Gold Plating on Spring

[Ramijreng]

I need to spec an appropriate MIL-SPEC-45204C gold plating to go over a Beryllium copper spring C172. The spring is going against a housing that is also gold plated so we are worried about galvanic compatibility. What would be the correct type, grade and class? How do you usualy come up with this data?

Thanks

 

[israelkk]

It is MIL-G-45204C or the new one MIL-DTL-45204D. Why are you worried about galvanic compatibility if gold plating is in contact with gold plating

 

[MiketheEngineer]

I beleive that is why he wants to do it. With gold on gold - no problem.

 

[Cockroach]

Some expensive spring. Check the gold prices lately? Surely there are other material possibilities, unless you're building a Rolex watch.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[Ramijreng]

The reason for the gold plating on the spring is to prevent the galvanic corrosion. Our electrical engineer specs the maximum resistance allowable by the spring. I evaluated the material choices, I.E. Music wire, SS, Phosphorus bronze, and beryllium copper. Both the beryllium copper and phosphorus bronze are the only materials that conform to the electrical spec. Once I apply the mechanical needs the phosphorus bronze gets eliminated. All I am left with is Beryllium copper. However the housing for the spring is also gold platted. From my documentation I see that Beryllium copper (group 3) will react with gold platting (Group 1). So I know I need Beryllium copper to conform to electrical and mechanical requirements, I now need the gold plating so that the spring can conform to harsh environmental requirements. But how do you specify the correct Type, Grade and Class???

 

[TVP]

The answer to your original question is that you need some combination of knowledge, intuition, and experience in order to properly specify materials and coatings for real-world applications. In the absence of these, you can either solicit opinions from others or just guess. Do you have any information about the end-use environment for this product, meaning temperature, exposure to chemicals, contamination with abrasive particles, etc.? One simple way to proceed would be to specify the same details that are used for plating the housing. If this were my responsibility, I would be looking for any other possible solution to provide corrosion resistance, due to the cost and other issues associated with gold plating.

 

[Ramijreng]

Hi TVP, I believe Beryllium copper is the best material for this application given all the parameters. Regarding the environment, this part is for outdoor use, temp -50 to +75°C, and humidity will be of concern, howeever the housing will keedp the part safe from particles. My orriginal guess was as you said to just use the same gold plating as the housing, however the gold plating of the housing was designed around cycles of use, electrical requirements and mating materials, there was some type of logic applied to it. However through internet searching I discovered an IEEE article "The Effect of Gold-Tin Intermetallic Compound on the Low Cycle Fatigue Behavior of Copper Alloy C72700 and C17200 Wires" The paper uses a class 0 gold thickness, and there seems to be some type of relationship between plating and fatigue limit. The reason why I was going with the beryllium copper is that the phosphorus bronze hits the fatigue limit well before my entended cycle lifespan. Now I am afraid that the wrong spec on the spring will decrease the lifespan of the spring. However the paper does not tell me the grade, type and class needed for springs.

 

[israelkk]

As I see it a lot of information is missing from your post.
"The spring is going against a housing" What do you mean by that? Is it just touching the housing or it is rubbing against ? If it is rubbing, is it an occasionally contact or continuously rubs and in what linear velocity? Is it a connector type product of a slip ring/motor type product? etc. If the spring made of strip or wire?

If it rubs against the housing the harder grade seams to be the choice (for the spring and the housing).

As to the "electrical engineer specs the maximum resistance allowable by the spring" does it includes the contact electrical resistance which is crucially depends on the contact surface pressure between the spring and the housing. Too much pressure will lower the life cycle of the gold plating. Too low will create more electrical resistance.

To assure the correct local contact surface pressure the contact area must be carefully designed.

To my opinion you can not avoid an extensive development and testing phase. Am I correct to assume that this is for aerospace/space use? You may even have to resort to more exotic platings such as rhodium or platinum if gold against gold will not deliver.

From my experience you have to tackle the contact area pressure first.

 

[Ramijreng]

The spring is inside a howsing, what I mean is think of a pogo test pin, the type you use for programming boards.

My instinct does indeed say type II grade D (The hardest choice). Nad I am also leaning towards class 0 based on the IEEE paper. However as you can see I feel a little over my head on this spec, I tend not to like to spec things I am not familiar with. The contact pressure was already calculated based on the minimum required for good contact under our conditions. Basically everything has been engineered, but just wanted to make sure I was not going to lower the fatique limit of the spring based on the coating.

Thanks,

 

[israelkk]

Can you tell us the contact "pressure" (not the spring force)?
I am unaware of any problem that pure gold plating may cause to beryllium copper spring? The article you mentioned talks about Gold-Tin, this is a different issue. What I am aware of (from memory) is an inner metallic corrosion that may occur when soldering bare beryllium copper and melted Tin.

 

[TVP]

Electroplating the spring will almost certainly reduce the fatigue strength, at least somewhat. This is true for almost any spring material and plating type. The best case is to minimize the decrease in fatigue strength, which can be done by thermally stress relieving the spring prior to plating. For this alloy, a typical treatment is 1 hour at 230 C (450 F). Specify the minimum coating thickness possible. Immersion plating (electroless or autocatalytic) is better than electroplating with regards to fatigue strength of the spring material.

 

[Ramijreng]

Thank you all for your help.