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Thermally Sprayed Aluminum-Zinc-Indium as a Corrosion Protection for Steel

Thermally Sprayed Aluminum-Zinc-Indium as a Corrosion Protection for Steel. Dr Chris J Wheatley. Terminology. Metalizing Thermal Spraying Arc Spraying (HVOF Spraying) Metal Spraying. History (I). Galvanizing Zinc Spraying (TSZ) Aluminum Spraying (TSA). History (II). Why Zinc ?

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Thermally Sprayed Aluminum-Zinc-Indium as a Corrosion Protection for Steel

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  1. Thermally Sprayed Aluminum-Zinc-Indium as a Corrosion Protection for Steel Dr Chris J Wheatley Palm Springs, August 2011

  2. Terminology Metalizing Thermal Spraying Arc Spraying (HVOF Spraying) MetalSpraying Palm Springs, August 2011

  3. History (I) Galvanizing Zinc Spraying (TSZ) Aluminum Spraying (TSA) Palm Springs, August 2011

  4. History (II) Why Zinc ? Reactive and reliable Cheap Trusted (from galvanizing) Easy to work with But soft and too active in aggressive environments Why Aluminum ? Excellent barrier Low density Not too reactive Harder than zinc Expensive and can it be trusted to act sacrificially? Palm Springs, August 2011

  5. What is new to drive change ? Zinc Now expensive - when density is considered. 220 million tonnes at 11 million tonnes per year = 20 years Aluminum New alloys 34 million tonnes per year primary output. 50% recycled. Almost limitless stock. Palm Springs, August 2011

  6. Aluminum – Zinc –IndiumNominal Composition:Aluminum 95%Zinc 5%Indium 0.025%Low FeLow Cu Palm Springs, August 2011

  7. Aluminum – Zinc –IndiumThis composition has been used for about 30 years under water in solid cast anodes – the basic chemistry is well understood. This will now be tested, sprayed onto steel as an alternative to zinc spraying. The same composition has been sprayed onto reinforced concrete for protection of the rebars. Palm Springs, August 2011

  8. Corrosion Testing • Salt fog (ASTM B117) 5% NaCl 1000 hrs ) 100 micron sprayed 0.5% NaCl 1000 hrs) 5mm scratch • Zero Resistance Ammeter 5% and 0.5% NaCl • Galvanic current • Potential Palm Springs, August 2011

  9. Salt Fog 5% NaCl Sprayed Aluminum Sprayed Al-Zn-In Palm Springs, August 2011

  10. Salt Fog 5% NaCl (corrosion product removed) Sprayed Aluminum Sprayed Al-Zn-In Palm Springs, August 2011

  11. Salt Fog (0.5% NaCl) Sprayed Aluminum Sprayed Al-Zn-In Palm Springs, August 2011

  12. ZRA - schematic Palm Springs, August 2011

  13. ZRA - actual Palm Springs, August 2011

  14. Potential versus SCE (5% NaCl) Palm Springs, August 2011

  15. Galvanic Current – 5% NaCl Palm Springs, August 2011

  16. Potential versus SCE (0.5% NaCl) Palm Springs, August 2011

  17. Galvanic Current - (0.5% NaCl) Palm Springs, August 2011

  18. Corrosion Test Summary 1.In 5% NaCl solution and in 0.5% NaCl the potential of the AlZnIn anode is about 100mV more negative than that of pure aluminum. 2. The corresponding galvanic couple between the coating and the steel substrate is more aggressive in the protection of the steel and has more throwing power. |It can therefore be trusted at very low chloride concentrations. Palm Springs, August 2011

  19. Mechanism for Activation by Indium (AlZn)In + Cl-InCl2+ + 3e- 2InCl2+ + 3Zn 3Zn2+ + 2Cl- + 2In Pitting InCl2+ + Al Al3+ + Cl- + In Palm Springs, August 2011

  20. What chloride (halide) levels will allow sprayed Al-Zn-In to work effectively to protect steel ? This paper 3,000 – 30,000 ppm Previous work on cast anodes: (1) > 2000ppm (2) > 1000ppm Palm Springs, August 2011

  21. When Should Al-Zn-In Always be Chosen compared to Zinc ? Erosion / corrosion – zinc is simply too soft compared to aluminum alloys. Where ambient temperatures can be higher than 60°C (140°F) where zinc goes into polarity reversal and becomes cathodic to steel. Palm Springs, August 2011

  22. Where should Al-Zn-In not be used for galvanic protection? At halide concentrations < 1000ppm it will only act like pure aluminum (TSA) and will be suitable as a barrier to corrosion. In high concentrations of sulfate or bicarbonate Ions where it is less effective – the same is true for zinc. A lot more work is required here. In river waters containing large concentrations of fertilizers – the same is true for zinc. Neither TSA nor TSZ should be used in the presence of high concentrations of nitrates. Palm Springs, August 2011

  23. Summary • Increasing cost of zinc will mean that new aluminum alloys will become more important for thermal spraying. • Al-Zn-In can be arc-sprayed instead of zinc in almost every normal environment and will galvanically protect steel, even in very low halide concentrations. • Al-Zn-In will be more effective than zinc in conditions of high erosion. • Al-Zn-In can only be used where temperatures exceed 60°C (140°F) Palm Springs, August 2011

  24. Thank you for listening. Palm Springs, August 2011

  25. ACKNOWLEDGEMENTS • London & Scandinavian Metallurgical Limited made the alloy wires. • Metallisation Limited did the spraying • Sheffield Hallam University did the corrosion testing. • BAC Corrosion Control Limited provided information, especially on concrete. Palm Springs, August 2011

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