Why cadmium plated bolts rather than stainless steel?

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peter-h

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peter-h
I wonder why there is the widespread use of cadmium plated steel fasteners, rather than stainless?

Cad plated stuff doesn't last for ever, but stainless does (unless it's crap stainless).

And the pricing seems comparable. Cadmium has been outlawed in most applications outside aviation and military, so all the parts are pricey.

Is there some electrolytic/corrosion issue with a stainless bolt going into a traditional cad plated steel nutlock (which is probably not available in stainless steel)?

I know stainless on stainless can over many years weld together (so it is claimed) though this seems to have been solved with current alloys, and stainless nuts and bolts are in widespread use outside aviation.
 
peter-h said:
I wonder why there is the widespread use of cadmium plated steel fasteners, rather than stainless?

Cad plated stuff doesn't last for ever, but stainless does (unless it's crap stainless).

And the pricing seems comparable. Cadmium has been outlawed in most applications outside aviation and military, so all the parts are pricey.

Is there some electrolytic/corrosion issue with a stainless bolt going into a traditional cad plated steel nutlock (which is probably not available in stainless steel)?

I know stainless on stainless can over many years weld together (so it is claimed) though this seems to have been solved with current alloys, and stainless nuts and bolts are in widespread use outside aviation.
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Where the exchanges are not allowed, the issue is a difference in strength and mechanical properties. In applications where exchange is allowed, the issue is cost. I'd open my plane up a lot, so every couple of annuals I'd buy a stainless hardware kit for my plane ($100 or so for a light twin at the time) and change it all out. There is a reactivity issue between aluminum and stainless which is why Duralac or similar barrier coat should be used.
 
Can Duralac be used between a bolt and a nutplate??

difference in strength and mechanical properties
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Can you be more specific? The vast majority of small fasteners on an aircraft do trivial stuff like holding cable-carrying P-clips to structures, so obviously strength is not an issue. I am not talking about the engine bolts :)
 
peter-h said:
Can Duralac be used between a bolt and a nutplate??



Can you be more specific? The vast majority of small fasteners on an aircraft do trivial stuff like holding cable-carrying P-clips to structures, so obviously strength is not an issue. I am not talking about the engine bolts :)
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Duralac can always be used as an insulator against electrolysis. The small hardware of which you speak is typically allowed to be exchanged with SS and there are companies that provide entire kits for types. They are cad to reduce manufacturing costs.
 
peter-h said:
Can you be more specific? :)
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The sheer strengh of steel is 186,000 pounds per square inch..

The sheer strengh of stainless is 137, 000 pounds per square inch
 
Tom-D said:
The sheer strengh of steel is 186,000 pounds per square inch..

The sheer strengh of stainless is 137, 000 pounds per square inch
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Can stainless be hardened like steel?
 
Tom-D said:
The sheer strengh of steel is 186,000 pounds per square inch..

The sheer strengh of stainless is 137, 000 pounds per square inch
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Since steel's single-shear strength is typically 70% of tensile, it would take some super steel to get 186KSI. AN bolts have a tensile of around 120ksi and would have a shear strength of 84ksi. Stainless bolts would be much weaker.

Dan
 
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Henning said:
Depends on the grade/alloy, some yes, some no. Shiny hardware grade stainlesses generally no.
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There's a wide range of "stainless" steels. My Machinery's Handbook devotes many pages to their various compositions and properties. Some are magnetic, some not. Some are austenitic, some not. Heat treatment depends entirely on the various alloys and carbon content and they're all over the map with the different stainlesses.

Stainless is good where corrosion is a real hassle. It's commonly found in hostile environments like refinery and pulp mill equipment that has to handle enormously corrosive chemicals at higher temperatures, and in aircraft exhaust components. But to substitute stainless hardware for steel without a total re-engineering of the whole item is asking for a failure. And it's horribly expensive, containing costly metals like chromium and nickel.

Dan
 
Dan Thomas said:
Since steel's single-shear strength is typically 70% of tensile, it would take some super steel to get 186KSI. AN bolts have a tensile of around 120ksi and would have a shear strength of 84ksi. Stainless bolts would be much weaker.

Dan
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The stainless bolts that I have been exposed to are very prone to galling as well.
 
Generally, the more "stainless" the alloy, the crappier it is for durability, tensile strength, fatigue resistance, and machinability. The only good thing about stainless is that it doesn't rust.

With such high pressure to optimize strength and weight in aircraft, it's no wonder that Cad-plated steel is a popular choice.
 
Dan Thomas said:
Since steel's single-shear strength is typically 70% of tensile, it would take some super steel to get 186KSI. AN bolts have a tensile of around 120ksi and would have a shear strength of 84ksi. Stainless bolts would be much weaker.

Dan
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Do your home work, the ASTM standard for aviation hardware is the source
 
I am still looking for a reason why SS is not used in the myriad places on an airplane where the tensile or shear strength is very obviously not an issue (by several orders of magnitude) and where cadmium plated screws corrode quite happily after a few years.
 
peter-h said:
I am still looking for a reason why SS is not used in the myriad places on an airplane where the tensile or shear strength is very obviously not an issue (by several orders of magnitude) and where cadmium plated screws corrode quite happily after a few years.
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I told you twice already, "Because it is cheaper" end of story. If you want to spend the money and replace them with SS, you are free to do so and there are even companies out there that will provide you with presorted "kits" of hardware for replacement on most popular types.
 
Tom-D said:
Do your home work, the ASTM standard for aviation hardware is the source
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Post a source, maybe? ASTM is for industrial hardware. AN/MS/NAS bolts use DoD specs, since they were all military to start with.

AN bolt tensile strength, section 12.4 ot this page:
http://www.auf.asn.au/scratchbuilder/hardware.html#bolted_joints

or

http://exp-aircraft.com/library/alexande/hardware.html

or

http://www.mechanicsupport.com/articleBoltTensileStrength.html

or

http://www.aircraftspruce.com/menus/ha/bolts.html

NAS bolts are considerably stronger than AN, around 160ksi. And much more expensive:
http://www.aircraftspruce.com/catalog/hapages/nas1103_1106.php

Relationship of tensile to shear, here:
http://www.roymech.co.uk/Useful_Tables/Matter/shear_tensile.htm
or
http://www.portlandbolt.com/faqs/bolt-shear-strength-considerations

There's considerable disagreement regarding the shear figure, but it's much less than 1.0 in any case. Installing the bolt in double (or more) shear gains the strength we're after.

Dan
 
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peter-h said:
I am still looking for a reason why SS is not used in the myriad places on an airplane where the tensile or shear strength is very obviously not an issue (by several orders of magnitude) and where cadmium plated screws corrode quite happily after a few years.
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It's my understanding that there can be some galvanic corrosion between the SS and the aluminum that may make removal difficult. Also SS isn't very paintable.
 
Dan Thomas said:
Stainless is good where corrosion is a real hassle. It's commonly found in hostile environments like refinery and pulp mill equipment that has to handle enormously corrosive chemicals at higher temperatures, and in aircraft exhaust components. But to substitute stainless hardware for steel without a total re-engineering of the whole item is asking for a failure. And it's horribly expensive, containing costly metals like chromium and nickel.

Dan
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Actually, in really hostile environments you typically see copper nickel alloys.

As the OP suggest, there are areas where there is no "rengineering" involved to replace the fastener. Nearly any fastener a person can handle is overkill for a typical GA inspection cover or fairing. These are steel because of cost factors and it is perfectly acceptable and actually commonplace for them to be replaced with SS hardware. I do suggest though not to go with store grade Chinese SS hardware as it is typically a 304 grade done with little QC at the mill and will rust. Even better one could order Monel fasteners, but most people balk, "for screws?" when they see the price tag (then I show them the price on Bronze:eek:)
 
gismo said:
It's my understanding that there can be some galvanic corrosion between the SS and the aluminum that may make removal difficult. Also SS isn't very paintable.
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You can get the same problems with cad steel fasteners that have been run a few time. Use Duralac or similar regardless. SS screw heads get a dip in weld pickle gel and the surface will now take paint nicely. It's the polish finish that causes the problems with paint adhesion, not the SS in particular.
 
Henning said:
Actually, in really hostile environments you typically see copper nickel alloys.

As the OP suggest, there are areas where there is no "rengineering" involved to replace the fastener. Nearly any fastener a person can handle is overkill for a typical GA inspection cover or fairing. These are steel because of cost factors and it is perfectly acceptable and actually commonplace for them to be replaced with SS hardware. I do suggest though not to go with store grade Chinese SS hardware as it is typically a 304 grade done with little QC at the mill and will rust. Even better one could order Monel fasteners, but most people balk, "for screws?" when they see the price tag (then I show them the price on Bronze:eek:)
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There are stainless "Trim" kits for most GA airplanes available from Skybolt.
http://www.skybolt.com/

These are not the sort of fasteners I was talking about. I'm referring to structural fasteners. The OP, I think, was talking about engine hardware, which is mostly structural.

