NO753662L - - Google Patents

Description

For phosphating of metal surfaces, e.g. iron and steel, zinc or aluminum, the so-called alkali phosphating is increasingly being used in recent times. We work with slightly acidic alkali or ammonium phosphate solutions, which react with the base metal to form the corresponding insoluble phosphates. The layers formed in this way improve paint adhesion. The layers formed on iron and steel consist of a mixture of iron phosphates and oxides and act as a good undercoat rust protection during painting or plastic coating. A prerequisite for this, however, is that the phosphating layer is uniformly thick and is formed to a sufficient thickness. Usually layers are produced with basis weights from 0.4 to 1 g/m.

Often, the phosphating agent contains additional accelerators such as e.g. oxidizing agent to enhance layer formation. In order for a reaction between the phosphating solution and the metal surface to start at all, it is necessary that the corresponding places are clean, i.e. not covered by a dense or dirty film. We therefore add to the solutions wetting and emulsifying agents or uses a phosphating agent that contains a wetting and emulsifying agent.

The treatment takes place in dipping or spraying facilities, the latter being preferred. One usually works with at least three treatment steps, namely 1. humectant-containing phosphating solution, pH below 6, 2a., rinsing with tap water,

2b. post-rinse with fully desalted water, possibly with the addition of a post-passivating agent, e.g. on basic chromic acid and phosphoric acid.

While fully developed spray zones with circulation of the solution are required for stages 1 and 2a, for stage 2b only an atomizing ring is often used, as the excess flushing liquid is discarded. It is true that a cleaned and passivated surface can be achieved in this way, however, the phosphate layer thus formed is uneven in thickness, as the thinner areas do not ensure sufficient under-rust protection. Most of the time, the uneven formation of the a-v layer can be seen in its mottled appearance or its iridescent colouring. If the phosphating is followed by a single-layer lacquering, then the irregularities of the phosphate layer are the reason for an irregular appearance of the lacquer surface. The reason for the different strong formation of the phosphate layer, as it occurs with the method just mentioned, is that the phosphating reaction can only take place in the fat-free places

of the metal surface. However, these spots only occur during cleaning where the dirt is easiest to remove, whether it is because it is too thin or because the spots are hit particularly hard by the spray jet. The phosphating that occurs here spreads over the entire metal surface as the cleaning progresses, if only the treatment time is long enough. Since, however, in all alkali phosphating methods, the layer thickness increases with the treatment time - at least in the usually available treatment times of a few minutes - the phosphating reaction is different in thickness on the different surface areas of the treatment material. Reference is made several times in the literature to this significant disadvantage of alkali phosphating (e.g. by G. Lorin in "La Phosphation des metaux", Paris 1973, page 222), however no truly satisfactory solution has been proposed so far.

When the goods to be treated are heavily soiled, there is also the danger that a treatment solution that is heavily charged with dirt or grease does not or insufficiently separates emulsified dirt and grease particles on the phosphated treatment goods and interferes with the subsequent varnishing. In order to avoid this back-fouling, facilities are often used which, in relation to the facilities mentioned at the outset, have one more zone and in which the same alkali phosphating solutions are used in both first zones. The order of the treatment steps is then 1. net agent-containing.. f os f fating solution, pH below 6,

2. net agent-containing phosphating solution, pH below 6,

3a. rinsing with tap water,

3b. After-rinsing with fully desalted water, possibly with the addition of a post-passivating agent, e.g. on

basic chromic acid + phosphoric acid.

With this precaution, it is achieved that the main amount of dirt accumulates in the first zone. You thus certainly avoid back-fouling and can also use the solution in another zone for a longer time without having to mix it again; however, uniformly thick phosphate layers are not obtained in this way. In these plants too, step 3b often only consists of an atomization nozzle, as excess liquid is discarded. In contrast, zones 1, 2 and 3a work with circulation of solutions or of the flushing water (at 3a during the influx of fresh water).

However, only an aqueous neutral solution of non-ionic wetting agent without phosphate (DAS 1,446,431) has already been used in the first treatment step; however, the cleaning power of such solutions in the absence of phosphate at uniform wetting agent concentration is clearly less. Also, salt-free solutions tend to non-ionic wetting agents, especially at high concentrations, strongly cause foaming and are therefore hardly applicable in spraying systems for metal treatment. ■ Large amounts of wetting agent also reduce the quality of the dried phosphate layer when added in the second stage, because the complete rinsing of this wetting agent is not always successful in subsequent cold water rinsing.

It is also already known for cleaning to use a solution of condensed phosphates and not ionic wetting agents (Austrian patent no. 196,201). However, it has been shown that this method is not feasible with alkaline phosphating, which is accelerated with oxidizing agents,

then condensed phosphates, when they are mixed in on cleaned but not rinsed processing goods in the second stage, where the layer formation is considerably affected so that the phosphating layers become too thin.

There was therefore the task, despite the omission of an intermediate rinse, to produce good phosphate layers.

The invention relates to a method for phosphating metal surfaces using orthophosphate solutions, the method being characterized by treating the metal surface without intermediate rinsing in two consecutive steps with orthophosphate solutions, the first step containing wetting agents and having a pH range from 7.1 to 738 and second stage contains an aromatic nitro compound and has a pH range of 4.0 - 6.0. Preferably, a pH range of 4.5 - 5»5 is set in the second step. Aromatic nitro compounds are preferably used which have sufficient water solubility, especially acidic water-soluble nitro compounds, e.g. 4-nitrobenzoic acid, 3-nitrobenzenesulfonic acid or 4-nitrophthalic acid. The term "orthophosphates" means ammonium phosphate and the phosphates of the alkali metals, especially sodium and potassium phosphates. The method according to the invention can be carried out in ordinary phosphating plants with only two treatment steps and leads to evenly formed phosphate layers with excellent under rust protection.

The method according to the invention offers the possibility that the leaching trials of orthophosphate in step 2 can be balanced by mixing in solutions from step 1. One can once start in both steps with the same orthophosphate concentration and from time to time or continuously in step 1 after add the drained amount of cleaner (orthophosphate and wetting agent). By adding borates, for example polyborates, the cleaning effect can be further strengthened in the first step. The losses in stage 2 caused by mechanical spillage are thereby completed by themselves. However, in step 2 when a phosphate layer is formed, a further consumption of phosphate occurs in the layer formation reaction, which is replaced.

By using a correspondingly higher orthophosphate concentration in stage 1, this extra phosphate consumption can be offset in stage 2. To keep the phosphating bath composition constant in stage 2, it is only necessary to occasionally or continuously add an accelerator (nitro compound), as well as to observe the pH value. In this case, the ratio to be chosen of the two stage phosphate concentrations depends decisively on the ratio of the mechanical extraction of phosphate to the chemical consumption of phosphate. If the subsequent addition of the acidic nitro compounds used as accelerators is not sufficient to keep the pH value constant, free phosphoric acid is subsequently added.

A significant advantage of the method according to the invention is that the phosphate used is effective in both baths, as in the first stage the phosphate enhances the cleaning effect of the wetting agent and then in the second stage (<y>ed lower pH value) forms the phosphate layer. In the first step, practically no phosphating takes place. It is true that by lowering the pH value (below 751) a phosphatization can also be achieved in the first step. Since in this case, however, the phosphating and the cleaning proceed next to each other, it is only possible to obtain spotted, uneven phosphating layers.

Usually, it is sufficient in both stages with a concentration of orthophosphate of 1-30 g/litre, preferably 4-20 g/litre. Second stage resolution can in an amount of

about. 0.8-1% of the total phosphate (calculated as and the ammonium compounds.

After the end of phosphating, the phosphated objects are rinsed in a manner known per se.

As a wetting agent, all substances common for metal cleaning can be used, such as e.g. ethylene oxide adducts of alkylphenols, alcohols, fatty acids, amines and amides; further alkanesulfonates or alkylarylsulfonates. For use in spray systems, the wetting agent must be chosen so that the phosphate solution does not foam too strongly. This condition is easily fulfilled by the method according to the invention. On the one hand, the presence of orthophosphate reduces the stability of the foam formed. On the other hand, you can work with significantly lower concentrations of wetting agents, as the formation of foam is then also less.

When no ionic wetting agent is used in the first step, it seems beneficial that its cloud point is lowered by adding phosphates. Since, however, it is precisely near the point of cloudiness where the greatest wetting and cleaning effect of non-ionic wetting agents exists (Ullmann, Encyklopådie der Technischen Chemie, volume 18, 3rd edition, page 331) the addition of phos fat causes a reduction of the optimal working temperature and an improvement of the cleaning effect.

Adding the solution from step 1 does indeed lead to a slow increase in the pH value in step 2, but this can be easily compensated for by adding the acidic accelerator or phosphoric acid without there being a significant change in the phosphating effect. according to the method according to the invention, good phosphate layers are also obtained at relatively high pH values (set to approx. 6.0). Thereby, it is considered an advantage that the rate of layer formation depends only slightly on the solution's pH value.

An increase of the first stage pH value to above

7.8 is in principle possible. However, the necessary addition of acid accelerator or phosphoric acid in the second step increases too strongly to keep the second step's pH value constant. A reduction of the pH value in the first step below 7.1 is not appropriate, as already in the range of 6.2 -

7.1 slowly forms a phosphate layer. A layer formation on a first partially cleaned surface, however, leads to an uneven phosphate layer. It is essential for the method according to the invention that cleaning and phosphating proceed one after the other.

A reduction in the temperature of the first stage reduces the cleaning effect. A reduction of the temperature in the second step leads to a slowing down of layer formation. In both stages, the temperatures used can be between room temperature and approx. 70°C. Preferably work at 60°C.

The addition of small amounts of concentrated phosphates significantly reduces the rate of layer formation. This can be advantageous in plants with long treatment times to keep the layer weight in the desired range of 500 - 1000 mg/m p. Even higher layer weights do not entail any benefits in under rust protection, reduce paint adhesion and lead to an unnecessarily high consumption of accelerators as well as increased sludge formation . In addition, the addition of condensed phosphates in small quantities prevents the formation of a white coating on the phosphate layer that can be wiped off. Larger amounts of condensed phosphates (below 1% referred to total P2O5) have the effect of such a strong reduction in the rate of layer formation that the phosphate layers formed in the usual treatment times (1 - 5 minutes) in the phosphating step are too thin for optimal under rust protection.

As already mentioned, the resolution in step 1

when incorporating in step 2, affect the phosphating as little as possible. It therefore contains no or at most as much condensed phosphate as the solution in step 2, i.e. in any case below 1% (referred to total PgO^).

The method according to the invention can advantageously also be used when each of the two steps or at least one of them is carried out in several treatment zones connected one after the other. Thus, one can e.g. in a spray phosphating plant in the first zone use the cleaner according to the invention in a concentration of 20 g/litre and in a subsequent zone with 5 g/litre, immediately followed by the phosphating solution with a concentration of 5 g/litre. On the other hand, two phosphating zones can also follow in a cleaning step. However, an intermediate rinse with water is always disregarded.

Oak sample 1.

Sheet steel in deep drawing quality (St 1304), which

was soiled with a layer of grease applied in a rolling mill was treated according to the following procedure: A) 2 minute spraying with a 60°C warm aqueous solution of 13.16 g/l Na2HP01|

6.00 g/l NaH2P<0>Jj

0.6 g/l C^Q-C^-alcohol reacted with 12 mol ethylene oxide and then 15 mol propylene oxide

0.24 g/l C-^j-C-j^g-alkanesulfonate.

The pH value of the solution formed with tap water was 7.1.

B) Without intermediate rinsing, 2 minutes of spraying with a 60°C hot aqueous solution followed

4.35<g/>l NaH2<PO>4

0.25 g/l 4-nitrobenzoic acid

0.35 g/l urea-phosphate

0.05 g/l non-ionic wetting agent (as in A).

The pH value of the fresh solution diluted with tap water) was 5.1. C) Flushing took place with tap water at room temperature, then flushing with fully desalted water, possibly at room temperature.

The then.drying look showed a uniform gray appearance, free from spots. The phosphating layer weight was 485 to 500 mg/m<2>.

Example 2.

Sheet steel in deep drawing quality (ST 1403), which

was soiled with a grease layer applied in a rolling mill was treated according to the following procedure: A) 2 minute spraying with a 60°C hot solution of 6.5 g/l. Na 2 HPO 4

2.5 g/l NaH2P<0>4

0.6 g/l C-^Q-C^-alcohol, reacted with 12 mol ethylene oxide and then 15 mol propylene oxide

0.24 g/l C1-5-C1g-alkanesulfonate.

The pH value of the solution in tap water was 7.4. B) Spray treatment corresponding to step B in example 1.

C) Flush treatment corresponding to C in example 1. The solution B) was thereby repeatedly added. each time 1% of its volume

to solution A to simulate the lagging effect of strongly deformed (so-called gall-like) parts.

The results appear in the following table:

The formed phosphate layers show a uniform gray appearance.

Example 3»

Sheet steel in deep drawing quality (St 1405), which had been soiled with grease applied in a rolling mill, was treated according to the following procedure: A) 2 minute spraying with a .60°C hot aqueous solution of 6.5 g/l Na2HPOi4

3.0 g/l NaH2POi|

0.5 g/l wetting agent CxH2x+10(C2H1|0)15(<C>3<H60>)12H with x = 10 - 14.

The pH value of the solution (prepared with tap water) was 7.2.

B) Without intermediate rinsing, spraying took place for 2 minutes with a 60°C warm aqueous solution of

4.33 g/l NaH2P<0>4

0.02 g/l Na2<H2P>2<0>7

0.5 g/l 4-nitrobenzoic acid

0.1 g/l non-ionic wetting agent of example 1 0.05 g/l Cg-C11 alcohol reacted with 5 mol ethylene oxide

0.02 g/l nonylphenol, reacted with 10 mol ethylene oxide The pH value of the fresh solution (prepared with tap water) was 4.8. C) Flushing was included. tap water at room temperature,

then rinsing with fully desalinated water, likewise at room temperature.

The eyes, which were then dried with hot air, showed a .

uniform gray appearance.

In a parallel experiment, corresponding tin was treated without cleaning according to A 2x2 minutes as under B) and then rinsed according to C) and dried. These tins were mottled and in some places not phosphated.

Example 4.

Deep drawing grade sheet steel (St 1405), which was soiled with an aged drawing agent, was treated according to the following procedure: A) 2 minute spraying with a 55°C warm aqueous solution of 6.50 g/l Na2HP04

2.50 g/l NaH2P<0>4

0.60 g/l non-ionic wetting agent according to example 1 0.24 g/l anionic wetting agent (alkanesulfonate) The pH value of the solution (prepared with tap water) was 7.4.

B) Without intermediate rinsing, 2 minutes of spraying with a 55°C hot aqueous solution followed

4.27 g/l NaH2P0lj<->

0.25 g/l 4-nitrobenzoic acid.

0.43 g/l urea-phosphate

0.05 g/l non-ionic wetting agent (as in A). The pH value of the solution (prepared with tap water) initially amounted to 4.5 and during the phosphating was kept in the range from 4.5 to 5.8. C) This was followed by rinsing with tap water at room temperature,

then 0.5 minute flushing with a solution of

0.18 g/l Cr03

0.04 g/l NaH2PO1|

in fully desalinated water at 60°C.

The tins, which were then dried with hot air, were electrophoretically coated with an epoxy resin ester-based primer. Paint adhesion (5 mm depth) and rust protection after a 120-minute salt spray test ASTM B 117-64 was very good.

Example 5-

Sheet steel in deep drawing quality (St 1405), which was soiled with grease applied in a rolling mill, was treated according to the following procedure: A) 2 minute spraying with a 6'0°C warm aqueous solution of 5.8 g/l Na2HPOi|

2.7 g/l NaH2POi|

0.5 g/l non-ionic wetting agent according to example

3 A

0.2 g/l nonylphenol polyglycol ether, terminally esterified,

The pH value of the solution in tap water was 7.2. The concentrations of the chemicals in this solution were kept constant by subsequent additions.

B) Without intermediate rinsing, 2 minutes of spraying followed at 60°C with an aqueous solution of

4.33 g/l NaH2POj4

0.5 g/l 4-nitrobenzoic acid

0.15 g/l non-ionic wetting agent from example 1 0.02 g/l nonylphenol polyglycol ether. The pH value of the solution was initially 4.8 and was kept between 4.5 and 5.8 by the addition of 4-nitrobenzoic acid. The content of this solution of phosphate amounted to 3.43 g/

liters (calculated' as PO^). Then the consumption of phosphate by this treatment material to approx. 43$ was due to the chemical. reaction with the iron surface and to approx. 57% on leaching of the solution, the phosphate content in stage 2 was just maintained by leaching from stage 1 (3.43=6.00 57:100). The accelerator content only had to be supplemented.

C) Flushing with tap water followed, then 0.5 minute flushing with a solution of

0.18 g/l CrOj

0.04 g/l NaH2P<0>4

in fully desalinated water at 60°C.

The dried sheets were then primed electrophoretically. Paint adhesion and rust protection were impeccable.

Example 6.

Sheet steel in deep-drawing quality (St 1304), which had been soiled with grease in a rolling mill, was treated as follows: A) 2 minute spraying with a 60°C hot solution of. 3.9 g/l Na2HPOil

6.8 g/l NaH2P<0>lj

0.6 g/l C^Q-C-^-alcohol, reacted with 12 mol of ethylene oxide and then with 15 mol of propylene oxide

0.24 g/l C 1 -C 2 -alkanesulfonate.

The solution's pH value was 6.5. Steps B) and C) (spraying and rinsing) took place as in Example 4. The phosphated tins obtained had a mottled appearance.

E xample 7-

Different foam ratios were investigated. solutions containing humectants which are suitable for cleaning metals. For this, the solution was filled in a measuring cylinder and foam was produced by moving a perforated plate up and down. Resolution. 1.

13 g/l Na2HPO1|

6 g/l NaH2POi|

0.6 g/l polyether 'from example 1

0.24<g>/l C13-C1g-alkanesulfonate

in tap water, pH 7.1, 6o°C.

Foam 15%, eventually collapses. Solution 2.

6.5 g/l. Na2HPOiJ

3. g/l NaH2POi)

0.5 g/l alkylphenol polyglycol ether, etherified

end-placed with a butyl residue in tap water, pH 7>2, 60°C.

Foam 5%, collapses quickly.

Solution 2a.

0.5 g of wetting agent from solution 2, in tap water pH 7j0, foam 10$, eventually collapses.

Resolution 3.

5 g/l ethylene oxide adduct of approx. 10 moles of ethylene oxide on fatty acid amide. 5 g/l conversion product of polypropylene oxide with

ethylene oxide (molecular weight approx. 2000).

in tap water, pH 7>0, 60°C.

Foam 225%, which collapses slowly.

Op solution 3a.

As resolution 3j, however, each time only

0.5 g/l of the wetting agent.

Foam 150%, which collapses slowly. -

Solution 5b.

13 g/l Na2HPO4

6 g/l NaH2PO4

0.5 g/l ethylene oxide adduct of solution 3 0.5 g/l polypropylene oxide adduct of solution 3

in tap water, pH 73lj 60°C.

Foam 35%, which eventually collapses. Sheet steel, which was equipped with grease from a rolling mill, was sprayed for 2 minutes with the cleaning solution at 60°C. After cleaning with solution 2), the tins were moistened with water, but not after cleaning with solution 2a. Wetting was examined after thorough rinsing under running water.

Claims (9)

1. Process for phosphating metal surfaces by means of orthophosphate solutions, characterized in that the metal surfaces without intermediate rinsing are treated in two successive stages with orthophosphate solutions, the first stage containing a wetting agent and having a pH range from 7.1 to 7.8 and the second stage containing an aromatic nitro compound and has a pH range from 4.0 to 6.0. 2. Method according to claim 1, characterized in that the pH range of the second stage is set at 4.5 to 5.5. 3. Method according to claim 1, characterized in that the second step also contains a wetting agent. 4. Method according to claim 1, characterized by dN that the concentration of phosphates (calculated as PgO^ ) in both stages amounts to 1 to 30 g/litre. 5. Method according to claim 4, characterized in that the phosphate concentration amounts to 4 to 20 g/litre. 6. Method according to claim 4, characterized in that in the second step 0.2 to 1% of the phosphate is present (calculated as P2 ^5 ^ as condensed phosphate, preferably pyrophosphate. 7. Method according to one of claims 1 to 6, characterized in that the concentration of orthophosphate in the first step is chosen such that the amount of solution entrained with the metal surface from the first to the second step is sufficient to maintain the initial concentration of the orthophosphate in the second step. 8. Method according to claim 1 or 3, characterized in that the solutions contain non-ionic wetting agents. 9. Method according to claim 8, characterized in that the solution additionally contains anion-active wetting agents.