Galvanic corrosion btwn black oxide coated hardened steel fasteners

[speedfoos]

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.

 

[unclesyd]

Usually what happens it case like yours is that the Al oxidizes and causes the bolt to bind. Then you turn this around and some say no anti-seize.
Here is information to a zinc-based anti-seize (ASC 1900 Zn) used around aircraft and motorcycles for steel to Al fasteners.

http://www.superior-industries.com/_1900_anti_seize_product_238.html

 

[TVP]

There are a couple of different issues here. First is using an off-the-shelf stud from Lowes. Second is using a black oxide coating on an exposed steel fastener threaded into an Al block. The first is a definite no-no, and I would make fun of you as well. There is a huge difference between grade 8 fasteners (SAE J429) and high quality engine studs like those from ARP, etc. as compared to the type of hardware that is readily available at a big box store. SAE J429 specifically requires rolled threads and the studs are ground prior to threading so that they have excellent straightness. Most fasteners at Lowes, etc. have cut threads, numerous defects (thread laps, excessive surface roughness, head and shank cracks, sharp thread runout, etc.), and poor dimensional control (head to shank perpendicularity, shank straightness, etc.).

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.

 

[speedfoos]

This is exactly the kind of information I was looking for from the folks who do this for a living. I'm already contending with HD owners who are set in their ways and simply repeat advice they've heard or read but with little practical application. The common one that makes my skin crawl is "an engine needs back pressure to make torque". Gah. I could throw up every time I read that. An engine needs exhaust velocity to make torque, not back pressure. There are others, but I wanted to make sure this wasn't the case before blindly following someone's advice.

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.

 

[blacksmith37]

How many answers do you want ? Even if you use the engine for a boat anchor ,the aluminum will be anodic (protect) the steel. At 380F you are even below "relaxation" temp for steels (unless it it is "file hard" , > HRC 60). Your anti seize should prevent galling (where bare-no oxide- Al micro welds to a mating surface) , so it is difficult to imagine a problem. Unless the threads do not have proper dimensions).

 

[unclesyd]

The above mentioned "micro-welding" is really thermite welding, Al + Iron Oxide. One way to minimize this is Anodize the Al or coat it with an anti-seize. We use all three approaches. We use a lot of self-energising Al gaskets for polymer at 600°F and up to 2200 psig, All these are required to be Anodized or Molykote. The gaskets work against A2, D2, the PH steels. The gasket will weld itself to the steels due to it trying to be extruded between the mating components under normal operating conditions due to a slight movement. If you have pressure excursion even more so due to the thinner part getting a little bow.
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.

 

[EdStainless]

Speed, you are doing right getting the correct studs.
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.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube