DE2103086A1 - Process for the surface treatment of iron and steel - Google Patents

Description

METALLGESELLSCHAFT Frankfurt / Main, Jan. 21, 1971

Public company DrBr / SL

6 Frankfurt / Main 1
Reuterweg 14

prov. No. 6643 M

Process for the surface treatment of iron and steel

The invention relates to the surface treatment of iron and steel workpieces and in particular to an improved way of working in the chemical pretreatment of the workpieces for the application of a organic coating in the electrodeposition process.

To prepare iron and steel for electrocoating it is common practice to clean the workpieces, rinse them one or more times with water and then rinse them to phosphate. Mildly alkaline cleaners in the pH range are generally used for cleaning used by 8-11. In practice, however, difficulties sometimes arise because these cleaners Do not always completely remove anti-corrosion oils and drawing greases from the metal surface, especially when the workpieces have been stored for a long time. The remaining remnants of the oily and fatty Impurities mean that the workpieces are not or only partially with after rinsing with water Are wetted with water. In these cases, during the subsequent phosphating, an un-

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even phosphate layer formed, which is also has an unfavorable effect on electrocoating.

This problem of uneven phosphating occurs particularly annoying when handling workpieces in continuous spray systems. Ground in these systems the workpieces either hanging on a conveyor or placed on a subframe continuously through transports the individual treatment zones of the system. There are between the spray registers of the individual zones Transition sections are available which are dimensioned so that mixing of the treatment solutions is avoided as far as possible will. Depending on the shape of the parts to be treated and the transport speed, for example the time interval between the rinsing zone and the phosphating zone in the range of 15-60 sec. In This transition zone, however, allows phosphating solution to act on the metal surface practically not completely ruled out. By adjusted nozzles or by deflected spray jets or by Fine misting of the phosphating solution is the metal surface in the transition zone from the phosphating solution met. Depending on the exposure time, more or less strong pre-passivation occurs. The formation of the phosphate layer in the phosphating zone then takes place unevenly. She has streaks, Veil, striped markings, some of which even shimmer bluish. In some cases these irregularities in the phosphate layer can still be seen after the electrodeposition coating and therefore highly disadvantageous and undesirable.

It has now been found that the difficulties outlined can be eliminated in a simple manner by after alkaline cleaning and before phosphating on the surfaces at a temperature below

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60 ° C a weakly acidic aqueous solution is brought into action, whose pH value with lactic acid is set to 4 to 5.5, preferably 4 to 4.8, and which contains at least one nonionic surfactant.

The multi-stage method for surface treatment according to the invention of workpieces made of iron and steel consists in the fact that the workpieces are initially alkaline cleaned (1), then at a temperature below 60 ° C with an aqueous solution whose pH value with Lactic acid to 4 to 5.5> preferably 4 to 4.8, one g | and contains non-ionic surfactant, brought into contact (2), then with a phosphate coating provided (3) and finally covered with an organic coating using the electrodeposition process (4).

The usual alkaline cleaners can be used to clean the workpieces; Their pH value is expediently between 8 and 11. The alkaline cleaner preferably also contains additives known to activate the phosphating, such as titanium orthophosphate. The cleaning can be carried out by spraying as well as by immersion. The detergent concentration is, for example, 3-5 g / l> im m when spraying

Dip 20-30 g / l. When using the process sequence according to the invention, as an additional advantage, the detergent concentration can in many cases be reduced to about 50 % of the stated values. The temperature of the cleaning bath can often also be reduced.

After cleaning, the workpieces can be rinsed with water to prevent the detergent solution from entering the Avoid lactic acid treatment bath. You can also use the lactic acid treatment without rinsing with water use and save rinse water. However, it then requires compliance with the pH value limits and the bath temperature a stronger monitoring.

In the acidic intermediate treatment carried out according to the invention after the alkaline cleaning, it is necessary to the pH of the aqueous solution with lactic acid to a pH between 4 and 5.5, preferably up to 4.8. A higher pH value leads to an uneven formation of the subsequently applied Phosphate layer. At pH values below 4 there is a risk of excessive pickling of the iron surfaces, which leads to coarse crystalline, for an electrocoating less suitable phosphate layers leads.

The adjustment of the required pH value by other inorganic or organic acids, e.g. Phosphoric acid, sulfamic acid, acetic acid, monochloroacetic acid, Oxalic acid, malonic acid, maleic acid, citric acid, does not lead to the desired beneficial Result.

The commercially available lactic acids, including technical grade, and their water-soluble salts can be used to produce the acidic treatment solution. Depending on the hardness of the water used, 0.01 - 1 g / l lactic acid (100 %) are generally required to set the necessary pH. The pH setting can be monitored with a glass electrode or with indicator paper. Commercially available products can be used as nonionic surfactants. First and foremost, there are ethylene oxide adducts with long-chain organic compounds with a reactive hydrogen atom, such as polyglycol ethers of alkylphenols, fatty alcohols, fatty acids, fatty amines and fatty acid amides. Low-foaming non-ionic surfactants are used for spray application. The addition products of ethylene oxide with polypropylene glycol have proven themselves because of their acid resistance and low

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Foaming particularly proven. With these products it is possible to obtain sufficiently concentrated lactic acid solutions without segregation taking place. The content of the solution is expediently at non-ionic surfactant at least 0.05 g / l.

When using the acidic treatment bath, the pH value can e.g. by adding the alkaline cleaning solution and increase slowly by reaction of the acidic solution with the iron surface. By discontinuous or continuous addition of a mixture of lactic acid and surfactants the So11 pH value is maintained. Especially before- J | Automatically operating pH measuring and metering devices can be beneficial be applied.

Too much iron and alkalis from the cleaning stage can reduce the effect of the acidic Treatment solution will be affected. In this case it is advisable to use the treatment bath from time to time to renew or to avoid excessive accumulation of foreign ions by adding water.

The temperature of the acid treatment bath must be below 60 ° C. At higher temperatures, lactate layers are formed on the workpiece surface W that may affect the subsequent application of the phosphate layer. This is particularly important if the lactic acid treatment solution is to be applied directly after cleaning with a hot alkaline solution and the workpieces are still at too high a temperature. The amount of rinsing water required for temperature control is in any case comparatively low because the workpieces only need to be cooled to below 60 ° C. At the same time, it is already sufficient to avoid a disruptive accumulation of foreign ions in the acidic treatment solution.

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Without performing the lactic acid treatment Solution, on the other hand, it is necessary after the alkaline cleaning, with the rinsing completely Use considerably larger amounts of water to prevent alkali residues from entering the phosphating bath to prevent and above all to cool the workpieces much more, thus in the transition from the rinsing zone to the phosphating zone, an undesirable reactive effect of phosphating solution on the workpieces, which leads to the formation of uneven coatings in the phosphating zone, as far as possible is prevented. Normally, the rinsing bath temperature in the corresponding systems is set to around Maintained 30 ° C, which requires relatively large amounts of rinse water. When using lactic acid treatment however, it is not necessary to keep the transition temperature so low. Because of the higher allowable In the process sequence carried out according to the invention, considerable amounts of rinsing water can therefore be heated can be saved.

When treating heavily scooping workpieces or after cleaning with strongly alkaline solutions it is advisable to rinse with water before the acid treatment. In this way the service life of the lactic acid treatment bath is increased, the consumption of chemicals is reduced and the bath management is reduced simplified.

The usual phosphating solutions can be used for phosphating based on layer-forming phosphates, preferably based on zinc phosphate, can be used. Even the subsequent electrodeposition coating can be carried out in the usual way.

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2Ί03086

The process sequence according to the invention allows in easier and safer way to get to thin, finely crystalline and hard phosphate layers, the streaky and mark-free and therefore excellently suited for electrocoating in every respect so that high quality end products can be obtained.

This surprisingly advantageous result was not to be expected on the basis of the known prior art. According to the method of German Patent 362, alkaline-cleaned surfaces should be cleaned before their Phosphating with an acidic solution with a pH from 4.2 to 6 are rinsed, the pH of the rinsing solution preferably with phosphoric acid or a acidic phosphate should be adjusted. In this way of working, however, there is the disadvantage of pre-passivation one that interferes with the formation of layers. According to the method of German patent specification 753 730 the surfaces before phosphating with an oxalic acid solution that also contains small amounts of a strong Mineral acid can be added, rinsed. This makes the packages considerably more acidic Pre-flushing, however, is very susceptible to rust, so that the achievement of good phosphate layers is severely hindered. It is also known prior to phosphating iron surfaces to activate and remove light Rust and scale a solution of substituted short-chain To use monocarboxylic acids (USA patent 3 118 793). The "solutions are used in such concentrations that a strong Pickling of the iron surface takes place. During the subsequent phosphating process, coarse, thick ones are grounded Phosphate layers obtained which are unsuitable for a subsequent electrodeposition coating.

-.- .. - ⁸ _U - BADORIGtNAt

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The sequence of procedures of the present invention is as follows explained using examples and comparative experiments:

example 1

Greased steel sheets of the quality class RRSt 1405 m (according to DIN 1623, sheet 1) were in a continuous spraying system pretreated and then coated with a base coat using the electrodeposition process.

In the continuous system, the sheets were cleaned in the first zone for 2 minutes at 60 ° C. with an aqueous cleaning solution splashed, the 3.4 g / l borax, 0.3 g / l activating titanium compound and 0.3 g / l non-ionic contained low-foam surfactant and that for adaptation to practical application conditions 3 g / l of a commercially available Corrosion protection oil (Rustilo 845) had been added.

In the second treatment zone, the sheets were treated for 30 seconds. sprayed at 45 ° C with one of the following solutions a) to i). After a transition of 30 seconds were the metal sheets in the third zone were sprayed with a nitrite-accelerated zinc phosphate solution for 90 seconds at 60 ° C. The panels were then rinsed with water and dried. The quality of the phosphate layer of the various Series was then checked.

The solutions used in the second treatment zone were made by adding 1 g / l acid and 0.2 g / l des same non-ionic low-foam surfactant to tap water (approx. 15 ° dH) and uniform setting made with NaOH to a pH of 4.5. The acids used were:

BATH ORIGINAL

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a) phosphoric acid
"b) sulfamic acid

c) acetic acid

d) monochloroacetic acid

e) oxalic acid

f) malonic acid

g) maleic acid
h) citric acid
i) lactic acid.

The test result showed that only in the sheets treated with the solution i) (lactic acid) completely flawless phosphate layer was formed. The sheets treated with solution a) (phosphoric acid) had poor, sometimes dusty layers with strong passivation phenomena. For all other sheets treated with solutions b) to h) the phosphate layer was poorly formed and interspersed with rust film.

In the subsequent electrodeposition coating, the phosphate layer was found, as was already the case on the basis of the assessment to be expected ^ only the sheets treated with the solution i) are suitable.

Example 2

Steel sheets were cleaned as indicated in Example 1, pretreated with a solution containing lactic acid and phosphated. The pH of the 0.5 g / l lactic acid and The solution containing 0.2 g / l of surfactant was adjusted to values between 3.5 and 6 with NaOH. The pH was measured with a glass electrode. The influence of the pH on the quality of the phosphate layers obtained can be found in the table below.

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pH value quality of the phosphate layer

6 streaky, blotchy

5 ", 5 little streaks and markings

5.0 almost perfect

4.8 e flawless i

4.5 e flawless i

4.0 e flawless i

3.5 line streaks, crystallinity of

Uneven layer, partly coarser crystals

In the subsequent electrodeposition coating, only the sheets treated in the pH range from 4 to 5.5 performed to good or satisfactory results.

Example 3

In a 5-zone continuous spray system were with Oil, lubricants and other contaminated automobile bodies are treated as follows:

aj 2 min. cleaning at 60 ° C. with a solution containing 3.4 g / l borax, 0.3 g / l activating titanium compound and 0.3 g / l nonionic, low-foam surfactant. h) _ Rinsing 1 min. at approx. 30 ° C with tap water, water inlet 8 m / h.

c_2 phosphating for 2 minutes at 55 ° C with a nitrite-accelerated zinc phosphate solution, concentration 14 points (consumption of 0.1 η WaOH when titrating 10 ml bath sample against phenolphthalein); Accelerator content approx. 100 mg / 1 NaNO ₂ .

c0 rinsing with tap water for 1 min. at 35 ° C. e_2 rinsing with deionized water for 1 min. at 25 ° C. f_2 Showering with fully demineralized water.

The phosphate layers of the bodies treated in this way were heavily streaked and streaked. At

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some spots, especially with aged grease had been affected, fat residues were still visible after phosphating.

In a second experiment in step b) instead of the water rinsing before the phosphating, an acidic rinse was used Rinsing carried out. The rinsing bath was for this purpose with a technical lactic acid and low foaming Concentrate containing surfactant adjusted to a pH value of 4.2 (glass electrode) and during operation maintained. The rinse bath contained approximately 0.12 g / l lactic acid and 0.09 g / l surfactant. The water inlet was at 1.8 m / h. throttled. The temperature settled on about 41 ° C. The one with this one The phosphate coatings obtained in the procedure were very uniform and finely crystalline and showed no streaks and stains on. There was no fat residue: no longer detectable.

To determine the suitability of the two types of procedures Bodywork for subsequent painting and in terms of corrosion resistance and To determine the paint ventilation, corresponding test sheets (steel of quality class RRSt 1405m) treated as well. For this purpose, the test panels were provided with a three-layer lacquer coating, with the base lacquer has been used up in the slectrodeposition process. The painted panels were then subjected to the salt spray test ASTM B 117-64 and tested in the so-called stone chip test. The latter was carried out as follows:

The sheets to be tested are first grounded in an air stream of 2 atmospheres with 500 g of steel shot (angular, 4-5 mm), then subjected to the salt spray test according to ASTM B 117-64 for 120 hours and again in the

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BATH ORIGINAL

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bombarded with steel shot as mentioned above. The assessment is carried out by comparison with standard metal sheets with a rating of 1 to 6, with grade 1 meaning very good and grade 6 meaning completely inadequate adhesion.
The following results were obtained:

Treatment salt spray test Stone impact test in stage b Paint infiltration paint adhesion

of 3 mm after hour grading

Water flush 360 3

Lactic acid 720 1-2

Flushing

The process sequence according to the invention thus leads to a substantial increase in the corrosion resistance and paint adhesion.

Example 4

In a 6-zone diving facility became larger units assembled sheet metal parts are treated as follows:

a) cleaning with an alkaline immersion cleaner at 90 ° C;

b) rinsing with cold water;

c) rinsing with cold water;

d) phosphating with a chlorate-accelerated zinc phosphate solution (23 points) at 60 ° C;

e) rinsing with cold water;

f) rinsing with deionized water;

g) drying.

The treated molded parts had phosphate layers heavily interspersed with stripes and markings.

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ORIGINAL BATHROOM

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Ir: a further series was then used instead of the water rinse c) a lactic acid-containing solution of the composition mentioned in Example 3 is used. the The phosphate layers contained in the first showed the phosphate layers Series did not show any defects and were uniform and finely crystalline. Especially on the cavities of the molded parts, the layer formation was much better than in the first series.

The quality of the subsequently electrodeposited molded parts was accordingly also better in the case of the second. ' m

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Claims (2)

PATENT CLAIMS [1 ) J Process for the surface treatment of workpieces made of iron and steel, characterized in that the workpieces a) cleaned with an aqueous-alkaline solution, b) at a temperature below 60 ° C with an aqueous Solution whose pH with lactic acid is 4 to 55> preferably 4 to 4.6, and contains nonionic surfactant, in contact brought, c) coated with a phosphate coating and d) covered with an organic coating using the electrodeposition method. 2) Method according to claim 1, characterized in that a solution containing 0.01 to 1 g / l lactic acid and at least 0.05 g / l nonionic surfactant is used in step b). 209835/0936 IAD origwaj