The present disclosure also relates to methods of manufacturing and applying sacrificial protective coatings of this type, and to supports coated thereby.
Conversion treatments lead to a superficial modification of the metallic support (alloys of aluminum, titanium, iron and other metals) by an anodizing process (electrolysis operation, for example chromic, sulfuric, or phosphoric anodic oxidation) or by a simple chemical conversion process (for example chromating or phosphating).
Chromating can be used to form a thin, highly adhesive deposit of metallic chromates by bringing the surface of the part to be treated (typically aluminum or zinc alloys or steels) into contact with an acidic solution based on dichromates and fluorinated activating agents. That treatment improves the corrosion resistance of the support and is also used as a keying base for paints.
However, such processes suffer from the disadvantage of using toxic compounds, particularly as regards the excess treatment and washing water used for the treated supports.
Further, the washing water and solutions employed need to be treated to eliminate the heavy dissolved CMR metals before discharging them or reusing them. Eliminating metals produces additional toxic waste, which is difficult to purify and to treat.
A support, in particular a support including iron and constituting a sub-assembly or a component of aircraft equipment, can be provided with sacrificial protection against corrosion, in particular against red rust corrosion, by applying a moist single- or two-component binder composition containing a solvent or dilutable in water, but that suffers from numerous constraints. Single-component compositions must be re-dispersed at least 24 hours (h) before they are used. Once such binder compositions have been applied to the support that is to be functionalized, possibly after a first evaporation step, it is necessary to carry out a first cure, then to allow the support to cool down, and then to carry out a second cure, known as an anneal, for several hours. A third cure or final cure might be necessary and could last 5 h.
Using such compositions also necessitates intermediate compacting when intermediate annealing is carried out. That operation consists in spraying particles such as corundum particles onto the portion of the surface of the support to be functionalized, at a pressure that is from 100 kilopascals (kPa) to 150 kPa, the dimensions of said particles being in the range 90-180 US mesh. A compacting operation known as final compacting, which is thus different from the intermediate operation in terms of pressure (kPa) and dimensions of the projected particles, is also carried out in order to finish the coating.
Binder compositions of that type also suffer from the major disadvantage of including trivalent and/or hexavalent chromates as well as borates.
In operation, binder compositions of that type are combined with a sacrificial metallic pigmentation, in particular of aluminum or of magnesium, these latter being associated with the chromium III or VI components and acting to inhibit corrosion and to passivate the support, as well as acting as the above-mentioned metal pigmentation.
Sacrificial performance, and thus corrosion resistance, can only be obtained after performing the above-mentioned treatment operations of annealing and of compacting, which operations are awkward and can be carried out with any guarantee of success only by qualified, certified operators.
EP 0 995 816 A1 is known, and discloses a binder composition comprising, in the aqueous phase, phosphate ions and chromium III ions and secondary cationic species such as aluminum ions, zinc ions, or indeed magnesium ions to which metal oxide particles are also added.
That binder composition does away with the use of hexavalent chromium, but still employs chromium III ions.
In addition, US 2006/0225613 is known, which describes the synthesis of an epoxysilane oligomer that is said to present good stability and good dispersion in an aqueous medium despite its high molecular weight. Said organosilane oligomer is mixed with metallic particles the quantity of which must not exceed 35% by weight of the total binder composition weight so that the film obtained retains its good appearance (see [0046]). The binder composition examples (4 to 18 and 21) for which the corrosion properties were tested comprise 28% by weight of zinc oxide and 3% of aluminum oxide. There is no indication as to the appearance of the films obtained. In all of those examples, the binder composition is dried at 70° C. for 20 minutes (min) then dried at 300° C. for 30 min in order to polymerize the organosilane oligomer and form the film of corrosion-protective paint. The corrosion resistance test consists in bringing the substrate coated with a film corrosion-protective paint into contact with a saline solution at a temperature of 35° C. as defined in accordance with ISO standard 7253:1984. The measured value corresponds to the time after which 5% of red rust compared with the total weight of the coating appears when the substrate coated with the film of corrosion-resistant paint is placed in a stove at 35° C. The red rust corresponds to oxidation of iron contained in the support to be protected against corrosion, which means that said salt spray passes through the film of corrosion-resistant paint, which no longer protects the substrate. The above-mentioned standard, which has been updated and now corresponds to ISO standard 9277-2012, defines the type of salt spray, but it does not define the specifications to be complied with as regards corrosion, nor does it define in precise terms the time for application of said salt spray. In US 2006/0225613, after only a few hours, 5% of red rust relative to the total weight of the coating has already been formed. A sacrificial corrosion-protective coating should allow red rust to appear only after a few hundred hours, or even more than 1000 h. Thus, US 2006/0225613 concerns a binder composition for the manufacture of a film of corrosion-resistant paint that is not suitable for the manufacture of a corrosion-protective coating that is sacrificial and that is thus intended to resist extreme corrosive conditions such as several salt spray cycles, each cycle lasting more than 15 h and comprising a period during which the coating is exposed to temperatures greater than or equal to 250° C. for several hours.