FIELD OF INVENTION
The present invention relates to a new product with excellent corrosion behaviour compared to regular galvanized coating (GI). More particularly the invention relates to Ni-Zn intermetallic layer over the steel substrate.
BACKGROUND OF THE INVENTION
Hot-dip galvanization is the process of applying zinc coating on the steel to protect from environmental damage or corrosion. It is one of the oldest and commercial techniques used for protection of steel against the corrosion. The galvanized coating serves as a barrier protection that guards the steel surface from contaminant attacks and mechanical damage, and provides cathodic protection as zinc acts as a sacrificial anode. It has many advantages against other metallic coatings in terms of longer protection, mode of application and cost. The current developments in hot-dip galvanization are modification of pre-treatments, bath composition, steel chemistry and various post treatment methods [1].
Composition of the molten zinc bath plays major role in development of galvanising coating. Recent studies on this particular area include addition of other alloying elements to the bath including Al, Mg and Ni etc. These elements addition to the bath improves the corrosion performance of the coating. Elements present such as Mn and Si in the recently developed high strength steels may segregate at the surface during continuous annealing and may result in selective oxidation of steel surface causing wettability and bare spot problem during galvanising. This can be minimised by using Ni prior coating before galvanisation [2, 3]. It was reported that Ni-Zn layer with 8-50 wt.% Ni plated on stainless steel [4], Ni and Co plated on high strength steels [5] can improve the zinc wettability to great extent. Another invention reveals the application of a first flash layer containing one or more of Al, Ag, Au, Cr, Cu, Mo, Ni, Sn and Zr adjacent to the steel substrate, and a second flash layer containing Fe for retaining the first flash layer during hot-dip galvanising can reduce the segregation of the alloying elements [6].

The Zn-Ni alloy based coatings have good corrosion resistance and good weldability. Ni addition to the zinc coatings can be done by following process of electro deposition, electroless deposition, PVD coating and addition of Zn-Ni master alloy to the bath [7]. Electro deposition of Nickel on the steel is one of the major routes which can be followed to observe the effect of nickel on the galvanization of steel. Galvanised steel containing nickel in under layer has higher hardness as compared to pure zinc galvanised steel. Addition of 0.06% Ni into the zinc bath has also been reported to be effective in reducing the coating weight. Although, controlling the right composition of the bath imposes the disadvantage of the process of adding Ni to the Zn bath.
It is reasonable to note that nickel can control the diffusion of iron and retard the direct interaction between zinc bath and steel substrate [2]. It can also be anticipated that the incorporated nickel layer may interact with zinc to develop alloys of enhanced corrosion properties. In the case of electrodeposited Zn–Ni, for example, it is known that 8–14 wt.% Ni leads to improvement of the corrosion resistance by 5 times over that of zinc, and the corrosion rate of the alloys generally decreases with Ni content [2]. It is attributed to the Zn-Ni intermetallic compounds which will have higher corrosion resistance.
References
[1]. Shibli S.M.A., Manu R., “Process and performance improvement of hot dip zinc coating by dispersed nickel in the under layer”, Surface and Coating technology, Vol.197., pp 103-108, 2005.
[2]. Zhong .N, Zhang .K, Li .J, and Hu W.B, “Improvement of the Galvanized Coating Quality of High Strength Dual Phase Steels by Pre-Electroplating Nickel Layer”, Steel research int, Vol. 82, No.3, 2011.
[3] Development of an intermediate metallic layer of Ni or Ni-P alloy by electro or electroless plating process on HSS and AHSS to improve zinc coatability, TATA STEEL, Indian Patent Appl. No. 231/KOL/2011 dt 23.02.2011 dated 23-02-2011

[4] Production of hot-dip galvanized stainless steel strip excellent in adhesive strength of plating and corrosion resistance, NISSHIN STEEL, JPH04224666A 1992-08-13.
[5] High strength galvanealed steel sheet with good wettability and adhesion and method for manufacturing the same, POSCO, KR20130131871 A 2013-12-04 [KR20130131871]
[6] Coated steel suitable for hot-dip galvanizing, TATA STEEL, WO2014124749 A1 2014-08-21 [WO2014124749].
[7] Shibli S.M.A., Manu R., Dilimon V.S., “Effect of nickel-rich barrier layer on improvement of hot-dip zinc coating”, Applied surface science, Vol. 245, pp 179-185, 2005.
[8] S. K. Rajagopalan, Characterization of electrodeposited zn-ni alloy coatings as a replacement for electrodeposited zn and cd coatings, Masters Thesis, Department of Mining & Materials Engineering, McGill University, Montreal, Quebec, Canada, August 2012
[9] Steel sheet for hot stamping, and process for manufacturing hot-stamped steel products using steel sheet for hot stamping, JFE Steel Corporation, Publication number EP2602359 A1, Publication date Jun 12, 2013
OBJECTS OF THE INVENTION
With reference to the above mentioned prior art, it is an object of the invention to produce galvanized steel with improved corrosion performance.
SUMMARY OF THE INVENTION
In one aspect, the invention provides a process for manufacturing a galvanized steel product with at least one layer of nickel rich Zn-Ni intermetallic over steel substrate comprising steps of coating a steel substrate with nickel (Ni) of thickness 1.0 µm - 5.0 µm for 30-60 min in a nickel bath; and galvanizing the Ni coated steel substrate in a zinc (Zn) bath for atleast 5 sec.

In another aspect, the invention provides a galvanized steel product comprising a steel substrate as a base layer, one or more Zn-Ni intermetallic layer over the steel substrate out which at least one layer is nickel rich Zn-Ni intermetallic and a zinc layer over the Zn-Ni intermetallic layer.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIG. 1 illustrates various steps of a process for manufacturing a galvanized steel product with a layer of nickel rich Zn-Ni intermetallic in accordance with an embodiment of the invention.
FIG. 2 illustrates a galvanized steel product in accordance with an embodiment of the invention.
FIGS. 3(a-d) shows the SEM images of various nickel electrodeposited steel substrate for (a, b) 10 minute and (c, d) 30 minute nickel plating time in accordance with an embodiment of the invention.
FIGS. 4(a-b) show the top surface average EDS elemental concentration of (a) 10 minutes and (b) 30 minute electro deposited nickel coated steel substrate in accordance with an embodiment of the invention.
FIGS. 5(a-d) show cross sectional optical micrograph for steel substrate nickel coated for (a) 10 minute (b) 30 minute (c) 45 minute and (d) 60 minute and galvanized for 10 seconds in accordance with an embodiment of the invention.
FIGS. 6(a-d) show cross sectional SEM/EDS point analysis for steel substrate nickel coated for (a) 10 minute (b) 30 minute (c) 45 minute and (d) 60 minute and galvanized for 10 seconds in accordance with an embodiment of the invention.
FIGS. 7(a-d) show cross sectional EDS mapping for steel substrate nickel coated for (a) 10 minute (b) 30 minute (c) 45 minute and (d) 60 minute and galvanized for 10 seconds in accordance with an embodiment of the invention.

FIGS. 8(a-d) show cross sectional line elemental intensity for steel substrate nickel coated for (a) 10 minute (b) 30 minute (c) 45 minute and (d) 60 minute and galvanized for 10 seconds in accordance with an embodiment of the invention.
FIGS. 9(a-d) shows elemental depth profile by GDOES for steel substrate nickel coated for (a) 10 minute (b) 30 minute (c) 45 minute and (d) 60 minute and galvanized for 10 seconds in accordance with an embodiment of the invention.
FIGS. 10(a-c) show comparison of the galvanostatic curves for (a) conventional galvanized steel substrate and steel substrate nickel coated for (b) 10 minute (c) 60 minute and galvanized for 10 seconds in accordance with an embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
Various embodiments of the invention provide a process for manufacturing a galvanized steel product with at least one layer of nickel rich Zn-Ni intermetallic over steel substrate, the process comprising steps of coating a steel substrate with nickel (Ni) of thickness 1.0 µm - 5.0 µm for 30-60 min in a nickel bath; and galvanizing the Ni coated steel substrate in a zinc (Zn) bath for atleast 5 sec.
In another embodiment the invention provides a galvanized steel product comprising a steel substrate as a base layer; one or more Zn-Ni intermetallic layer over the steel substrate out which at least one layer is nickel rich Zn-Ni intermetallic; and a zinc layer over the Zn-Ni intermetallic layer.
Shown in FIG. 1 are various steps of a process (100) for manufacturing a galvanized steel product (200) (shown in FIG. 2) with at least one layer of nickel rich Zn-Ni intermetallic over a steel substrate. At step 104, the steel substrate is coated with nickel (Ni) in a Ni bath for thickness 1.0 µm - 5.0 µm for 30-60 min.

The steel substrate is cold rolled steel sheet and its constituents are given in the Table 1.

