Nickel + Chrome coating on 17-4 PH -> Still corrodes!! 1

[cnuk]

I have an oilfield application where large 17-4 PH stainless steel part is exposed to a 100,000 ppm chloride solution at 150-175 degC. The part was initially only chrome plated and it was severely corroded after only 24 hours. Deep pits formed which is not that surprising.

To solve the problem it was suggested that we apply a .002" sulfamate nickel layer to the part before applying the chrome coating (chrome only for abrasion resistance). Due to the operation of the part, the threads on either end of the part are masked before the nickel and chrome plating process so the finished product still has bare 17-4PH exposed to the fluid. We did some corrosion testing and found that the nickel coating did essentially nothing to prevent corrosion.

My questions are:

1. Does the entire part have to be encased in nickel for it to be beneficial? ie: no exposed base metal, unlike what I have in my application? This is the critical question for me!

2. Some have suggested that removing the coating all together would have made for less corrosion. The cracks in the chrome layer provide an ideal site for crevice corrosion and pitting. Other than wear problem, I believe this. Do you agree?

3. What other base metal that does not cost a fortune would perform better? Some have suggested Monel, but a 7" diameter by 200" long bar of Monel must be worth a small fortune.

Any other comments or references would be greatly appreciated. I would like to send an email saying that Nickel coating using the method we are doing is useless and we should save our money. Before I send it, I'd like some confirmation.

Thank You!

 

[EdStainless]

1. It shouldn't. The exposed areas should corrode and the protected areas should be fine.
2. How bad is your plating? Yes, this is sort of true, but in the short time that you are talking about it is hard to belive that you would have that many cracks and flaws in the plating. This is often why Ni is used. It is a more ductile under layer to provide some protection when the Cr cracks.
3. What strenght level do you need? What product shape?

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Corrosion never sleeps, but it can be managed.

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[cnuk]

1. So you're saying that nickel, if done properly, should still be effective even though the entire product is not coated with nickel? Our internal corrosion expert said something different...hmmm. Is there not a galvanic corrosion cell setup between the exposed 17-4PH base and the surrounding material? He implied that any unprotected base matterial would be an anode and because of its small size relative to the large cathode, corrosion rate may actually increase.

2. How bad is our plating? Good question. We use the local vendors and rely on their expertise. Chrome is full of cracks, but I have never examined the sample to see what the nickel coating looks like.

3. Material strength have to be 110-120 ksi yield strength. The product shape starts with a round bar(7" dia x 200" long) and has a contour milled into the OD of the bar. Threaded connections on each end.

Thank You for your input.

 

[ApC2Kp]

To cnuk,
The high level of chlorides would be a problem even for some of the duplex stainless steels. The size of the part would definitely be high cost in a nickel alloy. Does the part need to be of solid alloy or Monel? Could a sleeve or a welded liner be placed over the part?
There is a coating, NEDOX, from General Magnaplate that applies a thin PTFE layer over a porous nickel base. It could extend the life somewhat, if it is less porous than the chrome plating. The size of the part could be limitation for their process. Maybe a plasma arc applied ceramic coating could provide the added corrosion protection and some enhanced wear resistance you need.

http://wwww.magnaplate.com

 

[EdStainless]

1. Yes, there will be galvanic attack. It should only be at the edge of the plating though.
2. I expect crack in the Cr, but the Ni should be intact. It sounds like it isn't.
3. At that strength and chloride level you should really look at some high alloy materials. I know that it is a lot of money, but they would work.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[cnuk]

We did a lab corrosion test with 1" x 2" x 1/8" coupons coated in chrome and nickel. Similar to our rotors, there is a tapped hole in the coupon that was not coated and has exposed base 17-4. It was used to hang the samples in our corrosion autoclave. Test was for 72 hours at 150 degC and 1000 psi (downhole oilfield condition) The corrosion pit that formed was no where near the exposed base metal (tapped hole), but at the other end of the coupon where is is coated in nickel and chrome. If I could figure out how to post a picture I could show you. This is why I was led to believe that nickel provided little benefit in terms of corrosion. From what I read in this thread, this is not what one would expect. I'm starting to wonder if we had poorly prepared samples.

 

[EdStainless]

There would be more driving force for the formation of a pit at a minor holiday in the plaiting than at a large exposed location. The relitive surface areas of annode/cathode will be worse.

I would guess that under low magnification there is a lot of etching and the start of pitting on the bare metal portion.

How long do you really want these to last? Even if you got a part with great corrosion resistance how likely is it to be damaged other ways?

A rotary forged bar of superduplex sure sounds like a better bet for this.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm