Claims:
We claim:

1. A method for forming a coating of a corrosion resistant Al-Zn-Si-Mg alloy on a metal, typically steel, strip, that is suitable, by way of example, for "acid rain" or "polluted" environments comprises:
(a) passing metal strip through a molten bath of the Al-Zn-Si-Mg alloy and forming a coating of the alloy on one or both surfaces of the strip,
(b) solidifying the coating on the strip and forming a solidified coating having a microstructure that comprises dendrites of Al-rich alpha phase and interdendritic channels of Zn-rich eutectic phase mixture, extending from the metal strip, and with particles of Mg2Si phase in the interdendritic channels,
and the method comprising controlling steps (a) and (b) and forming the solidified coating with an OT: SDAS ratio greater than 0.5:1, where OT is the overlay thickness and SDAS is the secondary dendrite arm spacing for the Al-rich alpha phase dendrites of the coating.

2. The method as claimed in claim 1 wherein the OT: SDAS ratio is greater than 1:1.

3. A metal strip with a coating of an Al-Zn-Si-Mg alloy on one or both surfaces of the strip that is suitable, by way of example, for "acid rain" or "polluted" environments , with the coating comprising a microstructure that comprises dendrites of Al-rich alpha phase and interdendritic channels of Zn-rich eutectic phase mixture extending from the metal strip, and with particles of Mg2Si phase in the interdendritic channels, and the coating having an OT: SDAS ratio greater than 0.5:1, where OT is the overlay thickness and SDAS is the secondary dendrite arm spacing for the Al-rich alpha phase dendrites of the coating .

4. The coated metal strip as claimed in claim 3 wherein the OT: SDAS ratio is greater than 1:1.

5. The coated metal strip as claimed in claim 3 or claim 4 wherein the coating has a total coating mass of less than 200 g per m2 coating on both surfaces of the strip, which equates to less than 100 g per m2 coating on one surface of the steel strip when the strip is coated on one surface only and the coating thickness is the same on both surfaces.

6. The coated metal strip as claimed in any one of claims 3 to 5 wherein the overlay thickness of the coating is greater than 3 µm.

7. The coated metal strip as claimed in any one of claims 3 to 5 wherein the SDAS of the Al-rich alpha phase dendrites in the coating is greater than 3 µm but smaller than 20 µm.

8. The coated metal strip as claimed in any one of claims 3 to 7 wherein the Al-Zn-Si-Mg alloy contains 20- 95% Al, up to 5% Si, up to 10% Mg and balance Zn with other elements in small amounts , typically less than 0.5% for each other element.

9. The coated metal strip as claimed in any one of claims 3 to 8 wherein the metal strip is a steel strip.

10. A method for forming a coating of a corrosion resistant Al-Zn-Si-Mg alloy on a metal, typically steel, strip, that is suitable, by way of example, for "acid rain" or "polluted" environments comprises:
(a) passing metal strip through a molten bath of the Al-Zn-Si-Mg alloy and forming a coating of the alloy on one or both surfaces of the strip,
(b) solidifying the coating on the strip and forming a solidified coating having a microstructure that comprises dendrites of Al-rich alpha phase and interdendritic channels of Zn-rich eutectic phase mixture extending from the metal strip, and with Mg2Si phase in the interdendritic channels in the solidified coating,
and the method comprising selecting the Mg and Si concentrations and controlling the cooling rate in step (b) to form particles of Mg2Si phase in the interdendritic channels.

11. The method as claimed in claim 10 comprises selecting the Mg concentration to be greater than 0.5%.

12. The method as claimed in claim 10 or claim 11 comprises selecting the Mg concentration to be greater than 1%.

13. The method as claimed in any one of claims 10 to 12 comprises selecting the Mg concentration to be less than 3%.

14. The method as claimed in any one of claims 10 to 13 wherein the step of selecting the Mg and Si concentrations and controlling the cooling rate in step (b) forms particles of Mg2Si phase in the interdendritic channels that have an appropriate size and morphology to block corrosion along the interdendritic channels.

15. The method as claimed in claim 14 wherein the morphology of the Mg2Si phase particles in the interdendritic channels is in the form of "Chinese script" when viewed in planar images and in the form of flower petals when viewed in 3-dimensional images.

16. The method as claimed in claim 15 wherein the petals have a thickness less than 5 µm.

17. The method as claimed in claim 15 wherein the petals have a thickness in the range of 0.5-2.5 µm.

18 The method as claimed in any one of claims 10 to 14 wherein the step of selecting the Mg and Si concentrations and controlling the cooling rate in step (b) to form particles of Mg2Si phase in the interdendritic channels forms Mg2Si phase particles in the interdendritic channels in the solidified coating having a size range and a spacial distribution that activates the Al-rich alpha phase to provide sacrificial protection.

