Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2~301~6
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PROCESS FOR THE CHEMICAL COI.v~-nSION OF METALLIC
SUBSTRATES, ~ONV~ION BATH ADAPTED TO BE USED
IN THE SAME PROCESS AND COh~k~ ~ATE ADAPTED
FOR THE PREPARATION OF THE BATH
The invention relates to a process for the chemical
conversion of metallic substrates, especially based on iron
or galvanized steel, in order to increase their resistance
against corrosion, their capacity against deformation or
simultaneously both properties.
The invention also relates to a chemical conversion
bath adapted to be used in the said process as well as a
concentrate adapted for the preparation of the bath.
It is already known to carry out chemical conver-
sion of metallic substrates by way of classic crystalline
phosphatization treatments due to which a coating of
insoluble phosphates is obtained on the surface of the
metal.
The crystalline phosphatization treatments which
are already known, generally use acid baths which contain
at least the following components :
- orthophosphoric acid H3PO4
- a primary phosphate ion H2PO4-
- one or several metals selected among zinc, iron,
manganese, nickel, copper, cobalt, calcium,
- at least one accelerating agent consisting of an
oxidizing agent such as chlorate ions, nitrate ions,
nitrite or metanitrobenzene sodium sulphonate.
It is possible to carry out these treatments, on
the substrates to be treated, by spraying or by immersion.
The said treatments are part of a sequence of
operational steps which may comprise:
- one or several degreasing steps,
- one or several rinsing steps,
- possibly one or several pickling steps, then
rinsing steps,
- possibly one surface conditioning step,
- the phosphatization step proper,
- one or several rinsing steps,
- possibly a passivation step, generally in chromic medium and a rinsing
step.
The thus treated substrates are adapted to be used as such or to receive
subsequently an application of lubricant, of protection oil or of paint.
The nature and the composition of the coating of insoluble phosphates
obtained essentially depend;
- upon the nature of the substrate,
- upon the composition of the phosphatization bath
- upon the kind of application of the product (spraying or immersion),
- upon the nature of the pretreatment steps.
The prior art discloses various improvements to chemical conversion
baths in order to improve the performances of the said baths.
In that connection, according to the French patent FR-A-l 362 202 (filed
June 28, 1963) the corrosion resistance of phosphatization coatings is
improved by the addition of a small quantity (lower than 2~ and
preferably comprised between 0.5 and 1.5~) of organic complexing agents;
in the said patent, resistances against the so-called "neutral salt
spray" (standard ASTM-B-117) lower or equal to 25 hours have been
recorded.
According to the French patent FR-A-l 585 660 (filed August 28, 1968)
the so-called coating weight can be increased by addition of an organic
polyacid into a phosphatization bath accelerated with chlorate and
almost free of ferrous ions. That patent relates in fact to
phosphatization treatments intended for metals which are to be cold
worked; now, the phosphatization treatments in question provide coatings
whose coating weights are comprised between 16.1 and 32.2 g/m2
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and about which it is known that they provide only a low resistance
against neutral salt spray when they are tested as such.
Finally, according to the French Patent FR-A-2 531 457 (filed August 3,
1982) the association of chelating agent and of a soluble polyphosphate
makes it possible to obtain coatings whose resistance against corrosion
is substantially improved, as resistances against neutral salt spray
which in some cases are equal to 310 hours are mentioned in connection
with metal parts which are only phosphated.
The last mentioned French patent discloses that an excess of
polyphosphates might prevent the formation of the crystalline coating
and proposes to "ripenN the baths as claimed by variation of the pH
before use.
The contemplated ripening being not instantaneous, the steps of
regeneration of the bath must be discontinuous, which constitutes a
drawback as most of industrial baths are regenerated continuously.
The object of the invention is above all to overcome said drawback and
to provide a chemical conversion bath which, while providing
performances equivalent to those of the process disclosed in the French
patent FR-A-2 531 457, can be continuously regenerated.
And Applicants had the merit of discovering that in order to achieve the
said object, it is necessary to introduce in a zinc conversion bath
containing phosphate ions, nitrates ions as well as ferrous or ferric
ions at least 20 g/l of an organic chelating agent.
Consequently, the chemical conversion process according to the invention
is characterized by the fact that it comprises the use, at the moment of
the phosphatization step proper, of a zinc conversion bath containing
phosphate and nitrate ions as well as ferrous or ferric ions and from
about 21 to about 100 g/l of at least one organic chelating agent.
The zinc conversion bath according to the invention
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which is characterized by the fact that it comprises phos-
phate and nltrate ions as well as ferrous and/or ferric
ions as well as from about 21 to about 100 g/l of at least
one organic chelating agent, the said chelating agent being
selected from the group comprising
- polycarboxylic acids, among which citric acid, oxalic
acid, malic acid, glutamic acid, tartric acid, aspartic
acid, malonic acid and their salts, citric acid being
preferred, and
- polyhydroxycarboxylic acids among which gluconic acid,
glucoheptonic acid, mucic acid and their salts, gluconic
acid being preferred.
The salts of the above-mentioned polycarboxylic and
polyhydroxycarboxylic acids are selected from the salts of
alkaline and earth-alkaline metals, ammonium, iron, zinc,
manganese, molybdene, cobalt and nickel.
The zinc ion of which a proportion of 0.5 to 25 g/l
is used, can be introduced by any convenient way and
especially in the form of one of its salts with nitric,
phosphoric, carbonic, gluconic, citric acid or in the form
of its oxide. It can be used in association with any metal
currently used in phosphatization bath such as for example
manganese, nickel, cobalt, calcium, magnesium.
The iron ions, which are used at the proportion of
0.1 to 15 g/l, can be introduced either by dissolution of
iron in solid state as for example iron chips, iron powder,
iron parts or iron strappings: they can also be introduced
by way of salts of iron with sulfuric acid, fluorohydric
acid, phosphoric acid or nitric acid; the iron ion can be
3o the ferrous ion or the ferric ion or a mixture of both.
The phosphate ions on the one hand and the nitrate
ions on the other hand are used in the said bath in a
proportion up to 40 g/l.
The bath according to the invention can also
contain fluorhydric acid, fluosilicic acid or fluoboric
acid or several of the said acids in order to regulate the
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equilibrium between the pickling of the substrate by the
acid solution and the growing of the crystalline coating.
The bath according to the invention is prepared by
way of dilution starting from a concentrate which is
chraracterized by the fact that it comprises
- up to 500 g/liter of organic chelating agent, espe-
cially the citrate or the gluconate ion,
- up to 200 g/l of phosphate ions,
- up to 125 g/l of zinc ions,
- up to 200 g/l of nitrate ions.
The coatings obtained when carrying out the process
according to the invention provide within the meaning of
the test according to the standard ASTM B-117, a resistance
against neutral salt spray which is comparable to that
obtained according to patent FR-A-2 531 457 while permit-
ting a continuous regeneration.
The substrates sub;ected to the process according
to the invention can be stocked or used without further
protection but it is also possible to paint the said
substrates in order to further increase their resistance
against corrosion; they may be cold worked after a possible
application of a lubricant.
A preferred conversion bath comprises :
- from 5 to 40 g/l of phosphate ions
~ from 21 to 100 g/l of citric ions and/or gluconic ions
- from 2 to 40 g/l of nitrate ions
- from 0.5 to 25 g/l of zinc ions
- from 0.1 to 15 g/l of iron.
According to a modification, the said bath com-
3o prises from 0.02 to 3 g/l of nickel, cobalt or copper.
A particularly preferred bath comprises :
- from 5 to 20 g/l of phosphate ions
- from 21 to 60 g/l of citric ions
- from 2 to 40 g/l of nitrate ions
- from 2 to 10 g/l of zinc ions
- from 0.1 to 15 g/l of iron.
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Another particularly preferred bath comprises : -
- from 5 to 20 g/l of phosphate ions
- from 30 to 80 g/l of gluconic ion
- from 2 to 40 g/l of nitrate ion
- from 2 to 10 g/l of zinc ion
- from 0.1 to 15 g/l of iron.
The chemical conversion bath according to the
invention is used by way of spraying on the metallic
substrate to be treated or by way of immersion of the
substrate within the bath, the immersion being preferred.
The chemical conversion process according to the
invention comprises a set of treatment steps, i.e.:
- one or several degreasing steps,
- one or several rinsing steps,
- possibly, one or several activation or pickling
steps,
- one rinsing step,
- the chemical conversion step proper using the
conversion bath according to the invention,
- a rinsing step,
- a drying step.
Due to the exceptional resistance against dry
corrosion of the phosphatization coating obtAine~ when
using the bath according to the invention, the process is
simplified by suppression of the chromic passivation and
protection steps, which follow the chemical phosphatization
conversion and which are necessary in connection with the
prior art processes.
Preferably, the temperature of the bath according
3o to the invention is comprised between about 40 and about
100~C and more particularly between 70 and 99~C.
The invention will be well understood thanks to the
non-limiting examples hereafter indicated and in which are
disclosed advantageous embodiments of the bath according to
the invention.
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EXEMPLE 1 (comparative)
A conversion bath according to the prior art is
prepared, the said bath comprising besides water the
following ions :
Zn++ : 6 g/l
total PO4 : 16.7 g/l
NO ~ : 10 g/l
total Fe : 1 g/l
Ni++ : 0.2 g/l.
The temperature of the said bath is brought to
95~C; the free acidity is brought within the requested
range corresponding to 8.0 points using concentrated sodium
hydroxide (the free acidity is determined by titration of
10 ml of cold solution with a N/10 sodium hydroxide
solution sufficient to bring the pH to 3.6).
Cold rolled plates of steel containing O . 02~ of
carbon and corresponding to the quality ZES normally used
in the motorcar industry, the lengths and the widths of the
said plates being 150 mm x 140 mm, are subjected to the
following set of treatments :
Step 1: Alkaline degreasing within an aquous solution based
on the degreasing agents commercialized by the
Applicants under the trademarks RIDOLINE 1550 CF/2
(1.5% v/v) and RIDOSOL 550 CF (1.5% v/v).
Temperature: 60~C
Duration of the treatment: 3 minutes.
Step 2: Cold rinsing with tap water during 2 minutes.
Step 3: Nitric pickling
Pickling agent: 60% nitric acid in aqueous solution
(10% v/v)
Temperature: 20~C
Duration of the treatment: 10 seconds.
Step 4: Cold rinsing with tap water during 1 minute.
Step 5: The conversion proper at a temperature of 95~C
during 30 minutes.
Step 6: Cold tap water rinsing during 10 seconds.
Step 7: Drying within a drying box at a temperature of
about 95~C during 10 minutes.
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The thus treated plates are sub~ected to the
neutral salt spray at a concentration of 5~ (according to
the standard ASTM B-117).
After 1 hour exposition to the salt spray, a very
homogeneous 50% rust forming is obtained.
EXAMPLE 2 (comparative)
A conversion bath according to the prior art is
prepared by addition to the bath according to example 1 of
a quantity of 10 g/l of citric acid. The free acidity of
the bath is brought to 15.0 points. The same treatment as
above-mentioned is carried out on the same cold rolled
steel plates. After 8 hours of exposition a homogeneous
rust formation of 50% is obtained.
EXAMPLE 3
A conversion bath according to the invention is
prepared by addition to the bath according to example 1 of
a quantity of 30 g/l of citric acid. The free acidity of
the new bath is brought to 18Jl9 points. In the same way as
in example 1 cold rolled steel plates are treated and there
is obtained locally after 150 hours of exposition a rust
formation of 10%.
TABLE I
Amount of citric Salt spray test
Example acid in the bath -------------------------------
25 No. (g/l)Beginning of 50% rust
the rust
1 0 immediate less than l h
2 10 5 h 8 h
3 30 haze after several rust
3o 100 h spots
after 150 h
EXAMPLE 4
This example is intended to show the possibility
of using the process on various industrial parts.
The treatment of the parts which consist of
cemented steel HRC 55 (containing 0.55% of carbon in the
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mass) and of cold rolled steel XC lO (containing 0.1% of
carbon).
The three following compositions are prepared:
Composition A
56.7 g Of water are mixed with 20 g of citric acid
and 22.3 g of phosphoric acid Of a concentration of 75%.
In the said mixture is dissolved an amount of 1 g of
metallic iron.
Composition B
46.7 g of water are mixed with 30 g of citric acid
and 22.3 g of phosphoric acid of a concentration of 75%.
An amount of 1 g of metallic iron is dissolved in this
mixture.
Composition C
An amount of 7. 5 g of zinc oxide and an amount of
1 g of nickel nitrate containing 20% of nickel are dis-
solved in a mixture cont~ ni ng 74.8 g of water and 16.7 g
of nitric acid of a concentration of 60%.
