Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to a method of a metal
surface treatment.
Considerable problems have hither-to been experienced
in effecting coating of some materials including paints
and some metals on metal surfaces.
As illustrative, but not limiting, enormous
difficulty is experienced in attempting to coat what is
termed hard chrome onto a metal surface of another type
for instance, steel, and while it is capable of being
achieved, in practice, it requires a number of steps in
an electroplating process which require both time and
expertise in effecting.
In another application, it is known that it is
very difficult to have a paint based upon a resin, adhere
to a zinc coated steel surface where this has been newly
prepared.
Perhaps because the paint whether this is based on
; an acrylic resin or some differing resin material requires
a mechanical interlocking bond or not, it is conventionally
i~
,.
~3~32
aceepted that such paint should not be applied to a
surface such as newly prepared zinc eoated steel until
the coat has weathered, which may -take some months or
in some cases, years.
In another instance, it is conventionally acknowledged
that it is most difficult to effect the coating either
of another metal or a protective coating on an aluminium
surface.
It is now believed that this is because of the
characteris-tic of the well acknowledged oxide formed on
the surfaee of the aluminium metal.
The above three illustrations indieate diffieulties
that have been experienced -thus far and are real problems
in the art in relation to the coating of metal surfaces.
This invention is concerned with the treatment of
metal surfaces including ferrous and non-ferrous metals
and alloys which assist in the subsequent bonding of
surface coatings and which incidentally assist in reducing
vulnerability of the surface to subsequent further
oxidisation.
The invention involving the method uses a solution
which is applied to the metal surface and which is so
eompounded that it will provide a benefieial coating
effect in relation to metal surfaces which will provide
a base upon which subsequent eoating can be more effective
than has hitherto been the case, and in some instances
enables a eoating that has hitherto not been able to be
as effeetively or eeonomically enabled to be secured to
the surface of the said metal.
3~3~
I have hitherto described a solution especially
in relation to a preferred arrangement which I found
was suitable for application to corroded metal surfaces.
This invention relates to firstly a broadening of
the general discovery of that first invention and the
discovery of i-ts wider applicability to enable coating
to be achieved subsequently which has hitherto not been
realized.
Conventionally, treatment of a metal surface
has been based on the concept of removing the oxidised
surface which almost inevitably exists on the surface
whether the metal appears to be bright clean or badly
corroded.
Treatment therefore has taken two main approaches
that is, ei-ther to apply to the surface a material which
can substantially attack and thereby dissolve and remove
the metal oxides as well as the metal underlying the
oxides on the basis that this will remove any seat of
corrosion or secondarily it relies upon the expectation
of most of the metal oxide to be physically removed and
then to cover the surface with a generally impermeable
membrane which will therefore seal the surface from
access of water or other corrosion inducing materials.
From experiments conducted thus far, neither of
the two conventional processes appear to provide significant
improvement and in a number of cases, according to
experiments conducted would appear to, at least in the
long term, worsen the vulnerability of the material to
attack.
30Typically, if steel is pickled by immersion in
hydrochloric acid, besides the fact that the surface can
3~3Z
be irregularly corroded by the hydrochloric acid, the
hydrochloric acld in i-tself must subsequently be
neutralized or removed by quenching and this in itself
leaves agents which can induce further oxidisation.
While an acid such as phosphoric acid can be used
Eor pickling, it is seldom used for scale removal because
it is an expensive acid and slow in operation.
However, some s-teel plates are often initially
de-scaled in sulphuric acid, and then, after rinsing,
immersed in a 2~ phosphoric acid containing very small
quantities of iron and at a high temperature, such as
80-90C. for several minutes.
This provides an iron phosphate coating which is
considered useful for afterwards coating with paint.
The difficulty with simply coating the material
with a material that forms in effect an impermeable
membrane is the difficulty that it is almost impossible
to ensure that there is no-t a humid atmosphere between
the membrane and the metal surface provided either by
20 ~ the drying of the setting membrane or by entrapped air
providing from time to time condensed water or perhaps
more commonly, the breakdown in the surface at one or
more places, and the subsequent ingestion between the
commonly rather loosely adhered membranes and the metal
surface, thus allowing water not only to be pulled into
the interface but to be kept there because of the membrane
and in fact therefore promote corrosion.
The first discovery of this invention is that it
is of significant value to apply to the metal surface
which will inevitably have oxide thereon upon any
exposure to air, a ma-terial which is formulated so that
3t~3~
rather -than attack the metal oxide, it will have no
or minimal reactivity with respect to the metal oxide
and hence much more readily than with other materials
perfuse through the oxicle without substantially
afEecting this or blocking this porous character of
the oxide by the react:ion products such as a ~Jas or
other materials.
The material however is selected or formulated so
-that when it reaches the base of the oxide, it is
reac-tive with respect to the metal providing the surface
from which the oxide is formed or perhaps in other words
is substantially more reactive with respect tothe me-tal
base than the oxides so that by applying such material
to the surface of a corroded material, the material will
perfuse withou-t slgnificant reaction through the pores
of the oxide and then attack the metal at the base
releasing or lifting off the oxides.
The next discovery of this invention relates to the
problem of limiting the corrosion and providing a further
base upon which subsequent coating can be effective.
By including in the formulation, materials with
significant concentrations having large molecular weights,
it has been found that this material can confine the acid
attack or the base attack so that firstly the rate of
reaction is limited so that the release of gas is at
a Iimited rate but at the same time, the reaction products
- are held within the vicini-ty of the area of reaction so
that by including metal iron in the solution, especially
those at a higher position in the electromotive series,
then with the acid or base attack occurring, the reaction
products will be taken into the solution with the result
displacement potential of these other metals and these
~1~a3~32
are very likely to form a strong attachment bond with the
then accessib].e metal bonds forming the base or insofar
that there are complex molecules in the solution, there
is a high likelihood of forming complex association bonds
at or around the site of the reaction.
The characteri.stics of the solution then provide
also in effec-t, protection from oxidation of the metal
being exposed by the acid or base attack.
By then treating the thus prepared surface in such
a way as to induce further coating either by metal
buildup or by reason of organic or inorganic coating
materials and i.n such a way as -to be able to accept
within the coating method the presence of the said solution
or in another way neutralising this, one can then obtain
a coating on metals in such a way and with materials that
have not hitherto been able to be so combined, or so
economically combined.
The invention can in one form then be said to reside
in a method of treatment of a metal surface including
the steps of preparing the surface and then building up
a protective coating on the metal surface so prepared,
the preparation of the surface including the step of
applying to the surface a solution formulated or selected
so as to be substantially non-reactive with any oxide of
the metal surface, adapted to perfuse through any oxides
on the surface of the metals insofar that these are
porous without thereby being blocked by reaction products,
and then adapted to react with the underlying metal
providin~ the metal surface, and then to cover such under-
lying metal so as to allow ion exchange between metalions such as those of higher electromotive series with
respect to the metal of the surface and those in the
formulation of the solution and the metal of the surface
32
but to substantially resist access to the metals or
yas so as to sigllificantly retard any forma-tion of
oxides of the metal forming the surface, and then applying
compatible coating materials either by way of electro-
deposition or otherwise onto the thus prepared surface.
It is to be emphasi~ed that the results of thepreparation appear to produce a sur:Eace which does not
appear to have previously been provided and its
compatibility -to subsequent coating either by way of
electro-deposition or to coating by applying a painting
solution or o-therwise is also a significant discovery
in that the prepared surface appears to provide significant
molecular bonding so that there is a better grip and
therefore a potential for applying metals by way of
1$ electro-deposition that could not previously be applied
with effective grip hitllerto, and similarly with
painting solutions ! provided these are compatible with
the solution tha-t will to a cer-tain extent be left on
the surface, then there can also be a significant increase
in bonding attachment.
The process therefore appears to have the joint
advantage that it will not only not depend upon having
to physically remove rust or to provide a first attack
of a very strong acjd with subsequent necessary treatments
but of course with a single application being available
and with the necessity of removal not being any longer
present, a number of materials can be treated in situ
and of course within a short period of time can be
immediately coa-ted without significant disadvantage by
reason of underlying great rust or other oxide potential.
