US2219977A - Process for producing coatings on zinc and galvanized articles - Google Patents

Description

num acid or a neutral or acid solution of a salt of Patented Oct. 29, 1949 PATENT} OFFICE rnooass FOR PRODUCING ooA'rmGs ON zmo AND GALVANIZED ARTICLES t Otto Brill, Oruro, Bolivia, assignor to Firm Mannesmann-Stahlblechbau A.-G., many, a company of Germany Berlin, Ger- No Drawing. Application January 10, 1938, Se

rial No. 184,227. 1936 5 Claims.

It is known to provide zinc and galvanised articles with coloured coatings in molybdate solutions without passing an electric current. For this purpose use is made of solutions of molybdemolybdenum acid, e. g. an alkali salt.

Further it is known that these coatings are shining and weather and acid resisting if heavy metal salts such as nickel sulphate are added to' the molybdate solutions, suitably in such quantity that the content of heavy metal sulphate is greater than the contentyof molybdenum compound (German patent specification 613,024).

' When working in these baths there is a gradual deposition of a sludge as a precipitate on the bottom. Investigations have shown that this sludge consists of molybdenum and heavy metal, which are thus withdrawn from the bath and lost. A -re-solution of the sludge involves great diificulties, so that the regeneration thereof-is never effected. Economically this fact is associated with the disadvantages that heavy metals like nickel which may have to be imported from abroad are lost unless use is made of inconvenient and tedious methods of regeneration.

Hitherto, the coloured coatings deposited from molybdate solutions have not exhibited adequate hardness and adhering capacity. Although to a considerably smaller extent, the same applies, for certain purposes, to those baths which have a greater content of heavy metal salt than of molybdates.

It has now been found that the vaFious defects exhibited by the existing colouring processes and coloured coatings can be obviated to a far-reaching extent by substituting a two-phase colouring process for the single-phase process. According to the invention the articles or goods of zinc or galvanised materials which are to be coated are first treated with the solution of one or more heavy metal salts. In a second operation the article coated with a layer of a heavy metal or a heavy metal compound is then coloured in a molybdate solution.

The treatment in the bath of the heavy metal salt may be effected with or without the use of electric current. Salts of nickel, copper, cobalt, etc. are suitable for the first treatment.

The second colouring bath which suitably is neutral or slightly acid, may contain molybdenum acid or one or more salts of molybdenum acid.

When employing the two-phase process, there is no or practically no separation of sludge containing nickel and molybdenum. From investigations made, the coloured coatings obtained by In Germany September 16,

this process have a greater hardness and adhering capacity than the depositions obtained in the known single-phase process. This phenomenon can perhaps be explained in that in the treatment in a solution of a heavy metal salt a comparatively permeable coating iszobtained which onthe one hand acts to a certain extent asan intermediate layer betweenfthe zinc surface and the coloured molybdenum coatingand has a greater adheringcapacity both'at the zinc surface and at the molybdenumfcoating, and on theother hand is sufiiciently permeable for the. molybdenum coating to come. into direct contact with the zinc surface.

When colouring in I molybdate solutions a 1 precipitate forms during operation and in the process of the invention it is ,free from heavymetal and consists of zinc andlmolybdenum.

I The formationand deposition of such a precipitate0r sludge of zinc and molybdenum can generally be avoided when colouring from molybdenum solutions by adding to the colouring bath before the commencement or in the first stage of its use, a small quantity of abuffer solution which keeps constant the pH-value of the colouring bath. Suitable buffering agents are, for example, sodium acetate and acetic acid, tartaric acid and sodium tartr'ate, glycocoll and hydrochloric acid, secondary sodium citrate and hydrochloric acid. Keeping the pH-value constant, e. g. at 5, has the result that the separation out of a sludge consisting of zincand molybdenum is avoided.

It is certainly already known to revive used baths by adding tartar, even repeatedly until the bath is fully sludged-up' and exhausted (German Patent 615,911) The present mode of working differs from this step in.-.that at the commencement or in the first stage of the use of the bath, a buffer mixture is added which has the effect that no sludge can form and separate out in the colouring bath. This surprising efiect is also obtained in the known colouring process in which the bath sludges-up during use.

Examples (a) A copper sulphate bath with 10-15% copper sulphate and 1% acetic acid is used for the first treatment. The desired article of zinc or Or the solution may contain 10 to 15% nickel 55 i sulphate 1% aceticac'lc l. The duration of immersion in this :case amounts to 1 minute at after previous rinsroom temperature. ing, the articleintrodnced into the bath containing molyhdemim.

The colouring loath contains about'0.5-5% ammonium molybdate and about Ll-0.5% tartaric acid. Colouring is effected at '50-'60 C; for 3-6 minutes accordingto thew-depth of the colour.

,giving, a nuance to the colour :or for other practical reasons, small additions "of materials afie'cting the colour ':or other materials may be added'to th'e second bath. v

"(-b) The article pretreated a nickel or copper sulphate bath is further treated .in a solution of 0;5-'5%. ammonium molybdate which is alkalised with ammonia and set to a vpH-yalue of i5 by a butter "solution of sodium acetate and acetic acid. The duration of the colouring "amountstof3--6. niinutesit 5199.

I v v 1. The process coating articles consisting at least Sweeney of zinc, comprising the steps "of treating therarticleswith a so'iutionof a heav-y metal salt :selected from the group consisting of copper, cobalt, and nickel salts, from which the, metalsimay be -.depos'ited upon the zinc upon immersion of saidl surface in said solution, and thereafter treating :said articles with (a molybdate -.-solution in which hydrogen "ion iconcentrafion :ma'intaineil at about :pH=5-.

'2. The :process 'orcoating :articles consisting at least superficially of zinc, comprising the steps of "treatingzthe articles -with a solution of 'a heavy :metal sulfate selected from the group consisting of copper, cobalt, and nickel sulfates, from which the metals may beideposited upon the zinc -upon immersionflof :said surface in said solution, and thereafter treating said articles with ;'a..molybdate solution, "while maintaining the hydrogen ion concentration of the solution at .a value of about pH=5 by means of a suitable buflier. agent.

3.. The process of coating articles consisting at least superficially of-zinc, comprising the steps of treating the articles with a solution of a heavy metal salt selected iromnthe group. consisting of copper, cobalt, and nickel salts, from whichithe metals may be deposited upon the .zinc upon immersion of said surface in said solution, and

thereafter treating them with :an ammonium molybd'at'e "solution which is maintained at :a. hydrogen ion concentration of 1pH=5 by means of :an acetic buffer agent.

4. The process of coating articles consisting at least superficially of 'zinc, comprising the steps of treating the articles with a solution of :alheavy metal salt selected from :the group "consisting of copper, cobalt, and nickel :salts, from which the metals may be deposited upon the zinc :upon immersion of said surface. in said solution and thereafter treating theni with an ammonium molybdate solution which has been first alkalized with ammonia and then set at :a hydrogen ion concentration of pH=5 by means of an acetic buffer agent.

5. The process of coating articles consisting at least superficially of zinc, comprising the steps of treating the articles with .a solution .of

a heavy metal salt selected from the :group conmolyhdate solution while maintaining the hydrogen ion concentration at a value of about pH=5 by means-of a bufier'agenth-selected irom. the group consisting of sodium vacetateanil acetic acid, and sodium tartrate :and tartaric acid.

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