EP0089455A1 - Method for phosphating metal surfaces in non-aqueous phosphating baths - Google Patents

Abstract

A process for phosphating metallic surfaces in nonaqueous phosphating baths comprising low-boiling halogenated hydrocarbons, aqueous phosphoric acid as the phosphating agent, alcohols as the solubilizer, and, optionally, further known components, e.g., stabilizers, inhibitors, or accelerators, comprising dipping the workpieces to be phosphated at least twice for at least 10 seconds into the boiling phosphating bath and, in the interval, leaving them for at least 20 seconds in the gaseous phase.

Description

The invention relates to the technique of phosphating metal surfaces in non-aqueous phosphating baths.

Non-layer-forming phosphating (see W. Rausch, The Phosphating of Metals, Eugen G. Leuze Verlag, Saulgau (1974), page 103) with conventional aqueous phosphating baths based on ammonium or alkali dihydrogen phosphates - known as Fe phosphating - is known. that by immersion processes, depending on the immersion time, from about 2 to 5 minutes, phosphate layer thicknesses from 0.3 pm to about 0.8 μm can be achieved. Extending the diving time does not result in a strengthening of the phosphate layer. Only one-time immersion processes are known.

Also in conventional layer-forming phosphating (see W. Rausch, The Phosphating of Metals, Eugen G. Leuze Verlag, Saulgau (1974), page 42) based on aqueous zinc phosphate, zinc iron phosphate or zinc calcium phosphate solutions - the so-called Zn phosphating - So far, only the single immersion of the object to be phosphated has become known. Phosphate layer thicknesses of about 1 μm to about 20 μm are produced with dipping times of 5 to 10 minutes or by spraying, depending on the intended use, those of about 2 μm to 3 μm being preferred.

Also for the phosphating processes based on organic solvents that have increasingly come to the fore in recent years - the so-called solvent phosphating - especially those based on low-boiling halogenated hydrocarbons, only one-time immersion processes are described. Here the diving time is generally 0.5 to 3 minutes, with layer thicknesses of 0.1 pm generally depending on the immersion time and the composition of the organic phosphating bath. up to about 1 µm. In individual cases, larger layer thicknesses can also be achieved.

To assess the quality of phosphate layers on metal surfaces as corrosion protection and / or as inorganic primers for subsequent painting, the layer thickness alone is not a sufficient criterion; rather, porosity, surface roughness, crystallinity, water solubility, adhesive strength on the metal surface, adhesion to the paint layer and the like play a role. a. surface-specific properties play a decisive role. Only the interaction of all surface and layer properties is decisive for corrosion protection and priming suitability.

For the assessment of phosphate layers, empirical test methods are generally used after a defined coating, such as. B. the salt spray test on scored test panels according to DIN 50 021 and DIN 53 167, the cross cut test according to DIN 53 151, the determination of the degree of rust according to DIN 53 210, the determination of the degree of bubbles according to DIN 53 209 and other application-related test methods.

The use of such test methods on conventionally Fe-phosphated surfaces shows that the aqueous Fe-phosphating offers little protection against corrosion. In many cases, the requirements for everyday objects or technical components are not met.

In such cases, conventional Zn phosphating is usually used today, which provides significantly better corrosion protection. Zn phosphating is, however, considerably more expensive than Fe phosphating and, as a result, is higher sludge is a major environmental burden.

The newer phosphating processes based on organic solvents, in particular based on low-boiling halogenated hydrocarbons, as described for example in DE-AS 26 11 789, DE-AS 26 11 790 or GB-PS 34 842, are non-layer-forming phosphating processes, the The quality of phosphate layers essentially corresponds to that of conventional Fe phosphating. In many cases, therefore, the phosphate layers from the solvent phosphating as well as the phosphate layers from the conventional Fe phosphating do not meet the requirements.

As is known, however, solvent phosphating offers considerable advantages over conventional aqueous phosphating processes. So there are no environmental problems from wastewater, the number of treatment steps is reduced due to the elimination of various washing and rinsing processes, and the energy-intensive oven drying is not necessary.

It is therefore the task to develop a solvent phosphating process that preserves the known advantages of solvent phosphating and at the same time provides phosphate layers that meet the higher requirements that are usually only met by Zn phosphating.

This object was achieved by a process for phosphating metal surfaces in non-aqueous phosphating baths based on low-boiling halogenated hydrocarbons with the addition of aqueous ones. Phosphoric acid as a phosphating agent, an alcohol as a solubilizer and optionally further components, known as stabilizers, inhibitors or accelerators, as was presented in the claims.

