fresh-water corrosion, pitting and erosion
05-23-2011, 10:12 PM,
#1
fresh-water corrosion, pitting and erosion
Dear SE members,
I need all of you suggestions.Already, Thank you for taking the time to ...

we have 2500 HP, fresh water cooled Diesel engine's cylinder piston liners. Picture of worn material is attached.

[Image: metallurgy-eng-1-thumb.jpg]

I will give you information about the material conditions of work.2 - 3 kg/cm2 water cooling pressure is cyclating arround the liner, and it takes the outside temperature of the liner 90 C "during engine running". And the max temp. inside the liner is 900 C.
The material of the liner is cast iron. And the problem is pitting corrosion and abrasion(erresion) on the liner. As you can see the pictures.

[Image: metallurgy-eng-2-thumb.jpg]

[Image: metallurgy-eng-3.jpg]

We want to coat outside the liner and extend the service life.Because After 600 hours the pitting corrosion occure outside the liner, and at the 700 hours the liner crack. we want to run for 2000 hours or more...

We are plannig to use HVOF for coating. Please help me, the choice of materials . Waiting for valuable comments.
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05-24-2011, 04:50 PM,
#2
RE: fresh-water corrosion, pitting and erosion
The picture seems to illustrate a curious pattern to the pitting. It appears as though this is only happening on one side of the liner(s). Is that accurate?

If it is only one side, as it appears, then I would suspect that either the piston is not running concentric to the bore in operation (therefore wearing one side more aggressively), or there was some flaw to the casting process that left greater porosity on one side (which would tend to show greater corrosion-erosion effects). The cracking may suggest that there is a temperature issue present. How does the crack occur? Is there a tendency for longitudinal cracks? Spiderweb? Circumferential? Is the 90C measured by water temps in operation? If it is, then there may be some sort of quench cracking in play, and this is a much more difficult problem to solve than the above.

I suppose a root cause needs to be determined before deciding on the solution. In the absence of clear evidence it might be best to use some kind of a high carbon steel (try to maximize bond while matching thermal expansion to the base metal) with a high temp sealer and see what happens.
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05-24-2011, 11:03 PM, (This post was last modified: 05-24-2011, 11:04 PM by Met.Eng..)
#3
RE: fresh-water corrosion, pitting and erosion
(05-24-2011, 04:50 PM)MichiganMan Wrote: The picture seems to illustrate a curious pattern to the pitting. It appears as though this is only happening on one side of the liner(s). Is that accurate?

If it is only one side, as it appears, then I would suspect that either the piston is not running concentric to the bore in operation (therefore wearing one side more aggressively), or there was some flaw to the casting process that left greater porosity on one side (which would tend to show greater corrosion-erosion effects). The cracking may suggest that there is a temperature issue present. How does the crack occur? Is there a tendency for longitudinal cracks? Spiderweb? Circumferential? Is the 90C measured by water temps in operation? If it is, then there may be some sort of quench cracking in play, and this is a much more difficult problem to solve than the above.

I suppose a root cause needs to be determined before deciding on the solution. In the absence of clear evidence it might be best to use some kind of a high carbon steel (try to maximize bond while matching thermal expansion to the base metal) with a high temp sealer and see what happens.

actually not on the one side, pitting starts high pressure water hitting area. after than pitting spread. if the pitting is much higherarea on the liner surface, then the damage occurs. But we never use up to this crack stage... Fx 700 hours, 800 h...
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05-26-2011, 09:01 PM,
#4
RE: fresh-water corrosion, pitting and erosion
(05-24-2011, 11:03 PM)Metallurgy_eng Wrote:
(05-24-2011, 04:50 PM)MichiganMan Wrote: The picture seems to illustrate a curious pattern to the pitting. It appears as though this is only happening on one side of the liner(s). Is that accurate?

If it is only one side, as it appears, then I would suspect that either the piston is not running concentric to the bore in operation (therefore wearing one side more aggressively), or there was some flaw to the casting process that left greater porosity on one side (which would tend to show greater corrosion-erosion effects). The cracking may suggest that there is a temperature issue present. How does the crack occur? Is there a tendency for longitudinal cracks? Spiderweb? Circumferential? Is the 90C measured by water temps in operation? If it is, then there may be some sort of quench cracking in play, and this is a much more difficult problem to solve than the above.

I suppose a root cause needs to be determined before deciding on the solution. In the absence of clear evidence it might be best to use some kind of a high carbon steel (try to maximize bond while matching thermal expansion to the base metal) with a high temp sealer and see what happens.

actually not on the one side, pitting starts high pressure water hitting area. after than pitting spread. if the pitting is much higherarea on the liner surface, then the damage occurs. But we never use up to this crack stage... Fx 700 hours, 800 h...

I think this type of pitting pattern is pretty common to liners. Like the original poster indicates, its the direciton of flow of coolant around the liners which causes the specific patterns. The real fix would be to change the original liner material. There are types that hold up much better than others. The good news is that I don't think it would be difficult to find a coating that would hold up better. Maybe alum bronze.
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05-27-2011, 08:39 PM,
#5
RE: fresh-water corrosion, pitting and erosion
How about inconel 625?
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05-29-2011, 05:45 PM,
#6
RE: fresh-water corrosion, pitting and erosion
Hi Metallurgy_eng

Looking at your photos and thinking about this for a while, I think the problem is not one of normal abrasion/erosion but one of cavitation (erosion caused by formation and collapse of vapour bubbles) caused by shock wave/vibration from combustion/piston movement and no doubt compounded by pressurised flow of coolant, high temperature differentials and corrosion.

The normal coating recommendations for cavitation erosion are for materials that resist surface fatigue and corrosion.

HVOF Inconel 625 I think is certainly worthy of consideration as with your thoughts on 316 austenitic stainless steel (good as its surface readily work hardens).

Other potential HVOF coatings that I think may be good and some recommended from other sources:
NiCrBSi, Stellite 6, WC/CoCr, aluminium bronze and CuNi.

My initial suggestion of aluminium in your first thread , does sound intuitively poor after seeing the photos, but I still think this is worth a try
1. cheap and cheerful (arc sprayed)
2. excellent corrosion protection (anodic coating and self sealing)
3. high thermal conductivity and high temperature resistance
4. Downside; poor wear resistance
I have developed aluminium coatings in the past for nucleate boiling surfaces for use in heat exchangers. Now I know this sounds counter intuitive to actually promote formation of vapour bubbles, but I think having much finer bubbles, more even dispersed over a larger area may reduce the damage compared to fewer larger bubbles that persistently target weak sites (graphite and pits). All too often I come across coatings that at first seem totally wrong for an application, but end up being the best solution. Murphy's law Happy0193 or maybe not being able to understand all the complex mechanism involved. Maybe totally wrong Ashamed0002 or a Eureka moment Sign0184
Some reading link
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