The present invention relates to a continuous chemical patination/satinising treatment process for zinc-titanium alloys used in the construction industry, in particular for roofs, facades, guttering, interiors or architectural works.
Zinc-titanium alloy laminates are widely used in the construction industry, especially for roofing and cladding. The natural appearance of these claddings can be varied by particular pre-patination surface treatments.
Said processes must give coil surfaces having a chemical composition complying with Standard EN 988 - 1998 (see Table 1) an attractive appearance with a patinated effect, specific colouring, such as antique-look zinc, excellent antifinger properties, and high corrosion resistance.
The known process normally comprises the following stages:
An improved process has now been found which is characterised, after the second rinse with water, by a pre-passivation stage followed by drying with hot air to prevent the formation of surface zinc oxides, and the use of a polyacrylic resin as antifinger agent in the final passivation treatment.
The process of the invention comprises the following stages in particular:
Degreasing serves to eliminate all trace of oils, fats, cleaning paste, oxides and any other impurities from the coil surface, in order to have 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 5 and 40 seconds, with spray pressures of between 1 and 4 bars. The main elements of the degreasing bath are listed in Table 2 below.
P2O5 is present either as orthophosphates (monosodium, disodium or trisodium phosphate) or as polyphosphates (tripolyphosphate or neutral pyrophosphate), all in the form of sodium or potassium salts.
The most commonly used surfactants are ethoxylated fatty alcohols with alcohol C9-C11 chain, C12-C13 or C12-C18, having different degrees of ethoxylation or containing different numbers of ethylene oxide moles.
The sequestering additives are organic compounds selected from nitriloacetic acid, sodium gluconate, gluconic acid, ethylenediaminetetraacetic acid disodium, ethylenediaminetetraacetic acid trisodium, phosphonates, acrylates and polyacrylates.
The first rinse with continuously fresh water serves to eliminate all trace of the preceding stage; the temperature is normally between 30°C and 60°C, with times ranging between 5 and 30 seconds, and spray application.
The chemical patination/satinising treatment is performed by pickling the zinc-titanium alloy with a surface attack between 5 and 20 g/m2.
The treatment is usually performed with a specific product at a pH of between 0 and 2, used at concentrations of between 10 and 15%. The temperature of the working bath can be between 50% and 70%, for a reaction time of between 10 and 30 seconds; the spray pressures must be between 1 and 4 bars.
The main elements of the patination/satinising bath are listed in Table 3 below.
The process accelerator can consist of NO3 donor compounds such as nitric acid, ammonium nitrate, nickel nitrate, sodium nitrate, zinc nitrate, iron nitrate or cobalt nitrate, or nitrogenous organic compounds such as nitroguanidine, used alone or mixed together. The surfactants and agents are the same as already described.
The second rinse with continuously fresh water is performed as for the first rinse, to eliminate all trace of the preceding stage; the temperature is normally between 30°C and 60°C, with times ranging between 5 and 30 seconds, and spray application.
Pre-passivation, which prevents the formation of zinc-oxides on the surface by increasing the anti-corrosion properties of the coil, is performed by treatment with a chromium-free bath containing zirconium and/or titanium and aluminium compounds.
The typical composition of a pre-passivation bath according to the invention is set out in Table 4 below. The concentration of the product ranges between 1 and 5%, the temperature between 30 and 60°C, and the treatment times between 1 and 10 seconds; the product can be applied by spraying or immersion, and the pH value is < 5.
The zirconium compounds are preferably selected from fluorozirconic acid, ammonium zirconium carbonate and potassium fluozirconate; the aluminium compounds are selected from aluminium oxide of various extractions (aluminas), aluminium fluoride and aluminium bifluoride; the titanium compounds are selected from fluotitanic acid, titanium oxalate, titanium oxide and potassium fluotitanate.
Drying is then performed with hot air from 50 to 90°C.
The antifinger passivating product is applied with a Chem-Coater machine in two separate application stages, with a wet film drying step between each stage.
The inorganic component is applied at the first stage. Depending on the process required, the inorganic component can be one of three different types:
The product consists of chromic acid, chromium phosphate, corrosion inhibitors such as zinc phosphate and zinc chromate, and reducing agents such as gluconic acid, citric acid, saccharose, hydroxylamine, methanol, hydrogen peroxide and starch. The application takes place continuously at a temperature of between 15°C and 40°C; the concentration used depends on the wet film applied, which ranges between 2 and 10 ml/m2; it is generally between 10 and 50%, so that the end result is a total chromium deposit of 20 to 100 mg/m2; its value is calculated from the mg of chromium, on the basis that 1 mg/m2 of Cr = 16 mg/m2 of total coating.
An example of the composition of the bath is shown in Table 5 below.
The product consists of phosphoric acid, chromium phosphate, chromium nitrate, complexing agents selected from salified phosphonates or acids, gluconic acid, citric acid, tartaric acid, EDTA, glycolic acid and/or polyacrylates, corrosion inhibitors such as colloidal silicon dioxides in suspension, possibly having different particle sizes, generally below 30 µm, and with a SiO2 content of between 10 and 50%, and finally, organic wetting agents based on alkoxypolypropoxypolyethoxy-ethyl-benzylether or belonging to the perfluoroderivative family. The application takes place continuously at a temperature of between 15°C and 40°C; the concentration used depends on the wet film applied, which ranges between 2 and 10 ml/m2; it is generally between 30 and 70%, so that the end result is a total chromium deposit of 30 to 80 mg/m2; its value is calculated from the mg of chromium, on the basis that 1 mg/m2 of Cr = 32 mg/ m2 of total coating.
An example of the composition of the bath is shown in Table 6 below.
The product consists of silanes, selected from the families of 1-propylamine,3-(triethoxysilyl)-silane and trimethoxy[3-(oxiranylmethoxy)-propyl]-silane, titanium salts deriving from fluotitanic acid, titanium oxalate, titanium oxide or potassium fluorotitanate, alkaline phosphates such as monobasic or dibasic ammonium phosphate, mono-, bi- or tribasic sodium phosphate, acid or neutral potassium pyrophosphate, alkaline nitrates such as ammonium nitrate, sodium nitrate or potassium nitrate, sequestering agents selected from nitriloacetic acid, sodium gluconate, gluconic acid, ethylenediaminetetraacetic acid disodium, ethylenediaminetetraacetic acid trisodium, phosphonates, acrylates and polyacrylates, and corrosion inhibitors based on heavy metals such as vanadium, molybdenum, tungsten and cerium. The product should be applied at room temperature, in concentrations ranging between 80 and 100%, in such a way that the titanium deposit is between 4 and 8 mg/m2 and the SiO2 deposit between 70 and 140 mg/m2; its value is calculated from the mg of titanium, on the basis that 1 mg/m2 of Ti = 160 mg/m2 of total coating.
An example of the composition of the bath is shown in Table 7 below.
Drying is then performed with hot air from 50 to 90°C.
An antifinger agent is applied at the second stage, namely an aqueous solution of corrosion inhibitors such as zinc oxide and P2O5 and an acrylic resin consisting of monomers based on butylacrylate, methylmethacrylate and acrylic acid, having the following characteristics:
In the case of version 1 only, the antifinger passivation product can also be applied in a single stage, with a Chem-Coater machine. Depending on the final chromium deposit desired, the bath is prepared with 1 part inorganic to 2 parts organic component, or 1 part inorganic to 3 parts organic component. The quantity of mixture applied is 2 to 6 ml/m2. There is a partial reduction of chromium from 6 to 3 during this stage.
The wet film is then dried at a belt temperature of 80 to 140°C.
Drying is typically performed with hot air at a temperature of between 60 and 80°C.
First stage: "Chromium 3 - Chromium 6 Version"
First stage: "Chromium 3 version"
First stage: "Chromium-free version"
Drying is typically performed with hot air at a temperature of between 60 and 80°C.
Second stage: all versions.
Drying can be performed with hot air at a temperature of between 100 and 120°C.
The laboratory tests were performed to compare the results obtained with (i) cycles not including the pre-passivation stage (conventional cycle) and (ii) other cycles which were identical to the preceding ones except that they included the pre-passivation stage and subsequent drying.
Two-stage application of the final passivating product and single-stage application were tested in the cycles relating to the "Chromium 3 - Chromium 6 version"; 1:2 and 1:3 dose ratios of the two components were tested in the latter case.
Both the above-mentioned silanes, subsequently marked α and β, were tested in the cycles relating to the "chromium-free version", both separately and in combined.
Only the acrylic resin according to the invention was used in the organic component of the final passivation stage in all the cycles.
For convenience, the way in which the various cycles tested were marked is shown in Table 8 below.
After application of the cycles, the zinc-titanium alloy panels treated were subjected to corrosion-resistance tests in a salt spray (fog) chamber, in accordance with Standard ASTM B 117, unprotected panels and panels with protected edges being checked for the appearance of the first white oxides.
Table 9 shows the results obtained, expressed as hours of exposure before the appearance of the first white oxidation phenomena.
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