WO2021115863A1

WO2021115863A1 - A process for the treatment of galvanized steel

Info

Publication number: WO2021115863A1
Application number: PCT/EP2020/084217
Authority: WO
Inventors: Monica Santamaria; Francesco DI FRANCO; Bartolomeo MEGNA; Barbara SEMINARA
Original assignee: Universita' Degli Studi Di Palermo
Priority date: 2019-12-11
Filing date: 2020-12-02
Publication date: 2021-06-17
Also published as: IT201900023661A1

Abstract

A process for covering galvanized steel with a ZnSn(OH)₆coating by immersion in a bath containing stannate ions under conditions of pH comprised between (12) and (13) and of temperature comprised between 70°C and 90°C until galvanized steel coated with a ZnSn(OH)₆ film is obtained as well as the products obtained and the use thereof for the concrete reinforcement are described.

Description

A PROCESS FOR THE TREATMENT OF GALVANIZED STEEL

Background of the invention

The present invention refers to the chemical sector and more precisely to a process for the treatment of galvanized steel in order to improve the adhesion between galvanized steel rods and concrete in reinforced concrete without compromising the active protection provided by zinc against corrosion.

Background Galvanized reinforcements are used in reinforced concrete structures exposed to aggressive environments such as industrial and/or marine atmospheres in order to increase the service life thereof. The zinc coating on the rods of the reinforcements acts as a barrier preventing the direct contact with the aggressive environment, the zinc corrosion products have a sealing effect on the discontinuities of the coating, improving the properties thereof and above all the zinc coating performs an active protection as a sacrificial anode for galvanic coupling [R A Cottis; L L Shreir, Shreir's corrosion, Amsterdam; London:Elsevier, 2010].

Zinc is an amphoteric metal that can dissolve as an ionic species in an acidic and basic environment. According to the Zinc Pourbaix diagram, zinc dissolves as Zn2+ in an acid 12 environment while it dissolves as galvanized ions, [Zn(OH) in an alkaline environment. In this last case the dissolution continues until the solution becomes supersaturated of these ions, thus the Zn(OH)₂ (or ZnO) precipitation takes place which form the passive film.

Galvanizing is commonly carried out on carbon steel to give it an active protection, i.e. a cathodic protection for sacrificial anode, as well as a physical barrier type protection. Hot dip galvanizing of steel can significantly extend the life of the construction exposed to carbonation or mild chloride contamination. The galvanized coating forms a physical barrier that prevents the contact of aggressive agents with the steel substrate and the zinc acts as a sacrificial anode, protecting the steel against corrosion.

The hot dip galvanizing process of steel rods features the following steps: i. Degreasing in which foreign substances such as oils or greases deriving from steel processing are removed, by immersion in a tank containing an acidic degreasing detergent in an aqueous solution at 35°C for about 15 minutes; ii. pickling, i.e. reaction with a strong inorganic acid, which removes from the surface of the metal the oxides that have formed after the hot processing of the metal, or any corrosion products, by immersion in a tank containing hydrochloric acid and water at 16% with the addition of pickling inhibitors, for a variable time; iii. washing in which the manufactured product is rinsed in water to avoid transporting acids in the subsequent tanks; iv. flushing in which the piece is immersed in a heated tank at 38°C containing a solution of zinc chloride, ammonium chloride and potassium chloride, and during this step a protective layer is created on the surface of the piece that prevents oxidation as long as the latter is immersed in the tank of molten zinc; v. preheating by immersion of the manufactured product in an insulated chamber with an average temperature of about 130°C in which the protective layer formed in the flushing tank has time to dry, and the humidity present on the surface of the piece evaporates; vi. galvanizing in which the prepared manufactured product is immersed in a tank containing liquid zinc at an average temperature of 445°C; different thicknesses will be obtained with respect to the duration of stay in the liquid zinc, which will increase as the immersion time increases, the bath has a purity of 98.7%; at each immersion, slags are formed which remain afloat and are removed manually before extracting the pieces; vii. cooling in which, before storage, the pieces are cooled in the air.