19. The method as claimed in any one of claims 10 to 18 wherein the cooling rate CR during coating solidification is less than 170 - 4.5CT, where CR is the cooling rate in °C/second and CT is the coating thickness on a surface of the strip in micrometres.

20. A metal strip with a coating of an Al-Zn-Si-Mg alloy on one or both surfaces of the strip that is suitable, by way of example, for "acid rain" or "polluted" environments, with the coating comprising a microstructure that comprises dendrites of Al-rich alpha phase and interdendritic channels of Zn-rich eutectic phase mixture extending from the metal strip, and with particles of Mg2Si phase in the interdendritic channels.

21. The coated metal strip as claimed in claim 20 wherein the Al-Zn-Si-Mg alloy contains 20-95% Al, up to 5% Si, up to 10% Mg and balance Zn with other elements in small amounts, typically less than 0.5% for each other element.

22. The coated metal strip as claimed claim 21 wherein the Mg concentration is greater than 0.5%.

23. The coated metal strip as claimed claim 21 wherein the Mg concentration is greater than 1%.

24. The coated metal strip as claimed claim 21 wherein the Mg concentration is less than 3%.

25. The coated metal strip as claimed in any one of claims 21 to 24 wherein, for coatings with Si concentrations from 0.5 to 2%, the volume fraction of interdendritic Mg2Si phase compared to other Si-containing phases is greater than 50%.

26. The coated metal strip as claimed in any one of claims 21 to 25 wherein the volume fraction of interdendritic Mg2Si phase compared to other Si-containing phases is greater than 80%.

27. The coated metal strip as claimed in any one of claims 21 to 26 wherein greater than 70% of the total volume fraction of Mg2Si phase in the coating is in the lower two thirds of the overlay thickness of the coating.

28. The coated metal strip as claimed in any one of claims 21 to 27 wherein greater than 60% of the interdendritic channels is "blocked" by Mg2Si phase particles.

29. The method as claimed in claim 1 or claim 10 wherein the Al-Zn-Si-Mg alloy contains 45-60 wt. % Al.

30. The method as claimed in claim 1 or claim 10 wherein the Al-Zn-Si-Mg alloy contains 39-48 wt. % Zn.

31. The method as claimed in claim 1 or claim 10 wherein the Al-Zn-Si-Mg alloy contains 1.3-2.5 wt. % Si.

32. The method as claimed in claim 1 or claim 10 wherein the Al-Zn-Si-Mg alloy contains 1.2-2.8 wt. % Mg.

33. The method as claimed in claim 1 or claim 10 wherein the Al-Zn-Si-Mg alloy contains 1.5-2.5 wt. % Mg.

34. The method as claimed in claim 1 or claim 10 wherein the Al-Zn-Si-Mg alloy contains 1.7-2.3 wt. % Mg.

35. The method as claimed in claim 1 or claim 10 wherein the overlay thickness of the coating is less than 30 µm.

36. The method as claimed in claim 1 or claim 10 wherein the overlay thickness of the coating is 5- 20 µm.

37. The metal strip as claimed in claim 3 or claim 20 wherein the Al-Zn-Si-Mg alloy contains 45-60 wt. % Al.

38. The metal strip as claimed in claim 3 or claim 20 wherein the Al-Zn-Si-Mg alloy contains 39-48 wt. % Zn.

39. The metal strip as claimed in claim 3 or claim 20 wherein the Al-Zn-Si-Mg alloy contains 1.3-2.5 wt. % Si.

40. The metal strip as claimed in claim 3 or claim 20 wherein the Al-Zn-Si-Mg alloy contains 1.2-2.8 wt. % Mg.

41. The metal strip as claimed in claim 3 or claim 20 wherein the Al-Zn-Si-Mg alloy contains 1.5-2.5 wt. % Mg.

42. The metal strip as claimed in claim 3 or claim 20 wherein the Al-Zn-Si-Mg alloy contains 1.7-2.3 wt. % Mg.

43. The metal strip as claimed in claim 3 or claim 20 wherein the overlay thickness of the coating is less than 30 µm.

44. The metal strip as claimed in claim 3 or claim 20 wherein the overlay thickness of the coating is 5- 20 µm.

Dated this 22nd day of May 2020.

(GAYATRI BHASIN)
IN/PA-1246
Of SUBRAMANIAM & ASSOCIATES
ATTORNEYS FOR THE APPLICANTS
, Description:-Please see the attached .pdf file-