Process for Obtaining an Electrolytic Deposit of Metallic Films on Aluminium.
Year: 1898

page_text: Date of Application, 8th Nov., 1898-Accepted, 21st Jan., 1899
COMPLETE SPECIFICATION.
Process for Obtaining an Electrolytic Deposit of Metallic Films on
Aluminium.
I, OTTO PAUL NAUHARDT; of No. 18, Boulevard Magenta, Paris, in the French
Republic, Engineer, do hereby declare the nature of this invention and in what
manner the same is to be performed, to be particularly described and ascertained
in and by the following statement:―
This invention relates to a process for obtaining an electrolytic deposit of
metallic films on aluminium, and the main object of it is. to prevent the bath
from attacking the aluminium and thus to obtain a good and thoroughly adherent
coating or him upon its surface which has hitherto been impracticable for reasons
hereafter explained.
1
The electrolytic baths employed hitherto for plating or coating certain metals
(copper, iron, etc.,) with other metals (zinc, nickel, copper, silver, etc.,) have certain
properties, either caustic or acid, which act on aluminium in such a manner
that it has been impossible, up to the present time, to obtain coatings of silver,
nickel etc., that will adhere well to that metal.
I have accordingly sought, and found, the composition for electrolytic baths
which combines the fundamental properties necessary for plating aluminium with
good adherent films or coatings of other metals, such as zinc, copper, nickel,
silver etc.
I obtain this result by forming electrolytic baths, from which I eliminate the
caustic or acid properties which attack the aluminium, by neutralising such baths
by adding to them derivatives of phosphorus, and more particularly by a pro-
portionate addition of ammonium phosphate,
For example, the coating of copper and its alloys with silver is effected, at
present, generally speaking, in an electrolytic bath containing 250 grammes of
nitrate of silver and 750 grammes of potassium cyanide. But, up to the present
time, it has been impossible to coat or plate aluminium with silver in baths thus
composed, as they injuriously affect the aluminium, in consequence of the large
excess of the cyanide.
Accordingly, in practically applying my process for plating or coating aluminium
with silver, I make use of a bath composed as follows:-
I prepare a solution of nitrate of silver and potassium cyanide, in about equal
quantities, the proportion of the cyanide not exceeding that of the nitrate to
an extent greater than is strictly necessary for the solution of the salts or that
would cause the bath to offer too high a resistance to the current. The causticity
of this bath, which would even then be sufficient to act on the aluminium, is
neutralised by the addition of an alkaline phosphate, preferably phosphate of
ammonia, in the proportion of about 10% of the weight of the other metallic
salts. The bath thus composed remains perfectly liquid and produces a coating

page_text: of aluminium which adheres well, the aluminium being previously prepared in
the usual manner by etching with caustic potash and rubbing with powdered
pumice stone.
The use of an alkaline phosphate (salts of phosphorus), for neutralising the
bath composed of approximately equal parts of silver and cyanide of potassium,
thus produces a new industrial effect viz., the possibility of covering or plating
aluminium with coatings of silver which adhere well, and which has never been
secured by any of the processes hitherto employed.
This possibility of a direct coating of aluminium with silver having been ascer-
tained, I proceeded to apply my invention for coating this metal with copper,
nickel, and zinc, and succeeded completely in forming electrolytic baths similar
to that employed by me in the silvering process, that is to say, by replacing the
chlorides and sulphates of copper, nickel, or zine, usually employed in electro-
plating operations, by the nitrates of those metals, and adding to the baths, com-
posed approximately of equal quantities of nitrate of copper, nickel or zinc, and
of cyanide of potassium, a salt derived from phosphorus, preferably, phosphate of
ammonia, for neutralising the causticity or acidity of the baths.
Baths so prepared effect the covering or plating of the aluminium with good
adhesive layers of the respective metals which they contain.
However, the excessively high prices of the nitrates of zinc and of nickel,
and of cyanide of potassium, form a certain obstacle to the carrying out of the
new process for commercial purposes in connection with the covering or plating
of aluminium with nickel or zinc; consequently I have been obliged to seek,
and have discovered, practical means whereby the operations are rendered less
expensive.
For the nickelling process I proceed as follows:-
I prepare a bath of chloride or sulphate of nickel, which I render non-injurious
to the aluminium by means of a suitable addition (about 10%) of ammonium
phosphate, and I acidify it slightly in order to re-dissolve any excess of the pre-
cipitate that may be formed. For the acidifying of the bath I employ hydro-
chloric acid, if the bath contains sulphate of nickel, or sulphuric acid, if the bath
contains chloride of nickel.
A bath so prepared covers or plates the aluminium with a good adhesive coat
or film of nickel, and which I can make of any thickness at will.
For "zincing" the aluminium, I prepare a bath of sulphate or chloride of zinc,
from which I eliminate the caustic or acid properties by the addition of about
10% of a salt of phosphorus, preferably phosphate of ammonia.
According to the nature of the metals with which the aluminium is to be coated,
I make use of electrolytic baths as described, either cold or heated:-It may be
observed generally that alkaline or caustic baths may be employed either hot or
cold, whilst acid baths must of necessity be used cold.
In employing a bath for gilding, from which I eliminate the properties which
injuriously affect the aluminium, by means of a salt of phosphorus, I could equally
effect the gilding of the aluminium direct, but having regard to the porosity of
that metal, it is always preferable, and less expensive to cover the aluminium first
with a layer of copper, nickel, or silver, and then to proceed with the gilding,
employing some known process.
Beyond the examples enumerated above, I originate generally speaking, the
employment of a combination or derivative of phosphorus, and especially phosphate
of ammonia for eliminating from electrolytic baths the caustic or acid properties
which hitherto have interfered with electrolytic processes for covering aluminium
with layers or films of any other metals.
Having now particularly described and ascertained the nature of my said
invention and in what manner the same is to be performed, I declare that what I
claim is:-
Electrolytic process or bath for covering or plating aluminium direct with

page_text: coatings or films of any other metals, by the employment or addition thereto
of a compound or a derivative of phosphorus, preferably ammonium phosphate
for rendering electrolytic baths harmless to the surface of the aluminium to be
coated, by eliminating the caustic or acid properties as the case may be from
such baths.
Red the 8th day of November 10
PHILLIPS & LEIGH,
No. 22, Southampton Buildings, Chaucery Lane, London, W.C.,
Agents for the Applicant.