EP1718779B1 - Metal coating for a kitchen utensil - Google Patents

Description

The present invention relates to a metal coating for a cooking utensil.

Various metals or metal alloys, for example aluminum alloys, are known for their good mechanical properties, their good thermal conductivity, their light weight, their low cost and they have found many applications for a long time, especially for utensils and appliances. cooking. However, most of these metals or metal alloys have drawbacks related to their insufficient hardness and wear resistance, or their low resistance to corrosion.

Attempts to obtain alloys with improved properties have been made, and have resulted in particular quasicrystalline alloys. for example FR-2 744 839 discloses quasicrystalline alloys having the atomic composition Al _a X _d Y _e I _g wherein X is at least one member selected from B, C, P, S, Ge and Si, Y is at least one member selected from V, Mo, Cr, Mn, Fe, Co, Ni, Ru, Rh and Pd, I represent the unavoidable impurities of elaboration, 0≤g≤2,0≤d≤5, 18≤e≤29, and a + d + e + g = 100%. The use of an alloy having the composition Al ₇₁ Cu ₉ Fe ₁₀ Cr ₁₀ as an internal coating of a cooking vessel made of Pyrex ^® glass is also disclosed. FR-2 671 808 discloses quasicrystalline alloys having the atomic composition Al _a Cu _b Co _b (B, C) _c M _d N _e I _f , wherein M represents one or more elements selected from Fe, Cr, Mn, Ru, Mo, Ni, Ru , Os, V, Mg, Zn, Pd, N represents one or more elements selected from W, Ti, Zr, Hf, Rh, Nb, Ta, Y, Si, Ge and the rare earths, and I represents the impurities of development inevitable, with a≥50, 0≤b≤14, 0≤b'≤22, 0 <b + b'≤30, 0≤c≤5, 8≤d≤30, 0≤e≤4, f≤ 2 and a + b + b '+ c + d + e + f = 100%. Alloys having the composition Al _a Cu _b Co _b , (B, C) _c M _d N _e I _f , with 0≤b≤5, 0 <b '<22, 0 <c <5, and M represents Mn + Fe + Cr or Fe + Cr are recommended as a coating for cooking utensils. According to Z. Minevski, et al., [MRS Fall 2003 Symposium, "Electrocodeposited Quasicrystalline Coatings for Non-Stick, Wear Resistant Cookware " the quasicrystalline alloys have good mechanical properties and surface characteristics which make them particularly useful for various applications, especially for coating cookware. Al ₆₅ Cu ₂₃ Fe ₁₂ alloy is mentioned in particular.

Although quasicrystalline alloys generally have good mechanical properties, good heat transfer properties, and good impact and abrasion resistance, not all of them can be used as a coating for food cookware. In this particular application, the quasicrystalline alloy is in contact with food, which is a saline medium (due to the addition of sodium chloride to most foods) and possibly acidic. It is therefore necessary that the quasicrystalline coating has a good resistance to corrosion caused by this type of medium. But generally recommended alloys contain copper, which is the cause of low corrosion resistance.

The object of the present invention is to provide a quasicrystalline alloy that can be used as a coating for the surface of a cooking utensil in contact with the food to be cooked, which has good mechanical properties, as well as a good resistance scratch and corrosion.

The present invention therefore relates to a coating for a utensil or a cooking appliance of food products, as well as utensils and appliances carrying said coating.

• X représente un ou plusieurs éléments isoélectroniques de Fe, choisis parmi Ru et Os ;
• Y représente un ou plusieurs éléments isoélectroniques de Cr, choisis parmi Mo et W ;
• Z est un élément ou un mélange d'éléments choisis parmi Ti, Zr, Hf, V, Nb, Ta, Mn, Re, Rh, Ni et Pd ;
• J représente les impuretés inévitables, autres que Cu ;
• a + b + c + z = 100
• 5 ≤ b ≤ 15 ; 10 ≤ c ≤ 29 ; 0 ≤ z ≤ 10 ;
• xb ≤ 2
• yc ≤ 2
• j<1.

