DE2103086C3 - 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 from 8 to 11 are generally used 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 when treating workpieces in continuous spray systems. In these systems, the workpieces are either suspended from a conveyor or placed on a subframe and are continuously transported through the individual treatment zones of 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 treatment solutions is avoided as far as possible. 60 seconds. In this transition zone, however, the effect of phosphating solution on the metal surface cannot practically completely be ruled out. The metal surface in the transition zone is hit by the phosphating solution through adjusted nozzles or deflected spray jets or through fine misting of the phosphating solution. | e after the exposure time, more or less strong pre-passivations arise. The formation of the phosphate layer in the phosphating zone then takes place unevenly. It shows streaks, veils, stripe-shaped markings, some of which even shimmer bluish. In some cases, these irregularities in the phosphate layer are still recognizable even after the electrodeposition coating and are 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 can be used to clean the workpieces; their pH is expediently between 8 and 11. The alkaline cleaner preferably also contains the to activate the Phosphating known additives, such as. B. Titanium orthophosphate The cleaning can be done by spraying as can also be carried out in diving. The detergent concentration is, for example, when spraying 3-5 g / l, while diving 20-30 g / l. When using the process sequence according to the invention can be used as An additional advantage in many cases is the detergent concentration up to around 50% of the specified values to be humiliated. The temperature of the detergent bath can often also 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

stronger surveillance.

In the acidic intermediate treatment carried out according to the invention after the alkaline cleaning it is necessary to adjust the pH of the aqueous solution with lactic acid 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 iron surfaces, which results in coarser crystalline surfaces, for electrocoating less suitable phosphate layers leads.

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

Commercial lactic acids, including technical ones, can be used to produce the acidic treatment solution Quality, and their water-soluble salts

, be det. Depending on the hardness of the water used, it is generally necessary to adjust the water pH value 0.01 - 1 g / l lactic acid (100%) required. The pH value can be adjusted with a cia electrode

b5 or with indicator paper. as Commercially available products are suitable for nonionic surfactants. It is primarily ethylene oxide adducts to long-chain organic compounds with

a reactive hydrogen atom, such as. B. Polyglycol ethers of alkylphetiols, fatty alcohols, Fatty acids, fatty amines and fatty acid amides. In the Spray application, low-foaming non-ionic surfactants are used. The addition products of ethylene oxide have aiii polypropylene glycol Particularly proven because of their acid resistance and low foaming. It is with these products It is possible to produce sufficiently concentrated lactic acid solutions without mixing taking place. The nonionic surfactant content of the solution is expediently at least 0.05 g / l

When using the acidic treatment bath the pH value e.g. through the entry of the alkaline Cleaner solution and slowly rise due to the reaction of the acidic solution with the iron surface. By discontinuous or continuous supplementation of a Lactic acid-surfactant mixture is used to maintain the target pH value. Automatically operating pH measuring and metering devices can be particularly advantageous be applied.

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 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, which can impair the subsequent application of the phosphate layer. This is particularly important if the lactic acid treatment solution is to be used immediately after cleaning with a hot alkaline solution 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 treatment 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 action 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 Spülbadtemperatur 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 treating heavily scooping workpieces or after cleaning with strongly alkaline Solutions, it is advisable to rinse with water before the acidic treatment. To this 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 can be used for phosphating

solutions based on layered phosphates, preferably based on zinc phosphate can be used. The subsequent electrodeposition coating can also be carried out in the customary manner. The process sequence according to the invention allows

ίο to get to thin, finely crystalline and hard phosphate layers in a simple and safe way streak and mark-free and are therefore excellently suited for electrocoating in every respect, so that end products have a high

ι s quality can be achieved.

This surprisingly advantageous result was not possible due to the known prior art expect. According to the method of German Patent 8 62 848, alkaline cleaned surfaces are to be used be rinsed with an acidic solution with a pH of 4.2 to 6 prior to their phosphating, the The pH of the rinsing solution can preferably be adjusted with phosphoric acid or an acidic phosphate However, this method of operation has the disadvantage the pre-passivation, which disrupts the layer formation According to the method of German patent specification 7 53 730, the surfaces are to be phosphated prior to phosphating with an oxalic acid solution to which small amounts of a strong mineral acid are also added can be rinsed. However, this considerably more acidic pre-rinse makes the workpieces very much susceptible to rust, so that the achievement of good phosphate layers is severely hindered. It is also known before Phosphating of iron surfaces for activation

J5 and removal of light rust and scale one Use solution of substituted short-chain monocarboxylic acids (USA Patent 31 18 793). the Solutions are used in such concentrations that vigorous heating of the iron surface before takes place. During the subsequent phosphating coarse, thicker phosphate layers are obtained which are unsuitable for a subsequent electrodeposition coating. The sequence of procedures according to the invention is next based on examples and comparative experiments explained:

example 1 Greased steel sheets of quality class RRSt 1405m

(according to DIN 1623, sheet 1) were pretreated in a continuous spray system and then coated with a base coat using the electrodeposition method.

In the continuous system, the sheets were in the first zone for 2 minutes at 60 ° C with an aqueous Splashed cleaning solution that contained 3.4 g / l borax, 0.3 g / l activating titanium compound and 0.3 g / l non-ionic, low-foaming surfactant and which was 3 g / l for adaptation to practical application conditions a commercially available anti-corrosive oil added

bo had been.

In the second treatment zone, the metal sheets were sprayed with one of the following solutions a) to i) ^{at 45 ° C. for 30 s.} After a transition of 30 sec., The sheets were in the third zone 90 s with at 6O ⁰ C a nitrite-accelerated zinc phosphate solution. 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 of the same non-ionic foaming agent to tap water (about 15 ° dH) and uniform adjustment with NaOH to a pH value of 4.5 The acids used were:

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. Those treated with solution a) (phospharic acid) Sheets had poor, sometimes dusty layers with severe passivation phenomena. 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

As indicated in Example 1, steel sheets were cleaned, pretreated with a lactic acid-containing solution and phosphated. The pH value of 0.5 g / l Lactic acid and 0.2 g / l surfactant-containing solution was mixed with NaOH to values between 3.5 and 6 adjusted The pH was measured with a glass electrode. The influence of pH on quality the phosphate layers obtained are shown in the table below.

pH value quality of the phosphate layer

6th streaky, blotchy 5.5 few streaks and marks 5.0 almost perfect 4.8 perfect 4.5 perfect 4.0 perfect 3.5 no streaks, crystallinity of the Uneven layer, ζ. Τ coarser Crystals

Containing non-ionic low-foam teasid Solution.

b) rinsing 1 min at 30 ⁰ C with tap water, water inlet 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 when titrating 10 ml bath sample against phenolphthalein); Accelerator content approx. 100 mg / 1 NaNO ₂ .

d) Rinse with tap water for 1 min at 35 ° C.

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 streaks and stains In some places that had been particularly contaminated with aged ■ grease showed up the phosphating still contains fat residues.

In a second experiment, instead of rinsing with water before phosphating in step b) an acidic rinse carried out The rinsing bath was for this purpose with a technical lactic acid and weak foaming surfactant-containing concentrate adjusted to a pH value of 4.2 (glass electrode) and maintained during operation. The rinsing bath contained about 0.18 g / l lactic acid and 0.09 g / l surfactant. The water feed was throttled to 13 m ³ / h. The temperature was set at about 48 ° C. The phosphate coatings obtained with this procedure were very uniform and finely crystalline and showed no streaks and stains on. Any fat residues could no longer be found.

In order to determine the suitability of the bodies treated by both types of process for the subsequent painting and with regard to corrosion resistance and To determine paint adhesion, corresponding test sheets (steel of quality class RRSt 1405m) treated with the test panels were provided with a three-layer lacquer coating, the Base coat was applied using the electrodeposition method de. The painted panels were then subjected to a salt spray test tested according to ASTM B117-64 and in the so-called stone chip test. The latter was carried out as follows: The sheets to be tested are initially in a Air flow of 2 atmospheres, with 500 g steel shot (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 the standard sheet metal with a grade from 1 to 6, whereby grade 1 is a very good and grade 6 means completely inadequate adhesion.

The following results were obtained:

55 Treatment in Stage b

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

Example 3

In a 5-zone continuous spray system were with oil, lubricant, etc. Treated contaminated automobile bodies as follows:

a) 2 min cleaning at 60 ° C with a 3.4 g / l borax, 03 g / l activating titanium compound and 0.3 g / l

Salt spray test Rockfall test

Infiltration of paint paint adhesion denoting

from 3 mm to h

Water flush 360

Lactic acid Flushing

720

1-2

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

Example 4

In a 6-zone immersion system, shaped sheet metal parts were assembled into larger units as follows treated:

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 (28 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.

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

The quality of the subsequently electrocoated molded parts was correspondingly also in the case of the second Series better.

Claims (2)

2! 03 086 claims: 1. A method 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 workpieces are cleaned after cleaning and before phosphating at a temperature below 60 ^° 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 a nonionic surfactant. 2. The method according to claim I, 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.