Corrosion-resistant plated composite steel strip and method of producing same

Abstract: An electroplated composite steel strip having a high corrosion resistance comprises a steel strip substrate (1) and a corrosion resistant coating layer (2) which comprises at least a base plating layer comprising a zinc-based metal matrix (2a), a number of corrosion-­ preventing fine solid particles (3) consisting essentially of core fine particles of, for example, chromate, phosphate or aluminum, molybdenum or titanium compounds, and encapsulated by very thin coating membranes consisting of, for example, SiO₂, Al₂O₃, ZrO₂ or TiO₂ , and optionally a number of additional fine particles (4) consisting essentially of, for example, SiO₂, TiO₂ , Cr₂O₃ , Al₂O₃, ZrO₂, SnO₂ or Sb₂O₅.

Description: The present invention relates to a corrosion resistant plated composite steel strip and a method of producing the same. More particularly, the present invention relates to a corrosion resistant plated composite steel strip having a corrosion-resistant zinc-based plating layer containing corrosion-resistant fine particles in the form of microcapsules having coating membranes, and to a method of producing the same. It is known that, in the winter in North America and Europe, the freezing (icing) of road surfaces is prevented by sprinkling rock salt powder or calcium chloride powder on the road surface, and that the above mentioned icing-preventing material causes a corrosion and rusting of the bodies of cars traveling on those roads. Accordingly, there is a demand for a high corrosion resistant plated steel strip for car bodies which can be used under the above-mentioned circumstances, without allowing the forming of red rust on the car bodies, over a long period. There are two approaches for meeting the above-mentioned demand. In countries, for example, the U.S.A. and Canada, where the cost of electricity is relatively low, the corrosion resistance of the steel strip is promoted by forming a thick corrosion resistant coating layer on the steel strip. This is thick coating layer, however, causes the resultant coated steel strip to exhibit a reduced weldability, paint adhesion, and plating properties. In other countries, for example, Japan, where electricity is expensive and enhanced weldability, paint adhesion, and plating properties are required for the steel strip to be used for car bodies, a plated steel strip having a thin corrosion resistant electroplating layer has been developed. The plated steel strip of the present invention belongs to the above-mentioned category of plated steel strips having a thin corrosion resistant electroplating layer. In this type of conventional electroplated steel strip having a thin electroplating layer, a zinc alloy, for example, a zinc-iron, zinc-nickel of zinc-manganese alloy, is plated on a steel strip substrate, or zinc or a zinc-nickel alloy is electroplated on a steel strip substrate and a chromate treatment and an organic resinous paint are then applied to the electroplating layer. The zinc alloy-electroplated or zinc or zinc alloy-electroplated and painted steel strips have a thin coating layer at a weight of 20 - 30 g/m². The conventional electroplated steel strips having the above-mentioned thin coating layer are not considered satisfactory for attaining the object of the domestic and foreign car manufacturers, i.e., that the car bodies should exhibit a resistance to corrosion to an extent such that rust does not form on the outer surfaces of the car bodies over a period of use of at least 5 years, and perforation from the outer and inner surfaces of the car bodies does not occur over a period of use of at least 10 years. In particular, a 10 year resistance to perforation is demanded. Under the above-mentioned circumstances, investigations have been made into ways and means of obtaining a high corrosion resistant steel strip having a coating layer in which corrosion resistive fine solid particles are co-deposited with a plating metal matrix and are evenly dispersed within the plating metal matrix, i.e., a high corrosion resistant plated composite steel strip. The co-deposited, dispersed fine solid particles can impart various properties to the plating layer of the plated composite steel strip, and thus this co-deposition type plating method has been developed as a new functional plating method. Namely, this type of plating method has been recently disclosed in Japanese Unexamined Patent Publication Nos. 60-96786, 60-211094, 60-211095 and 60-211096. Japanese Unexamined Patent Publication No. 60-96786 discloses a method of producing a plated composite steel strip in which fine solid particles of rust-resistant pigments, for example, PbCrO₄ , SrCrO₄ , ZnCrO₄ , BaCrO₄ , Zn₃ (PO₄)₂ are co-deposited with a plating metal matrix, for example, Zn or a Zn-Ni alloy, to be evenly dispersed in the plating metal matrix. This type of plated composite steel strip is considered to have an enhanced resistance to rust and perforation. Nevertheless, according to the results of a study by the inventors of the present invention, the plated composite steel strip of Japanese Unexamined Patent Publication No.60-96786, in which the fine solid particles dispersed in the plating layer consist of rust-resistant pigments consisting of substantially water-insoluble chromates, for example, PbCrO₄ , SrCrO₄ , ZnCrO₄ or BaCrO₄ , cannot realize the above-mentioned corrosion resistance level of no rust for at least 5 years and no perforation for at least 10 years. This will be explained in detail hereinafter. Generally, the rust resistant pigment fine particles of the substantially water-insoluble chromates dispersed in a zinc-plating liquid exhibit a surface potential of approximately zero, and accordingly, when a steel strip is placed as a cathode in the zinc-plating liquid and is electrolytically treated, zinc ions are selectively deposited on the steel strip surface but there is a resistance to the deposition of the rust resistant pigment fine particles into the zinc-plating layer, and therefore, it is very difficult to obtain a plated composite steel strip having an enhanced corrosion resistance. Japanese Unexamined Patent Publication No. 60-211095 discloses a plated composite steel strip having a Zn-Ni alloy plating layer in which fine solid particles of metallic chromium, alumina (Al₂O₃) or silica (SiO₂) are co-deposited with and dispersed in a Zn-Ni alloy matrix. According to the disclosure of this Japanese Publication 1095, the metallic chromium is obtained from chromium chloride (CrCl₃); i.e., chromium chloride is dissolved in the plating liquid and releases chromium ions (Cr³⁺), and when the steel strip is immersed and electrolytically plated as a cathode in the plating liquid, metallic chromium particles and chromium oxide (Cr₂O₃·nH₂0) particles are deposited into the plating layer to form a Zn-Ni alloy plating layer containing metallic chromium (Cr) and chromium oxide (Cr₂O₃.nH₂O) particles. When alumina or silica particles are further co-deposited into the Zn-Ni-Cr-Cr₂O₃·nH₂O plating layer, the resultant plated composite steel strip exhibits an enhanced corrosion resistance compared with the plated composite steel having the Zn-Ni-Cr-Cr₂O₃·nH₂O layer, but the degree of enhancement of the corrosion resistance is small, and the Al₂O₃ or SiO₂ particle-containing, plated composite steel strip cannot realize a perforation resistance for at least 10 years. Under the above-mentioned circumstances, it is desired by industry, especially the car industry, that a high corrosion resistant plated composite steel strip having a rust resistance for at least 5 years and a perforation resistance for at least 10 years, and a method of producing the same, be provided. The present invention provides a corrosion-resistant electroplated composite steel strip comprising a steel strip substrate having on at least one surface a corrosion-resistant coating comprising at least an electroplated corrosion-resistant base layer which preferably weighs from 5 to 50 g/m² and which comprises a matrix of zinc or zinc alloy (e.g. with at least one of Fe, Co, Mn, Cr, Sn, Sb, Pb, Ni and Mo) having dispersed therein, preferably in an amount of from 0.1 to 30% of the weight of the base layer, fine solid anti-corrosion particles having (i) solid cores comprising at least one material which is soluble or sparingly soluble in water and selected from chromates, aluminium compounds, phosphates, molybdenum compounds, titanium compounds, and polypropylene and fluorine-containing polymer resins encapsulated by (ii) membranes which have a thickness of 1µm or less and which comprise at least one material which is insoluble in water or less soluble in water than the core material and which is selected from SiO₂, TiO₂, Al₂O₃, ZrO₂, ethyl cellulose, amino resins, polyvinylidene chloride resins, polyethylene resins and polystyrene resins. It also provides a method for producing a corrosion-resistant electroplated composite steel strip comprising electroplating a base layer onto at least one surface of a descaled steel strip substrate using an electroplating liquid containing (a) matrix-forming ions of zinc optionally together with ions of at least one other metal to be alloyed with zinc, (b) dispersed in the electroplating said liquid fine solid anti-corrosion particles, and (c) co-deposition-promoting agent for promoting the co-deposition of the anti-corrosion particles with the matrix-forming metal. The anti-corrosion particles can have solid cores of corrosion-resistance material, with the membranes being of material with better electrophoretic properties and so improving the co-deposition of the particles with the matrix metal from the electroplating liquid. The membrane material may control (e.g. slow down or even out) dissolution and/or reaction of the core material to improve co-deposition of the particles in the metal matrix and/or to improve or extend anti-corrosion activity in the plated base layer; thus the membrane material may be less soluble than the core material in the electroplating liquid and/or when the plated product is under corrosive attack; instead or in addition the membrane may not fully seal the core. The membrane material may itself have anti-corrosion activity, to supplement that of the core material. The encapsulating of the solid core particles can be effected by known chemical and physicochemical methods. Thus the core particles may be dispersed in an aqueous solution of water-soluble compound which is then converted to water-insoluble compound which deposits on the outer surfaces of the particles and forms the coating membrane thereon. For example, solid core particles can be encapsulated by SiO₂ membranes by suspending the solid particles in an aqueous solution of water-soluble silicate, adding acid or alkali to the solution to convert the silicate to SiO₂ which is insoluble in water and forms coating membranes of SiO₂ on the particles. The method can be applied to an organic coating membrane. The suitable encapsulating methods are disclosed in, for example, "New Engineering Review of Coating", pages 101 to 103, published on December 21, 1987 and "New Enmicrocapsulating Technology", pages 13 to 36, published on December 21, 1987. A corrosion resistant plated composite steel strip of the present invention can comprise : (A) a substrate consisting of a steel strip; and (B) at least one corrosion resistant coating formed on at least one surface of the steel strip substrate and comprising a base plating layer which comprises (a) a matrix consisting of a member selected from the group consisting of zinc and zinc alloys; and (b) said corrosion-preventing fine solid particles dispersed in the matrix and consisting essentially of fine core solid particles encapsulated by very thin organic or inorganic membranes. The fine core particles preferably comprise at least one member selected from chromates, aluminum compounds, phosphates, molybdenum compounds and titanium compounds. Such corrosion resistant plated composite steel strip can be produced by a method of the present invention which comprises; coating at least one surface of a substrate consisting of a descaled steel strip by at least first electroplating the substrate surface with a first electroplating liquid containing (a) matrix-forming metal ions selected from the group consisting of zinc ions and mixtures of ions of zinc and at least one metal other than zinc to be alloyed with zinc, (b) said corrosion-preventing fine solid particles dispersed in the electroplating liquid and consisting of fine core solid particles encapsulated by very thin organic or inorganic coating membranes, and (c) a co-deposition-promoting agent for promoting the co-deposition of the corrosion-preventing fine particles together with the matrix-forming method, to form a base plating layer on the substrate surface. In the accompanying drawings: Figure 1 shows the corrosion resistances of an embodiment of the corrosion resistant plated composite steel strip of the present invention, two comparative conventional plated composite steel strips, and a comparative conventional zinc-galvanized steel strip; Fig. 2 shows the relationship between the pH of plating liquids and the amounts of substantially water-insoluble chromate particles deposited from the plating liquids; Fig. 3 shows a relationship between concentration of Cr⁶⁺ ions in a plating liquid and the amount of substantially water-insoluble chromate particles deposited from the plating liquid; Fig.4 shows the relationship between oxidation-reduction reaction time of metallic zinc grains with Cr⁶⁺ ions in a plating liquid and concentration of Cr⁶⁺ ions in the plating liquid; and, Figs. 5A, 5B, 5C, and 5D, respectively, are explanatory cross-sectional views of an embodiment of the plated composite steel strip of the present invention. In the corrosion resistant plated composite steel strip of the present invention, at least one surface of a steel strip substrate is coated with a corrosion resistant coating layer comprising at least a base electroplated layer. The base layer comprises a plating matrix consisting of zinc or zinc alloy and anti-corrosion fine solid particles evenly dispersed in the matrix. These fine particles have solid cores encapsulated by organic or inorganic membranes and are in the form of microcapsules. The base layer is preferably formed on the steel strip substrate surface in a total amount of from 5 to 50 g/m², more preferably from 10 to 40 g/m². The base layer matrix consists of zinc or a zinc alloy with at least one additional metal, preferably selected from Fe, Co, Mn, Cr, Sn, Sb, Pb, Ni, and Mo. The content of additional metal in the zinc alloy is not limited to a specific level. The base layer optionally contains additional fine or colloidal particles comprising at least one member selected from SiO₂ , TiO₂ , Cr₂O₃ ,Al₂O₃ , ZrO₂ , SnO₂ and Sb₂O₅. The anti-corrosion particles in the form of microcapsules can have fine solid cores (of, for example, water-soluble or slightly water-soluble chromates, aluminum compounds, phosphates, molybdenum compounds, and titanium compounds) and very thin organic or inorganic coating membranes formed around the core particles. The water-soluble chromates include, for example, CrO₃, Na₂CrO₄, K₂CrO₄, K₂0.4Zn0.4CrO₃. Suitable partially or difficultly water-soluble chromates include, for examplle, PbCrO₄, BaCrO₄, SrCrO₄, and ZnCrO₄. The aluminum compounds include, for example, Zn-Al alloys and Al₂O₃·2SiO₂·2H₂O. The phosphates include, for example, Zn₃(PO₄)₂·2H₂O. The molybdenum compounds include, for example, ZnO·ZnMoO₄ , CaMoO₄·ZnOMoO₄ and PbCrO₄·PbMoO₄·PbSO₄. The titanium compounds include, for example, TiO₂·NiO·Sb₂O₃. The particle cores may be organic, of fluorine-containing polymer resins or polypropylene resins. The coating membrane formed around the particle core has a thickness of 1.0 µm or less and comprises at least one member selected from inorganic materials SiO₂ , TiO₂ , Al₂O₃ and ZrO₂) and organic materials (ethyl cellulose, amino resins, polyvinylidene chloride resins, polyethylene resins, and polystyrene resins). Preferred corrosion-resistant microcapsules for this invention can have one or more of the following effects and advantages. (1) Conventional corrosion-resistant fine particles, for example, chromate and phosphate particles, exhibit a surface potential of substantially zero or a very small value in an electroplating liquid. Accordingly, in an electroplating process relying on electrophoretic particle properties, the co-deposition performance of the conventional corrosion-resistant fine particles is unsatisfactory. SiO₂ , TiO₂ , Al₂O₃ , or ZrO₂ exhibit satisfactory surface potential in the electroplating liquid, even when in the form of a very thin membrane. Therefore, corrosion-resistant particles having solid cores of corrosion-resistant but non-electrophoretic material (for example chromate, phosphate, aluminum compound, molybdenum compound or titanium compound) and membranes of electrophoretic material (for example SiO₂ , TiO₂ , Al₂O₃ , ZrO₂) exhibit satisfactory electrophoretic and co-deposition properties. (2) The corrosion-resistant cores, (for example chromate or phosphate) have a relatively high solubility in the electroplating liquid and the thin coating membranes have substantially no or very low solubility in the electroplating liquid. For example, a sparingly water-soluble chromate particle is dissolved in small amount in the electroplating liquid and generates Cr⁶⁺ ions. When the concentration of Cr⁶⁺ ions in the electroplating liquid reaches or exceeds a predetermined level, the particle deposition is decreased, and the resultant plated layer on a substrate exhibits an undesirable black powder-like appearance and a low adhesion to the substrate. When the corrosion resistant core particles are coated with insoluble thin membranes, the resultant microcapsulated particles exhibit satisfactory resistance to dissolution in the electroplating liquid, and the electroplating liquid is maintained in a satisfactory stable condition over a long period and can produce plated composite steel strip of high quality. (3) The microcapsulated particles dispersed in the base layer enhance the corrosion resistance of the plated composite steel strip over conventional plated composite steel strip containing non-microcapsulated corrosion-resistant particles, because the anti-corrosion activity of the core particles is promoted by the thin coating membranes (for example, SiO₂ , TiO₂ , Al₂O₃ or ZrO₂ membranes) which have high corrosion-resistance. Referring to Fig. 1 which shows decreases in thickness of four different plated composite steel strips by a corrosion test, sample No. 1 is a plated composite steel strip which was produced in accordance with the method disclosed in Japanese Unexamined Patent Publication (Kokoku) No. 60-96,786 and had 23 g/m² of an electroplated layer consisting of a zinc matrix and 0.3% by weight of BaCrO₄ particles dispersed in the matrix. Sample No. 2 is a plated composite steel strip which was produced in accordance with the method disclosed in Japanese Unexamined Patent Publication (Kokai) No. 60-211,095 and had 20 g/m² of an electroplated layer of a matrix of zinc-nickel alloy containing 1% by weight of Ni with 1% by weight of metallic chromium (Cr) and chromium oxide particles and 1% by weight of Al₂O₃ particles dispersed in the matrix. Sample No. 3 is a plated composite steel strip of the present invention having 21 g/m² of an electroplated layer consisting of a matrix of zinc-cobalt alloy containing 10% by weight of Co with 4.0% by weight of corrosion-resistant fine solid particles(consisting of BaCrO₄ cores and SiO₂ coating membranes)and 1% by weight additional TiO₂ particles dispersed therein. Sample No. 4 is a zinc-galvanized steel strip which has a thick (90 g/m²) zinc-galvanizing layer and is believed to exhibit a high perforation resistance over a long period of 10 years or more. A corrosion treatment cycle with the successive steps of a salt water-spraying procedure at 35°C for 6 hours, a drying procedure at 70°C at a relative humidity of 60%RH for 4 hours, a wetting procedure at 49°C at more than 95%RH for 4 hours, and a freezing procedure at -20°C for 4 hours, was repeatedly applied 50 times to each sample. In Fig. 1, the perforation resistances of Sample No. 1, the plated zinc layer of which contained BaCrO₄ particles, and Sample No. 2, the plated zinc-nickel alloy layer of which contained metallic chromium and chromium oxide particles and Al₂O₃ particles, are poorer than that of Sample No. 4 having a thick (90 g/m²) galvanized zinc layer. Also, Fig. 1 shows that the perforation resistance of Sample No. 1, the plated zinc layer of which contains only sparingly water- soluble chromate (BaCrO₄) particles in a small amount of 0.3% by weight, is unsatisfactory. That is, by the method of Japanese Unexamined Patent Publication (Kokoku) No. 60-96786, it is difficult to co-deposit a large amount of the rust-resistant pigment consisting of substantially water-insoluble chromate particles from the electroplating liquid in the zinc plating layer, because the chromate particles in the plating liquid have a surface potential of approximately zero. Further, Fig. 1 shows that Sample No. 3, i.e., the plated composite steel strip of the present invention, exhibited a higher perforation resistance than Sample No. 4. Namely, in the plated composite steel strip of the present invention, the microcapsule-like corrosion-preventing fine particles promote the perforation resistance-enhancing effect of the substantially water-insoluble chromate particles in the base layer. Conventional corrosion resistant particles dispersed in the base plating layer promote the corrosion resistance of the plating layer in the following manner. For example, when sparingly water-soluble chromate particles are co-deposited with a matrix-forming metal on a steel strip substrate to form a plating layer, and the resultant plated composite steel strip is placed under corrosive conditions, the chromate particles are decomposed with development of the corrosion and generate Cr⁶⁺ ions. The Cr⁶⁺ ions react with the metal in the plating layer to form corrosion resistant chromium compounds and chromium oxides and chromium hydroxide. This phenomenon is effective for providing a corrosion resistant layer in the plating layer and for enhancing the corrosion resistance of the plating layer. When the chromium compound layer in the plating layer is decomposed, a new corrosion resistant chromium compound layer is formed in the plating layer, because chromate particles are evenly distributed in the plating layer. The re-formation of the corrosion-resistant chromium compound layer is repeated. When microcapsule-like particles as in the present invention are used, the corrosion resistant plating layer exhibits a promoted corrosion resistance by the following mechanism. With microcapsule-like particles comprising core particles consisting of low water- soluble chromate and thin coating membranes consisting of SiO₂ , a portion of the chromate is very slowly dissolved through the thin coating membranes, because the thin coating membranes do not completely seal the core particles. The generating rate of Cr⁶⁺ ions in the plating layer of the present invention is significantly smaller than that of the conventional plating layer in which the chromate particles are not encapsulated, and thus the corrosion resistance of the plating layer can be maintained at a satisfactory level over a longer period. We find that the Cr⁶⁺ ion-forming rate in the plating layer of the present invention is about 1/3 to 1/10 times that in the conventional plating layer. Thus the plated composite steel strip of the present invention has long term corrosion resistance; it may withstand a corrosion test over 1 to 3 months, and meet the demand of a 10 year resistance to perforation for car bodies. The other types of core particle, for example, phosphate particles which generate PO₄³⁻ ions and molybdenum compound particles which generate MoO₄²⁻ ions, can exhibit the corrosion-preventing effect by the same mechanism as that of the chromate particles. The corrosion resistant fine particles in the form of microcapsules are preferably contained in a total amount of 0.1% to 30%, more preferably 0.1% to 20% by weight, based on the weight of the base layer. When the content of the corrosion-preventing fine particles is less than 0.1%, the resultant base plating layer sometimes exhibits an unsatisfactory corrosion resistance. When the content of the corrosion-preventing fine particles is more than 30% by weight, the resultant base plating layer sometimes exhibits unsatisfactory bonding to the steel strip substrate. The additional fine or colloidal particles optionally dispersed with the corrosion-preventing fine particles, for example, SiO₂ , TiO₂ , Cr₂O₃ , Al₂O₃, ZrO₂ , SnO₂ , and Sb₂O₅ , promote the corrosion resistance of the base plating layer as follows. The additional fine or colloidal particles exhibit a lower corrosion-resistant property than the corrosion-preventing fine particles, but in the base layer are distributed between the corrosion-preventing fine particles, and thus can restrict the corrosion of the portion of base layer around the additional particles. Namely, the additional particles exhibit a barrier effect against corrosive action. In the base plating layer of the present invention, the additional fine or colloidal particles preferably amount to from 0.1% to 30%, more preferably from 0.1% to 20%, of the total weight of the base electroplated layer. When the content of additional particles is less than 0.1% by weight, the improvement in the corrosion resistance of the base plating layer due to the additional particles is sometimes unsatisfactory. When the content of the additional particles is more than 30% by weight, the resultant base plating layer sometimes exhibits poor bonding to the steel strip substrate. Preferably, the total content of corrosion-preventing fine particles and additional particles does not exceed 30% based on the weight of the base plating layer. In an embodiment of the composite steel strip of the present invention, the corrosion resistant coating layer has an additional thin electroplated layer formed on the base plating layer. The additional electroplated layer preferably comprises at least one member selected from Zn, Fe, Co, Ni, Mn and Cr, and preferably has a weight of 1 to 5 g/m². In another embodiment of the composite steel strip of the present invention, the corrosion resistant coating layer has a surface coating layer formed on the base plating layer. The surface coating layer may have a single layer structure comprising a member selected from organic resinous materials and mixtures of at least one of the organic resinous materials and chromium ions. The organic resinous materials include, for example, epoxy resins, epoxy-phenol resins and water-soluble type and emulsion type acrylic resins. Instead, the surface coating layer can have a double layer structure consisting essentially of an under layer formed by applying a chromate treatment to the base plating layer surface and an upper layer formed on the under layer and comprising an organic resinous material as mentioned above. In still another embodiment of the composite steel strip of the present invention, the above-mentioned surface coating layer is formed on the above-mentioned additional thin electroplated layer on the base layer. The additional electroplated layer and the surface coating layer will be explained in detail hereinafter. In one method of the present invention, at least one surface of a substrate consisting of a descaled steel strip is coated by at least first electroplating the substrate surface in a first electroplating liquid. The surface of the steel strip to be first electroplated is cleaned by an ordinary surface-cleaning treatment, before the first electroplating step. The first electroplating liquid contains (a) matrix-forming metal ions selected from zinc ions and a mixture of zinc ions and at least one other metal ion than zinc ions to be alloyed with zinc, (b) a number of the above-mentioned corrosion-preventing fine solid particles in the form of microcapsules, dispersed in the first electroplating liquid and (c) a co-deposition-promoting agent for promoting the co-deposition of the corrosion-preventing particles together with the matrix-forming metal, to provide a base electroplating layer on the substrate surface. The first electroplating liquid optionally contains at least one type of additional fine or colloidal particles consisting of a member selected from SiO₂ , TiO₂ , Cr₂O₃ , Al₂O₃ , ZrO₂ , SnO₂ , and Sb₂O₅. The co-deposition-promoting agent is used to promote the co-deposition of the corrosion-preventing particles, and optionally the additional particles, together with the matrix-forming metal, from the first electroplating liquid into the base electroplating layer. The co-deposition-promoting agent preferably comprises at least one member selected from Ni²⁺ ions, Fe²⁺ ions, Co²⁺ ions, Cr³⁺ ions, TiO₂ colloid, Al₂O₃ colloid, SiO₂ colloid, ZrO₂ colloid, SnO₂ colloid, and Sb₂O₅ colloid. The role of the above-mentioned ions or colloids as the co-deposition-promoting agent will be explained below. As stated above, the surface potential of the corrosion-preventing particles in the electroplating liquid can be controlled by the thin coating membranes. When the corrosion-preventing particles have thin SiO₂ coating membranes, the resultant microcapsule-like particles have a negative surface potential. In an electroplating process in which a steel strip serves as a cathode, it is difficult to deposit the microcapsule -like particles having the thin SiO₂ coating membranes into the plating layer on the steel strip substrate. Accordingly, the deposition of the microcapsules-like particles into the plating layer must be promoted by using the co-deposition-promoting agent. Where Ni²⁺ ions are used as the co-deposition-promoting agent, the Ni²⁺ ions are absorbed on the surface of the SiO₂ coating membranes surfaces of the microcapsule-like particles so that the surfaces of the microcapsule-like particles have a positive potential. The microcapsule-like particles having the positive surface potential can be readily drawn to and deposited into the plating layer on the cathode (steel strip). Co²⁺, Fe²⁺ and Cr³⁺ ions in the electroplating layer exhibit the same co-deposition-promoting effect as Ni²⁺ ions. The metal ions Ni²⁺, Co²⁺, Fe²⁺ and Cr³⁺, are also deposited to form a zinc alloy matrix which is effective for enhancing the corrosion resistance of the first electroplating layer. SiO₂ , TiO₂ , Al₂O₃ , ZrO₂ , SnO₂ and Sb₂O₅ colloids added to the electroplating liquid serve as a co-deposition-promoting agent in the same manner as that of the Ni²⁺ ions, etc. When added to the electroplating liquid, the colloid particles exhibit a positive or negative potential and are absorbed on the surfaces of the corrosion-preventing microcapsule-like fine particles. For example, at a pH of 1 to 2.5, Al₂O₃ , ZrO₂ , SnO₂ , and TiO₂ colloid particles exhibit a positive potential, and SiO₂ and Sb₂O₅ colloid particles exhibit a negative potential. Accordingly, the nature and intensity of the potential of the fine particles in the electroplating liquid can be adjusted to a desired level by controlling the type and amount of the colloid particles to be added to the electroplating liquid, in consideration of the type of the electroplating method. That is, the composition of the co-depositionpromoting agent should be determined in view of the composition of the corrosion-preventing microcapsule-like particles, especially the type and nature of the thin coating membrane. The co-deposition of the corrosion-preventing particles can be promoted by using another type of co-deposition-promoting agent which is very effective for the accelerated codeposition of the corrosion-preventing particles and for stabilizing the electroplating step for the base plating layer. This co-deposition-promoting agent comprises at least one member selected from amine compounds having a cationic polar structure of the formula (1): ammonium compounds having a cationic polar structure of the formula (2): wherein R¹ , R² , R³ , and R⁴ represent, respectively and independently from each other, a member selected from a hydrogen atom, and alkyl and aryl radicals, and polymers having at least one type of the cationic polar radical. The amine compounds, ammonium compounds and the cationic polymers are selected, e.g. from ethylene imine and ethylene imine-containing polymers, diallylamine and diallylamine-containing polymers, polyaminesulfons which are copolymers of diallylamine and SO₂ , trimethylammonium chlorides diallyldimethylammonium chloride and alkyl betaines The base plating layer of the present invention has a satisfactory rust-resistance and corrosional perforation resistance, but it was found that, when some types of the plated composite steel strips are subjected to a chemical conversion treatment prior to a paint coating step, the base plating layer tends to hinder the growth of chemical conversion membrane crystals. That is, the chemical conversion membranes are formed only locally and the crystals in the membrane are coarse, and therefore the chemical conversion membrane exhibits poor adhesion to the paint coating. This disadvantage is serious when the base plating layer contains chromium-containing particles. Accordingly, where a paint coating is required, for example on a steel strip to be used for forming outer surfaces of car bodies, preferably the base electroplated layer is coated with a thin additional electroplated layer, preferably in a weight of 1 to 5 g/m². The additional electroplated layer preferably comprises at least one type of metal selected from Zn, Fe, Co, Ni, Mn, and Cr. The base plating layer in the plated composite steel strip of the present invention may have a surface coating selected from simple coating layers comprising organic resinous material and optionally chromium ions evenly mixed therein, and composite coating layers with an under layer formed by applying a chromate treatment to the base layer surface and an upper layer formed on the under layer and comprising organic resinous material. The surface coating effectively enhances the firm adhesion of paint to the plated composite steel strip. The above-mentioned surface coating may be formed instead on an additional electroplated layer on the base layer. In the method disclosed in Japanese Unexamined Patent Publication (Kokai) No. 60-96786, the first electroplating operation is carried out with a first electroplating liquid having a pH of 3.5 or more. Where the steel strip serves as a cathode and the electroplating liquid has a pH of 3.5 or more, the pH at the interface between the cathode and the electroplating liquid is easily increased to a level at which a membrane of Zn(OH₂) is formed; the Zn(OH)₂ membrane hinders the deposition of metal ions and of rust-resistant pigment particles of larger size than the metal ions onto the cathode surface through the Zn(OH)₂ membrane. That is, the formation of the electrocoating layer containing the corrosion-resistant dispersoid particles is obstructed by the Zn(OH)₂ membrane formed on the cathode surface. Therefore, the resultant plating layer has an unstable composition, contains a very small amount of the corrosion resistant dispersoid particles, and thus exhibits unsatisfactory corrosion resistance. Referring to Fig. 2, which shows the relationship between the pH of the electroplating liquid and the amount of low water-soluble chromate fine particles deposited from the electroplating liquid, it is clear that, at a pH of 3.5 or more, the amount of the deposited chromate fine particles becomes very small. Also, it should be noted that a portion of the chromate particles is dissolved in the electroplating liquid to generate Cr⁶⁺ ions. If the electroplating operation is carried out in an electroplating liquid containing a large amount of Cr⁶⁺ ions, the resultant electroplated layer is a black colored powder and exhibits very poor adhesion to the steel strip substrate. Where the content of Cr⁶⁺ ions in the electroplating liquid is in the range of from 0.1 to 0.25 g/l, the black colored deposit is not formed in the resultant electroplated layer. However, the electroplated layer contains a very small amount of the low water-soluble chromate fine particles deposited therein. Figure 2 suggests that, in the range of Cr⁶⁺ ion content of from 0.1 to 0.25 g/l in the electroplating liquid, an increase in the content of Cr⁶⁺ ions results in remarkable decrease in the amount of low water-soluble chromate fine particles deposited. Also, referring to Fig. 3 showing the relationship between the content of Cr⁶⁺ ions in an electroplating liquid and the amount of low water-soluble chromate fine particles deposited from the electroplating liquid, it is clear that the increase in the content of Cr⁶⁺ results in a remarkable decrease in the amount of deposited chromate fine particles, and at a Cr⁶⁺ ion content of 0.3 g/l or more, practical electroplating becomes impossible. In the method of Japanese Unexamined Patent Publication (Kokai) No. 60-96786, an attempt is made to resolve the Cr⁶⁺ ion problem in the following manner. Where an electroplating liquid contains BaCrO₄ fine particles as substantially water-insoluble chromate fine particles, a portion of the BaCrO₄ is dissolved in the electroplating liquid and is dissociated by the following reaction. BaCrO₄⇄Ba²⁺ + CrO₄²⁻(Cr⁶⁺) The reaction in the → direction causes the BaCrO₄ to be dissolved in the electroplating liquid. To restrict the dissolution reaction, the ionic dissociation of the BrCrO₄ should be prevented by, for example, adding Ba²⁺ ions. The addition of Cr⁶⁺ ions should be avoided, because the increase in the Cr⁶⁺ ion content in the electroplating liquid results in a decrease in the plating utility of the electroplating liquid. To add Ba²⁺ ions, BaCl₂ , which has a relatively large solubility in water, is preferably added to the electroplating liquid. In the method of Japanese Unexamined Patent Publication No. 60-96786, the electroplating liquid contains chlorides including BaCl₂. However, when a non-soluble electrode is used as an anode in a chloride-containing electroplating liquid, chlorine gas is generated from the electroplating liquid. Therefore, a soluble electrode must be used as an anode in the chloride-containing electroplating liquid. However, in most of the recent electroplating apparatus, the electrode is a fixed type, and thus is a non-soluble electrode, because generally, in most recent electroplating methods, a horizontal, high flow speed type electroplating cell is used, the distance between the steel strip and electrode is made short to increase the current density to be applied to the electroplating process, and the plated steel strip is produced at a very high efficiency which corresponds to several times that obtained in a conventional electroplating process. The method of the present invention is very useful for electroplating a steel strip substrate in a horizontal, high flow speed type electroplating apparatus at a high current density and at a high efficiency. In this type of electroplating process, when a non-soluble electrode is used, the electroplating liquid is preferably a sulfate type plating bath. In the sulfate type plating bath, the generation of Cr⁶⁺ ions cannot be prevented by adding Ba²⁺ ions to the bath, because the added Ba²⁺ ions are converted to BaSO₄ which is insoluble in water and deposits from the bath. Accordingly, where the sulfate type plating liquid is used as a first electroplating bath for the method of the present invention, it is preferable to convert the dissolved Cr⁶⁺ ions to Cr³⁺ ions by adding grains or a plate of a metal, for example, metallic zinc or iron, or a reducing agent, for example, sodium sulfite, in an amount to reduce the dissolved Cr⁶⁺ ions to Cr³⁺ in the first electroplating liquid. In this manner, an oxidation-reduction reaction is utilized. Figure 4 shows the relationship between the reaction time (minutes) of metallic zinc grains added in an amount of 20 kg/m³ in an electroplating liquid and the concentration (g/l) of Cr⁶⁺ ions dissolved in the electroplating liquid. In view of Fig. 4, it is clear that, after the metallic zinc grains are added to the electroplating liquid, the Cr⁶⁺ ions are reduced to Cr³⁺ ions by reduction by the zinc grains, and thus the concentration of the Cr⁶⁺ ions decreases with lapse of reaction time. That is, it was around that a high corrosion resistant plated composite steel strip, in which a stable dispersion of the corrosion-resistant solid particles in a satisfactory amount in a base plating layer is ensured, can be easily produced by the method of the present invention in which, preferably, the pH of the first electroplating liquid is controlled to a level of 3.5 or less, more preferably from 1 to 2.5, and the concentration of the dissolved Cr⁶⁺ ions is restricted to a level of 0.1 g/l or less, more preferably 0.05 g/l or less, by adding metal grains or plate or reducing agent to the first electroplating liquid, for a wide range of current density from a low level to a high level. The resultant high corrosion resistant plated composite steel strip of the present invention exhibits excellent metal plating and adhesion, weldability, and painting properties. Referring to Fig. 5A, a plated composite steel plate is composed of a steel strip substrate 1 descaled by ordinary surface cleaning treatment and a base plating layer 2, which consists of a metal matrix 2a of zinc or a zinc alloy, for example an alloy of zinc with at least one member selected from Fe, Co, Mn, Cr, Sn, Sb, Pb, Ni and Mo, and a number of corrosion-preventing microcapsule-like fine particles 3 of the present invention and additional fine or colloidal particles 4 consisting of a member selected from SiO₂ , TiO₂ , Cr₂O₃ , Al₂O₃ , ZrO₂ , SnO₂ and Sb₂O₅. Referring to Fig. 5B, a base plating layer 2 formed on a steel strip substrate 1 is coated by a thin additional electroplated layer 5, which comprises at least one member selected from Zn, Fe, Co, Ni, Mn and Cr. Preferably, the additional electroplated layer 5 is in an amount of 1 to 5 g/m². In Fig. 5C, base electroplated layer 2 is coated with a coating 6. The coating 6 may be a single coating layer structure made of an organic resinous material, which optionally contains chromium ions evenly mixed in the resinous material, or a double coating layer structure consisting of an under layer formed by applying a chromate treatment to the base plating layer surface and an upper layer formed on the under layer and comprising an organic resinous material as mentioned above. As shown in Fig. 5D, the same coating layer 6 as mentioned above is formed on the additional electroplated layer 5 formed on the base electroplated layer 2. The coating 6 is preferably formed when the base or additional electroplated layer contains chromium. When a chromium-containing compound, for example low water-soluble chromate or metallic chromium, is contained in an electroplated layer, and a chemical conversion treatment is applied as a pre-paint coating step to the surface of the electroplated layer, it is known that the resultant chemical conversion membrane contains coarse crystals. The coarse crystals cause the chemical conversion membrane to exhibit poor paint coating property. Therefore, preferably a surface layer to be chemical conversion-treated is free from chromium compound or metallic chromium, The organic resinous material usable for the surface coating may be selected from epoxy resins, epoxy-phenol resins, and water-soluble polyacrylic resin emulsion type resins. The organic resinous material may be coated by any conventional coating method, for example a roll-coating method, electrostatic spraying method, and curtain flow method. From the aspect of ensuring the weldability and processability of the resultant plated composite steel strip, the thickness of the organic resinous material layer is preferably 2 µm or less. In the surface coating, the organic resinous material layer is also effective for preventing the undesirable dissolution of chromium from the chromate-treated under layer, which is very effective for enhancing the corrosion resistance of the plated composite steel strip. The dissolution of chromium sometimes occurs when the plated composite steel strip having the chromate treatment layer is subjected to a degreasing procedure or chemical conversion procedure, and can be prevented by coating the chromium compound-containing layer with the resinous material layer, which optionally contains chromium ions. Recently, a method of applying a new surface coating layer having a thickness of about 2 µm and containing SiO₂ particles, etc, to the electroplated layer has been developed. This surface coating layer consisting of an organic resinous material and the SiO₂ particles can exhibit high corrosion resistance without the chromate treatment or using chromium ions. The present invention will be further explained by way of specific examples which, however, are representative and do not restrict the scope of the present invention in any way. Examples 1 to 38 and Comparative Examples 1 to 7 In each of the examples and comparative examples, a cold-rolled steel strip having a thickness of 0.8 mm, a length of 200 mm, and a width of 100 mm was degreased with an alkali aqueous solution, pickled with a 10% sulfuric acid aqueous solution, and washed with water. The descaled steel strip was subjected to a first electroplating procedure wherein the steel strip served as a cathode, a first electroplating liquid containing necessary metal ions, corrosion-preventing fine particles, additional fine or colloidal particles and a co-deposition-promoting agent, as shown in Table 1, was stirred and circulated through an electroplating vessel and a circulating pump, while controlling the amounts of the above-mentioned components to a predetermined level, and while maintaining the pH of the first electroplating liquid at a level of 2, and the electroplating operation was carried out at a temperature of about 50°C at a current density of 40 A/dm² for about 22 seconds to provide base electroplating layers in a targeted weight of 22 g/m² formed on both surfaces of the steel strip. For example, in each of Examples 22 to 25 in which the resultant base electroplating layer was composed of a matrix consisting of a zinc (90%) - cobalt (10%) alloy and corrosion-preventing fine particles consisting of 4% by weight of BaCrO₄ core particles capsulated with a SiO₂ membrane and 1% of weight of additional TiO₂ colloidal particles, the first electroplating liquid had the following composition. ZnSO₄·7H₂O 180 g/l CoSO₄·7H₂O 10 to 450 g/l BaCrO₄ core particle encapsulated by SiO₂ membrane 5 to 60 g/l TiO₂ 0.5 to 60 g/l In each of Example 2, 6 to 12, 16 to 19, 23, 27, 28, 30 to 32, 35, 37 and 38, an additional electroplating layer in the total amount of 1 to 5 g/m² and the composition as shown in Table 1 was formed on the base electroplating layer surface by using a second electroplating liquid containing necessary metal ions, for example, Zn ions or a mixture of Zn ions with Fe, Co, Ni, Mn and/or Cr ions in the form of sulfates. In each of Examples 3, 4, 6, 8, 10, 13 to 15, 20, 21, 24, 25, 28 to 30, 32, and 35 to 38, a surface coating layer having the composition and the thickness as shown in Table I was formed on the base electroplating layer or the additional electroplating layer. In the formation of the surface coating layer, the organic resinous material layer or chromium-containing organic resinous material layer was formed by a roll-coating method and by using a water-soluble polyacrylic resin emulsion. Also, the chromate treatment was carried out by coating, reaction or electrolysis. The resultant plated composite steel strip was subjected to the following tests. 1. Cyclic corrosion resistance test A painted specimen, which was prepared by a full-dip type chemical conversion treatment and a cationic paint-coating, and an unpainted specimen, were scratched and then subjected to a 50 cycle corrosion test. In each cycle of the corrosion test, the specimens were subjected to salt water-spraying at 35°C for 6 hours, to drying at 70°C at 60%RH for 4 hours, to wetting at 49°C and at a 95%RH or more for 4 hours, and then to freezing at -20°C for 4 hours. After the 50 cycle corrosion test, the formation of red rust and the depths of pits formed in the specimens were measured. 2. Paint adhesion property A specimen was subjected to a full-dip type chemical conversion treatment, was coated three times with paint, and was then immersed in hot water at 40°C for 10 days. After the completion of the immersion step, the specimen was subjected to a cross-cut test in which the specimen surface was scratched in a chequered pattern at intervals of 2 mm to form 100 squares. Then an adhesive tape was adhered on the scratched surface of the specimen and was peeled from the specimen. The number of squares separated from the specimen was then counted. The rust resistance was evaluated as follows. Class Rust formation R (%) 5 R = 0 4 R ≦ 5 3 5 < R ≦ 20 2 20 < R ≦ 50 1 50 < R The depth of corrosion was evaluated as follows. Class Depth C (mm) of pits 5 C = 0 4 C ≦ 0.1 3 0.1 < C ≦ 0.3 2 0.3 < D ≦ 0.5 1 0.5 < C The paint-adhesion property was evaluated as follows. Class Peeled squares D (%) 5 D = 0 4 D ≦ 5 3 5 < D ≦ 20 2 20 < D ≦ 50 1 50 < D The results of the tests are shown in Table 1. Table 1 clearly shows that the plated composite steel strips of Examples 1 to 38 in accordance with the present invention exhibited an enhanced corrosion resistance and a satisfactory paint-adhesion in comparison with the comparative plated composite steel strip. Namely, the specific corrosion-preventing fine particles in the form of microcapsules are effective for promoting the corrosion resistance of the resultant plated composite steel strip.

Claims: A corrosion-resistant electroplated composite steel strip comprising a steel strip substrate having on at least one surface a corrosion-resistant coating comprising at least an electroplated corrosion-resistant base layer which comprises a matrix of zinc or zinc alloy having dispersed therein fine solid anti-corrosion particles in the form of microcapsules having (i) solid cores comprising at least one material which is soluble or sparingly soluble in water and selected from chromates, aluminium compounds, phosphates, molybdenum compounds, titanium compounds, and polypropylene and fluorine-containing polymer resins encapsulated by (ii) membranes which have a thickness of 1µm or less and which comprise at least one material which is insoluble in water or less soluble in water than the core material and which is selected from SiO₂, TiO₂, Al₂O₃, ZrO₂, ethyl cellulose, amino resins, polyvinylidene chloride resins, polyethylene resins and polystyrene resins. A strip according to claim 1 wherein the corrosion-resistant coating includes an additional thin layer electroplated on the base layer. A strip according to claim 2 wherein the additional thin electroplated layer (i) has a weight of 1 to 5 g/m² and/or (ii) comprises at least one member selected from Zn, Fe, Co, Ni, Mn and Cr. A strip according to any preceding claim wherein the corrosion-resistant coating includes a surface coating which comprises (a) organic resinous material optionally having chromium ions mixed therewith or (b) an outer layer comprising organic resinous material over an under layer obtainable by applying a chromate treatment to the electroplated layer beneath it. A strip according to any preceding claim having one, two or all of the features: (i) said base layer weighs from 5 to 50 g/m²; (ii) said base layer comprises zinc alloy with at lease one of Fe, Co, Mn, Cr, Sn, Sb, Pb, Ni and Mo; (iii) said microcapsules constitute from 0.1 to 30% of the weight of said base layer. A method for producing a corrosion-resistant electroplated composite steel strip comprising electroplating a base layer onto at least one surface of a descaled steel strip substrate using an electroplating liquid containing (a) matrix-forming ions of zinc optionally together with ions of at least one other metal to be alloyed with zinc; (b) dispersed in the electroplating liquid fine solid anti-corrosion particles in the form of microcapsules having (i) solid cores comprising at least one material which is soluble or sparingly soluble in water and selected from chromates, aluminium compounds, phosphates, molybdenum compounds, titanium compounds, and polypropylene and fluorine-containing polymer resins encapsulated by (ii) membranes which have a thickness of 1µm or less and which comprise at least one material which is insoluble in water or less soluble in water than the core material and which is selected from SiO₂, TiO₂, Al₂O₃, ZrO₂, ethyl cellulose, amino resins, polyvinylidene chloride resins, polyethylene resins and polystyrene resins; and (c) co-deposition-promoting agent for promoting the co-deposition of the anti-corrosion particles with the matrix-forming metal. A method according to claim 6 wherein the co-deposition promoting agent comprises at least one member selected from Ni²⁺ ions, Fe²⁺ ions, Co²⁺ ions, Cr³⁺ ions, TiO₂ colloid, Al₂O₃ colloid, SiO₂ colloid, ZrO₂ colloid, SnO₂ colloid, Sb₂O₅ colloid, amine and ammonium compounds of cationic polar structures (1) and (2) wherein R¹, R², R³ and R⁴ are the same or different and selected independently from a hydrogen atom and alkyl and aryl radicals, and polymers having at least one of the cationic polar radicals (1) and (2). A method according to claim 6 or 7 wherein the anti-corrosion particles contain chromium, a portion of the chromium is dissolved into the electroplating liquid to form Cr⁶⁺ ions and the Cr⁶⁺ ions are reduced to Cr³⁺ ions by adding metal grains, metal plate or reducing agent. A method according to claim 6 or 7 or 8 wherein the electroplating liquid contains zinc sulfate. A method according to claim 9 wherein (i) said liquid has a pH of 3.5 or less and/or (ii) an insoluble electrode is used. A method according to any of claims 6 to 10 wherein the electroplating liquid contains additional fine or colloidal particles comprising at least one member selected from SiO₂, TiO₂, Cr₂O₃, ZrO₂, SnO₂ and Sb₂O₅. A method according to any of claims 6 to 11 including electroplating an additional thin layer onto the base layer from an electroplating liquid. A method according to claim 12 wherein the electroplating liquid for said additional layer contains at least one type of metal ion selected from Zn, Fe, Co, Ni, Mn and Cr ions. A method according to any of claims 6 to 13 including forming a surface coating on the base electroplated layer, or on the additional thin electroplated layer when present, by coating said electroplated layer with organic resinous material optionally having chromium ions evenly mixed therein or by applying a chromate treatment to said electroplated layer and then forming over the treated surface a coating comprising organic resinous material. A strip according to any of claims 1 to 5 wherein the base layer matrix also has dispersed therein additional fine or colloidal particles selected from SiO₂, TiO₂, Cr₂O₃, Al₂O₃, Zro₂, SnO₂ and Sb₂O₅. A strip according to claim 15 wherein (i) said additional particles constitute from 0.1 to 30% of the weight of said base layer, and/or (ii) the total of said microcapsules and said additional particles constitutes not more than 30% of the weight of said base later.

Inventors: HIGUCHI SEIJUN, IZAKI TERUAKI, OSAWA MASAMI, SATO HISAAKI, YOSHIDA MAKOTO, HIGUCHI, SEIJUN, IZAKI, TERUAKI, OSAWA, MASAMI, SATO, HISAAKI, YOSHIDA, MAKOTO
Applicants: NIPPON STEEL CORP, NIPPON STEEL CORPORATION
Application year: 1988
Main classification: C25D15
IPC codes: C25D15, C25D5