Patent Application
20130142941
The invention relates to phosphating treatment processes applicable for various purposes, such as anticorrosion protection prior to oiling or waxing, anticorrosion protection prior to painting (vehicle bodywork, household appliances and the like), reducing stresses in the cold deformation of semi-finished products (drawing of tubes, wires, extrusions and the like), reduction of friction between sliding surfaces (manganese phosphating), and electrical insulation.
Whatever the purpose for which it is used, the process comprises various steps, and the reactions that take place comprise two main steps.
The reaction begins with an acid attack on iron, which passes into solution in ion form, by means of an electrochemical mechanism comprising the anode reaction of iron oxidation and a simultaneous cathode reaction of development of molecular hydrogen. As a result of this attack the concentration of hydrogen ions falls (the pH increases) in the diffusion boundary layer (a few microns) close to the microcathodic zones, because the more the pH value increases, the lower the solubility of the phosphates becomes. The least soluble phosphates begin to a precipitate in these zones, and small crystals of zinc phosphate (or iron, zinc-iron, zinc-calcium, or the like) form after only a few seconds (less than 10). The initial nuclei then enlarge, but doesn't increase in number.
Phosphating is the most widespread pre-treatment used on metals prior to painting. Although it is specifically designed for iron, carbon steel and galvanised surfaces, it can also be successfully applied to aluminium, especially in cases where that metal needs to be treated together with others in the same factory.
Before a metal is painted, pre-treatment is nearly always needed to eliminate protective grease and oil, lubricants of various kinds, oxides and calamine, dust, unconsolidated materials and the like. Paint cannot always be applied if the surface of the metal is contaminated by alkaline residues originating, for example, from alkaline degreasing which is not thoroughly rinsed.
In the case of iron surfaces, it is also necessary to ensure that, after such cleaning, the surface does not reoxidise in the short time between pre-treatment and painting. If a cleaning solvent is used, the problem of reoxidation does not arise; however, it can occur when decontamination is performed in aqueous phase.
For the purpose of pre-treatment, the properties which a paint must possess after application to a given substrate can be divided into two classes:
In practice, therefore, pre-treatment should not worsen (and if possible should improve) the mechanical properties of the metal, and should improve its anticorrosive properties as much as possible: phosphating is ideal for both purposes.
As regards mechanical properties, the coatings must be as thin as possible, because high coating weights can cause the film of paint to flake off under stress, such as bending or drawing of the metal substrate.
There is no correlation between corrosion resistance and coating weight; rather, anticorrosion efficacy is correlated with porosity and the content of metals other than zinc (iron, manganese and nickel) in the coating.
As regards porosity, it seems logical that the larger the metal surface exposed to the coat of paint (which is also porous), the more easily corrosion can occur.
The iron, manganese and nickel content of the coating also affects its solubility in alkalis: zinc phosphate, an amphoteric metal, is readily soluble in caustic soda, whereas iron, manganese and nickel phosphates are insoluble, or less and more slowly soluble therein.
In industrial practice, two main types of process have been in widespread use for some time:
The choice between the two pre-treatments is a compromise between quality and economic and environmental costs: in industrial practice, crystalline phosphating is mainly used in the automobile and household appliance industries; the other ferrous, galvanised and, to a lesser extent, aluminium products are pre-treated before painting by amorphous phosphating. An important characteristic of this latter process is the possibility of adding a suitable mixture of surfactants to the phosphating product, so that the metal surface is cleaned and phosphated in a single treatment. Surfactants facilitate the removal of any oils and fats which may be present, thus preparing the metal surface for contact with the phosphating solution. Their choice must take account of the type of application proposed: they must not produce foam if they are to be used in a spray system, whereas this limitation does not apply to lance or immersion applications.
The chemical mechanism is the same for both types of process, as described above.
All modern zinc phosphating baths consist of zinc acid phosphate and accelerators (oxidising agents), plus various additives; due to the action of the accelerators, and the effect of depolarising metals, the molecular hydrogen that forms at the cathode is immediately reoxidised to ion, thus restoring the local acidity of the bath and guaranteeing the duration of the process.
An amorphous phosphating bath generally contains monosodium phosphate, free phosphoric acid in small quantities to maintain the pH in the required range of values, surfactants, accelerators and additives. The pH of the baths is much higher than that typical of crystalline phosphating, because the precipitation of neutral ferrous phosphate, which takes place at the expense of the phosphoric ion of the solution and of the iron originating from the metal surface, requires mildly acid conditions.
In amorphous phosphating, especially in the case of spray or lance application, the accelerant plays a slightly different role from that of “oxidiser” as in the case of crystalline phosphating. In these applications, the oxidation of the iron from bivalent to trivalent still takes place through the oxygen in the air, and the accelerant mainly acts as catalyst towards the coating formation reaction; in other words, its operating mechanism does not necessarily depend directly on oxidising power.
Patents relating to the field of the invention include the following:
The invention relates to a phosphating process for multi-metal pre-painting surface treatments which, with different application procedures, provides an alternative to traditional zinc phosphating processes and phosphodegreasing processes.
The process of the invention offers, for both applications:
This aspect appears particularly important, and constitutes an important innovation compared with other products alternative to the conventional zinc phosphating and phosphodegreasing products currently used, paving the way for their industrial use. While conventional products, due to the colour acquired by the conversion layer obtained, immediately show whether the quality of the coating is good or not, the alternative products applied to date on industrial production lines give a colourless or slightly yellowish coating, the colour of which can easily be mistaken for rust, which means that it is very difficult, if not impossible, to evaluate the quality correctly.
The process according to the invention therefore produces a significant reduction in operating costs, greater operational safety, and is more environment-friendly.
The process can be applied, by spray or immersion, to all types of substrate, such as cold-rolled steel (CRS), electrogalvanised steel (EG), hot-dip galvanised steel (HDG) or aluminium (AL), and is compatible with the subsequent application of all the main painting processes now known (electrophoresis, powder paints and liquid paints).
The mechanical performance and corrosion resistance of these products are at least comparable to those obtained with conventional cycles.
In a first embodiment thereof, the invention provides a process that replaces zinc phosphating, comprising:
Degreasing (step a) serves to eliminate all trace of oils, fats, cleaning paste, oxides and any other impurities from the coil surface, in order to leave a perfectly clean metal surface ready for subsequent treatments.
Normally, said degreasing is performed with liquid products in aqueous solution at an alkaline pH (10-14). The use concentration is between 1% and 10%, and the temperature of the working bath between 50° C. and 70° C., for a treatment time of between 30 and 120 seconds.
The degreasing bath typically contains 2 to 20 g/l of KOH or NaOH, 2 to 20 g/l of P2O5, 200 to 3000 ppm of surfactants, and 1 to 10 g/l of sequestering additives.
P2O5 is present in the form of sodium or potassium orthophosphates (monosodium, disodium or trisodium phosphate) or polyphosphates (tripolyphosphate or neutral pyrophosphate).
The surfactants most commonly used are selected from ethoxylated and/or ethoxy-propoxylated fatty alcohols with C9-C11, C12-C13 or C12-C18 alcohol chain, with different degrees of ethoxy-propoxylation.
The sequestering additives are preferably selected from nitriloacetic acid, sodium gluconate, gluconic acid, ethylenediaminetetraacetic acid disodium, ethylenediaminetetraacetic acid trisodium, phosphonates, acrylates and polyacrylates.
The wash with tap water (step b) serves to eliminate all trace of the preceding step; the temperature is normally between 30° C. and 60° C., with times ranging between 15 and 60 seconds.
Washing with demineralised water (step c) completes the action of the preceding step, and the operating conditions are the same; the temperature ranges between 30° C. and 60° C. for times of 15 to 60 secs.
The conversion treatment (step d) is the characteristic feature of the invention. It is usually performed at a temperature of between 15° C. and 50° C., for times ranging between 20 a 120 seconds, depending on the speed of the line, the type of application (spray or immersion) and the quality/reactivity of the metal. The treatment is normally performed with the bath described above, based on zirconium salts and phosphates with a pH of between 4 and 5, used at concentrations of between 10 and 30 g/1.
The zirconium salts are usually present in concentrations of 100 to 5000 mg/l, and are preferably selected from fluorozirconic acid, ammonium zirconium carbonate and potassium fluorozirconate.
The phosphates, typically present in concentrations of 10-500 mg/l, are ammonium orthophosphates (monosodium, disodium or trisodium phosphate) or polyphosphates (tripolyphosphate or neutral pyrophosphate).
The fluoride complexes are present in concentrations of 100-10000 mg/l, while ammonia is present in concentrations of 100-1000 ppm.
The titanium compounds comprise, for example, fluorotitanic acid, titanium oxalate, titanium oxide and potassium fluorotitanate, and can be present in concentrations of 100-5000 mg/l.
Other metals, such as vanadium, molybdenum and antimony, can be present in acid or salified form in concentrations of between 10 and 10000 mg/l.
The corrosion inhibitor, present in concentrations of 100-500 ppm, can be a more or less branched amine, an alkine derivative or a thiourea derivative, and has the basic function of preventing the appearance of oxidative phenomena during accidental or intentional stoppages of the treatment line.
The process accelerator is typically a donor compound of inorganic NO3, such as ammonium nitrate, or nitrogen organic compounds such as nitroguanidine or benzene derivatives, used alone or mixed together, in concentrations of 100-1500 ppm.
The system that limits the quantity of sludge and makes it friable, and therefore easily removable, consists of a suitably balanced combination of a polysaccharide and a glycol.
The sequestering agents are selected from those specified above for the degreasing bath, at concentrations of 10-5000 ppm.
The morphology of the phosphate coating obtained, mostly consisting of zirconium and/or titanium phosphates, is compact, uniform and highly insoluble. Depending on the type of application (spray or immersion) and the type of metal, the thickness of the phosphate coating layer can range between 50 and 200 nm, and the colour of the layer can vary from iridescent yellow to dark red or blue.
In a second embodiment thereof, the invention provides a process that replaces phosphodegreasing, comprising:
Step a) is similar to step d) described above, in terms of the components and their concentrations, with the sole difference that the conversion bath also contains at least one surfactant able to eliminate traces of oils, fats, cleaning paste, oxides and all other impurities from the surface of the material. The same surfactants as described above for the degreasing step can conveniently be used.
Similarly, washing steps b) and c) are performed under the same conditions as for the corresponding washing steps of the zinc phosphating replacement process described above.
The invention is described in greater detail in the examples below.
The laboratory tests were conducted so as to compare the results obtained with those of conventional cycles.
Cold-rolled steel plates (CRS), electrogalvanised steel (EG), hot-dip galvanised steel (HDG) and aluminium (AL) were tested; after the cycles, they were painted with 2 types of paint for both cases of pre-treatment, according to the normal conditions of industrial application.
The treated and painted plates were subjected to corrosion-resistance tests in a salt spray (fog) chamber, in accordance with Standard ASTM B 117. Panels on which a deep cross-cut was made down to the basic metal, with protected edges, were inspected for the appearance of the first signs of corrosion.
For convenience, Table 1 shows the ways in which the various cycles tested were distinguished. The results obtained are expressed as hours of exposure in the salt spray chamber until the appearance of the first signs of oxidation, such as sub-corrosion or flaking of the paint at a distance of >1 mm from the cut.
In view of the results obtained, the two alternative processes were further tested to evaluate the quantity of sludge formed, which was compared, once again, with that obtained in the corresponding conventional processes. The results are shown in Table 2 below.
When the laboratory tests had been performed, and the very good results objectively evaluated, it was necessary to ensure that after industrialisation, the process would guarantee the same performance on the production line.
For this purpose, the product according to the invention was tested confidentially, for a period required to assess its real benefits, on two production lines in the field of household appliances; the first used traditional trication multi-metal zinc phosphating, and the second used normal multi-metal phosphodegreasing.
In all cases it was found that compared with conventional cycles:
The product is cheaper, guaranteeing lower electricity consumption, less maintenance of the tanks, and lower logistical and waste water disposal costs.
| Number | Date | Country | Kind |
|---|---|---|---|
| ML2010A000094 | Jan 2010 | IT | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2011/050583 | 1/18/2011 | WO | 00 | 2/15/2013 |
