Note: Descriptions are shown in the official language in which they were submitted.
~05~6~;~
The present invention relates to a zinc based coating
material which is suitable for use in protecting ferrous sur-
faces against corrosion and to metal bodies having such a
coating. More specifically, the invention can be used, for
example, in proteffl ing against corrosion the surfaces of
steel sheets and the inner and outer surfaces of steel pipes.
The coating material is in the ~orm of a protective alloy
containing zinc, magnesium, alummnum and chromium which gives
good resistance to widespread corrosion, to localized corro-
sion occurring in systems using hot water, to the granularcorrosion produced by steam at high temperature and to
the corrosion resulting from any inversion in polarity with
respect to a steel base layer. The coating alloy of the
invention adheres well to the base layer7 it has good con-
tinuity features and a shiny and smooth surface. ` -
It is common practice to protect ferrous surfaces
against a hostile environment by coating them with a pro-
tective layer of a non-ferrous metal, for instance by ~-
immersin~ them in a bath of such molten non-ferrous metal. ;
It is also known that the protection given by the coating
depends on the following characteristics:
1) good adhesion to the ferrous base, that is to say a
minimum number of weak regions in the base metal to
coating interface;
2) continuit~, i.e. a uniform thickness and good appearance;
3) good resistance to widespread corrosion for the duration ;;
of the protection required;
4) good galvanic pxotection;
5) that it has a minimum susceptibility to inversion in
polarity with respect to a ferrous base;
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~951692
6) that it is stable against localised attacks such as
pitting and undershield (or crevice) corrosion;
7) that it is resistant to selective and intergranular
corrosion.
Of the many non-ferrous metals used for this purpose,
the most common is ~inc, both for its relatively low cost
and for its position with respect to iron in the electro-
chemical table of elements. So far however, use of the
known methods and alloys in proviaing a zinc coating only
affect the problems referred to in the paragraphs numbered
1, 2, 3 and 4 above.
Thus for instance in U.S. Patent No. 3,393,089 granted
to Bethlehem Steel Company, there is described a zinc based
protective alloy, containing from 25 to 70% Al for use a-
gainst widespread corrosion. An alloy described in the
British Patent No. 1,125,965, in the name Inland Steel,
serves the same purpose and contains from 1 to 4~ Mg and
from 0.05 to 5~ Al, it being clearly stated that the best
results are obtained with about 2.5% Mg and about 4.4% Al.
It is also expressly stated that such better results refer to
protection against widespread corrosion.
2Q In German Patent Application No. 2,146,376, in the
name of Fredericia Galvaniseringsanstalt, there is described
a process of zinc coating by means of double immersion,
wherein ths second bath contains a zinc alloy containing 5%
Al and 4% Cu or 20% Al, 5% Mg and 1% Si. Such coating is
stated to be resistant to atmospheric corrosion namely to
widespread corrosion. Additionally, in British Patent No.
1,057,285, in the name of Armco Steel Co. there is claimed
a coating for protection against widespread corrosion con-
taining from 0.04 to 0.35% Al and from 0.01 to 0.1% Mg, pre-
ferably from 0.1 to 0.2 Al and from 0.01 to 0.04~ Mg the
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~C~5169Z
remaind2r being zinc and minor impurities. On the other
hand there is a recent Czechoslovakian Publication in the
name of ~. Teindl, translated by B~ .I. in ~ugust 1972
and numbered 10140 in which it is sta~ed that it is a mis-
take to add magnesium to a bath for zinc coating steel be~ -
cause, when this is done, the coating is fragile and easily
comes away from the steel base. There is also a report sub~
mitted at the 7th Internàtional Galvanizing Conference in
Paris in 1964 inthe-name of J.J. Sebisty in ~hich it is
stated that magnesium has no positive effects on the per-
formance of zinc based galvanizing coatings in respect of
many types of corrosion.
This being the state of the art, it seemed obvious
that there was no point in making an~ further investigations
15into zinc based galvanizing coatings containing magnesium. ~
It was therefore a great surprise to us that, during -
an experiment, we found that a suitable addition of magne-
sium to a bath containing molten zinc and aluminium enhanced
the qu~lity of the coating to such an e~tent that it acquired
to some degree all of the seven features mentioned above
upon which the protectivity of the coating is dependent,
such ~eatures being obtained by putting into the zinc coating
bath mixtures rather different from those indicated in the
above mentioned patents and stated in them to he the best.
It is therefore an aim of the present invention to provide a
zinc based coating for use with ferrous surfaces having
improved characteristics of resistance against widespread
corrosion, localised corrosion, and selective and inter-
granular corrosion, as well as reduced suscep ibility to
polarity inversion, good adhesion to a ferrous base, a more
_ ~ _
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uniform thickness and a shiny and good appearance.
According to the present invention, there is provided
a zinc based coating material for use in protecting ferrous
surfaces against corrosion, said material also including
magnesium, aluminum and chromium in which the weight percentage
ratio between magnesium and alumin~n is between 1.5 and 5, the
weight percentage ratio between chromium and magnesium is
between 0.03 and 0.2, and the amount of magnesium is between
1% and 5~ by weight, the balance element of said material being
zinc. Preferably, the ratio between the magnesium and aluminum
weight percentage values is between 1.5 and 3 and the magnesium
content is between 1% and 3% by weight. The coating material
may further comprise up to 2% aluminum by weight.
All the percentage values given in this specification
and the claims are relative to the molten composition contained
in the bath and are given by weight. The chromium assists
in increasing the resistance of the composition to corrosion,
especially to inter-granular corrosion in particularly
hostile environments, especially for those alloys which
contain almost the maximum aluminum content allowed according
to the present invention. Coatings obtained according
to the present invention are much more resistant to
corrosion than those previously known, as can be seen from
Table I in which a comparision is provided between results ~ ;
obtained by using samples of steel sheet (2 mm. thick) and
of pipes ~outside diameter 21 mm. and wall thickness 3 mm.)
having compositions including 0.07% C, 0.32% Mn, 0.01% p, ~
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~L051692
0.016~ S, the remainder being iron and including minor im-
purities, such samp~les having been coated with zinc based
alloys made in accordance with the prior art and the present
invention, as indicated,
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~05~69Z
The contents of the water used for the corrosion tests
in hot water is shown in Table II.
TABLE :[I
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Ion ~CO3 CO3 NO3 Cl SO4 Ca ~ Ug++ K Na :
. . ._ _ . ~ _ _ _ _ :
Concentration ppm 439 _ 0.70 65 29 99 21 20 80
..... . . _ . - ~ ' , . ,
pH 7.2
... . . . _ ...... _ . _ . .
Table III shows the data referring to tests against
inter-crystal corrosion and corrosion caused by hot water ~
in respect of coatings made according to the present inven- -
tion.
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As far as the tendency to inversion in ~olarity is
concerned, Figure l shows a ~raph (a) relating to zinc coated
samples, a graph (b) relating to samples coated with an alloy
including 1% Mg and 0.5~ Al and a graph (c) relating to s-am-
S ples c~ated with an alloy including 5~ Mg and 2% Al. The
measurements were carried out in cool compartment pyrex
cells. The testing electrolyte was a O.OlN solution of
NaHCO3 at 65C. In one compartment an uncoated steel test
piece was ~lushed with CO2, the pH being between 5.5 and 6 ~;
approximately. In another compartment a coated test piece
was flushed with 2 The current intensities shown refer `
to the steel surface. As can be seen from the graph in Fig-
ure l, ~y using a coating alloy according to the present in-
vention a clear decrease in the current intensity relating -
to the inverted pair, that is wi~h the coating acting as a
cathode and the steel acting as an anode, is achieved. It
has been found that after 100 hours under test, the coating
according to the present invention containing the highe~t
amount of magnesium has a cur~ent intensity of the order of
a few ~A/cm2, whereas for the zinc coating it is of the order -
of appro~imately 300 ~A/cm . ~'
If one considers the effective speed at which corro-
sion occurs over a steel surface which has a protective
coating according to the inve~ion, as opposed to a surface
without such a coating, it will be seen that with a coating
according to the present invention ~here would be an annual
steel corrosion of the order of a few hundredths o~ a milli-
metre whereas with a simple zinc coating there would be an
annual corrosion of between 3 and 3.5 mm.
~L~5~69Z
As far ~s the resistance of the coating to localised
attack from water chlorides and resistance to interstitial
under-screen attacks is concerned, Table IV sets out data
referring to the passivity break potential, showing that
the less negative the recorded break potential the better is
the resistance to localised attack, and to the amplitude of
the peak of polarisa~tion, showing that the smaller the ampli-
tude of the peak the better is the resistance to undershield
corrosion.
:
TABLE IV
~ . . . _ ..................... ... ,,.
Ty~ of coating Break Potential Amplitude of
(mV, S.H.E.~ passivation peak
(mV)
.. ,.. ~ . .. .. .. _ _
~n - 770 120
Zn, Mg 1%, Al 0.5% ~ 620 60
Zn, Mg 3%, Al 1~ - 560 50
Zn, Ng 5%, Al 2% ~ __ _ _ _
:
The data shown in Table IV have been obtained from ;
anode polarisation graphs obtained using water whose con-
tents are given in Table II, at 65C.
Contrary to the standard practice for Zn-Al coatings
using the Sendzmir process, the coatings mentioned above
were applied by a method involving a double immersion, first
in a molten zinc bath and then in a bath of a chosen alloy.
By way of example Table V gives below data relating to
the formation of slag in the bath, adherence and the coating
thickness determined according to UNI-5741-66 standards
(Aupperle Method).
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As shown by the previous Tables, the best coatings of
zinc alloy according to the present invention give a much
higher resistance to the various types of corrosion than
has been given by the coatings previously known. Resistance
to widespread corrosion has been particularly improved, as
can be seen from Table I, which enables the results of tests
for exposure in salt vapour to be compared.
As far as inter-granular corrosion, susceptibility to
inversion in polarity and resistance to locali~ed attacks are
concerned, the improved coatings according th the present
invention give quite unexpected results, as compared wikh
the known coatings.
The advantages given by the improved coatings accord- --
ing to the present inveniOQn are not only limited to an im-
proved resistance to corrosion, but include-ease of appli-
cation. In fact coatings according to the present invention
may be conveniently applied in accorddnce with the following
method, which is already well knownl:- remove the grease from
the ferrous piece, -~ pickle in HCl, ~ > wash,~
flush at 80C in zinc and ammonium chloride ---~ immersion in
a mol en:~t~c W~ WL~ bath of the alloy
Zn Mg Al Cr ~ cooling off.
Pipes can be treated inside by the same method, a
rather difficult operation when traditional methods such as
Sendsmir's, or metallisation in a vacuum or electrolytic
sedimentation are used.