The use of galvanized reinforcements in reinforced concrete has two main drawbacks: the modification of the mechanical properties of steel due to the exposure in a molten zinc bath at high temperature, during zinc deposition, in hot dip galvanizing and loss of adhesion between the galvanized rods and the concrete due to the development of hydrogen during the hardening of the concrete and due to the modification of the profile of the ribs due to the corrosion of the metal.Both of these phenomena are caused by the initial corrosion processes involving the dissolution of Zinc as an anodic reaction and the reduction of water with consequent development of hydrogen as a cathodic reaction [R. Pernicova, D. Dobias, P. Pokorny, "Problems connected with use of hot-dip galvanized reinforcement in concrete elements" Procedia Engineering 172 (2017) 859 - 866; A.Cheng, R.Huang, J.K.Wu, C.H.Chen, Effect of rebar coating on corrosion resistance and bond strength of reinforced concrete Construction and Building Materials Volume 19, Issue 5, June 2005, Pages 404-414].

Zinc is an amphoteric metal, stable over a wide pH range (6- 13).The pH values of the solution present in the pores of concrete, in the first hours of the casting, is higher than 13, and under these conditions zinc reacts with the water of the wet concrete generating zinc hydroxide which interacts with Ca(OH)₂, freed from the hydration of the cement, giving rise to calcium hydroxy-zincate with the development of hydrogen. During this step, a consumption of the zinc layer of about 10 mpimay occur, and furthermore, the developed hydrogen can increase the porosity at the interface between the reinforcement and fresh concrete, causing a reduction in the cohesion between the galvanized reinforcement and the concrete.

Surface treatments based on Cr(VI) species as active inhibitors are known in the art, to solve said problems, but today their use is not recommended as they are carcinogenic and harmful to the environment.

A passivation process based on molybdenum-phosphorus compounds of hot-dip galvanised coatings that forms a hydro-galvanised film on the surface is also known. [D. D. N. Singh, R. Ghosh, Molybdenum-phosphorus compounds based passivator to control corrosion of hot dip galvanized coated rebars exposed in simulated concrete pore solution, Surface and Coatings Technology, Volume 202, Issue 19, 25 June 2008, Pages 4687- 4701].

Technical problem

As previously explained, galvanizing could also be used for reinforcement rods in reinforced concrete but this has the side effect of decreasing the adhesion of the steel rods to the concrete, causing problems to the structure.

In light of what is known in the art, the inventors of the present invention have developed a surface treatment for galvanized steel rods, obtained with the processes known in the art, which allows to overcome the known drawbacks, and in particular to reduce corrosion phenomena and improve the adhesion of galvanized steel to concrete, without modifying the cathodic protection action induced by the zinc coating. The process of the present invention does not require the use of electricity and is carried out without the aid of dangerous reagents such as chromates [Alberto FRANCHI, Romeo FRATESI, Giacomo MORICONI, Giovanni A. PLIZZARI, CARATTERISTICHE MECCANICHE E DI ADERENZA AL CALCESTRUZZO DI BARRE DI ARMATURA IN ACCIAIO ZINCATO, AICAP 99 Days - Turin, 4-6 November 1999. b) Standard ASTM A 767/A 767/M-90].The process of the present invention can easily implement the known galvanizing processes and plants.

Object of the invention

With reference to the attached claims, the technical problem is therefore solved by providing a process for the treatment of galvanized steel which envisages immersing the galvanized steel in a bath containing stannate ions under conditions of pH comprised between 12 and 13 and of temperature comprised between 70°C and 90°C until galvanized steel coated with a ZnSn(OH)₆ film is obtained.

A further object of the present invention are the processes for galvanizing steel which comprise as a last step the immersion of galvanized steel in a bath containing stannate ions under conditions of pH comprised between 12 and 13 and of temperature comprised between 70°C and 90°C until galvanized steel coated with a ZnSn(OH)₆ film is obtained.

The galvanized steel coated with a ZnSn(OH)₆ film obtained by the process of the present invention is also the object of the present invention.

The use of galvanized steel coated with a ZnSn(OH)₆ film obtained by the process of the present invention to reinforce concrete structures, as well as concrete structures reinforced with galvanized steel coated with a ZnSn(OH)₆ film obtained by the process of the present invention is also an object of the present invention.

Further characteristics of the present invention will be clarified from the following detailed description, with reference to experimental data reported and the appended figures.

Brief description of the figures Figure 1 graphically shows the dependence of the open circuit potential of the zinc specimens, UOC, with the time during the chemical conversion process.

Figure 2 graphically shows the variation in mass as a function of the immersion time of the zinc samples during the chemical conversion process.

Figure 3 shows the diffractogram relating to the zinc samples on which the conversion layer was grown by means of a chemical conversion bath, for different immersion times. Figure 4 shows the Raman spectra relating to the zinc samples on which the conversion layer was grown by means of a chemical conversion bath for different immersion times.

Figure 5 shows the open circuit potential (corrosion potential) of galvanized steel samples with and without conversion treatment in a chloride-free concrete simulating solution.

Figure 6 shows the open circuit potential (corrosion potential) of galvanized steel samples with and without conversion treatment in a chloride-free concrete simulating solution.

Figure 7 shows the electrochemical impedance spectra recorded at the open circuit potential for galvanized steel samples with and without conversion treatment in a chloride-free concrete simulating solution. Figure 8 shows the electrochemical impedance spectra recorded at the open circuit potential for galvanized steel samples with and without conversion treatment in a chloride-free concrete simulating solution.

Figure 9 shows the polarization curves recorded for galvanized steel samples with and without conversion treatment in a chloride-free concrete simulating solution. Figure 10 shows the polarization curves recorded for galvanized steel samples with and without conversion treatment in a chloride-free concrete simulating solution.

Figure 11 shows the polarization curves recorded for galvanized steel samples with and without conversion treatment in a chloride-free concrete simulating solution.

Detailed description of the invention

The present invention relates to a process for the treatment of galvanized steel which envisages immersing the galvanized steel in a bath containing stannate ions under conditions of pH comprised between 12 and 13 and of temperature comprised between 70°C and 90°C until galvanized steel coated with a ZnSn(OH)₆ film is obtained.

Preferably the bath containing stannate ions comprises:

NaOH in a concentration comprised between 5 and 10 gl^_1,

K₂Sn0_3*3H₂0 in a concentration comprised between 25 and 50 gl^_1,

NaC₂H₃0₂ ·3H₂0 in a concentration comprised between 5 and 10 gl 1

Na₄P₂0₇ in a concentration comprised between 25 and 50 gl^_1,

More preferably, the bath containing stannate ions has the following composition

NaOH 7.5 gl¹

K₂Sn0₃·3H₂037.5 gl^_1

NaC₂H₃0₂ ·7.5 gl^_1

Na₄P₂0₇ 37.5 gl

Preferably the process for the treatment of galvanized steel is carried out at a temperature of 80°C.

Preferably the process for the treatment of galvanized steel is carried out at a pH value of 12.5. Preferably the process for the treatment of galvanized steel is carried out for a time comprised between 30 to 90 minutes, more preferably between 30 and 60 minutes, still more preferably for 30 minutes.

A further object of the present invention are the processes for galvanizing steel which further comprise a last step that permits the immersion of galvanized steel in a bath containing stannate ions under conditions of pH comprised between 12 and 13 and of temperature comprised between 70°C and 90°C until galvanized steel coated with a ZnSn(OH)₆ film is obtained.

Preferably, the process for the galvanization of steel comprises the following steps: a)Degreasing by immersion in an acidic degreasing detergent in an aqueous solution b)pickling in the presence of an aqueous solution of an inorganic strong acid and pickling inhibitors c)washing in water d)flushing by immersion in a solution of zinc chloride, ammonium chloride and potassium chloride, e)preheating at a temperature comprised between 120°C and 140°C; f)galvanizing in which by immersion in liquid pure zinc at a temperature comprised between 435°C and 455°C, g)cooling; h)immersion of the galvanized steel in a bath containing stannate ions under conditions of pH comprised between 12 and 13 and of temperature comprised between 70°C and 90°C until galvanized steel coated with a ZnSn(OH)₆ film is obtained.

Preferably in step e) temperature is equal to 130°C.

Preferably in step f) temperature is equal to 445°C. The galvanized steel coated with a ZnSn(OH)₆ film obtained by the process of the present invention is also the object of the present invention.

