FIELD OF THE INVENTION
The present invention generally relates to coating of an article with a metallic aluminium based alloy coating. The aluminium based alloy coating making the article more corrosion resistant in atmospheric, marine and soil environment. More particularly, the invention relates to metallic alloy coating with enhanced corrosion resistance.
BACKGROUND OF THE INVENTION
Corrosion of article preferably made of iron and / or steel has increased in recent times due to ever increasing industrialization and release of more and more corrosive gases and toxic waste in the atmosphere, water bodies and soil. The sacrificial coatings like zinc and zinc alloy coatings corrode at a faster rate resulting in lower overall life cycle of coated steel structure.
Iron and / or steel structures are protected by cathodic protection route. The potential of iron and/or steel is 600 mV and by providing additional electric current OR by galvanic coupling i.e. using sacrificial materials like zinc (having potential of 1200 mV), the potential of iron and/ or steel is brought down below 750 mV to reduce its corrosion. Zinc or other such sacrificial material corrodes in preference to iron and / or steel.
The process of galvanic coupling of iron and / or steel substrate can be done
by one of the following routes:
Hot dip galvanizing
Vapour deposition
Electro deposition
Thermal spraying route

Hot dip galvanizing of steel sheet is known from long time and people have used different combinations of Zinc, zinc-aluminium and zinc-aluminium with third element like magnesium or copper or nickel to impart corrosion resistance. The primary element of hot dip galvanizing is zinc with excess 70 wt.% or more in the coating, as reported in U.S. Pat. Nos. 3,505,043, US6235410B1, JPA-8-60324, JPB-64-8702 and JPB-64-11112. Most of the continuous hot dip galvanized material is used in construction, fencing or automotive application where atmospheric corrosion resistance is most important. Also, hot dip galvanizing of sheet metal is widely used but the process becomes expensive and difficult when the article to be coated is of complicated shape. The wiping of extra molten material becomes difficult and it can result in excess loss of material. Further, addition of material like Al and Mg in molten zinc is difficult and also they tend to oxidize very fast resulting in material loss and poor surface coating quality.
Vapour deposition and electro deposition, as reported in Patent No. US5135817A and US6214420B1 respectively, are both energy intensive, costly processes and difficult to scale up for large sized components. Also, electro deposition process produces sludge which needs additional treatment before discharge.
Thermal spraying route is popular in industry for metallizing due to its
versatile nature and easy of application. Pseudo coating route is generally
adopted to produce alloy coatings with varying composition to provide
sacrificial property to iron / steel using zinc based alloy coatings as reported
in Patent Nos. US20160195216A1 and 501/DEL/1999/243606. However,
there is a need to develop more corrosion resistant coating which can ascertain higher life of critical structural components like bridges, DI pipes, etc.

OBJECT OF THE INVENTION
An object of the invention is to design an alloy coating which can provide significantly higher corrosion resistance compared to traditional Zinc and zinc alloy coatings, especially in marine and soil environment.
Another object of the invention is to propose metallic alloy coating with enhanced corrosion resistance.
SUMMARY OF THE INVENTION
A coating on an article, preferably made of iron / cast iron / steel, with Aluminium preferably in the range of 50 or more weight percent and specifically in range of (51 to 75) weight percent, while zinc being the second element in coating preferably in the range of 20 to 49 weight percent and specifically in the range of (21 to 46) weight percent and magnesium and/ or silicon and / or nickel being the third element of coating, preferably in the range of 0 to 5 weight percent and specifically in the range of 0.01 to 4 weight percent, to yield higher corrosion resistance compared to traditional sacrificial zinc and zinc alloy coatings. The coating is deposited by preferably arc spraying route using twin wire arc spray technique.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 substrate, preferably iron / cast iron / steel is shown as 1 and
the alloy coating described in the invention is shown as 2
Fig. 2 Cross section of Zn-Al-Mg coating
Fig. 3 Comparative corrosion rate of Zn-Al and Zn-Al-Mg coatings in
different simulated corrosive media
Fig. 4 Comparative behaviour of Zn-Al-Mg and Zn-Al samples in SST test

DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to aluminium alloy coating to improve corrosion resistance of iron/cast iron/steel article. Aluminium alloy coating is produced through in-flight alloying using the preferable method of twin wire arc spraying. In present case one wire was of Zn-Al where the weight of aluminium varies from (0.3 to 22) % and other wire were of Al-Mg / Al-Si where the weight of magnesium varies from (3 to 5)%.
In the invented coating, aluminium acts as barrier coating and also acts as a
source of highly stable and chemically inert aluminium oxide. Zinc acts as a
sacrificial material as compared to substrate while the third component like
magnesium and / or silicon and / or nickel acts as a filler material which
forms stable oxides and plugs the pores in the coating. Magnesium in
addition to zinc acts as a sacrificial material and its corrosion products are
also highly stable. These coatings can contain zinc and aluminium rich
phases. Magnesium often is found in combination with aluminium and
thereby imparts additional sacrificial property to aluminium rich phase.
In the present work, the corrosion data for the synthesized coatings were tested in Salt spray test as per ASTM standard B117, tafel test and weight loss method. The comparative study of data with conventional zinc coatings shows the improved behaviour of the invented coating.
Example:
Metallized coating was sprayed using twin wire arc spray system to achieve aluminium preferably within range of 54 to 75 wt.%, zinc preferably in the range of 21 to 45 wt.% magnesium preferably in the range of 1.2 to 4 wt.% and silicon preferably in the range of 0.2 to 1 wt.%. The as sprayed coating was compared with as sprayed zinc coating as reference material.