Cold rolled steel sheet is prepared for nickel coating by electroplating. Before nickel coating, the steel substrate is subjected to surface cleaning by degreasing and pickling. Degreasing is done by alkaline solution which is prepared using 2.5 to 3 wt. % Redoline powder in one litre of distilled water. The degreasing solution is heated up to 60 ͦC and the steel substrate is cleaned. Pickling solution (10 wt.% HCl) is heated up to 50 ͦC and etched for 10 – 15 seconds. In between two steps samples are rinsed in distilled water. After pickling the steel substrate is electroplated with nickel. Electroplating bath composition and operating conditions are given in the following Table 2.


The temperature of the Ni bath is maintained at 60-80C. Two nickel electrodes are used in electro plating in order to have both side plating and purity of nickel electrodes are above 99.99%. The distance between the electrode and steel substrate are maintained at 3 cm.
The electroplating is done for 30-60 min so that coating thickness of nickel (Ni) is developed upto 1.0µm - 5.0µm.
The nickel coated steel substrate is dipped in fluxing solution containing zinc chloride (80%) and ammonium chloride (20%) in order to remove any oxide particles if formed on the coated surface. It helps in getting good coating adhesion.
At step (108), the Ni coated steel substrate is galvanized in a molten/liquid zinc (Zn) bath. The purity of zinc is maintained at 99.995%. The temperature of the molten zinc bath is maintained at 460±5°C. Nickel coated steel substrate are dipped in zinc bath for atleast 5 seconds for galvanizing.
Due to interaction between nickel and zinc, one or more Ni-Zn intermetallic layers are formed between steel and the pure zinc. Out of the many layers, the layers closer to steel have higher nickel composition. Nickel content of at least one of the layer of Zn-Ni intermetallic is >15% (by wt.). The melting point of such layer is around 881C.
Shown in FIG. 2, is the galvanized steel product (200), comprising a steel substrate (204) as a base layer, one or more Zn-Ni intermetallic layer (208a, 208b and 208c) over the steel substrate (204) and a zinc layer (212) over the Zn-Ni intermetallic layer (208).
At least one layer out of the one or more Zn-Ni intermetallic layer is nickel rich having Ni content > 15% (by wt.). Generally the layer close to the steel substrate is Ni rich.
The one or more Zn-Ni intermetallic layer, in other embodiments can be made up of one single layer.
The consolidated thickness of one or more Zn-Ni intermetallic layer (including the Ni rich intermetallic) is 10µm - 20µm.

The nickel electrodeposited layer of thickness of 1.0µm - 5.0µm actually hinders the diffusion of iron, promoting the formation of nickel-zinc intermetallic layers and inhibiting the formation of Fe-Zn intermetallic layers. Since the nickel-zinc intermetallic layers provide superior corrosion resistance as compared to Fe-Zn intermetallic layers, hence the corrosion resistivity property of the steel substrate improves.
Experimental Analysis:
The following experimental analysis should not be construed to limit the scope of the invention.
A composition of cold rolled steel sheet which is used for experiment work was given in the Table 3.

Cold rolled steel sheets of 7×4.5 cm2 for electroplating were prepared. Before coating, samples were subjected to surface cleaning by degreasing and pickling. Degreasing was done by alkaline solution which was prepared using 2.5 to 3.0 wt.% Redoline powder in one litre of distilled water. This solution is heated up to 60 ͦC and sample was cleaned. Pickling solution (10 wt% HCl) was heated up to 50 ͦC and etched for 10 – 15 seconds. In between two steps samples were rinsed in distilled water.

Electroplating bath composition and operating conditions are given in the following Table 4.

Two nickel electrodes were used in electro plating in order to have both side plating and purity of nickel electrodes are above 99.99%. The distance between the electrode and steel sample were maintained at 3 cm.
The steel substrate was coated with nickel (Ni) of thickness .5µm - 5µm for 10, 30, 45 and 60 min in a Nickel bath. The temperature of the Ni bath was maintained at 80 ̊C.
Then nickel coated steel samples were dipped in fluxing solution containing zinc chloride (80%) and ammonium chloride (20%) in order to remove any oxide particles if formed on the coated surface.
Then samples were dipped into molten zinc bath. The purity of zinc was 99.995%. The temperature of the molten zinc bath was kept at 460±5 ͦC. Nickel coated samples, prepared at varying plating time of 10 min, 30 min, 45 min and 60 min., were dipped in zinc bath for 10 seconds.