A coating according to the present invention is constituted by an aluminum-based alloy containing more than 80% by weight of one or more quasicrystalline or approximate phases, having the atomic composition Al _a (Fe _1-x X _x ) _b (Cr _1-y Y _y ) _c Z _z J _j in which:

• X represents one or more isoelectronic elements of Fe, selected from Ru and Os;
• Y is one or more isoelectronic elements of Cr, selected from Mo and W;
• Z is a member or a mixture of elements selected from Ti, Zr, Hf, V, Nb, Ta, Mn, Re, Rh, Ni and Pd;
• J represents unavoidable impurities other than Cu;
• a + b + c + z = 100
• 5 ≤ b ≤ 15; 10 ≤ c ≤ 29; 0 ≤ z ≤ 10;
• xb ≤ 2
• yc ≤ 2
• j <1.

• a + b + c + j = 100
• 5 ≤ b ≤ 15 ; 10 ≤ c ≤ 29 ; j<1.

In a particular embodiment, the quasicrystalline alloy has an atomic composition Al _a Fe _b Cr _c J _j , in which:

• a + b + c + j = 100
• 5 ≤ b ≤ 15; 10 ≤ c ≤ 29; j <1.

A coating according to the present invention can be obtained from a pre-elaborated ingot, or ingots of separate elements taken as targets in a cathode sputtering reactor or by vapor deposition produced by the vacuum melting of the bulk material. in all cases from copper-free materials.

The coating may also be obtained by thermal spraying, for example using an oxy-gas torch, a supersonic torch or a plasma torch, from a powder consisting of an alloy having the composition final desired.

The coating may further be obtained by electro-deposition from a quasicrystalline alloy powder having the desired composition for the final coating.

An alloy intended to be used in bulk form or in powder form for the preparation of a coating according to the invention can be obtained by conventional metallurgical production processes, that is to say which comprise a phase of slow cooling (ie ΔT / t less than a few hundred degrees per minute). For example, ingots may be obtained by melting the separated metal elements or pre-alloys in a brazed graphite crucible under a protective gas blanket (argon, nitrogen), conventionally used blanket flux in metallurgy, or in a crucible kept under empty. It is also possible to use crucibles made of refractory ceramic or copper cooled with heating by high frequency current. The preparation of an alloy powder can then be carried out by mechanical grinding. A powder consisting of spherical particles may also be obtained by atomization of the liquid alloy by an argon jet according to a conventional technique, such a powder being particularly suitable for the preparation of thermal spray coatings.

Another object of the present invention is a food cooking utensil or apparatus, wherein the food contact surface is coated according to the present invention.

The present invention is illustrated by the following example, to which it is however not limited.

Example Preparation of an AlFeCr coating by plasma spraying

An alloy having the composition Al atomic Fe _{≈70 ≈10 ≈20} Cr (that is to say a weight composition Fe Al _{≈54,2 ≈16,0 ≈29,8} Cr) was formed into powder by atomization, with a capillary diameter of 4 mm and a nitrogen pressure of 4 bar. The powder was separated into granulometric slices and the powders having a grain size between 20 μm and 90 μm were kept. The actual mass composition of the powder after atomization is Al _{53.8 ± 0.5} Fe _{16.4 ± 0.2} Cr _{29.9 ± 0.3} .

With the aid of the powder thus obtained, a coating deposit was carried out on a 316L stainless steel substrate preheated to 250 ° C., using a plasma torch with a hydrogen flow rate of 0.4 l. / min. The coating obtained has a thickness of 200 to 300 microns.

By way of comparison, plasma spraying was carried out on 316L stainless steel substrates from the composition Al ₇₁ Cr _10.6 Fe _8.7 Cu _9.7 ("Cristome A1") relatively rich in copper, and of the composition Al _69.5 Cu _0.54 Cr _20.26 Fe _9.72 (A11) in which the copper content is very low.

Corrosion tests (galvanic test, impedancemetry and immersion test) were performed on 25 mm diameter disc samples that were metallographically polished to the 3 μm diamond-filled felt.

Galvanometric tests

Galvanic tests simulate accelerated corrosion. They were carried out on a coating according to the invention of Example 1, as well as for comparison on alloy coatings A1 and A11 according to the following procedure. 0.35 M NaCl aqueous solution was immersed at 60 ° C., a test sample to be used as a working electrode, a platinum plate to serve as a counter electrode and a reference electrode. Increasing potential has been imposed between the reference electrode and the sample. ΔE represents the offset between the abandoning potential (that is, the potential that exists intrinsically between the sample and the reference electrode), and the potential from which dissolution of the coating begins. The results of the galvanic tests carried out are summarized in the table below.

Impedancemetry measurements

The impedance measurements are performed in a cell similar to that used for the galvanic tests. From the equilibrium potential, the cell is imposed with a sinusoidal potential around the equilibrium potential, and the complex impedance is measured as a function of the frequency of the sinusoid. A Nyquist diagram is modeled using equivalent circuits that give interfacial capacities (related to the developed surface of the sample) and transfer resistances (related to the resistance to the passage in solution of the metal ions. ). The corrosion current I _c is determined by the relation I _c = 0.02 / Rt, where Rt is the transfer resistance.

Immersion tests

For the immersion tests, the samples were placed for 20 h in a 0.35 M NaCl aqueous solution at 60 ° C. After extraction of the samples, the surface condition was examined and the immersion solutions were analyzed.

The results of all the tests are given in the table below. Sample Example 1 A1 A11 Vickers hardness (under 100 g) 462 400 Corrosion tests I _c 9 20 21 ΔE (in V) 1.36 0.40 Transfer resistance after 2 h (Ω / cm ² ) 65300 15500 Immersion test, dissolution measure Al (mg / l) 0.50 1.10 Cr (mg / l) <0.01 0, 14 Fe (mg / l) <0.01 0.10 Cu (mg / l) <0.01

These results show that the absence of Cu makes the alloy less sensitive to corrosion in 0.35 M NaCl medium and less susceptible to dissolution in salt water. A very small quantity of Cu, of the order of 0.54 atomic%, that is to say an order of magnitude which is that of the impurities, is sufficient to significantly reduce the corrosion resistance of a alloy. It thus appears that it is imperative that the alloys used for cooking utensil coatings are completely free of copper.

Claims (3)

1. A coating for a utensil or vessel for cooking food products, characterized in that it consists of an aluminum-based alloy containing more than 80% by weight of one or more quasicrystalline or approximant phases, having the composition Al_a(Fe_1-xX_x)_b(Cr_1-yY_y)_cZ_zJ_j in which:

   • X represents one or more elements isoelectronic with Fe, chosen from Ru and Os;

   • Y represents one or more elements isoelectronic with Cr, chosen from Mo and W;

   • Z is an element or a mixture of elements chosen from Ti, Zr, Hf, V, Nb, Ta, Mn, Re, Rh, Ni and Pd;

   • J represents the inevitable impurities other than copper;

   • a + b + c + z = 100

   • 5 ≤ b ≤ 15; 10 ≤ c ≤ 29; 0 ≤ z ≤ 10;

   • xb ≤ 2

   • yc ≤ 2; and

   • j<1.
2. The coating as claimed in claim 1, characterized in that the quasicrystalline alloy has an atomic composition Al_aFe_bCr_cJ_j, in which:

   • a + b + c + j = 100; and

   • 5 ≤ b ≤ 15; 10 ≤ c ≤ 29; j<1.
3. A utensil or vessel for cooking food products, characterized in that the surface of said utensil or vessel that is in contact with the food products has a coating as claimed in either of claims 1 or 2.