Progressing cavity pump - unusual rotor corrosion - why

[NickJ67]

Hi,

Long time no post - and first post in the Corrosion area.

Needing advice on some rather unusual corrosion on the rotor of a progressing cavity pump. I've been working with these more than 20 years now (applications support, tech sales and some service work) and not seen anything quite like this before. We've previously used this materials combination successfully on the "same" product in several other locations. I'd previously considered bentonite to be moderately abrasive (depends on source quality) but not chemically aggressive. Closest I've seen to this in the past is when a client started dosing ferric chloride upstream without telling any one. That is definitely not the case here.

For scale, the rotor diameter is 45mm. It's a 2-stage pump.
The rotor material is induction-hardened C45 tool-steel running against a nitrile stator.
Product is 2% sodium bentonite mixed with towns water (site is in western Austria). Ambient temperature - approx 10 - 15ºC at present.
Pump running speed is ~200 rpm
Flooded suction 1 - 2m with a short, well-sized pipe. Pump runs very smoothly - almost silent. This is not cavitation.
Discharge is 2 Bar against a loading valve and the pump runs 50% 24/7 (duty standby, 2 hour switch over)

First picture (less damage) is after 8 days running. Performance still ok.

Second picture is after 3 weeks. It's still moving product, but the rate is well down.

Also attached is a scan of an analysis done - we were looking for chlorides - didn't find much < 30mg/l. The towns water alone was 4.0 - 5.6 mg/l. pH measured at 8.6

We've switched the rotor on the first pump to 316ss - haven't seen the result of that yet. Also have a 316/hard-chrome plate waiting for the second pump. Hopefully this will make the problem go away, but would really like to understand what the root cause is! Any ideas?

Thanks

Nick

 

[EdStainless]

Was the 8.6 the pH of the slurry being pumped?
So this is just 1045 carbon steel. What is the specific heat treatment?
This really resembles high pressure CO2 attack on steel.

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P.E. Metallurgy, consulting work welcomed

 

[NickJ67]

Yes, 8.6 is the pH of the slurry.

Yes, 1045 class carbon steel. Simple induction hardening where the machined item is passed through an induction loop heating to approx 850 - 900ºC. I don't have any detail on this.

It's our typical selection for general waste-water and slurry work where product abrasivity is a bigger concern that corrosivity. It's pretty hard (55Hrc typically), but its not corrosion resistant.

Also worth noting that on this same site there are also two identical pumps handling lime slurry for pH correction elsewhere in the process. The lime slurry is mixed using the same water as the bentonite system and these pumps are working just fine and show no sign of corrosion to date. Pretty sure the bentonite is the source of the problem

Thanks
Nick

 

[EdStainless]

Some trace impurity in the clay? Br or Fl?

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P.E. Metallurgy, consulting work welcomed

 

[NickJ67]

Yes, that's our working theory - though exactly what remains a mystery. Your Br or F maybe. Nobody is very keen to pay for analysis.

Pumps will be opened and inspected week after next. Reckon the tool steel rotor will be done by then. It will be replaced by 316ss/hard-chrome plated. Curious to see how the 316 one is doing. Abrasion could be problem, there are more hard solids in this than usual too.

Also curious to see how the cast-iron casings are doing....

 

[EdStainless]

If 316 works why not use 17-7PH? You could likely get away without plating it.
But honestly I am a little concerned about using an austenitic SS.
Do you work in any high alloy ferritic grades (446) or duplex SS?

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P.E. Metallurgy, consulting work welcomed

 

[NickJ67]

Got to work with the options the manufacturer provides. Neither of two major German manufacturers I've worked for have ever showed any interest in 17/4PH. 304 or 316ss are the mainstream, with or without hard-chrome plating.

Duplex in 22% Cr form is also semi-standard, but pricey. 25% Cr is possible too be generally only in petrochem industry and priced accordingly. Current lot also do a full ceramic version (with bonded metallic joint head), which is astonishingly hard-wearing but really expensive. What is the concern with austenitic?

 

[EdStainless]

Austenitic SS is a softer substate and will likely not support a hard coating under high loads.
And with voids and cracks in plating the risk of crevice corrosion and SCC is fairly high.
There is no justification for 2205 being much more expensive than 316, especially when the 316 has to be plated.
If you want an austenitic SS why not Nitronic 50?
It would have similar corrosion resistance to 316 and the surface will work harden to some very high levels.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed