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protecting steel surfaces against atmospheric oxidation
Compositions for protecting steel surfaces against
atmosferic oxidation comprising the quercitine derivative of
monogallerylellogic acid, phosphoric acid, monoacid Zn phosphate, Zn nitrate,
ascorbic acid, a water-miscible organic solvent ed water, said water-miscible
organic solvent consisting of isopropyl glucol and ethoxypropanol. Said
compositions are applied directly to the oxidized, treated or non-treated,
Pedrazzini; Giuseppe (Segrate, IT)
ITALBONDER Spa (Caleppio de Settala, IT)
Application Priority Data
Field of Search:
Foreign Patent Documents
Primary Examiner: Silverbert; Sam
Attorney, Agent or Firm: Stevens, Davis, Miller & Mosher
1. Compositions for protecting steel surfaces against atmospheric oxidation,
consisting essentially of the quercetin derivatives of monogalloyllellagic
acid, contained in a quantity of between 15 and 30% by weight, phosphoric acid
contained in a quantity of between 2 and 3.2% by weight, monoacid Zn phosphate
contained in a quantity of between 1.1 and 2% by weight, Zn nitrate contained
in a quantity of between 7 and 12% by weight, ascorbic acid contained in a
quantity of between 0.05 and 0.5% by weight, an organic solvent mixture
consisting of isopropyl alcohol, propylene glycol and ethoxypropanol,
contained in a quantity of between 18% and 32% by weight, and water.
2. Compositions as claimed in claim 1, characterised in that the isopropyl
alcohol is present in a quantity of between 5 and 15% by weight, the propylene
glycol is present in a quantity of between 5 and 15% by weight, and the
ethoxypropanol is present in a quantity of between 2 and 8% by weight of the
This invention relates to new compositions for protecting steel surfaces
against atmospheric oxidation.
The invention also relates to steel objects having a protective layer formed
from a said composition as an external layer or as an intermediate layer below
the paint, and to the process for protecting steel from atmospheric corrosion
by applying a layer of said composition.
Various known processes are currently used for protecting steel surfaces from
atmospheric corrosion and, in the case of painted metal surfaces, for
preventing oxidation of the metal surface causing the separation and rapid
flaking of the overlying paint layer.
The process most widely used and which has given best results in this field up
to the present time is undoubtedly the phosphating process. This process
consists essentially of treating steel surfaces, whether oxidised or not, with
aqueous solutions containing phosphoric acid. The phosphoric acid attacks the
iron to form Fe phosphates. Whereas primary iron phosphate is soluble and
secondary iron phosphate is slightly soluble, tertiary iron phosphate is
totally insoluble. The main purpose of phosphating is therefore to form a
surface layer of insoluble tertiary iron phosphate, which protects the
underlying metal from any further attack by atmospheric agents.
In practice, during reaction by the phosphating solution, the pH rises because
of the reduction in the concentration of hydrogen ions in the boundary layer,
and consequently the insoluble tertiary phosphates precipitate.
The phosphate layer formed in the reaction between the phosphating solution
and the steel closely adheres to the treated surface, and is characterised by
strong resistance to electronic conduction so that it protects the underlying
metal from further oxidation processes and prevents incoherence and flaking of
pre-existing corrosion products.
The phosphating solutions utilised are rather complicated in that in addition
to phosphoric acid and possibly phosphates, they comprise surface-active
agents, accelerators, inhibitors of acid attack against zerovalent metal,
solvents, antioxidants etc. Said phosphating solutions can be applied to any
type of object.
The critical aspect of the phosphating process is the concentration of the
phosphoric acid in the phosphating solution. In this respect, if the
phosphoric acid is not completely consumed by its reaction with the oxides
present on the treated surface and by the surface reaction with the iron, it
produces, even if present in small concentrations, a strongly acid reaction
against the subsequently applied layers such as oil, wax or paint, and
consequently can result in negative reactions in these layers and in the
finishing layer which alter and disintegrate them.
As it is very difficult to exactly calculate the quantity of phosphoric acid
required, and as an acid deficiency leads to an unsatisfactory phosphated
layer, an acid excess is generally used and the phosphated object is then
washed with abundant water before applying the finishing layers.
