ES344368A1 - Method of electrodepositing corrosion resistant coating - Google Patents

Abstract

A process for preparing a nickel surface adapted to receive a chromium plate so that the product possesses a high degree of corrosion resistance comprises electro-depositing or otherwise depositing on to a basis metal a first nickel plate having a tensile stress of less than 2,100 kg/cm<2> , and electro-depositing on to the said first nickel plate an outer nickel plate having a tensile stress of at least 8,400 kg/cm<2> and a thickness of 0À1-0À5 micron, wherein the said outer nickel plate is electro-deposited from an electro-plating bath containing 0À1-0À8 g/l of an additive heterocyclic compound having at least two nitrogen atoms in an otherwise carbocyclic ring structure and the said first nickel plate is electrodeposited or otherwise deposited on the basis metal in the absence of the said additive heterocyclic compound whereby the said different tensile stress properties are obtained for the said first and outer nickel plates. A basis metal of Fe, Fe alloy such as steel, Cu, Ni, brass, bronze, Zn or an alloy of any of these metals, e.g. a Znbase die casting, may be electro-plated with (a) Cu, (b) semi-bright Ni and/or bright Ni each having a tensile stress of less than 2,100 kg/cm<2>, (c) Ni having a tensile stress of at least 8, 400 kg/cm<2> and (d) Cr. The Ni having the lower tensile stress, instead of being electro-plated, may be provided by decomposing nickel carbonyl. The Cu may be electro-deposited from a bath containing Cu CN and either KCN or Na CN. The semi-bright Ni plate, which may be sulphurfree, may be electro-deposited from baths of the following types:- Watts, sulphamate, fluoborate, chloride-free sulphate or chloride-free sulphamate- Specified semi-brightening additives are coumarin, butyne diol, chloral hydrate, formaldehyde, piperonal, diethoxylated butyne diol and bromal hydrate. The bright Ni plate may be electrodeposited from a Watts bath, a high chloride bath or baths containing combinations of nickel fluoborate, sulphate and chloride or nickel sulphamate and chloride. The bright Ni bath may contain primary brighteners such as acetylenic compounds or pyridinium compounds, secondary brighteners such as sulpho-oxygen compounds, e.g. saccharin or benzene monosulphonate, and secondary auxiliary brighteners e.g. sodium allyl sulphonate or sodium 3-chloro butene-1-sulphonate. The higher tensile stress outer Ni plate may be electrodeposited from a bath containing NiCl2 6H2O, Ni(BF4)2, primary and secondary brighteners, and the additive heterocylcic compound having at least two (e.g. 2, 3 or 4) N atoms in an otherwise carbocyclic ring structure. The additive may be mono-, bi- or tri-cyclic. Typical ring structures are pryazole, 2-isoimidazole, 1, 2, 3- and 1, 2, 4- triazoles, pyridazine, pyrimidine, pyrazine, piperazine, s-as-and v-triazines, isoindazole, cinnoline, quinazoline, napthyridine, pyrido-pyridines, purine, hexamethylene tetramine and bis-pyridinium compounds in which the two N atoms form part of an otherwise carbocylcic ring, e.g. 1:1 ethylene 2:2 dipyridinium dichloride. Inertly substituted compounds having the above ring structure may be employed e.g. 2, 6 dimethyl pyrazine. The heterocyclic compounds may have one or more of their nitrogen atoms quaternized as by reaction of the heterocyclic compound with HCl HBr or H2 SO4. The Cr may be electro-plated from a bath containing CrO3 and either Na2SO4 or H2 SO4. It may also contain SrSO4, SrCl2 or K2 SiF6.