Tafel test is done by varying the voltage across an area of 1 cm2 of sample in a corrosive media. The voltage is scanned at rate of 1mV/sec from anodic to cathodic region. The potential vs current density diagram provides critical information about the mechanism of corrosion of coating and the corrosion rate is determined through a formula.
In our case, to simulate marine conditions, the tests were conducted in 3.5% NaCl solution and to simulate soil conditions the tests were are also conducted in 2 different pore solutions. The composition of pore solutions is given below in Table 1.
Table 1 Different types of pore solution to simulate soil conditions in Tafel test

Pore solution KCl (g/l) CaCl2 (g/l) MgSO4 (g/l) NaHCO3 (g/l)
Type 1 0.5 0.5 0.5 0.5
Type 2 1 1 0.5 0.2
The SEM cross section of the coating is shown in Figure 2. One can observe the interconnected lamellar structure of zinc and aluminium rich phases constituting the coating. The lighter lamella are zinc rich while the darker ones are aluminium rich.
Fig. 2 Cross section of Zn-Al-Mg coating
The corrosion rate, as determined from the tafel test is shown in Figure 3. The data shows that there is about 29 to 60 percent reduction in corrosion rate of Zn-Al-Mg alloy coating in comparison to Zn-Al coating. The increase in corrosion resistance for the Zn-Al-Mg coating is due to the formation of Al2O3.MgO product which fills the pores and stops the percolation of corrosive media.

Fig. 3 Comparative corrosion rate of Zn-Al and Zn-Al-Mg coatings in different simulated corrosive media.
The salt spray test was performed as per ASTM B117 standard. The results
are shown in Table 3 and Figure 4.
Fig. 4 Comparative behaviour of Zn-Al-Mg and Zn-Al samples in SST test
Table 3 SST visual observations for Zn-Al-Mg and Zn-Al coating

Observation Zn-Al (hrs) Zn-Al-Mg (hrs)
5% white rust 10 12
100% white rust 36 360
5% Red rust 450 1700
50% Red rust 1240 2440
100% Red rust 1820 3350
The results show that the time taken by Zn-Al-Mg sample to get covered by
white rust is ten times higher than Zn-Al. This is mainly attributed to
presence of Al and Mg compounds on the top surface. Similarly, the time to red rust of samples also shows nearly 50% higher time for Zn-Al-Mg coating as compared to Zn-Al.
Zn-Al-Mg and Zn-Al samples were dipped in 3.5 % NaCl solution for 7 days and weight loss was reported for the samples. The samples were cleaned in acetone solution under ultrasonigation effect at room temperature for 5 minutes to ensure that the corrosion products sticking on the surface are uniformly removed for all the samples. The initial and final weight of samples is reported in Table 4.
Table 4 Weight loss measurement of samples in NaCl

Sample Zn-Al Zn-Al-Mg
Initial weight (g) 112.45 113.42
Final weight (g) 112.35 113.35
∆W (g) 0.10 0.07

The weight loss results of coatings in NaCl solution are having similar trend as the tafel test results. Zn-Al-Mg reports about 30% lower weight loss as compared to Zn-Al. This shows that the corrosion products formed on Zn-Al-Mg coating are much more stable and adherent as compared to corrosion product of Zn-Al coating. On basis of SST, tafel and weight loss results one can conclude that Zn-Al-Mg coating is 1.3 to 1.6 times superior than Zn-Al coating.
Advantages:
The invented coating provides following advantages over conventional zinc metallized coatings:
1. Substantially higher corrosion resistance as compared to zinc coatings in simulated marine and soil environment.
2. Process modification makes it easy to make the coating in one step.
3. In flight alloying plays critical role in providing a balance of sacrificial and barrier property to the coating.
4. Higher life cycle time of coated iron/cast iron/steel structure can be achieved.

WE CLAIM :
1. A coating composition for a substrate, the coating composition
comprising (in wt.%):
aluminium (Al) >50, zinc (Zn) 20 - 49 and at least one of magnesium (Mg), silicon (Si), nickel (Ni) being 0 – 5, rest- unavoidable impurities.
2. The coating composition as claimed in claim 1, wherein the substrate is preferably made of iron / cast iron / steel.
3. The coating composition as claimed in claim 1, wherein the aluminium is 54 to 75 by wt.%.
4. The coating composition as claimed in claim 1, wherein the zinc is 21 to 46 by wt.%.
5. The coating composition as claimed in claim 1, wherein the magnesium and/ or silicon and / or nickel is 0.01 to 4.
6. The coating composition as claimed in claim 1, wherein density of the coating composition lies in the range of (4 to 4.7) g/cc.
7. The coating composition as claimed in claim 1, wherein colour of the coating composition is greyish.
8. The coating composition as claimed in claim 1, wherein microstructure of the coating composition is interconnected lamellar structure with alternate zinc and aluminium rich phases.
9. A process of coating a substrate using a coating composition as claimed in claim 1, wherein the substrate being formed of iron/cast iron/steel, the process comprising depositing the coating composition over the substrate by thermal spray technique in which the wire used

is pure Al, pure Zn, Al-Zn alloy, Al-Mg alloy, Al-Si alloy and cored wire with Zn,Al,Mg,Si and Ni powder or any combination of alloy powder thereof.
10. A process of coating a substrate using a coating composition as claimed in claim 1, wherein the substrate being formed of iron/cast, iron/steel, the process comprising depositing the coating composition over the substrate by flame spray technique using Zn, Al, Mg, Si and Ni powder or any combination of alloy powder thereof.
11. The process as claimed in claim 10, wherein an arc spray technique using single or twin wire is used.
Dated this 13TH day of JUNE , 2017