Post on 23-Feb-2016
description
Palm Springs, August 2011
Thermally Sprayed Aluminum-Zinc-Indium as a Corrosion Protection for Steel
Dr Chris J Wheatley
Palm Springs, August 2011
Terminology• Metalizing• Thermal Spraying• Arc Spraying• (HVOF Spraying)• Metal Spraying
Palm Springs, August 2011
Galvanizing
Zinc Spraying (TSZ)
Aluminum Spraying (TSA)
History (I)
Palm Springs, August 2011
Why Zinc ?Reactive and reliableCheapTrusted (from
galvanizing)Easy to work with
But soft and too active in aggressive
environments
Why Aluminum ?Excellent barrierLow densityNot too reactiveHarder than zinc
Expensive and can it be
trusted to act sacrificially?
History (II)
Palm Springs, August 2011
Zinc
Now expensive - when
density is considered.
220 million tonnes at11 million tonnes peryear = 20 years
Aluminum
New alloys
34 million tonnes per year primary output. 50% recycled. Almost limitless stock.
What is new to drive change ?
Palm Springs, August 2011
Aluminum – Zinc –IndiumNominal Composition:
Aluminum 95%Zinc 5%Indium 0.025%Low FeLow Cu
Palm Springs, August 2011
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
Salt fog (ASTM B117) 5% NaCl 1000 hrs ) 100 micron sprayed 0.5% NaCl 1000 hrs ) 5mm scratch
Zero Resistance Ammeter5% and 0.5% NaCl
Galvanic current Potential
Corrosion Testing
Palm Springs, August 2011
Salt Fog 5% NaCl
Sprayed Aluminum Sprayed Al-Zn-In
Palm Springs, August 2011
Salt Fog 5% NaCl (corrosion product removed)
Sprayed Aluminum Sprayed Al-Zn-In
Palm Springs, August 2011
Salt Fog (0.5% NaCl)
Sprayed Aluminum Sprayed Al-Zn-In
Palm Springs, August 2011
ZRA - schematic
Palm Springs, August 2011
ZRA - actual
Palm Springs, August 2011
Potential versus SCE (5% NaCl)
0 20 40 60 80 100 120 140 160 180-1.2
-1.1
-0.999999999999999
-0.899999999999999
-0.799999999999999
-0.699999999999999
-0.599999999999999
Elapsed Time (h)
Pote
ntia
l (V
vs S
CE)
Aluminum
AlZnIn
Palm Springs, August 2011
Galvanic Current – 5% NaCl
0 20 40 60 80 100 120 140 160 1800E+00
1E-05
2E-05
3E-05
4E-05
5E-05
6E-05
7E-05
8E-05
Elapsed Time (h)
Curr
ent
(A)
AlZnIn
Aluminum
Palm Springs, August 2011
Potential versus SCE (0.5% NaCl)
0 20 40 60 80 100 120 140 160 180-1.2
-1.1
-0.999999999999999
-0.899999999999999
-0.799999999999999
-0.699999999999999
-0.599999999999999
Elapsed Time (h)
Pote
ntia
l (V
vs S
CE)
Aluminum
AlZnIn
Palm Springs, August 2011
Galvanic Current - (0.5% NaCl)
0 20 40 60 80 100 120 140 160 1800E+00
1E-05
2E-05
3E-05
4E-05
5E-05
6E-05
7E-05
8E-05
Elapsed Time (h)
Curr
ent
(A)
AlZnIn
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.
Corrosion Test Summary
Palm Springs, August 2011
(AlZn)In + Cl- InCl2+ + 3e-
2InCl2+ + 3Zn 3Zn2+ + 2Cl- + 2In
PittingInCl2+ + Al Al3+ + Cl- + In
Mechanism for Activation by Indium
Palm Springs, August 2011
This paper 3,000 – 30,000 ppm
Previous work on cast anodes:(1) > 2000ppm(2) > 1000ppm
What chloride (halide) levels will allow sprayed Al-Zn-In to work effectively to protect steel ?
Palm Springs, August 2011
Erosion / corrosion – zinc is simply too softcompared to aluminum alloys.
Where ambient temperatures can be higher than 60°C (140°F) where zinc goes into
polarityreversal and becomes cathodic to steel.
When Should Al-Zn-In Always be Chosen compared to Zinc ?
Palm Springs, August 2011
At halide concentrations < 1000ppm it will only act like pure aluminum (TSA) and will besuitable as a barrier to corrosion.
In high concentrations of sulfate or bicarbonateIons where it is less effective – the same is true forzinc. A lot more work is required here.
In river waters containing large concentrations of fertilizers – the same is true for zinc. Neither TSA nor TSZshould be used in the presence of high concentrations of nitrates.
Where should Al-Zn-In not be used for galvanic protection?
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)
Summary
Palm Springs, August 2011
Thank you for listening.
Palm Springs, August 2011
ACKNOWLEDGEMENTS1. London & Scandinavian Metallurgical
Limited made the alloy wires.2. Metallisation Limited did the spraying3. Sheffield Hallam University did the
corrosion testing.4. BAC Corrosion Control Limited provided
information, especially on concrete.