The use of galvanized steel coated with a ZnSn(OH)₆ film obtained by the process of the present invention to reinforce concrete structures, as well as the concrete structures reinforced with galvanized steel coated with a ZnSn(OH)₆ film obtained by the process of the present invention is also an object of the present invention. Examples

Pure zinc electrodes (99.95% Goodfellow Metal, Cambridge) were used as rods (7mm in diameter) and as foils (150 pm thick).The rod electrodes were incorporated in a Teflon cylinder and sealed with an epoxy resin (Torr Seal Varian Ass.), leaving an area of metal zinc equal to 0.385 cm² exposed. Before each experiment, the surface of the electrodes was mechanically cleaned with abrasive papers using papers of different finishes (320, 800 and 1200 Grit) supplied by LECO®; the samples were subsequently degreased in an ultrasonic bath containing de-ionized water for 5 min. Then a pickling was carried out in a 39.5 vol.% HF acid solution for 30 seconds at room temperature and subsequently the sample was rinsed in deionized water and dried. The ZnSn(OH)₆ films were grown by means of a chemical conversion process by immersing the galvanized steel rods in a bath containing stannate ions whose composition is reported in table 1.

Table 1

During the immersion in the bath, the curves of the open circuit potential vs. time were recorded using a mercury/mercury oxide (Hg/HgO) reference electrode. The samples were then rinsed with deionized water and dried. Gravimetric tests were performed to measure the mass of the samples before and after conversion; measurements were made for a minimum of three samples for each type of grown layer. The morphologies of the obtained layers were analysed by Scanning Electron Microscopy (SEM) using a Philips ESEM microscope (operating at 30 keV) coupled to an EDX (Energy dispersive X-Ray) analyzer. X-ray diffractograms were recorded using an X-ray diffraction (XRD) diffractometer which uses CuKa (Pan Analytical Empyrean) radiation. Raman spectra on grown coatings were recorded at room temperature using a Raman Renishaw Invia spectrometer equipped with a microprobe (5Ox) and a CCD detector. The sample was irradiated with a solid state laser (Nd:YAG) at 532 nm. The power of the beam incident on the sample was 5 mW and the width of the spot used for the analysis was 2 pm. Raman analyses were carried out in different areas of the surface of each sample in order to verify the homogeneity of the coating. The polarization curves were recorded at 0.5 mV s^_1 in borate buffer (pH = 8.9) and in 0.1 M NaOH (pH 12.5) at room temperature. A three-electrode cell configuration with a Pt wire as counter electrode and mercury/mercury oxide (Hg/Hg0₉₎ as a reference electrode was used.In order to verify whether the conversion treatment had increased the adhesion between the cement conglomerate and the reinforcement, pull-out tests were carried out.150 mm cubic concrete specimens were prepared and axially reinforced with a 12 mm round bar for reinforced cement. The samples were tested in the Zwick/Roell machine setting a preload of 300 N and a preload speed equal to 10 mm/min. The specimen was placed on a perforated plate in the middle through which the round bar for reinforced cement could pass. This test makes it possible to estimate the slippage of the reinforcement rod with respect to concrete, evaluating the raising of the crosspiece with respect to the force imposed by the machine. To accelerate the cathodic process of hydrogen development, two samples were immersed in the solution that simulates the pores of the fresh concrete and a current was applied thereto. Four DSAs (dimensionally stable anodes) were applied to the four faces of the specimen to carry out the anodic process on them. The rod immersed in the concrete was made to work cathodically in order to speed up the hydrogen development process. The test was carried out for 2 hours by imposing a voltage equal to 50 V and by connecting a resistance equal to 10 W in series, it was possible to measure the circulating current. Figure 1 shows the dependence of the open circuit potential of the zinc specimens, UOC, with the time during the chemical conversion process. In particular, in the first 5 minutes of immersion the open circuit potential is approximately constant (equal to -1.35 V vs.Hg/HgO); in the following 10 min there is a slight increase, reaching a stationary value equal to - 1.32 V vs.Hg/HgO after an immersion equal to 120 min. This indicates the rapid formation of a conversion layer on the surface of the zinc. At high pHs near the surface of the zinc electrode, zinc dissolves as galvanized in accordance with the following anodic reaction:

Zn+30H ® HZn0₂ +H₂0 +2e (la)

The balance potential of this reaction is Eeq = - 1.59 V (Hg/HgO) considering a pH equal to 12.5 and an ion activity equal to 85°C. The following cathodic processes can be associated with the zinc dissolution reaction:

0₂ +4e + 2H₂0 ® 40H (lb)

2H₂0 +2e ®H₂+ 2OH (lc) whose thermodynamic potentials assuming a pH equal to 12.5 and T = 85°C are respectively 0.25 V vs.Hg/HgO and -0.98 V vs.Hg/HgO.

The high overvoltage values for oxygen development and the occluded geometry of the cathodic surface determined by the morphology of the coating suggest that the reaction (lb) is under kinetic control as to material transport and that consequently the overall cathodic overvoltage is attributable to the hydrogen development process. Due to the increase in the concentration of galvanized ions, the corrosion process is followed by the ZnSn(OH)6 precipitation in accordance with the following reaction:

HZnO + SnO/ + AH/) ® ZnSn(OH)₆ i +30H _{1) The samples were weighed before and after immersion in the conversion bath in order to evaluate the possible variations of the mass thereof. Figure 2 shows the variations in the mass of the coating as a function of the time for the immersed samples. In the first 60 min, the mass of the coating increases approximately linearly with the immersion time (with a value equal to ~ 0.047 mg cm-2 min-l.In subsequent times the mass of the samples turns out to be almost constant (about 2.8 mg cm ²) although a slight decrease is observed at 90 min and 120 min.

The mass loss can be attributed to zinc dissolution not followed by ZnSn(OH)₆ precipitation, due to the reduction in the concentration of stannate ions near the electrode surface and to the difficulty of diffusive transport of stannate ions from the bulk of the electrolyte to the surface of the electrode which increases as the time of the conversion process increases. The morphology of the grown conversion layers on zinc samples after 5 and 30 min of immersion in the chemical conversion bath were evaluated with SEM micrographs.

After 5 min of immersion in the conversion bath, the surface of the zinc appears partially covered by crystals, whereas this is completely covered after 30 min of immersion. Suggesting that the nucleation of ZnSn(OH)₆ crystals is triggered in some areas of the sample and the corrosion of the substrate further promotes the growth of the crystal by forming a layer formed by truncated octahedra.

The morphology of the conversion layers grown on zinc samples after 5 (a), 15 (b), 30 (c) and 60 (d) min of immersion in the chemical conversion bath was evaluated with SEM micrographs.

The crystal size increases as the time increases up to 60 min. The increase in crystal size for conversion times higher than 30 min takes into account the increase in mass measured by the gravimetric tests up to 60 min despite the completely covered surface indicating that the nucleation process has stopped. The growth process stops when the zinc dissolution no longer occurs due to the formation of a layer on the surface that does not allow the electrolyte to come into contact with the metallic Zinc (for conversion times ³ 90 min, see fig. 2).EDX analyses indicate that the conversion layer is composed of oxygen, zinc, tin with an atomic ratio Zn/Sn close to unity, while the hydrogen contained in the layer cannot be measured. In order to identify the composition of the coating, an XRD and Raman characterization was performed as a function of the conversion time.

From the diffractogram of figure 3 the peaks relating to the substrate (Zinc and Zinc Oxide) and different characteristic peaks relating to ZnSn(OH)₆ are identified in accordance with the ICCD card.73-2384.The relative intensity of peaks suggests that [200] is the preferred orientation of growth.

Figure 4 shows the Raman spectra relating to the conversion layers grown on the zinc substrate as a function of the time of the conversion process. The vibrational modes at 297, 374 and 608 cm-1 for ZnSn(OH)₆ originate from the M-OH and M-OH-M bonds.

Figures 5 and 6 show the trend of the open circuit potential as a function of the time of galvanized steel samples without and with the chemical conversion treatment, in a solution simulating concrete in the absence and presence of chlorides. The corrosion potential measured for the treated galvanized rods is equal to or even more cathodic than that measured for untreated galvanized rods; this confirms that the proposed chemical conversion treatment does not compromise the cathodic protection imparted by galvanizing.

Figures 7 and 8 show the EIS spectra recorded at the open circuit potential in the absence and in the presence of chlorides for galvanized steel and treated galvanized steel samples. In both cases, from the EIS spectra (Electrochemical Impedance Spectroscopy) a higher Polarization Resistance is measured in the case of galvanized steel rods with chemical conversion treatment; this indicates a higher corrosion resistance of the treated samples.

Figures 9 and 10 compare the polarization curves recorded in the absence and presence of chlorides for the galvanized steel and treated galvanized steel samples. The polarization curves show a more cathodic corrosion potential and a lower passivity current in the case of the treated samples.