It has been shown that - in contrast to aqueous phosphating - multiple dips in solvent phosphating lead to a significant improvement in the corrosion protection properties. Multiple dives with specific diving times in the liquid phase and coordinated hanging intervals in the gas phase above the liquid phase provide better results than a single dive, even if the total diving time is the same. It is particularly advantageous for this process if the liquid phase contains surface-activating components with high vapor pressure, such as, for example, formic acid esters, which also pass into the gas phase and are effective there.

• Das gereinigte, vorentfettete Werkstück wird zunächst in die Gasphase unmittelbar über die leicht siedende Phosphatierbadflüssigkeit gehängt, wobei solange Kondensat abläuft, bis das Werkstück die Temperatur der Gasphase erreicht hat. Das kann je nach Wärmekapazität des Werkstücks unterschiedlich lange dauern. Danach wird das Werkstück etwa 10 bis 60 sec, vorzugsweise 20 bis 30 sec, in das siedende Phosphatierbad getaucht, anschließend in die Gasphase gehoben und dort etwa 20 bis 120 sec, vorzugsweise 30 bis 90 sec, hängen gelassen. Längere Tauchzeiten und Intervalle sind zwar möglich, verbessern das Ergebnis aber nicht. Dieser Zyklus wird noch mindestens einmal, vorzugsweise zweimal, gegebenenfalls öfter wiederholt. Die Gesamttauchzeit beträgt vorzugsweise 30 bis 90 sec, mit besonderem Vorzug 30 bis 60 sec.

In the process according to the invention, a typical phosphating process takes place as follows:

• The cleaned, degreased workpiece is first suspended in the gas phase directly over the low-boiling phosphating bath liquid, condensate draining off until the workpiece has reached the temperature of the gas phase. This can take a long time depending on the heat capacity of the workpiece. The workpiece is then immersed in the boiling phosphating bath for about 10 to 60 seconds, preferably 20 to 30 seconds, then lifted into the gas phase and left there for about 20 to 120 seconds, preferably 30 to 90 seconds. Longer diving times and intervals are possible, but do not improve the result. This cycle is repeated at least once, preferably twice, possibly more often. The total diving time is preferably 30 to 90 sec, with particular preference 30 to 60 sec.

This phosphating technique therefore requires phosphating with a boiling phosphating bath that has a sufficiently large vapor space above the liquid phase. The method according to the invention therefore preferably relates to phosphating baths with low

according to the boiling point of, for example, about 40 ° C., as is the case with phosphating baths based on dichloromethane as the main solvent.

Other low-boiling halogenated hydrocarbons suitable as main solvents are: dichloromethane, chloroform, trichlorotrifluoromethane, dichloroethane, trichlorethylene, 1,1,1-trichloroethane, 1,1, 3-trichlorotrifluoroethane and mixtures thereof.

Possible low-boiling alcohols which can be used as solubilizers are: methanol, ethanol, propanol, isopropanol, butanol, sec-butanol, tert. -Butanol and their mixtures. Higher homologs such as n-pentanol, sec. Pentanol, n-hexanol, sec. Hexane, isohexanol, heptanol, n-octanol, 2-ethylhexanol, nonanol, decanol, undecanol, dodecanol or mixtures thereof can also be used.

The following may optionally be used as stabilizers: quinones, phenols, nitrophenols, nitromethane and other customary stabilizers for chlorinated hydrocarbons.

The following compounds may be considered as inhibitors: urea, dimethylurea, diethylurea, nitrourea, thiourea, methylthiourea, ethylthiourea, dimethylthiourea, diethylthiourea and other alkylated urea and thioureas.

The following compounds can optionally be used as accelerators: nitrobenzene, dinitrobenzene, nitrotoluene, dinitrotoluene, nitroethylbenzene, pyridine, prikric acid and mixtures thereof.

The main solvent will generally be present at 60 to 85 percent by weight, preferably 70 to 80 percent by weight, based on the total phosphating bath, while the aqueous phosphoric acid should be used in such an amount that an H ₃ PO ₄ concentration of 0.1 to 2 , 0 percent by weight, preferably 0.3 to 1.0 Weight percent, based on the total phosphating bath, is present. The concentration of the water in the phosphating bath should be 0.5 to 7 percent by weight, preferably 3.0 to 6.0 percent by weight.

Methanol or a mixture of alcohols with a predominant proportion of methanol is used as a solubilizer. The concentration of the methanol or the alcohol mixture with a predominant methanol content should be 10 to 30 percent by weight, preferably 15 to 25 percent by weight, based on the total phosphating bath.

The accelerators, stabilizers and inhibitors can each be present in a concentration of 0.01 to 1.0 percent by weight, preferably 0.05 to 0.3 percent by weight, based on the total phosphating bath.

The formic acid ester can be present in a concentration of 0.01 to 2.0 percent by weight, preferably 0.1 to 1.0 percent by weight, based on the total phosphating bath. The formic acid ester used is preferably formic acid methyl ester, but also the use of formic acid ethyl ester, propyl ester, isopropyl ester, butyl ester, sec-butyl ester, and tert. -butyl ester and mixtures thereof is possible. It can also use higher homologous formic acid esters, such as. B. formic acid pentyl ester, sec. -pentyl ester, -isopentyl ester, -n-hexyl ester, -sec. -hexyl ester, -isohexyl ester, -heptyl ester, -n-octyl ester, -2-ethylhexyl ester, -nonyl ester, -decyl ester, -undecyl ester, -dodecyl ester or mixtures thereof. The formic acid esters can therefore contain 1 to 12 carbon atoms in the alcohol part.

• 74 % CH₂Cl₂, 20 % CH₃OH, 5 % H₂O, 0,7 % H₃PO₄, 0, 1 % 2, 4-Dinitrotoluol, 0, 1 % Harnstoff, 0, 3 % HCOOCH₃
• 73 % CH₂Cl₂, 21 % CH30H, 5 % H20, 0,7 % H₃PO₄, 0, 1 % 1,3-Dinitrobenzol, 0, 1 % Harnstoff, 0,1 % HCOOCH₃
• 72 % CCl₃CF₃, 22 % CH₃OH, 4, 5 % H₂0, 0,8 % H₃PO₄, 0,2 % Harn- stoff, 0, 2 % 1, 3-Dinitrobenzol, 0,1 % HCOOCH₃
• 70 % CH₃ CCl₃, 24, 5 % C₂H₅OH, 4 % H₂O, 0, 7 % H₃PO₄, 0, 1 % Dimethylharnstoff, 0,1 % 2, 4-Dinitrotoluol, 0,6 % HCOOCH₃
• 35 % CH₂Cl₂, 36 % CCl₃CF₃, 20 % CH₃OH, 4 % i-C₃H₇OH, 4,0 % H₂0, 0, 6 % H₃PO₄, 0,1 % 2, 4-Dinitrotoluol, 0, 1 % Harnstoff, 0, 2 % HCOOCH₃.

Typical formulations for phosphating baths based on low-boiling halogenated hydrocarbons are as follows (percentages are always percentages by weight):

• 74% CH ₂ Cl ₂ , 20% CH ₃ OH, 5% H ₂ O, 0.7% H ₃ PO ₄ , 0.1% 2, 4-dinitrotoluene, 0.1% urea, 0.3% HCOOCH ₃
• 73% CH ₂ Cl ₂ , 21% CH30H, 5% H20, 0.7% H ₃ PO ₄ , 0.1% 1,3-dinitrobenzene, 0.1% urea, 0.1% HCOOCH ₃
• 72% CCl ₃ CF ₃ , 22% CH ₃ OH, 4.5% H ₂ 0, 0.8% H ₃ PO ₄ , 0.2% urea, 0.2% 1, 3-dinitrobenzene, 0, 1% HCOOCH ₃
• 70% CH ₃ CCl ₃ , 24, 5% C ₂ H ₅ OH, 4% H ₂ O, 0, 7% H ₃ PO ₄ , 0, 1% dimethyl urea, 0.1% 2, 4-dinitrotoluene, 0, 6% HCOOCH ₃
• 35% CH ₂ Cl ₂ , 36% CCl ₃ CF ₃ , 20% CH ₃ OH, 4% iC ₃ H ₇ OH, 4.0% H ₂ 0, 0, 6% H ₃ PO ₄ , 0.1% 2 , 4-dinitrotoluene, 0.1% urea, 0.2% HCOOCH ₃ .

The following examples are intended to explain the process according to the invention in more detail.

Low-carbon, cold-rolled deep drawing sheets St 1405 with the dimensions 10 x 20 cm are used as test workpieces, which are degreased with steam or immersion using standard metal degreasing baths. Two series of steel sheets have been used, which have been designated A and B and which differ only in their surface roughness. Series B has the greater roughness. After degreasing, these test panels are weighed in a dry state and then subjected to phosphating. Phosphating baths with dichloromethane as the base solvent have been selected, although phosphating baths with other low-boiling halogenated hydrocarbons or mixtures thereof are also suitable in principle.

A heated jacket is used as the phosphating vessel, half of which is filled with phosphating solution and which is provided with cooling coils at the top of the vessel to avoid evaporation losses and is somewhat narrowed. The cooling medium is kept at -10 ° C. The vessel can also be provided with a cover with a passage for a hanging device for the sheets.

example 1

The phosphating bath specified in Table 1 is kept in a boiling state in a half-filled jacket vessel of the type described above such that the space up to the cooling coils consists of a gas phase which is in equilibrium with the liquid phase. The prepared test sheets are then hung in the gas phase for preheating until no more condensate runs off. The test panels are then immersed in the liquid phase and left for a certain time (see Table 1) in the boiling liquid phase for phosphating. Then the test sheets are hung in the gas phase again for a certain time (see Table 1). During this hanging interval, the excess phosphating solution drips off and the remaining phosphating bath film, which is in equilibrium with the gas phase, acts on the metal surface. This process is repeated once or twice (see Table 1). The sheet is then lifted into the atmosphere through the cooling zone, where it dries immediately.

After determining the increase in mass, the sheets are subjected to a test coating in the manner customary in production. In principle, all commercially available paint systems can be used for test painting. Here, a baked enamel based on an alkyd resin was used, which is baked at 100 C for 6 min after coating. The dry layers of paint have a uniform thickness of approx. 30 pm.

After scoring, the painted metal sheets are subjected to a 240-hour salt spray test in accordance with DIN 50 021 and 53 167, and the width of the rusting is then determined and the cross-cut test in accordance with DIN 53 151 is carried out.

The process steps and test results are listed in Table 1.

The results show that unexpectedly, despite the same total diving time, the layer thickness of the phosphate layer increases with the number of dives and a significant improvement in corrosion protection results, as evidenced by salt spray testing and cross-cut test.

Example 2

In the same way as in Example 1, test plates with different immersion times in the liquid phase and hanging times in the gas phase were phosphated using the phosphating bath shown in Table 2. For quality control of the phosphate layers produced, a test coating with a stoving lacquer based on an alkyd resin was used, which is stoved for 6 min at 100 ° C after coating. The dry layers of paint have a uniform thickness of approx. 30 pm.

The process steps and test results are listed in Table 2.

The results show that despite the same total diving time, the layer thickness of the phosphate layer increases significantly with the number of dives and the corrosion protection properties are significantly improved in multiple dives. The greater increase compared to Example 1 can be attributed to the presence of the formic acid ester. In comparison to Example 1, it is also clear that the presence of the formic acid ester has a positive effect on the corrosion protection properties in the case of multiple dips.

Example 3

In the same way as in Example 1, test plates with different immersion times in the liquid phase and hanging times in the gas phase were phosphated with the phosphating baths given in Table 3. For quality control of the phosphate layers produced, a test coating with a stoving lacquer based on saturated polyester resins was used, which is baked at 150 C for 20 min after coating. The dry layers of paint have a uniform thickness of approx. 30 µm.

The process steps and test results are listed in Table 3.

The results show that by using multiple dipping according to the invention, the corrosion protection properties of the phosphated sheets are surprisingly improved in a remarkable manner.

Claims (4)

1. Process for phosphating metal surfaces in non-aqueous phosphating baths based on low-boiling halogenated hydrocarbons, with the addition of aqueous phosphoric acid as a phosphating agent, an alcohol as a solubilizer and, if appropriate, other components known as stabilizers, inhibitors or accelerators,
characterized,
that, after preheating, the workpieces to be phosphated in the gas phase are immersed in the boiling phosphating bath for at least twice for at least 10 seconds and in the meantime are left in the gas phase for at least 20 seconds. 2. The method according to claim 1,
characterized,
that the total dive time is at least 30 seconds. 3. The method according to claims 1 and 2,
characterized,
that the hanging times in the gas phase between dives are at least 30 seconds. 4. The method according to claims 1 to 3,
characterized,
that the phosphating bath contains a formic acid ester as an activating component and methanol or an alcohol mixture consisting predominantly of methanol as a solubilizer.