THREE TECHNIQUES ARE AVAILABLE
Thermal spraying or spray metallization
Before metallization, abrasive blast stripping in order to:
a. to eliminate the oxides that would have formed;
b. to create a roughness of the order of Ra 8 to 12 microns, to ensure the bonding of the metallization.
To avoid damaging the galvanization at the periphery of the surface to be repackaged, use a mask, for example a window, in a sheet of steel. For small areas to be repacked (a few cm²), the surface can be prepared by grinding the abrasive disc. However, the roughness obtained being insufficient, to ensure adhesion, it will heat for at least 10 seconds the area to repackage with the flame of the metallization gun.
The finished surface preparation will be metallized immediately, with a thickness in accordance with the NF EN ISO 1641 standard, ie 100 μm minimum for steels with a thickness of 6 mm and more.
Application of zinc-rich paint
To carry out an abrasive blasting Sa 2 1/2 according to NF EN ISO 12944-4, either by projection or by grinding, or to eliminate the welding slag with a hammer to be stitched, completed by a brushing ST 2 with the metallic brush.
Then, using a brush, apply a zinc-rich paint that meets the definition of the ISO 12944-5 standard of May 1998: “For zinc-rich primers, the minimum zinc dust content of the dry extract of the paint is 80% in mass, whether organic or mineral binders … “
The adhesion of this paint to galvanized steel should be checked. The thickness applied will comply with the NF EN ISO 1461 standard, ie 100 μm minimum for steels with a thickness of 6 mm and more.
The application must comply with the manufacturer’s data sheet, in particular the recovery time in the case of a two-layer application.
The use of aerosol cans can only be considered in finishing film over repackaging if you want a closer look to galvanizing.
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Application of low melting point solder
Use abrasive blasting, or abrasive grinding.
The surface to be repacked will be preheated to a temperature of about 300 ° C, then the solder applied and distributed evenly with a spatula.
A zinc-tin-lead alloy with a melting point between 230 and 260 ° C will be used. An alloy with a high tin content should not be used because the tin is cathodic with respect to the steel and rust will occur if there is porosity in the deposit.
The best application results are obtained on horizontal surfaces. The appearance of the deposit is quite similar to that of galvanization, as well as its evolution over time.
As for the corrosion resistance of this type of repackaging, it is less good than those of other systems. In practice, it is sometimes difficult to apply the thickness recommended in standard NF EN ISO 1461.
It is perfectly possible to repackage the galvanization and obtain homogeneous corrosion resistance results. The quality of the result will depend on the care given to the surface preparation and the respect of the coating thickness in the reconditioned area, which can be controlled by the usual non-destructive measuring method.
There are ACQPA certified paints that meet the criteria of zinc-rich repair paint according to NF EN ISO 12944-5.
Other paints for repair can perfectly meet the criteria of the standard quoted above without having made the certification procedures.
Also, the key is to ensure that the zinc dust content of the dry extract of the paint is at least 80% by mass. It goes without saying that the performance is proportional to the zinc content beyond the minimum.
Source: Galvazinc, Reconditioning, 13/09/2012
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