CA1255246A - Corrosion resistant surface-treated steel strip and process for making - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A steel strip having an Fe-P alloy with a phosphorus content of 0.0003% to 15% by weight electrodeposited as an uppermost layer on at least one surface of the strip is provided as well as a process for making. The Fe-P alloy may be electrodeposited onto a cold rolled steel strip either directly or via an underlying layer of zinc or zinc alloy previously electrodeposited onto the strip. The uppermost Fe-P plating layer on the strip may be chemically treated with a phosphate to form a phosphate film which provides improved corrosion resistance without paint coating thereon.

Description

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Corrosion Resistant Surface-Treated Steel Strip and Process for Making This invention relates to highly corrosion resistant plated steel strips or sheets adapted for phosphate treatment and cathodic electrodeposition and intended for use in automobiles; and chemically treated steel strips prepared by subjecting the plated steel strips to an ordinary phosphate treatment as generally employed for cold rolled steel and zinc or zinc alloy plated steel to thereby form a surface film having improved corrosion resistance without paint coating; as well as a process for making such plated and treated steel strips.
BACKGRO~ND OF THE INVENTION
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F'or steel strips intended for use in automobiles, electrophoretic deposition or painting is often used to form a primer coating. Cationic paint particles are electrodeposi-ted on the surface of a workpiece to be coated during deposition, creating defects in the coating. At the same time, H2 gas is concomitantly generated due to electrolysis of the medium, that is, water so that hydrogen gas bubbles break the previously electrodeposited coating, also creating defects in the coating. The occurrence of such coating deEects generally called craters is a phenomenon inherent to steel strips plated with zinc or its a].loy.
Zinc or zinc alloy plated steel strips further exhibit 5~

inferior secondary wet adhesion of coating after they are triply coated with a cathodic elec-trophoretic painting, a sealer coating, and a top coating. By the (secondary) wet adhesion o~ (paint) coating is meant the adhesion to the treated steel of a paint coating which has been deteriorated through any appropriate process under moist environment. One -test method is to dip a triple coated s-teel strip in water at 40C for 10 days and to conduct a scribed adhesion test on the strip immediately aE-ter its removal from the water.
In order to improve the wet adhesion of paint coa-ting and to prevent crater formation, an Fe plating treatment was proposed as disclosed in Japanese Patent Appl.ication Kokai No. 57-671.~)5, laid open to the public on April 23, 19~2 (Applicant: Kawasaki Steel Corporation), Japanese Patent Application Kokai No. 57-198293, laid open to the public on December ~, 1982 (Applicant: Kawasaki Steel Corportion) and Japanese Patent Application Kokai No. 5~-34192, laid open to the public on February 28, 1983 (Applicant: Kabushiki ECaisha Kobe Seikosho). The treatment by pure Fe p:Lating is, however, incompatible wi-th bonderizincJ or phosphate treatment to be followed.
A relatively smaLl number of nuclei of phosphate generate on a pure Fe platincJ, resulting in a phosphate ~ilm of relatively rough or large phosphate crystals. Some phosphate trea-ting solutions result in a lack of coating and are unsucaessEul in improving the wet aclhesion oE paint coating. ~o bene:Eicial e~:Eec-t is achievecl particularly by the phosphate treatment oE spray type.

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The hasic consideration about rus-t preventive steel mus-t involve no-t only the rust preventive ablli-ties tha-t an elec-troplating on steel strips and a paint coating thereon individually possess, but also -the overall rust prevention -2a-;~

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resulting from the ~ynerglstic effect o~ both the electroplatiny and the paint coating. More particularly, zinc or zinc alloy plated steel strips, which have improved corrosion prevention because of the sacrificial protection of the underlyin~ steel by the plating and the protection by corrosion products, have found a wide variety o~ applications in automobiles, electric appliances, building materials and the like. On the other hand, cathodic electrophoretic deposition or painting has been spread which provides very hlgh corrosion resistance. However, applying the cathodic electrophoretic deposition to zinc or zinc alloy electroplated s-teel does ~ot achieve such a remarkable effect as enco~mtered when the cathodic electrophoretic deposition is directly applied to cold rolled steel strips.
Although the zinc or zinc alloy pl~ting and the cathodic electrophoretic deposition provide excellent corrosion prevention when applied alone, their combination does not lead to satisfactory rust preventive ability because a phosphate film i8 formed on the plating of zinc or zinc alloy. This film consists es~entially of hopeite, zn3(Po4)2~4H2o~ ~hic~ is not re~istant to alkali and is dissolved due to an increase in pH
during cathodic electrophoretic deposition or caused by corrosion und~r paint coatings. This results in the poor adhesion and reduced blister resistance of paint coatlng under moi~t environment. On the other hand, a phosphate film consistin~
es~entially of phosphophyllite, Zn2Fe~PO4)2.4H2O

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is formed on co:Ld rollecl steel strips. Phosphophylli-te is resis-tant ~o alkali and th~s subs-tan-ti.ally lmproves the adhesion and bllster resistance of p~in-t coa~ings under mois-t environment. It is -thus believed that the wet adhesion o~ paint coa-ting can be improved by ~orming a phosphate film of phosphophyllite on zinc or zinc alloy plated s-teel strips.
Among methods for rendering steel strips more adaptable to chemical conversion or phosphate trea-tment are known an Fe plating treatmen-t as disclosed in Japanese Patent Application Kokai No.
S6-1~2885, laid open to the public on November 7, 1981 (Applicant:
Shin Nippon Steel Corporation) ancl Japanese Patent Application Kokai No. 57-67195, laid open -to the public on April 23, 1982 (Applicant:
Kawasaki Steel Corpora-tion), and a relatively iron rich Fe-Zn pla-ting treatment as disclosed in Japanese Patent Application Kokai ~o. 58-52483, laid open to the public on March 28, 19~3 (Applicant:
Shin ~ippon Steel Corporation). Such prior art Fe plating treatments yield a relatively stable plating surface and thus, some trea-ting solutions Eail to ~ully improve phosphatability. The iron rich Fe-Zn plating trea-tment is eE~ective in ~orming phosphophyllite only in -the limited optimum range o:E plating conditions and often results in a nonurl:i.forJn appearance. Although the ~orrnat:ion o:E
phosphophy:Llite improves the wet adhesion of paint coating ancl ''' ,r hence, the corrosion resistance a:Eter p~.~r-~t, the result:ing phosphate :E.i.Lm itself has little corrosion :resistance and consequently, the phosphated steel strips only exhibit corrosion res:istance suhstant:ially equal to tha~ o~ uncoated, untrea-ted s-teel strips. In general, steel strips are marketed after they are configured to a variety o:E shapes and pain-ted. Many such con~igu:red ~55~

parts include ~ome sites whe~e paint does not flow or spread well. For this reason the corrosion resistance without paint coating is o:E substantial importance to steel strips even though they are normally coated with paint.
Apart from the traditional concept that phosphate films are underlying or intervening films on which paint is applied, the inventors have made investigations about surface-treated steel strips from the novel point of view that novel and improved surface treated steel strips are produced by imparting corrosion resistance without paint coating to phosphate films themselves.
SUMMARY OF T IE INVENTION
It is, therefore, an object o:E the present invention to provide an Fe-P plated steel strip adapted for subsequent phosphate treatment and cathodic electrophoretic deposition.
Another object of the invention is to provide a surface treated steel strip having improved corrosion resistance without paint coating or just after it is plated with an Fe-P alloy and :Eormed with a phosphate film.
A further object of the invention is to provide a process for making such surface treated steel strips adapted for subsequent phosphate treatment and cathodic electrophoretic deposition and having a phosphate .Eilm exhib.iting improved corrosion resistance without paint coating.

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In one broad aspect, the present invention relates to an Fe-P plated steel strip adapted for phosphate treatment, comprising a steel strip, a lower layer of zinc or a zinc alloy electrodeposi~
ted on at least one surface of the steel strip, and an upper layer of an Fe-P system with a phosphorus content of 0.0003 to 15% by weight electrodeposited on said lower layer to a build-up of at least 0.5 g/m2.
In another broad aspect/ the present invention relates to an Fe-P plated steel strip adapted for phosphate treatment, comprising lo a steel strip, a lower layer of zinc or a zinc alloy electrodepo-sited on at least one surface of the steel strip, an upper layer of an Fe-P system with a phosphorus content of 0.0003 to 15% by weight electrodeposited on said lower layer to a build-up of at least 0.5 g/m2, and a topcoat of one element selected from the group consisting of Ni, Zn, Mn, and Ti, deposited on said upper layer to a build-up of 5 to 50 Mg/m2.
In a further broad aspect, the present invention relates to a phosphate treated steel strip having improved corrosion resis-tance without paint coating, comprising a steel strip, a lower layer of zinc or a ~inc alloy electrodeposited on at least one surface of the steel strip, an upper layer of an Fe-P system with a phosphorus content of 0.0003 to 15% by weight electrodeposited on said lower layer to a build-up o~ at least 0.5 g/m2, and a surface film formed by chemically treating said upper layer with a phosphate.

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~55~f~i In another broad aspect, the present invention relates to a phosphate treated steel strip having improved corrosion resistance without paint coating, comprising a steel strip a lower layer of zinc or a zinc alloy electrodeposited on at least one surface of the steel s~rip, an upper layer of an Fe-P system with a phosphorus content of 0.0003 to 15% by weight electrodeposited on said lower layer to a build-up o~ at least 0.5 g/m2 and a surface film formed by chemically treating said topcoat with a phosphate.
In a further broad aspect, the present invention relates to lo a process Eor making an Fe-P plated steel strip, comprising electrodepositing a lower layer of zinc or zinc alloy on at least one surface of a steel strip, thereafter electroplating said zinc or zinc alloy plating layer with an Fe-P system alloy containing 0.0003 to 15% by weight of phosphorus in a plating bath containing Fe2+ ions in an a~ount of from 0.3 mol/litre to the solubility limit and a hypophosphite in an amount of 0.001 to 25 g/litre as expressed in NaH2PO2.H2O at a pH of 1.0 to 5.0, a tempexature of 30 to 60C, and a current density greater than 20 A/dm2 up to and including 200 ~/dm2.
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The above an~ other objects, ~eatures, and advantages of the present invent:ion Will be more ~ul.~y unders~ood by reading the following qescript.i~n tak~ con-~unction with t~e accompanying drawings, in whic~J

3~55~Z ~6 FIG 1 is a graph in which the R content of Fe-P platings is plotted in relation to the number of crystal nuclei at the initial (after 5 seconds);
FIGS.2a and 2b are scanning electron photomicrographs (750X) of a s-teel strip treated according to the present invention (a cold rolled steel coated with 2.5 g/m2 of an Ee-P plating with a P
content of 1.5% by weight) and a steel strip bonderized in a conventional manner (SPCC, using Granodine SD-2000) after 120 seconds, respectively;
FIG 3 is a graph showing P content vs corrosion resistance wherein Fe-P plated steel strips are prepared to varying P contents by changing the concentration oE NaH2P02 H20 under the conditions of Example E1, treated with Granodine SD-2000, and subjected to a salt spray test, the corrosion resistance being expressed as the length o~ time required for the area of red rust to exceed 10%: and FIG.4 is a yraph showing the corrosion resistance of various plated steel strips wherein steel strips are plated and phosphate treated under the condit1ons of Example E3 -/
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~5Z~i _9_ and subjectecl to a 50 cycle test according to the saltspray testing procedure of JIS Z 2371, the corrosion resistance being expressed as a reduction in thickness measured using a point micrometer after removal of rust.

DETAILED DESCRIPTION OF THE INVENTION
The Fe-P plated steel strips of the prevent invention will be described in further detail.
The Fe-P plating to be applied in the practice of the present lnvention is effective as long as the phosphorus content is in the range of 0.0003 to 15% by weight. First we did not find this overall range to be effective, but have found certain ranges to be effectlve in the progress of our research.
In the initial stage, we made research from the point of view of formation of phosphate crystal nuclei at an initial stage of phosphate treatment and found that the preferred phosphorus content is in the range of 0.0003 to 0.5~ by weight of the Fe-P plating.
Nevertheless, we have continued research from a broader aspace of eventually forming a phosphated film to find that evaluation in terms of initial nucleus formation is somewhat severe to obtain an acceptable phosphated film.
We have foun~ that phosphorus contents in the range of 0.5 7 ~ r71~ 1b to 15% by ~L~ht -i~ satiæfactory.

These diEferent ranges of phosphorus content are defined for different reasons and will be separately discussed below.

iZ'~6 The Fe-P plating should be carried out in the practice of ~he present inv~ntion according to the descrlbed aspects thereof. When the number of phosphate crystal nuclei formed at an lnitial stage of subsequent pho~phate treatment or bonderizing is taken into account, the Fe-P plating layer should contain 0.0003 to 0.5% by weight of phosphorus. A pur~ Fe plating layer yield~ a stable oxide film on the surface which retards the initial reaction of the pho~phate treatment and causes crystals to grow rough. The presence of phosphorus in such a minor proportion is effective to substantially promote the initial reaction of the phosphate treatment and to increase the initial number of crystal nuclei.
FI~. 1 is a graph showing the initial numbèr of crystal nuclei V8 P content. Steel strips were plated with various Fe-P
plating layers to a build~up of 2.0 g/m2 and then immersed for 5 seconds in a phosphatin~ solution (Bonderite ~3004, (trade-mark), manufactured and solcl by Nihon Parkerizing K.K.).
The number of phosphate crystal nuclei formed at the end of the 5 second immersion was determlned. It was found from FIG. 1 that the initial reaction of phosphate treatment is sub~tantially promoted when the pho~phorus content in the Fe-P plating ranyes from 0.0003% to 0.5% by weight.
When the quality of a final phosphate film resulting from phosphate treatment i~ evaluated in terms of the P proportion and crystal size which are considered most important by those skilled in the art, the Fe-P plating ,,~

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layer should contain phosphorus in an amount of 0.5% to 15.0% by weight~
As previously mentioned for Fe-P plating layers having a phosphorus content of 0.0003% to 0.5% by weight, the initial number of crystal nuclei after 5 secon~s is substantially increased within this higher phosphorus content range.
In the practice of phosphate treatment, even when the initial number of crystal nuclei after 5 seconds of phosphate trea~ment is small, crystal nuclei continue to generate thereafter. It was found that evaluation in terms of the initial number of crystal nuclei after 5 seconds is too severe to apply to most treated steel strips intended for commercial use.
Phosphatability, that i5 adaptability to phosphate treatment is generally evaluated on the basis of the phosphate L5 crystals formed after 120 seconds of phosphate -treatment. In general, the surface film resulting from the phosphate treatment is of most importance. Likewise, in the present invention, the quality of the final surface film is evalua-ted in terms of the phosphorus proportion and crystal size which are at present considered most important by those skilled in the art.
When the phosphorus content oP Fe-P plating e~ceeds 0.5%
by wei~ht, a small number of crystal nuclei have been generated after 5 seconds oP phosphate treatment, but a fur~her number of crystal nuclei are continuously ~enerated thereafter, - 12 - ~ ~ S 5~
eventually yieldlng a film of high quality, that is, a dense nonporous film characterized by a reduced crystal size. The phosphate content range has been hroadened because steel strips having such a f.ilm are also within the scope of the present inventlon.
With the increasing content of phosphorus, the efficiency of cathodic Fe-P ele~troplating decli~es, which is economically disadvantageous. Fe-P platings having an extremely high phosphorus content are platings of amorphous type which are often less reactive during -the phosphate treatment.
For this reason, the upper limit of the phosphorus content is limited to 15% by weight although the preferrecl phosphorus content is not more than 10% by weight and not preferably not more than 5% by weight.
To apply such an Fe-P plating directly onto a steel strip or sheet, the build-up of the Fe-P plating should be at least 0.01 grams per square meter. Build-ups of less than 0.01 g/m2 are insufficient to uniformly and continuously cover the steel ' surface with an Fe-P plating and thus useless. The minimum eEfective build-up is 0.01 g/m2 partlally because Fe atoms are supplied from the underlying steel when a phosphate film is modified into a film of phosphophyllite Zn2Fe(P04)2.H20 during the phosphate treatment.
~s is well known in the art, when steel strips plated with zinc or a zinc based alloy are sub~ected to a phosphate treatment or bonderizing, a film of hopeite Zn3(P04)2~4~20 i5 formed, resulting in the inferior secondary wet adhesion of a coating layer formed by triple coatings including cathodic electrophoretic deposition as well as the inferior crater prevention during cathodic electrophoretic deposition. According to the present invention, by applying an Fe-P plating as defined above to a ~inc or zi.nc alloy plated steel strip, the phosphate film formed during the subsequent phosphate treatment is modified into a film o:E phosphophyllite Zn2Fe(PO4)2-4H2O, which is highly effective in improving the crater prevention during cathodic electrophoretic deposition and the secondary wet adhesion of a coating layer.
The build-up of the Fe-P plating to be deposited on a zinc or zinc alloy plated steel strip is preferably at least 0.5 g/m2. In order to improve the secorldary wet adhesion of paint coating and the crater prevention during cathodic electrophoretic deposition, it is a ke~ -to convert the film resulting from phosphate treatment into a :Eilm of phosphophyllite. With build-ups of Fe-P plating of less than 0.5 g/m2, phosphophyllite is formed in an ~mount insuEficient to achieve the desired eEEect. In some embodiments, the upper limit Of 3 g/m2 is irnposed on the Fe-P plating because a substantial quantity of a thicker Fe-P plating might be left unconverted upon its conversion into phosphophyllite during phosphate treatment.

In one preferred embodiment of the present invention, one element selected from the group consisting of Ni, Zn, Mn, and Ti is deposited to a build-up of 5 to 50 mg/m2 on the Fe-P plating layer which is previously deposited on a steel strip directly or on a zinc or zinc alloy plated steel strip. This metallic topcoat containing numerous microcells at the surface facilitates chemical conversion with phosphate. The metallic topcoat in a build-up of less than 5 mg/m2 is not effective. With build-ups of more than 50 mg/m2 the elemental topcoat entirely and uniformly covers the surface to leave few microcells and is le~t in the phosphate film in the form of phosphate salt, undesirably reducing the ratio of phosphophyllite to phosphophyllite plus hopeite. Similar results will be obtained when phosphorus in the Fe-P plating according to the present invention is replaced by another element in the same Periodic Group, that is, As, Sb or Bi.
Next, the phosphate treated steel strip prepared by subjecting the Fe-P plated steel strip to phosphate treatment or bonderizing according to the fifth aspect of the present invention will be described.
More speciEically, the phosphate treated steel strip is prepared by subjecting a steel strip having an uppermost la~er of an Fe-P system with a phosphorus content of 0.0003 to 15~ by weight electrodeposited thereon to a phosphate treatment or bonderizing. The phosphate treated steel strip exhibits improved corrosion resistance without paint coating probably for the following reason.
FIGS. ~a and 2b are scanning electron microscope (SEM) photographs of the phosphate treated steel strip according to the present invention and a phosphate treated cold rolled steel strip, respectively. Crystals of chemically converted phosphate on the cold rolled strip are as rough as 14 micrometers while crystals on the phosphate treated steel strip according to the preseant invention are as fine as 2.5 micrometers and form a dense film over the steel surface. Because of the denseness of crystals, the phosphate film according to the present invention is free of pinholes, Eirmly bonded and has improved corrosion resistance as opposed to conventional phosphate films.
Some phosphorus contained in the Fe-P plating is oxidized into phosphoric acid which acts as an inhibitor at the plating surface. Since the firmly bonded phosphate film is present on the plating surface, freshly formed phosphoric acid is retained on the plating surface s~lch that its effect is Eully exerted, resulting in outstanding improvement in corrosion resistance without paint coating.
As long as the content of phosphorus in the iron plating is at least 0.0003% by weight, phosphate treated steel strips exhibit improved corrosion resistance. The corrosion resistance increases with an increase in phosphorus content, reaches the maximum at phosphorus contents o~
about 0~5~ by weight, and then remains at a constant high level. The upper limit is set to 15~ by weight. Although ~s~

phosphorus contents e~ceeding 15% by weight do not adversely afEect the corrosion resistance without paint coating, electric current efficiency is lowered to undesirably increase the operating cost in actual manufacture lines. An increase in the amount of phosphorus compound added to the treating solution is also undesirable n economy.
The Fe-P plating should be deposited to a build-up of at least 0.01 g/m2 when deposited directly onto cold rolled steel. Fe-P platings in build-ups of less than O.Ol g/m2 do not fully cover the steel surface or exert the above-mentioned effect on the phosphate film. Corrosion resistance is improved with build-ups of at least 0.01 g/m2, and preferably at least 0.1 g/m2.

When the Fe-P plating is applied onto a zinc or zinc alloy plating on the steel, the Fe-P plating should be at least 0.5 g/m2.
As described above, the phosphate treated, Fe-P plated steel strips according to the present invention have signiEicantly improved corrosion resistance without paint coating, and this effect is enhanced particularly when a plating layer oE zinc or a zinc alloy intervenes between the Fe-P plating layer and the underlying steel. When flaws reach the underlying steel through the paint and plating layer~, the intervening zinc or zinc alloy provides electrochemical protective action to the underlying steel, minimizing red rust formation. In addition, the corrosion .~5~Z~6 products of zlnc afford protective action, improving corroeion resi~tance without paint coating at no sacrifice of the effect of Fe-P plating.
Although the foregoing description refers to steel strips plated with Fe-P followed by phosphate treatment, similar results are obtained when the Fe-P platin~ layer further contains an impurity of one or more elements selected from Zn, Cu, Ni, Cr, Co, Mn, V, Sn, Cd and the like.

The phosphate treating or bonderizin~ solution which can be used herein may be selected from conventional treating solutions which principally contain phosphate ion, zinc ion, alkali metal ion, heavy metal ion, and accelerators. Such typical examples are phosphate treating solutions of dip and spray types including Bonderite 3030 (trade mark, manuEactured and sold by Nihon Parkerizing K.K., Japan) and Granodine SD 2000 and 16NC (trade marks, manufactured and sold by Nihon Paint ~.K., Japan). The steel strips according to the present invention ensure when phosphate treated, excellent corrosion resistance without paint coating, and exhibit further improved corrosion resistance without paint coating when the phosphate treatment is followed by chromate treatment Eor sealing.
As opposed to E'e-Zn plati~gs liable to be nonuniEorm, the Fe-P plating is relativèly easily adapted to Eorm a uniform layer which ensures that the final phosphate film be uniEorm and good in appearance. This is one of the great advantages of the present invention.

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Next, the process Eor making such Fe-P plated steel strips will be described.
According to the process of the present invention, a steel strip is electroplated on at least one surface thereof with an Fe-P system alloy containing 0.0003% to 15%
by weight of phosphorus. ~ plating bath containing Fe2 ions in an amount of from 0.3 mol/liter up to the solubility limit and hypophosphorous acid or a hypophosphite in an amount o 0.001 to 25 g/liter as expressed in NaE12PO2~H2O is used at a pH of 1.0 to 5.0 and a temperature of 30 to 60C while electroplating is carried out with a current density of from more than 20 to 200 A/dm2 .
The basic plating bath may be selected from chloride baths, sulfate baths, and mixed baths well known in the art. The ferrous (Fe2 ) ions in the bath is available either in the form of a compound such as ferrous chloride FeCl2~nH2O and ferrous sulfate F`eSO4 7H2O or by dissolving metallic iron. Conduction aids may be added to increase electric conductivity and critical current density and reduce solution resistance, and they may be added to the solubility limit, for example, KCl, NH4Cl, NaCl, CaC12,

2 4~ ( 4 2 2~
The concentration of ferrous (Fe2 ) ions ran~es from 0.3 mol/]iter, and pre~erably from l.0 mol/liter to the solubility limit. Burnt deposits often form at concentrations below the lower limit.
The amount of NaH2P02~H2O is limited to the range of 31 2~5~'~6 0.001 to 25 g/liter which ensures deposition of an Fe-P
plating having a phosphorus content of 0.0003 to 15% by weight. The phosphorus source may include a wide variety of phosphoric acids and salts thereof, the preferred source being hypophosphorous acid or a hypophosphite.
The pH of the ba-th is in the range of l.0 to 5.0, and preferably 1.5 to 4.0 because cathodic deposition efficiency is lowered at lower pH and precipitation of iron hydroxide resulting from oxidation oE Fe2 becomes excessive at higher pH.
The bath temperature is in the range of 30 to 60C
because the ingredients become less soluble at lower temperatures and excessive oxidation of Fe2 and P occurs as higher temperatures.
The current density is in the range of from more than 20 to 200 A/dm2, and preferably from 40 to 150 A/dm2. With lower current densities, platings of poor appearance (discolored and nonuniform) are deposited in low yields, eventually cletracting :Erom phosphatabi].ity. Contrary, higher current densities cause platings to be burnt and reduce cathodic deposition efficiency.
In order that those skilled in the art will more readily understand how to practice the present invention, examples of the present invention will be presented by way Oe illustration and not by way of limitation. In the following description, g/l is yram per liter, A/dm2 is ampere per square decimeter, and C/m2 is coulomb per square meter.

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[I] Fe-P platin~ with a P content of 0.0003 to 0.5 wt%
Cold rolled steel strips were electrolytically degreased and pickled in a conventional manner before an Fe-P plating was applied to them under the following conditions. For some strips, an elemental metal, Ni, Znr Mn or Ti was applied onto the Fe-P plating layer by a flash plating technique. The resultant Fe-P plated steel strips were subjected to the following tests~ The results are shown in Table 1.
(1) Fe-P plating (1-1) Bath composition FeC12 150 g/l KCl 200 g/l Citric acid10 g/l NaH2P02~H200.001 - 2 g/l (1-2) Plating conditions pH 3.0 Bath temperature 50 C
Current density40 - 150 A/dm2 The concentration of NaH2P02 and the current density were changed to control the P contenk.
(2) Flash plating (2-1) Ni plating Bath composition NiS04250 g/l NiC1245 g/l Boric acid 30 g/l i5;~

Pla-ting conditions pH 3.5 Bath -temperature 60~C
Anode Ni plate Electricity28 C/m2 (2-2) Zn plating Bath composi-tion : ZnC12 210 g/1 KCl 360 g/l Pla-ting conditions pH 5.0 Bath temperature 50C
Anode Zn plate Electricity60 C/m2 (2-3) Mn plating Bath composi-tion MnSo4.4H20150 g/l (N~4)2S04100 g/l ~ Na2S3 2 g/l Glyc:ine 15 g/l Plating conditions pH 3 Bath temperature 20C
ArlodeInsoluble carbon F,lectricity110 C/m (2-4) ti plating Plating was carried out by immersing for 5 seconds a strip in a bath containing 0.001 mol/liter of K2rriO3 at room temperature.

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~ 7 Additionally, Fe-P platings were applied under the following conditions onto steel strips which had previously been electroplated with Zn, Zn-Fe alloy, Zn-Ni alloy and Zn-Al alloy in an ordinary manner. For some strips, an elemental metal, Ni, Zn, Mn or Ti was applied to the Fe-P
plating layer by a flash plating technique. The resultant Fe-P plated steel strips were subjected to the following tests. The results are shown in Table 2.
(l) Fe-P plating (l-l) Bath composition FeCl2 200 g/l KCl 200 g/l Citric acid20 g/l NaH2PO2 H20.001 - 2 g/l (1-2) Plating conditions pH 3.0 Bath temperature 50 C
Current density ~0 - lS0 A/dm2 The concentration of NaH2PO2 in the bath and the current density were changed to control the P content.
(2) Elash plating Pl.ating was carried out as described above.
hate trertment The plated steel strips were degreased, rinsed and surEace conditioned under standard conditions correspondi.ng to the phosphate treating soluti.ons before they were subjected to phosphate treatment, rinsed with water, and ~s~

dried. The phosphate treating solutions used are Bonderite 3030 (trade namé of dip -type phosphate solution, manufactured and sold by Nihon Parkerizing K.K., Japan), Bonderite 3128 (-trade -~ame of spray type phosphate solution, manufactured and sold by Nihon Parkerizing K.K.), and Granodine SD 2000 (trade na-me of dip type phosphate solution, manufactured and sold by Nihon Paint K.K., Japan).
(1) Etched quantity An etched quantity was determined by the weight of a degreased specimen minus the weight of the phosphated specimen from which the phosphate film was dissolved away.
(2) Film weight The weight of the phosphate film was determined by dissolving away the film with a 5~ chromic acid solution.

(3) P proportion The proportion of phosphophyllite relative to hopeite was determined by the :Eormula:
PhosphophYllite peak x 100%
Phosphophyllite peak ~ Hopeite peak where Phosphophyllite and Hopeite peaks means the peak height~ of phosphophyllite and hopeite in X ray analysis, respect.ively.

(4) Crystal size ~fter ordinary phosphate treatment, the phosphate film was observed under a scanning electron microscope (SEM) to determine the size of crystals. The maximum lengths of ~;~S~6 crystals are averaged.

(5) Corrosion resistance of phosphated steel withou-t paint coating A phosphate treated specimen was sealed at its edges and subjected to a salt spray test according to JIS Z 2371.
The corrosion resistance without paint coating was evaluated in terms of the time required for red rust to spread in excess of 10% of the surface area.
~6) Secondary wet adhesion of paint coating A phosphate treated specimen was coated with an under coat of 20 ~um by cathodic electrophoretic deposition, a sealer coat, and a top coat to a total coating thickness of 90 to 100 ~m. The coated specimen was immersed in water at 40C for 10 days. Immediately after removal from water, the specimen was scribed in mutually perpendicular directions to a depth reaching the underlying steel to define one hundred (100) square areas of 2 ~n x 2 mm and an adhesive tape was applied to the scribed coating. The adhesive tape was removed to determine the number of coating pieces separated.
(7) Corrosion resistance A phosphate treated specimen was coated to a total coating thickness of 90 to 100 ~rn in the same manner as described for the wet adhesion test. The coated specimen was cross cut to a depth reaching the underlying steel, and then subjected to one hundred (100) cycles of salt water dip test, each cycle consisting of immersing in 5~ sodium :~ZSS~'~6 chloride in water for lS minutes, drying for 75 minutes at room temperature, and holding for 22.5 hours in a moisture box at a temperature of 49C and a relative humidity of 85%. Blisters grew from the cross cuts during the test.
The width of blisters in mm was measured and the Elow of rust was observed.
(8) Crater prevention A cathodic electrophoretic deposition solution, U-30 ~, (trade ~me, manufactured and sold by Nihon Paint K.K.) was prepared and stirred for one week. Using this solution, specimens were electrophoretically painted under conditions: electrode distance 4 cm; voltage 350 volts; and deposition time 180 seconds without soft touch. Evaluation was made according to the following criterion.

Number of craters, /cm2 (excellent) none O (yood) 1 - 5 (fair) 6 - 20 X (rejected) more than 20 As evident from the results shown in Tables 1 and 2, steel strips are improved in phosphatability by applying an Fe-P plating having a phosphorus content of 0.0003 to 0.5~
by weight of the plating onto the steel strips directly or via a plating of zinc or zinc alloy according to the present invention.

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[II] ~e-P platinq with a P contel1t of 0.5% to 15% by we.iqht Cold rolled steel strips were electrolytically degreased and pickled in a conventional manner befor2 an Fe-P plating was applied to them under the following conditions. For some strips, an elemental metal, Ni, Zn, Mn or Ti was applied onto the Fe-P plating layer by a flash plating technique. The resultant Fe-P plated steel strips were subjected to the following tests. The results are shown in Table 3 and FIGS. 2a and 2b.

(l) Fe-P plating (l-l~ Ba-th composition FeCl2 150 g/l KCl 200 g/l Citric acidlO g/l NaH2PO2~H2OO.OOl - 25 g/l (l-2) Plating conditions pH 3.0 Bath temperature 50 C
Current density 40 - lS0 A/dm2 The concentration of NaH2PO2 and the current density were changed to control the P content.
(2) Flash plating (2-l) Ni plating Bath composition NiS04250 g/l NiCl245 g/l Boric acid 30 g/l Plating condi.tions p~ 3.5 Bath temperature 60 C
AnodeNickel plate Electricity 28 C/m (2-2) Zn plating Bath composition ZnCl2 210 g/l KCl 360 g/l Plat.ing conditions pH 5.0 Bath temperature 50C
Anode Zn plate Electricity 60 C/m2 (2-3) Mn plating Bath composition MnSO .4H2O150 g/l (NH4)2SO4lO0 g/l 2 3 2 g/l Glycine15 g/l Plating conditions pH 3 Bath temperature 20 C
AnodeInsoluble carbon Electricity llO C/m2 (2-4) Ti plating Plating was carried out by immersing for 5 seconds z~

a strip in a bath containing O.OOl mol/liter oE K2l'iO3 at room temperature.
Additionally, Fe-P platings were applied under the following conditions onto steel strips which had previously been electroplated with Zn, Zn-Ni alloy and Zn-Fe alloy in an ordinary manner. For some strips, an elemental metal, Ni, Zn, Mn or Ti was applied to the Fe-P plating layer by a flash plating technique. The resultant Fe-P plated steel strips were subjected to the following tests. The results are shown in Table 4.
(1) Fe-P plating (l-l) Bath composition FeCl2 200 g/l KCl 200 g/l Citric acid20 g/l NaH2PO2 H2OO.OOl - 25 g/l (1-2) Plating conditions pH 3.0 Bath temperature 50 C
Current density 40 - 150 A/dm2 The concentration oE NaH2PO2 in the bath and the current density were changed to control the P content.
~2) Flash plating Plating was carried out as described ahove.
Phos~ e treatment The plated stee] strips were degreased, rinsed and surface conditioned under standard conditions corresponding ~S5~

to the phosphate treating solutions before they were subjected to phosphate treatment, rinsed with wa-ter, and dried. The phosphate treating solutions used are Bonderite ~= 3030 (trade-name of dip type phosphate solution, manufactured and sold by Nihon Parkerizing K.K., Japan3, Bonderite 3128 (trade -~ame of spray type phosphate solution, manuEactured and sold by Nihon Parkerizing K.K.), and Granodine SD 2000 (trade-~a-mé of dip type phosphate solution, manufactured and sold by Nihon Paint K.K., Japan).
(l) Crystal size Specimens were phosphate treated in a usual manner and observed under a scanning electron microscope (SEM) to determine the size of crystals. A mean value of the maximum lengths oE crystals was determined.
FIGS. 2a and 2b are SEM photographs of a specimen according to the present invention and a specimen phosphate treated without Fe-P plating, respectively.
(2) Film weight l'he weight of the phosphate Eilm was determined by dissolving away the film with a 5% chromic acid solution.
(3) P proportion The proportion oE phosphophyllite relative to hopeite was determined by the formula:
Phosphophyllite peak _ x 100 Phosphophyllite peak + Hopeite peak where Phosphophyllite and Hopeite peaks mean the peak Z~

heights of phosphophyllite and hopeite in X ray analysis, respectively.
(4) Wet adhesion of paint coating A phosphate treated specimen was coated with an under coat oE 20 um by cathodic electrophoretic deposition, a sealer coat, and an overcoat to a total coating thickness of 90 to lO0 um~ The coated specimen was immersed in water at 40C for lO days. Immediately after removal from water, the specimen was scribed in mutually perpendicular directions to a depth reaching the underlying steel to define one hundred (lO0) s~uare areas of 2 mm x 2 mrn and an adhesive tape was applied to the scribed coating. The adhesive tape was removed to determine the number of coating pieces separated.
(5) Blister prevention A phosphate treated specimen was coated to a total coating thickness of 90 to lO0 um in the same manner as described for the secondary adhesion test. The coated specimen was cross cut to a depth reaching the underlying 2U steel, and then subjected to one hundred (lO0) cycles of salt water dip testl each cycle consisting oE immersing in 5% sodium chloride in water Eor 15 minutes, drying for 75 minutes at room temperature, and holding for 22~5 hours in a moisture box at a temperature of A9C and a relative humidity of 85~. Blisters grew from the cross cuts during the test. The width of blisters in mm was measured.

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(6) Crater prevention A cathodic electrophoretic deposition solution, ~-30 (trade ~mé, manufactured and sold by Nihon Paint K.K.) was prepared and stirred for one week. Using this solution, specimens were electrophoretically painted under conditions: electrode distance 4 cm; voltage 350 volts; and deposition time 180 seconds without soft touch. Evaluation was made according to the following criterion.
Number of craters, /cm2 ~ (excellent) none O tgood) l - 5 a ~ fair) 6 - 20 X (rejected) more than 20 As evident from the results shown in Tables 3 and 4 and FIGS. 2a and 2b, steel strips are improved in phosphatability by applying an Fe-P plating having a phosphorus content of 0.5% to 15.0~ by weight of the plating onto the steel strips directly or via a plating of zinc or zinc alloy according to the present invention.

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[III] Chemically Treated Fe-P Plated Steel The following e~amples demonstrate that steel strips which are plated with an Fe-P plating as described above and then chemically treated with a phosphate exhib.it improved corrosion resistance without paint coating.
Example El Cold rolled steel strips were electrolytically degreased and pickled in a conventional manner before an Fe-P plating was applied to them under the following conditions.
Bath composition FeSO4~7H2O 140 g/l (NH4)2S4 .lO0 g/l KCl 20 g/l NaH2PO2~H2O O.l - 25 g/l Plating conditions pEI 2.5 Bath temperature 50 C
Current dens.ity 40 - lO0 A/dm2 The thus plated steel strips were treated with Granodine SD 2000 (dip type) i.n a usual manner.
E~ le E2 Fe-P platings were applied to Zn electroplated steel strips, Zn-Ni alloy electroplated steel strips, galvanized steel strips, and galvannealed steel strips under the following conditions. (Electroplating, galvanizing and galvannealing were as usual.) ~2~5~

Bath composition FeC1~'nE12 200 g/l KC1 150 g/l NaH2PO2 H2O 0.5 - 25 g/l Plating conditions pH . 3~5 Bath temperature 50 C
Current density 40 - 100 A/dm2 Fe-P platings were applied to steel strips which were previously electroplated with a Zn-Fe plating in a usual manner.
Bath composition FeSO4 7H2O 250 g/l (NH4)2SO4 150 g/l NaH2PO2 H2O 0.4 g/l Plating conditions p~l ~
Bath temperature ~0C
Current density 60 A/dm Thereafter, a E)hosphate treatment was carried out in a usual manner using Granodine SD 2000.
The thus obtained treated steel strips were subjected to the following tests.
(1) Corrosion resistance without paint coatinq_(a:Eter phosphate treatment) A phosphate treated specimen was sealed at its edges ~55~

and subjected tv a salt spray test according to JIS Z 2371.
The corrosion resistance without paint coating was evaluated in terms of the time required for red rust to spread in excess of 10~ of the surface area.
(2) Wet adhesion A phosphate treated specimen was triple coated with paints as described above and immersed for 10 days in pure water at 5nC. Immediately after removal from water, the specimens was scribed in mutually perpendicular directions to a depth reaching the underlying steel to define one hundred (100) square areas of 2 mm x 2 mm and an adhesive tape was applied to the scribed coating. The adhesive tape was removed to determine the number of coating pieces separated.

(3) Crater prevention A cathodic elestrophoretic deposition solution, U-30 (trade name,, manufactured and sold by Nihon Paint K.K.) was prepared and stirred Eor one week. ~sing -this solution, specimens were electrophoretically painted under conditions: electrode distance 4 cm; voltage 350 volts; and deposition time 180 seconds without soft touch. Evaluation was made according to the Eollowing criterion.
Number oE craters, ~ (exceLlent) none O (good) 1 - 5 (fair) 6 - 20 X (rejected) more than 20 ~2~5~

The ~esults are shown in Table 5~ As evident from Table 5, steel strips having a phosph~te -treated Fe-P
plating as the uppermost layer according to the present invention exhibit improved corrosion resistance without pain-t coatin~ as well as excellent corrosion resistance with paint coating.
FIG. 3 is a graph showing P content vs corrosion resistance wherein Fe-P plated steel strips are prepared to varying P contents by changing the concentration of NaH2PO2 H2O under the conditions of Example El, treated with Granodine SD-2000, and subjected to a salt spray test, the corrosion resistance being expressed as ~he length oE time required for the area of red rust to exceed 10%.
FIG. 4 is a graph showing the corrosion resistance o~

various plated steel strips wherein steel strips are plated and phosphate treated under the conditions of Example E3 and subjected to a 50 cycle test according to the salt spray testing procedure of JIS Z 2371, the corrosion resistance being expressed as a reduction in thickness measured using a point micrometer after removal of rust.
These figures prove khat corrosion resistance is substantially improved with phosphorus contents of more than 0.5~ by weight and the process of the invention is greatly effectively.

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Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An Fe-P plated steel strip adapted for phosphate treatment, comprising a steel strip, a lower layer of zinc or a zinc alloy electrodeposited tan at least one surface of the steel strip, and an upper layer of an Fe-P system with a phosphorus content of 0.0003 to 15% by weight electrodeposited on said lower layer to a build-up of at least 0.5 g/m2.

2. An Fe-P plated steel strip adapted for phosphate treatment, comprising a steel strip, a lower layer of zinc or a zinc alloy electrodeposited on at least one surface of the steel strip, an upper layer of an Fe-P system with a phosphorus content of 0.0003 to 15% by weight electrodeposited on said lower layer to a build-up of at least 0.5 g/ma, and a topcoat of one element selected from the group consisting of Ni, Zn, Mn, and Ti, deposited on said upper layer to a build-up of 5 to 50 Mg/m2.

3. A phosphate treated steel strip having improved corrosion resistance without paint coating, comprising a steel strip, a lower layer of zinc or a zinc alloy electrodeposited on at least one surface of the steel strip, an upper layer of an Fe-P system with a phosphorus content of 0.0003 to 15% by weight electrodeposited on said lower layer to a build-up of at least 0.5 g/m2, and a surface film formed by chemically treating said upper layer with a phosphate.

4. A phosphate treated steel strip having improved corrosion resistance without paint coating, comprising a steel strip a lower layer of zinc or a zinc alloy electrodeposited on at least one surface of the steel strip, an upper layer of an Fe-P system with a phosphorus content of 0.0003 to 15% by weight electrodeposited on said lower layer to a build-up of at least 0.5 g/m2, a topcoat of one element selected from the group consisting of Ni, Zn, Mn, and Ti, deposited on said upper layer to a build-up of 5 to 50 mg/m2 and a surface film formed by chemically treating said topcoat with a phosphate.

5. A process for making an Fe-P plated steel strip, comprising electrodepositing a lower layer of zinc or zinc alloy on at least one surface of a steel strip, thereafter electroplating said zinc or zinc alloy plating layer with an Fe-P system alloy containing 0.0003 to 15% by weight of phosphorus in a plating bath containing Fe2+ ions in an amount of from 0.3 mol/litre to the solubility limit and a hypophosphite in an amount of 0.001 to 25 g/litre as expressed in NaH2PO2.H2O at a pH of 1.0 to 5.0, a temperature of 30 to 60°C, and a current density greater than 20 A/dm2 up to and including 200 A/dm2.