Characterization and property evaluation of coated steel substrate
Coating Characterization
To characterize the galvanized steel substrate optical microscopy, scanning electron microscopy (SEM) is done to observe the coating thickness, intermetallic layer thickness, intermetallic layer morphology. The compositional analysis is done by using Energy dispersive spectroscopy (EDS).
Glow discharge optical emission spectroscopy (GDOES) for compositional analysis of coatings and galvanostatic and anodic polarization tests is done as well.
FIGS. 3a-3d shows the top surface morphology of the nickel electro-deposited steel substrate. The nickel plating time was (a) & (b) 10 minutes and (c) and (d) 30 minute. The nickel crystallites were much smaller for 10 minutes dipping time whereas it has experienced significant growth for 30 minutes plating time. The higher magnification image FIG. 2(d) shows that the surface coverage was satisfactory with 30 minutes of nickel plating time.
FIGS. 4a-4b shows that average elemental composition of nickel electroplated steel substrate. For nickel electro-deposition time of 10 minutes, the nickel concentration at the top surface was much less, about 15 wt. % whereas for 30 minute nickel plating the nickel concentration was increased to more than 90 wt.%, confirming presence of considerable amount of nickel at the top surface before galvanizing.
FIGS. 5a-5d shows the optical micrograph of galvanised samples having pre electrodeposited nickel layer of (a) 10 minutes (b) 30 minute (c) 45 minute and (d) 60 minute respectively. The optical micrograph indicates the presence of the steel substrate, an intermetallic layer above the steel substrate and overlay zinc on the top of the coating. There are difference in contrast within the intermetallic layer for samples shown in FIGS. 5b, 5c and 5d indicating the presence of different intermetallic layers which is further confirmed by SEM/EDS point analysis.

FIGS. 6a-6d shows the SEM image and corresponding elemental compositions for the points as marked in the image. It is noteworthy that for lower thickness of nickel i.e. for 10 minutes dipping time only, the nickel-zinc intermetallic was not present at the interface rather it was precipitated at various points within the coating as shown in FIG. 6a.
The Ni-Zn intermetallic compositions are shown by black rectangle in the figures. The nickel composition suggested that the probable phase precipitated is δ-phase (7-12% nickel) according to the nickel-zinc equilibrium phase diagram. The Fe-Zn phases are formed near the steel substrate as marked by the red rectangle. Due to lower thickness of nickel the iron diffused in to the zinc and Fe-Zn intermetallic was precipitated.
However if the nickel thickness is gradually increased for (b) 30 minute (c) 45 minute and (d) 60 minute and then the sample is galvanized for 10 seconds then the nickel-zinc intermetallic are formed at the substrate-coating interface. Depending on the thickness of the nickel layer one/two or more than two different nickel-zinc layers have been formed. The nickel zinc intermetallic layer-1 is of δ-phase (8-12% nickel, melting point 490oC) and
nickel-zinc intermetallic layer-2 is of Y-phase (15-25% nickel, melting point 881oC) nickel and the nickel rich layer can be the β1-phase (about 50% nickel, melting point 840oC) and terminal solid solution of zinc in nickel (upto 30% soluble zinc). The Fe-level in the samples shown in FIGS. 6b-6d is very less near the interface and eliminate the possibility of formation of any Fe-Zn intermetallic layer unlike the conventional galvanized sample with Fe-Zn intermetallic at the interface and overlay zinc on top of that.
FIGS. 7a-7d shows the cross sectional SEM/EDS mapping of zinc, nickel and iron near the interface layer. The presence of different intermetallic layers is marked in the corresponding elemental maps. The total intermetallic layer thickness is about 10-20 μm. With increase in nickel plating time the intermetallic layer thickness is increased. The overlay zinc thickness is not captured as this is not a function of nickel coating rather the wiping system which is not varied in the present case.

FIGS. 8a-8d shows the SEM/EDS elemental line scanning for nickel plated galvanized sample. Near the iron substrate the nickel concentration is increased with increase in nickel plating time. The images need to be read from bottom to top i.e. bottom being steel substrate and top being pure zinc. At the lowest time i.e. (a) 10 minute nickel plating, there was very low amount of nickel at the interface whereas the nickel peak intensity becomes stronger with increase in nickel plating time. For the highest plating time i.e. (d) 60 minute considerable nickel was found at the substrate coating interface.
FIGS. 9a-9d shows the GDOES elemental depth profile for nickel coated galvanized samples. The increase in nickel concentration at the interface was confirmed by the GDOES analysis with increase in nickel plating time. The probable phases are identified in the profile as also observed from cross sectional SEM/EDS analysis.
Corrosion Behaviour
Two different electrochemical tests were performed to study the corrosion performance of the product. The galvanostatic experiment was carried out using current density of 0.7 mA/cm2 and the solution used was of the following composition, 250 g/l NaCl, ZnSO4 (50 g/l) and pH: 5).
The conventional samples without any prior nickel coating, were prepared by degreasing and pickling the steel substrate and then dipped in fluxing solution and then dipped in molten zinc bath for 10 seconds.
FIGS. 10a-10c shows the galvanostatic curve for conventional sample as well as nickel plated samples. The constant voltage plateaus indicate the presence of different phases. The conventional galvanized coating reveals one plateau which is of the overlay zinc coating FIG. 10a) which is also present for nickel coated galvanized samples (FIG. 10b and 10c). The plateau at about -1.1 volts indicates that the phase is pure zinc.

With progress of time there are one plateau observed for the conventional sample which is of the Fe-Zn phase. However there are different constant voltages plateaus for nickel coated galvanized samples shown in FIGS. 10b and 10c. The plateaus are at the voltage positive than zinc. So the phases formed are nobler than zinc and will be corrosion resistant as compared to zinc only.
The number of plateaus increases with the increase in nickel plating time from 10 to 60 minutes as already seen that there are three probable nickel-zinc layer present for 60 minutes nickel coated galvanized samples.
Advantages
The invention provides galvanized steel substrate having better corrosion performance. The nickel-zinc intermetallic formed at the substrate-coating interface has higher hardness. The nickel-zinc intermetallic is of higher melting point as compared to the conventional galvanized. Therefore the nickel-zinc intermetallic can be useful in hot stamping application where the liquid metal embrittlement is a serious issue. The nickel coated and then galvanized can be used in hot stamping satisfying all the required properties like.

We claim:
1. A process for manufacturing a galvanized steel product having one or more Zn-Ni
intermetallic layer out of which at least one layer is nickel rich, the process
comprising steps of:
coating a steel substrate with nickel (Ni) of thickness 1.0 µm - 5.0 µm for 30-60 min in a nickel bath; and
galvanizing the Ni coated steel substrate in a zinc (Zn) bath for atleast 5 sec.
2. The process as claimed in claim 1, wherein consolidated thickness of the Zn-Ni intermetallic layer(s) is 10 µm -20 µm.
3. The process as claimed in claim 1, wherein nickel content of nickel rich Zn-Ni intermetallic layer > 15% (by wt.).
4. The process as claimed in claim 1, wherein temperature of the nickel bath temperature for coating the steel substrate is maintained at 60-80 ̊C.
5. The process as claimed in claim 1, wherein zinc (Zn) bath temperature for galvanising the Ni coated steel substrate is 460 + 5 ̊C.
6. The process as claimed in claim 1, wherein composition of the steel substrate is (in wt. %) carbon (C) 0.05 - 0.06, phosphorus (P) 0.0100 - 0.0110, Sulphur (S) 0.0105 - 0.0110, silicon (Si) 0.0125 - 0.0135, vanadium (V) 0.0120 - 0.0130, chromium (Cr) 0.015 - 0.016, iron & unavoidable impurities rest.
7. The process as claimed in claim 1, wherein the steel substrate is cold rolled.
8. The process as claimed in claim 1, wherein the nickel coating over the steel substrate is done by electroplating.
9. A galvanized steel product comprising:
a steel substrate as a base layer;
one or more Zn-Ni intermetallic layer over the steel substrate out which at least one layer is nickel rich; and
a zinc layer over the Zn-Ni intermetallic layer.

10. The galvanized steel product as claimed in claim 9, wherein consolidated thickness of one or more Zn-Ni intermetallic layers) is 10 urn -20 urn.
11. The galvanized steel product as claimed in claim 9, wherein nickel content of nickel rich Zn-Ni intermetallic > 15% (by wt).
12 The galvanized steel product as claimed in claim 9, wherein composition of the steel
'substrate(in wt.%)carbon(C)0.05-0.06, phosphorus (P) 0.0100 - 0.0110,
Sulphur (S) 0.0105 - 0.0110, silicon (Si) 0.0125 - 0.0135, vanadium (V) 0.0120
- 0.0130, chromium (Cr) 0.015 - 0.016, iron & unavoidable impurities rest.
13. The galvanized steel product as claimed in daim 9, wherein the steel substrate is cold rolled.