This procedure is however not free from drawbacks in that the protective layer
of tertiary iron phosphates is very thin, and does not properly withstand the
water washing action, to give rise to the formation of new incoherent oxide by
To obviate the drawbacks and limitations of phosphating, steel surface
treatments have been in force for some years using formulations based on
tannic acid derivatives of very high molecular weight which instead of
eliminating the rust existing on these steel surfaces, form a continuous
covering film over the rust.
This film consists of the product of the reaction between the tannic acids and
the iron, and is in the form of a chelate of variable composition bonded to
The drawbacks of this type of surface protection derive mainly from the fact
that if an iron oxide layer is present on the steel surface it remains
incorporated between the support and the protective layer without being
eliminated, and this can lead to separation of the chelate layer by virtue
mainly of its different coefficient of anisotropic expansion with respect to
the support and the protective layer.
Moreover, under cover of the organic coating, the interchange reaction Fe.sup.++
.revreaction.Fe.sup.+++ in the underlying layer of uneliminated Fe oxides
continues, with consequent variation in the composition of the oxide layer.
This results overall in system instability, creating stresses which affect the
organic chelate layer and produce discontinuity.
Difficulties are also encountered in this process because of the pH of the
applied formulations based on tannic acid, this pH often being insufficiently
low to produce a significant initial rate of reaction.
In accordance with the present invention we have now found a new composition
for protecting oxidised, treated or non-treated surfaces which has none of the
drawbacks of compositions of the known art, and moreover ensures a higher
degree of protection, is very stable during storage and is absolutely free of
The process for protecting steel surfaces according to the present invention
is characterised by the use of mixed inorganic/organic compositions in which
each component performs a specific function and is present in a quantity which
is critical for system equilibrium. If the components are used outside the
critical limits or if unscheduled components are added, the system becomes
blocked and loses is functionality.
The compositions according to the invention are characterised by comprising:
the quercetin derivative of monogalloylellagic acid
monoacid Zn phosphate
a water-miscible organic solvent consisting of isopropyl alcohol, propylene
glycol and ethoxypropanol water.
The possibility of forming such a mixed system for protecting steel surfaces
was unpredictable beforehand because as the organic component is an ester, it
is sensitive to the hydrolysing action of phosphoric acid.
In practice, the hydrolysing and thus deactivating action of the phosphoric
acid occurs only if the system deviates from the limits defined for its
Furthermore, in said composition the phosphating components as described
hereinafter are present in proportions which are ineffective for the
phosphating process, and also the organic component is absolutely ineffective
if used alone, because of its low molecular weight. It was therefore in no way
predictable that said composition could provide an anticorrosive action far
superior to that obtainable by phosphating or by known tannin treatment.
The new compositions according to the invention act both by attacking any iron
oxides present on the steel surface, and by forming a protective surface layer
of organometallic chelate.
The final protective layer consists mainly of the organic coordination layer.
In applying said compositions, there is however no cause for preoccupation
that the layer may not be absolutely continuous, in that any discontinuity
regions are protected by the underlying phosphate layer, consisting of
tertiary Zn, Fe and Fe/Zn phosphates in various proportions.
The purpose of the organic solvents, by dissolving the organic component but
not the inorganic salts, is to protect the organic components from prolonged
contact with phosphoric acid during storage of the formulations. When applying
them, they in no way hinder the formation of a perfectly homogeneous,
uniformly applicable system, because of their perfect and complete miscibility
with water. They also eliminate heterogeneous substances such as fats, oils,
workshop dust and the like from the surface to be treated, provided they are
present in limited quantities.
The aforelisted components are present in the new compositions in quantities
within the critical limits specified hereinafter. The percentages are by
weight of the total weight of the composition, organic solvents and water
The quercetin derivative of monogalloylellagic acid is present in the
composition in a quantity of between 15 and 30%. A quantity of less than 15%
leads to a final incoherent layer, whereas a quantity exceeding 30% makes the
stability of the composition in solution questionable.
The phosphoric acid is present in a quantity of between 2 and 3.2%; the
Zn(HPO.sub.4).sub.2 in a quantity of between 1.1 and 2%; and the
Zn(NO.sub.3).sub.2 in a quantity of between 7 and 12%. Any deficiency in one
of these three latter components results in a lower initial reaction rate and
the formation of a final incoherent protective layer. Any excess in one or
more of said components excessively slows down the various competing processes
for the formation of the protective layer, and thus slows down overall the
formation of the protective layer on the treated surface.
The ascorbic acid is present in the composition in a quantity of between 0.05
and 0.5%. The minimum indicated limit corresponds to the minimum necessary for
acting as an accelerator for the layer formation process. Any excess beyond
0.5% is of no advantage, and in fact can be damaging in that it begins to
interfere with the other components.
The organic solvent as heretofore defined is a mixture of solvents, in which
each component has a specific purpose. Said mixture consists of isopropyl
alcohol the main purpose of which is to degrease the metal surface, propylene
glycol which retards the drying of the layer and thus favours uniformity, and
ethoxypropanol which has a considerable solvent power for said quercetin
derivative and therefore protects it within certain limits from the aqueous
acid phase. In total, the organic solvent constitutes between 18 and 32% of
the composition, and its individual components are distributed in the
following manner: isopropyl alcohol 5-15%, propylene glycol 5-15%,
ethoxypropanol 2-8% of the composition. Water and the quercetin derivative are
added to the total of the stated organic solvent in its indicated percentages,
to arrive at 100%. In this manner, a homogeneous solution is obtained.
The compositions according to the present invention can be prepared in various
ways, all suitable for the purpose.
The following series of steps has given positive results, and is indicated by
way of example only:
preparation of a premix consisting of a solution of the phosphoric acid, Zn
phosphate and Zn nitrate in water
addition of the monogalloylellagic acid quercetin derivative under agitation
to the uniform premix obtained in this manner
addition of the organic solvents to the obtained aqueous solution, addition of
the ascorbic acid and finally addition of the water necessary to obtain the
The composition according to the present invention can be applied to steel
surfaces by any procedure of the known art, such as by spraying, immersion or
This enables the new antirust process to be used for any type of object
whether of large dimensions such as ships in shipyards, gasholders, tanks,
reaction columns and the like, and of small dimensions such as automobile body
The compositions are applied at ambient temperature, preferably between
15.degree. C. and 30.degree. C., in layers of variable thickness depending on
the state of the steel surface.
Generally, 3-5 micron layers are sufficient. Under normal conditions, drying
is complete and the protective layer stable after about 24 hours. It is
preferable however to wait at least 48 hours before applying any subsequent
The mixed organic-inorganic antirust layers obtained according to the present
invention have proved compatible with any finishing layer applied to them, and
in particular with any type of paint. They ensure electrical insulation of the
metal surfaces, perfect bonding of subsequent finishing layers and
particularly of paint, and a considerable increase in the corrosion resistance
of the object.
In order to demonstrate the technical progress made in the field of
anticorrosive coatings by the compositions according to the invention,
comparative tests were carried out with other types of metal surface
pretreatment, applied before final covering with large-thickness paint layers.
The accompanying FIG. 1 shows the test results obtained according to the type
Specifically, the FIGURE comprises three curves, of which curve (a) relates to
a steel test piece without pretreatment, curve (b) relates to a steel test
piece pretreated with a known commercial phosphating agent (Gabrol C2 of
Italbonder, Milan) and curve (c) relates to a steel test piece pretreated with
an antioxidant according to the present invention, having the following
2.5% by weight
1.5% by weight
11.0% by weight
quercetin derivative of a
22.0% by weight
isopropyl alcohol, propylene
25.0% by weight
0.2% by weight
37.8% by weight
In all cases the steel test pieces, of dimensions 10.5.times.19.5 cm, were
covered with a final paint layer using a ship painting procedure, and were
then subjected to a resistance test in a corrosion chamber in accordance with
the ASTM B 117-64 procedure.
In the FIGURE, the abscissa represents the Schuster-Krause reading and the
ordinate represents time in hours.
As can be seen from the FIGURE diagrams, phosphating treatment considerably
improves the corrosion resistance of the steel, but the results obtained with
the new treatment according to the invention are much better.
Further tests were carried out on the shelf life of the composition according
to the invention used for the aforesaid tests, in comparison with the
phosphoric acid + Zn phosphate + Zn
tannin extract 22.3%
CH.sub.2 O 2.0%
ethylene glycol 10.5%
butyl cellosolve 2.5%
Samples of the two compositions were kept standing at ambient temperature for
80 hours. The amount of bottom sediment was 4% in the case of the composition
according to the invention against 8% for the comparison composition.
In another test, samples of the two compositions were centrifuged at 1500
r.p.m. for 15 minutes. The bottom sediment was 2.5% in the case of the
composition according to the invention, against 5% for the comparison
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