The set of treatments is the same as the set
disclosed in example 1, except step 3, the nitric pickling
being replaced by an aqueous phosphoric pickling carried
out within an aqueous solution containing 7.75% v/v of
phosphoric acid of a concentration Of 75% during 2 minutes
at a temperature of 60 ~ C .
The baths used at the moment of the conversion
step proper are constituted as follows :
. Bath 1: - 800 ml of water
- 100 ml of composition A
- 100 ml of composition C
3o . Bath 2: - 800 ml of water
- 100 ml of composition B
- 100 ml of composition C.
Two series of steel parts corresponding respecti-
vely to the quality XC 10 and HRC 55, which had been
previously subjected to the set Of treatments according to
example 1 within which bath 1 is used with the parts
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consisting of XC 10 steel and bath 2 with the parts of HRC
55 steel, are then subjected to the neutral salt spray test
according to the standard ASTM B-117; the duration at the
end of which the red rust appears is recorded.
The results are recorded in table II.
TABLE II
Parts Treatment according to example 1 with
consisting ---------------------------------------------
of steel bath 1 bath 2
10 of the type (20 g/l of citric (30 g/l of citric
acid) acid)
XC 10 beginning of the rust no rust
after 400 h after 700 h
_____________ _____________________________________________
HRC 55 beginning of the rust beginning of the rust
after 10 h after 40 h
It is noticed that, when using in the bath accord-
ing to the invention a concentration of citrate ion higher
than 20 g/l, a significant implo~e.nent of the resistance
against corrosion is obtained, even than using a steel
difficult to be treated as a cemented steel of the type HRC
55.
EXAMPLE 5
An aqueous bath is prepared containing (weight/
volume):
- 25 g/l of citric acid
- 10 g/l of N03 ion
- 16.7 g/l of P04 ion
- 1 g/l of Fe++ ion
- 6 g/l of Zn++ ion
- 1 g/l of Mn++ ion.
The bath is heated to 95~C and the free acidity is
brought to a value equal to 20 points using sodium hydro-
xide.
Cold rolled galvanized steel sheets of the type ZES
are subjected to a sequence of treatments identical to that
disclosed in example 1 (the conversion step using the
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above-mentioned bath), except steps 3 and 4 which are
abandoned. After that treatment, the steel sheets are
subjected to the neutral salt spray test (standard ASTM
B-117) and, after an exposition during 400 h, no rust is
observed.
EXAMPLE 6
Cold rolled galvanized steel parts of the type ZES,
parts made of steel having a low carbon content of the type
XC 10 and parts consisting of steel having a high carbon
content of the type XC 55 (0.55~ of carbon) are subjected
to the conversion process comprising the following steps:
Step 1: Alkaline degreasing with an aqueous solution based
on degreasing agents commercialized by the Appli-
cants under the tr~ rkS RIDOLINE 1550 CF (2 to
1.5% v/v) and RIDOSOL 550 CF (0.15~ v/v).
Temperature: 60~C.
Duration of treatment: 4 minutes.
Step 2: Cold rinsing with tap water during 2 minutes.
Step 3: Phosphoric pickling within an aqueous solution.
Pickling agent: phosphoric acid having a concen-
tration of 75% (7.75% v/v).
Temperature: 55~C.
Duration of treatment: 4 minutes.
Step 4: Cold rinsing with tap water during 1 minute.
Step 5: Nitric pickling in an aqueous solution.
Pickling agent: nitric acid having a concentration
of 60% (10% v/v).
Temperature: 20~C.
Duration of treatment: 20 seconds.
Step 6: Cold rinsing with tap water during 1 minute.
Step 7: Conversion proper at 95~C during 15 minutes.
Step 8: Rinsing with cold tap water during 10 seconds.
Step 9: Drying in the drying box at a temperature of about
95~C during 10 minutes.
In that example, the conversion bath mentioned at
step 7 has the following composition:
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- 51 g/l of gluconate ion
- 3.6 g/l of zinc ion
- 8.2 g/l of phosphate ions
- 16.g g/l of nitrate ion
- 0.5 g/l of iron ions.
Its pH is equal to 2.7.
The parts and the sheets treated under these condi-
tions are sub~ected to the neutral salt spray according to
standard ASTM B-117, the duration after which rust appears
being recorded.
The results are indicated in Table III.
TABLE III
Steel of which Duration at the end of which
1the tested parts red rust appears
are consisting of
Strips of the ZES type higher than 480 h
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XC 10 about 400 h
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XC 55 about 400 h