Dan
 
Dan Thomas said:
There are stainless "Trim" kits for most GA airplanes available from Skybolt.
http://www.skybolt.com/

These are not the sort of fasteners I was talking about. I'm referring to structural fasteners. The OP, I think, was talking about engine hardware, which is mostly structural.

Dan
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He addressed both in the OP and I addressed both in post #2 actually he even modified to "not engine fasteners"..
 
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Henning said:
He addressed both in the OP and I addressed both in post #2 actually he even modified to "not engine fasteners"..
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Ah. I went back and re-read that. You're right. I was getting all excited about nothing.

Dan
 
Apologies for resurrecting this thread, but I thought instead of posting a whole new one, I'd check with you all on this. So I'm still a little fuzzy after reading through all of this. I'm looking at replacing the MS27039-0809 Machine Screws that hold my fuel tanks to the wing and am trying to decide which will be best in the long run, cadmium plated or stainless. The difference seems to have grown since this original post. I can buy 250 cadmium plated screws for $30, but the same number of stainless costs $150:

http://www.aircraftspruce.com/catalog/hapages/ms27039.php

Does anyone shed some more light on this topic? I did not plan on painting them if that matters. I prefer the metallic look to them.
 
BGF_Yankee said:
Apologies for resurrecting this thread, but I thought instead of posting a whole new one, I'd check with you all on this. So I'm still a little fuzzy after reading through all of this. I'm looking at replacing the MS27039-0809 Machine Screws that hold my fuel tanks to the wing and am trying to decide which will be best in the long run, cadmium plated or stainless. The difference seems to have grown since this original post. I can buy 250 cadmium plated screws for $30, but the same number of stainless costs $150:

http://www.aircraftspruce.com/catalog/hapages/ms27039.php

Does anyone shed some more light on this topic? I did not plan on painting them if that matters. I prefer the metallic look to them.
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If you don't paint cadmium, it will rust. If you use stainless, be sure to use an insulating compound to prevent electrolysis.
 
Thanks Henning. For this particular application (holding fuel tanks in a wing), do you have a particular suggestion? Each tank (4) gets roughly 50 screws, plus or minus.
 
BGF_Yankee said:
Thanks Henning. For this particular application (holding fuel tanks in a wing), do you have a particular suggestion? Each tank (4) gets roughly 50 screws, plus or minus.
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I use a product called Duralac, I have no idea why it's nearly impossible to find in the US. Any galvanic isolation compound will work, I've seen a couple of Teflon based ones.
 
peter-h said:
And the pricing seems comparable. Cadmium has been outlawed in most applications outside aviation and military, so all the parts are pricey.
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Have you ever priced corrosion resistance (stainless) aircraft bolts vs cad aircraft bolts? :idea:


Have you noticed any corrosion surrounding the stainless screws in aluminum sheets? :dunno:

See page 2 of 4. https://www.fastenal.com/content/feds/pdf/Article - Corrosion.pdf

Stainless fasteners into aluminum - "The corrosion of the base metal is increased by the fastener."
 
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Hawker (and a few others I'm sure) literally bolt the wings skins to the spars and ribs using a countersunk steel bolt that has a coin slot head (not phillips or straight blade) It is not uncommon to remove several hundred of them for corrosion cleanup during a full repaint. Replacing the fastener and finding minor pitting of the base metal.

I can't imagine how bad the corrosion of the aluminum would be if there were all stainless bolts. :hairraise:


Citation X uses about 500 stainless screws to hold the lower forward wing-to-fuselage composite fairing on. It seems like I had to drill out a 20+ of them every time those fairings were removed. I soaked all screws in oil and set the screw gun to low to avoid galling but sometimes the nutplates just screw you anyway.
 
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Henning said:
Duralac can always be used as an insulator against electrolysis. The small hardware of which you speak is typically allowed to be exchanged with SS and there are companies that provide entire kits for types. They are cad to reduce manufacturing costs.
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Mastinox was the bomb on Hawkers. A chromated yellow paste that Boeing banned the use of after a union member saw the skull & bones on the label.

A great option would be P/S 870. Available pretty much anywhere.


http://www.ppgaerospace.com/Products/Sealants/Corrosion-Inhibitive.aspx
 
bnt83 said:
It seems like I had to drill out a 20+ of them every time those fairings were removed. I soaked all screws in oil and set the screw gun to low to avoid galling but sometimes the nutplates just screw you anyway.
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When a nutplate spins I get that :mad2: feeling...
 
peter-h said:
I wonder why there is the widespread use of cadmium plated steel fasteners, rather than stainless?

Cad plated stuff doesn't last for ever, but stainless does (unless it's crap stainless).

And the pricing seems comparable. Cadmium has been outlawed in most applications outside aviation and military, so all the parts are pricey.

Is there some electrolytic/corrosion issue with a stainless bolt going into a traditional cad plated steel nutlock (which is probably not available in stainless steel)?

I know stainless on stainless can over many years weld together (so it is claimed) though this seems to have been solved with current alloys, and stainless nuts and bolts are in widespread use outside aviation.
Click to expand...

Because stainless and aluminum and a little moisture create a battery, corroding the aluminum. The plating on steel fasteners is designed to resist corrosion.
 
When replacing the screws holding the fuel cell covers on your Cessna product, either the stainless kit from spruce and use nickel antiseze from NAPA

OR

the AN525 screws and nickel antiseze from NAPA. (much cheaper)

then paint.
 
I'm surprised by all of this. The electronics industry switched from cad to zinc coating on normal duty fasteners many years ago because of toxicity issues. Are zinc coated fasteners ever used/approved for aircraft use?
 
BGF_Yankee said:
Thanks Henning. For this particular application (holding fuel tanks in a wing), do you have a particular suggestion? Each tank (4) gets roughly 50 screws, plus or minus.
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There are kits available for just that purpose. AS sells them.
 
Old Geek said:
I'm surprised by all of this. The electronics industry switched from cad to zinc coating on normal duty fasteners many years ago because of toxicity issues. Are zinc coated fasteners ever used/approved for aircraft use?
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That would be called "galvanized" coating and while it may be fine in real life I'm sure the snobbie types would cringe at the thought. I'm sure it would work well as we use zinc blocks for sacrificial anodes on aluminum boat parts to prevent them turning into AlO2. On the other hand we're required to use spec'd hardware and I dont know of any aircraft grade hardware for structural use that is galvanized rather than cad plated. Since a lot of the structural stuff uses close tolerance or MS hardware I'm not sure if you could find legal stainless replacements on top of the fact that there isn't much room for any sort of paint or coating if you could use it. The other thing about stainless is that as others noted it doesn't have the shear strength of high strength alloy steel used for bolts and on top of that it tends to work harden or crack in critical places that even hardened steel alloys won't do. There is some stainless hardware out there in use in Airliners but a lot of that hardware gets tossed or ndt'd during inspections which drives up the cost even more. Wing bolts in your Cessna or Beech? Not a chance on my toy. I'm fine with cad plate. Its worked for a long time and nothing so far is any better. As for the covers on your wing tanks stainless should be fine and its available. Cost is up there though.....


Frank
 
Just be sure to use antiseize on the threads - and don't allow any to get near the phillips head or they will cam out next time. I suggest using a toothpick to put a little on the internal threads and otherwise keep it away from the screws. Use a new unplated driver bit to minimize the head damage when tightening. The next guy will appreciate it.
 
Sorry for being late for the show. Stainless is a grade of corrosion resistant steel. Often corrosion resistant steel (CRES) are confused as stainless. There are numerous grades of corrosion resistant steels and stainless steels. Some corrode. Surgical Stainless is the highest grade of stainless.
 
GlennAB1 said:
Sorry for being late for the show. Stainless is a grade of corrosion resistant steel. Often corrosion resistant steel (CRES) are confused as stainless. There are numerous grades of corrosion resistant steels and stainless steels. Some corrode. Surgical Stainless is the highest grade of stainless.
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First post....

Welcome to POA...

:cheers::cheers:
 
Thanks for the welcome! This may be my last post, LOL. I stumbled upon this thread out on the Internet while doing other research. I'm not a pilot, I'm a QC Maintenance Inspector for an airline, and saw a need for a little more input about the subject.
 
The screws holding the fuel tanks in the wings of cherokees are structural, not trim. Not familiar with other types.
 
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