How to pass the acid test?
10-09-2009, 07:22 AM,
#1
How to pass the acid test?
Dear all,
I have some questions about TAFA1310VM HVOF coating, our customer required us to do acid test on this coating, the acid test condition is that the test coupon with this coating shall be dipped in 28% HCL (room temp.) for 336hours, the coating on test coupon should not show any signs of blistering or flaking after 336 hours acid exposure, otherwise the acid test will be considered fail. We have already failed two times of such acid test. Our HVOF machine is PRAXAIR JP-8000 and the spraying parameter is as: Oxygen: 1900 SCFH, Nitrogen: 23 SCFH, Kerosene: 6 GPH, spray distance:14 inches, powder feedrate:76g/min, so could you give us any suggestions on the spray parameter settings to get a better quality of coating and help us to pass the acid test? Thanks
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10-10-2009, 04:47 PM,
#2
RE: How to pass the acid test?
Hi jurongwei

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Can you gives us more detail on the test coupon and coating? Is coupon 100% covered by coating? Is coating sealed? coating thickness? any finishing? etc.

Have been able to do metallography of the coating to check porosity, cracks and anything that will effect permeability to that harsh environment?
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10-12-2009, 03:46 AM,
#3
RE: How to pass the acid test?
Thank you, Mr. Gordon

The coupon size is OD of 3? with ID of 2?, the length is 6.5?, and coating starts 0.5? from one end and progress for the next 5.5?. The coupon material is 4130.
The coating finished with grinding to 20 microinches and a thickness of 0.008 inches. Sealer (DICHTOL WFT 1532) is applied after spraying and after grinding (2times). Have already done the coating porosity and adhesion checking and the results are 1% and 12444 psi min. respectively.

Acid Testing Requirement:

The test coupon shall be tested such that approximately 50% of the coating is exposed to the liquid phase and 50% to the vapor phase. The test solution shall be uninhibited 28% HCl (weight % of reagent grade) at room temperature. The test coupon must not show any signs of blistering or flaking after 336 hrs acid exposures.

Before testing, coupon is plugged and tape off ends with multiple layers of acid resistant tape, must prevent acid and vapors from contacting un-coated ends.
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10-12-2009, 03:03 PM,
#4
RE: How to pass the acid test?
Hi jurongwei

Do you know if this coating system has ever passed this test before?
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10-13-2009, 04:16 AM,
#5
RE: How to pass the acid test?
Hi Mr. Gordon,

From our customer's U.S.A mother company, their coating supplier has passed this test, I'm sure the powder composition they used is 90% WC plus 10% Ni, but I'm not sure which brand of powder they used (TAFA 1310VM or Metco WA3302 or other brand), can you give any recommendations about the powder? Thanks
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10-13-2009, 02:53 PM,
#6
RE: How to pass the acid test?
Hi jurongwei

Quote:I'm sure the powder composition they used is 90% WC plus 10% Ni, but I'm not sure which brand of powder they used (TAFA 1310VM or Metco WA3302 or other brand), can you give any recommendations about the powder?

Well, only from the aspect that if I was trailing these materials for JP HVOF, I would put the praxiar/tafa powders at the top of my list for testing. Reason being that the material will be tailored towards use firstly with the manufacturers own equipment. This does not always guarantee that it will always be the best though, just more likely towards top.

Just thinking aloud - Two critical criteria needed to pass test:
* coating needs to resist environment
* coating needs to completely isolate substrate from environment

Assuming coating has been successful before in this test, lets assume coating has sufficient resistance in itself. That tends to suggest that it is a barrier problem.

Question - What does the test failure show? any corrosion to coating itself? or is it just corrosion at the coating/substrate interface?

Coatings will invariably have porosity/micro-cracks/pathways through coating to some degree, hence the reason for sealing. Your sealing and sealers certainly sound like a reasonable choice, but in this situation I would question its performance and suitability.

I know the corrosion resistance WC/Co and WC/CoCr coatings can be effected by the different phases that appear. Coatings which suffer break down of WC through over heating/oxidation of particles producing metastable phases tend to be very much less corrosion resistant to certain chemicals. I don't know if WC/Ni coatings can suffer in the same way.

May be if still failing with coating and sealer improvements, consider using a inconel 625 or other acid resistant bond/buffer layer under your WC/Ni coating.
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10-14-2009, 06:54 AM,
#7
RE: How to pass the acid test?
Hi, Mr. Gordon
From the photo attached, the corrosion occurs at coating/substrate interface. Seems HCL liquid pass through the coating and react with substrate material, that means the porosity of this coating too high, but 1% of porosity is the optimum value for this HOVF coating based on the powder supplier's TECHNICAL DATA SHEET. How to improve anymore on the porosity?
If an acid resistant buffer layer (May I say 1310VM also is acid resistant coating?) is applied first, this buffer layer must have a porosity lower than 1%, otherwise HCL liquid will also pass through this buffer layer and react with substrate. Meantime this buffer layer must have a good adhesion between substrate and top layer (at least 10,000PSI) according to the customer's specification. What materials do you recommend to use as a buffer layer? Thanks


Attached Files
.bmp   acid test sample (Custom).bmp (Size: 300.05 KB / Downloads: 731)
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10-14-2009, 07:59 AM,
#8
RE: How to pass the acid test?
Hi jurongwei,

From the pictures that you have attached, I dont think the problem persists with the coating but it actually has to do with the procedure of testing. Im not very sure but the colouration suggests that corrosive liquid has attacked the substrate-coating interface. You mentioned that you had sealed the ends (non-coated area) with acid resistant tape but did you seal it such that the acid could not enter the ID of the pipe?

Also, if you are unsure about the corrosive resistant properties of the powder, ask your vendor to provide a technical spec-sheet with proper indication of corrosion resistance against various corrosive mediums.

I have attached one of the spec-sheets that i had with me for your reference. They have clearly mentioned how the coating behaves in diffrent aqueous corrosive mediums. and whats even more convinient is that they have give the parameters with respect to JP5000..
.pdf   WOKA 3652 - JP5000 - Kerosene.pdf (Size: 250.51 KB / Downloads: 7,601)
Regards

K09Mad0223
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10-15-2009, 02:54 AM,
#9
RE: How to pass the acid test?
Hi jurongwei

Quote:Seems HCL liquid pass through the coating and react with substrate material, that means the porosity of this coating too high, but 1% of porosity is the optimum value for this HOVF coating based on the powder supplier's TECHNICAL DATA SHEET.

Yes, but also that the sealer has failed. Also check that the finish grinding is not introducing cracks into your coating.

Quote:How to improve anymore on the porosity?

Well, I think this will be down to doing some coating development work, using the recommended parameters as your base line, also maybe trying alternative powders. No quick fix I'm afraid. I have my doubts as to whether anybody who has spent time and money doing development work on this, will be happy to divulge the information for free.
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10-16-2009, 10:24 AM,
#10
RE: How to pass the acid test?
Hi Mr. Gordon, K09

The test coupon was sprayed in Singapore and was tested in US, the acid test procedure as:
1. Clean part thoroughly with solvent to remove any oils that might prevent tape from sealing off ends.
2. Plug and tape off ends with multiple layers of acid resistant tape, must prevent acid and vapors from contacting un-coated ends
3. Mix up 28% uninhibited Hydrochloric Acid
4. Half submerge sample in glass beaker containing Acid
5. Cover the top of beaker in order to contain the acid vapors that will work on upper portion of sample.
6. Leave part in acid for test duration with periodic visual checks.
7. Typically the clear acid solution will turn color when the base metal is contacted � test over
8. If acid remains clear for the duration of test, then remove part from acid, remove tape and plugs, wash thoroughly and examine closely for pin holes.
9. Typically pass or fail is very easy to see.

From the photo can see the coating failed in area that was in acid, can also see in the photo that the coating is still in place where the tape was so we can say no leaks on the seal tape.
In the customer's specification, they indicate the HVOF powder must have a chemical composition of 90% WC plus 10% Ni, but they don�t specify the powder brand. From attached Metco WOKA 3302 data sheet, can see this coating has a poor corrosion resistance in 0.5M H2SO4, does this mean this coating also has a poor corrosion resistance in HCL liquid? They don't indicate there.
Our HVOF equipment is JP-8000, could you tell me what the difference is between JP-8000 and JP-5000? In our hand all the spray parameters TAFA recommended is for JP-5000.
Will continuously do some coating development work on powder TAFA1310VM, to get better coating porosity, is it a correct way to increase oxygen flow/fuel flow and reduce the powder feedrate? Thanks


Attached Files
.pdf   WOKA 3302 - JP5000 - Kerosene.pdf (Size: 261.21 KB / Downloads: 3,401)
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10-17-2009, 05:27 PM,
#11
RE: How to pass the acid test?
Hi jurongwei

Quote:From attached Metco WOKA 3302 data sheet, can see this coating has a poor corrosion resistance in 0.5M H2SO4, does this mean this coating also has a poor corrosion resistance in HCL liquid?

Difficult one - Nickel is claimed to be fairly resistant to both hydrochloric and sulphuric acids as is tungsten. Tungsten carbide and any other phases in between along with being mixed altogether is anybodies guess, without doing the specific tests. Also remember that passing this test, does not necessarily mean the coating is suitable for long term service in these conditions.

Quote:Our HVOF equipment is JP-8000, could you tell me what the difference is between JP-8000 and JP-5000? In our hand all the spray parameters TAFA recommended is for JP-5000.

I think essentially the same spray gun technology, just more advanced process control equipment for JP 8000. Please correct me if I'm wrong Happy0193

Quote:Will continuously do some coating development work on powder TAFA1310VM, to get better coating porosity, is it a correct way to increase oxygen flow/fuel flow and reduce the powder feedrate? Thanks

You may see a statement on the powder technical bulletin - " Spray parameters are presented as a starting point and may require adjustment to suit specific spray applications". Well I would like to think that these parameters are developed to give an all-round compromise between coating qualities, deposit efficiency and production rate, the more cynical may say it is a get out clause to save on effort/money in development. Anyway, there is sometimes scope to improve upon these recommended parameters, particular if you have a specific goal in mind. In your case, coating quality is paramount and perhaps you can sacrifice some of the economic considerations such as deposit efficiency and spray rate.

As you are probably aware, parameter development can be quite complex. Make changes from your TAFA recommended parameters (use this as your baseline for comparison, as it is this coating that you want to improve upon). Don't be tempted to try to change to many things at once. Best to make one change at a time, say reduction in spray rate to see if this has any desirable effect. Then perhaps Changing oxygen/fuel flows or just oxygen flows, spray distance, barrel length........ as you can imagine it gets complex with all the permutations. You need to feel your way, with each test the direction you need to take will hopefully become clear. It will be necessary to have very good metallurgical testing facilities and acid testing facilities to hand.

Don't disregard the aspect of sealing and sealers. It seems obvious to me (even if coating is slightly more porous/micro-cracked than it could be) that your sealer is failing. I would also include tests on the sealing, using say just your standard coating to establish best sealing method, before any lengthy spraying parameter development.

Good luck Smile
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