plated layer is from 5 to 50% by weight, pre-ferably from 10 to 30% by weight. If the iron content is outside this range, the corrosion resistance and paintability become poor. In the case where a zinc-nickel plated steel sheet is employed, the nickel content in the plated layer is from 5 to 20% by weight, preferably from 12 to 13% by weight. If the nickel ~L2~ 6 content is less than 5%, the corrosion resistance be-comes poor. On the other hand 9 if the nickel content is more than 20%, it simply adds to the cost and such is not economically practical.
CHROMATE FILM
A chromate film is formed onthe above mentioned plated base steel sheet as the first layer.
The chromate treatment to form this first layer of the chromate film may be conducted in accordance with chromate treatment known per se in the art. The amount of chromium deposited on the base steel sheet must be from 10 to 150 mg/m2, preferably from 40 to 100 mg/m2. If the amount is less than 10 mg/m2, the film tends to be uneven. On the other hand~ an excessive amount over 150 mg/m2 is not desirable as it facilitates degradation of the treatment solution and adds to the costs. As a typical example, a reaction type chromate treatment solution comprises from 1 to 100 g/l, as calculated as metal chromium, of a water soluble chromium compound9 and from 0.2 to 20 g/l of sulfuric acid, as major components, in which the tri~
valent chromium content in the total chromium is not more than 50% by weight, preferably from 20 to 35% by weight. Proper amounts of heavy metal ions, for example, Zn2 , Co2 or FE3 , or other mineral acids, ~- for example, phosphoric acid or hydrofluoric acid, may also be added.
With respect to the chromium compound of the major components, if the amount, calculated as metal chromium, is less than 1 g/l, it is difficult to obtain the desired chromate film in a short period of time. On the other hand, if the amount exceeds 100 g/l, the stability of the treatment bath will be impaired to a conside~able extent.

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With respect to sulfuric acid, if the amount is less than 0.2 g/l the desired chromate film cannot easily be obtained in a short period of time, and uni-formity in the treatment becomes poor. On the other hand, if the amount exceeds 20 g/l, the etching rate of zinc tends to be too fast to be desirable.
If the proportion of CR 3 in the total chromium exceeds 50%, the stability of the bath will be dis-turbed, whereupon the bath tends to undergo gelation, and the corrosion resistance of the steel sheet before painting will be poor so that even when the coating amount of the chromate film is at the predetermined level, the desired high level of corrosion resistance is not obtainable.
The function of the heavy metal ions, for example, Zn2+, which may be added as well as the afore-mentioned major components, is to improve the treating efficiency of the treatment solution, and their amounts may suitably be determined depending upon the pro-portions of the major components.
A~ a typical example of a coating type chromate treatment solution, there may be mentioned a treatment solution prepared by adding to the above-mentioned reaction type chromate treatment solution, an organic polymer resin containing in its molecule a substantial number o~ carboxyl groups and which is water soluble and compatible with the above-mentioned reaction-type chromate treatment solution and adjusting the pH to from 2.0 to 3.5. The organic polymer preferably has an average molecular weight of from 1,000 to 500,000.
The amount of the organic polymer is selected within a range of from 0.02 to 30 gtl calculated as the resin.
If the amount is less than 0.02 g/l, it is not possible to completely trap chromium in the film when the film is formed. On the other hand, if the amount exceeds g _ .

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30 g~l, the stability of the bath tends to be poor. If the pH is less than 2.0, the bath tends to have characteristics similar to the reaction-type chromate treatment solution, and if the pH exceeds 3.5, the stability of the bath tends to be impaired.
In any event, the chromium amount in the first layer of the chromate film should be within a range of from 10 to 150 mg/m2. It is important that when the treatment solution for the second layer is applied, there should not be elution of chromium from the first layer. If an eluted component from the first layer enters the composite silicate resin treatment solution during the treatment of the second layer, the balance of the resin treatment solution will be disturbed, and in an extreme case, gelation of the treatment solution will be led. Accordingly, it is necessary to conduct forcible drying and washing with water after the treatment of the first layer so as to prevent elution of the components from the first layer.
COMPOSITE SILICATE RESIN FILM
The composite silicate resin film is formed on the above mentioned first layer of the chromate film.
The composike silicate resin is composed of a resin or a mixture of resins, which are prepared by reacting a water dispersible silica with a water soluble or water dispersible organic polymer resin having in its molecule a hydroxyl group (for example, polyvinyl alcohol, hydroxyethyl cellulose, starch, polyester, alkyd, epoxy ester or acrylic copolymer) in the presence of a silane compound. As the silica, so-called colloidal silica having a particle size of 7 to 100 nm, pref`erably from 10 to 50 nm, is used. The resin to be used may be any resin so long as it can react and bond with the silica. Further, ultraviolet or electron beam curing type functional groups may be introduced into the structure of the resin.

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The role of the silane compound is to act as a catalyst during the composite forming reaction of the silica with the organic resin and to perform an important function as a cross-linking agent ~or the reactants and as a cross-linking agent to establish a firm bond between the second layer and the first layer. Suitably, silane compounds include commercially available products.
The ratio of the water dispersible silica to the water soluble or water dispersible organic resin in the composite silicate resin composition, must be from 5:95 to 95:5, based on the weight ratio of the solids content. If the ratio is less than the lower limit, a high level of corrosion resistance is not obtainable even when applied onto the first layer of the chromate film. On the other hand, i~ the ratio exceeds the upper limit, good paintability is not obtainable. The amount of the silane compound is suitably fromO.5 to 15% by weight, preferably from 1 to 10% by weight, based on the weight of the total solids content of the silica and organic resin. If the amount is less than 0.5% by weight, adequate cross-linking with the chromate film cannot be expected. On the other hand, if the amount exceeds 15% by weight, no further improvement of the effect-iveness is observed.
Further, by an addition of an alkoxide chelate compound to the above mentioned composite silicate resin treatment solution, the corrosion resistance can be improved. The alkoxide chelate compound suitably has the structure R2MR2, Rm3', MR4', or R3MR', where R
is an allyl group or an aryl group, which may have an amino group or a mercapto group on its side chain, M
is titanium, zirconium or aluminum, and R' is a radical selected from an alkoxy group having ~rom 1 to 8 carbon atoms or an alkoxyalkoxy group having from 2 ~LZ~ 6 - to 10 carbon atoms, which is condensed with a di-carboxylic acid, a hydroxycarboxylic acid, a di-ketone, an ester or an alkanolamine, as the ligand.
The alkoxide chelate compound is added to the composite silicate resin in a solids content weight ratio of the resin: the alkoxide chelate compound being from 97:3 to 80:20. If the amount of the alkoxide chelate compound is less than the lower limit, free hydrcxyl groups left in the cured film becomes sub-stantial and it is impossible to improve the cor-rosion resistance or water repelling property to an adequate degree. On the other hand, if the amount is in excess of the upper limit, the condensation of the alkoxide chelate compound itself occurs preferentially, and it becomes difficult to form a uniform film. Further, the alkoxide chelate compound tends to undergo self-polymerization and becomes viscous as time passes thus leading to thickening of the composite silicate resin treatment solution whereby the treatment solution becomes difficulty applicable after several days from its preparation. In order to avoid, such a difficulty, at least one of the additives selected from the group consisting of oxy acids of molybdenum, tungsten, vanadium, tin, boron and silicon and salts of such oxy acids may be added to the composite silicate resin treatment solution in place of the alkoxide chelate compound.
The amount of such additives should be not more than 10% by weight, preferably from 0.3 to 5% by weight, based on the solids content weight of the composite silicate resin treatment solution. If the amount exceeds 10% by weight, there is the possibility of deterioration in the stability of the treatment solution, which is undesirable. ~

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Further, one or more such additives may be added together with the above mentioned alkoxide chelate compound. In such a case, the total amount of such additives and the alkoxide chelate compound should be not more than 10% by weight, preferably, from 0.3 to 5% by weight, based on the solids content weight of the composite silicate resin, and the ratio of the alkoxide chelate compound to such additives, should be from 95:5 to 5:95, preferably from 80:20 to 20:80.
Effectiveness is obtainable within the above ranges, and if the total amount of additive exceeds the upper limit, there is the possibility that the stability of the treatment solution is disturbed.
In the case where the composite silicate resin is composed of a reaction product of polyvinyl alcohol and a silica sol, it is possible to improve the drying characteristic of the film by incorporating, together with the alkoxide chelate compound, one or more additives selected from the group consisting of water soluble salts of copper, zinc, aluminum, zirconium, chromium, cobalt, and nickel (for instance, zinc chloride), and coordination compounds of such elements (for instance, ethylenediamine tetraacetic acid zinc complex salt). The amount of such additives is preferably fromO.3 to 5% by weight, based on the solids content weight of the composite silicate resin.
If the amount exceeds this upper limit, there is a possibility that the stability of the composition is disturbed. Further9 the ratio of the composite silicate resin plus such additives to the alkoxide chelate compound is from 97:3 to 80:20, based on the solids content weight.

