FR2522023A1 - BATHS AND METHODS FOR USING CHROMATE ZINC AND CADMIUM SURFACES - Google Patents

Abstract

THE PRESENT INVENTION CONCERNS BATHS THAT CAN BE USED FOR CHROMATING SURFACES IN ZINC, ZINC ALLOYS OR CADMIUM, CHARACTERIZED IN THAT THEY CONTAIN: -5 TO 40GL, EXPRESSED IN HEXAVALENT CHROME; -A SUFFICIENT QUANTITY OF SULPHATE IONS; - 0.025 TO 30GL OF COPPER ION; -0.1 TO 5GL OF IONS CHOSEN FROM SILVER AND MOLYBDIIC ION; -From 30 to 120GL OF ACETIC ACID; -From 0 TO 6GL OF FORMIC ACID.

Description

2 '22023

  Baths and processes that can be used to make chroma

  tage of zinc and cadmium surfaces.

  The present invention relates to new

  baths allowing, by chromating, to obtain a coating

  shiny black on zinc alloy surfaces

based on zinc or cadmium.

  the protection of surfaces of zinc, zinc-based alloys or cadmium has already been described by

chromating of these surfaces.

  The solutions used to achieve this

  matting contain essentially chromium (in the form for example of sodium or potassium dichromate and / or chromic acid), sulphate ions and, optionally a metallic compound giving rise to the formation

  of a suitably colored chromate deposit.

  Among the metallic compounds which can be used, mention may be made of silver salts (for example silver nitrate) which lead to the formation of a glossy black chromating film, of pleasant appearance, the corrosion resistance of which is

  rapidly altered by metallic pollution (zinc, iron).

  However the baths usable for the.

  realization of these silver-based coatings are

  quite expensive for their preparation and for their

  tièn This is why it was investigated if there were no metal compounds other than silver, which used in chromating baths, could give rise to black, shiny and resistant coatings.

corrosion.

  A chromating bath has recently been described which contains, in addition to chromium 6 and sulphate ions, a copper salt and a mixture of acetic acid and formic acid Unfortunately the deposit obtained, when baths of this type are used, is dull

  which takes away part of their industrial interest.

  The present invention aims to remedy this difficulty. The present invention therefore relates to a process for chromating surfaces of zinc, zinc alloys or cadmium, said process giving rise to a shiny black coating having good adhesion to the support and ensuring good resistance to corrosion of said support, characterized in that the support to be chromated is brought into contact with a bath containing: at 40 g / l, expressed as Cr, of hexavalent chromium, a sufficient quantity of sulfate ions, from 0.025 to 40 g / l, of cupric ions, from 0.1 to 5 g / l of ions chosen from silver and molybdic ion, from 30 to 120 g / l of acetic acid,

and from 0 to 6 g / l of formic acid.

  The invention also relates to the baths used

  sés in the implementation of this process.

  It will be noted: that the quantities of chromium ions to be used are substantially the same as those used

  today in chromating baths.

  that we use as known sulfur ions

  qoes dnt the role, all doses equal, is to anlio-

  sea speed ch dzcmatage and secondly, the adhesion of the deposits on the supports, especially when these sulfate ions are

  amnesates in the form of sodium sulphate and sul-

  furious; these sulfate ions can also be brought, for example, in the form of a copper sulfate, that in all cases we will use cupric ions which will give rise to a chromate

  with a very pleasant coloring.

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about.

  a contact time of the part to be treated with the bath which can vary, depending on the bath, between seconds and 10 minutes,

use of agitation or not.

  The parts treated according to the invention naturally have a coating of black chromate, shiny, very adherent and constituting effective protection.

against corrosion.

  It is however possible to further improve these coating properties, in particular in certain

  case the shine of the deposit and its effectiveness as a

  anti-corrosion guard subjecting the part carrying said coating to a post-rinsing using

  of a phosphate bath The phosphate baths used

  sands are known in themselves and have already been preco-

  used for certain applications, in particular for the glossing of certain deposits, it is believed that in the case of deposits according to the invention (copper-silver or copper-molybdate) the effectiveness of these rinsing baths is specific and particularly advantageous in that

  that they improve the protective effect of coatings.

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Claims (4)

  1 -Baths usable to carry out the chromating   surfaces of zinc, zinc alloys or cadmium, charac-   terized in that they contain: at 40 g / l, expressed as hexavalent chromium, a sufficient amount of sulfate ions, 0.025 to 30 g / l of cupric ions, 0.1 to 5 g / 1 of selected ions among silver and molybdic ion, from 30 to 120 g / l of acetic acid, from O to 6 g / 1 formic acid.   2 baths according to claim 1, characterized in that they contain: at 40 g / l, expressed as hexavalent chromium i 5, a sufficient amount of sulfate ions, 0.025 to 0.2 g / l of cupric ions, 0 , 1 to 0.2 g / l silver ions, 120 g / l acetic acid.   3 baths according to claim 1, characterized in that they contain: at 40 g / l, expressed as chromium, of hexavalent chromium, a sufficient quantity of sulphate ions, 15 to 30 g / l of cupric ions, _ 0 , 15 to 5 g / 1 of molybdic ions, to 120 g / 1 of acetic acid, about 3 g / l formic acid.   4 Process for carrying out surface chromating in zinc, zinc alloys or cadmium, characterized in that the part which is to be chromated is immersed   in a bath according to one of claims 1 to 3,   the operation taking place at a temperature close to ambient temperature and for a period of between 10 seconds and 10 minutes.   Method according to claim 4, characterized in that after rinsing of the part obtained, said part   is immersed in a phosphate bath.