Galling of SS hardware 1

[jnims]

When specifying outdoor electrical equipment, we typically specify stainless steel hardware to prevent/reduce corrosion. I have had one manufacturer request the substitution of silicon bronze nuts to prevent galling. In this instance, the hardware is used on a component that might be removed in several years. Using dissimilar metals concerns me given that some stainless steels are seperated from silicon bronze on a galvanic table I have seen. Note: This hardware is not used to conduct current.

Is this an appropriate application of these two metals? What problems can occur as a result of their use?

Thanks

 

[israelkk]

You could use a silver plated thread nuts. This is common practice with commercial stainless fluid valves (used by companies such as Swagelok) and used by NASA.

 

[jnims]

I have read that silver plating and special lubricants can be used to minimize galling (and I may suggest one of these), but the manufacturer has suggested the silicon bronze nuts as their usual way. The only place I have found that references this way is a supplier of underground tanks. I am interest if this method has drawbacks

Thanks for your input.

 

[israelkk]

jnims

I am mostly experienced with the military and aerospace industry but done few projects for industrial and coammercial indusrtries.

As you pointed out "using dissimilar metals concerns me given that some stainless steels are seperated from silicon bronze on a galvanic table" (MIL-STD-889 titled dissimilar metals is the reference).

In the Aerospace industry the use of copper alloys is very restricted, just look in the MIL-HDBK-5 and see that almost the only copper alloy listed there is Copper Berylliun Alloy and they are usually plated against corrosion.

In the military they use more copper alloys such as aluminum bronze, phosphore bronze and naval brass but always they are protected with grease, for example in the cannons, fire arms etc.

If you can plate/coat the silicon bronze nuts with a coat such as Zinc etc. or keep them always coated with a grease then it may be OK. You should consult a corrosion specialist for the exact environment you have or follow the leads in MIL-STD-889.

There is one more option and it is to use anti gall alloys such as Nitronic 60 etc.

However the common practice in the industrial fittings bussiness to use stainless fittings with silver plated thread nuts. I am familiar with the products of the Ham-let compny who trys to compete with Swagelok and they use the silver coating to avoid galling. There web address is

http://www.ham-let.com/

you may find more info there.

 

[unclesyd]

It is not unusual to see the combination of SiBronze with SS. The SiBronze/SS fastener combination will not corrode very little if any, in any application that does not stay wet.
More common is the use if SiBronze bolts and nuts on Electrical equipment. In our plant this was the standard in all substations, silver plated bus bars bolted with SiBronze fasteners.

 

[metman]

jnims,
Some very interesting and helpful comments. To reiterate unclesyd "...that does not stay wet." Keep in mind that for the couple to conduct, there needs to be an electrolyte to complete the circuit. This could be moisture (not necessarily plain H2O)in the air.

Since this application is not "...used to conduct current." this means there is reduced chance for electrical pressure to drive the galvanic couple.

Given these two points, and unclesyss' comment "...It is not unusual..." and if it will always be used in a controlled environment, then it would appear to be a fair choice of materials. However, I would suggest that you supply a disclaimer(s) internally and for end users to dissuade misapplication.

I am not a corrosion expert just my 2 cents so maybe my suggestion is overcautious.

Jesus is THE life,
Leonard

 

[YLWEA]

Recognizing that your specific service environment could be anything from rural to urban to industrial, one thing is worth noting about copper alloys in electrical service. In environments where there is even small amounts of H2S present in the atmosphere then copper and many copper alloys and silver plated components can be a problem. Some geothermal power plant operators learned that the hard way as far back as the 1960s-70s. (Not sure, but I suspect that some areas of waste water treatment plants might have similar issues?)
I agree with the previous comments regarding galvanic corrosion probably not being an issue in electrical connections.

 

[TEV]

Most manufacturers of stainless tube fittings (Swagelok, HamLet, etc.) use silver plating on the ID threads of the nuts to avoid galling. Parker Hannifin uses Molybdenum Disulfide on their CPI fittings, which is particularly effective both as a lubricant and preventing galling. It is available in a number of forms, including spray-on, greases, vapor deposition, etc.

It is also very important to use a thread form that eliminates thread crest-to-root interference. This is accomplished by selecting a thread form with some truncation of the crest. If the crest is too high and "digs into" the root of the mating thread, you will have galling no matter what you put on the threads.

Carpenter Technology makes a stainless steel called "Gallthough" that is specifically developed to reduce galling. It is similar to Nitronic 60.

 

[jnims]

Thanks for all the information.

At first glance, asking for stainless steel hardware to reduce rust on outdoor electrical equipment seemed to be an easy solution. So much for first glances.

These forums are a great place to benefit from the experience of others.

Thanks again.