Further, in the case where an ultraviolet or electron beam curing type resin is employed, it is possible to facilitate the curing of the composite silicate resin film by incorporating a photo-sensitizer, for example, zinc oxide, titanium oxide (anatage type) or titanic acid, and an oxy acid of molybdenum, tungsten or vanadium (for instance, vanadium trioxide) or its salt (for instance, lithium orthovanadate). This is attributable to the fact that an oxidation reduction reaction occurs among the three components, whereby the functional groups in the resin and a cation formed by the-reaction,form a salt bond or a coordination hond.
The amount of the oxy acid or its salt is from 0.1 to 6% by weight, based on the solids content weight of the composite silicate resin, and the amount of the photosensitizer is from 30 to 200%
by weight, based on the amount of the oxy acid or its salt.
RELATION BETWEEN THE THICKNESS OF THE CHROMATE FILM AND
THE THICKNESS OF THE COMPOSITE SILICATE RESIN FILM
Now, the relation between the first layer of the chromate film and the second layer of the composite silicate resin film will be described. An interrelation exists, as shown in Fig. 1, between the coating amount ~ of chromium for the first layer and the film thickness ; of the second layer. For a coating amount of chromium from 10 to 150mg/m2, the film thickness of the second layer must be from 0~01 to 4~. Therefore, in order to obtain a predetermined level of corrosion resistance, it is necessary to select the coating amount of chromium for the first layer and the film thickness of the second layer based on this relationship. For instance, in ;
I - 13a -, ~2~ 6 order to obtain corrosion resistance of 500 hours, if the coating amount of chromium in the first layer is 40mg/m2, the film thickness of the second layer must be at least 1.5~, and if the coating amount of chromium is 150mg/m2, the film thickness of the second layer must be at least 0.4~. Generally, for the purpose of practical application, it is desirable that for a coat-ing amount of chromium from 10 to 150mg/m2, the film thickness of the second layer is set to be from 0.4 to 4~. The above ranges are desirable from the standpoint of the manufacture of the products, and as is apparent from the graph of Fig. 1, if the film thickness of the second layer is set to be less than 0.4~, it is necessary to increase the coating amount of chromium, which tends to lead to degradation of the chromate treatment solution. On the other hand, if the film thickness of the second layer is set to be greater than 4~, the costs will increase to an uneconomical level and the products become difficult to weld under commonly employed welding conditions, although the degra~dation of the chromate treatment solution will be reduced.
However9 in a case where such a high level of corrosion resistance is not required, the film thickness of the second layer will be less than 0.4~ and at least 0.01~ for the coating amount of chromium in the first layer being from 10 to 150mg/m2. In this case, the conditions for both layers to provide the desired corrosion resistance may be presented in a more obvious manner by modifying the graph as shown in Fig.

2, in which the vertical axis is the film thickness of the second layer and the horizontal axis is the coating amount of chromium of the first layer, and the corrosion resistance represented by the spraying time of a salt spray test is presented as a parameter. From Fig. 2, it will be understood that in order to obtain corrosion resistance of 200 hours, for instance, the film thick-ness of the second layer may be set to be O.l~ when the coating amount of chromium of the first layer is 80mg/m2, and 0.02~ when the coating amount of chromium is 120mg/m2.
In a particular embodiment the chromate film has a thickness of from 70 to lO0 mg/m2 calculated as metal chromium and the composite silicate resin film has a thickness of from 0.5 to O.9~m.
Having thus described the relationship between the coating amount of the first layer and the film thick-ness of the second layer, it should be added that it is essential to provide a double layer structure composed of the chromate film and the composite silicate resin film. Without this double layer structure, a high level of corrosion resistance is not obtainabIe.
Now, the invention will be described with reference to the Examples.

-: ~lZ1~6 EXAMPLE~
Tests for white rust forma-tion. paintability and corrosion resistance af-ter paint coating were conducted with respect to the steel sheets No. 1 to No. 37 of the present in-vention which had various coating amounts of chromium in the respective first layers and various film thicknesses of the respective second layers, as indicated in the Table below. The results thereby obtained are shown in comparison with comparative steel sheets No. 38 to No. 53. As -the chromate treatment solu-tion for the first layer, the following three compositions were used as representatives:
( 03. H2~04. ~3~04) = (10 g/l, 2 g/l, 2 g/l) ,,,,~.,"i ,. ~
B: (CrO3. H2~04. crJ , Zn~ ) = (10 g/l, 2 g/l, 2 g/l. 3 g/l) C: To the composition ~, 2 g/l of a polyacrylic acid having a molecular weight of about 100,000 was added, and then the pH
was adjusted to 3 with aqueous ammonia.
Further, as the composite silicate resin treatment solution, the following three compositions were used as repre-sentatives:
(a): As the organic resin, an acrylic copolymer and an epoxy resin were mixed in a solid content ratio of 70:30, and the mixture was reacted and bonded with silica sol in a solid content ratio of 60:40.
(b): To the treatment solution (a), trifunctional dibutyl titanate prepared by reacting butyl titanate with triethanol c~ Iqft: ' amine was added as the alkoxide ch~te compound in a solid content weight ratio of 90:10.
(c)s To the treatment solution (a), ammonium metavanadate was - 16 _ added in a solid conten-t weight ratio of 100:1.
However, it should be unders-tood that the treatment solutions are not limited to those men-tioned above.
As the plated base steel sheets, there were used, an electro-galvanized steel sheet, an zinc-nickel alloy plated steel sheet, and an electrolytically formed zinc-iron alloy plated steel sheet. As the plated base sheet, other zinc alloy plated steel :sheets or aluminum plated steel sheets may also be used.
~he steps o~ the treatments were as follows: ~
Weak alkaline degreasing _ washing with water --~ drawing _~ i reaction type chromate treatment (A, B) --~ drawing --~ washing with water _ drying __~ application of the composite silicate resin ___ drying.
Weak alkaline degreasing _ washing with water ~ drawing _~
coating type chromate treatment (C) __~ forcible drying application of the composite silicate resin ~ drying.
It will be seen from the Table below, that the steel sheets No. 1 to No. 37 of the present invention have distinctly superior corrosion resistance to the conventional comparative steel sheets r~O. 38 to ~3. It will also be seen that they are superior to the conventional phosphate treated steel sheet tlei~
. (comparative steel sheet No. 44~ in ~ paintability.
rhe comparative steel sheet No. 45 iB a steel sheet on which the composite silicate resin film was formed without the chromate treatment. ~rhe comparative steel sheet No. 46 is a steel sheet on which the composite silicate resin film was formed O.o~par~ ;v~
after phosphate treatment. The ompar&~e~ steel sheet No. 45 has good paintability comparable to the steel sheets of the `` ~2~il 4~E~

present invention, but it is inferior in corrosion resistance and in corrosion resistance after painting.
The comparative steel sheet No. 46 does not have adequate corrosion resistance and paintability and it is inferior in corrosion resistance after painting.

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Claims

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

1. A composite coating steel sheet having good corrosion resistance, paintability and corrosion resistance after paint coating, which comprises a plated base steel sheet, a chromate film formed on the surface of the plated base steel sheet and a composite silicate resin film formed on the chromate film, said composite silicate resin film being derived from a colloidal silica an organic resin and a silane compound; said organic resin containing hydroxyl groups and being reactable with said colloidal silica, the weight ratio of resin to colloidal silica being 95:5 to 5:95, based on the solids content, and said silane compound being present in a catalytic and cross-llnking amount of at least 0.5%, by weight, based on the solids content of colloidal silica and organic resin, effective to catalyse reaction between said colloidal silica and said organic resin, and to cross-link said colloidal silica and resin and form a bond between said composite film and said chromate film, provided that when said plated sheet comprises an alloy plating containing at least one metal selected from iron and nickel, said at least one metal is present in an amount of at least 5% by weight of said plating.

2. The steel sheet as claimed in claim 1, in which said plated base steel sheet is a galvanized steel sheet.

3. The steel sheet as claimed in claim 1, in which said plated base steel sheet is a zinc-iron alloy plated steel sheet.

4. The steel sheet as claimed in claim 3, in which the zinc-iron alloy layer of said zinc-iron alloy plated steel sheet contains from 5 to 50% by weight of iron.

5. The steel sheet as claimed in claim 3, in which the zinc-iron alloy layer of said zinc-iron alloy plated steel sheet contains from 10 to 30% by weight of iron.

6. The steel sheet as claimed in claim 1, in which said plated base steel sheet is a zinc-nickel alloy plated steel sheet.

7. The steel sheet as claimed in claim 6, in which the zinc-nickel alloy layer of said zinc-nickel alloy plated steel sheet contains from 5 to 20% by weight of nickel.

8. The steel sheet as claimed in claim 6, in which the zinc-nickel alloy layer of said zinc-nickel alloy plated steel sheet contains from 12 to 13% by weight of nickel.

9. The steel sheet as claimed in claim 1, in which said plated base steel sheet is a zinc-manganese alloy plated steel sheet.

10. The steel sheet as claimed in claim 1, in which said plated base steel sheet is an aluminum plated steel sheet.

11. The steel sheet as claimed in claim 1, in which said plated base steel sheet is a multi-layer plated steel sheet having at least two layers selected from a zinc layer, a zinc-iron alloy layer, a zinc-nickel alloy layer, a zinc-manganese alloy layer and an aluminum layer.

12. The steel sheet as claimed in claim 1, 3 or 4, in which the coating amount of the chromate film is from 10 to 150 mg/m2, calculated as metal chromium.

13. The steel sheet as claimed in claim 5, 6 or 7, in which the coating amount of the chromate film is from 10 to 150 mg/m2, calculated as metal chromium.

14. The steel sheet as claimed in claim 8, 9 or 10, in which the coating amount of the chromate film is from 10 to 150 mg/m2, calculated as metal chromium.

15. The steel sheet as claimed in claim 1, 2 or 3, in which the coating amount of the chromate film is from 40 to 100 mg/m2, calculated as metal chromium.

16. The steel sheet as claimed in claim 5, 6 or 7, in which the coating amount of the chromate film is from 40 to 100 mg/m2, calculated as metal chromium.

17. The steel sheet as claimed in claim 8, 9 or 10, in which the coating amount of the chromate film is from 40 to 100 mg/m2, calculated as metal chromium.

18. The steel sheet as claimed in claim 1, 2 or 3, in which said composite silicate resin film has a thickness of from 0.04 to 4 µm.

19. The steel sheet as claimed in claim 4, 5 or 6, in which said composite silicate resin film has a thickness of from 0.04 to 4 µm.

20. The steel sheet as claimed in claim 8, 9 or 10, in which said composite silicate resin film has a thickness of from 0.04 to 4 µm.

21. The steel sheet as claimed in claim 11, in which said composite silicate resin film has a thickness of from 0.4 to 4 µm.

22. The steel sheet as claimed in claim 1, 2 or 3, in which said composite silicate resin film has a thickness of from 0.5 to 0.9 µm.

23. The steel sheet as claimed in claim 1, 3 or 4, in which the coating amount of said chromate film is from 70 to 100 mg/m2 calculated as metal chromium, and said composite silicate resin film has a thickness of from 0.5 to 0.9 µm.

24. The steel sheet as claimed in claim 6, 7 or 8, in which the coating amount of said chromate film is from 70 to 100 mg/m2 calculated as metal chromium, and said composite silicate resin film has a thickness of from 0.5 to 0.9 µm.

25. The steel sheet as claimed in claim 9, 10 or 11, in which the coating amount of said chromate film is from 70 to 100 mg/m2 calculated as metal chromium, and said composite silicate resin film has a thickness of from 0.5 to 0.9 µm.

26. The steel sheet as claimed in claim 1, in which said composite silicate resin film was formed by application of a composite silicate resin treat-ment solution prepared by adding, to a treatment solution composed of a mixture of a colloidal silica and an organic resin in a solids content weight ratio of from 5:95 to 95:5, from 0.5 to 15%
by weight, based on the weight of the total solids content of said colloidal silica and organic resin, of a silane compound.

27. The steel sheet as claimed in claim 26, in which said silane compound was added in an amount of from 1 to 10% by weight, based on the weight of the total solids content of said colloidal silica and organic resin.

28. The steel sheet as claimed in claim 26, in which an alkoxide chelate compound was further added to said composite silicate resin treatment solution, the solids content weight ratio of the treatment solution to the alkoxide chelate compound being from 97:3 to 80:20.

