DE1621487B2 - Process for chemical surface treatment of steel, zinc and aluminum by pickling and drying a chromium-containing solution - Google Patents

Description

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The invention relates to a method for chemical timely or successively without special adaptation of the

Surface treatment of steel, zinc and aluminum - bath composition and treatment method to the

nium, which can be used in particular to prepare this single metal, is sufficient, however

Surfaces for the application of organic coatings none of the known working methods the very high

suitable is. 5 Requirements that apply in particular to the pretreatment

There are already various chemical surfaces before the application of organic coatings

Treatment methods to improve the Korro practice are provided. You can therefore also see in the

sion protection and / or paint adhesion known, the latest and most modern systems for chemical

in the same way both for steel and for surface treatment prior to painting or

zinc material as well as for aluminum io plastic coating is still used separately

can be. For example, according to the action zones for the individual metals in which

Method of German patent specification 853 695 to the most appropriate for the particular metal

The above-mentioned surfaces are applied in the same apparatus, treatment solution and manner (cf.

and with the same solution essentially from this z. B. Sheet Metal Industries, Vol. 44, No. 481,

Zinc phosphate existing coatings applied May 15, 1967, p. 338). So z. B. the treatment of

With the help of aqueous zinc monophosphate solutions, steel with solutions of acidic alkali metal phosphates

the soluble compounds of each of these three metals, iron oxide-iron phosphate coatings are formed,

a double fluoride and common oxidizing agent, for zinc are chromating processes or treatments

preferably nitrate. So pretreated with alkaline aqueous solutions, in addition

Metal surfaces can then be painted or 20 complexing agents and certain heavy metals

be coated with plastic films. The Korro hold, usual. For aluminum, very forged

Sion resistance of such parts is very good. The dige, easily deformable top layers also with

Adhesion of organic films to the chromating process thus prepared. The bathroom

ridden metal bases, however, is often not settlement, however, depends on the zinc chromating

sufficient. In particular, when the sheet 25 is deformed, proceed differently.

After coating, the organic film can therefore be used in a variety of treatment

peel off the surface. zones, of which those except

After a further operation, which can be used for general purposes, is set for the metal in question

The multistage processes drawn up should not be required. This disadvantage is one strong

which surfaces first cleaned with alkaline and 30 increased equipment costs and a larger one

then with an aqueous conditioning solution, which will take up space in spite of the known working methods

Taken 0.3 to 10 ° / ₀ of ortho-phosphoric acid compound in the purchase, as they put the requirements

z. As H3PO4, and 0.02 to 0.10 ° / ₀ of a polyoxyethylene not approximations suffice.

contains substituted hydrocarbon, wetted. There is thus still the desire to finally use a surface treatment process containing hexavalent chromium, with which the solution is dried. The zinc, steel and aluminum used in this case without any special adaptation, conditioning solutions, take effect with the bath composition being kept constant for the individual metal, temperature and exposure time, the metals steel, zinc and aluminum can be prepared with organic materials at the same time or one after the other for the coating at very different speeds, with the 40 and the good corrosion protection combined with aluminum being the least reactive. Good adhesion to iron is guaranteed with all three metals, even after a relatively short throughput. A two-step process has now been found, muddy coatings made of iron phosphate, the adhesion that meets the requirements. The subsequently applied coatings impair the methods according to the invention for chemical mastery. *. Furthermore, the enrichment of surface treatment of steel, zinc and aluminum, iron phosphate sludge in the treatment bath, in particular before the application of organic coatings, incrustations, clogging of nozzles, pumps and the like, is that the surfaces are initially noticeable with a disturbing effect. aqueous solution containing 0.2 to 3 g / l CrO ₃ and 0.5 to

According to another known procedure, it contains 5 g / l HF. The pickling solution (e.g. German Auslegeschrift 1176 958, USA.-Patent 50 as long as and at such a temperature applied / ends, 3 053 693) are aqueous, compounds of the sixth that a pickling removal of the metal of 0.2 up to 2 g / m ^{a of} valuable chromium and organic reducing agents is achieved. After a water rinse under z. If, for example, certain conditions containing sugar, organic polymers, etc. are omitted, dispersions on the various metals can be applied, the metal surface is then wäßbracht and dried with a. In the case of steel 55 rinse containing hexavalent chromium, a previous stain is moistened with nitric acid solution, and the film that remains is recommended. That dries up on the metal surfaces on the surface,
water-insoluble, fairly dense film that forms. The pickling solution is used in the Cr ^m compounds and organic polymerization processes according to the invention on products that have been as largely pre-degreased as possible and is not compatible with every type of post-surface. For degreasing the treatment. So difficult z. B. the film made of metal surfaces are suitable, among other things, the reaction of acidic components in the paint, ζ. Β. . knew aqueous solutions of alkaline cleaners, phosphoric acid, with the base metal. Water-soluble alkali hydroxide, alkali carbonates, alkali ortho and lacquers do not wet the surface well. The electro-condensed phosphates, alkali silicates, alkali borates, phoretic paintwork are impaired by the noticeable complexing agents and surface-active substances, the surface resistance of the layer. may contain and those in diving, flooding or

Although the most diverse methods are thus known - spraying, preferably at an elevated temperature

have become to be applied to zinc, steel and aluminum alike- the metal surface. The development

Fatting stage can be omitted if the metal to be treated is at least largely free of grease, as is the case z. B. is the case with freshly hot-dip galvanized steel.

The pickling attack of solutions containing chromic acid and hydrofluoric acid on aluminum is known (W. Wiederholt in "METALL", Vol. 7 [1953], pp. 343 to 347), However, it was surprising that the pickling solution used according to the invention in the specified concentration range steel, zinc and Pickling aluminum at the same temperature at approximately the same speed. There is one in this essential advantage of the method according to the invention. Provide an overview of the pickling removals Tables 1 and 2, in which the abrasions for steel and hot-dip galvanized steel when spraying the Surfaces are listed for 11 seconds at room temperature.

Tabel 1 0.0 0.5 CrO ₃ (g / l) i steel 5.0 Pickling removal (g / m ² ) be 0.00 0.00 1.0 0.00 HF (g / D 0.12 0.00 0.00 2.5 0.00 0.12 0.32 0.00 0.00 0.00 0.0 0.12 0.40 0.05 0.00 0.00 0.5 0.12 0.50 0.82 0.00 0.70 1.0 0.82 0.52 2.5 2.25 5.0

Tabel 0.5 2 2.5 5.0 Pickling removal (g / m ² 0.00 0.02 0.05 HF (g / 0 0.20 0.30 0.32 0.0 0.42 ι for hot-dip galvanized steel 0.65 1.15 0.0 0.00 0.55 CrO ₃ (g / l) 1.53 1.58 0.5 0.05 0.67 1.0 3.00 3.15 1.0 0.05 0.00 2.5 0.07 0.27 5.0 0.10 0.55 0.82 1.00

Cations of the treated metals are formed in them. In addition, Cr ^{111 is formed} , which is produced by reducing the chromic acid. The baths are usually tested for free acid content and Cr ^ ¹ content using known methods. The supplement is preferably carried out with HF to _{ensure that the free acid remains constant and with CrO 3} to ensure that the CrVT content is constant. The make-up chemicals can be added to the bath individually or premixed.

ίο The accumulating cations have a different influence on the way the bath works. Table 3 shows their behavior in a bathroom. 0.5 g / l CrO ₃ + 2.5 g / l HF, applied by spraying at room temperature, listed. ^{Since Cr 111} accumulates in addition to its cations during the treatment of each metal, the various cations were each tested ^{in conjunction with Cr 331.}

Table 3
Zn + Cr ¹¹¹

a) for steel: practically no influence up to approx. 20 g / l Zn + approx. 13 g / l Cr ^m .

b) for hot-dip galvanized steel: practically no influence up to approx. 20 g / l Zn + approx. 13 g / l Cr ^m .

Aim -j-

a) for steel: practically no influence up to approx. 1.9 g / l Al + approx. 1 g / l Cr ^m .

b) for hot-dip galvanized steel: practically no influence up to approx. 1.9 g / l Al + approx. 1 g / l Cr ^m .

c) for aluminum: practically no influence up to about 1.9 g / l Al + about 1 g / l Cr ^m .

When sprayed with a solution of 0.5 g / l CrO ₃ + 2.5 g / l HF over 11 seconds at room temperature, ^{aluminum shows an abrasion of about 0.3 g / m 2} and thus corresponds to a large extent to the values for steel ( 0.40 g / m ² ) and hot-dip galvanized steel (0.55 g / m ² ). When the temperature is increased and the treatment time is extended, the pickling attacks increase. The conditions must therefore be chosen so that the removal is between 0.2 and 2.0 g / m ² . Below 0.2 g / m ² , the roughening of the metal surface is too small, so that adhesion problems can occur after the subsequent organic coating. Removal of more than 2.0 g / m ^2, especially with aluminum and steel, easily causes pickling sludge to be deposited on the surface, which can also impair paint adhesion. On the other hand, the consumption of chemicals increases. In the range between 0.2 and 2.0 g / m ² , the metal surfaces showed a uniform roughening without any signs of noticeable layer formation or sludge deposition.

The pickling stage according to the invention can be used in dipping, flooding or spraying. In general, however, the spray method is preferred. Furthermore, it is preferably carried out at room temperature to about 4O ⁰ C, although higher temperatures may also be employed. Enrich during the operation of the pickling baths according to the invention

a) for steel and hot-dip galvanized steel: significant increase in pickling attack from (0.3 to 1) g / l Fern _ | _ (0.15 to 0.45) g / l Cr ¹¹¹ .

b) for aluminum: practically no influence up to about 3.0 g / l Fe ¹¹¹ + 1.4 g / l Cr ^m .

To control the content of interfering cations in the bathroom, you can use the bath z. B. send continuously over a cation exchanger in the H ^{+ form while working.} Furthermore, it is also possible to artificially increase the mechanical bath losses to prevent the increase in concentration beyond the interference limit. Good results were z. B. obtained when 100 to 300 ml of bath solution were discarded ^{per 1 m 2 of treated metal surface.}

The pickling bath used according to the invention is preferably used in continuously operating systems for used for the treatment of strip and sheet metal. For use in the usual spray and Diving systems with longer transition times from bath to bath, it is less suitable as it is in these during the transition to rusting on the surfaces of the treated material.

Following the pickling stage, rinsing is preferably carried out with water. However, if the The metal surface was vigorously squeezed off, thus keeping the pickling bath entry into the next bath low flushing can be dispensed with if necessary.

An aqueous solution containing Cr ^ is used to rinse the metal surfaces. The Cr ⁷¹ content, calculated as CrO ₃ , is preferably selected between 0.6 and 6 g / l, the higher being the higher

Levels are particularly used when the metal surface is subsequently squeezed off to reduce mechanical discharge and to even out the adhering film of solution. The hexavalent chromium can e.g. B. be introduced into the bath as chromic acid and / or zinc, calcium, aluminum and chromium ^{111 chromate.} For rinsing, it is sufficient to bring the metal surfaces into contact with the Cr ^ -containing solution for a few seconds. The temperature of the rinsing bath can be ιό ζ. B. between room temperature and about 60 ° C. After rinsing, the film of liquid remaining on the metal surface is dried up. After that, paints, adhesives for plastic films and the like can be applied. The method of operation according to the invention also gives the metal surface a certain storage protection without further post-treatment.

The method according to the invention will be explained briefly using an example.

Sheets made of steel, sendzimir galvanized steel and aluminum were produced in the following process Syringes treated.

a) degreasing with an aqueous solution containing 3 g / l NaOH, 3 .g / l Na _s P ₃ O _10, 0.2 g / l of organic complexing agent, 0.7 g / l of non-ionic emulsifier, 22 seconds at 65 ° C.

b) Rinse with cold water, 4 seconds.

c) pickling with an aqueous solution containing 0.5 g / l of CrO _3, 2.5 g / l of HF, 11 seconds at '25 ⁰ C.

d) Rinse with cold water, 4 seconds.

e) Rinsing with an aqueous solution containing 3.4 g / l CrO ₃ , 0.6 g / l Cr ^m for 5 seconds at 45.degree.

f) Squeezing between rubber rollers and drying.

After this treatment, all types of material showed a clean, evenly matt surface. The pickling

The removal rate was between 0.3 and 0.6 g / m ² . The sheets were coated with acrylate stove-enamels known for coil coating and tested for corrosion protection in the salt spray room and adhesion by means of Erichsen cupping and cup pulling. The tests showed that the steel sheets treated according to the invention behave in the same way with regard to corrosion protection and better adhesion than sheets treated with the known alkali metal phosphate processes. The sendzimir galvanized sheets according to the process according to the invention corresponded to the chromated sheets in terms of corrosion protection and paint adhesion. The same picture emerged with the aluminum sheets.

Claims (3)

Patent claims: 1. A process for the chemical surface treatment of steel, zinc and aluminum, especially before the application of organic coatings, by pickling and drying an aqueous solution containing hexavalent chromium, characterized in that the surfaces are treated with an aqueous solution containing 0.2 to 3 g / l CrO ₃ and 0.5 to 5 g / 1 HF, are pickled to such an extent that the pickling rate is 0.2 to 2 g / m ^a , and then, preferably after rinsing with water, with a hexavalent chromium, preferably from 0.6 to 6 g / l, calculated as CrO ₃ , containing aqueous solution are brought into contact and then dried. 2. The method according to claim 1, characterized in that the pickling solution for removal interfering cations, preferably continuously, pumped over a strongly acidic cation exchanger will. 3. The method according to claim 1, characterized in that the pickling solution is subjected to a mechanical bath loss of 100 to 300 ml bath per 1 m ^{2 of} treated surface to regulate a stationary content of the cations below the interference limit.