dr chris j wheatley palm springs, august 2011. metalizing thermal spraying arc spraying (hvof...

Palm Springs, August 2011

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


Galvanizing

Zinc Spraying (TSZ)

Aluminum Spraying (TSA)

History (I)


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)


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 ?


Aluminum – Zinc –IndiumNominal Composition:

Aluminum 95%Zinc 5%Indium 0.025%Low FeLow Cu


Aluminum – Zinc –Indium

This 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.


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


Salt Fog 5% NaCl

Sprayed Aluminum Sprayed Al-Zn-In


Salt Fog 5% NaCl (corrosion product

removed)



Salt Fog (0.5% NaCl)



ZRA - schematic


ZRA - actual


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

nti

al (V

vs

SC

E)

Aluminum

AlZnIn


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


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

nti

al (V

vs

SC

E)

Aluminum

AlZnIn


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


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


(AlZn)In + Cl- InCl2+ + 3e-

2InCl2+ + 3Zn 3Zn2+ + 2Cl- + 2In

PittingInCl2+ + Al Al3+ + Cl- + In

Mechanism for Activation by Indium


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 ?


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 ?


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?


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


Thank you for listening.


ACKNOWLEDGEMENTS

1. 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.

dr chris j wheatley palm springs, august 2011. metalizing thermal spraying arc spraying (hvof...

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