DE2103086B2 - Process for the surface treatment of workpieces made of iron and steel - Google Patents

Description

The invention relates to a method for the surface treatment of workpieces made of iron and steel, in which the workpieces are cleaned with an aqueous alkaline solution, with a phosphate coating coated and covered with an organic coating using the electrodeposition method.

To prepare iron and steel for electrodeposition painting, it is common practice to coat the workpieces clean, rinse one or more times with water and then phosphate. For cleaning Mildly alkaline cleaners in the pH range of 8-1 are generally used. Step into practice However, this sometimes leads to difficulties because these cleaners do not have anti-corrosion oils and drawing greases Always remove it completely from the metal surface, especially if the workpieces are to be used for a long time were stored. The remaining residues of the oily and fatty impurities lead to the Workpieces are not or only partially wetted with water after rinsing with water. In these cases it will During the subsequent phosphating, a non-uniform phosphate layer is formed, which is also has an unfavorable effect on electrocoating.

This problem of uneven phosphating occurs particularly troublesome in the treatment of Workpieces in continuous spray systems. In these systems, the workpieces are either on a Conveyor suspended or placed on a subframe continuously through the individual treatment zones transported by the system. There are transition sections between the spray registers of the individual zones, which are dimensioned in such a way that mixing of the trading solutions is avoided as far as possible. ] c depending on the shape of the parts to be treated and the transport speed, for example, the time required Space between the rinsing zone and the phosphating zone in the range of 15-60 seconds. 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 taken. Depending on the exposure time, more or less strong pre-passive struggles. The formation of a phosphate layer in the The phosphating zone then takes place unevenly. It has streaks, veils, stripe-shaped markings, some of which even shimmer bluish. In some cases these are unevenness of the phosphate layer still recognizable even after electrodeposition and therefore highly disadvantageous and undesirable.

It has now been found that the difficulties described can be eliminated in a simple manner if the method of the type mentioned is designed according to the invention in such a way that the workpieces after cleaning and before phosphating at a temperature below 60 ⁰ C with an aqueous Solution, the pH of which is adjusted to 4 to 5.5, preferably 4 to 4.8, with lactic acid and which contains nonionic surfactant, can be brought into contact.

The usual alkaline cleaners are used; Their pH is expediently between 8 and 11. Preferably Enihäit the alkaline cleaner also for activating the phosphating additives known such. B. 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 when spraying, and 20–30 g / l when diving. When applying the process sequence according to the invention can, in many cases, be the detergent concentration as an additional advantage can be reduced to about 50% of the specified values. Often times, the temperature of the Detergent bath can be reduced.

After cleaning, the workpieces can be rinsed with water to prevent the entry of cleaning solution Avoid going into the lactic acid treatment bath. But you can also use the Use lactic acid treatment and save rinse water. However, compliance then requires 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 adjust the pH of the lactic acid aqueous solution to a pH between 4 and 5.5, preferably set 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 ice-snacking surfaces. what to achieve coarse crystalline, for an electrocoating less suitable phosphate layers leads.

The adjustment of the required pH value by other inorganic or organic acids, such as. B. Phosphoric acid, / midosulfonic acid, acetic acid, monochloroacetic acid, Oxalic acid, malonic acid, maleic acid. Citric acid, does not lead to the desired one beneficial result.

Commercial lactic acids, including technical ones, can be used to produce the acidic treatment solution Quality, and their water-soluble salts are used. Jc according to the hardness of the water used are generally required to set the necessary pH value 0.01 - I g / l lactic acid (100%). The pH setting can be made with a glass electrode or with indicator paper. as Non-ionogenic surfactants are the commercially available products. It is primarily ethylene oxide adducts to long-chain organic compounds with

a reactive hydrogen sioffaiom such as /, B. Polyglycol ethers of alkylphenols, fatty alcohols, Fatty acids, fatty amines and fatty acid amides. At tier Spray application, low-foaming non-ionic surfactants are used. Here the addition products have of ethylene oxide on polypropylene glycol because of its acid resistance and low foaming particularly proven. With these products it is possible to obtain sufficiently concentrated lactic acid solutions to produce 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 can, for. B. by entering the alkaline Cleaner solution and slowly rise due to the reaction of the acidic solution with the iron surface. By discontinuous or continuous addition of a lactic acid-surfactant mixture becomes the target pH value maintained. Automatically operating pH measuring and metering devices can be particularly advantageous applied wenäea.

Too much iron and alkalis from the cleaning stage can reduce the effect of the acid treatment solution. In this case it is advisable to remove the treatment bath from To be renewed from time to time or to avoid an excessive accumulation of frenuiions by adding water.

The temperature of the acid treatment bath must be below 60 ° C. At higher temperatures, lactal layers are formed on the surface of the workpiece, which impair the subsequent application of the phosphate layer ί. Inside. This is particularly important if the lactic acid treatment solution is to be used in a hot alkaline solution immediately after cleaning and the workpieces are still at too high a temperature. The required for temperature control of flush water is relatively low but in any case, because the workpieces need only be cooled to below 60 ⁰ C. At the same time, it is already sufficient to avoid a disruptive accumulation of foreign ions in the acidic processing solution.

In contrast, without carrying out the treatment with the lactic acid solution, following the alkaline cleaning, it is necessary to use considerably larger amounts of water during rinsing in order to prevent alkali residues from entering the phosphating bath and, above all, to cool the workpieces much more 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, is prevented as far as possible. Normally, in the corresponding systems, the Spülbadtcmperatur is maintained at about 30 ⁰ C, which requires relatively large quantities of flush water. When using the lactic acid treatment, however, it is not necessary to keep the temperature in the transition so low. Because of the higher permissible temperature, considerable amounts of rinsing water can be saved in the process sequence carried out according to the invention.

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

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

The process sequence according to the invention gestatui it, to get to thin, finely crystalline and hard phosphate layers in a simple and safe manner Free of streaks and marks and therefore excellent in every respect for electrocoating are suitable so that high quality end products can be obtained.

This surprisingly advantageous result was not possible due to the known prior art expect. According to the German P-itentschrift procedure 8 62 848 should be alkaline-cleaned surfaces with an acidic solution before they are phosphated A pH of 4.2 to 6 can be rinsed, the pH of the rinsing solution preferably with phosphoric acid or an acidic phosphate should be set. However, there is a disadvantage with this method of operation the pre-passivation, which disrupts the layer formation. According to the method of the German patent specification 7 53 730, the surfaces should be treated with an oxalic acid solution, which is also low, before phosphating Amounts of a strong mineral acid can be added, rinsing. The workpieces are through However, this considerably more acidic pre-rinse is very susceptible to rust, so that good phosphate layers can be achieved is severely disabled. It is also known prior to phosphating iron surfaces for activation and removal of light rust and scale a solution of substituted short-chain monocarboxylic acids to be used (USA patent specification 31 18 793). The solutions are used in such concentrations used so that vigorous heating up of the iron surface takes place. During the subsequent phosphating Coarse, thicker phosphate layers are obtained, which are unsuitable for subsequent electrocoating are.

The process sequence according to the invention is verified explained using examples and comparative experiments:

Example I.

Greased steel sheets of quality class RRSt 1405m (according to DIN 1623, sheet I) were pretreated in a continuous spray system and then in the electro-dipping process coated with a base coat.

In the continuous system, the sheets were in the first zone for 2 minutes at 60 "C with an aqueous Splashed cleaning solution containing 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 had been added.

The sheets were in the second handling zone Sprayed for 30 s at 45 ° C. with one of the following solutions a) to i). After a transition of 30 seconds 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 different series was then examined.

The ones used in the / wide treatment zone Solutions were made by adding 1 g / l acid and 0.2 g / l of the same nonionic foam Surfactant to tap water (about 15 "dH) and uniform adjustment with NaOH to a pH value of 4.5 manufactured. The acids used were:

a) phosphoric acid

b) sulfamic acid

c) Essi _fa acid

d) monochloroacetic acid c
c) 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. Those treated with solution a) (phosphoric acid) Eleche pointed out bad ones, to the part! dusty layers with strong passivation phenomena r-i. At 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, as was already the case on the basis of the assessment, the Phosphate layer was to be expected, only the sheets treated with the solution i) as suitable.

Example 2

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

PH value

Quality of Phusphaischichi

6 streaky, blotchy

5.5 few streaks and marks

5.0 load flawless

4, H flawless

4.5 flawless

4.0 flawless

3.5 no streaks. Crystal initiation of

Uneven layer. /. T. coarser
Crystals

In the subsequent electrodeposition coating, only those treated in the pH range from 4 to 5.5 resulted Sheet metal to good or satisfactory results.

Example 3

In an S-zone continuous spray system were with oil, lubricant, etc. contaminated automobile bodies treated as follows:

a) 2 min cleaning at 60 ° C with a 3.4 g / l borax, 0.3 g / l activating titanium compound and 0.3 g / l Solution containing non-ionic low-foaming surfactant.

b) Rinsing for 1 min at approx. 30 C with tap water, water supply 8 mVh.

c) Phosphating for 2 min at 55 "C with a nitrite-accelerated Zinc phosphate solution, concentration 14 points (consumption of 0.1 N NaOH during titration of IO ml bath sample against phenolphthalein). Accelerator content approx. 100 mg / 1 NaNOi.

d) Rinsing with tap water for 1 min at 35.degree.

e) Rinsing with deionized water for 1 min at 25 ° C.

f) Showering with fully demineralized water.

The phosphate layers of the bodies treated in this way were heavily interspersed with stripes and stains. In some places that had been particularly afflicted with aged grease showed up the phosphating still contains residual fat.

In a second experiment, instead of rinsing with water before phosphating in step b) an acidic rinse through; ' . t. For this purpose, the rinsing bath was made with technical lactic acid and was weak foaming surfactant-containing concentrate adjusted to a pH value of 4.2 (glass electrode) and maintained during operation. The rinse bath contained approximately 0.18 g / l lactic acid and 0.09 g / l surfactant. The water supply was throttled to 1.8 mVh. the The temperature settles at about 48 ° C. The phosphate coatings obtained by this procedure were very good uniform and finely crystalline and showed no streaks or spots. There weren't any residues of fat to find out more.

About the suitability of the bodies treated by both types of process for the subsequent painting and to determine the corrosion resistance and paint adhesion were simultaneously appropriate Test panels (steel of quality class RRSt 1405m) also treated. The Tesl sheets were to do this with a provided with a three-layer lacquer coating, the base lacquer being applied using the electrodeposition method. The coated panels were then subjected to the salt spray test according to ASTM B 117-64 and the so-called stone impact test checked. The latter was carried out as follows:

The sheets to be tested are first placed in an air stream of 2atü. with 500 g .'jtahlschrot (angular, 4-5 mm), then subjected to the salt spray test according to ASTM B 117-64 for 120 h and again in shot at the aforementioned manner with steel shot. The assessment is made by comparison with standard sheets A grade from 1 to 6. With grade 1 a very good and grade 6 a completely unsatisfactory adhesion mean.

The following results were obtained:

Treatment in
Stul'e h

Siil / sprühcsl

Lacquer infiltration!
of 3 mm me h

Subsequent test

l.iiftung

licnolung

Flushing water

Lactic acid

Flushing

360

720

1-2

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 chloratbeschleunigten in zinc phosphate solution (28 points) at 60 ⁰ C;

c) rinsing with cold water;

f) Rinsing with volient salted water:

g) drying.

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

In a further series, a lactic acid-containing solution of the composition mentioned in Example 3 was then used instead of the water rinse c). The phosphate layers obtained did not have the defects found in the first series and were uniform and finely crystalline. The layer formation was significantly better than in the first series, especially on the cavities of the molded parts.

The quality of the subsequently electrocoated f ormteilc was accordingly also at the / expanses Series better.

Claims (2)

Patent claims: 1. Process for the surface treatment of workpieces made of iron and steel, in which the> workpieces are cleaned with an aqueous alkaline solution, coated with a phosphate coating and covered with an organic coating using the electrodeposition process, characterized in that the We: .-. Pieces according to before cleaning and before phosphating at a temperature below 60 ⁰ C with an aqueous solution whose pH is adjusted to 4 to 5.5, preferably 4 to 4.8, with lactic acid and contains nonionic surfactant, are brought into contact, ι · 2. The method according to claim 1, characterized in that that a solution containing 0.01 to 1 g / l lactic acid and at least 0.05 g / l non-ionic Contains surfactant is used.