Corrosion issue with OTEVA 70 steel springs

[ta5ari]

In an automotive application we have a spring made of OTEVA 70 (55SiCr6); the spring is clamped between two Ergal plates. During its life the component is subject to pressurized air up to 4 bar; there could be moisture in the air.
On a 30% basis we find that the spring terminals (were the spring is in contact with the Ergal plates)are heavily corroded (red rust). This compromises completely the spring functionality.
I would like to understand:
1) why this happen?
2) why on 30% basis(I am investigating also the customer operating conditions)?
3) and what action I can take to prevent this from happening (coating or different material ecc).
Thank You

 

[swall]

I'm not familiar with the "Ergal" base plate material, but my initial assessment is that spring cycling is generating fretting (wear) debris and this fine debris is oxidizing, creating the rust that you are seeing. Application of an anti-seize compound or dry film lube would be one way of mitigating this problem, if your assembly can tolerate the presence of lube.

 

[TVP]

I agree with swall unless Ergal is a more noble metal like nickel or platinum.

 

[CoryPad]

Ergal is 7075 aluminium.

I think your spring may need a more functional coating like zinc flake + organic or PTFE, expecially if you can't have contamination.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[EdStainless]

Zn sounds like a good option.
I like the idea of a synthetic lube also. This might prevent the fretting in the first place.

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Rust never sleeps
Neither should your protection

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[swall]

Another possibility would be to anodize the 7075 aluminum and use the anodize film as a base for some sort of dry film lube or polymer coating. But I would like to hear from ta5ari to see what lubes or other coatings might be tolerated in his system. I would also like to know if the springs are located by some sort of pocket or abutment.

 

[kenvlach]

ta5ari, please provide a correct link to photo. Please also show the Al/steel contact detail. Strange that steel corrodes in contact with Al. Is the Al 7075 anodized or otherwise protected?

A Zn-5%Ni coating offers more corrosion & wear resistance than plain Zn and is galvanically compatible with the Al. A higher Ni content offers even more protection, but above 12% Ni no longer galvanically protects steel.
Per ASTM B841 (electrodeposited Zn-Ni alloys), to avoid hydrogen embrittlement, a spring steel of hardness > 31 HRC requires both a pre-plating stress relief (per ASTM B849 or ISO/DIS 9587) and a post-plating hydrogen bake-out (per ASTM B850 or ISO/DIS 9588).

 

[ta5ari]

Dear all, thank you for your help.
I would like to add some more info.
The application is quasi-static, i.e. the spring displacement is in the order of 0,1 mm therefore there is no friction at all with the plates. The spring is retained by two Ergal plates guided by inside diameter. Spring dimensions are roughly,
external diameter 14mm
internal diameter 8mm
length uncompressed 24mm.
The spring is located in a chamber were compressed air is blown by the engine turbocharger.

 

[CoryPad]

I suspect the Ergal plates are anodized, which provides them some corrosion resistance, plus it is a hard surface that may abrade the steel spring.

I would investigate a zinc + aluminium flake coating like:

Delta MKS 3000 series from D[ö]rken

http://www.doerken-mks.de

or Magni 565 from the Magni Group:

http://www.themagnigroup.com

They offer excellent corrosion resistance for the steel, and they are formulated to prevent galvanic corrosion of aluminium parts that are in contact. Plus, they offer versions that are lubricated, which will reduce the tendency for wear.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[EdStainless]

I like the sound of the Zn-Ni plating. Good call Ken. This would take care a multiple problems at once.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm