Dr Chris J Wheatley Palm Springs, August 2011

Metalizing Thermal Spraying Arc Spraying (HVOF Spraying) Metal Spraying Palm Springs, August 2011

Galvanizing Zinc Spraying (TSZ) Aluminum Spraying (TSA) Palm Springs, August 2011

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

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

 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

Sprayed Aluminum Sprayed Al-Zn-In Palm Springs, August 2011

Sprayed Aluminum Sprayed Al-Zn-In Palm Springs, August 2011

Sprayed Aluminum Sprayed Al-Zn-In Palm Springs, August 2011

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

(AlZn)In + Cl - InCl e - 2InCl Zn 3Zn Cl - + 2In Pitting InCl 2+ + AlAl 3+ + Cl - + In Palm Springs, August 2011

This paper 3,000 – 30,000 ppm Previous work on cast anodes: (1) > 2000ppm (2)> 1000ppm Palm Springs, August 2011

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

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

1. Increasing cost of zinc will mean that new aluminum alloys will become more important for thermal spraying. 2. 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. 3. Al-Zn-In will be more effective than zinc in conditions of high erosion. 4. Al-Zn-In can only be used where temperatures exceed 60°C (140°F) Palm Springs, August 2011

1. London & Scandinavian Metallurgical Limited made the alloy wires. 2. Metallisation Limited did the spraying 3. Sheffield Hallam University did the corrosion testing. 4. BAC Corrosion Control Limited provided information, especially on concrete. Palm Springs, August 2011