Corrosion resistant coating for 4140 shaft

[Thorley]

Hello All.

The subject of this topic is corrosion resistant coatings for 4140 base material. The application is a shaft of a linear actuator used in a saltwater spray environment (aft deck of a ship). The current shaft is processed with a conversion coating known as QPQ (quench polish quench), which I understand is a salt bath nitriding process used to develop an oxide layer at the surface of the base material to aide in wear and corrosion resistance.

We have recently observed surface rust on some of the shafts, leading us to believe perhaps this is not the appropriate coating for the saltwater spray environment. I have further researched the QPQ coating, and found data suggesting ASTM B117 corrosion test results yield ~ 500-600 hours prior to the onset of visible rust for this coating on a 4140 substrate.

We are looking to select a different coating for the shafts, however I am unable to find data on what is most suitable. Our goal is to just swap out the coating process, i.e. NOT have to change shaft dimensions to accommodate a thicker, non-conversion coating (there are a lot of shafts machined already which would require further machining if we begin to mess with tolerances).

My question is: What is the suggested coating for my application, and what test results (i.e. ASTM B117) should I look at in order to get an objective measurement of the coating performance with respect to corrosion resistance?

Thank you

 

[EdStainless]

How thick is the current coating?
There are a number of plating options, but they would need to be thick enough to be effective.
We have supplied cold finished 6%Mo stainless tube for seawater applications, it lasts forever. Alloys like 316 or a PH grade would be better than what you have now, but they would likely rust also eventually.

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P.E. Metallurgy, Plymouth Tube

 

[tbuelna]

The molten salt bath ferritic nitrocarburizing with post-oxidation (Melonite QPQ) you describe is a case hardening process that also provides corrosion resistance. The process temperatures involved will limit the core strength of your 4140 shaft to around 180ksi UTS. If you do not require a case hardened surface on your shaft, there are many corrosion resistant steel alloys having adequate strength that you might consider using instead.

 

[Thorley]

Thank you both for your suggestion. As indicated in the original post, our goal is to select a different coating for the existing 4140 shaft to provide better corrosion resistance. That said, would any of the following work?

1. Nickel Plating
2. Diamond Like Coating (DLC)
3. Ceramic coating (ceram-kote) *This is additive so will require rework to the existing shafts
4. PTFE coating (Xylan or equal) *this is additive, but usually pretty thin (less than 1 mills DFT)

With respect to #1, is there a guide to for thickness vs. performance? I am not even quite sure how to approach the problem, i.e. what measurement I can use to verify the shaft will exhibit corrosion for X years.

Thank you

 

[tbuelna]

One nice thing about the LSB nitrocarburize process you are currently using is that it does not result in any significant dimensional change. As for electroless nickel or thin-dense chrome plating, they will create some dimensional change. Here is a graph showing some salt spray test results of hard chrome, electroless nickel and an LSB nitrocarburize QPQ process. DLC would do the job, but would be very expensive. Ceram-Kote is applied by spraying and oven cured, so it can be difficult to achieve a uniform coating thickness on parts with complex surface geometry.

One type of coating you might take a look at is IVD aluminum. It has good corrosion resistance and very uniform thickness, but may not provide adequate abrasion resistance for your application.

 

[EdStainless]

The real problem with any of these coatings gets down to the holes in them.
With Ni or Cr you get a very hard corrosion resistant surface, but any minor imperfections will result in localized corrosion of the underlying steel.
Ceramic coatings come in many varieties. Some go down wet and are then cured. These are not hard (for ceramic), durable, or well sealed. Others go down using a flame or HVOF process. These are high density and very hard, but also expensive.
A soft coating like PTFE or a urethane would provide protection but it would not be mechanically robust.
A cold or hot sprayed Al/Zn coating would provide great corrosion protection by virtue of galvanic protection, but they are not hard or wear resistant.
You need to decide which performance characteristics matter the most to you and work from there.

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P.E. Metallurgy, Plymouth Tube

 

[Thorley]

Tbuelna and EdStainless,

Thank you for the thoughtful replies. I have used CERAM-KOTE for an actuator on a subsea XMASS tree. What we did was applied it (ceram-kote) thick then precision ground to final dim's. This was applied onto INCONEL 718, so the base material was already corrosion resistant. The function in this case was to prevent bio accumulation. Either way, for my current application I would have to alter the part geometry, which I want to avoid.

Tbuelna: If I am reading your graph correctly, the QPQ process yielded the best results? If so, it looks like I should avoid chrome or nickel plating as it will yield poorer results (at least with respect to ASTM B117 salt spray testing)

EdStainless: Yes I am looking at a few praxair coatings, specifically a PTFE coating. I have used XYLAN coatings for fasteners and found good results with respect to corrosion, however wear would be a concern as you indicated. There is another coating (SERMETEL 2F1) which seems to offer good abrasion resistance. This may be a good trade-off.

 

[WKTaylor]

Thorley...

A salt-spray environment on steel part is brutal... Can we 'assume' that this actuator/shaft design already has a moisture/dust-seal boot... and that You are looking for added protection???

IF-so, then engineering chrome [over flash-nickel] or hard/dense-nickel plating on EXPOSED surfaces might do the trick. Are the nitride surfaces 'as-machined' or 'as-shot-peened'?

Typical info on 'seal-boots' that I mentioned are as follows...

NOTE.
The interiors of Seal-boots can be coated with corrosion preventive grease, oil, VCI etc, to further enhance corrosion protection.

http://machinedesign.com/archive/give-cylinder-rod-contaminants-boot

https://www.google.com/search?q=hyd...v&ved=0ahUKEwjOkL3s19HRAhUKxmMKHSNKDn4QsAQIGQ

http://www.sealsaver.com/

http://www.dynatect.com/protective-covers/bellows

http://www.customadvanced.com/rubber-boots-and-sleeves.html

etc...

Regards, Wil Taylor

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