ZA200605133B

ZA200605133B - Hydroxysulfate surface treatment

Info

Publication number: ZA200605133B
Application number: ZA200605133A
Authority: ZA
Inventors: Bello Alain; Jacqueson Eric; Arnoux Claude; Petitjean Jacques
Original assignee: Arcelor France
Priority date: 2003-12-24
Filing date: 2006-06-22
Publication date: 2007-09-26
Also published as: FR2864552A1; MXPA06007253A; BRPI0418188B1; CN1914354A; UA89368C2; CA2551492A1; WO2005071140A1; US20080308192A1; BRPI0418188A; FR2864552B1; RU2371516C2; RU2006126698A; JP2007517135A; EP2450470A1; KR101022683B1; KR20060129306A; EP1699947A1

Description

Hydroxy sulfate surface treatment } The present invention relates to the use of a treatment solution for treating the surface of a steel sheet } 5 coated with a metal coating based on zimc or its alloys. It also relates to a method of lubricating such a coated sheet.

Steel sheet coated with zinc or its alloys is widely used in the automotive field and in industry in general, as it exhibits excellent corrosion resistance.

However, such galvanized steel sheet has a number of difficulties when Ht is being formed, for e=xample by drawing, in order to manufacture parts.

Usually, to give galvanized steel sheet better tribological properties, a film of lubricatirg oil is applied to its surface so as to facilitate time forming operation.

However, despite applying a suitable lubricating oil film, very substantial friction exerted by forming tools on the surface of the sheet causes, at the surface of the sheet, powder or particles base d on zinc or its alloys that are generated by degradation of the coating. The accumulation and/or agglomeration of these particles or this powder in the forming tools may damage the formed parts, by the formation of barbs and/or constrictions .

Furthermore, because of the high friction co efficient that characterizes tthe sliding of a galvanized surface in contact with the surface of a forming t-ool, the sheet runs the risk of fracturing should there be insufficient sliding of the sheet in the forming tool gap. Such fractures may appear even when an oil film of sufficient weight, i.e. greater than 1 g/m? iss applied to the surface of the sheet as it is not possible to maintain a uniform distribution of the oil film over

- 2 = the surface of the sheet. This is du e to the phenomenon of dewetting, which corresponds to the presence of } areas deficient in oil. . 5 However, the fact of depositing a relatively thick oil film on the surface of the sheet poses problems of contamination of the workshops and drawing tools, and requires the use of large quantit-ies of degreasing agents in order to clean the shee# and considerable means for treating the effluents coming from the cleaning operation.

Moreover, the oil deficiency in cer-tain areas of the oil film due to the dewetting effect is also responsible for lesser temporary coerrosion protection of the steel sheet while it is being stored.

The object of the present inventiom is therefore to propose a treatment solution which, wshen applied to the surface of a steel sheet coated woth a metal layer based on zinc or its alloys, makes it possible to reduce the degradation of the galv-anized surface of this sheet while it is being formed, to reduce the quantity of lubricating oil to be deposited on the sheet before it is formed, and to improve the temporary corrosion protection of the sheet.

For this purpose, the subject of the invention is the use of an aqueous treatment solution containing sulfate ions S042” with a concentration of not less than 0.01 mol/l in order to treat the swirface of a steel sheet treated on at least one of its sides with a metal coating based on zinc or its alloys, for the purpose of reducing the formation of metal powder or particles based on zinc or its alloys generated by the degradation of the coating while sa=zid sheet is being formed.

The expression “metal coating based or a zinc alloy” is understood t=o mean a zinc coating comprising one or more alloyirg elements, such as for example but not i being restr icted thereto, iron, a luminum, silicon, magnesium an.d nickel. . 5

According tc the invention, the use of a steel sheet coated with san almost pure zinc coatimg is preferred.

When the sur-face of a steel sheet co ated with a metal coating basecd on zinc or its alloys iss treated by means of an aqueous treatment solution according to the invention, a layer forms on the surface of the sheet that is base=d on zinc hydroxysulfate and zinc sulfate and is botzh sufficiently thick and sufficiently adherent. However, such a layer cannot be formed when the $0,” con centration is less than 3.01 mol/l, but it has also beem found that too high a concentration does not substant ially improve the rate of deposition and may even slightly reduce it. : In a first embodiment of the inventieon, the treatment solution is applied in a conventional manner, for example by d=ipping, by spraying or by coating, both on electrogalvarmized sheet and on ho t-dip galvanized sheet.

In a preferred embodiment, the a queous treatment solution fu rthermore contains zn?" ions with a concentratiom of not less than 0.01 mmol/l which makes it possible t.o obtain a more uniform deposition.

For example, the treatment solution is prepared by dissolving zzinc sulfate in pure wateer; for example, zinc sulfate heptahydrate (ZnSQ4 7H,0) is used, the 2zZn®* ion concentra:stion then being equal to that of the S0,%” anions.

Preferably, the PH of the tre-atment solution corresponds t=o the natural pH of the solution without the additiom of either base or acid - the value of this

PH is generally between 5 and 7.

To minimize the formation of powder or particles of . 5 zinc or its alloys resulting from degracidation of the coating on the sheet while it is being: formed, the conditions under which the treatment solution is applied to the surface of the sheet, namely the temperature, the time during which the sc>luticen is in contact with the galvanized surface, t-he S0,%” ion concentraticen and the zn?' ion concentration, are adjusted so as to form a layer based on zinc hydroxysulfa te and zinc sulfate, the sulfuar content of which is not less than 0.5 mg/m?. This is because when the sulfur c ontent is less than 0.5 mg/m®, -the reduction in degradati on of the coating is less subst-antial.

Thus, the tome during which the treatment. solution is in contact wvith the galvanized surface is between two seconds and two minutes and the temperamture of the treatment solution is between 20 and 60°C.

Preferably, the treatment solution wus ed contains between 20 &nd 160 g/l of zinc sulfate Ieptahydrate, which corresponds to a zn?" ion concentration and an

S042” ion ceoncentration that lie between 0.07 and 0.55 mol/l. In fact, it has been found t=hat in this concentratiorn range the rate of depositicon is barely influenced byw the value of the concentratior.

Advantageousl.y, the conditions under which the treatment so_lution is applied, namely the temperature, the time dur ing which the solution is in «contact with the galvanizead surface and the S0,2 ion .and 2Zn?* ion concentratioms, are adjusted so as to formn a hydroxy- sulfate/sulfa te-based layer having a sulfur content of between 3.7 a.nd 27 mg/m.

According to a variant of the invention, t¥he treatment solution contains an oxidizing agermt for zinc, such as hydrogen peeroxide. This oxidizing acgent may have a very . pronounced. sulfation/hydroxysulfation accelerator effect at low concentration. It has been found that the . 5 addition of only 0.03%, i.e. 8 x 102 mol/l of hydrogen peroxide, or of 2 x 107 mol /1 of potassium permangana te, to the solution makes it possible for the rate of deposition to be doubl ed (approximately).

However, dt has been found that concentrations 100 times higher no longer allow this improvement in deposition rate to be obtained.

After the treatment solution has been applied, but before dr=ying, the layer deposited on the sheet is adherent. The drying is adjusted in order to remove the residual 1 iguid water from the deposit.

Between tlie application step and t he drying step, the sheet is preferably rinsed so as to remove the soluble portion of the deposit obtained. The= absence of rinsing and the formation of a resulting deposit that is partially soluble in water do not greatly prejudice the reduction in degradation of the galvanized coating during the operation of forming tlie sheet, since the deposit ob tained does indeed compris=se a water-insoluble zinc hydro xysulfate/zinc sulfate-bassed layer in contact with the s heet.

According to a second embodiment off the invention, the aqueous “treatment solution having an S042" ion concentrat ion of not less than 0. Ol mol/l is applied under anodic polarization and the pH of the treatment solution i s equal to 12 or higher, k>ut less than 13.

If the pH of the solution is less than 12, adherent hydroxysul fates are not formed on the surface to be treated. If the pH of the solutiomm 1s equal to 13 or higher, the hydroxysulfate r-edissolves and/or decomposes into zinc hydroxides.

When sodiumm sulfate is used in the treat ment solution, . if the seodium sulfate concentration is less than 1.4 g/1 Fn the solution, little formation of . 5 hydroxysulffates on the surface is observed. More generally, it is therefore important for- the S0,*° ion concentrat# on to be not less than 0.01 mol/l, and preferably not less than 0.07 mol/l.

Furthermore, the sulfate ion conc entration is preferably not more than 1 mol/l. If sodDdum sulfate is used, at concentrations greater han 142 g/1 (equivalent to 1 mol/l of S04%7), for exampole 180 g/l, a reduction =in the efficiency of formatior of the zinc hydroxysulffate/zinc sulfate-based layer iss observed.

It has beern found that the reduction in degradation of the galvani zed coating of the sheet whil e it is being formed is -obtained only if the thickness of the zinc hydroxysulf ate/zinc sulfate-based layer deposited corresponds to more than 0.5 mg/m? of. equiwalent sulfur, preferably at least 3.5 mg/m? of equivalent sulfur.

However, it has been found that the reduction in degradation of the galvanized coating decreases if the amount of sulfur in the zinc hydrosysulfate/zinc sulfate-base~ed layer deposited greatly exceeds 30 mg/m?, owing, it wwould seem, to the deterioratio-n in adhesion of this layeer.

Thus, to r educe the deterioration of the galvanized coating, si gnificantly, it is necessary for the total deposited ammount of hydroxysulfates and s-ulfates to be not less tlman 0.5 mg/m’ but not to exceed 30 mg/m? of equivalent sulfur, preferably to be bet ween 3.5 and 27 mg/m? of equivalent sulfur.

The zinc neseded to form the zinc hydroxysulfate/zinc sulfate-basesd deposit comes from the anodi ¢ dissolution of zinc under the effect of the polarization of the galvanized surface

It is therefore ne«cessary for the density of electri cal . 5 charges flowing during the treatment through the surface of the sheet to be adjusted so as to forrm a. zinc hydroxysulfat e/zinc sulfate-based layer having a sulfur content of rot less than 0.5 mg/m?.

Thus, the applied charge density is preferably between 10 and 100 C/dm? of surface to be treated.

If the charge den sity exceeds 100 C/dm?, it has b een found that the amount of sulfur deposited on the surface no longer increases, and even decreases.

Thanks to the anodic polarization of the galvani zed surface to be treated, there is rapid dissolution of the zinc in the immediate vicinity of the galvani zed surface, which prormotes the precipitation of zinc sa lts : on this surface. :

Thus, to carry out this treatment in as productive a manner as possib le with a satisfactory coulom_ bic x 25 efficiency, it is necessary for the zinc hydro xy- sulfate/zinc sulfate-based layer to be deposited un der a high polarizatiom current density, especially grea ter than 20 A/dm?, for example 200 A/dm’.

At a current densitty of less than or equal to 20 A/&n?, the deposition effi® ciency is very low and the amount of sulfur in the depossited layer does not make it possible to significantly wxreduce the degradation of the =z inc coating on the sheest while it is being formed.

As counter-electrode, it is possible to use a titan jum cathode.

The temperature of the treatment solution is genera lly between 20°C and 6 0°C. Preferably, the treatment is carried out at a temdmperature of 40°C or higher, so as . to increase the cormductivity of the sol.ution and to reduce the ohmic los ses. } 5

The flow rate of tke solution at the suarface of the sheet does not, here, have any major i.mpact on the treatment according -to the invention.

After the hydroxysu_lfate/sulfate-based l=ayer has been formed on the surfac e, the coated surface is thoroughly rinsed with demineralized water. This rirmsing step is important in order t-o remove the alkaline reactants at the surface of the deposit, reactants tha t might cause corrosion problems.

The subject of the: invention is also a method of lubricating a steel sheet coated wi th a layer consisting of a mettal coating based on zinc or its alloys, in which method: - said sheet i s coated with an uppe—r layer based on zinc hydroxysulfate and zinc sulfate , said upper layer having been obtained by using a treatment solution according to the invention; and then = a lubricating oil film is applied to the upper ’ layer with a weight of less than 1 g/m?.

The weight of lulbricating oil film applied is preferably less than 0.9 g/m? and is more particularly between 0.2 and O0 .5 g/m?, since such weights are sufficient to obtadin excellent tempora.ry corrosion protection and to awoid any risk of cont-amination of workshops and formingy tools.

Finally, the subject of the invention is t—he use of an aqueous treatment sol ution comprising sulfate ions with a concentration of n.ot less than 0.01 mol. /l, in order to improve temporary~ corrosion protectiom of a steel sheet coated with a metal layer based on zinc or its alloys. . This aqueous treatment solution is applied to the steel sheet according to the embodi ments described in the . 5 paragraphs relating to the use of an aqueous treatment solution containing sulfate ions for treating a galvanized steel sheet for the purpose of reducing degradation of the zinc coating while the sheet is being formed. For this purpose, the reader may refer to the paragraphs relating thereto.

As will be seen in the exa mples illustrating the invention, the inventors have sshown that the temporary corrosion protection of a alvanized steel sheet firstly treated with a treatmen.t solution according to the invention and then coated w- ith an oil film is very much better than that of a galwanized steel sheet that has not been treated beforehand.

The invention will now be described by examples given by way of nonlimiting indicatio n and with reference to : the appended figures in which: - Figure 1, with refe rence to Example 2, illustrates the results of the friction tests carried out on various sheet test piecess treated according to ’ the invention or untreated; and - Figure 2, with refe_rence to Example 3, illustrates the results of the hot/wet corrosion tests carried out on various sheet te=st pieces according to the invention or untreated. 1. Reduction in the formatiora of coating powder ox particles when drawing a galvani zed sheet

Test pieces were cut from a ste el sheet, of “aluminum- killed steel” grade and of ES quality, with a thickness of 0.7 mm and coated on each off its sides with a zinc coating produced by hot dipping _.in a zinc bath.

An aqueous treatment solution according to the invention, obtained from 125 g/l of zinc sul fate . heptahydrate ZnS0,-7H,0 was prepared. . 5 Next, this treatment solution was applied to some of the test pieces by spraying the treatment solution at a temperature of 40°C. After the sheet was in coratact with the solution for a time of 3 to 4 s, the treated sheet was drained amd then dried.

A lubricating oil film was then applied to the zinc hydroxysulfate/zinc sulfate-based layer formed on the surface of the galvanized steel sheet test pieces, said 0il being either QUAKER 6130 oil (from Quaker) or

FUCHS 4107S oil (fxom Fuchs), with a film weight of 1.5 g/m?.

The other test pieces that had not been pretreated with the treatment solut ion according to the invention were oiled, either with the QUAKER 6130 o0il or with the

FUCHS 4107S oil, again with a film weight of 1.5 g/rn?.

The two series of test pieces were then subjected to a controlled deformation test by means of a [pporess comprising a punch, a die and a blank hol.der, recreating in the laboratory the stresses undergon e by the sheet during a drawing operation, especially in. the die radii and/or in the retaining rings with which the drawing tools are equipped. Various Dblank-hoslder clamping forces were applied to the test pi.eces undergoing the test.

Each of the test pieces of the two series was wei ghed before the oiling operation and then at the end of the test, after de-oiling, by means of a balance accurate to 0.0001 grams. The measured difference in weight was normalized to a weight loss per square meter, ta king into account the area affected by the friction during the simulation of drawing the test piece, which was identical for each of the test piecess. - Moreover, after having formed one t est piece and before the n ext test piece was formed, the press was wiped so : 5 as tc identify the zinc coating powder or particles lost koy the test piece in the press -

The weight loss results of the test pieces after drawirmng and the identification of zinc powder and/or particles coming from the coating a re given in Table 1.

The p.articles and/or powder are idesntified by a rating in th e following manner, according to a scale ranging from 71 to 4, where: mating 1: very few particles or very little powdem:; mating 2: few particles or litt=le powder; mating 3: many particles or a lot of powder; and mating 4: very high level of particles or powder.

Table 1: Test results

Type of oil Identification on

Clamping Weight (quantity: the tools force losss 1.5 g/m? (daN) (g/m?) Powder | Particles : per side)

Steel sheet 0.63

QUAKER oil 400 3 3 coated with 0.024 lubriczating 0.55 £

FUCHS oil 400 3 3 oil fi_ lm 0.0-4

Steel sheet 0.12: = , . coated with 0.71

QUAKER oil a hydr-oxy- 0.22: * 750 3 1 to 2 sulfat. e 0.71 layer and with a. 0.20: =

FUCHS oil 750 3 1 lubric=ating 0.71 oil fi 1m

The measured weight losses and the amounts of pow er and particles observed when Wiping the tools show that ] the loss of material from the zinc coating, due to the galvanized steel sheet passing over the punch, is } 5 = ‘significantly reduced when the sheet, before bei ng oiled, is treated with the treatment solution according to the invention. 2. Reduction in the effect of dewetting - effect on the tribological behavior — friction test

Test pieces 1 cm? in area wexe cut from a steel shee:t, of “aluminum-killed steel” grade and of ES qualit vy, with a thickness of 0.7 mm and coated on each of i ts sides with a zinc coating produced by hot dipping in a zinc bath.

Some of these test pieces wer e treated with a treatme nt solution according to the invention, under the sa me conditions as those indicated in Example 1, so as to form a zinc hydroxysulfate/zinc sulfate-based layer. A lubricating oil film (QUAKER 6130 oil) is then appli ed to this layer in amounts ranging between 0.25 a nd 2.5 g/m.

The other test pieces were oi led in the same manner as previously, but without havin«g been pretreated with t he treatment solution according to the invention.

The friction behavior of each of the test pieces w.as then characterized using a tribology tester in the following manner.

The tester was a flat-on-flatt tribometer known per see,

The test pieces to be tested were clamped with a clamping force F. between two high-speed steel platess offering a bearing (or slidzdng) surface to the tes=st pieces. The friction coefficient N was measured while moving the test piece relative to the plates over a total travel D of 180 mm and at a speed of 10 mm/s, whil e progressively increasing the clamping force F..

A curve showing the variation in fr iction coefficient . 5 as & function of the clamping force F. for different lubricating oil film weights can then be plotted (see

Figwere 1).

The various curves are identified by the following symb=ols: t+: sheet treated according to the invention and then coated with 0.25 g/m? per side off a QUAKER 6130 oil filmu; x: sheet treated according to the invention and then coated with 1.0 g/m? per side of a QUAKER 6130 oil; ¢: sheet treated according to the invention and then. coated with 2.5 g/m® per side of a QUAKER 6130 oil film ;

BM: untreated sheet coated with 0.25 g/m? per side of a QUAKER 6130 oil film;

A: untreated sheet coated with 1 _0 g/m? per side of a QU AKER 6130 oil film; and ¢: untreated sheet coated with 2_5 g/m? per side of a QU. AKER 6130 oil film.

Given in Table 2, for each of the te=st pieces tested, is t—he mean value of the friction coefficient for a give n clamping force F..

Table 2 . Weight of 0il on the

Clamping sheet treated Weight of oil on the force according to the untreated sheet (MPa) invention (g/m) | (g/m?) | (g/m®) | (g/m?) | (g/m?) | (g/m?)

The results obtained show tlmat a reduction in the weight of oil results in a sub stantial increase in trae friction coefficient when no treatment soluticon according to the invention is applied before applyimg the oil film.

However, when the treatment solution according to thlme invention has been applied t o the galvanized shee-t : prior to application of the lwabricating oil film, th_e friction coefficients obtained are very low, even wit.h oil weights of less than 0.5 g/ct’. 3. Reduction in the effect of the dewetting - th.e effect on temporary corrosion pmotection

Test pieces were cut from a stesel sheet, of “aluminum. - killed steel” grade and of ES gq uality, with a thicknes s of 0.7 mm and coated on each o f its sides with a zin c coating produced by hot dipping in a zinc bath.

Some of these test pieces were treated with a treatment solution according to the inv-ention, under the same-e conditions as those indicated in Example 1, so as teo form a zinc hydroxysulfate/zincs sulfate-based layer. A lubricating oil film (QUAKER 6130 oil) was then applied to this layer in weights ranging between 0.25 and 1.0 g/m.

The otherr test pieces were ciled in the same manner as . 5 previously, but without having been pretre ated with the treatment. solution according to the invention.

The lubri cating oils applied to the steel sheets coated with the zinc-based metal coating guaranteed corrosion protection during the period of time that elapsed between m:anufacture of the sheets and thei _r processing, for examp le by drawing.

The conformity of the product delivered up to this point was checked via the results of arm accelerated hot/wet ceorrosion test.

Specifically, the test pieces to be testecd were placed in an environmental chamber correspond_ing to the

DIN 50017 standard, which simulates the conditions for corrosion of the external turn of a coil ofS sheet or of an indivicdual cut sheet during storage.

The details of the hot/wet cycle (one cycle = 24 hours) are given below: - 8 h at 40°C and 95-100% RH (relative humidity) ; - 15 h at 20°C and 75% RH.

The indivi dual test pieces were suspended veartically.

The results of the test, given in Table 3, were obtained Koy measuring the number of succe ssive cycles before any traces of corrosion appeared on the test piece.

A curve showing the variation in the percen_tage content of white r-ust as a function of the number o f cycles for each of tloe test pieces tested can then be plotted (cf.

Figure 2).

- 16 =

The various curves are identified by the foll owing : symbols: +: sheet treated according to the inventiorh and : 5 then coated with 0.25 g/m? pe:r side of a QUAKER 613 0 oil film; *X: sheet treated according toc the inventiorm and then coated with 0.5 g/m? per- side of a QUAKER 613 0 oil . film; 4: sheet treated accorcling to the inventiora and then coated with 1.0 g/m’ per side of a QUAKER 613 0 oil film; ¢: untreated sheet coateed with 0.25 g/m? per side of a QUAKER 6130 oil film;

MB: untreated sheet coate:d with 0.5 g/m? per si de of a QUAKER 6130 oil film; and e: untreated sheet coate:d with 1.0 g/m’ per si de of a QUAKER 6130 oil film.

Table 3

Weight of cil on the

Number sheet treated Weight of oil on the of according to the untreated shee t cycles invention (g/m) | (g/m?) | (g/m?) | (g/m®) | (g/m?) | ( g/m®) 1s [0 Ff os [ess [| as [2s | 0 | 2s [ os [TT as ss | 20s | 2s | os | 00 | 0 | as { «os | sos [| os [| aos | os ss | sss | [os | | sos | 2s § os | [wes | os [0 | ess | 2s

It has been found that it 1s possible to very significantly improve the temporary corr osion protectiorm of galvanized steel sheets to which a treatment solution according to the invent ion has been . applied before application of the lubricating oil film, this being so even when the weight of oil is less than . 5 1 g/m?.

Claims

CLAXMS . 1. The use of an aqueous treatment solut=ion containing sulfate ions S042 with a concentration of - 5 not less than 0.01 mol/l in order to treat the surface of a steel sheet treated on at least one of its si_des with a metal coating based on zinc or its alloys, for the purpose of reducing the formation of metal powder Or particles based on zinc or its alloys generated by the degradation of the coating while said sheet is being formed.

2. The use as claimed in claim 1, in which the aqueous treatment solution furthermore contains .Zn%" ions with a concentration of not less than 0.01 mol/M.

3. The use as claimed in either of claims 1 and 2, in which the conditions under which the treatment solut.ion is applied to the surface of the sheet, namely the temperature, the time during which the solution is in contact with the galvanized surface, the S0,%” ion concentration and the 2Zn?* ion concentration, are adjusted so as to form a layer based on z inc hydroxysulfate and zinc sulf ate, the sulfur content of which is not less than 0.5 mg/m?. 4, The use as claimed in army one of claims 1 to 3, in which the 2zn?* ion concentration and the S042” ion concentration are between 0.07 and 0.55 mol/l.

5. The use as claimed in amy one of claims 1 to 4, in which the pH of the treatment solution is between 5 and

7.

6. The use as claimed in amy one of claims 1 to 5, in which the conditions under which the treatment solut ion is applied, namely the temperature, the time dur ing which the solution is in contact with the galvani zed surface and the $0, ion armad Zn?" ion concentratioms,

are adjusted so as to form a hydro=ysulfate/sulfate- based layer having a sulfur content of between 3.7 and : 27 mg/m°. ) 5 7. The use as claimed in any one of claims 1 to 6, in which, after the treatment solution has been applied to the sheet, said sheet is dried, after having optionally been rinsed in order to remove the s oluble portion of the hydroxysulfate/sulfate layer.

8. The use as claimed in «claim 1, in which the treatment solution is applied under arocdic polarization and the pH of the treatment sclution is equal to 12 or higher, but less than 13.

9. The use as claimed in claim 8, in which the density of electrical charges flowing during the treatment through the surface of the sheet is adjusted in order to form a zinc hydroxysulffate/zinc sulfate- based layer, the sulfur content of whi ch is 0.5 mg/m? or higher.

10. The use as claimed in either of claims 8 and 9, in which the S044 ion concentration is greater than : 25 0.07 mol/l.

11. The use as claimed in any one o f claims 8 to 10, in which the electrical charge density 1s adjusted in order to form a zinc hydroxysulfate/=inc sulfate-based layer, the amount of sulfur of which =is between 3.7 and 27 mg/m°.

12. The use as claimed in any one o f claims 8 to 11, in which the polarization current density applied during the treatment is greater than 2:0 A/dm®.

13. The use as claimed in any one o f claims 8 to 12, in which, after the treatment solutior has been applied to the sheet, said sheet is rinsed.

14. A method of lubricating a steel sheet coated with

. a layer consi sting of a metal coating based on zinc or its alloys, im which method: : 5 —- said sheet is coated with an upper layer based on zinc hydr oxysulfate and zinc sulfat-e, said upper layer having been obtained by usings a treatment solution as defined in any one of claim= 1 to 13; and then - a lubmricating oil film is applie d to the upper layer with a wveight of less than 1 g/m2.

15. The meth od as claimed in claim 14, characterized in that the weight of the oil film is less than

0.9 g/m?.

16. The meth od as claimed in claim 15, characterized in that the wreight of the oil film is l»etween 0.2 and

0.5 g/m?.

17. The use= of an agueous treatmnent solution containing SO 4° sulfate ions with a co ncentration of not less than 0.01 mol/l, in order to treat the surface of a steel skhieet coated on at least one of its sides with a metal coating based on zinc or i ts alloys, for the purpose of improving the tempor-ary «corrosion protection of said sheet. p