NO764272L - PROCEDURES FOR THE MANUFACTURE OF A CORROSION-RESISTANT ZINC-BASED COATING ON IRON-BASED SURFACES - Google Patents

Description

The present invention relates to a method

for the industrial production of zinc-based coatings on iron-based substrates, whereby the iron surface is protected against corrosion, as well as the resulting metal articles.

More particularly, the invention relates to a method for corrosion protection of iron-based materials, in particular the surface of steel plates and in particular the outer and inner surfaces of steel pipes. This protection is achieved by coating the surfaces with a zinc-based protective alloy that also contains magnesium, aluminum and chromium. The thus coated surfaces show an improved resistance to general corrosion and especially to local corrosion in hot water, to intergranular corrosion in the vapor phase at high temperatures and polarity inversion in relation to an iron-based substrate, as the coating exhibits very good adhesion to said substrate, optimal continuity properties, and smooth and shiny surface.

When it comes to corrosion protection of iron-based surfaces, it is known to coat such surfaces with a protective layer of a non-ferrous metal, for example, by dipping in a bath of molten non-ferrous metal, typically zinc. Furthermore, it is known that the corrosion protection achieved by such a coating depends to a large extent on the following coating properties: 1) continuity, homogeneity and evenness of thickness; 2) satisfactory adhesion to the iron-based substrate; 3) satisfactory properties with regard to resistance to general corrosion in connection with the required duration of protection; 4) satisfactory galvanic protection; 5) no tendency for polarity inversion in relation to the iron-based substrate; 6) stability against local attacks, such as pitting corrosion and under-rusting (interstitial attacks); .

7) resistance to selective and intergranular corrosion.

In the Italian patent no. 984 964 it is described

a zinc-based coating which essentially exhibits all of the above-mentioned desirable properties.

This zinc-based coating, which shows a reduced tendency to polarity inversion and is resistant to widespread, local and intergranular corrosion, and is particularly well suited for corrosion protection of iron surfaces, especially steel plates and pipes, is characterized by the fact that it includes magnesium, aluminum and chromium, where the ratio between the magnesium content and the aluminum content, calculated in % by weight, is below 5, the ratio between the chromium content and the magnesium content, calculated in % by weight, is between 0 and 0.2, and where the magnesium content is between 1 and 5 % by weight.

The above-mentioned Italian patent application states that the said corrosion-resistant coating is applied by conventional, well-known methods, which essentially consist of the following steps: (a) degreasing the iron-based substrate; (b) pickling the substrate with mineral acids; (c) washing the substrate;

(d) treatment of the substrate in zinc chloride and ammonium chloride

at 80°C; (e) the substrate is treated in a bath of molten zinc; (f) the substrate is treated in a bath of molten zinc, magnesium,

aluminum (and optionally chrome) alloy; and

(<g>) cooling.

This conventional coating process is particularly well suited for treating the inner surfaces of pipes, which treatment is particularly difficult when traditional methods, such as Sendzmirs, for vacuum metallization or electrolytic deposition are used.

It was now surprisingly found that coatings exhibiting to a particularly high degree all the above-mentioned desired properties 1) to 7) can be obtained by a method which essentially comprises the already known, above-mentioned steps, but where the parameters relating to the temperature of the molten baths and the iron-based substrate's residence time in the molten baths are carefully regulated within defined areas.

Furthermore, it was surprisingly found that particularly satisfactory results are obtained if, at the same time as the regulation of the mentioned parameters within the special ranges, to be specified below, special melt bath compositions are used.

The method according to the invention for producing on an iron-based substrate a zinc-based coating with a minimum tendency to polarity inversion and with resistance to widespread, local and intergranular corrosion, for protection against corrosion of iron-based surfaces, in particular surfaces of steel plates and external and internal surfaces of steel pipes, where magnesium, aluminum and chromium are added as additives to zinc to improve corrosion resistance, and where the ratio between the magnesium content and the aluminum content calculated in % by weight is lower than 5, while the ratio between the chromium content and the magnesium content calculated in % by weight is between 0 and 0.2, and the magnesium content is between 0.5 and 5% by weight, comprising the following steps: (a) degreasing the iron-based substrate; (b) pickling the substrate with mineral acids; (c) washing the substrate; (d) the substrate is treated in zinc chloride and ammonium chloride at 80°C; (e) the substrate is treated in a first bath consisting mainly of molten zinc, after which it is withdrawn from this first bath; (f) the substrate is treated in another molten bath of zinc, magnesium, aluminum and possibly chromium, where the ratio between the magnesium content and the aluminum content calculated in % by weight is lower than 5, the ratio between the chromium content and the magnesium content calculated in % by weight is between 0 and 0 ,2, the magnesium content being between 0.5 and 5% by weight, after which the substrate is removed from the aforementioned second bath, and (g) the substrate is cooled, and the method is characterized by: (1) step (e) where the substrate is treated in the aforementioned first bath and taken from this, is carried out at a temperature in the first bath of 440-460°C, preferably at 450°C, with a residence time in the first bath of 5-40 seconds, preferably 10 sec.; (2) step (f) where the substrate is treated in the aforementioned second bath and removed from this, is carried out at a bath temperature between 440°C and 500°C, with a residence time of 20-60 seconds, preferably 50 seconds.

According to a particularly preferred embodiment of the method according to the invention, the first bath mentioned in step (e) consists of zinc and contains 0.02% by weight of aluminum, while it. in step (f) said second bath consists of zinc, magnesium and aluminium, where magnesium may be present in an amount of 0.5-3% by weight, aluminum in an amount of 0.5% by weight for which preferably magnesium content up to 2% by weight, while the aluminum content can go up to 1% by weight for any magnesium content above 2% by weight.

The method according to the invention will now be explained in more detail by the following examples in connection with the drawing, which show the results of corrosion tests carried out in natural water at 65°C. Fig. 1-4 apply to repeated cycles of anode polarization in deaerated water, while fig. 5 applies to tests carried out by free immersion in aerated circulating water.

Fig. 1 shows curves regarding anode polarization i

natural water at 65°C on the zinc-based coating;

Fig. 2 shows curves regarding anode polarization i

natural water at 65°C on the duplex coatings Mg 1%-Al 0.5%;

Fig. 3 shows curves regarding anode polarization i

natural water at 65°C on the duplex coatings Mg 3%-Al 1%;

Fig. 4 shows curves regarding anode polarization i

natural water at 65°C on the duplex coatings Mg 5%-Al 2%; and

Fig. 5 shows curves regarding the corrosion potential as a function of time, measured on zinc-plated pipes and coated with a zinc, magnesium, aluminum alloy, the pipes being immersed in natural water at 65°C.

Experiments concerning the coating of steel plates with zinc

EXAMPLE 1

The experiment comprised a single immersion treatment in a bath of Zn-Mg 1% and Zn-Mg 2%, using steel plates that had been degreased in vapors of hot trichlorethylene, stained in 1:1 HC1 for 15 minutes, washed with water, treated in 10% NH^Cl-Zn Cl^ solution at 80 C for 30 seconds, followed by drying in hot air.

Some experiments were carried out in the molten bath temperature range between 460 and 480°C, where the steel plates' residence time in the bath was between 60 and 300 seconds. The bath had a very high rate of oxidation, which led to the formation of slag both on the surface and inside the coating, which are therefore heterogeneous with poor adhesion to the substrate. Application of a stream of argon in contact with the bath surface during the immersion test does not improve the properties of the coating.

EXAMPLE 2

Experiment with two immersions in a Zn bath (the first step) and Zn-Mg 1% (second step).

The pre-treatment of the steel plates was as in example 1. The bath temperature was 460°C in the first stage and 480°C in the second stage, and the treatment time was 10 and 10+30 seconds respectively. •

The coatings thus obtained are more homogeneous than the earlier ones, particularly as a result of the shorter immersion time in the second bath; in any case, the adhesion of the<p>egg to the substrate is still poor.

EXAMPLE 3

Experiment with two immersions in the zinc bath (first step) and Zn-Mg 1%-Al 0.2% bath (second step).

Aluminum is added to the second bath to regulate the bath's oxidation rate. The bath temperatures are the same as in example 2; 460°C (in the first bath) and 480°C

(in the second bathroom); the immersion time was 10 and 30 seconds respectively. The amount of slag formed on the surface of the second bath is negligible in the case of Examples 1 and 2; however, the adhesion of the coating to the substrate is still insufficient.

EXAMPLE 4

Experiment with two immersions in a Zn bath (first step) and a Zn-Mg 1%-Al 0.5% bath (second step).

The bath temperature and immersion time were as in example 3. The coatings thus obtained were homogeneous, glossy and adhered well to the substrate even after the steel plates had been subjected to a bending test. Table 1 below shows the coating thicknesses obtained in the various experiments.

EXAMPLE 5

Experiment with two immersions in Zn-0.2% Al bath (first step) and Zn-1% Mg-0.5% Al (second step).

The addition of small percentage amounts of aluminum to the first bath is related to the known effect that this element exhibits when it comes to preventing the growth of Fe-Zn phases and thereby reducing the thickness and fragility of the coating formed during treatment in the first bath.

Table II below shows the thicknesses obtained with two different immersion times (respectively 50 and 80 seconds) in the second bath.

EXAMPLE 6

Experiment with two immersions in a Zn-0.02% Al bath (first step) and Zn-2% Mg-0.5% Al (second step)

The first step is unchanged; in the second step, the alloy bath's minimum temperature for obtaining homogeneous coatings was found to be 490°C.

Table III below shows the thicknesses corresponding to the three different immersion times in the second bath, namely 40, 50 and 60 seconds.

EXAMPLE 7

Experiment with two immersions in Zn + 0.02% Al bath (first step) and Zn+3% Mg + 0.5% Al (and 1% Al) (2nd step).

The first step is unchanged; in the second stage the temperature was 465°C. The Zn+3% Mg+0.5% Al baths were very reactive, and the coatings obtained at different treatment times (from 20 to 100 seconds) therefore showed poor homogeneity and adhesion.'

A marked improvement in both the regulation of the bath's oxidation rate and the properties of the coatings was obtained by increasing the aluminum content in the second bath from 0.5 to 1% with treatment times from 40 to 7o seconds. The corresponding thicknesses are shown in table IV below.

EXAMPLE 8

Experiment with two times immersion in Zn+0.2% Al (first step) and Zn+5% Mg+1%; 1.5% and 2% Al (2nd step).

The first step is unchanged. Among the different baths that were tested for the 2nd immersion treatment, the bath with a relatively high aluminum concentration (2%) was chosen, as this has a lower oxidation rate, which however still remained high.

For this reason, when coatings with good adhesion strength are to be achieved, it was necessary to remove the large amount of slag that was formed from the bath surface before each immersion. Considering the temperature to which the alloy bath had to be heated to achieve sufficient fluidity and homogeneity (approx. 500°C), the duration of the second immersion had to be limited to 10 seconds in order to regulate the thickness of the coating. From Table V below, it will be seen that already after just

20 second immersion treatment, the coating is far too thick; under these conditions, the coating is also uneven and inhomogeneous.

Experiments concerning the coating of pipes with zinc

EXAMPLE 9

Some experiments with two times immersion treatment were carried out using tubes manufactured by Dalmine (length = 50 mm, inner diameter 16 mm, outer diameter 21 mm) whose elemental content is given in Table VI below.

Two alloys with the following composition were chosen: Zn+1% Mg+0.5% Al and Zn+3% Mg+1% Al.

Table VII below shows the thicknesses of the internal and external coatings obtained with the aforementioned two alloys.

Corrosion test in natural water at 65°C

(a) Repeated cycles of anode polarization in deaerated water.

Figs 1 and 4 show curves regarding repeated anode polarization for the coating materials Zn 1% Mg 0.5% Al,

Zn 3% Mg 1% Al, Zn 5% Mg 2% Al and a standard zinc coating.

The meaning of these curves in connection with the definition of the tendency to local corrosion of the coating will be understood by experts in the field.

For each coating, the evolution of the kinetic parameters characteristic of the curves was determined as a function of the number of polarizations: maximum potential and current density,

■Ec and Ic; the passivation current density 1^; passivation fracture potential or pitting corrosion potential, E pitting (see e.g. Fig. 1).

(b) Experiments regarding free immersion in aerated circulating water (0.01 l/second)

These tests were carried out using a series of zinc-plated tubes coated with Zn 1%.Mg 0.5M Al and Zn 3%Mg 1% Al alloys and a reference sample consisting of a zinc-plated tube.

The trial period was 3 months. Two tubes were used for

each type of coating.

Fig. 5 shows how the corrosion potentials vary with time, and weight data is given in table VIII below.

Tests regarding intercrystalline corrosion (on steel plates)

in water vapor from distilled water

Table IX below shows the results obtained

in terms of average penetration of the intercrystalline attack on zinc-plated coatings and coatings of Zn 1% Mg 0.5% Al; Zn 3% Mg 1% Al and Zn 5% Mg 2% Al.

Claims (3)

1. Method of producing on an iron-based substrate a zinc-based coating with a minimal tendency to polarity inversion and with resistance to widespread, local and intergranular corrosion for the protection of the iron surface against corrosion, especially surfaces of steel plates and external and internal surfaces of steel pipes, where magnesium, aluminum and chromium are added as additives to zinc to improve corrosion resistance, and where the ratio between the magnesium content and the aluminum content calculated in % by weight is lower than 5, while the ratio between the chromium content and the magnesium content calculated in % by weight is between 0 and 0.2, the magnesium content being between 0.5 and 5% by weight, which method comprises the following steps: (a) degreasing the ferrous substrate; (b) pickling the substrate with mineral acids; (c) washing the substrate; (d) the substrate is subjected to treatment in zinc chloride and magnesium chloride at 80°C; (e) the substrate is treated in a first bath consisting mainly of molten zinc and withdrawn from the bath; (f) the substrate is treated in another molten bath of zinc, magnesium, aluminum and possibly chromium, where the ratio between the magnesium content and the aluminum content calculated in % by weight is lower than 5, the ratio between the chromium content and the magnesium content calculated in % by weight. is between 0 and 0.2, and where the magnesium content is between 0.5 and 5% by weight, and the substrate is taken from said second bath, and (g) the substrate is cooled; characterized by that (1) step (e) in which the substrate is treated in said first bath and removed from it, is carried out at a temperature in the first bath between 440 and 460°C, preferably at 450°C, the residence time of the substrate in the first bath is from 5 to 40 seconds, preferably 10 seconds; (2) step (f) in which the substrate is treated in the aforementioned second bath and removed from it, is carried out at a bath temperature between 440 and 500°C, the residence time being from 20 to 60 seconds, preferably 50 seconds. 2. Method according to claim 1, characterized in that (I) the first bath consists of zinc and 0.02% by weight aluminum; and (II) said second bath consists of zinc, 0.5-3% by weight magnesium and 0.5% by weight aluminum for magnesium content up to 2% by weight, or 1% by weight aluminum for magnesium content above 2%. 3. Iron-based substrates comprising a zinc-based coating with a reduced tendency to polarity inversion and with resistance to widespread, local and intergranular corrosion for the protection of the iron surfaces against corrosion, in particular surfaces of steel plates and external and internal surfaces of steel pipes, when produced by the method according to the preceding requirements.