EP0572534A1

EP0572534A1 - Ferrous product with metal coating having improved corrosion resistance.

Info

Publication number: EP0572534A1
Application number: EP92907268A
Authority: EP
Inventors: Bernard Gailliez
Original assignee: Fabrique de Fer de Maubeuge
Current assignee: Fabrique de Fer de Maubeuge
Priority date: 1991-02-22
Filing date: 1992-02-21
Publication date: 1993-12-08
Anticipated expiration: 2012-02-21
Also published as: DE69202700D1; AU1439692A; ATE123077T1; ES2072757T3; EP0572534B1; WO1992014856A1; DE69202700T2

Abstract

Le produit ferreux à revêtement métallique et de résistance à la corrosion améliorée est tel que le revêtement de surface se présente sous forme stratifiée avec, dans une matrice fer-zinc-aluminium (zone 2) comprenant 10 à 15 % de fer et moins de 3 % d'aluminium, des nodules (zone 3) d'un composé Fe-Al-Zn comprenant entre 60 et 70 % de fer, 20 à 25 % d'aluminium et le solde de zinc.The ferrous product with a metallic coating and improved corrosion resistance is such that the surface coating is in laminated form with, in an iron-zinc-aluminum matrix (zone 2) comprising 10 to 15% iron and less than 3 % aluminium, nodules (zone 3) of an Fe-Al-Zn compound comprising between 60 and 70% iron, 20 to 25% aluminum and the balance zinc.

Description

Ferrous metal coated product with improved corrosion resistance

The present invention relates to a ferrous metal-coated product having a corrosion resistance far superior to that of an ordinary galvanized product.

Protection against corrosion of a ferrous product is achieved in various ways. The oldest and the least expensive is galvanization and in particular dip galvanization in a bath of liquid zinc alloy.

Attempts have been made to improve the protection thus obtained by modifying the composition of the bath, mainly by adding aluminum to it. With regard to alloys containing less than 10% of aluminum in zinc, there are numerous compositions of galvanizing baths which all have their specific properties as regards the adhesion of the coating to the substrate or the surface appearance. On the other hand as regards the resistance to corrosion, they are little different from a zinc bath alone because, on the one hand, zinc remains largely dominant in the composition of the coating and on the other hand, the coating structures are heterogeneous and contain zones rich in addition aluminum and others rich in zinc which remains in the unalloyed state.

Alloys have also been proposed in which the aluminum is present from 20 to 50%, or even more. The corrosion protection of surfaces coated with this alloy is markedly improved. However, these alloys have several drawbacks: the energy expenditure to apply it is greater than that of galvanizing; the coating is not very sacrificial so that corrosion occurs on the edges of the substrate and propagates slowly at the coating / substrate interface; the higher the proportion of aluminum, the more the coating faults such as uncovered points.

Finally, most of these alloys with a high aluminum content are present in the coating in the form of heterogeneous two-phase structures both on the surface and in the mass of the coating, which requires:

- a treatment aimed at carrying out fine recrystallization in an attempt to avoid intergranular corrosion,

- an appearance treatment to make the product suitable for receiving a paint, we know that these treatments are difficult to carry out and have random results and that, even with high crystallization rates, the heterogeneities are present without organization which could be favorable for the protection of the substrate.

To remove the flowers from a zinc coating, a heat treatment is carried out to diffuse the iron in the zinc. The operation known as "galvanealing" gives the alloy a satin appearance which gives it good quality for receiving a paint and / or for being weldable. It is known that the formation of this iron-zinc alloy is hampered if the coating comprises aluminum. Many documents relate this characteristic: a /. Haugton (MA) in Sheet Métal Industries, 1958 page 453-38. b /. Babli (H) et al. in Korrosion und Metallschutz 21 (1945) p.1 / 9 ZF Metallkunde 40 (1949) p.141 and 176. c /. Belgian patent n ° 897788. d /. United States Patent No. 3,190,768. It appears, at least according to the aforementioned American patent, that the ternary alloy Fe-Zn-Al is blocked at the interface and constitutes a barrier to the diffusion of iron in zinc. There is nevertheless an interest in the presence of aluminum in zinc - in small quantity: 0.15 to 0.30% - by the very fact of this inhibition because it prevents the formation of an Fe-Zn alloy at 1 * interface which is very brittle and is the main cause of a possibility of cracking of the coating. These characteristics have always been a prejudice unfavorable to the effectiveness of a simple diffusion treatment of a coating of Zn-Al alloy. And this, despite the existence of some documents which relate laboratory experiments or which are simple intellectual speculations on the possibility of improving the corrosion resistance of a Zn-Al coating by subjecting it to a diffusion treatment.

Thus document JP-52131934 proposes to subject a heat treatment to a product coated with a Zn-Al alloy; the aluminum content is between 3 and 22% and the temperature is between 550 and 700 ° C. The product obtained is, according to this silver gray appearance document, which tends to prove that the surface layer consists essentially of zinc. The only exemplary embodiment described (aluminum content: 10% - holding time at a temperature of 600 ° C.: 60 seconds) and the summary corrosion resistance tests clearly show that the product has not really been satisfactory. It is indeed likely that the improvement in corrosion resistance was not significant given the existence of an almost pure zinc layer on the surface (silver gray color). The aluminum is probably concentrated at the substrate / coating interface, blocking diffusion, all the more so since the heat treatment was carried out after freezing and cooling of the coating, that is to say on a structure little sensitive to significant dissemination.

Document JP-59,193,258 clearly states that it is very difficult to carry out an alloy treatment of a ferrous substrate covered with a Zn-Al alloy. To overcome this technical difficulty, this document proposes to mechanically treat (brushing) the interface just before its penetration into the bath so as to promote the formation of intermetallic alloy which can then diffuse during the heat treatment. subsequent. The presence of aluminum in the bath is, according to this document, the factor which prevents the diffusion of iron in the coating alloy.

Finally, mention will be made of document BE-A-897,788 which relates to a coating based on iron-zinc-aluminum which it can be said is not of a nature to give the skilled person an exploitable teaching. Indeed, a first embodiment of this coating resides in immersing the substrate to be coated in a molten bath of iron-zinc-aluminum alloy which may contain up to 25% of iron, which would imply a temperature of the bath quite impossible to maintain industrially. Another embodiment still consists, as unrealistically, from an industrial point of view, in passing the metal strip through two successive baths, the first having less than 0.5% of aluminum so as not to encounter the difficulties in the interface discussed above, born from the presence of too much aluminum in the alloy, the second bath having a significantly higher aluminum content. These embodiments do not concur in granting credit to the teaching of this document, especially since it contains no practical example showing the structure and the characteristics of the coating thus obtained.

The present invention relates to a metallic coating of a ferrous substrate which has a surprising resistance to corrosion, especially when it is covered with an organic coating such as one or more layers of paint.

The coating of the invention is remarkable in that it is in laminated form with, in an iron-zinc-aluminum matrix comprising 10 to 15% of iron and less than 3% of aluminum, nodules of a compound Fe-Al-Zn comprising between 60 and 70% iron, 20 to 25% aluminum and the balance of zinc. In a preferred embodiment of this coating, the nodules are located on the surface of the coating to form a very tormented surface layer which is of excellent quality for the attachment of the paint which it would be caused to receive. The invention also has for its object and a process for obtaining this coating which consists in covering a ferrous substrate by soaking in a bath of liquid alloy containing between 85% and 97% by weight of zinc, the remainder consisting of the aluminum, to adjust the thickness of the layer of alloy deposited on the substrate, to heat the substrate thus coated while the coating is not yet set, to maintain the product at temperature for an interval of time, the length of which is in inverse proportion to the temperature and in cooling the product thus treated.

This treatment cycle, immediately after leaving the galvanizing bath, makes it possible to obtain the product described above. Everything seems to happen as if, while the coating is still in the liquid state, the diffusion heat treatment caused a migration of the aluminum towards the substrate / coating interface. The most probable hypothesis is that there form nodules of a compound Fe-Al which, very eager to fer, deprive, first of all the zinc of this iron which it requires for the formation of an iron-zinc alloy. These nodules, however, have a limited exchange capacity, so that, on the one hand, the aluminum present in the zinc is not: fully alloyed with the iron and there remains a residual fraction thereof in the zinc matrix and , on the other hand the depleted zinc greedy for iron, causes the diffusion of the latter between the nodules. The iron-zinc compound created at this time is a compound δ which, by its crystallization, detaches the nodules from the interface and pushes them back to the surface.

Preferably, the processing temperature of the coated material will be maintained between 580 ° and 670 ^C C well that these limits are not intangible and that a lower value can be taken for the lower if the residence time is extended and higher for the higher if this residence time for the strip is reduced in the treatment enclosure.

It is preferable to proceed with the diffusion while the coating is still liquid because an intermediate solidification of the zones rich in aluminum delays significantly the diffusion during the annealing, which leads to heterogeneities in the surface of the coating.

The residence time of the strip in the thermal reprocessing enclosure depends on the temperature of this enclosure. For a temperature at the band holding level of between 600 and 650 ° C., the residence time is of the order of one to three tens of seconds and rather in the low values of this range if the heating is carried out by induction. One of the criteria making it possible to assess the adequacy of the residence time and to adjust it if necessary, is the color of the surface of the coating at the outlet of the treatment oven. This strip must have a dark, matt gray appearance, which is a sign of a good transformation of the coating, the surface of which has become microrough due to the nodules which it presents, eliminating the reflection and giving it this matt gray appearance. For reasons of congestion in the production line, it is desirable that the heating part of the thermal cycle is as rapid as possible. The temperature holding time then becomes the most important part of the strip's residence time in the enclosure of the diffusion treatment, the cooling being able to be natural or forced.

Other characteristics and advantages will emerge from the description given below of the structure of the coating of the invention, of the process which has made it possible to reproducibly obtain numerous examples thereof and tests and corrosion resistance tests which were carried out in parallel on samples coated in accordance with the invention and on simply galvanized samples.

Reference will be made to the appended drawings in which:

FIG. 1 is a photograph of a typical section of the coating according to the invention,

FIG. 2 is the graph of the mass composition of the coating of FIG. 1 as a function of the depth of the point of analysis, carried out from an analysis with a scanning electron microscope,

- Figure 3 is a graph illustrating the temperature of the strip during the treatment according to several heating protocols, - Figures 4, 5 and 6 illustrate by diagrams the different installations implemented having led to the different temperature curves of the figure 3,

FIGS. 7 to 10 are external views of the various results observed during corrosion resistance tests carried out in a salt spray,

- Figures 11 to 14 are external views of the results observed on samples subjected to a corrosive atmosphere containing ammonia.

Figures 1 and 2 illustrate the structure of the coating according to the invention and its composition. In the figure we recognize four distinct zones:

- on the left of the figure, the substrate (zone i) which is steel,

- then, moving to the right, a zone 2 which has a crystallization in the form of rods and which essentially contains an iron-zinc-aluminum alloy in proportions of the order of 10 to 15% of iron, less than 3 % aluminum and the balance of zinc,

- a zone 3 in which there is the presence of substantially spherical nodules first embedded in the previous zone whose analysis with the scanning electron microscope reveals an iron-aluminum alloy having about 60% iron and 25% aluminum, the rest being zinc,

- The fourth zone 4 is simply the layer of resin having been carried on the surface of the sample to allow micrographic analysis.

Figure 2 illustrates the evolution of the concentration of the three components of the alloy from the base substrate to the surface. The Zn curve traces the evolution of the zinc concentration from the substrate to the surface; the Fe curve, that of iron; and the curve Al, that of aluminum. The thickness of the coating at the measurement is of the order of 12 to 13 μ.

In FIG. 3, various thermal curves have been shown illustrating the thermal cycle of the different samples produced in accordance with the invention with the different installations shown in FIGS. 4, 5 and 6. Curve 5 is the image of a thermal cycle obtained with the installation of FIG. 4. This comprises, at the outlet of the galvanizing bath, behind the wiping nozzles 6 of the strip 7, a first inductor 8 a second inductor 9, a thermal holding sleeve 10 and a cooler there. The installation, experimental, treats a metal strip 140 millimeters wide. The two inductors are 700 millimeters long and are separated by a space of the order of 300 millimeters. During the experiment, we realized that the energy supply to the strip was not satisfactory, so we added a cuff to keep the strip at temperature at the outlet. of the second inductor.

The curve 5 therefore has a first part 5a which corresponds to the temperature rise of the strip 7 up to approximately 650 ° C. in the first inductor 8. The second inductor 9 maintains the strip at this temperature of 650 ° C. , which corresponds to part 5b of the curve 5. Having not, during the first experiments, sufficiently long and powerful inductors, it was necessary to artificially extend the temperature holding stage of the strip by means of the insulating sleeve 10 to slow the cooling of the band. The evolution of the temperature of the strip is represented by part 5c of curve 5. Part 5d of this curve illustrates the evolution of the temperature of the strip in the cooler 11. The sample in Figure i comes from a heat treatment of the type illustrated by curve 5.

The curves 12, 13 and 14 of Figure 3 were obtained in the installation of Figure 5 where the second inductor 15 has a length of 1.25 meters. This inductor 15 has made it possible to have a sufficiently long temperature holding level so that the cuff is no longer necessary. The temperature is maintained in an area between 580 and 670 ° C and the strip then undergoes natural cooling (part 16 common to all the curves 12, 13 and 14). The products from this series of production tests have the same structure as that illustrated in Figure l.

Finally, tests have been carried out on the treatment of the strip illustrated by the temperature curve 17 which corresponds to a rise in temperature by the first inductor 8 and insulation and a slowing down of cooling by the second inductor 15HS (FIG. 6) has not been put into service. The temperature holding level was in this case more imperfect and the structure of the coating less characterized; everything seems to indicate that there is a transformation of the coating under the effect of the heat treatment which is more or less completed depending on whether the temperature maintenance is more or less well assured or more or less sufficient. The advantage of the products produced by the invention lies in improving their resistance to corrosion, in particular when they are intended to be painted. Corrosion resistance tests were carried out in parallel on samples of products resulting from the invention and on samples of normally galvanized products. These tests are described below.

A) Corrosion tests in a salt spray atmosphere.

Samples of the invention: They come from a pilot production line 140 mm wide. They were then treated on the surface on the same pilot line or in the laboratory in the same way as the hot galvanizing products are treated continuously before being painted. Finally, a polyester paint layer of 15 to 18 μ was applied to each side of these samples. Control sample :

This sample comes from a production line where it is produced in about one meter wide and prepared on the surface to receive a paint. This paint is applied in the laboratory as for the previous samples.

Preparation of tests:

A plate of each product is cut and folded at 90 °. Three of its banks are hermetically protected from contact with the atmosphere by means, for example, of an adhesive film. Figures 7 to 10 show two of these samples at different stages of corrosion. Sample A is that from conventional galvanization while sample B is that of the invention. The protected edges of each of these two samples are denoted 20, 21 and 22. Only the edge 23 is exposed to the outside atmosphere. Execution of tests:

The samples are placed in a salt spray oven. Such an oven and the protocol of experiments comply with AFNOR X41002 Standard. Thus the test temperature is 35 ° C, the humidity is 85-90% and the concentration of sodium chloride is 5%. At specified time intervals, the samples are removed from the oven for examination. For this purpose they are rinsed with ordinary water and dried. The exam involves taking photographs of each sample. Figures 7 to 10 are drawings made from these photographs (A for the galvanized control sample; B for the sample of the invention). The samples are then reintroduced into the oven for a further period of exposure. Explanation of observations: After 100 hours of drying oven (FIG. 7), it is observed that sample A presents a beginning of corrosion in the vicinity of the unprotected section 23. It is white rust over a width of 1 or 2 mm (zone 24). This white rust (zinc oxides and hydoxides) tends to lift the paint layer which allows corrosion to progress under the paint layer. Sample B shows no signs of corrosion.

After 300 hours (FIG. 8), the sample A is attacked over an area 25 mm wide from the unprotected edge 23; some bites appear here and there and affect 1% of the surface. Sample B shows some pitting 26.

After 1300 hours of exposure (FIG. 9), the part A sees its affected area 27 reaching approximately 20% of the surface of the part. Red rust (oxidation of the substrate) appears on the side of the unprotected edge. For part B, the number of punctures 26 has increased slightly.

Finally, after 3500 hours of oven (FIG. 10), the part A is pierced (zone 28) on the side of the edge 23 and the surface is practically entirely affected by corrosion. Exhibit B has more 26 bites and the start of corrosion 29 (similar in dimensions) to that observed after 100 hours on the part A, on the unprotected edge 23 as well as on the fold.

B) Corrosion tests in an ammonia atmosphere

The inventive and control samples are prepared as before.

Preparation of tests:

The atmosphere is created in a closed enclosure inside which is placed a container containing diluted ammonia. The upper part of this container is open and supports a grid on which the samples are placed. More precisely, the parts being in V are retained by parallel grid elements with the tip of the V turned downwards, that is to say directly exposed to the ammonia vapors escaping from the container. These are conditions of use of materials close to those of livestock buildings.

As in the previous tests, the samples are taken at determined intervals, washed and photographed.

The observations made are as follows and are the subject of a schematic representation in FIGS. 11 to 14 (sample C being the control sample, sample D being that of the invention).

After 140 hours of exposure (Figure 11), a start of corrosion (white rust) appears on the fold, at locations 30, of sample C with lifting of the paint. On piece D, there is also a slight tendency for white rust to form in places 31.

950 hours of exposure (figure 12): Exhibit C has a blistering of the paint at locations 32 and, at locations 33, an uplift thereof, while the exhibit

D shows no sign of significant change compared to the previous survey. The statement made after 2700 hours of exposure

(Figure 13), shows a large corrosion 33 on part C (more than 40% of the surface affected by white and red rust). The part D presents some points 34 of separation of the paint.

Finally, after 4400 hours (FIG. 14), the corroded zone 35 of the part C reaches more than 60% of the total surface whereas the part D does not show any significant evolution of the corrosion as far as it is concerned. It appears that the coating according to the invention is of certain interest in the case where the product is intended to be painted or lacquered.

It will also be noted that the transformation of the coating during its diffusion treatment increases its thickness due to the diffusion of the iron in the alloy; for example, having deposited 140 g / m ² double-sided coating, the product after treatment is equivalent to a deposit of 180 g / m ² .

Claims

1 - Ferrous product with metallic coating and improved corrosion resistance, characterized in that the surface coating is in laminated form with, in an iron-zinc-aluminum matrix comprising 10 to 15% of iron and less than 3% aluminum, nodules of a Fe-Al-Zn compound comprising between 60 and 70% iron, 20 to 25% aluminum and the balance of zinc.

2 - Ferrous product according to claim i, characterized in that the nodules are located on the surface of the coating to form a very tormented surface layer, of dark matt gray appearance, which is of excellent quality for the attachment of the paint that she would receive. 3 - Process for obtaining the coating according to one of the preceding claims, characterized in that it consists in covering a ferrous substrate by dipping in a bath of liquid alloy containing between 85% and 97% by weight of zinc, the the remainder consisting of aluminum, adjusting the thickness of the layer of alloy deposited on the substrate, heating the substrate thus coated while the coating is not yet fixed, maintaining the product in temperature during a time interval the length of which is in inverse ratio to the temperature and to cool the product thus treated.

4 - Method according to claim 3, characterized in that the treatment temperature of the coated material is maintained between 580 ° and 670 ° C.