Figure 11 reports the data obtained from the tests indicating in the ordinates the tangential stresses t in (MPa) at the steel-concrete interface and in the abscissas the recorded creeps s (in mm).The obtained results highlight that the maximum shear stress withstood by the specimens with rods immersed in the conversion bath for 30 minutes (AZT30) is twice that of galvanized steel rods (AZ), but also slightly higher than those with steel rods (A).The failure mode of all the tested specimens is the slippage of the rod except for some treated specimens in which, with values of shear stresses and creeps higher than the previous ones, the concrete breaks. This phenomenon can be attributed to the fact that these specimens exerted a very high adhesion force, and consequently, once the strength of the concrete was reached, they broke.

Claims

1.Process for the treatment of galvanized steel providing immersion of galvanized steel in a bath containing stannate ions at pH comprised between 12 and 13 and temperature comprised between 70°C and 90°C to obtain galvanized steel coated by a ZnSn(OH)₆ film wherein the bath containing stannate ions comprises

NaOH in an amount comprised between 5 and 10 gl^_1

K₂Sn0_3*3H₂0 in an amount comprised between 25 and 50 gl^_1

NaC₂H₃0₂ ·3H₂0 in an amount comprised between 5 and 10 gl^_1

Na₄P₂0₇ in an amount comprised between 25 and 50 gl^_1

2.Process for the treatment of galvanized steel according to claim 1 wherein the bath containing stannate ions comprises

NaOH 7.5 gl¹

K₂Sn0₃·3H₂037.5 gl^_1

NaC₂H₃0₂ · 7.5 gl^_1

Na₄P₂0₇ 37.5gl

3.Process for the treatment of galvanized steel according to claim 1 carried out at a temperature of 80 °C.

4.Process for the treatment of galvanized steel according to claim 1 carried out at pH 12.5.

5.Process for the treatment of galvanized steel according to claim 1 carried out for a time comprised between 30 and 90 minutes.

6.Process for the treatment of galvanized steel according to claim 5 carried out for a time comprised between 30 e 60 minutes.

7.Process for the treatment of galvanized steel according to claim 6 carried out for a time of 30 minutes.

8.Process for the galvanization of steel comprising a final step of immersion of galvanized steel in a bath containing stannate ions at pH comprised between 12 and 13 and temperature comprised between 70°C and 90°C to obtain galvanized steel coated by a ZnSn(OH)₆ film wherein the bath containing stannate ions comprises

NaOH in an amount comprised between 5 and 10 gl^_1

K_2Sn0_3*3H₂₀ in an amount comprised between 25 and 50 gl^_1

NaC₂H₃₀₂·3H₂₀ in an amount comprised between tra 5 and 10 gl-¹

Na₄P₂₀₇ in an amount comprised between tra 25 and 50 gl^-1

9.Process for the galvanization of steel according to claim 8 comprising the following steps: a) degreasing by immersion in a acidic degreasing detergent in an aqueous solution b) pickling in the presence of an aqueous solution of an inorganic strong acid and pickling inhibitors c) washing in water d) flushing by immersion in a solution of zinc chloride, ammonium chloride and potassium chloride e) preheating at a temperature comprised between 120 and 140°C; f) galvanization by immersion in liquid pure zinc at a temperature comprised between 435°C and 455°C; g) cooling; h) immersion of the galvanized steel as obtained at the end of step g) in a bath containing stannate ions at pH comprised between 12 and 13 and temperature comprised between 70°C and 90°C to obtain galvanized steel coated by a ZnSn(OH)₆ film.

10. Process for the galvanization of steel according to claim 9 wherein in step e) temperature is 130 °C.

11. Process for the galvanization of steel according to claim 9 wherein in step f) temperature is 445 °C.

12. Galvanized steel coated by a ZnSn(OH)6 film as obtained by a process comprising the following steps: a) degreasing by immersion in a acidic degreasing detergent in an aqueous solution b) pickling in the presence of an aqueous solution of an inorganic strong acid and pickling inhibitors c) washing in water d) flushing by immersion in a solution of zinc chloride, ammonium chloride and potassium chloride e) preheating at a temperature comprised between 120 and 140°C; f) galvanization by immersion in liquid pure zinc at a temperature comprised between 435°C and 455°C; g) cooling; h) immersion of the galvanized steel as obtained at the end of step g) in a bath containing stannate ions at pH comprised between 12 and 13 and temperature comprised between 70°C and 90°C to obtain galvanized steel coated by a ZnSn(OH)₆ film.

13. Use of the galvanized steel coated by a ZnSn(OH)6 film of claim 12 to reinforce concrete structures.

14. Concrete reinforced with the galvanized steel coated by a ZnSn(OH)6 film of claim 12.