steel bolt and nut colors

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Hi

I have a noob question for material experts. I was shopping some bolts and nuts for my exhaust system today. I notice that even though all those bolts and nuts are made of steel, they have different colors. Some are black; some are shining silver; some are grey and some are brown yellowish. When I asked the clerks about the difference, they all reply me with typical and vague answers such as "they are suitable for general uses".

If you know why they had different colors, please share. What I like to know the most are their mechanical properties and their corrosion resistance.

thx
 
Black is some "black oxide" protective coating. Shining silver is usually zinc coated. Iridescent yellow-green-red-brown used to be cadmium plated, now is yellow zinc. Not sure I've seen a dull grey to my memory, but I may not be thinking straight.

All are corrosion protection to different levels/for different looks.
 
Regardless of color or protective coatings, 300 series austenitic stainless steels, which make great exhaust fasteners since they are totally rust free forever; and easy to identify even if thrown in a bucket of "steel" bolts, they are nonmagnetic. https://www.sandmeyersteel.com/austenitic-family.html

You don't say anything about what kind of car/engine, but I have stainless exhaust bolts on my vehicle: https://zzperformance.com/collections/3800/products/arp-stainless-header-bolts

I also used Loctite 771 Nickel Antiseize on them; label says it's good to 2400 F....Overkill, but they're not going to seize if I need to disassemble. Word of caution: Stainless STRETCHES almost twice as much as carbon steel; don't overtighten them, you can cause other problems you don't want....Galled threads or broken off bolts in a cylinder head. I torqued mine to factory specs minus 10% (since the antiseize lubed the threads). Been on there for 5+ years and I promise they'll loosen if needed, I know this because I've turned them occasionally while under the hood for other stuff.
 
I've seen the brown/yellow more on higher(strength) grade fasteners. Could be a coincidence.

Be sure to dope up whatever you get in anti-sieze.
 
Originally Posted by Ihatetochangeoil
Regardless of color or protective coatings, 300 series austenitic stainless steels, which make great exhaust fasteners since they are totally rust free forever; and easy to identify even if thrown in a bucket of "steel" bolts, they are nonmagnetic. https://www.sandmeyersteel.com/austenitic-family.html

You don't say anything about what kind of car/engine, but I have stainless exhaust bolts on my vehicle: https://zzperformance.com/collections/3800/products/arp-stainless-header-bolts

I also used Loctite 771 Nickel Antiseize on them; label says it's good to 2400 F....Overkill, but they're not going to seize if I need to disassemble. Word of caution: Stainless STRETCHES almost twice as much as carbon steel; don't overtighten them, you can cause other problems you don't want....Galled threads or broken off bolts in a cylinder head. I torqued mine to factory specs minus 10% (since the antiseize lubed the threads). Been on there for 5+ years and I promise they'll loosen if needed, I know this because I've turned them occasionally while under the hood for other stuff.


It's not only for my exhaust system that I like to know why they have different colors. If their specifications are known, it would be easier for us to choose the "right" bolts and nuts for other mechanic and home jobs. They have the similar price (depends the stores), but I like to know why they are made differently.

btw, my beater is a honda civic 2000 dx. I also bought a bottle of high temperature nickel anti-seize.
 
Last edited:
Originally Posted by JHZR2
Black is some "black oxide" protective coating. Shining silver is usually zinc coated. Iridescent yellow-green-red-brown used to be cadmium plated, now is yellow zinc. Not sure I've seen a dull grey to my memory, but I may not be thinking straight.

All are corrosion protection to different levels/for different looks.


I was not aware the industry was moving away from cadmium. This is good to know. Another warning about stainless steel- unless otherwise specified, SS is weak and brittle compared to generic steel. Make sure if you use SS you know exactly what alloy, and is it correct for that use. Pretty much the only "strong" SS I use would be from ARP.
 
Originally Posted by rubberchicken
Originally Posted by JHZR2
Black is some "black oxide" protective coating. Shining silver is usually zinc coated. Iridescent yellow-green-red-brown used to be cadmium plated, now is yellow zinc. Not sure I've seen a dull grey to my memory, but I may not be thinking straight.

All are corrosion protection to different levels/for different looks.


I was not aware the industry was moving away from cadmium. This is good to know. Another warning about stainless steel- unless otherwise specified, SS is weak and brittle compared to generic steel. Make sure if you use SS you know exactly what alloy, and is it correct for that use. Pretty much the only "strong" SS I use would be from ARP.


Yeah... I always ask why it's so hard to find SS exhaust bolt kit in any general auto part store. Probably the mechanical properties of standard SS are not suitable for exhaust system.
 
Grade 8 , 5 bars on the head. gr 5 has 3 bars and gr 2 is plain. The only place I use gr 8 exhaust is the manifold /down pipe connection. Best nuts were the long brass ones. Close second was the ovaled out copper nuts used on my 528e I cheap out and go with gr2 stuff on other exhaust stuff. Easy shear ,one time use.No fan of stainless. You won't be either if you shear a stainless bolt and have to drill it out.
grin2.gif
 
I don't think anything above a Grade 2 should be used for the exhaust system. If the fastener rusts in place or snaps off, you'll be tasked with cutting or drilling the fastener.

I don't understand the comment of stainless steel being weak and brittle compared with steel. It's only true if designed that way. Some stainless steels are quite strong. Some are even used as roller bearing steel.

Cadmium is a poison. It is in the same class as lead and hexavalent chromium. I can't imagine it being used outside of military applications.
 
Originally Posted by ST2008
Originally Posted by rubberchicken
Originally Posted by JHZR2
Black is some "black oxide" protective coating. Shining silver is usually zinc coated. Iridescent yellow-green-red-brown used to be cadmium plated, now is yellow zinc. Not sure I've seen a dull grey to my memory, but I may not be thinking straight.

All are corrosion protection to different levels/for different looks.


I was not aware the industry was moving away from cadmium. This is good to know. Another warning about stainless steel- unless otherwise specified, SS is weak and brittle compared to generic steel. Make sure if you use SS you know exactly what alloy, and is it correct for that use. Pretty much the only "strong" SS I use would be from ARP.


Yeah... I always ask why it's so hard to find SS exhaust bolt kit in any general auto part store. Probably the mechanical properties of standard SS are not suitable for exhaust system.


If anyone had read my post and followed my links, you would see that ARP MANUFACTURED my 304 ss exhaust studs. I know what alloy they are and how correct they are for the use. And yes, it is true that while 300 series stainless is "weak" compared to hardened bolts, it IS strong enough and corrosion resistant to make excellent exhaust fasteners, else why would ARP make them? And stainless is ANYTHING but "brittle," having a percentage of elongation of 55%, over twice that of carbon steel, metallurgically speaking, it's more like a rubber band. https://www.machinedesign.com/materials/comparing-stainless-steel-and-other-metals

My guess as to why it's so hard to find SS exhaust bolts in a general auto parts store is simply cost. My local Lowe's has a good supply of stainless bolts and nuts, and if you're unsure of what stainless alloy generic stainless bolts are, a pocket magnet will separate them. 400 series stainless is slightly magnetic and will rust a little, while 300 series is totally nonmagnetic and will NOT rust.
 
I am going through the stainless steel nut and bolt search for exhaust myself. Some SS is rated at grade 5 strength; Bumax 88 Stainless Steel Tensile Strength 110,000 psi is one example. I like the strength, that it will not rust then seize in place but for this, it is expensive $3 or more per bolt depending on size.

https://www.mcmaster.com/stainless-steel-bolts
 
Originally Posted by Ihatetochangeoil
Originally Posted by ST2008
Originally Posted by rubberchicken
Originally Posted by JHZR2
Black is some "black oxide" protective coating. Shining silver is usually zinc coated. Iridescent yellow-green-red-brown used to be cadmium plated, now is yellow zinc. Not sure I've seen a dull grey to my memory, but I may not be thinking straight.

All are corrosion protection to different levels/for different looks.


I was not aware the industry was moving away from cadmium. This is good to know. Another warning about stainless steel- unless otherwise specified, SS is weak and brittle compared to generic steel. Make sure if you use SS you know exactly what alloy, and is it correct for that use. Pretty much the only "strong" SS I use would be from ARP.


Yeah... I always ask why it's so hard to find SS exhaust bolt kit in any general auto part store. Probably the mechanical properties of standard SS are not suitable for exhaust system.


If anyone had read my post and followed my links, you would see that ARP MANUFACTURED my 304 ss exhaust studs. I know what alloy they are and how correct they are for the use. And yes, it is true that while 300 series stainless is "weak" compared to hardened bolts, it IS strong enough and corrosion resistant to make excellent exhaust fasteners, else why would ARP make them? And stainless is ANYTHING but "brittle," having a percentage of elongation of 55%, over twice that of carbon steel, metallurgically speaking, it's more like a rubber band. https://www.machinedesign.com/materials/comparing-stainless-steel-and-other-metals

My guess as to why it's so hard to find SS exhaust bolts in a general auto parts store is simply cost. My local Lowe's has a good supply of stainless bolts and nuts, and if you're unsure of what stainless alloy generic stainless bolts are, a pocket magnet will separate them. 400 series stainless is slightly magnetic and will rust a little, while 300 series is totally nonmagnetic and will NOT rust.


I used the term brittle when perhaps I should have used the term weak. Like you, when I need SS that is strong, I use ARP products. Most people that go to hardware stores and get SS fasteners are looking for corrosion resistance and are very price sensitive, and these products are not nearly as good as grade 8 , probably not even as good as grade 3. I did have a huge fup with ARP bolts in a boat engine exhaust manifolds, and ARP gave me excellent technical support for the problem. In the end, I had to pull the engine, cut off the heads of the bolts with a cut-off wheel which in turn ruined the manifolds. The bolts I used were too long by .25 inch, bottomed against blind holes, and could not be removed by impact wrench, and cutting the heads off then drilling the ARP bolts with cobalt bits was too expensive- I was going through about 3 cobalt bits for each fastener, so at that rate it would be about $500 for bits, or buy an expensive drill sharpener that can handle cobalt bits, or just write off the exhaust manifolds and replace them. Very costly mistake, and I should have spent about $20 more for proper exhaust studs rather than bolts.
 
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