Surface corrosion when HVOF spraying
08-19-2021, 04:55 PM,
#1
Surface corrosion when HVOF spraying
Dear all, how are you?

As can be seen in the attached photo, I'm having surface oxidation problem when applying carbide coatings by HVOF, with a DJ-2700 gun.

The region adjacent to the coating is turning greenish. This problem is very serious in pieces similar to the one in the photo, as I have to coat both sides (a and b). It can be seen in image (b) - side I coated first - that the entire surface looked like this after side (a) was coated.

Before coating, the piece was heated to 80°C and then blasted with brown aluminum oxide.

The coating was carried out as follows:
O2: 280 lpm;
C3H6: 80lpm;
Air: 440 lpm;
N2: 15-20 lpm;
Powder: WOKA 7205;
FR: 45 gpm;
Distance: ~250mm (manual application);
Start temperature: ~50°C;
Standby temperature: ~150°C;
Part speed: 80-150 m/min (ring).

The problem appears right at the beginning of the coating and several pieces (which I don't have a photographic record) have this problem.

Have any of you been through something similar or do you have any information you can share about it?

Thank you very much for your attention.


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08-19-2021, 07:54 PM, (This post was last modified: 08-19-2021, 08:05 PM by Vadim Verlotski.)
#2
RE: Surface corrosion when HVOF spraying
(08-19-2021, 04:55 PM)vcduramais Wrote: Dear all, how are you?

As can be seen in the attached photo, I'm having surface oxidation problem when applying carbide coatings by HVOF, with a DJ-2700 gun.

The region adjacent to the coating is turning greenish. This problem is very serious in pieces similar to the one in the photo, as I have to coat both sides (a and b). It can be seen in image (b) - side I coated first - that the entire surface looked like this after side (a) was coated.

Before coating, the piece was heated to 80°C and then blasted with brown aluminum oxide.

The coating was carried out as follows:
O2: 280 lpm;
C3H6: 80lpm;
Air: 440 lpm;
N2: 15-20 lpm;
Powder: WOKA 7205;
FR: 45 gpm;
Distance: ~250mm (manual application);
Start temperature: ~50°C;
Standby temperature: ~150°C;
Part speed: 80-150 m/min (ring).

The problem appears right at the beginning of the coating and several pieces (which I don't have a photographic record) have this problem.

Have any of you been through something similar or do you have any information you can share about it?

Thank you very much for your attention.

What you got is quite a typical case. Immediately after spraying, the coating is clean, free of dust and oxides, therefore it is gray in color. However, over time or after heating, it darkens, as it oxidizes and becomes covered with dust. On the rough coating of the first side of the part, burnt dust (overspay), which is formed when the second side is sprayed, settles well. Since the oxidation of chromium carbide produces green chromium oxide, all surfaces in the spraying chamber gradually acquire this color. If you want both sides to have the same beautiful appearance, both sides after spraying must be sandblasted with glass beads (150-200 microns, air pressure up to 4 bar). Such sandblasting does not harm the coating, but only removes the outer rough layer (2-5 microns) and all dust. After processing with glass beads, the coating becomes smooth and acquires a dark metallic luster.
Reply
08-19-2021, 11:06 PM,
#3
RE: Surface corrosion when HVOF spraying
(08-19-2021, 07:54 PM)Vadim Verlotski Wrote:
(08-19-2021, 04:55 PM)vcduramais Wrote: Dear all, how are you?

As can be seen in the attached photo, I'm having surface oxidation problem when applying carbide coatings by HVOF, with a DJ-2700 gun.

The region adjacent to the coating is turning greenish. This problem is very serious in pieces similar to the one in the photo, as I have to coat both sides (a and b). It can be seen in image (b) - side I coated first - that the entire surface looked like this after side (a) was coated.

Before coating, the piece was heated to 80°C and then blasted with brown aluminum oxide.

