Galvanic corrosion btwn black oxide coated hardened steel fasteners
Galvanic corrosion btwn black oxide coated hardened steel fasteners
(OP)
and cast aluminum?
Help me answer a question folks who are much more technically savvy than I am. Here's my situation.
I'm an amateur hot-rodder/engine-builder and recently had the opportunity to get a Harley (thanks wife!). Due to necessity and personal experience, I had to replace the chromed mild steel exhaust studs with some hardened, black oxide coated grade 8 bolts. I don't know they exact alloy, but I got them at Lowes so whatever a standard bolt steel is. If you aren't familiar with Harley Davidson motors, they run an air-cooled, cast aluminum cylinder head with a fairly large surface area. I coated the fasteners with Permatex anti-seize to help with the corrosion and thermal expansion.
Now every "old school" HD owner is screaming at me to get those terrible fasteners out of my head "before they destroy the holes" and "cause me all kinds of problems". Right. This makes little sense to me, but you folks probably know better than anyone else.
The fasteners are 5/16" x 1" long, there are four of them. The motor is an airhead so there is zero chance of any contact with any fluid aside from maybe some road spray on the front cylinder which would be cleaned off that day. Head temperatures will not exceed about 380°F.
How much (if any) galvanic corrosion am I looking at here?
My gut and previous experience with Grade 8 fasteners tells me I don't have anything to worry about. I called the folks at ARP fasteners and they agree with me. However, you folks probably have a lot more technical experience than I do.
Help me answer a question folks who are much more technically savvy than I am. Here's my situation.
I'm an amateur hot-rodder/engine-builder and recently had the opportunity to get a Harley (thanks wife!). Due to necessity and personal experience, I had to replace the chromed mild steel exhaust studs with some hardened, black oxide coated grade 8 bolts. I don't know they exact alloy, but I got them at Lowes so whatever a standard bolt steel is. If you aren't familiar with Harley Davidson motors, they run an air-cooled, cast aluminum cylinder head with a fairly large surface area. I coated the fasteners with Permatex anti-seize to help with the corrosion and thermal expansion.
Now every "old school" HD owner is screaming at me to get those terrible fasteners out of my head "before they destroy the holes" and "cause me all kinds of problems". Right. This makes little sense to me, but you folks probably know better than anyone else.
The fasteners are 5/16" x 1" long, there are four of them. The motor is an airhead so there is zero chance of any contact with any fluid aside from maybe some road spray on the front cylinder which would be cleaned off that day. Head temperatures will not exceed about 380°F.
How much (if any) galvanic corrosion am I looking at here?
My gut and previous experience with Grade 8 fasteners tells me I don't have anything to worry about. I called the folks at ARP fasteners and they agree with me. However, you folks probably have a lot more technical experience than I do.





RE: Galvanic corrosion btwn black oxide coated hardened steel fasteners
Here is information to a zinc-based anti-seize (ASC 1900 Zn) used around aircraft and motorcycles for steel to Al fasteners.
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RE: Galvanic corrosion btwn black oxide coated hardened steel fasteners
The second is questionable, and it depends somewhat on the details (frequency of exposure to rain/salt/etc., frequency of cleaning, use of anti-seize, etc.). Did you tell ARP your exact application and that you purchased the studs from Lowes? While black oxide may be commonly used on parts for automobile engines where they are constantly exposed to engine oil and always within some type of cover or housing, it is rarely/never used for highly-stressed fasteners that are exposed to the elements such as on engines for motorcycles, ATV's, construction/farm equipment, etc. At the very least I would buy some ARP studs with black oxide to replace the junk that is currently on your hog.
RE: Galvanic corrosion btwn black oxide coated hardened steel fasteners
Now armed with accurate information from a reputable source, I will change out the shelf-stock Lowes Grade 8 oxide bolts with a proper set of studs.
I appreciate the time folks.
RE: Galvanic corrosion btwn black oxide coated hardened steel fasteners
RE: Galvanic corrosion btwn black oxide coated hardened steel fasteners
There is enough heat generated when you tighten close tolerance fasteners into Al. It is just like bolting SS you open up the tolerances to minimize galling. Most of the problem comes when one is try overcome the breakaway torque.
Addendum:
When these gaskets were in the development stage a total of 96 gaskets were installed on one machine. The line ran about 9 months and when they start to disassemble the components would com apart, welded to gather on the gasket surface. The R&D people went into a panic as it seems the bench scale tests the parts were anodized and dyed to check the various designs and the first lot of 6,000 production gaskets were not. To get the gaskets into service on the second line I used a spray Molykote material to coat about 1,000 and setup to chromized the rest. The parts that were welded togather were put in a 1100°F oven and separated with no problem. The Al was removed in modified cleaning bath.
RE: Galvanic corrosion btwn black oxide coated hardened steel fasteners
Now get the correct anti-seize. The link that Syd listed is where you want to go. One made for steel-on-steel or for stainless will not work for this application.
Keep an eye on these. If it looks like you are getting any corrosion then take them apart to clean and replace.
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