CA1051692A - Anticorrosion zinc based coating material - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A zinc based coating material useful for protecting ferrous surfaces against corrosion, also including magnesium, aluminum and chromium, wherein the percentage ratio Mg/Al is less than 5, the percentage ratio Cr.Mg is comprised between 0.03 and 0.2 and the amount of magnesium is between 1% and 5% by weight.

Description

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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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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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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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The contents of the water used for the corrosion tests in hot water is shown in Table II.

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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.

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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.
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TABLE IV

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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.

Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. 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 aluminum 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.

2. The coating material of claim 1 in which 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.

3. The coating material of claim 1, further comprising up to 2% aluminum by weight.