Method of fabricating a bath of electrolyte for plating a platinum-based metallic underlayer on a metallic substrate

A fabrication method for a bath of electrolyte, the method comprising:

• a) providing a first system having ligands and amine functional groups, said first system comprising an aqueous solution of an amino ligand comprising:
• (i) at least one compound X—(NH2)n, where X is selected from the group consisting of (CH3, CH3—CH2, CH3—(CH2)m;
• (ii) NH3;
• or
• (iii) an xP−(NH4)+p salt where x is an acid radical selected from the group consisting of PO43−, HPO42−, H2PO4−, SO42−, HSO4−, and CH3COO−, where n, m, and p are non-zero integers;
• b) providing a second system forming a buffer system;
• c) providing a third system providing a metallic salt, and comprising an aqueous solution of platinum;
• the metallic salt being at least one salt of platinum of degree of oxidation IV;
• d) providing a fourth system suitable for imparting the property of conduction to the bath of electrolyte;
• forming a first solution B by grouping the first system, the second system, and the fourth system together in a single solution;
• forming a second solution A comprising sodium hydroxide and the least one salt of platinum of degree of oxidation IV of the third system;
• and
• e) mixing together the four systems so as to obtain the electrolyte bath of electrolyte;
• wherein
• the mixing comprises:
• e1) covering the first solution B and raising the first solution B to a temperature of at least 50° C. for at least 1h30 and
• e2) adding the second solution A to the first solution B and mixing the second solution A with the first solution B thereby forming a platinum amino complex and thus obtaining said bath of electrolyte.