The concept then is to formulate the solution and
treat the surface so that in fact quite contrary to
what has been the conventional concept, the oxide of the
~3~
metal is not attacked and it is the concept that one
should arrange -the formulation that -there is most
decidedly preferrential attack -to the base metal so that
access to the base metal is not impeded by any reaction
with the oxides this of course assuminy as has been
discovered by experiMent that all of the oxides of metal
forming on the surface in a corrosion type situation
appear to be porous and of course that the formulation
is suitable for the selected metal and the oxide in
question. There is also an assumption that the formulation
will wet the oxides and if this is not generally possible
additional material such as appropriate surfactants may
be necessary to assist such wetting and therefore perfusion
of the formulation through the oxides.
An understanding of the action is perhaps better
achieved by reference to a preferred formulation which
also happens to be useful for a number of metals with
their oxides on their surface.
The preferred formulation also perhaps surprisingly,
has materials which individually provide a single function
or a multiplicity of functions but it is well understood
that such functions can be provided by two or more
materials acting in concert.
The reference to the preferred formulation then
is not to be taken as necessarily limiting the applicability
of the method -to such formulation although the components
of the formulation have provided significant advantages
but clearly many o-ther formulations, once the underlying
concept of the method has been realized, can be devised
to provide the same function and significant advantages.
According to the preferred solution then, this
includes a acid and preferably also phosphoric acid H3PO4
`-' 1().
~3~;3Z
procluced from commercial acid of 82'-~ H3PO~ known as
syrupy phosphoric acid. Because orthophosphoric acid
is a triprotic acld, i-t forms three series of salts
eorresponding to three stacJes oE ionisation. The
primary phosphates are more soluable in water than
tertiary pho.sphates.
The solution also eontains a signifieant propor-tion
of urea. Typically a mixture of 82'~ orthophosphoric acid,
mixed with an aqueous solution of urea in the proportions
- one part by weight of urea, to two par-ts by weight of
the phosphoric acid solution - produces a crystalline
addition product which is strongly acid and which is
soluable in water. In the solution preferred in the
embodiment, there is an excess of phosphoric aeid over
that which is required to produce this addition product.
The preferred solution also contains significant
amounts of the sulphates of copper, nickel, chromium,manganese,
cobalt, and in some eases zine. These ean have
several functions, one of whieh is to act as an activator
in assistiny the attacking or underlying metal providing
the metal surface and they also of eourse provide for
sequential deposition of the metal on the surface and
subsequent passivating of the surface.
The depositions also include phosphates which are
of eourse formed wnen this preferred solution is formed
which grow on the phospha-te ion primary layer.
Typically then, when the preferred solution is
applied to a metal surfaee such as iron, the metals in
the solution, for example eopper, being eathodie to the
dissolvincJ iron, increase the rate of solution of iron
by depositing on the iron and forming local cells with
i-t. Thus a large number of centres for crystal growth
~1~3~3~2
are produced which result in rapid formation of
phosphate coatincJ.
In a preferred instance then, a copper film is
formed in relation to a still surface with -the copper
film beiny uniformly adherent when the preferred
solution is applied by wiping, or if the surface
temperature o:E the steel is raised to about 80C. For
the later building up of the coat, the steel sheet is
then immersed after draining in either nickel sulphate
or chromium sulphate for several minutes so that nickel
or chromium phosphate is deposited on the iron phosphate
underlayer and finally there becomes a layer of pure
nickel or chromium phosphate.
The term phosphate is given as a simple terminology
reference but of course, especially with the urea
present, there would be a mixed phosphate with the
crystalline addition product of urea and orthophosphoric
acid. An absorption barrier is accordingly formed in
the unrusted areas of the specimen by the urea phosphoric
acid complex and there is of course a lower reaction
rate than that of the solution absorbed into the rust.
In the metal oxide, autophoretic or electrophoretic
separation of the excess phosphoric from the completed
reaction appears to occur, accelerating at-tack on the
underlying iron compared with the unoxidized areas
providing a reservoir of acid.
. .
.
1'''.
3~
The action of the urea therefore can be likened
to an inhlbitor, but it also has additional character-
istics which are of significant advanta~e in the method.
A Eirst oE these is that it assis-ts in we-tting of
oxidized metal surfaces and it has a characteristic
perhaps termed an ability to creep over surfaces which
has been suggested as being caused by an ability to
crystallize and recrystallize at its edges, thereby
achieving this creeping feature.
Another advantageous feature is the character of
the combination of urea and phosphoric acid and also
another feature is the fact that it provides especially
in the higher concentrations at the metal face, this
more viscous layer which significantly inhibits the
action which firstly reduces the size of any gas bubble
evolution which means that the action is much more uniform
over the whole of the surface and the effec-t appears to
be somewhat akin to a micro-etch rather than a macro-etch
or significant pitting and it also appears to provide an
association comple~ with the materials at the reacting
face which is substantially insoluble subsequently but
provides a good binder for subsequent coating. It is
also useful because i-t will be compatible when used as a
complex associating molecule with subsequent electro-
plating techniques.
Reference has been made to the applications toespecially steel surfaces, but it is -to be unders-tood
that it would appear that the concept applies to any
m~tal surface.
As illistrative, aluminium alloy is very suitable
for this treatment, and as with steel, an aluminium alloy
should be appropriately degreased by application of
32
appropriate solvents which allow for removal of fatty
compounds that are typ:ically found on -the surface of
me-tals.
Witll relation to the preferred solution, an
aluminium par-t, iE heated to approx:imately 90C and
then immersed in the preferred solu-tion for approx-
imately 5 minutes, will have significantly corrosion
products removed and there will be observed a minor
etching.
With the preferred solution, a copper deposit will
be noticed on the surface of the aluminium in a friable
form. This can be simply removed by washing and
scrubbing the surface leaving a clean aluminium surface
but preferably in relation to this invention, the part
can be immediately plated in copper, nickel or chromium
plating baths, leaving the copper. The copper will be
replaced in the action but it provides a good basis upon
which the action can proceed.
A selection of the various compounds or materials
forming the solution will depend upon many factors
including effectiveness in relation to the material to
be used, the costing, safe-ty of usage and of course the
long term effectiveness of the answer provided.
It is apprecia-ted that there are many combinations
of solutions that would be suitable and that at least in
the wider concept of this invention, it is the discovery
of thé mechanism so that compounds-can be effectively
formulated and when applied, can be understood as to
provide a useful basls for subsequent coating.
The preferred solution has significant advantages
in sofar that with relatively safe materials from a
.,. 1~1.
~3~3;~
toxicity poin-t of view and from a cost point of view,
a solution which is relatively simple and economic
can be widely used ln relation to a large variety of
metal surfaces.
It is to be emphasized however, that this is
realized as beincJ typical and not limi-ted to the
inventive concept at least in its wider concept.
Typically then, acids or bases with relatively
weak dissociation constants may be used in a formulation
either with or in replacement of phosphoric acid typically
a dissociatlon constant of 7.5 x 10 3 or as in the
case of ascetic acid 1.8 x 10 5 or in the case of
formic acid 1.8 x 10 4 all could be suitable.
Typically, other acids that are considered useful
provided the other formulation aspects are satisfactory,
can include oxalic acid, carbonic acid, hydrogen
selenide, hydrofluoric acid and so on.
Typically, bases can also be used including ammonia,
dymethylamine, methylamine, trimethylamine and the like.
It is significant that as well as a weakly dissociated
acid or base, there needs to be a significant concentration
of a material which conventionally can be useful as an
inhibitor.
It is to be emphasized that the action is not only
an inhibitor in this application, but it is a name used
to indicate *he type of material suitable in this case.
Large molecules such as the polysaccharides or urea
or substituted ureas, amides, thiourea and subs-tituted
632
thioureas may also be suitable.
This of course ls not to suggest a limitation to
the scope bu-t simply to indicate an illustrative example
o:E type oE molecu].e and the fact that a significant
concentration will :Eirs-tly assist to inhibit the acid
action on -the oxide of the metal, assisting hopefully
in also perfusinc3 through the oxide to the underlying
me-tal and -then assisting firstly in -the limiting of the
reaction base and access of oxide forming materials from
ex-ternally and finally, being either compatible with or
forming an association complex with the underlying metal
in combination with other portions of the compound
provided.
Finally, it is a necessary feature -to assist in -the
later passiva-tion -tha-t there be metal ions provided in
the material and typically these are transitional elements
sulphates.
Once again however, this is illustrative in that
the sulphates are generally water soluable and we have
been talking generally of aqueous solutions and also
that these salts are more economic than others which are
also water soluable~
Having now described in general terms aspects of
the invention, the invention will be better understood
with reference to examples which shall now be given, of
the way the invention can be preferrably carried out.
The composition and preparation of the solution used
in the firs-t application to the metal surface in one
preferred form is as follows:
3~
16
Eour hundred grams dry urea (46%N 0.4 biuret)
is dissolved in 1600 millilitres of hot water (85C)
and to this is added 200 millilitres of a sulphate
solution, this being made by dissolving 40 grams of
each of the followiny metal sulpha-tes in 11~0 millilitres
of water - these sulphates being Copper Sulphate, Cobalt
Sulphate, Chromium Sulphate, Nickel Sulphate, Manganese
Sulphate and Zinc Sulphate - and to this mixture adding
3200 millilitres oE phosphoric acid 82% technical grade.
This provides approximately 5 litres of the solution.
The above solution contains by weight then:
Urea 5.4%
Phosphoric Acid 57.3%
Water 36.7%
Copper Sulphate 0.08%
Cobalt Sulphate 0.08%
Chromium Sulphate 0.08%
Nickel Sulphate 0.08%
Manganese Sulpha-te 0.08%
Zinc Sulphate 0.08%
In the forthcoming examples this solution will be
referred to as the solution of the preferred embodiment
of the inven-tion.
EXAMPLE 1
The metal artic e to be plated may first be cleaned
to remove grease, oil, and loose solids by any of the
common methods such as solvent cleaning, emulsion
cleaning or alkaline cleaning. Where mill scale and
rust are present, it is of course feasible to use
weathering and wire brushing, acid pickling, grit blasting
or flame cleaning. In-the present invention it is found
that new steel may be cleaned with solvent such as
3t~32
perchlorethylene 80% and butanol 20% by weight, or
methyl ethyl ketone, particularly if the initial temperature
of -the specimen is for example from 20C - 100C.
Adequate cleaning may be given by immersing the specimen
5. in -the solution of the preEerred embodiment of the
invention at temperatures of 40C - 100C for times
depending on the temperature but generally of the
order of 10 seconds to 1 minute a-t 100 C. If the
specimen is hea-ted to 100C - 140C first and dipped
10. in the solution of the preferred embodiment of the
invention a cloud of fine bubbles emanates from the
specimen and indicates that micro-etching has occurred
and the surface is ready for further treatment.
The article may then be placed i.n the selected
15. electroplating bath, for example a copper plating bath
comprising:
Sodium cyanide NaCN 37 grams per litre
Copper cyanide CuCN 30 grams per litre
Rochelle salt I~NaC4H4O6 4H2O 50 grams per litre
20. Sodium carbonate Na2CO3 38 grams per litre
Operating conditions:
50 - 70 C, 2-6 amps per dm , 2 - 6 volts, pH
12.2 - 12.8, current efficiency 50 - 60%, anodes
copper rolled and annealed, steel vat, free cyanide
25. 3 - 6 grams per litre.
Or the article may be placed in a selected
electroplating bath for example a chromium plating bath
comprising:
Chromic acid CrO3 450 grams per litre
30. Sulphuric acid H2SO4 4.5 grams per litre
3~3~
The solution should be boiled with citric acid
12.5 grams per litre, tartarie aeid 18 grams per litre
or oxalic aeid 25 grams per litre to give some reduction.
The operating conditions are, ~l0 - 50C 12 - 20 amps
5. per dm , current e:Eficiency 12 - 15%, volts ~ - S, 7%
antimonial lead anodes, in a vat made of steel, lined
with 7~ antimonial lead.
EXAM2LE 2
Where the me-tal article to be heated is zinc or
10. zinc based alloys such as used in diecastings, the
application of the solution of the preferred embodiment
of the invention may be carried out by brushing,
spraying or immersion. As the solution rate for the
zinc is much higher than that for steel, temperatures
15. and times of immersion are reduced to prevent undue
etching of the surface, for example at 40C, 5 - 10
seconds may be adequate. The eopper plating bath shown
in example 1 is recommended for zinc and zine based
dieeastings; the pH value should be eontrolled by addi.ng
20. sodium hydroxide if it is too low, tartarie acid if it
is too high.
A chromium plating solution for direet plating on the
zinc and zinc based diecastings is as follows:
Chromic acid CrO3 331 grams per litre
25. Sulphuric aeid H2SO~ 0.6 gxams per litre
Sodium hydroxide NaOH 48 grams per litre
Operating conditions:
15 - 21C, current density 100 amps per dm ,
6 - 12 volts.
19,
~L3~;~3~
EXAMPLE 3
When the metal article to be heated is aluminium
or aluminium alloys, as in sheet or castings, -the
application of the solution of the preferred en~odiment
5, of present invention to the article is best achieved
at temperakures in the range of 20C - 40 C and the
progress oE treatment can be gauged by the appearance
of a friable thin film of copper on the specimen.
The specimen can then be transferred to a copper plating
10. solution such as alkaline copper of composition:
Copper sulphate CuSO4 5H2O 15 grams per litre
Sodium oxalate Na2C2O4 10 grams per litre
Triethanolamine N(CH2CH2OH)3 22 grams per litre
Operating conditions:
15. 20C, 0.3 - 0.6 amps per dm , copper anodes, vat:
earthenware or polymer lined.
Or transferred direct to a chromium plating
solution such as tha-t in example 1.
Or transferred to an autocatalytic copper plating
20. that is current-less bath such as that proposed by
A.E. Cahill in U.S. Patent 2,874~-,072 (1959).
For example a solu-tion could be:
~-Copper nitra-te Cu(NO3)23H2O 15 grams per litre
Sodium hydroxide NaOH20 grams per litre
-25. Formaldehyde HCHO100 millilitres of
a 36~ solution
Sodium bicarbonate NaHCO3 10.grams per litre
Sodium potassium tartrate
4H4 6 230 grams per li-tre
Water remainder to 1 litre
20.
Or transferred -to a zincate solu-tion con-taining
525 grams of sodium hydroxide and 100 grams of zinc
oxide per litre and immersed for 1 - 3 minutes at
20C, rinsed and copper plated in the previously mentioned
copper baths, followed by plating with hard chromium
from so:Lution as in example 1.
E XAMP LE 4
Treatment of the metal articles with the solution
of the preferred embodiment of the invention is followed
by dipping in a concentrated solution of metal sulphate
or phosphate at 40 - 100C for 5 seconds - 1 minute
at 100 C, which produces a metal phosphate coating
such as chromium phosphate. A suitable solution is
for example:
15. Chromium sulphate 280 grams
Water 600 millilitres
Orthophosphoric acid (82%) 600 millilitres
A further solution is ~or example:
Chromium phosphate200 grams
Water 600 millilitres
Orthophosphoric acid (82%) 20 millilitres
EXAMPLE 5
.
In example 4, metal sulphates or phosphates are used
such as cobalt, nickle, manganese, silver, gold,
platinum and zinc.
EXAMPLE 6
mis example involves the direct plating in a
single solution of copper onto a metal in single stage
process.
3~
A suitable solu-tion is as follows:
Orthosphosphoric acid (82~) 58%
Urea or thiourea 6%
Copper sulphate 10~
5. The copper sulphate may be replaced by nickel,
cobalt, manganese, zinc, silver or gold, or by
chromic acid to direct]y plate these metals onto a
metal surface.
EXAMæLE 7
10. The solution of the preferred embodiment of the
invention is used to prepare the surface of aluminium
alloy castings for hard chrome plating from a normal
plating solution o chromic acid 250 grams per litre,
sulphuric acid 25 grams per litre and is operated
15. at 300 amps per square foot at 4 - 8 volts.
These examples are by way of illustra-tion only
and are not in-tended to limit the invention.