29. The steel sheet as claimed in claim 26, in which one or more additives selected from the group consisting of oxy acids of molybdenum, tungsten, vanadium, tin, boron and silicon, and salts of such oxy acids, were added to said com-posite silicate resin treatment solution, in a solids content amount of not more than 10% by weight, based on the solids content of the com-posite silicate resin.

30. The steel sheet as claimed in claim 29, in which said one or more additives were added to the composite silicate resin solution, in a solid content amount of from 0.3 to 5% by weight, based on the solid content of the composite silicate resin.

31. The steel sheet as claimed in claim 29 or 30, in which said additive is ammonium metavanadate.

32. The steel sheet as claimed in claim 26, in which an alkoxide chelate compound and one or more additives selected from the group consisting of oxy acids of molybdenum, tungsten, vanadium, tin, boron and silicon, and salts of such oxy acids,were added to said composite silicate resin treatment solution in the following solids content weight ratios:
Alkoxide chelate compound: said one or more additives = 95:5 to 5:95 Composite silicate resin > 10 Alkoxide chelate compound + said one or more additives.

33. The steel sheet as claimed in claim 26, in which said organic resin is a polyvinyl alcohol, and one or more additives selected from the group con-sisting of water soluble salts and coordination compounds of copper, zinc, aluminum, zirconium, chromium, cobalt and nickel, were added to the composite silicate resin treatment solution, in a solids content amount of from 0.3 to 5% by weight, based on the solid content of the treatment solution, and an alkoxide chelate compound was further added in the following solids content weight ratio:
Composite silicate resin + said one or more additives:
alkoxide chelate compound = 97:3 to 80:20.

34. The steel sheet as claimed in claim 26, in which said organic resin is an ultraviolet or electron beam curing type resin, and one or more additives selected from the group consisting of oxy acids of molybdenum, tungsten and vanadium and salts of such oxy acids, were added to said com-posite silicate resin treatment solution, in a solids content amount of from 0.1 to 6% by weight, based on the solids content of the treatment solution, and a photosensitizer selected from the group con-sisting of zinc oxide, titanium oxide (anatase type), and titanic acid, was added in a solids content amount of from 30 to 200% by weight, based on said one or more additives.

35. A composite coating steel sheet having good corrosion resistance, paintability and corrosion resistance after painting, which comprises a plated base steel sheet, a chromate film formed on the sur-face of the plated base steel sheet and a composite silicate resin film formed on the chromate film;
said chromate film being present in a coating amount of from 40 to 100 mg/m2, calculated as metal chromium; said composite silicate resin film being derived by reaction of a colloidal silica and a hydroxyl group-containing organic resin, in the presence of a silane compound; the weight ratio of resin to colloidal silica being 95:5 to 5:95, based on the solids content and said silane com-pound being present in an amount of 0.5 to 5%, by weight, based on the solids content of colloidal silica and organic resin; said silane compound being effective to catalyse reaction between said colloidal silica and organic resin for formation of said composite film, and being effective for cross-linking said colloidal silica and organic resin and to establish a bond between said chromate film and said composite silicate film, provided that when said plated sheet comprises an alloy plating containing at least one metal selected from iron and nickel, said at least one metal is present in an amount of at least 5% by weight of said plating.

36. A steel sheet as claimed in claim 35, in which said plated base steel sheet is a zinc-iron alloy plated steel sheet having a zinc-iron layer containing from 5 to 50%, by weight, or iron.

37. A steel sheet as claimed in claim 35, in which said plated base steel sheet is zinc-nickel alloy plated steel sheet having a zinc-nickel alloy layer containing 5 to 20%, by weight, of nickel.

38. A process of forming a composite coating steel sheet which comprises:
forming a chromate film on a surface of a plated base steel sheet, and applying a composite silicate resin treatment solution to said chromate film, said solution comprising a mixture of a colloidal silica and an organic resin reactable with said silica, in a solids content weight ratio of from 5:95 to 95:5, and a silane compound in a catalytic and cross-linking amount of at least 0.5%, by weight, based on the solids content of colloidal silica and organic resin, provided that when said plated sheet comprises an alloy plating containing at least one metal selected from iron and nickel, said at least one metal is present in an amount of at least 5% by weight of said plating.

39. A process according to claim 38, wherein said solution contains from 0.5 to 15%, by weight, of said silane compound based on the weight of the total solids content of said silica and resin.