The coating was carried out as follows:
O2: 280 lpm;
C3H6: 80lpm;
Air: 440 lpm;
N2: 15-20 lpm;
Powder: WOKA 7205;
FR: 45 gpm;
Distance: ~250mm (manual application);
Start temperature: ~50°C;
Standby temperature: ~150°C;
Part speed: 80-150 m/min (ring).

The problem appears right at the beginning of the coating and several pieces (which I don't have a photographic record) have this problem.

Have any of you been through something similar or do you have any information you can share about it?

Thank you very much for your attention.

What you got is quite a typical case. Immediately after spraying, the coating is clean, free of dust and oxides, therefore it is gray in color. However, over time or after heating, it darkens, as it oxidizes and becomes covered with dust. On the rough coating of the first side of the part, burnt dust (overspay), which is formed when the second side is sprayed, settles well. Since the oxidation of chromium carbide produces green chromium oxide, all surfaces in the spraying chamber gradually acquire this color. If you want both sides to have the same beautiful appearance, both sides after spraying must be sandblasted with glass beads (150-200 microns, air pressure up to 4 bar). Such sandblasting does not harm the coating, but only removes the outer rough layer (2-5 microns) and all dust. After processing with glass beads, the coating becomes smooth and acquires a dark metallic luster.

Hi Vadim, how are you?

Thank you very much for sharing! 

In this particular case after coating we grit blast the surface with aluminum oxide with low working pressure (3-4 bar) and higher distance (> 250 mm) but glass beads seams a better option.

Is this oxide bad for coating cohesion? And air cooling can prevent this to happen by removing the dust?

I usually have this problem when applying chromium carbide, but recently I'm experiencing it with tungsten carbide as well. And when spraying on flat surfaces it gets worse and sometimes cause delamination.

There is something to prevent it to happen at all?

Again, thank you very much!
Reply
08-20-2021, 06:59 AM,
#4
RE: Surface corrosion when HVOF spraying
Hi,
The main reason for the phenomenon you are observing is that the topmost layer of the sprayed coating is always very porous. This porous layer (uppermost 2-10 microns) oxidizes very easily and absorbs dust and other contaminants. Moreover, this layer is very fragile. Unfortunately, it is impossible to avoid the formation of a porous upper layer during thermal spraying, since it is based on the very principle of layer-by-layer deposition of particles upon impact on the substrate: the uppermost particles cannot be compacted with another layer on top and therefore lie freely on the surface.
Typically, parts with carbide coatings are sanded after spraying and the porous top layer is removed automatically.
The porous top layer does not in any way affect the service of the main, dense, coating, but only worsens the appearance of the product and makes the surface rough. In some cases, the top layer does not interfere with the service of the part and can be left as is. In the presence of abrasive wear, this layer will quickly wear off itself.
Reply
08-20-2021, 11:25 AM,
#5
RE: Surface corrosion when HVOF spraying
(08-20-2021, 06:59 AM)Vadim Verlotski Wrote: Hi,
The main reason for the phenomenon you are observing is that the topmost layer of the sprayed coating is always very porous. This porous layer (uppermost 2-10 microns) oxidizes very easily and absorbs dust and other contaminants. Moreover, this layer is very fragile. Unfortunately, it is impossible to avoid the formation of a porous upper layer during thermal spraying, since it is based on the very principle of layer-by-layer deposition of particles upon impact on the substrate: the uppermost particles cannot be compacted with another layer on top and therefore lie freely on the surface.
Typically, parts with carbide coatings are sanded after spraying and the porous top layer is removed automatically.
The porous top layer does not in any way affect the service of the main, dense, coating, but only worsens the appearance of the product and makes the surface rough. In some cases, the top layer does not interfere with the service of the part and can be left as is. In the presence of abrasive wear, this layer will quickly wear off itself.

Hi Vadim. Thank you very much.

For this parts I think will be better to blast with glass beads. Usually we finish our coatings so as sprayed appearance is not very important.

I was only concerned about the coating quality as some coatings that exhibit this condition show delamination. I sometimes have problems even after blasting the surface (especially when re-applying carbide over unfinished parts, as in figure 2).

Best regards
Reply




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