2009 C2S 186K miles

Occasionally, I will remove a bolt and it will have surface rust. Example, the upper bolts holding my engine mounts were pretty darn corroded... surface rust, no indentations, but ugly. Actually the two right side bolts were, the left were fine... really odd. This bolt is not critical in that it is not taking big loads but I still don't like this.

What do you do?
Why do some corrode and others not?
Is this OK?

Normally, I would just purchase new ones just to be safe, but I never discover this until I am into a project and don't want to stop and wait four days for shipping. So.... I spray on some PB Blaster, soak, use a wire brush to lightly remove what is left, inspect for integrity, and re-use the bolt. I usually will re-position the bolt with another like one assuming that the position contributed to the corrosion. I never had say a brake bolt corrode.

Peace
Bruce in Philly (now Atlanta)

 

Soak in CLR for a while, will be very clean.
you can also use apple cider vinegar.

 

The best way to remove rust is Oxalic acid bath or Evaporust. I've restored some of 80's BMX chromoly bikes and soaking them in Oxalic acid removed the rust to the point where you can wipe it right off. I follow up with LPS-3 rust inhibitor

 

The reason some bolts rust whereas others do not might be that the (perhaps galvanized?) coating was thin or scratched, allowing rusting to start.

For potentially rusty fasteners, replace with stainless steel fasteners.

For rusty non-bolts, there are some products out there that convert rust to a more inert iron compound that no longer rusts. I used some in the 80s and 90s and cannot remember its name for the life of me but there are a host of modern equivalents out there today.

 

Por15?

 

One option is the black oxide finish gunsmiths use to blue a firearm. There's one called ospho blue by burnell and it's decent. The one I used when I had to knurl a bar for the gym was permatex rust treatment. It help up amazing. Two years later and the hand sweat hasn't even rubbed it off yet.

 

Replace.

 

Replace where possible. Navel Jelly works well at removing rust, then coat w oil or a clear coat to prevent it from happening again.

 

Unless it's a hardened bolt like a Grade 8 or 12 that holds a seat bracket or has another high strength requirement, every fastener I remove gets replaced with a stainless one. Looks better and no issues when removing it the next time since I'll be the one doing that.

 

Generally, bolts that are not pretty do not need to be replaced. If the wrench still fits the head and the nut screws off of the exposed threads, you should be good to go. Rusted beyond recognition or just plain ugly then go ahead and replace. fine thread metric bolts are readily available in the correct size and cheap. This was not the case 20 years ago. If you want to reuse them clean them up as noted above, wire brush the threads and use anti-sieze or thread locker depending on application. Most bolts are electro zinc plated , gold or silver and many new automotive fasteners are a green coating which is very durable. For exhaust work use stainless or bronze fasteners. Stainless works in almost every application but it will lock up into aluminum just like steel without a coating. Since I am also into restorations of classic cars, I like the original look if possible. Many concours have been lost by only one bolt having the incorrect head markings.
For metric series, the 10.8 is sorta equal to grade 5 and 10.9 equal to grade 8, both are hardened. Home Depot is not an automotive supply house and stainless steel (general purpose) is not hardened and not as high strength as most people think.
Stainless is a good replacement for garden variety stuff as Pete mentioned. General purpose stainless bolts that you will find are greater than grade 2 and less than grade 5.